nuevas aplicaciones electrocatalÍticas para

UNIVERSIDAD DE CASTILLA-LA MANCHA

FACULTAD DE CIENCIAS Y TECNOLOGÍAS QUÍMICAS

DEPARTAMENTO DE INGENIERÍA QUÍMICA

NUEVAS APLICACIONES ELECTROCATALÍTICAS PARA

PROCESOS ENERGÉTICOS Y DE REMEDIACIÓN

MEDIOAMBIENTAL

Memoria que para optar al grado de Doctor en Ingeniería Química

presenta:

NURIA GUTIÉRREZ GUERRA

Directores: Dr. José Luis Valverde Palomino

Dr. Antonio de Lucas Consuegra

Composición del tribunal: Dr. Jesús Arauzo Pérez

Dra. Paula Sánchez Paredes

Dr. Juan Carlos Serrano Ruíz

Profesores que han emitido informes favorable de la tesis: Dra. Amaya Romero Izquierdo

Dr. Ángel Caravaca Huertas

Ciudad Real, Noviembre de 2015

D. José Luis Valverde Palomino, Catedrático de Ingeniería

Química de la Universidad de Castilla-La Mancha, y D. Antonio de

Lucas Consuegra, Profesor Contratado Doctor de Ingeniería

Química de la Universidad de Castilla-La Mancha,

CERTIFICAN: Que el presente trabajo de investigación titulado:

“Nuevas aplicaciones electrocatalíticas para procesos energéticos y de

remediación medioambiental” constituye la memoria que Dª. Nuria

Gutiérrez Guerra para aspirar al grado de Doctor en Ingeniería

Química y que ha sido realizada en los laboratorios del

Departamento de Ingeniería Química de la Universidad de Castilla-

La Mancha bajo su supervisión.

Y para que conste a los efectos oportunos, firman el presente

certificado

En Ciudad Real, a 15 de Octubre de 2015

D. José Luis Valverde Palomino D. Antonio de Lucas Consuegra

i

TABLE OF CONTENTS

DESCRIPCIÓN DEL TRABAJO REALIZADO 1

A. Introducción 2

A.1. El hidrógeno como vector energético 2

A.2. El gas de síntesis como fuente de combustibles sintéticos 11

A.3. El dióxido de carbono como gas de efecto invernadero 16

A.4. Reactores de membrana de electrolito sólido (SEMRs) 19

A.5. Reactores electroquímicos de membrana polimérica (PEM) 28

A.6. Objetivo del presente trabajo 32

B. Instalaciones experimentales 33

B.1. Reactor SEMR de cámara sencilla 33

B.2. Reactor SEMR de doble cámara 34

B.3. Reactor PEM 35

C. Resultados y discusión 36

D. Conclusiones y recomendaciones 44

E. Bibliografía 46

CHAPTER 1: Direct Production of Flexible H2/CO Synthesis Gas

via Steam Electrolysis and Ethanol Partial Oxidation 55

Abstract 55

1.1. Introduction 56

1.2. Experimental 58

1.2.1. Catalytic activity measurements 58

1.2.2. Preparation of the solid electrolyte cell 60

1.2.3. Characterization measurements 61

1.3. Results and discussion 61

1.4. Conclusions 74

1.5. References 74

ii

CHAPTER 2: Simultaneous Production and Separation of H2 and

C2 Hydrocarbons via Steam Electrolysis and Methane Partial

Oxidation.

81

Abstract 81







2.4. Conclusions 99

2.5. References 100

CHAPTER 3. Electrochemical Reforming vs. Catalytic

Reforming of Ethanol: A Process Energy Analysis for Hydrogen

Production

107

Abstract 107


3.2. Methodology 111

3.3. Process description 112

3.3.1. Catalytic steam reforming of ethanol-water process 112

3.3.2. Electrochemical reforming of ethanol-water process 117

3.4. Process simulation and energetic evaluation 119

3.5. Conclusions 129

3.6. References 130

CHAPTER 4: Electrochemical Promotion of Ni with Alkali ions

in the CO2 Hydrogenation Toward CO and CH4 137

Abstract 137



4.2.1. Catalytic activity measurements and EPOC parameters 143



iii


4.3.1. Influence of the preparation method of the catalyst film 148

4.3.2. Kinetic study and electrochemical promotion experiments 158


4.5. References 170

CHAPTER 5: Gas Phase Electrocatalytic Conversion of CO2 on

Cu Carbon-based Catalyst-electrodes Toward Fuels 177

Abstract 177




5.2.2. Preparation of the electrochemical catalyst 183



5.3.1. Characterization of the Cu cathodic-catalyst and deposited

electrodes 186

5.3.2. Electrocatalytic experiments for CO2 conversion 197


5.5. References 206

CHAPTER 6: General Conclusions and Recommendations 213


6.2. Recommendations 214

List of Publications and Conferences 219

Anexo. Patente 221

A. Introducción

A.1. El hidrógeno como vector

energético

A.2. El gas de síntesis como fuente de

combustibles sintéticos

A.3. El dióxido de carbono como gas de

efecto invernadero

A.4. Reactores de membrana de

electrolito sólido (SEMRs)

A.5. Reactores electroquímicos de

membrana polimérica (PEM)

A.6. Objetivo del presente trabajo

B. Instalaciones experimentales

B.1. Reactor SEMR de cámara sencilla

B.2. Reactor SEMR de doble cámara

B.3. Reactor PEM

C. Resultados y discusión

D. Conclusiones y recomendaciones

F. Bibliografía

Descripción del Trabajo

Realizado

1

ste trabajo forma parte de un amplio programa de investigación

sobre la aplicación de sistemas electrocatalíticos en procesos de

interés energético y medioambiental que se viene desarrollando,

durante los últimos años en el Departamento de Ingeniería Química de la

Universidad de Castilla-La Mancha.

En particular, esta Tesis Doctoral tiene como objetivo el uso de nuevos

sistemas electrocatalíticos para la producción de hidrógeno e hidrocarburos

y la valorización de CO2. Este trabajo ha sido financiado por el centro de

investigación ABENGOA Research, a través del proyecto Aplicación de la

electrocatálisis en la producción de H2 acoplada a otros procesos de interés

industrial y medioambiental (proyecto Electrocatal).

Esta tesis doctoral se ha realizado en colaboración con el instituto de

Ciencia de Materiales de Madrid (CSIC) y con el centro de investigación

ABENGOA Research.

E

Descripción del trabajo realizado

2

A. Introducción

A.1. El hidrógeno como vector energético

El modelo energético actual, basado en combustibles fósiles, presenta

serios problemas de sostenibilidad. En primer lugar, por el agotamiento

progresivo del petróleo y las sucesivas crisis que afectan periódicamente a

su producción, y que repercuten en la estabilidad internacional y afectan al

precio del crudo, alterando, de este modo, el equilibrio económico mundial.

En segundo lugar, por los impactos ambientales que se derivan del uso

intensivo de los recursos energéticos fósiles. Por todo ello, resulta evidente

la necesidad de buscar nuevas alternativas energéticas.

Una de ellas se basa en el uso del hidrógeno como vector energético

(“economía del hidrógeno”). Grandes potencias como Estados Unidos,

Japón y la Unión Europea apuestan por el cambio del modelo energético

actual, basado en el uso de combustibles fósiles, por otro basado en el

hidrógeno. Tres son las causas que motivan este escenario [1]:

- Elevada eficiencia energética: la energía química del hidrógeno puede

convertirse de forma directa en electricidad, sin necesidad de emplear un

ciclo termodinámico intermedio, alcanzándose de este modo elevados

rendimientos energéticos. Esta transformación directa se lleva a cabo en

las llamadas celdas de combustible, capaces de convertir, de manera

electroquímica, la energía química del hidrógeno en energía eléctrica.

- Reducción de la dependencia energética: En la actualidad hay una

fuerte dependencia a escala mundial de los combustibles fósiles.

Numéricamente, los hidrocarburos aportan más de la mitad de la energía

primaria consumida a escala planetaria. El petróleo, en particular, aporta

el 32 % del consumo energético primario global, lo que la convierte en la

fuente energética más utilizada [2]. Las reservas de petróleo podrían


3

agotarse en el escenario actual de producción y de reservas conocidas en

40 años, las del gas natural en 60 y las de carbón en 200 [2]. Esta

limitación de reservas va unida en muchas ocasiones a una elevada

concentración de los yacimientos, lo que facilita presiones políticas por

parte de los países productores.

- Razones medioambientales: La combustión de hidrógeno con oxígeno

puro solo libera vapor de agua, libre de CO2 [3].

A.1.1. Procesos de producción de hidrógeno

Debido a que el hidrógeno no se encuentra aislado en la naturaleza, es

preciso obtenerlo a partir de otras materias primas. En la actualidad

existen procesos muy diferentes que permiten la obtención de hidrógeno a

partir de una gran variedad de materias primas. A continuación se

exponen las formas de producción más relevantes.

i) Procesos biológicos

Estos métodos están menos desarrollados que los que se derivan del

uso de hidrocarburos, aunque su importancia aumenta debido a las

bajas emisiones de CO2 que generan.

- Conversión de biomasa

La valoración de la biomasa puede realizarse a través de cuatro

procesos básicos (combustión, digestión anaerobia, gasificación y

pirólisis) mediante los cuales se puede transformar en calor y

electricidad.

- Producción biológica de hidrógeno

La producción biológica de hidrógeno se lleva a cabo en biorreactores

usando cianobacterias y algas verdes que aprovechan la luz solar. La

presencia de la enzima hidrogenasa en condiciones anaeróbicas permite

a estos organismos realizar, con la ayuda de la fotosíntesis, la reducción

catalítica de protones a H2.


4

ii) Procesos de conversión química

Los procesos de reformado son los más habituales para la obtención de

hidrógeno. La producción de hidrógeno mundial está alrededor de 500

billones Nm3/año, de la que cerca del 96 % corresponde con procesos de

reformado con vapor de agua de combustibles fósiles, como gas natural

(cuyo componente principal es el metano) que es en la actualidad la

principal fuente para la producción de hidrógeno (49 %) [4].

- Reformado con vapor de agua (SR)

Como se ha comentado anteriormente, la principal vía industrial de

producción de hidrógeno es el reformado de metano con vapor de agua.

Sin embargo, el hidrógeno también se puede obtener por reformado de

otras materias primas como el carbón, el metanol y el etanol. La

reacción, altamente endotérmica, que tiene lugar en estos procesos de

reformado con vapor de agua es la siguiente:

CnHm + nH2O → nCO + (n +m

2)H2 (A.1)

En los procesos convencionales de reformado con vapor de agua, el

combustible reacciona a elevada temperatura y presión (entre 500-950

ºC y 1-25 atmósferas de presión) con la finalidad de producir una mezcla

de H2 y de CO (gas de síntesis) [5]. Los catalizadores más utilizados son

los basados en Ni soportados sobre sílice, alúmina, ceria o zirconia [6].

El uso de este tipo de catalizadores conlleva sin embargo una serie de

inconvenientes como son: las elevadas temperaturas de operación

necesarias y los problemas de desactivación del catalizador originados

por deposición de filamentos de carbono [7].

En las últimas décadas se han incrementado los esfuerzos por

desarrollar catalizadores que sean eficientes a temperaturas de

operación inferiores y reduzcan los problemas asociados a la


5

desactivación. En este sentido, el uso de catalizadores nobles como Pt [8]

o Rh [9] ha demostrado que puede satisfacer ambos requerimientos.

Asociados a los procesos industriales de reformado de hidrocarburos

con vapor de agua, suelen coexistir otros procesos de conversión como el

de desplazamiento de CO con vapor de agua, convencionalmente

conocido como “Water Gas Shift” (WGS). Este proceso permite

incrementar la producción de hidrógeno mediante la reacción del

monóxido de carbono presente en el medio de reacción con vapor de

agua: CO + H2O ↔ CO2 + H2. Debido a motivos termodinámicos

(reacción exotérmica) y cinéticos, la reacción WGS se realiza con ayuda

de catalizadores en dos pasos: el primero a alta temperatura (HTS) y el

segundo a baja temperatura (LTS). El reactor HTS opera a elevadas

temperaturas (473-523 K) [10] mientras que el reactor LTS lo hace a

temperaturas inferiores (473-523 K) [10]. En el proceso a elevada

temperatura suelen utilizarse catalizadores de Fe3O4/Cr2O3, mientras

que en el de baja temperatura suelen emplearse catalizadores de Cu-

ZnO-Al2O3. En conjunto, se obtiene una conversión final de CO superior

al 99,5 % [7].

- Oxidación parcial (PO)

En los procesos de oxidación parcial de hidrocarburos, el combustible

se hace reaccionar con oxígeno en una proporción inferior a la requerida

para la combustión completa del mismo. La reacción que tiene lugar en

la oxidación parcial es la siguiente:

CnHm + n

2O2 → nCO +

m

2H2 (A.2)

Este proceso se puede llevar a cabo en ausencia de catalizador a

temperaturas comprendidas entre 1150 y 1350ºC [11-13], obteniéndose

un gas constituido por H2, CO, CO2 y CH4 [13]. Sin embargo, el uso de

catalizadores presenta ciertas ventajas operativas: disminución de la

temperatura de trabajo y de los problemas asociados a la deposición del


6

carbono (bloqueo del reactor, incremento de la presión, etc.). Los

catalizadores más utilizados en este proceso son los basados en Ni, Rh o

Pt, aunque también se utilizan aleaciones metálicas (Al2O3, La2O3 y

MgO) que aseguran condiciones apropiadas de actividad y selectividad

durante 100 horas de operación [11]. La reacción de oxidación parcial es

una reacción exotérmica que no requiere de sistemas auxiliares para

mantener la temperatura de reacción. Por otro lado, el control de la

temperatura es complicado como consecuencia de la exotermicidad de la

reacción ya que se pueden formar puntos calientes [14], que pueden

originar desactivación de los catalizadores por fenómenos de

sinterización.

Igual que en el proceso de reformado descrito anteriormente, este

proceso se puede acoplar a una unidad de desplazamiento de CO con

vapor de agua (WGS) con la que incrementar la producción de

hidrógeno.

Atendiendo a la economía de hidrógeno, los procesos de reformado

resultan más eficientes, ya que la cantidad de hidrógeno producida en

estos es el doble que la producida mediante su oxidación parcial [1].

- Reformado autotérmico (ATR)

El reformado autotérmico es un proceso muy estudiado y se ha usado

por la industria durante más de 50 años [15]. Se trata de un proceso que

combina los de reformado y oxidación parcial, anteriormente descritos.

En este caso se utiliza la exotermicidad de la reacción de oxidación

parcial para aportar el calor necesario a la reacción de reformado con

vapor de agua, buscando un balance de energía neto nulo. Al igual que

en los procesos anteriores, el CO producido puede ser desplazado con

vapor de agua lo que permite incrementar la producción de hidrógeno.

La reacción que se produce se muestra a continuación:

CnHm +n

2H2O +

n

4O2 → nCO + (

n

2+

m

2) H2 (A.3)


7

El valor de la temperatura de operación oscila entre los máximos de

las temperatura de los otros dos procesos mientras que los catalizadores

empleados son muy similares a los que se utilizan en el reformado con

vapor de agua y en el proceso de oxidación parcial [16]. Al igual que en

este último proceso, se requiere de complejas unidades de separación

con objeto de que el oxígeno alimentado al reactor sea puro.

iii) Procesos electroquímicos. Electrólisis

El proceso de electrólisis consiste en la ruptura de moléculas por la

acción de una corriente eléctrica. Este es el caso, por ejemplo, de la

molécula de agua con la que se obtendrían corrientes puras de H2 y O2. La

electrólisis del agua presenta como principales ventajas la ausencia de

emisiones nocivas para el medio ambiente siempre que se use la fuente de

energía adecuada (eólica, solar, etc.) y la enorme disponibilidad del agua

(constituye aproximadamente 3/4 de la superficie terrestre). La electrólisis

del agua se puede llevar a cabo en tres tipos de electrolizadores:

electrolizadores alcalinos, que operan a bajas temperaturas,

electrolizadores basados en membranas poliméricas (electrolizadores de

baja temperatura, tipo PEM) y electrolizadores de electrolito sólido

(electrolizadores de alta temperatura, tipo SOEC). La electrólisis alcalina

de agua es, debido a su sencillez, uno de las tecnologías más desarrolladas

y aplicadas por la industria para la producción de H2. Sin embargo,

presentan un alto consumo energético, elevados inmovilizados y costes

mantenimiento, siendo limitadas la durabilidad y la seguridad de uso. Esto

está provocando que se estén desarrollando tecnologías más eficientes para

la producción de hidrógeno [17]. Una de estas tecnologías es la basada en

el uso de electrolizadores tipo PEM. A diferencia de los electrolizadores

alcalinos, los PEM requieren un electrolito no líquido, lo que simplifica el

diseño significativamente. Además, los electrolizadores PEM presentan


8

más ventajas frente a los electrolizadores alcalinos como son: un diseño

más compacto, elevada pureza del H2 obtenido, menor consumo energético,

alta conductividad protónica, mayor nivel de seguridad, un fácil manejo y

mantenimiento [18]. Los electrolizadores de alta temperatura, por su

parte, permiten producir hidrógeno con un menor rendimiento energético

debido a las mayores temperaturas de operación [4].

Existen otros métodos de producción de hidrógeno que se basan en la

ruptura de la molécula de agua mediante procesos de termólisis (por acción

del calor) y fotoelectrólisis (por acción de la luz solar). Sin embargo, ambos

procesos presentan eficiencias inferiores a las obtenidas con el proceso de

electrólisis.

A.1.2. Aplicaciones del hidrógeno

i) Materia prima de procesos químicos

En la actualidad, el hidrógeno se utiliza a gran escala como materia

prima en procesos de hidrogenación de la industria química y

petroquímica [19]. Se utiliza también en otras industrias como la

electrónica, la metalúrgica y la farmacéutica [20]. En este sentido, el 95 %

del hidrógeno producido se consume in-situ, utilizándose

fundamentalmente como materia prima de procesos químicos. En la Figura

A.1 se indican las fuentes de producción y las aplicaciones del hidrógeno.


9

FUENTE

DEMANDA

Figura A.1. Fuentes de producción y aplicaciones del hidrógeno.

Dentro de los usos del H2 en procesos químicos, destaca la síntesis de

amoniaco por el proceso Haber-Bosh (N2 + 3H2 → 2NH3) que opera a

elevadas presiones (alrededor de 250 atm) y temperaturas de operación de

450 ºC [21].

Como se ha comentado anteriormente, la aplicación más importante del

hidrógeno es en procesos de hidrogenación de la industria del refino. El

objetivo principal es la obtención de fracciones ligeras de crudo a partir de

fracciones pesadas, aumentando su contenido en hidrógeno y

disminuyendo su peso molecular. Estos procesos permiten la eliminación

de forma simultánea de elementos indeseados como azufre, nitrógeno y

metales [20].

Como se ha mencionado anteriormente, la mayor parte del hidrógeno

consumido a escala mundial se produce a partir de combustibles fósiles,

principalmente a partir del reformado con vapor de gas natural o metano.

La producción de hidrógeno a partir de hidrocarburos conduce a una

mezcla de gases formada principalmente por hidrógeno y monóxido de

carbono (con trazas de CO2), lo que comúnmente se denomina gas de

síntesis. El gas de síntesis se utiliza fundamentalmente en procesos


10

químicos, como la síntesis de metanol, síntesis de Fischer-Tropsch,

hidroformilación de olefinas (síntesis oxo) y síntesis de metanol y etileno,

entre otros compuestos [22].

ii) Generación de energía

El hidrógeno se puede transformar en energía de manera indirecta a

través de su combustión en turbinas de gas y ciclos combinados o

directamente como combustible de motores. Las principales ventajas del

uso del hidrógeno en estos procesos se relacionan con el elevado

rendimiento que puede alcanzarse en su combustión, el hecho de obtener

vapor de agua como único producto de reacción, y la ausencia de NOx en los

gases efluentes al poder controlar la temperatura a la que reaccionan el

nitrógeno y el oxígeno atmosféricos, y de CO2, menor impacto sobre el

calentamiento global [20].

La ruta más directa para convertir el hidrógeno en energía es a través

de pilas de combustible [23]. El descubrimiento de la pila de combustible

por el jurista inglés sir William Robert Grove en 1839 supuso un punto de

inflexión en la tecnología del hidrógeno, siendo numerosos los expertos que

coinciden en su gran importancia futura como sistema de generación de

energía eléctrica. La pila de combustible es un sistema electroquímico que

produce energía por la combinación de hidrógeno y oxígeno en una reacción

química. Esta conversión de energía química en forma de energía eléctrica

se realiza separando el combustible y el comburente (oxígeno o aire) con

una membrana conductora iónica (por ejemplo, de iones oxígeno o

protones) que no permita el transporte de electrones. Esta membrana es

un electrolito. Los electrones necesarios para completar la reacción se

transportarán a través del circuito externo, donde realizarán el trabajo

útil. Por ejemplo, en una pila de combustible de baja temperatura

(PEMFC), el hidrógeno se alimenta al ánodo, donde un electrodo de Pt


11

facilita su electro-oxidación, formándose electrones y protones. Estos

últimos atraviesan una membrana conductora de protones, permitiendo su

combinación con oxígeno y electrones. Esta reacción está catalizada por Pt

[24]. Los electrones fluyen del ánodo al cátodo a través del circuito externo

pudiendo alimentar a algún dispositivo eléctrico [24, 25]. Este hidrógeno

debe ser de elevada pureza, conteniendo menos de 20 ppm de CO, lo que

evita la desactivación del catalizador [26], tal y como ha sido descrito en el

Capítulo 3 de esta memoria. Las principales ventajas que presenta el uso

industrial de pilas de combustible frente a otros sistemas tradicionales son:

admisión de diversos combustibles, flexibilidad de emplazamiento,

capacidad de cogeneración (pilas de alta temperatura), carácter modular,

seguridad energética e independencia de la red de suministro eléctrico.

A.2. El gas de síntesis como fuente de combustibles sintéticos

El uso del gas de síntesis (mezclas gaseosas de CO + H2) como materia

prima para la producción de combustibles sintéticos y de otros productos

importantes de síntesis supuso a lo largo del siglo XX el comienzo de una

nueva era de la industria química mundial [27]. La obtención de

combustibles líquidos sintéticos ofrecen alternativas interesantes a los

combustibles tradicionales. Estos combustibles suelen ser producidos a

partir de gas de síntesis, mediante las tecnologías X-To-Liquids (XTL).

Según la procedencia del gas de síntesis se distingue entre el proceso Gas-

To-Liquids (GTL), cuando el gas de síntesis proviene del gas natural; el

proceso Coal-To-Liquids (CTL), cuando el gas de síntesis proviene del

carbón; y el proceso Biomass-To-Liquids (BTL), cuando el gas de síntesis

procede de la biomasa. El interés en estas tecnologías está aumentando en

los últimos años debido a la disponibilidad de gas natural y carbón, las

ventajas medioambientales derivadas del uso de la biomasa, y los

inconvenientes e incertidumbres que existen alrededor del petróleo.


12

A.2.1. Procesos de producción de gas de síntesis

El gas de síntesis empezó a producirse a partir de la gasificación del

coque de hulla o de la destilación de lignitos con aire y vapor de agua.

Después de la segunda Guerra Mundial se introdujeron combustibles

fósiles líquidos y gaseosos, petróleo y gas natural. La principal ventaja de

estos combustibles reside en su contenido en hidrógeno. La proporción

aproximada de H/C del carbón es de 1:1, del petróleo 2:1 y de los gases

naturales ricos en metano 4:1 [27]. Como se verá posteriormente, un

parámetro importante es la razón H2/CO del gas de síntesis producido ya

que determina el tipo de aplicación para la que se va a utilizar éste. A

continuación, se describirán con más detalle los principales métodos

químicos para la producción de gas de síntesis:

i) Reformado del carbón

Este proceso consiste en hacer reaccionar vapor de agua con cualquier

material carbonoso de origen natural, según el siguiente esquema global

endotérmico:

H2O + C → CO + H2 (A.4)

La proporción de H2 y CO puede modificarse, como se ha comentado en

apartados anteriores, por medio de la reacción de desplazamiento con

vapor de agua, water gas shift (WGS) (H2O + CO → CO2 + H2), que

consume CO y genera H2. Los procesos de gasificación de carbón se

caracterizan por la elevada necesidad de energía del proceso que, por otra

parte, requiere de elevadas temperaturas (entre 900 y 1000 ºC) con las que

alcanzar una velocidad de reacción satisfactoria [27].

ii) Reformado de metano con vapor de agua

En la actualidad, el proceso más extendido para la producción de gas de

síntesis es el de reformado de metano (gas natural) con vapor de agua


13

sobre un catalizador metálico, debido a la elevadas disponibilidades y

reservas existentes de este gas [6]:

CH4 + H2O → CO + 3H2 (A.5)

La conversión completa del metano permite alcanzar razones H2/CO en

la corriente efluente del reactor de reformado de 3. Esta relación es mayor

que la necesaria para la síntesis de productos como metanol o los obtenidos

por medio del proceso de Fischer-Tropsch. Es por ello necesario ajustar la

relación H2/CO hasta un valor adecuado a través de la reacción WGS, lo

que encarece el proceso global.

Debido a la naturaleza endotérmica de la reacción (reacción A.5) se

necesitan temperaturas elevadas para maximizar la conversión de metano,

lo que implica el aporte de calor al sistema mediante la combustión de gas

natural usado como alimentación.

iii) Oxidación parcial de metano

Consiste en la oxidación parcial del CH4 con oxígeno a alta temperatura

mediante la siguiente reacción [28]:

2CH4 + O2 → 2CO + 4H2 (A.6)

Sin embargo, este método no ha sido usado industrialmente debido a

problemas derivados de la desactivación del catalizador. Sin embargo, se

ha desarrollado una variante electroquímica de este proceso que mejora

algunos aspectos del mismo [11]. En esta configuración, se utiliza un

reactor electroquímico de membrana con electrolito sólido (SEMR) al que

se alimenta CH4 (ánodo) y O2 (cátodo). Este sistema será estudiado en

detalle en el Capítulo 2.


14

A.2.2. Aplicaciones del gas de síntesis

El nombre de “gas de síntesis” proviene de su uso para la obtención de

gas natural sintético en la producción de amoniaco o metanol. El hidrógeno

presente en dicho gas, una vez purificado, puede ser utilizado directamente

en pilas de combustible tanto para la generación de electricidad como

combustible de vehículos eléctricos [27].

Durante muchos años, los procesos de gasificación del carbón se

utilizaron en la producción de gas de alumbrado (gas de hulla) usado en las

ciudades antes de que la iluminación eléctrica fuera una realidad.

El gas de síntesis también puede ser utilizado como combustible. Sin

embargo, posee menos de la mitad de densidad energética que el gas

natural lo que limita su utilización en este tipo de aplicaciones. Por este

motivo, se utiliza principalmente en la producción de combustibles para el

transporte y como producto intermedio para la síntesis de otros

compuestos químicos. Además, el gas de síntesis producido en las grandes

instalaciones de gasificación de residuos puede ser utilizado para generar

electricidad in-situ, disminuyendo los costes operativos de estas plantas.

El gas de síntesis también se utiliza como producto intermedio en la

producción de petróleo sintético, para su uso como lubricante o

combustible, a través del proceso de Fischer-Tropsch. Esta etapa puede

representar más del 50 % de los costes totales de inversión y una gran

parte de los costes operativos [29]. Del gas de síntesis también se puede

obtener gasolina a partir de metanol mediante el proceso Mobil. En la

Figura A.2 se resumen las principales aplicaciones del gas de síntesis.


15

Gas de síntesis

Metanol

Nafta

IntermediosAmoniaco

Metano- Olefinas

- Aromáticos

- Tolueno

- Etilenglicol

- Isobutano

Haber-Bosch

- Urea

- Hidrazina

- Metilaminas

- Ácido Nítrico

- Acrilonitrilo

- Fertilizantes

H2

CO

Metanación

Proceso

MOBIL- Olefinas aromáticas

(gasolinas)

- Metanol

- Ácido acético

- Formaldehido

- Metilamina

Figura A.2. Principales aplicaciones del gas de síntesis

La relación molar H2/CO del gas de síntesis es un parámetro

fundamental a la hora de determinar su posterior uso, ya que en función

de esta relación se podrán sintetizar unos compuestos determinados u

otros, tal y como se muestra en la Tabla A.1

Tabla A.1. Composición química del gas de síntesis y presencia de co-

reactantes en la producción de varios compuestos químicos y combustibles a partir

del mismo.

Proceso/producto Composición óptima Co-reactantes

Metanol H2/CO = 2 -

Fischer-Tropsch H2/CO = 2 -

Ácido acético CO Metanol

Etanol H2/CO ≈ 2 Metanol

Alcoholes de cadena larga H2/CO ≈ 1 Olefinas

H2 industrial 99,99% H2 -

H2 para celdas de combustible < 20 ppm de CO -


16

A.3. El dióxido de carbono como gas de efecto invernadero

Según la Convención Marco de las Naciones Unidas existen fuertes

evidencias de que las actividades antropogénicas son la causa del aumento

de la temperatura media en la Tierra debido al incremento de emisiones de

gases de efecto invernadero (GEIs) [30]. En la actualidad, estas emisiones

de gases de efecto invernadero son las mayores de la historia. Se cree que

como consecuencia del aumento de la temperatura media global, el clima

del planeta está cambiando, efecto que se conoce como cambio climático.

Los principales gases de efecto invernadero responsables del aumento de

temperaturas son: dióxido de carbono, metano, óxidos de nitrógeno y un

grupo de compuestos gaseosos que contienen cloro y flúor, como

perfluorocarburos, halogenuros de carbono y hexafluoruro de azufre. Entre

estos, el CO2 es el GEIs más importante debido principalmente a su mayor

emisión.

A.3.1. Principales fuentes y evolución de las emisiones

El consumo global de energía y las emisiones de CO2 asociadas a este

consumo continúan creciendo en los primeros años del siglo XXI. Los

combustibles fósiles son la fuente de energía más utilizada en el mundo; el

86 % de la energía mundial procede de fuentes fósiles y su combustión es

responsable del 75 % de las actuales emisiones antropogénicas. En la

Figura A.3 se representan los porcentajes de emisiones totales

antropogénicas de GEIs por sectores económicos relativas a 2010.


17

Energía Industria Transporte Construccion Agricultura, silvicultura y otros usos resto

Electricidad y

producción de calor

25 %

Otras energías

9.6 %

EMISIONES DE GEIS POR SECTORES ECONÓMICOS

Electricidad y produccion de calor Agricultura, silvicultura y otros usos Construcción

Transporte Industria Otras energias

Industria

21 %

Transporte

14 %

Construcción

6.4 %

Agricultura,

silvicultura y otros usos

24%

Energía

1,4 %Industria

11 %

Transporte

0,3 %

Construcción

12 %

Agricultura,

silvicultura y otros usos

0,87 %

Total: 49 Gt CO2-eq

(2010)

Emisiones directas de GEIs Emisiones indirectas de GEIs

Figura A.3. Porcentaje de emisiones antropogénicas de GEIs por sectores

económicos relativas a 2010 [30].

En el año 2010, el 35 % de las emisiones de GEIs provenían del sector

de la energía, el 24 % (emisiones netas) de la silvicultura y de otros usos

agrícolas, el 21 % de la industria, el 14 % del transporte y el 6.4 % del

sector de la construcción.

A.3.2. Mecanismos para la eliminación y reducción de las emisiones

En la actualidad, se han propuesto diversas opciones de adaptación y

mitigación que pueden ayudar a abordar el problema del cambio climático.

Sin embargo, una única opción no es suficiente por sí misma desde un

punto de vista tecnológico o económico. Según la Convención Marco de las

Naciones Unidas para el Cambio Climático (UNFCCC, United Nations

Framework Convention on Climate Change) la estabilización de las


18

emisiones de GEIs se alcanzará cuando las emisiones de GEIs

antropogénicas sean equiparadas a las que pueden ser absorbidas de forma

natural por el planeta.

Existe una gran variedad de tecnologías que pueden reducir la

concentración de CO2 en la atmósfera. Los objetivos de reducción, los

costes, el potencial de cada tecnología, el impacto ambiental, y factores

sociales, como la aceptación pública, serán determinantes para la elección

de la tecnología más adecuada para cada situación concreta. Las diferentes

opciones para la reducción de emisiones de efecto invernadero puede ser,

de acuerdo al Intergovernmental Panel on Climate Change (IPCC) [30], las

siguientes:

- Mejora de la eficiencia energética en la conversión, transporte y uso

final de la energía.

- Aumento de las fuentes de energía de baja emisión: renovables y

nuclear. Aunque el desarrollo de ambas tecnologías ha sido muy

significativo en los últimos años, especialmente en el campo de las

energías renovables, éstas presentan todavía grandes limitaciones.

Por un lado, las fuentes de energía renovables no van a ser capaces

de aportar toda la demanda energética que se requiere para los

próximos años, especialmente en los países con economías

emergentes, a lo que se suma los problemas asociados a su

intermitencia. Por otro lado, la energía nuclear presenta dificultades

de aceptación política y pública debido a problemas relacionados con

la seguridad y el posterior almacenamiento de residuos.

- Captura y secuestro del CO2 (CCS, CO2 Capture and Sequestration)

y almacenamiento de CO2 en sumideros naturales o biológicos. La

primera tecnología consiste en capturar el CO2 emitido por una

central de generación de energía convencional y su posterior


19

almacenamiento en el subsuelo. Por otro lado, el secuestro del CO2

en sumideros naturales o biológicos como son las plantas y los

océanos, permite la fijación del CO2 en las primeras gracias a

procesos fotosintéticos y a la absorción de este gas por parte de los

segundos. La fijación fotosintética puede potenciarse con prácticas

correctas de agricultura y reforestación.

- Conversión química del CO2 y almacenamiento de la energía en

moléculas orgánicas. El almacenamiento de energía en moléculas

orgánicas reduce los problemas de transporte y almacenamiento.

Además la conversión de CO2 se podría utilizar como un

intermediario para la incorporación de energías renovables en el

proceso industrial. Los procesos catalíticos de CO2 se postulan como

una de las soluciones más prometedoras para la reducción de las

emisiones del CO2 y, por tanto, de la mitigación del cambio climático.

Existen numerosos procesos de conversión catalítica de CO2:

reformado de metano con CO2, síntesis de dimetil carbonato a partir

de metanol y CO2, síntesis de carbonatos cíclicos a partir de CO2 y

epóxidos y sales de amonio, y procesos de hidrogenación para

obtener metanol [31]. En los últimos años este último proceso ha

adquirido gran relevancia ya que permite reemplazar el uso de CO

por CO2 en la producción de metanol.

A.4. Reactores de membrana de electrolito sólido (SEMRs)

Los reactores de membrana de electrolito sólido (Solid Electrolyte

Membrane reactor, SEMRs) son un tipo de reactores compuestos por un

electrolito sólido cerámico y por dos electrodos porosos depositados a ambos

lados del mismo. Los electrolitos sólidos son materiales que deben ser

químicamente estables, tener una elevada conductividad iónica y ser

impermeables a cualquier otra especie no cargada eléctricamente [32]. Las


20

membranas de electrolito sólido se pueden clasificar en: membranas de

conductores mixtos de iones-electrones (Mixed Ion-Electron Conductors,

MIEC) y membranas de electrolitos sólidos puros. Mientras que las

primeras se caracterizan por tener valores comparables de conductividad

iónica-electrónica, en las segundas la conductividad electrónica es al menos

dos órdenes de magnitud inferior a la conductividad iónica. Es por ello, que

los electrolitos sólidos puros requieren un circuito externo para el

transporte de los electrones. Con este tipo de configuraciones se puede

controlar de un modo eficiente el transporte de iones hacia el electrodo que

se desee mediante la aplicación de potenciales eléctricos o intensidades en

un sentido o en otro [32-34].

A.4.1. Tipos de reactores de membrana de electrolito sólido y modos de

operación

En función de cómo estén expuestos los electrodos a la atmósfera de

reacción, los SEMRs se pueden clasificar en reactores de cámara sencilla y

reactores de doble cámara. Los reactores de cámara sencilla se

caracterizan por tener ambos electrodos (ánodo y cátodo) expuestos a la

misma atmósfera de reacción. Por el contrario, los reactores de doble

cámara se caracterizan por tener los electrodos expuestos a diferentes

mezclas de reacción.

Una de las principales ventajas de los reactores de cámara sencilla

respecto a los de doble cámara es su mayor sencillez, así como la mayor

facilidad para ser implementados en sistemas en los que el soporte

catalítico se sustituye por el electrolito sólido. Por el contrario, los

reactores de doble cámara ofrecen la posibilidad de separar los productos

obtenidos al mismo tiempo que son formados, incrementando de este modo

la economía del proceso.


21

Los SEMRs pueden operar de acuerdo a los siguientes modos de

operación [33]:

Condiciones de circuito abierto (O.C.C, Open Circuit Conditions).

En este modo de operación no se aplica corriente eléctrica a

través del electrolito sólido, siendo la diferencia de potencial

químico la verdadera fuerza impulsora de la celda.

Condiciones de circuito cerrado. En estas condiciones se aplica

una corriente eléctrica que permite transferir los iones a través

del electrolito sólido, de un electrodo a otro, y reaccionar así con

los reactivos presentes en la fase gas. De este modo, el reactor

podría trabajar como una celda de combustible si el objetivo es

obtener energía. Por otro lado, si el objetivo fuera obtener un

determinado producto de reacción, se podría aplicar cierta

corriente eléctrica (equivalente al flujo de iones transferidos a

través del electrolito sólido) en la dirección deseada, con el

objetivo de favorecer la reacción de dichos iones y de los reactivos

presentes en la atmósfera de reacción.

A.4.2. Electrolitos sólidos

Se conocen numerosos electrolitos sólidos que se suelen clasificar de

acuerdo al ion móvil que se desplaza a su través. A día de hoy se han

descubierto conductores de O2-, F-, H+, K+, Na+, Cu+, Ag+ and Li+ [35].

Debido a la importancia industrial de los procesos catalíticos de oxidación e

hidrogenación, son precisamente los electrolitos sólidos conductores de

iones de oxígeno (O2-) y protones (H+) los más ampliamente utilizados en

los SEMRs.

Los conductores de O2- son disoluciones en estado sólido de cationes

divalentes o trivalentes (Y2O3, CaO, Yb2O3) en óxidos de metales


22

tetravalentes (ZrO2, ThO2, CeO2) [33]. La conductividad de los iones

oxígeno está basada en las vacantes de O2- creadas en la matriz del óxido

metálico tetravalente cuando se dopa con el óxido metálico divalente o

trivalente. Debido a su estabilidad química y mecánica, el conductor de O2-

más popular es el conocido como YSZ (Yttria Stabilized Zirconia, 6-10 %

Y2O3 e ZrO2).

Por otro lado, los conductores protónicos de alta temperatura consisten

en óxidos de la familia de las perovsquitas, normalmente basados en

SrCeO3 o BaCeO3. A principios de la década de 1980, el grupo de trabajo de

Iwahara y col, [36] descubrieron que estos materiales presentaban

conductividad protónica a elevadas temperaturas (T > 600 ºC). La principal

aplicación de este tipo de conductores protónicos es la obtención de

hidrógeno. Este tipo de perovsquitas presentan unas conductividades del

orden de 10-2-10-3 S cm-1 entre 600 - 1000 ºC cuando se exponen a

atmósferas de reacción que contiene hidrógeno [36].

Otra familia importante de electrolitos solidos son los basados en β”-

alúmina. Este tipo de materiales presentan conductividad de iones Na+ y

K+ a baja temperatura (> 150 ºC). Cabe destacar que los materiales de esta

familia permiten la promoción electroquímica sobre superficies metálicas

[37], fenómeno que será explicado en detalle en apartados posteriores.

A.4.3. Aplicaciones de los SEMRs

i) Conducción selectiva de iones

Las primeras aplicaciones de los SEMRs estaban relacionadas con su

capacidad de conducción selectiva de iones [33]. Existen celdas basadas en

conductores de iones O2- que se han utilizado en la separación del oxígeno

del aire o de cualquier otra mezcla gaseosa. Otra ventaja que ofrece la

conductividad selectiva de iones es que favorecen el suministro de especies


23

libres de impurezas que puedan afectar al proceso. Un ejemplo de ello es la

síntesis industrial de amoniaco. En este proceso, una parte significativa

del coste proviene de la preparación y purificación de la corriente gaseosa;

por ejemplo, el proceso de síntesis de amoniaco mediante el uso de

conductores protónicos permite el suministro de H+ libres de O2 [38], que es

el causante del envenenamiento irreversible del catalizador.

ii) Estudio del mecanismo de reacción en procesos catalíticos

Se puede extraer información potenciométrica cuando los sistemas

SEMR operan en condiciones de circuito abierto, es decir con una

intensidad eléctrica nula, que combinada con medidas catalíticas, permite

estudiar los mecanismos de la reacción que se está produciendo, De este

modo, se ha demostrado que si el contraelectrodo de una celda de doble

cámara está expuesto al aire (con una presión parcial de oxígeno PO2 = 0,21

bar), la actividad termodinámica del oxígeno atómico adsorbido viene dada

por la siguiente ecuación [33, 39, 40] :

ɑ0 = (0,21)0,5exp (2FE

RT) (A.7)

donde F es la constante de Faraday, R es la constante de los gases ideales,

T es la temperatura absoluta, E es la fuerza electromotriz de la celda (emf)

y α0 es la actividad del oxígeno adsorbido. Esta técnica, denominada como

Potenciometría de Electrolito Sólido (Solid Electrolyte Potenciometry,

SEP), se ha utilizado en el estudio de importantes sistemas catalíticos:

oxidación de óxido de azufre [41], CO [42], etileno [43] y propano [44] sobre

catalizadores de Pt y Ag. Esta técnica también ha sido utilizada para

estudiar cambios de fase.


24

iii) Promoción electroquímica de la catálisis

El fenómeno de promoción electroquímica de la catálisis

(Electrochemical Promotion Of Catalysis, EPOC) o efecto NEMCA (Non-

Faradaic Electrochemical Modification of Catalytic Activity) ha sido

definido como “una herramienta importante de la electroquímica que

permite alterar de manera pronunciada, reversible y predecible la actividad

y selectividad catalítica de catalizadores conductores soportados sobre

electrolitos sólidos” [37]. En este sentido, se puede afirmar que es un tipo

especial de catálisis en el que las propiedades catalíticas se modifican

mediante la aplicación de un campo eléctrico.

Este fenómeno fue descubierto en el año 1981, por el grupo del profesor

Vayenas, quienes observaron que la actividad y la selectividad de un

catalizador depositado sobre un electrolito sólido podía ser

electroquímicamente modificada in-situ durante el propio proceso de

reacción [45].

El origen del efecto NEMCA tiene lugar cuando se aplica un voltaje o

una intensidad eléctrica entre un electrodo metálico (que actúa como

catalizador y electrodo de trabajo) depositado a un lado de un electrolito

sólido, y un segundo electrodo (contra-electrodo) depositado en el lado

opuesto de dicho electrolito. De este modo, si sobre el electrodo de trabajo

está teniendo lugar una reacción catalítica heterogénea, la aplicación de

corriente eléctrica puede provocar un incremento pronunciado de la

velocidad de reacción, pudiéndose llegar a ser de 10 a 105 veces el valor que

predice la ley de Faraday. Este hecho motivó que este fenómeno también

sea conocido como modificación electroquímica no faradaica de la actividad

catalítica (efecto NEMCA en sus siglas en inglés).


25

- Origen y mecanismo del fenómeno de promoción electroquímica

En las últimas décadas, la comunidad científica ha emprendido

investigaciones con las que determinar el origen del fenómeno de

promoción electroquímica de la catálisis. Así, el origen ha sido atribuido al

movimiento de especies promotoras controlado electroquímicamente [37].

Estas especies son generadas en la región conocida como tbp (three-phase

bounderie; la interfase entre el electrolito sólido, catalizador-electrodo de

trabajo y la fase gas). En determinadas condiciones, estas especies migran

hacia la superficie del metal distribuyéndose a lo largo de él y modificando

así la capacidad de quimisorción de las moléculas de reactivo sobre la

superficie del catalizador. Esta teoría, que ha sido demostrada mediante

una gran variedad de técnicas de caracterización tanto catalíticas,

electrocatalíticas y de análisis de superficies (XPS, TPD, PEEM, STM,

Voltamperometrías cíclicas) [37], ha puesto de manifiesto que este

fenómeno es análogo al de la promoción química convencional de la

catálisis heterogénea. Sin embargo, el fenómeno EPOC cuenta con la

ventaja adicional de poder controlar, de un modo preciso y reversible, la

cantidad de promotor en el catalizador durante el propio proceso de

reacción. En el caso de la promoción electroquímica, las especies

promotoras son iones que migran de forma controlada desde el electrolito

sólido al metal, y viceversa, y cuyo sentido depende del signo de la

intensidad aplicada. Así por ejemplo, la aplicación de un potencial o

intensidad negativa genera la migración de iones positivos (como por

ejemplo Na+ o K+) desde el electrolito sólido hasta el metal en un conductor

catiónico. De la misma forma, se producirá la migración de iones negativos

(como por ejemplo O2-) desde el metal hasta el electrolito sólido si se trata

de un conductor aniónico.


26

Por otro lado, durante el proceso de migración, el movimiento de estos

iones está acompañado por el correspondiente ión de compensación de

carga, lo que da lugar a dipolos neutros superficiales. Estos dipolos se

distribuyen a lo largo de toda la superficie metálica, constituyendo lo que

se conoce como doble capa efectiva. La formación de la doble capa efectiva

produce una modificación de la función de trabajo del metal, es decir, una

variación de la densidad electrónica, modificando de este modo su

capacidad de enlace con cada una de las moléculas de reactivo tal y como

se muestra en la siguiente ecuación:

∆Φ=e∆UWR (A.8)

donde ∆Φ es la variación de la función de trabajo, e es la carga del electrón y ∆UWR

es la modificación de la diferencia de potencial. Así pues, cambiando la

función de trabajo de la película metálica se puede alterar la capacidad de

enlace con cada una de las moléculas de reactivo, lo que se traduce en una

modificación del comportamiento catalítico del metal, que dependerá de la

naturaleza del promotor [46]. De este modo, si la especie promotora es

electronegativa (O2-) se produce un incremento en la función de trabajo lo

que favorece la quimisorción de adsorbatos donadores de electrones y se

desfavorece la de aceptores de electrones. Por el contrario, una

disminución de la función de trabajo mediante la adición de promotores

electropositivos (Na+) favorece la quimisorción de especies aceptoras de

electrones.

- Tipos de reacción basadas en el fenómeno de promoción

electroquímica

Se pueden distinguir cuatro tipos de reacciones basadas en el fenómeno

de promoción electroquímica teniendo en cuenta las interacciones

atractivas o repulsivas entre los promotores y adsorbatos, [37]:


27

- Reacciones electrofóbicas: son aquellas reacciones que muestran un

incremento de la velocidad de reacción para valores positivos de

potencial. Este tipo de comportamiento tiene lugar cuando la cinética

es de orden positivo en el donador de electrones y de orden cero o

negativo en el aceptor de electrones, es decir, el donador de electrones

es el que se encuentra más débilmente adsorbido sobre el catalizador.

- Reacciones electrofílicas: son aquellas reacciones que muestran un

incremento de la velocidad de reacción para valores negativos del

potencial. Este tipo de reacciones se dan cuando la cinética es de

orden positivo en el aceptor de electrones y de orden cero o negativo

en el donador de electrones, es decir, el aceptor de electrones se

muestra más débilmente adsorbido sobre el catalizador.

- Reacciones tipo volcán: estas reacciones presentan un máximo local

de la velocidad de reacción respecto al potencial aplicado. Este

comportamiento tiene lugar cuando tanto el donador como el aceptor

de electrones se encuentran fuertemente adsorbidos sobre el

catalizador.

- Reacciones tipo volcán invertido: son aquellas que presentan un

mínimo en la velocidad de reacción respecto al potencial aplicado. En

este caso, este tipo de comportamiento tiene lugar cuando el donador

y el aceptor de electrones se encuentran débilmente adsorbidos sobre

el catalizador.

- Aplicaciones del fenómeno de promoción electroquímica

Desde su descubrimiento, el fenómeno de promoción electroquímica ha

sido demostrado en más de 80 sistemas catalíticos diferentes, lo que parece

indicar que no está limitado a ningún tipo de catalizador, electrolito sólido

o reacción catalítica en particular [37, 39]. De este modo, el fenómeno

EPOC ha sido estudiado en diversos procesos de interés industrial y


28

medioambiental, sobre una amplia variedad de catalizadores (Pt, Pd, Rh,

Ag, Au, Ni, Cu, Fe, IrO2, RuO2) depositados sobre distintos electrolitos

sólidos (Na-βAl2O3, K-βAl2O3, YSZ, Nasicon, V2O5-K2S2O7, CaZr0,9In0,1O3-a).

En particular, la promoción electroquímica ha sido investigada en

reacciones de hidrogenación de CO2 [47-58]. La mayoría de los estudios de

hidrogenación se han desarrollado utilizando YSZ como electrolito sólido y

metales nobles como Pt, Pd, Ru y Rh [47-50, 52, 53, 55, 58]. En menor

medida, esta reacción también se ha llevado a cabo con electrolitos sólidos

catiónicos [48, 51, 53, 55-57] y utilizando metales no nobles como Ni o Cu

[47, 50, 51, 54, 56].

A.5. Reactores electroquímicos de membrana polimérica (PEM)

A.5.1. Características generales de los reactores de membrana

polimérica

En los últimos años, como consecuencia del auge en el estudio de los

procesos de obtención de hidrógeno se han desarrollado configuraciones con

membranas de intercambio protónico (Proton Exchange Membrane/

Polymer Electrolyte Membrane, PEM). Estas membranas son poliméricas

y consisten en una estructura semipermeable diseñada para conducir

protones (H+) a temperaturas relativamente bajas (20-200 ºC). Si bien es

cierto que el principal uso de estos reactores reside en la obtención de

energía, en modo celda de combustible (tal y como se comentó en apartados

anteriores), estos también pueden ser utilizados para la producción de

hidrógeno a través de un proceso de electrólisis.

En los años 60, General Electric desarrolló el primer electrolizador de

agua basado en electrolitos poliméricos [59]. El modo de operación de los

electrolizadores tipo PEM es el inverso al llevado a cabo en el modo celda

de combustible. De este modo, se aplican potenciales eléctricos superiores

al potencial de equilibrio dando lugar a la ruptura de la molécula de agua


29

en el ánodo, produciendo O2, H+ y electrones de acuerdo con la siguiente

ecuación:

2H2O → O2 + 4H+ + 4e- (A.9)

A continuación, los protones migran hacia el cátodo de la celda a través

de la membrana polimérica, donde reaccionan con los electrones que se

mueven desde el ánodo al cátodo a través de un circuito eléctrico externo,

dando lugar a la producción de hidrógeno puro [4]:

2H+ + 2e- → H2 (A.10)

Un electrolizador tipo PEM presenta una configuración muy similar a la

de una celda de combustible tipo PEM. Dichos electrolizadores están

formados por un membrana polimérica y dos electrodos porosos situados a

ambos lados de dicha membrana formando el conjunto denominado MEA

(Membrane Electrode Assembly). También se compone de colectores de

corriente y placas bipolares, que permiten la polarización del sistema y el

flujo de gas o líquido a través de los electrodos.

A.5.2. Membranas de intercambio protónico

La función de las membranas de intercambio iónico (normalmente

protónicas) es permitir el paso de cargas iónicas desde un electrodo a otro

de la celda, cerrando el circuito iónico de la celda electroquímica y

actuando como barrera para el paso de los gases a través de la misma.

Estas membranas deben poseer elevadas estabilidades mecánicas,

químicas (debido a que están en contacto con medios de reacción extremos

como gases oxidantes y reductores en un entorno ácido [17]), y

conductividades iónicas elevadas para evitar pérdidas óhmicas y cierta

durabilidad y fiabilidad.

Los materiales utilizados en este tipo de membranas son polímeros

fluorocarbonados parecidos al teflón, que dotan a la membrana de una


30

gran resistencia química, mecánica y térmica, y la hacen insoluble al agua.

A estos polímeros se añaden grupos sulfónicos (SO3) que permiten el

tránsito de cargas iónicas a través de la membrana y le proporcionan cierto

carácter hidrófilo [60]. El tipo de membrana más usada es conocida como

Nafion® desarrollada por Dupont [4] y cuya estructura se muestra en la

Figura A.4.

Figura A.4. Estructura química del Nafion®

Además de las membranas Nafion®, existen otras membranas

comerciales perfluorosulfonadas como son Dow® (Dow Chemicals),

Flemion® (Asashi Glass), Aciplex® (Asashi Chemicals) [61] y Sterion®

(Hydrogen Works) [62].

A.5.3. Aplicación de los reactores PEM en procesos de producción de

hidrógeno y valorización de CO2

i) Electrólisis de agua.

Los procesos de electrólisis de agua en configuraciones tipo PEM han

sido muy estudiados en los últimos años. Los electrolizadores tipo PEM se

suelen construir con membranas de intercambio protónico

perfluorosulfonadas. En cuanto a los electrodos, se suele utilizar Pt

soportado sobre materiales carbonosos como catalizador catódico. Sin

embargo, gran parte de los trabajos realizados hasta la fecha han estado


31

enfocados hacia el estudio del catalizador anódico. En este sentido, el óxido

de Ru es conocido por ser el electrolizador más activo para la evolución de

O2 en el ánodo. Sin embargo, suele ser bastante inestable, por lo que se ha

de estabilizar con óxidos como el de iridio [63].

ii) Electrólisis o reformado electroquímico de alcoholes

Los procesos de reformado electroquímico de alcoholes (electrólisis de

alcoholes) han sido propuestos como una alternativa prometedora para la

producción de hidrógeno puro a partir de energía eléctrica, solventando los

problemas relacionados con los procesos de electrólisis de agua.

Se han reportado costes energéticos asociado a la electrólisis de

alcoholes inferiores a los de la electrólisis de agua (17-30 kWh·kg-1 H2 vs.

53-70 kWh·kg-1 H2, respectivamente [5, 64]) debido a que los primeros

suelen llevarse a cabo a potenciales muy inferiores. También se ha

estudiado la electrólisis de mezclas metanol-agua [65-68], y glicerol agua

[69, 70].

iii) Reducción electrocatalítica de CO2

Los procesos de reducción electrocatalítica de CO2 han sido propuestos

en los últimos años como una alternativa para disminuir las emisiones de

gases de efecto invernadero. Por otra parte, el uso reactores tipo PEM en

reacciones de valorización de CO2 pretende solventar los problemas

relacionados con los procesos convencionales de hidrogenación de CO2. Los

primeros operarían a temperaturas menores de reacción, no requerirían de

alimentación alguna de hidrógeno y podrían utilizar energía eléctrica de

origen renovable.

Hasta el momento, se han llevado a cabo pocos estudios de reducción

electrocatalítica en fase gas utilizando membranas de intercambio

protónico. Genovese y col. [71] usaron electrocatalizadores basados en


32

nanoestructuras soportadas sobre nanotubos carbono que difieren de los

convencionales utilizados en operaciones en fase acuosa [72]. Modificando

las propiedades de los electrodos nanoestructurados durante su síntesis es

posible mejorar la eficiencia del proceso de reducción y de su productividad

y ajustar la selectividad hacia la formación de mayores cadenas de

hidrocarburos y de otros productos químicos [73].

A.6. Objetivo del presente trabajo

El objetivo global del presente trabajo era estudiar y explorar nuevas

configuraciones electrocatalíticas, basadas tanto en configuraciones SEMR

como configuraciones PEM para la producción de hidrógeno y gas de

síntesis así como para la valorización de CO2 para obtener combustibles

líquidos. De este modo, se desarrolló un programa de trabajo con las

siguientes etapas:

- Revisión bibliográfica y puesta a punto de las distintas instalaciones

experimentales (equipos de análisis y polarización, configuraciones de los

reactores, calibración de equipos de análisis, etc).

- Desarrollo de celdas de electrolito sólido para la producción de gas de

síntesis con una razón H2/CO variable en un reactor SEMR de cámara

sencilla. Este proceso combinaba la electrólisis de vapor de agua (para la

obtención de H2) y la oxidación parcial de etanol (para la obtención de gas

de síntesis).

- Desarrollo de celdas de electrolito sólido en configuraciones SEMR de

doble cámara para la producción y separación simultánea de H2 e

hidrocarburos C2S. Este proceso combinaba la electrólisis de vapor de agua

(para la obtención de H2) y la reacción de acoplamiento selectivo de metano

(para la obtención de C2S).


33

- Comparativa energética de la producción de H2 a partir de reformado

electroquímico y del reformado catalítico de etanol, mediante su simulación

con Aspen HYSYS.

- Investigación del fenómeno de la promoción electroquímica de la

catálisis con catalizadores no nobles (Ni) sobre conductores de K+ en el

proceso de hidrogenación de CO2 hacia la formación de CO y CH4.

- Desarrollo de celdas tipo PEM para la conversión en fase gas de CO2 a

combustibles líquidos sobre catalizadores de Cu.

B. Instalaciones experimentales

B.1. Reactor SEMR de cámara sencilla

Los experimentos relacionados con la producción de gas de síntesis de

razón variable, vía reformado de vapor de agua y oxidación parcial de

etanol (Capítulo 1) y el estudio de la promoción electroquímica en el

proceso de hidrogenación de CO2 para la producción de CO y CH4 (Capítulo

4) fueron llevados a cabo en una instalación como la que se describe a

continuación. Dicha instalación consta de cuatro partes bien diferenciadas:

sistema de alimentación, sistema de reacción, sistema de polarización y

sistema de análisis. El esquema de la instalación está descrito en detalle

en el Capítulo 1 de esta memoria.

El sistema de alimentación estaba constituido por cuatro líneas de flujo

continuo, análogas e independientes para la alimentación de los diferentes

gases de reacción. CO2, H2 y N2 como gas portador para los experimentos

de hidrogenación de CO2 y N2, H2 y N2 como gas portador para los

experimentos de gas de síntesis. Además, estos últimos experimentos se

realizaron en presencia de vapor de agua y de etanol. El contenido de

ambos compuestos en la corriente alimento fue regulado mediante

saturación a temperatura controlada. Todas las líneas a la salida del


34

saturador fueron precalentadas con la finalidad de evitar la condensación

del vapor de agua.

El sistema de reacción estaba constituido por un reactor de membrana

de electrolito sólido de cámara sencilla (como los descritos en el apartado

A.4) construido en cuarzo, de tal forma que todos los electrodos se

encontraban bajo la misma atmósfera de reacción. A la salida del reactor,

los gases atravesaban una unidad de frío en la que se condensaban los

posibles productos líquidos. Los métodos de preparación de las celdas de

electrolito sólido, dependiendo de los estudios a realizar, son muy variados

por lo que serán explicados en detalle en los diferentes capítulos de esta

memoria.

El sistema de polarización consistía en un potenciostato-galvanostato

que permitía la aplicación controlada de corrientes eléctricas o potenciales

sobre la celda electroquímica.

Por último, el sistema de análisis estaba constituido por un micro-

cromatógrafo de gases con dos canales que permitía la separación y

cuantificación de los distintos reactivos y productos obtenidos a la salida

del sistema de reacción.

B.2. Reactor SEMR de doble cámara

Los experimentos para la producción/separación simultánea de H2 e

hidrocarburos C2s (Capítulo 2) se llevaron a cabo en una instalación muy

similar a la anterior. La principal diferencia radica en el sistema de

reacción que en este caso estaba constituido por un reactor de doble

cámara dotado de dos entradas y dos salidas, cada una de ellas a un

compartimento. De este modo, mientras que el vapor de agua fue

alimentado mediante saturación de una corriente de nitrógeno a

temperatura controlada a la cámara interior, la cámara exterior fue


35

alimentada con una corriente de CH4/N2. El hidrógeno puro en la cámara

interior, obtenido mediante el proceso de electrólisis fue calculado de

acuerdo con la ley de Faraday, mientras que los productos obtenidos en la

cámara exterior fueron medidos con el micro-cromatógrafo de gases

anteriormente descrito.

B.3. Reactor PEM

Los experimentos de reducción electrocatalítica en celda PEM (Capítulo

5) fueron llevados a cabo en una instalación formada por cuatro secciones:

sistema de alimentación, sistema de reacción, sistema de polarización y

sistema de análisis y que se describirá en detalle en el Capítulo 5 de esta

memoria.

El sistema de alimentación está constituido por dos líneas de flujo

continuo, análogas e independientes para la alimentación de los diferentes

gases de reacción (CO2 y N2). Todos los experimentos de reacción se

realizaron en presencia de vapor de agua. Como en el apartado anterior, el

contenido de vapor de agua de la corriente alimento fue regulado mediante

saturación en una corriente de N2 a temperatura controlada. Igualmente,

la línea a la salida del saturador fue precalentada con la finalidad de evitar

la condensación de vapor de agua.

El sistema de reacción estaba constituido por una celda PEM. Tanto el

sistema como los métodos de preparación de los electrodos y de ensamblaje

de la membrana-electrodo se describen en detalle en el Capítulo 5 de esta

memoria.

El sistema de polarización estaba formado por un potenciostato-

galvanostato que permitía la aplicación de forma controlada de potenciales

o corrientes eléctricas a la celda PEM.


36

Finalmente, el sistema de análisis estaba constituido por un

cromatógrafo de gases formado por dos canales de análisis independientes

en los que se analizaba los diferentes productos de reacción mediante

inyección manual con una jeringa de gases.

C. Resultados y discusión

En el Capítulo 1 se ha estudiado la posibilidad de obtener gas de

síntesis de razón variable a partir de la combinación de los procesos de

electrólisis de vapor de agua y oxidación parcial de etanol. Para ello, se

desarrolló un catalizador electroquímico con la configuración Pt/YSZ/Pt.

Cada una de las películas catalíticas de Pt, depositadas a ambos lados del

pellet circular de YSZ, fueron preparadas mediante la técnica de

deposición de arco catódico (CAD), en colaboración con el Instituto de

Ciencia de Materiales de Madrid (CSIC), dando lugar a un contenido final

de 0,8 mg Pt cm-2.

La caracterización del sistema mediante difracción de rayos X (DRX) y

microscopía electrónica de barrido (SEM) demostró la formación de una

película porosa de Pt con un tamaño de cristal de 15 nm con una adecuada

conductividad eléctrica y adhesión al electrolito sólido.

En primer lugar se realizó un experimento galvanostático mediante la

aplicación de intensidades eléctricas (-80 y +80 mA) en la atmósfera de

reacción: C2H5OH/H2O=0,7 %/2 %. En condiciones de circuito abierto (sin

aplicar intensidad) se produjo la reacción de reformado de etanol en ambos

electrodos de Pt dando lugar a H2, CO y CO2 y, en menor medida, a trazas

de CH4, C2H6 y C2H4. Adicionalmente, en el sistema de reacción, compuesto

por un reactor de cámara sencilla, se pudieron producir reacciones como la

reacción de water gas shift o las reacciones de hidrogenación de CO y CO2

dando lugar a una razón molar H2/CO comprendida entre 2,5-3. Por otro

lado, se pudo observar un incremento de la velocidad de producción de


37

todos los productos, principalmente de H2, durante la imposición de

corrientes eléctricas (-80 y +80 mA). Este incremento de la producción de

hidrógeno fue consecuencia del proceso de electrólisis de vapor de agua que

ocurrió en el cátodo de Pt y a la acción de los iones O2- suministrados

electroquímicamente a través del electrolito sólido hacia el ánodo de Pt que

reaccionaron con el etanol vía oxidación parcial y total. De esto modo, se

obtuvo una razón H2/CO de 8,2 y 8,7 para una intensidad de +80 y -80 mA,

respectivamente, demostrando la posibilidad de modificar la razón H2/CO

a partir de la intensidad aplicada.

A continuación se estudió el comportamiento del sistema a diferentes

temperaturas (T = 600, 650 y 700 ºC) en la atmósfera de reacción

anteriormente mencionada, tras la aplicación de distintos potenciales. Se

pudo observar un incremento de la velocidad de producción de H2, CO y

CO2 al incrementar el potencial aplicado debido a un incremento de la

actividad electrocatalítica. Adicionalmente, se pudo apreciar un

incremento de la velocidad de producción de los compuestos anteriormente

mencionados al incrementar la temperatura para un potencial fijo, debido

a efectos cinéticos. Se calcularon las energías de activación aparentes tanto

en condiciones de circuito abierto como en condiciones de polarización,

obteniéndose una disminución prácticamente lineal de dichas energías al

incrementarse el potencial aplicado.

A continuación se calculó la eficiencia faradaica del proceso de

producción de hidrógeno, observándose que esta fue para todos los casos

menor que 1 indicando que no hay efecto de promoción electroquímica en la

reacción de reformado catalítico de etanol. Además, parte del hidrógeno

producido en el reactor de cámara sencilla reaccionó con otros productos

como CO, CO2, C2H4 y O2 disminuyendo la eficiencia faradaica respecto al

hidrógeno.


38

Finalmente, se estudió la variación de la razón H2/CO con el potencial

aplicado a diferentes temperaturas observándose que aquella puede ser

fácilmente modificada y, por tanto, ajustada para la producción de

diferentes compuestos de interés industrial.

En el Capítulo 2 se utilizó un SEMR de doble cámara (Pt-

YSZporosa/YSZ/Pt) para llevar a cabo la producción y separación simultánea

de H2 e hidrocarburos C2S. En la cámara interna formada por una película

metálica de Pt depositada sobre YSZ densa se introdujo vapor de agua

mediante saturación de una corriente de nitrógeno; mientras que por la

cámara exterior, formada por Pt impregnado es una capa porosa de YSZ, se

alimentó una corriente de metano. La aplicación de corriente eléctrica en

la cámara interior provoca la electrólisis de vapor de agua, produciéndose

H2 e iones O2-. Estos iones migran hacia la cámara exterior, a través de la

membrana, que está alimentada por una corriente de CH4 y que conduce a

la formación de H2, CO, CO2 y C2S.

Este sistema catalítico fue caracterizado mediante SEM y DRX. Con las

micrografías SEM se pudo comprobar la buena distribución de las

partículas de Pt impregnadas sobre la matriz de la YSZ porosa. Por otro

lado, la técnica de difracción de rayos X demostró la presencia de

partículas de Pt metálico con un tamaño de cristal de 32 nm.

En primer lugar se realizó un experimento galvanostático mediante la

aplicación de intensidades eléctricas de +50 mA. En condiciones de circuito

abierto, en la cámara externa se produjo la reacción de descomposición

térmica de metano TCD (CH4 (g) → C(s) + 2H2 (g)) para producir hidrógeno

y carbón. El carbón se depositó sobre los centros activos del Pt provocando

una disminución progresiva de la actividad catalítica con el tiempo.

Durante esta etapa también se produjo gas de síntesis con una razón

H2/CO ≈ 2 por la reacción del carbón depositado con el oxígeno que


39

provenía de la YSZ (migración térmica) y el hidrógeno obtenido a partir de

la reacción TCD. Esta razón H2/CO ≈ 2 es la típicamente usada en la

producción de combustibles sintéticos a partir de la síntesis de Fischer-

Tropsch. La aplicación de intensidades positivas permitió obtener

hidrógeno en la cámara interna (por electrólisis de agua) y CO2, CO y C2 en

la cámara externa, atribuidos a la reacción de los iones O2- con el metano y

con el carbón previamente depositado sobre la superficie del catalizador,

regenerando el sistema in-situ.

A continuación, el sistema se caracterizó electroquímicamente mediante

voltamperometrías lineales para diferentes condiciones de reacción. De

esta forma se demostró el efecto depolarizante del carbón y del metano.

Por tanto, la presencia de estos compuestos permite la disminución del

potencial de la celda (para la misma intensidad), reduciendo la cantidad de

energía que hay que suministrar para realizar la electrólisis del vapor de

agua.

Finalmente, el sistema fue chequeado durante largos periodos de

operación con objeto de analizar su durabilidad y reproducibilidad

mostrando unos resultados prometedores con vistas a su posible aplicación

práctica.

Resumiendo, el sistema desarrollado en este capítulo fue capaz, por un

lado, de producir y separar H2 y C2S con elevados rendimientos mediante la

introducción separada de metano y vapor de agua. Además, esta

configuración y modo de operación permitió la valorización in-situ del

carbón producido como agente depolarizante del proceso de electrólisis de

vapor de agua, disminuyendo así el consumo eléctrico asociado al mismo.

Continuando con los diversos métodos de producción de hirógeno. En el

Capítulo 3 se realizó un análisis energético de la producción de hidrógeno

mediante dos métodos muy diferenciados: reformado catalítico de etanol y


40

reformado electroquímico de etanol. Se propuso un diagrama de flujo

completo para ambos sistemas que se simularon mediante Aspen HYSYS

empleando condiciones de operación y resultados reportados por la

literatura.

Durante el proceso de reformado catalítico, además de la reacción

catalítica de reformado de etanol para obtener hidrógeno y CO2 (C2H5OH +

3H2O → 2CO2 + 6H2), se produce una segunda reacción de reformado que

produce CO (C2H5OH + H2O → 2CO + 4H2). Esta situación implica el que

haya que introducirse dos etapas de reacción adicionales al proceso con el

objetivo de reducir la cantidad de CO presente en la corriente efluente del

proceso principal: reacción de desplazamiento de vapor de agua (Water Gas

Shift, WGS) y oxidación preferencial de CO (COPROX) con la que se

obtiene como subproductos etileno y metano. Por otro lado, en el reformado

electroquímico de etanol, la principal reacción que tiene lugar es la de

electro-oxidación de etanol para producir protones y acetaldehído (C2H5OH

→ C2H4O + 2H+ + 2e-). Estos protones han de ser selectivamente

transportados hacia el cátodo de la membrana de intercambio protónico

para formar hidrógeno (2H+ + 2e- → H2).

En el reformado catalítico se obtuvo el mayor consumo energético en el

heater el cual proporciona la energía necesaria para calentar la corriente

alimento a la temperatura requerida en el reactor de reformado catalítico

(T = 800 ºC). Por otro lado, en el reformado electroquímico de etanol, el

mayor consumo energético correspondió a la energía consumida en la

celda PEM (173,6 kJ mol-1 H2).

Del análisis de los balances de materia se pudo comprobar que el mayor

rendimiento hacia la formación de hidrógeno se obtuvo con el proceso de

reformado electroquímico (0,0436 kg H2/kg C2H5OH) frente al obtenido con

el proceso de reformado catalítico (0,0304 kg H2/kg C2H5OH). Asimismo, se


41

pudo determinar que el menor consumo energético se obtuvo con el proceso

de reformado electroquímico (29,2 vs. 32,70 kWh / kg H2).

De este modo, los resultados obtenidos en este capítulo demostraron el

interés del proceso de reformado electroquímico de etanol para obtener

hidrógeno de alta pureza en una simple etapa de reacción/separación,

presentando una interesante alternativa al clásico proceso de reformado

catalítico.

En capítulos anteriores se ha puesto de manifiesto la importancia de la

producción de hidrógeno libre de emisiones de CO2. Por ello, en el

Capítulo 4 se estudió la aplicación del fenómeno de la promoción

electroquímica (efecto EPOC, anteriormente descrito en el apartado A.4.3)

en procesos de hidrogenación de CO2. Se desarrollaron tres nuevos

catalizadores electroquímicos con la siguiente configuración: Ni/K-

βAl2O3/Au, Ni-αAl2O3/K-βAl2O3/Au y Au-Ni(30%)-αAl2O3/K-βAl2O3/Au).

Estos catalizadores fueron preparados mediante deposición, sobre un

electrolito sólido de K-βAl2O3, de una pasta de Ni (catalizador denominado

“N”) y de una pasta sintetizada a partir de la mezcla de pasta comercial de

Ni y un polvo de αAl2O3 (catalizador “NA”). Finalmente, se preparó un

tercer electrodo mediante la deposición de una pasta sintetizada a partir

de la mezcla de pasta comercial de Au y un catalizador en polvo de Ni

(30%) impregnado sobre αAl2O3 (catalizador “GNA”).

En primer lugar, se comprobó la influencia de la temperatura de

reducción del catalizador-electrodo de trabajo de Ni con un experimento de

reducción a temperatura programada (TPR) in-situ. El consumo de

hidrógeno fue medido mediante un micro-cromatógrafo de gases situado a

la salida del sistema de reacción. Simultáneamente, se midió la resistencia

de la película metálica de Ni. De esta forma, se demostró que la reducción

de la película metálica tenía lugar a temperaturas superiores a 350 ºC,


42

aumentando notablemente el consumo de hidrógeno a partir de la misma y

disminuyendo simultáneamente la resistencia eléctrica hasta hacerse cero

lo que indicaba la presencia de una película totalmente conductora

(resistencia < 0,3 Ω). Esta presencia de Ni metálico fue verificada mediante

la técnica difracción de rayos X.

A continuación, se llevaron a cabo experimentos de reacción en

condiciones de circuito cerrado, pudiéndose observar que la aplicación de

potenciales eléctricos negativos dio lugar a la activación del catalizador en

el proceso de reverse water gas shift, para la producción de CO. Este

fenómeno puede ser explicado de acuerdo al fenómeno EPOC. La aplicación

de polarizaciones negativas indujo la migración de especies de K+ desde el

electrolito sólido hacia el catalizador-electrodo de trabajo, favoreciendo la

quimisorción del CO2 con respecto al H2 y originando un incremento

notable de la producción de CO, especialmente con el catalizador NA, y una

disminución de la producción de CH4.

Se realizó un estudio cinético confirmándose que la velocidad de

producción de CO presentó un orden positivo con respecto al reactante

aceptor de electrones (CO2) y un orden negativo u orden cero con respecto

al donador de electrones (H2).

Finalmente, se estudió el efecto del potencial aplicado y de la

concentración de hidrógeno sobre la selectividad de los productos (CO y

CH4). De esta forma se pudo comprobar, como la selectividad y la actividad

podían ser controladas y modificadas mediante el efecto EPOC.

El estudio llevado a cabo en este capítulo supone un avance importante

en lo que se refiere a la aplicación del fenómeno de promoción

electroquímica para la activación de metales no nobles (como el Ni) en


43

procesos de hidrogenación de CO2, debido a la posibilidad de controlar in-

situ la formación de gas de síntesis o de metano,

Continuando con los métodos de valorización del CO2 en productos de

interés industrial y medioambiental, en el Capítulo 5 se desarrolló un

sistema para llevar a cabo la reducción electrocatalítica a baja

temperatura del CO2 en fase gas. Productos como gas de síntesis, metano,

monóxido de carbono, metanol, acetaldehído, acetona, formato de metilo,

etanol, 2-propanol y n-propanol pueden actuar como vector energético,

permitiendo, en cierto modo, el almacenamiento de energía eléctrica en

forma de energía química. Con este objetivo, se prepararon tres

catalizadores de cobre soportados sobre: grafito (G), carbón activo (AC) y

nanofibras de carbono (CNF) y caracterizados mediante difracción de rayos

X, reducción a temperatura programada, microscopía de transmisión

electrónica, adsorción de N2 y voltamperometrías cíclicas. Se demostró que

el catalizador de cobre soportado sobre carbón activo presentó una mayor

área superficial y tamaño de poro, así como una mayor dispersión de la

fase activa y un menor tamaño de cristal de Cu. La presencia de Cu

metálico fue verificada mediante difracción de rayos X.

Una vez caracterizados los catalizadores se procedió a su ensayo en

reacción. Se observó un incremento progresivo de la velocidad de reacción

al aplicar una intensidad constante de I = -20 mA y que la mayor velocidad

de reacción de CO2 se obtuvo con el catalizador de cobre soportado sobre

carbón activo que era, precisamente, el que presentaba mayores valores de

de área superficial, porosidad y dispersión. Los principales productos

obtenidos fueron: metanol a partir del catalizador de cobre soportado sobre

grafito (Cu-G), y acetaldehído a partir de los catalizadores de cobre

soportados sobre nanofibras de carbono (Cu-CNF) y carbón activo (Cu-AC).


44

Se pudo demostrar que los productos de reacción procedían de procesos

esencialmente electrocatalíticos y no catalíticos.

A continuación, se estudió la influencia de la intensidad eléctrica

aplicada en estado estacionario y de la temperatura de reacción sobre el

comportamiento de los diferentes sistemas catalíticos ensayados. Se pudo

verificar que el consumo de CO2 aumentaba al hacerlo las intensidades

eléctricas y la temperatura de reacción.

Estos experimentos ponen de manifiesto la posibilidad de modificar y

controlar la actividad catalítica y la selectividad hacia ciertos productos de

interés variando las condiciones de operación.

D. Conclusiones y recomendaciones

De los resultados obtenidos en esta investigación se pueden obtener las

siguientes conclusiones finales:

o El catalizador electroquímico desarrollado en el Capítulo 1

(Pt/YSZ/Pt) permitía producir gas de síntesis y controlar in-situ,

mediante la intensidad aplicada, la razón H2/CO.

o El sistema de doble cámara Pt-YSZporosa/YSZ/Pt desarrollado

permitía producir y separar, simultáneamente, H2 y C2S. Esta

configuración y modo de operación permitía la valorización in-situ

del carbón producido como agente depolarizante del proceso de

electrólisis de vapor de agua, disminuyendo el consumo energético

asociado a éste.

o El estudio desarrollado en el Capítulo 3 permitió la comparación del

consumo energético de dos métodos para la producción de H2:

reformado catalítico de etanol y reformado electroquímico de etanol.

El mayor rendimiento en la producción de hidrógeno y el menor


45

consumo energético se obtuvo con el reformado electroquímico de

etanol.

o La celda electroquímica Ni-αAl2O3/K-βAl2O3/Au desarrollada

permitía la activación del proceso reverse water gas shift de acuerdo

al fenómeno EPOC, incrementando la producción de CO.

Adicionalmente, este sistema permitía la posibilidad de controlar la

actividad y selectividad catalítica del Ni hacia la producción de CO

y CH4 mediante la migración controlada de iones K+ desde el

electrolito sólido hacia la película de catalizador.

o Los diferentes catalizadores sintetizados Cu-G, Cu-AC y Cu-CNF

permitieron, sin alimentar directamente hidrógeno, la obtención de

compuestos como metanol, acetaldehído, metano, a partir de la

reducción electrocatalítica del CO2 en fase gas a baja temperatura.

Con el objetivo de ampliar y completar los resultados obtenidos en esta

investigación se recomienda:

o Investigar el uso de electrodos basados en metales no nobles en el

proceso de electrólisis de vapor de agua y oxidación parcial de

etanol, así como para la producción/separación simultánea de H2 y

C2S, incrementando, así, la economía del proceso.

o Realizar un estudio exergético que permita seleccionar las

condiciones óptimas de operación para el proceso de reformado

catalítico de etanol y reformado electroquímico de etanol así como

localizar los sumideros de energía y su magnitud.

o Incrementar la producción de CH4 frente a la de CO en el proceso de

hidrogenación de CO2 mediante la activación de la reacción de

metanación de CO2 a través de procesos EPOC utilizando YSZ como

electrolito sólido.


46

o Incrementar la eficiencia del proceso de valorización de CO2 en la

celda PEM utilizando diferentes catalizadores preparados a partir

de distintas técnicas y aumentando la temperatura del proceso

utilizando membranas Sterion® dopadas con H3PO4 con mayor

resistencia térmica que las usadas en la presente investigación.

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Chemical Engineering Transactions, 2013, pp. 289-294.

1.1. Introduction

1.2. Experimental

1.2.1 Catalytic activity measurements

1.2.2. Preparation of the solid

electrolyte cell

1.2.3. Characterization measurements

1.3. Results and discussion

1.4. Conclusions

1.5. References

CHAPTER 1:

Direct Production of Flexible H2/CO

Synthesis Gas via Steam Electrolysis

and Ethanol Partial Oxidation

INTRODUCTION

EXPERIMENTAL CHARACTERIZATION

SEM analysis

RESULTS

C2H5OH/H2O = 0.7%/ 2 %, N2 balance, T = 600 ºC

Mixtures of hydrogen and carbon monoxide (syn-gas) are used in a wide range

of industrial applications such as petrochemicals, ammonia, petroleum

refining, methanol synthesis etc. Both kinds of products are the primary

feedstock streams in the refinery and biorefinery industry for the different

equipment and conversion process from petroleum and biomass.

The development of novel configurations for the production of synthesis gas (syn-gas) of flexible H2/CO ratio is of great importance to reduce the cost for the

synthesis of synfuels and high-value chemicals.

Single chamber solid electrolyte cell reactor

- The cross section revealed a good

contact between the porous layer

and the dense electrolyte.

XRD analysis

-Pt particle size = 15 nm

-Cathodic arc deposition method allows to better

control the deposition conditions and hence a better

reproducibility of the catalyst/electrode films.

Influence of the applied current Variation of the H2 Faradaic Efficiency vs. current

C2H5OH/H2O = 0.7%/ 2 %, N2 balance

Influence of the applied potential and temperature on the H2/CO ratio

- The ratio H2/CO can be largely modified and

controlled via the applied polarization

In this work, we propose a radically novel approach to the direct production of syn-gas with flexible H2/CO ratio by means of a solid electrolyte membrane

reactor (SEMRs). For that purpose, a single chamber solid electrolyte membrane reactor based on yttria-stabilized zirconia (YSZ), has been developed

(Pt/YSZ/Pt).

Outlet Inlet

Counter Electrode (CE)

Catalyst-working

Electrode (WE)

Reactor cap Cooling

Quartz

tube

Au wires

Alumina tube

with 4 bores 20 30 40 50 60 70 80 90

YS

Z (

4 2

0)

YS

Z (

4 0

0)

YS

Z (

2 2

2)

Pt

(2 2

2)

Pt

(3 1

1)

Pt

(2 2

0)

Pt

(2 0

0)

Pt

(1 1

1)

YS

Z (

3 3

1)

YS

Z (

2 2

0)

YS

Z (

2 0

0)

Inte

nsi

ty,

a.u

.2 º

YS

Z (

1 1

1)

Pt

YSZ

0

5

10

15

20

25

30

35

0 15 30 45 60 75 90 105 120 135 150

0.00

0.04

0.08

0.12

0.6

0.9

1.2

1.5

1.8

rH2

rCO

rCO2

H2/CO = 2.5H2/CO = 8.7H2/CO = 2.5H2/CO = 8.2

I= -80 mAI = 80 mA O.C.CO.C.C

(r /

mol·

s-1)

x 1

08

O.C.C

H2/CO = 2.8

rCH4

rC2H4

rC2H6

(r /

mol·

s-1)

x 1

08

Time / min

- H2/CO ratio between 2.5 in open circuit

transitions regimes to 8.7 in closed circuit

transitions regimes

0

30

60

90

120

150

180

0

1

2

3

4

3

6

9

12

15

0.0 0.5 1.0 1.5 2.0

0

2

4

6

I /

mA

(rH

2

/ m

ol·s

-1)·

10

7

(rC

O /

mol

·s-1)·

10

8

T = 600 ºC

T = 650 ºC

T = 700 ºC

(rC

O2

/ m

ol·s

-1)·

10

8

UWC

/ V

Influence of the applied potential and temperature

- An increase in the applied potential

led to an increase in the production rate

of the main products: H2, CO and CO2

- An increase in the production rate of

different products at fixed potential with

the explored temperatures.

0 30 60 90 120 150 180 210

0.3

0.4

0.5

0.6

0.7

0.8

0.9

I / mA

T = 600 ºC

T = 650 ºC

T = 700 ºC

Faraday Law: Λ= (r -r0) /(I/nF)

0.0 0.5 1.0 1.5 2.0

0

2

4

6

8

10

12

H2/C

O

Potential / V

T = 600 ºC

T = 650 ºC

T = 700 ºC

CHAPTER 1. DIRECT PRODUCTION OF FLEXIBLE

H2/CO SYNTHESIS GAS VIA STEAM ELECTROLYSIS

AND ETHANOL OXIDATION

55

Abstract

n this chapter, a novel approach for the direct production of syngas

with flexible H2/CO ratio by means of a solid electrolyte membrane

reactor (SEMRs) is proposed. This way, a single chamber solid

electrolyte membrane reactor based on yttria-stabilized zirconia (YSZ) was

developed (Pt/YSZ/Pt), where both active Pt catalysts-electrodes were

exposed to the same reaction atmosphere (C2H5OH/H2O = 0.7 %/2 %). The

application of different polarizations at a range of temperatures (600-700

ºC) allowed to control the H2/CO ratio of the obtained syngas, i.e., the ratio

was varied between 1.5-12 under polarization conditions. Unlike

conventional catalytic partial oxidation processes, the H2/CO adjustment

was tuned without the requirement of external O2 feeding to the reactor. An

increase in the applied current or potential caused the H2/CO ratio to

increase if compared to that of the open circuit conditions. Ethanol

reforming occurred on the Pt catalyst/electrodes which is due to an increase

in the rate of the electrocatalytic processes. On the other hand, a decrease in

the H2/CO ratio with increasing temperatures at fixed potentials was

achieved as a consequence of the occurrence of further reaction of the

produced H2 with the rest of species present in the gas phase, leading to a

decrease in the faradaic efficiency. The proposed configuration may be of

great interest especially for biorefinery applications where H2, syngas and

electricity may be produced from bioethanol.

I

Chapter 1

56

1.1. Introduction

Hydrogen is considered a promising candidate to be the energy carrier

of the future as it can be used to produce electricity via fuel cells. In

addition, precise mixtures of hydrogen and carbon monoxide (syngas) are

also used in a wide range of industrial applications, such as

petrochemicals, ammonia, petroleum refining, methanol synthesis, etc [1-

3]. Both kinds of products (hydrogen and syngas) are the primary feedstock

streams in refinery and biorefinery industries. In the latter case,

bioethanol (a renewable resource) could be used for the production of fuels,

power, heat, and value-added chemicals [4]. As mentioned in a previous

chapter (Descripción del trabajo realizado), autothermal reforming (ATR),

steam reforming (SR), catalytic partial oxidation (CPOX), and a

combination of two latter are the most common paths to convert such fuels

into syngas [5]. Each process has advantages and drawbacks. Briefly,

steam reforming is a strong endothermic process so a significant amount of

energy is required to drive the process. The main advantage of the partial

oxidation and the autothermal reforming, is that they can be carried out at

lower reaction temperatures without any external heat supply due to that

they are exothermic. However, the addition of an O2 stream free of N2 is

required for the reaction, which implies a previous O2/N2 air separation

unit. On the contrary, depending on the use of the stream exiting from the

ethanol reformer, several additional reaction and separation steps, such as

water-gas-shift, preferential oxidation (PROX), methanation and pressure

swing adsorption, are required for the purification and further adjustment

of the H2/CO ratio. If the final application is the electricity production in a

fuel cell, a high value of H2/CO ratio is required to decrease the further CO

separation/purification additional steps. If the syngas is required for

Direct Production of Flexible H2/CO Synthesis Gas via Steam Electrolysis and Ethanol Partial Oxidation

57

synthesis applications, a H2/CO ratio below 3 is usually required. The

direct production of flexible H2/CO synthesis gas could avoid or limit the

H2/CO adjustments units and thus energy conservation and cost reduction

should be achieved. Consequently, there is a growing interest to find

solutions for providing flexible H2/CO ratio [6, 7].

In this chapter, a radically novel approach for the direct production of

synthesis gas with flexible H2/CO ratio based on the use of a solid

electrolyte membrane reactor (SEMRs) is proposed [8]. As described in

previous chapter, SEMR mainly consists of a ceramic solid electrolyte

(ionic conductor material e.g., O2- or H+ conductor in most of cases) in

which two metal or metal oxide catalysts-electrodes are deposited on both

sides of the solid electrolyte [9]. These kinds of configurations allow to

supply one of the reactives electrochemically (e.g., O2− for an oxidation

reaction) by a Faradaic operation [9-14]. The option of using this kind of

reactors presents several advantages such as it is possible to enhance the

catalytic activity and selectivity, to better integrate the different processes

and to control the reaction rate in a fast and easy way [15]. In addition, for

the case of the ethanol partial oxidation reaction, pure O2 free of N2 can be

in-situ supplied through an integrate steam electrolysis process, which

would allow a further production of H2, avoiding an energetic intensive air

separation unit. These reactor configurations have already been used for

other catalytic reactions such as the oxidative coupling of methane, the

oxidation and reduction of NOx and the ammonia synthesis [10, 16-19].

Chapter 1

58

Vent

N2

H2

N2

FIC

FIC

FIC

FIC

N2

PC Potenciostat-

galvanostat

Flowmeter

300

300

300 32

300

BROOKS INSTRUMENT

Micro gas

cromatograph

Temperature controller

Cooler

Flow controller

Furnace

Reactor

PGZ 301

VoltaLab

cell

FIC

FIC

FIC

FIC

Nitrógeno

Hidrógeno

Oxígeno/

Aire

Nitrógeno

TIC

TIC

Salida de gasesRegistro

temperatura

TIC

Saturador

Agua

Saturador

Metanol

REACTOR

AUTOLAB

PGSTAT320-N

BRUKER 450-GC

Flujómetro

de burbuja

BRONKHORST EL-FLOW

S-1

S-1

S-1

S-1

S-1

S-1

S-1

Water saturator

Heating wire

FIC

FIC

FIC

FIC

Nitrógeno

Hidrógeno

Oxígeno/

Aire

Nitrógeno

TIC

TIC

Salida de gasesRegistro

temperatura

TIC

Saturador

Agua

Saturador

Metanol

REACTOR

AUTOLAB

PGSTAT320-N

BRUKER 450-GC

Flujómetro

de burbuja

BRONKHORST EL-FLOW

S-1

S-1

S-1

S-1

Ethanol saturator

-2 ºC

S-1

S-1

S-1

S-1

S-1S-1S-1S-1S-1S-1 S-1

S-1

1.2. Experimental

1.2.1. Catalytic activity measurements

The catalytic activity measurements were carried out in an

experimental set-up as shown in Figure 1.1.

Figure 1.1. Scheme of the experimental set-up for the steam reforming and

ethanol partial oxidation experiments

The catalytic experiments were carried out at atmospheric pressure

with an overall gas flow rate of 100 Nml·min-1, in the temperature range

between 600 and 700 ºC. The feed stream composition was: C2H5OH/H2O =

0.7 %/2 %, N2 balanced. The gas flow and composition was controlled by

two N2 calibrated mass flow-meters (Brooks 5850 E) flowing through two

independent ethanol and water temperature controlled saturators in order


59

Outlet Inlet


Catalyst-working

Electrode (WE)

Reactor cap Cooling

Quartz

tube

Au wires

Alumina tube

with 4 bores

to achieve liquid–vapour equilibrium. Thus, the content of C2H5OH and

H2O in the final reaction mixture was controlled by using their vapour

pressure at the temperature of the saturators (38 ºC and 45 ºC,

respectively). All lines placed downstream from the saturator were heated

above 100 ºC to prevent condensation.

The reactor was a single chamber type since all the two electrodes

(catalyst-working (WE) and counter (CE) electrodes) are in the same

chamber and are simultaneously exposed to the reactants and products

[20]. Figure 1.2 shows a schematic drawing of the single chamber reactor

and the corresponding electrode configuration.

Figure 1.2. Schematic drawing of the single chamber reactor

The cell reactor with a volume of 30 cm3 was made of quartz. The tube

was closed at one end and exhibited CSTR (continuous stirred tank

Chapter 1

60

reactor) behaviour. The open-end of the tube was mounted on a stainless

steel cap, which had provisions for the introduction of reactants and

removal of products as well as for the insertion of a thermocouple and the

electrical connection with the electrodes of the cell. The electrical contact of

the two Pt electrodes was carried out by gold wires, which were in turn

connected to a potentiostat-galvanostat Voltalab 21 (Radiometer

Analytical).

Reactant and product gases were analysed with an online microgas-

chromatograph (Varian CP-4900) equipped with two columns (Molsieve

and Poraplot Q column) and two thermal conductivity detectors (TCD).

The molsieve column used Ar as the carrier gas and operated at T=80 ºC

and 20 psi. On the other hand, Poraplot Q column operated at T = 70 ºC

and 20 psi, using He as the carrier gas. Before the analysis, the water and

ethanol were trapped by a condenser at -2 ºC. The main detectable

products were H2, CO, CO2, CH4, C2H6 and C2H4. All analysis outputs, as

well as the potentiostat-galvanostat ones (current and potential), were

continuously monitored and recorded.

1.2.2. Preparation of the solid electrolyte cell

The solid electrolyte cell, also called electrochemical catalyst, consisted

of a continuous Pt thin film (geometric area of 2.01 cm2) deposited on both

sides of a 19-mm-diameter, 1-mm-thick YSZ disc (Tosoh-Zirconia), as

shown in Figure 1.3. Both Pt electrodes, which were identical, were

symmetrically deposited on both sides of the YSZ solid electrolyte using

the pulsed cathodic arc technique [21]. The individual pulses were 1 µs

long and had a current of approximately 300 A. A total of 2800 pulses were

used for the deposition of each Pt layer. The substrate holder was at

ground during the deposition and was rotated at 2 r.p.m.


61

Catalyst-working

electrode (Pt)Counter

electrode (Pt)

Solid electrolyte

(YSZ)

20 mm

1 mm

Figure 1.3. Scheme drawing of the Pt/YSZ/Pt electrochemical catalyst.

This electrode preparation technique [22] allowed the preparation of a

thin reduced state Pt film with high adhesion to the substrate that did not

require of any further calcination procedure. The final Pt loading was

around 0.8 mg Pt/cm2.


The cathodic Pt catalyst-electrode film was characterized by X-ray

diffraction (XRD) with a Philips PW 1710 instrument using Ni-filtered Cu

K radiation. The diffractograms were compared with the JCPDS-ICDD

references. The Pt catalyst film was also characterized via scanning

electron microscopy (SEM) using a JEOL 6490 LV microscope.


In this chapter a single chamber solid electrolyte membrane reactor

based on yttria-stabilized zirconia (YSZ), which is an ionic O2- conductor

material, has been developed (Pt/YSZ/Pt), where both active Pt catalysts-

Chapter 1

62

electrodes were exposed to the same reaction atmosphere (a mixture of

C2H5OH and H2O). In this case, no O2 was directly fed from the gas phase

for the partial oxidation of ethanol, and hence, the active O2- ions and O2

molecules for the oxidation reactions were in-situ electrochemically

produced from the supplied H2O, through a steam electrolysis process.

Hence, steam was electrolyzed in the Pt cathode of the cell to form

hydrogen gas and oxygen ions (O2-), which were transported through the

YSZ solid electrolyte (O2- ionic conductor material) to the Pt anodic

electrode, leading to the ethanol electro-catalytic partial oxidation reaction

(Figure 1.4). As discussed below, an overall modification of the H2/CO ratio

of the produced syngas, which could be in-situ controlled via the applied

potential or current (H2 and O2- ions production rate), could occur.

Figure 1.4. Scheme of the solid electrolyte single chamber reactor.

XRD patterns of the Pt cathodic catalyst-electrode film are given in

Figure 1.5. The peak assignments based on JCPDS standards were

consistent with a Pt face-centered cubic (FCC) structure and a YSZ phase

corresponding to the solid electrolyte. This analysis indicated the purity of

both phases with no contamination of the catalyst-electrode film during

the preparation stage. According to previous studies [23], the Pt mean

particle size can be estimated by using the Scherrer’s equation

corresponding to the (1 1 1) peak, resulting in a Pt crystal size of 15 nm.


63

20 30 40 50 60 70 80 90

YS

Z (

4 2

0)

YS

Z (

4 0

0)

YS

Z (

2 2

2)

Pt

(2 2

2)

Pt

(3 1

1)

Pt

(2 2

0)

Pt

(2 0

0)

Pt

(1 1

1)

YS

Z (

3 3

1)

YS

Z (

2 2

0)

YS

Z (

2 0

0)

Inte

nsi

ty,

a.u

.

2 º

YS

Z (

1 1

1)

Figure 1.5. XRD patterns of the Pt catalyst electrode prepared by cathodic

arc deposition

Remarkably, this value is considerably lower than that typically

obtained (i.e., around 60 nm) by direct impregnation of Pt precursor

solution salts [24] or the application of metal pastes (values up to 500 nm)

[25], which are the most common techniques for the preparation of such

kind of catalytic/electrocatalytic films. It is worth noting that the

preparation of Pt catalyst/electrodes films with small grain sizes is of great

research interest due the higher kinetic activity [25]. In addition, the

cathodic arc deposition method allowed to better control the deposition

conditions and hence to obtain a better reproducibility in the manufacture

of the catalyst/electrode films. This reactor configuration based on a 19-

mm-diameter, 1-mm-thick YSZ pellet of low geometric area (2 cm2) is

typically employed for lab-scale studies. However, the reproducibility of

the used preparation technique may simplify, in view of practical

applications, by scaling it up to other configurations.

Chapter 1

64

a)

b)

c) Pt

YSZ

Pt

Figure 1.6 shows the top (a) and cross section SEM analysis (b and c) of

the Pt catalyst-electrode film.

Figure 1.6. Top (a) and cross section SEM micrographs (b and c) of the Pt

catalyst-electrode film

The micrograph shows a porous Pt layer with a foam structure where

the Pt particles were networked with each other, leading to a suitable

value of electrical conductivity (in-plane measured electrical resistance

value was below 2 ohms). No cracks or delimitations of the porous layer

were observed after the preparation. The cross section analysis also

revealed a good contact between the porous layer and the dense electrolyte.


65

Figure 1.7 represents the rate of the different obtained products (e.g.,

H2 CO2, CO, CH4, C2H4 and C2H6) with the time on stream under both open

circuit conditions (O.C.C) and current imposition of 80 mA and -80 mA at

C2H5OH/H2O = 0.7 %/2 %, N2 balanced, T=600 ºC. The Figure 1.7 also

indicates the H2/CO ratio obtained in each polarization regime (30 min of

duration). Firstly, it can be observed that the reforming products (e.g., H2,

CO and CO2) were the main products in the outlet reaction. Hence, under

O.C.C conditions (no current application) the catalytic ethanol reforming

reaction occurred on both Pt catalyst-electrodes, which symmetrically

contacted with the reaction mixture [26] via:

C2H5OH + H2O 4H2 + 2CO (1.1)

C2H5OH + 3H2O 6 H2 + 2 CO2 (1.2)

It can also be observed the presence of certain amount of CH4, C2H4 and

traces of C2H6 that can be attributed to the direct ethanol hydrogenation

reaction under the explored conditions:

C2H5OHH2+CO+CH4 (1.3)

C2H5OH C2H4 + H2O (1.4)

C2H4 + H2 C2H6 (1.5)

Other related catalytic processes may also occurred in the reaction

mixture such as the water gas shift and the CO and CO2 hydrogenation

reactions, leading to the experimentally H2/CO ratio between 2.5-3

observed under different open circuit transition regimes. The occurrence of

the water gas shift reaction is not very significant due to its exothermic

Chapter 1

66

0

5

10

15

20

25

30

35

0 15 30 45 60 75 90 105 120 135 150

0.00

0.04

0.08

0.12

0.6

0.9

1.2

1.5

1.8

rH2

rCO

rCO2

H2/CO = 2.5H2/CO = 8.7H2/CO = 2.5H2/CO = 8.2

I= -80 mAI = 80 mA O.C.CO.C.C

(r /

mol·

s-1)

x 1

08

O.C.C

H2/CO = 2.8

rCH4

rC2H4

rC2H6

(r /

mol·

s-1)

x 1

08

Time / min

character. In addition, the use of high temperatures (> 600 ºC) led to a

shift of the equilibrium to the left, limiting the conversion of CO.

Figure 1.7. Influence of the applied current (where O.C.C. denotes Open Circuit

Conditions) on the dynamic value of the reaction rates of the obtained products.

Conditions: C2H5OH/H2O = 0.7 %/2 %, N2 balanced, T=600 ºC.

On the other hand, it can be observed an increase in the reaction rates

of all the obtained products during the current impositions (80 and -80 mA.

The effect of the polarizations could be explained attending to the steam

electrolysis process on the Pt cathode and to the electrochemical supplying

of O2- ions to the Pt anode during the polarizations [13]. It is well known


67

[27] that YSZ solid electrolyte behaves as a pure oxygen anion conductor

material (O2-). Then, under polarization conditions, a considerable amount

of H2 is released into the gas phase at the cathode via reaction (1.6)

H2O + 2e- H2 + O2- (1.6)

while O2- ions were transported through the YSZ solid electrolyte to the Pt

anodic electrode leading to the electrocatalytic partial oxidation reactions

of ethanol (see Figure 1.4), increasing the production rate of the different

ethanol oxidation products:

C2H5OH+O2-3H2+2CO+2e- (1.7)

C2H5OH+3O2-3H2O+2CO2+6e- (1.8)

In addition, other reaction such as the oxygen formation, the ethanol

and CO catalytic oxidation and H2 electro and catalytic oxidation may also

occur at the explored conditions:

O2-O2+2e- (1.9)

C2H5OH+O2H2+CO+CO2 (1.10)

CO+1/2O2 CO2 (1.11)

H2 + O2- H2O + 2e- (1.12)

H2 + 1/2O2 H2O (1.13)

Hence, under the steady state conditions, a H2/CO ratios of 8.2 and 8.7

were achieved at 80 and -80 mA, respectively, showing the possibility of

increasing the overall H2/CO ratio . On the other hand, it can be observed

that similar production rate transitions behaviors were obtained under the

Chapter 1

68

positive and negative polarization. It indicates not only a suitable

geometry of the electrochemical cell and a similar morphology of both Pt

catalyst electrodes which can, therefore, act as cathode or anode depending

on the applied potential but also the reproducibility of the catalyst-

electrode preparation method on the two faces of the YSZ solid electrolyte

by the technique of cathodic arc deposition. It can also be observed that for

all the transitions, the system returned to the initial OCC production rate

values, demonstrating the reversibility of the polarization effect and the

stability of the electrodes under the explored reaction conditions.

According to these remarks, henceforth positive polarizations will be

applied for the subsequent reaction experiments, the rate of the primary

reaction products (H2, CO and CO2) being uniquely displayed.

Figure 1.8 shows the steady state variation of the main reaction

products: H2, CO, and CO2, and the obtained steady state current during

different potentiostatic impositions carried out at different reaction

temperatures (600-700 ºC) by using the same reaction mixture:

C2H5OH/H2O = 0.7 %/2 %, N2 balanced. Firstly, an increase in the applied

potential led to an increase in the production rate of the main products: H2,

CO and CO2. This effect can be easily explained considering that an

increase in the applied potential and hence in the current led to an

increase in the rate of the different electrocatalytic processes [13]: steam

electrolysis at the cathode (reaction 1.6) and ethanol electro-oxidation at

the anode, which increased the amount of partial and complete ethanol

oxidation products (reactions 1.7-1.11). Consequently, at higher current

values more amount of H2 was produced since higher amount of oxygen

ions (O2-) were supplied to the anodic electrode, which in turn increased

the rate of the electrocatalytic and catalytic partial oxidation of ethanol.


69

0

30

60

90

120

150

180

0

1

2

3

4

3

6

9

12

15

0.0 0.5 1.0 1.5 2.0

0

2

4

6

I /

mA

(rH

2

/ m

ol·

s-1)·

10

7

(rC

O /

mol·

s-1)·

10

8

T = 600 ºC

T = 650 ºC

T = 700 ºC

(rC

O2

/ m

ol·

s-1)·

10

8

UWC

/ V

Figure 1.8. Influence of the applied potential and the reaction temperature on the

steady state current and reaction rates of the main products. Conditions:

C2H5OH/H2O = 0.7 %/2 %, N2 balanced.

On the other hand, it can also be observed an increase in the production

rate of different products with the temperature at fixed potential. This

increase could be attributed to the increase of both the kinetics of the

catalytic and electrocatalytic activity, when increasing the temperatures,

and the ionic conductivity of the YSZ solid electrolyte. Hence, the obtained

current at fixed potential increased with the reaction temperature which

would again explain the increase in the electrocatalytic kinetics [12, 15].

Chapter 1

70

The apparent activation energy calculated via Arrhenius plot under

open circuit conditions and closed circuit conditions was obtained. The

apparent activation energy calculated under O.C.C was 106.83 kJ·mol-1.

This value is in the same range as that reported in previous works of

catalytic steam reforming of ethanol. For instance, an activation energy of

187.7 kJ·mol-1 was reported for the steam reforming of ethanol over Ni

catalyst [28]. On the other hand, the apparent activation energies under

closed circuit conditions (0.5 V, 1 V, 1.5 V and 2 V) were 98.50 kJ·mol-1,

87.30 kJ·mol-1, 55.96 kJ·mol-1 and 49.00 kJ·mol-1, respectively. As it can be

observed, calculated activation energy linearly decreased with increasing

applied potential. The activation energy under open circuit conditions was

106.83 kJ·mol-1 whereas that obtained at closed circuit conditions (2 V) was

49.00 kJ·mol-1. This value is in the same range that the activation energy

obtained under closed circuit conditions for the electro-oxidation of ethanol

over Pt in a phosphoric acid media (36 kJ·mol-1) [29].

Figure 1.9 shows the variation of the obtained H2 faradaic efficiency ().

This factor was calculated via Faraday´s law from the experimentally

measured H2 production rate under current imposition. This way, the

overall hydrogen consumption and production reactions were taken into

account.

= (r -r0) /(I/nF) (eq. 1.1)

where r is the H2 production rate under the applied current, ro is the open-

circuit catalytic rate due to the catalytic steam reforming of ethanol, I is

the applied current, n is the charge of the ionic species (2) and F is the

Faraday’s constant.


71

0 30 60 90 120 150 180 210

0.3

0.4

0.5

0.6

0.7

0.8

0.9

I / mA

T = 600 ºC

T = 650 ºC

T = 700 ºC

Figure 1.9. Variation of the H2 Faradaic Efficiency () vs. current at diferent

temperatures.

This parameter related the H2 produced at each current (I/nF) with the

overall hydrogen production reactions (r-r0). It can be observed that all the

displayed faradic efficiencies values were lower than 1, which indicated

that no electrochemical promotion effect (NEMCA) occurred on the Pt

catalyst/electrode for the ethanol reforming reaction [30] or at least was

negligible vs. the previous explained electrocatalytic processes. These

results are in agreement with EPOC studies which clearly demonstrated

that the desorption of O2- promoting ionic species on Pt is quite fast at

temperatures above 600 ºC. Vernoux et al [31] reported an O2-TPD study,

in which the promoting ionic oxygen species desorbed from the Pt surface

at 600 ºC. That the O2 evolution reaction is a fast reaction at temperature

above 600 ºC is evident from the high value of the applied current (high

electrocatalytic activity) in comparison with some EPOC studies where low

current values are used [20]. Furthermore, the high operating

temperatures used in this chapter did not favor the formation of the

Chapter 1

72

effective double layer, which is prerequisite to observe the NEMCA effect.

Additionally, part of the H2 produced in the single chamber reacted with

other products such as CO, CO2, C2H4 and O2, lowering the faradaic

efficiency value below 1. It can be observed that an increase in the applied

current and the reaction temperature decreased the H2 faradaic Efficiency

value. At higher currents, more H2 and ethanol electro-oxidation products

were formed which reacted between them decreasing the amount of H2

obtained in the reactor (r) vs. the corresponding faradaic H2 production

rate via steam electrolysis (I/nF). For a fixed current, the rise in the

reaction temperature promoted the further reaction, in the single chamber

reactor, of H2 with the other obtained molecules, which also contributed to

the decrease in the Faradaic efficiency. On the other hand, a decrease in

the Faradaic efficiency can also be attributed to the oxygen evolution

reaction (reaction 1.9) due to an increase in the temperature. However, the

most interesting part of the experiment comes from the experimentally

measured modification in the overall H2/CO ratio of the obtained syngas.

Figure 1.10 shows the variation of the obtained H2/CO ratio of the

produced syngas vs. the applied potential for the experiment inFigure 1.8.

It can be again observed that the H2/CO ratio can be largely modified and

controlled by applied polarization. Values between 1.5-12 were obtained

depending on the conditions (applied potential and temperature). As

expected, the higher H2/CO ratio modification was attained at the lowest

explored temperature (600 ºC) where the higher H2 faradaic efficiencies

values were measured. However, at higher temperatures, where higher

rates of syngas were obtained (Figure 1.9), the H2/CO ratio can also be

controlled between 1.5-3, which is very interesting from the petrochemical

viewpoint [7].


73

0.0 0.5 1.0 1.5 2.0

0

2

4

6

8

10

12

H2/C

O

Potential / V

T = 600 ºC

T = 650 ºC

T = 700 ºC

Figure 1.10. Influence of the applied potential and reaction temperature on the

obtained H2/CO ratio of the produced syngas.

For instance, H2/CO ratio of 2 is typically required for the Fischer-

Tropsch synthesis while an optimal H2/CO ratio around 1 is required for

the oxo-synthesis process. On the other hand, large values of H2/CO ratio

are desired to reduce the purification steps of H2 streams to be used in fuel

cells. Finally, a comparison of the cost of the produced synthesis gas via

electrochemical reactions with that obtained with conventional processes

was performed. In this sense, the cost of synthesis gas in the

electrochemical process was set at 22 kW·h/kg syngas at 700 ºC and 2 V.

This value is very similar to those reported in literature, for instance for

the theoretically production of syngas from coal reforming (18.22 kW·h/kg

syngas) [32]. Hence, it can be suggested that the proposed reactor

configuration may be of great interest in biorefinery applications where

Chapter 1

74

high-quality H2 and syngas of flexible ratio may be required from

renewable resources such as biogas and bioethanol.

1.4 Conclusions

A novel solid electrolyte membrane reactor (Pt/YSZ/Pt) has been

developed for the direct production of synthesis gas with flexible H2/CO

ratio. The technique of cathodic arc deposition was used to obtain

nanocrystalline Pt catalyst/electrode films with excellent stability under

the explored reaction conditions.

The application of different polarizations at different temperatures

ranging from 600 to700 ºC allowed to produce syngas from an (ethanol-

water stream) with a flexible H2/CO ratio (between 1.5-12). The amount of

syngas increased with the applied potential and the reaction temperature

as a result of an increase in the rate of the electrocatalytic processes:

steam electrolysis at cathode and electrocatalytic partial oxidation of

ethanol at the anode. A decrease in the H2/CO ratio at fixed potentials was

achieved at higher temperatures due to the further reaction of the

produced H2 with the rest of species present in the gas phase, thus leading

to a decrease in the faradaic efficiency.

1.5. References

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190.

[2] M. Conte, A. Iacobazzi, M. Ronchetti, R. Vellone, Journal of Power Sources, 100

(2001) 171-187.

[3] I. Dincer, International Journal of Hydrogen Energy, 27 (2002) 265-285.

[4] J. Comas, F. Marino, M. Laborde, N. Amadeo, Chemical Engineering Journal, 98

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75

[5] L.V. Mattos, G. Jacobs, B.H. Davis, F.B. Noronha, Chemical Reviews, 112 (2012)

4094-4123.

[6] Y. Cao, Z. Gao, J. Jin, H. Zhou, M. Cohron, H. Zhao, H. Liu, W. Pan, Energy and

Fuels, 22 (2008) 1720-1730.

[7] X. Song, Z. Guo, Energy Conversion and Management, 47 (2006) 560-569.

[8] G.-C.J. De Lucas-Consuegra A, Valverde J.L, Jiménez-Borja C, Gutiérrez-Guerra

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313-318.

[10] G. Marnellos, M. Stoukides, Solid State Ionics, 175 (2004) 597-603.

[11] E. Panagos, I. Voudouris, M. Stoukides, Chemical Engineering Science, 51

(1996) 3175-3180.

[12] A. Caravaca, A. de Lucas-Consuegra, J. González-Cobos, J.L. Valverde, F.

Dorado, Applied Catalysis B: Environmental, 113-114 (2012) 192-200.

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J.L. Valverde, F. Dorado, International Journal of Hydrogen Energy, 38 (2013) 3111-

3122.

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[15] A. Caravaca, A. de Lucas-Consuegra, V.J. Ferreira, J.L. Figueiredo, J.L. Faria,

J.L. Valverde, F. Dorado, Applied Catalysis B: Environmental, 142–143 (2013) 298-

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Stoukides, Industrial and Engineering Chemistry Research, 50 (2011) 431-472.

[17] M. Stoukides, Catalysis Reviews - Science and Engineering, 42 (2000) 1-70.

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(2007) 507-513.

Chapter 1

76

[19] P. Vernoux, L. Lizarraga, M.N. Tsampas, F.M. Sapountzi, A. De Lucas-

Consuegra, J.L. Valverde, S. Souentie, C.G. Vayenas, D. Tsiplakides, S. Balomenou,

E.A. Baranova, Chemical Reviews, 113 (2013) 8192-8260.

[20] C.G. Vayenas, S. Bebelis, C. Pliangos, S. Brosda, D. Tsiplakides,

Electrochemical Activation of Catalysis: Promotion, Electrochemical Promotion, and

Metal-Support Interactions, 2001.

[21] A. de Lucas-Consuegra, J. González-Cobos, Y. Gacia-Rodriguez, J.L. Endrino,

J.L. Valverde, Electrochemistry Communications, 19 (2012) 55-58.

[22] A. Anders, Cathodic Arcs: From Fractal Spots to Energetic Condensation,

Springer Series on Atomic, Optical, and Plasma Physics, 2008.

[23] A. De Lucas-Consuegra, J. González-Cobos, V. Carcelén, C. Magén, J.L.

Endrino, J.L. Valverde, Journal of Catalysis, 307 (2013) 18-26.

[24] A. de Lucas-Consuegra, F. Dorado, J.L. Valverde, R. Karoum, P. Vernoux,

Journal of Catalysis, 251 (2007) 474-484.

[25] A. de Lucas-Consuegra, A. Caravaca, P.J. Martínez, J.L. Endrino, F. Dorado, J.L.

Valverde, Journal of Catalysis, 274 (2010) 251-258.

[26] M. Kourtelesis, P. Panagiotopoulou, X.E. Verykios, Catalysis Today, (2014).

[27] L. Malavasi, C.A.J. Fisher, M.S. Islam, Chemical Society Reviews, 39 (2010)

4370-4387.

[28] C. Zhang, S. Li, G. Wu, Z. Huang, Z. Han, T. Wang, J. Gong, AIChE Journal, 60

(2014) 635-644.

[29] L. Jiang, A. Hsu, D. Chu, R. Chen, International Journal of Hydrogen Energy, 35

(2010) 365-372.

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(1992) 303-438.

[31] P. Vernoux, M. Guth, X. Li, Electrochemical and Solid-State Letters, 12 (2009)

E9-E11.

[32] E.J. Bair, Connecting the Dots to Future Electric Power, AuthorHouse2010.

2.1. Introduction

2.2. Experimental



electrolyte cell



2.4. Conclusions

2.5. References

CHAPTER 2:

Simultaneous Production and

Separation of H2 and C2 Hydrocarbons

via Steam Electrolysis and Methane

Partial Oxidation

INTRODUCTION


SEM analysis

RESULTS

Outer chamber: CH4 = 1 %,

Inner chamber H2O = 3 %, N2 balance, T = 750 ºC

Hydrogen and syngas can be used in a wide range of industrial applications, such us

petrochemicals, ammonia, petroleum refining, and methanol synthesis

The thermal catalytic decomposition of methane (TCD) into hydrogen and carbon is

attracting much research interest since it appears to be a direct, mildly endothermic,

attractive way for producing highly “pure” hydrogen with reduced CO2 emissions

Thermo-catalytically decomposed of methane

CH4 (g) C (s) + 2 H2 (g)

H2O/N2

inner chamberPure H2

inner chamber

Fritted quartz

Double chamber

quartz reactor

Platinum deposited

in dense YSZ

Counter electrode

Platinum impregnated

in porous YSZ

Solid electrolyte

tube(YSZ)

Working electrode

H2, CO, CO2,

C2H4, C2H6

outer chamber

CH4/N2

outer chamber

Thermocouple

Au wires

Double chamber solid electrolyte cell reactor

- Porous layer thickness of around 60 μm.

- Pt impregnated particles were dispersed

on the YSZ porous layer by the presence of

lighter dots along the cross section

micrograph.

XRD analysis

20 30 40 50 60 70 80 90

YS

Z (

4 2

0)

YS

Z (

4 0

0)

YS

Z (

2 2

2)

Pt

(2 2

2)

Pt

(3 1

1)

Pt

(2 2

0)Pt

(2 0

0)

Pt

(1 1

1)

YS

Z (

3 3

1)

YS

Z (

2 2

0)

YS

Z (

2 0

0)

Inte

nsi

ty, a.

u.

2º

YS

Z (

1 1

1)

-Pt particle size on porous solid

electrolyte = 32 nm

-Porous YSZ interlayer over the dense

solid electrolyte strongly increase the

dispersion of the Pt particles.

Influence of the applied current

0

1

2

3

0

1

2

3

0 40 80 120 160 200 240 280

0.0

0.5

1.0

1.5

50 mA50 mA O.C.CO.C.CO.C.C

(rH

2oute

r /

mol·

s-1·

cm-2

) x108

50 mAO.C.C

0

6

12

(r

H2

inner

/ m

ol·

s-1·c

m-2

) x108

(r C

Oo

ute

r /

mol·

s-1·c

m-2

)x108

(rC

O2

ou

ter

/ m

ol·

s-1·c

m-2

) x108

0.0

0.6

1.2

Time / min

(rC

2H

6oute

r /

mol·

s-1·c

m-2

) x109

(rc 2

H4

oute

r /

mol·

s-1·c

m-2

) x101

0

0.0

0.3

0.6

Influence of the reaction atmosphere

Symbol Outer chamber Deposited Carbon

N2 No

1 vol.% CH4/N2 No

N2 Yes

1 vol.% CH4/N2 Yes

0 20 40 60 80

0.0

0.5

1.0

1.5

2.0

2.5

UW

C (V

)

I (mA)

3 vol.% H2O (deposited C)

3 vol.% H2O (non deposited C)

3 vol.% H2O / 1 vol.% CH

4 (non deposited C)

0 20 40 60 80

0.0

0.5

1.0

1.5

2.0

2.5

UW

C (V

)

I (mA)




4 (non deposited C)

0 20 40 60 80

0.0

0.5

1.0

1.5

2.0

2.5

UW

C (V

)

I (mA)




4 (non deposited C)

0 20 40 60 80 100 120

0.0

0.5

1.0

1.5

2.0

2.5

UW

C (V

)

I (mA)




4 (non deposited C)

0 20 40 60 80 100 120

0.0

0.5

1.0

1.5

2.0

2.5

UW

C /

V

Current / mA

T = 700 ºC.

100 mVs-1

Reproducibility experiment

CH4 = 1 %, H2O = 3 %, N2 balance,

T = 750 ºC

0

10

20

30

40

50

60

0

2

4

6

0 100 200 300 400 500 600

0.0

0.5

1.0

1.5

2.0

(100 % N2) (100 % N2)

0% CH4

0% CH4

O.C.C

I / m

A

I = 50 mA

1% CH4

rCO

rCO

2

(rC

O, r

CO

2oute

r / m

ol·s

-1·c

m-2

) x1

08

(rH

2 oute

r /m

ol·s

-1·c

m-2)x

108

Time / min

T = 700 ºC.

- Stability of the Pt/YSZ/Pt catalyst exposed

to the reactions conditions

In this study, we propose a novel approach based on the ability of SEMRs to both, produce carbon

and H2, and to electrochemically oxidize the carbon deposited over an electrode. It consists of a

solid electrolyte membrane reactor that allows to electrochemically regenerate a Pt catalyst from

the carbon deposition in the TCD reaction.

0.0

0.4

0.8

1.2

1.6

2.0

0.00

0.15

0.30

0.45

0.60

0

1

2

3

4

5

0.0 2.5 5.0 7.5 10.0 12.5 15.0 17.5 20.0 22.5

0

5

10

15

20

25

30

35

(rH

2 o

ute

r/m

ol·

s-1·c

m-2

) x1

08

(r C

O2

ou

ter/

mol·

s-1

·cm

-2) x1

08

(rC

O o

ute

r/m

ol·

s-1

·cm

-2)

x1

08

0.0

0.3

0.6

0.9

1.2

1.5

1.8

(r C

2H

6ou

ter/

mol·

s-1·c

m-2

) x1

01

0

(rC

2H

4ou

ter/

mol·

s-1

·cm

-2)

x1

010

0

2

4

6

8

10

12

Cu

rren

t /

mA

Time / h

-0.85

-0.84

-0.83

-0.82

-0.81

-0.80

-0.79

UW

C (

V)

Gas de síntesis

Metanol

Nafta

IntermediosAmoniaco

Metano- Olefinas

- Aromáticos

- Tolueno

- Etilenglicol

- Isobutano

Haber-Bosch

- Urea

- Hidrazina

- Metilaminas

- Ácido Nítrico

- Acrilonitrilo

- Fertilizantes

H2

CO

Metanación

Proceso

MOBIL- Olefinas aromáticas

(gasolinas)

- Metanol

- Ácido acético

- Formaldehido

- Metilamina

CHAPTER 2. SIMULTANEOUS PRODUCTION AND

SEPARATION OF H2 AND C2 HYDROCARBONS VIA STEAM

ELECTROLYSIS AND METHANE PARTIAL OXIDATION

81

Abstract

his chapter reports the production of H2 via the catalytic

methane decomposition on Pt supported on yttria-stabilized

zirconia together with its electrochemical regeneration in a solid

electrolyte membrane reactor. Hence, a Pt-YSZporous/YSZ/Pt

double chamber solid electrolyte cell was prepared and tested under two

reaction regimes. In the first regime, under open circuit conditions,

hydrogen and carbon was produced on the catalytically active Pt-YSZ

porous catalyst via methane decomposition reaction (CH4 (g) C (s) + 2

H2 (g)). In the second regime, under polarization conditions, steam was

electrolysed at the Pt cathode of the cell (H2O + 2e- H2 + O2-) and the

produced O2- ions were simultaneously electrochemically pumped to the

Pt/YSZ porous catalyst (anode), thereby allowing removal of the previously

deposited carbon (C (s) + O2- CO2 (g)) and finally regenerating the

Pt/YSZ porous catalyst film. We demonstrated that the carbon generated in

the methane decomposition step served as a depolarizating agent in the

steam electrolysis process, thus decreasing the electrical energy input

required for electrochemically producing pure H2. In addition, during the

regeneration step, C2 hydrocarbons (e.g., ethane and ethylene) were obtained

as a result of the electro-catalytic methane oxidative coupling on the

Pt/YSZ porous catalyst film. The performance and durability of the system

was also verified for long operation times in view of the possible practical

application of this novel reactor configuration, which combines gas phase

catalysis and electro-catalysis for hydrogen production.

T

Chapter 2

82

2.1. Introduction

As already mentioned in Chapter 1, hydrogen and syngas can be used in

a wide range of processes, such as petrochemicals processing, ammonia

and methanol synthesis, and petroleum refining [1-3]. If attention is

focused on the use of hydrogen as clean fuel to produce energy by fuel cell

systems, the development of efficient processes for the production of “COx-

free” H2 streams becomes important in order to overcome hurdles

associated with the use of a conventional multi-step process for its further

separation and purification. In this sense, the thermal catalytic

decomposition of methane (TCD) into hydrogen and carbon is attracting

much research interest since it appears to be a direct, mildly endothermic,

attractive way for producing highly “pure” hydrogen with reduced CO2

emissions [4]. This process become more interesting taking into account

the reserves of natural gas and the recent advances in the technology for

the extraction of natural gas from impermeable shale formations

(commonly named shale gas) [5]. Hence, methane can be thermo-

catalytically decomposed into carbon and hydrogen without producing CO2

according to the following reaction:

CH4 (g) C (s) + 2 H2 (g) (2.1)

When compared to conventional methane reforming technologies, TCD

has a number of positive aspects, as recently stressed by some authors. For

instance, based on the life-cycle assessments, Dufour et al. [6] claimed

TCD as the most environmentally friendly process for hydrogen production

as it presented the lowest total environmental impact and CO2 emissions

in comparison with methane steam reforming coupled with CO2 capture

and methane thermal cracking technologies. Accordingly, different metal-

based catalysts have been widely used to decrease the high temperature

Simultaneous Production and Separation of H2 and C2 Hydrocarbons via Steam and Methane Partial Oxidation

83

required for methane decomposition [7-10]. These metal catalysts are

generally deactivated by deposited carbon during the course of the

reaction, thereby requiring frequent regeneration treatments for

recovering initial activity. This technical barrier has motivated researchers

to develop novel reactor designs and specifically membrane reactors [11-

13]. In this sense, Karagiannakis et al. reported the electrochemical double

chamber solid electrolyte membrane reactor (SEMR) using a proton

conducting ceramic membrane, SrCe0.95Yb0.05O3d (SCYb) [11]. In this

approach, the hydrogen produced by the catalytic hydrocarbon

decomposition is electrochemically transported through the proton

conducting membrane. In a similar work, the experimental verification

and evaluation of a barium cerate mixed conducting membrane perovskite

(BaCe0.9Y0.1O3−δ, BCYO) containing Pd catalyst was provided, leading to

high purity hydrogen and carbon production by TCD [14]. In addition, the

SEMRs have already been used in other related hydrogen production

processes, e.g., via steam reforming or partial oxidation reactions [15] and

have been described in detail by means of excellent reviews [16-19]. On the

other hand, the electrochemical oxidation of carbon in SEMRs (as Solid

Oxide Fuel Cells, SOFC) has been previously reported in literature [20-22].

In this chapter, a novel approach based on the ability of SEMRs to both

produce carbon and H2 and electrochemically oxidize the carbon deposited

over an electrode is proposed. It consists of a solid electrolyte membrane

reactor that allows to electrochemically regenerate a Pt catalyst from the

carbon deposition in the TCD reaction. Hence, a Pt-YSZporous/YSZ/Pt (YSZ =

Yttria-Stabilized Zirconia) double chamber cell operating under two

reaction regimes have been developed. In the first regime, under open

circuit conditions, hydrogen and carbon is produced on the catalytically

Chapter 2

84

active Pt/YSZ porous film via TCD. In the second regime, under close

circuit conditions, steam is electrolyzed at the Pt cathode of the cell

(reaction 2.2) thereby simultaneously generating O2- ions that are

electrochemically pumped to the Pt/YSZ porous catalyst film (anode)

assisting in the removal of the previously deposited carbon (reaction 2.3):

H2O + 2e- H2 + O2- (2.2)

C (s) + O2- CO2 (g) (2.3)

This way, the second reaction regime allows to electrochemically

regenerate the active Pt/YSZ porous film by a carbon assisted steam

electrolysis process leading to the further production of hydrogen. The use

of carbon containing molecules such as CH4 [23], CO [24] or Carbon [25] as

depolarizating agents is of great interest for the electrolytic production of

H2, since it allows to strongly decrease the required electrical power input

for the process. Consequently, the proposed double chamber configuration

allows the in-situ valorization of the produced carbon as a depolarizating

agent during the steam electrolysis process.

2.2. Experimental


The reaction experiments were carried out in the experimental setup

described in Chapter 1. The main difference respect to the previous

chapter is the use of a double chamber solid electrolyte membrane reactor.

The reaction gases (Praxair, Inc.) were certified standards of 10% CH4/N2,

and N2 (99.999% purity) was used as the carrier gas. The gas flow was

controlled by a set of calibrated mass flow-meters (Brooks 5850 E and 5850

S) while water was introduced into the inner chamber feed stream, as

previously explained, by flowing N2 through a saturator. All lines placed


85

downstream from the saturator were heated above 100 ºC to prevent

condensation. The inner Pt working electrode (W) was exposed to H2O/N2

mixture (3 %), whereas the outer Pt/YSZ porous catalyst film was exposed

to a CH4/N2 mixture (1%). The overall flow rate was 100 ml.min-1 in both

reaction chambers for all the performed experiments. Both reactions

atmospheres were completely isolated and no gas permeation occurred

between the two chambers of the electrochemical cell. The contributions of

both, the homogeneous reaction (activity without electrochemical catalyst)

and the catalytic rate on the current collector, were found to be negligible

under the operating conditions used. All the catalytic experiments were

carried out at atmospheric pressure and at reaction temperatures between

700-800 ºC. Hydrogen produced in the inner chamber was calculated on

the basis of the Faraday’s law from the applied current [15]. Reactant and

product gases of the outer chamber were analysed with a micro gas-

chromatograph (Varian CP-4900). All the products reaction rates were

normalized per catalyst-electrode area (2.01 cm2), thus being expressed in

units of mol s-1 cm-2. The main detectable products during the TCD

experiments in the outer side stream were H2 and CO (Open Circuit

Conditions) whereas during the regeneration step CO2, CO, C2H6 and

C2H4 were quantified in the outlet stream (Close Circuit Conditions).


The solid electrolyte cell consisted of an Yttria-Stabilized Zirconia (YSZ)

tube closed at one end, with 15 cm length, 1.8 cm internal diameter, and

1.5 mm thickness (supplied by CERECO). On both faces of the closed side

tube (Figure 2.1), two kind of Pt catalyst were prepared.

Chapter 2

86

H2O/N2

inner chamberPure H2

inner chamber

Fritted quartz

Double chamber

quartz reactor

Platinum deposited

in dense YSZ

Counter electrode

Platinum impregnated

in porous YSZ

Solid electrolyte

tube(YSZ)

Working electrode

H2, CO, CO2,

C2H4, C2H6

outer chamber

CH4/N2

outer chamber

Thermocouple

Au wires

Figure 2.1. Scheme of the double-chamber solid electrolyte cell reactor.

Firstly, in the inner side of the tube a continuous Pt catalyst film was

prepared on the dense YSZ by application of a thin coating of Pt paste

(METALOR), followed by two calcination steps, at 300 ºC (2 h) and 850 ºC

(2 h). The final Pt loading was around 5 mg Pt/cm2. A Pt/YSZ porous

catalyst film based on Pt nanoparticles supported on YSZ was furnished on

the outer side of the YSZ tube. For that purpose, a porous YSZ interlayer

was firstly deposited as follows. YSZ powder was mixed with an organic

binder (Decoflux, Zschwimmer and Schwartz) in a 1:1 (wt./wt.) ratio and

then spin-coated on the outer face of the tube. The assembly was dried at

100 °C for 1 h in an oven and then calcined at 850 °C for 6 h in order to

ensure good adherence. Then, the Pt active catalytic particles were

deposited on the porous YSZ interlayer. The Pt particles were prepared by

an impregnation technique consisting of successive steps of deposition and


87

thermal decomposition of the 2-propanol solution of 0.1 M H2PtCl6.

Initially, 10 μl of precursor solution were deposited on the YSZ substrate

using a plastic circular mask in order to obtain a 2.01 cm2 geometric area

of the catalytic film. Then, evaporation of the solvent took place at 70 °C

for 10 min, followed by drying of the sample at 120 °C overnight and then

calcination at 850 °C for 2 h. Several successive steps of this deposition

procedure followed by drying and heating were repeated until a final metal

loading of 0.6 mg Pt was obtained. It led to a highly dispersed Pt/YSZ

electrode, with a poor electrical conductivity. Then, a catalytically inert Ag

current collector was added to the Pt/YSZ porous catalyst film for

polarization purposes. Hence, the electrical contact of the Pt electrode and

the Au current collector was carried out by gold wires, which were in turn

connected to a potentiostat-galvanostat Voltalab 21 (Radiometer

Analytical). The prepared solid electrolyte cell was placed on a quartz tube

with appropriate feed-through for both reaction sides as shown in Figure

2.1. Before the catalytic activity measurements, the Pt porous catalyst film

was reduced under H2 stream at 450 ºC for 1 h.


The Pt porous catalyst film was characterized by X-ray diffraction

(XRD) with a Philips PW 1710 instrument using Ni-filtered Cu Kα

radiation. The diffractograms were compared with the JCPDS-ICDD

references. The Pt porous catalyst film was also characterized via scanning

electron microscopy (SEM) with a BSE signal using a JEOL 6490 LV

microscope. Linear voltammetry measurements were also performed with

the potentiostat-galvanostat Voltalab 21 under different reaction

atmospheres and different Pt catalytic states (regenerated and

deactivated) and were recorded at a sweep rate of 100 mV/s.

Chapter 2

88

dense YSZ

pellet

porous

Pt/YSZ

catalyst

layer


Figure 2.2 shows the cross section SEM analysis of the Pt-YSZ porous

catalyst film taken with BSE signal. The micrograph shows a porous layer

thickness of around 60 μm. Higher resolution images revealed good contact

between the porous layer and the dense electrolyte. No cracks or

delimitations of the porous layer were observed after preparation. On the

other hand, it is also interesting to note that the Pt impregnated particles

were dispersed on the YSZ porous layer as can be observed by the presence

of lighter dots along the cross section micrograph.

Figure 2.2. Cross-section SEM micrographs of the porous Pt-YSZ catalyst-

electrode supported over the YSZ solid electrolyte.

XRD analysis of the Pt-YSZ porous catalyst after reduction at 450 ºC is

shown in Figure 2.3. The peak assignments based on JCPDS standards

were consistent with a Pt face centered cubic (FCC) structure and the YSZ


89

20 30 40 50 60 70 80 90

YS

Z (

4 2

0)

YS

Z (

4 0

0)

YS

Z (

2 2

2)

Pt

(2 2

2)

Pt

(3 1

1)

Pt

(2 2

0)

Pt

(2 0

0)

Pt

(1 1

1)

YS

Z (

3 3

1)Y

SZ

(2

2 0

)

YS

Z (

2 0

0)

Inte

nsi

ty /

a.u

.

2º

YS

Z (

1 1

1)

phase corresponding to the porous layer [26]. No patterns of platinum

oxides appeared in XRD analysis showing that all Pt particles were on its

reduced state after H2 reduction. According to previous studies [27], Pt

mean particle size can be estimated by using the Scherrer’s equation [28]

corresponding to the (1 1 1) peak resulting in a Pt crystal size of 32 nm.

Remarkably, this value is considerably lower than that typically obtained

by direct impregnation of the Pt precursor solution on dense solid

electrolytes (i.e., around 60 nm) [27], thereby demonstrating that the

addition of a porous YSZ interlayer over the dense solid electrolyte

strongly increased the dispersion of the Pt particles, positively affecting

its catalytic activity.

Figure 2.3. XRD pattern of the fresh porous Pt-YSZ catalyst electrode.

Figure 2.4 represents the rate of the different obtained products

reaction rates (e.g., H2 CO2, CO, C2H4 and C2H6) with the time on stream

under open circuit conditions (O.C.C) and current imposition of 50 mA

Chapter 2

90

0

1

2

3

0

1

2

3

0 40 80 120 160 200 240 280

0.0

0.5

1.0

1.5

50 mA50 mA O.C.CO.C.CO.C.C

(r

H2

ou

ter

/ m

ol·

s-1·

cm-2)

x1

08

50 mAO.C.C

0

6

12

(r

H2

inn

er

/ m

ol·

s-1·c

m-2)

x1

08

(r C

Oou

ter

/ m

ol·

s-1·c

m-2)x

10

8

(rC

O2

ou

ter

/ m

ol·

s-1·c

m-2)

x1

08

0.0

0.6

1.2

(rC

2H

4

ou

ter

/ m

ol·

s-1·c

m-2)

x1

010

Time / min

(rC

2H

6

ou

ter

/ m

ol·

s-1·c

m-2)

x1

09

0.0

0.3

0.6

a)

b)

c)

during the regeneration steps. The duration of each reaction regime was

40 min.

Figure 2.4. Influence of the applied current (from O.C.C to 50 mA) on the

dynamic value of the inner (pure H2) and the outer chamber products (H2, C2s and

COx) obtained in the double-chamber solid electrolyte membrane reactor.

Conditions: outer chamber CH4 = 1 %, inner chamber H2O = 3 %, N2 balance in

both cases, temperature T=750 ºC.


91

Under O.C.C, TCD-derived hydrogen was produced at the outer side of

the solid electrolyte cell over porous Pt-YSZ catalyst. This result is in

agreement with previous works that have demonstrated the suitable

catalytic performance of Pt particles on the TCD reaction under similar

conditions [7]. During the O.C.C. regime, a strong deactivation of the

catalytic activity of the system caused by the deposition of Carbon on the

Pt active sites, which progressively decrease the activity of the catalyst

along the 40 min of duration of the experiment, was observed. On the other

hand, it can also be observed certain amount of CO production under open

circuit conditions during the TCD step. This observation could be mainly

attributed to the reaction between part of the carbon deposits and the

lattice oxygen of the YSZ porous catalyst film due to its oxygen storage

capacity and ionic conductivity at the temperature conditions of the

present study. The production of syngas in the absence of oxygen via TCD

over Pt supported on different oxygen storage materials such as CeO2 or

Gd-doped CeO2 has been recently reported [29]. In addition, the syngas

produced under open circuit conditions (H2/CO ≈ 2) suits with the

conditions required for liquid fuel production based on the Fischer–

Tropsch synthesis [30]. The possibility of syngas generation in the absence

of oxygen by methane decomposition offers an interesting route to decrease

reactor size and costs because methane is the only reactant in the gas

phase. Consequently, it could be considered a new application of ionically

conducting ceramics as active catalyst supports [31]. In the second reaction

regime at t= 40 min, a constant current of 50 mA was applied for another

40 min under the same reaction atmosphere. During this current

imposition step, CO2, CO, C2H4 and C2H6 were detected in the outer side of

the solid electrolyte cell. Furthermore, pure hydrogen was simultaneously

Chapter 2

92

obtained at the inner side of the solid electrolyte cell as a result of the

steam electrolysis process over the Pt cathode.

The scheme shown in Figure 2.5 summarizes the main processes

occurring under the different reaction regimes.

Figure 2.5. Schematic representation of the main processes involved during

the different operation regimes.

The YSZ solid electrolyte could be considered as a pure O2- conductor

[31]. Thus, the application of positive currents (being the outer Pt porous

catalyst the working electrode of the cell) led to the steam electrolysis

process in the Pt inner electrode with the corresponding production of pure

gaseous H2, and O2- ions (reaction 2) [15]. The H2 production rate in the

inner chamber was calculated on the basis of the Faraday’s law by I/2 F

and normalized per catalyst-electrode area (2.01 cm2), where I is the


93

applied current (50 mA), and F is the Faraday’s constant (see Figure 2.4a).

Simultaneously, the electrochemically supplied O2- ions to the outer Pt-

YSZ porous catalyst led to the formation of CO2, CO and C2s products (e.g.,

C2H4 and C2H6). The formation of these products was mainly attributed to

the removal of the previous deposited carbon along with the

electrocatalytic reaction of CH4 with the O2- ions, according to the reaction

scheme shown on Figure 2.5b. However, although the electrochemical

nature of these reactions will be explained later, the O2 evolution reaction,

as well as the catalytic methane oxidation, oxidative coupling and steam

reforming catalytic processes (see Figure 2.5b) is worth to consider. The

production of H2 and carbon derived products might be the sum of the

catalytic and electro-catalytic processes. On the other hand, it is evident

that the applied current allows to completely regenerate the Pt/YSZ porous

catalyst film from the previous deposited carbon since the catalytic activity

for the thermal decomposition of methane was again recovered in the next

OCC reaction step (Figure 2.4). A reproducible behavior in the hydrogen

production rate was obtained along the different cycles, being the slight

differences observed to be only attributed to the µGC injection delay time

after the current imposition (due to the fast nature of the deactivation

process). The observed deactivation process can be defined by the

deactivation index of the catalyst based on the H2 production. It refers to

the ratio between final and initial hydrogen production under O.C.C.

regime, (rH2f,O.C.C./ rH2o,O.C.C.)x100. In this chapter, the average deactivation

index of the catalyst based on the H2 production was around 55 %. Finally,

it is also interesting to note the formation of C2s products: ethane and

ethylene due to the activity of the Pt/YSZ porous catalyst in the methane

oxidative coupling reaction shown in Figure 2.5b. It is well known that

methane oxidative coupling reaction for C2s production is catalyzed by

Chapter 2

94

oxides materials and hence very likely most of the activity comes from the

porous YSZ interlayer [32]. However, as it can be observed in Figures 2.4 b

and c, due to the high catalytic activity of Pt in methane combustion

processes, the production of CO and CO2 (CO + CO2 yield ≈ 6.6 %) under

constant current imposition (50 mA, which corresponds to an O2- flux of

12.8 x 10-8 mol s-1 cm-2), is more than a magnitude order higher than that

of C2 hydrocarbons (C2s yield ≈ 0.44 %) coming from the oxidative coupling

reaction [33, 34].

In order to support the previous explained mechanism, a transient gas

atmosphere-potential experiment was carried (Figure 2.6). The experiment

was carried out at a constant temperature of 700 ºC. The outer porous

catalyst was initially kept under Open Circuit conditions (O.C.C) and

exposed to CH4 for 90 min. Subsequently, the outer chamber was purged

with N2 for 190 min in order to ensure the CH4 desorption from the

catalyst and to remove all the gaseous CH4 from the reactor. Finally, under

the same N2 atmosphere, a current of 50 mA was applied for 320 min. In

good agreement with the previous experiments, syngas was produced

during the initial TCD step. A strong deactivation was again observed due

to the deposition of Carbon on the Pt active sites. However, the final

application of 50 mA led again to the occurrence of CO and CO2 peaks as a

result of the removal of the previous deposited carbon adsorbed on the Pt

active sites. The absence of chemisorbed CH4 and CH4 coming from the gas

phase (after the purge state) evidenced the previous explained reaction

scheme (Figure 2.5). This experiment clearly demonstrated the ability of

the O2- ions to remove the previous deposited carbon from the Pt/YSZ

porous catalyst layer, leading to the electrochemical regeneration of the

catalyst.


95

0

10

20

30

40

50

60

0

2

4

6

0 100 200 300 400 500 600

0.0

0.5

1.0

1.5

2.0

(100 % N2) (100 % N2)

0% CH4

0% CH4

O.C.C

I /

mA

I = 50 mA

1% CH4

rCO

rCO

2

(rC

O, r C

O2

ou

ter

/ m

ol·

s-1·c

m-2

) x

10

8

(rH

2 o

ute

r /m

ol·

s-1·c

m-2)x

10

8

Time / min

Figure 2.6. Polarization programmed oxidation experiment after the

deactivation of the porous Pt-YSZ catalyst. Conditions: Temperature = 700 ºC. a)

Deactivation stage (O.C.C.): Outer chamber CH4 = 1 %, Inner chamber H2O = 3 %,

N2 balance in both cases. b) Purge (O.C.C.): Outer chamber 100 % N2, Inner

chamber H2O = 3 % (N2 balance). c) Polarization: Current = 50 mA, Outer

chamber 100 % N2, Inner chamber H2O = 3 % (N2 balance).

Chapter 2

96

0 20 40 60 80 100 120

0.0

0.5

1.0

1.5

2.0

2.5

UW

C (V

)

Current / mA

Symbol Outer chamber Deposited carbon

N2 No

1 vol.% CH4/N2 No

N2 Yes

1 vol.% CH4/N2 Yes

0 20 40 60 80 100 120

0.0

0.5

1.0

1.5

2.0

2.5

U

U

U

U

UW

C (V

)

Current / mA

0 20 40 60 80 100 120

0.0

0.5

1.0

1.5

2.0

2.5

U

U

U

U

UW

C (V

)

Current / mA

0 20 40 60 80 100 120

0.0

0.5

1.0

1.5

2.0

2.5

U

U

U

U

UW

C (V

)

Current / mA

0 20 40 60 80 100 120

0.0

0.5

1.0

1.5

2.0

2.5

U

U

U

U

UW

C (V

)

Current / mA

The electrochemical nature of the previous mentioned reactions can be

further supported by the characterization of the electrochemical cell by

linear voltammetry under different gas exposed atmospheres of the Pt-YSZ

porous catalyst and different Pt-YSZ state (activated or deactivated).

Figure 2.7 shows the different current-potential curves obtained at 700 ºC.

All the lineal voltammograms were performed under 3 % H2O/N2

atmosphere at the inner chamber of the electrochemical cell while the

outer chamber was exposed to two different reaction atmospheres: N2 or 1

% CH4/N2.

Figure 2.7. Influence of the reaction atmosphere and the deactivation state on

the Current-Potential curves obtained during a linear voltammetry. Temperature

700 ºC. Sweep rate = 100 mVs-1

In addition, as shown in this figure, the Pt porous catalyst film was

initially either clean and regenerate from Carbon deposited (by applying


97

50 mA for 30 min, denoted in Figure 2.7 as “Deposited Carbon: No”), or

previously deactivated under TCD conditions under the same conditions

shown in Figure 2.4 for 90 min (denoted in Figure 2.7 as “Deposited

Carbon: Yes”).

The trend of the current-potential curves clearly demonstrated the

depolarization effect of both CH4 and the carbon supported on the Pt-YSZ

porous catalyst layer. Accordingly, the presence of these species allows to

perform the steam electrolysis process with lower energy input (lower

potential is required for the same current). This fact confirms the

electrochemical nature of the previous mentioned reactions (as shown in

Figure 2.5), showing that these reactions involve the O2- ions (versus O2

coming from the gas phase) as a result of the O2- evolution reaction (2 O2-

O2 + 4 e-). Moreover, it is worth to note that the depolarization effect of

carbon deposited on the Pt-YSZ porous catalyst layer is higher than that of

chemisorbed CH4, although the highest depolarization effect took place

when both of them were present on the catalyst surface. The use of carbon

containing molecules such as: CH4 [23], CO [24] or Carbon [25] as

depolarizating agents is of great interest for the electrolytic production of

H2 since it allows to strongly decrease the required electrical power input.

On the other hand, it is worth to note that the enhanced electrochemical

properties of the system could be attributed to an improved electrical

conductivity of the anode due to the presence of carbon. However, further

experiments should be carried out to clarify the nature of the improved

electrochemical properties of the system. Anyway, this set of experiments

clearly demonstrated that the novel mode of operation proposed during the

regeneration stage of the Pt-YSZ porous catalyst (previously discussed on

Chapter 2

98

0.0

0.4

0.8

1.2

1.6

2.0

0.00

0.15

0.30

0.45

0.60

0

1

2

3

4

5

0.0 2.5 5.0 7.5 10.0 12.5 15.0 17.5 20.0 22.5

0

5

10

15

20

25

30

35

(rH

2 ou

ter/

mol

·s-1

·cm

-2)

x108

(rC

O2ou

ter/

mol

·s-1

·cm

-2) x

108

(rC

O o

ute

r/m

ol·s

-1·c

m-2

) x1

08

0.0

0.3

0.6

0.9

1.2

1.5

1.8

(r C

2H6ou

ter/

mol

·s-1

·cm

-2)

x1010

(rC

2H4ou

ter/

mol

·s-1

·cm

-2)

x1010

0

2

4

6

8

10

12

d)

c)

b)

Cur

rent

/ m

A

Time / h

a)

-0.85

-0.84

-0.83

-0.82

-0.81

-0.80

-0.79

UW

C (V

)

Figure 2.5b) allows the production of pure H2 by a high efficient C and CH4

assisted steam electrolysis process.

Finally, in order to check the reproducibility and durability of the solid

electrolyte electrochemical cell, a medium term operation experiment of 24

h was carried out at 750 ºC (Figure 2.8).

Figure 2.8. Reproducibility experiment under step changes in the applied current

(from O.C.C. to 30 mA). Conditions: Outer chamber CH4 = 1 %, Inner chamber

H2O = 3 %, N2 balance in both cases, Temperature = 750 ºC.


99

A complete number of 18 cycles, which included the Open Circuit TCD

step and the steam electrolysis for the regeneration of the Pt-YSZ catalyst,

were carried out. A reproducible behavior was observed in both catalytic-

electrocatalytic activity and obtained potential curves during open and

close circuit conditions. It demonstrates the stability of the inner Pt

electrode (exposed to the steam electrolysis reaction atmosphere) and the

Pt-YSZ porous catalyst layer (exposed to the CH4 atmosphere) as well as

the YSZ solid electrolyte for the proposed mode of operation under the

explored reaction conditions. Finally, an average potential under open

circuit conditions of -0.82 V was observed in Figure 2.8d. This potential

difference could allow operating the electrochemical cell as a Solid Oxide

Fuel Cell, leading therefore to the co-generation of energy and reaction

products. Hence, further studies could be performed in order to develop the

multiple applications given by this system.

2.4 Conclusions

The system proposed in this chapter showed two different techniques

for hydrogen production: the catalytic methane decomposition and the

electro-catalytic steam electrolysis process, which can be successfully

coupled in a double solid electrolyte membrane reactor.

The study carried out in this chapter allows to regenerate a catalyst

from carbon deposition by an assisted steam electrolysis process, which led

to the production of produce hydrogen with a lower energy requirement

demand. Therefore, this configuration and mode of operation allowed the

in-situ valorization of the produced carbon as a depolarizating agent in the

steam electrolysis process. Furthermore, the regeneration step yielded C2s

hydrocarbons by the oxidative coupling of methane reaction on the Pt-YSZ

porous catalyst.

Chapter 2

100

Finally, the performance and durability of the system was also verified

for long operation times in view of the possible practical application of this

novel reactor configuration.

2.5. References

[1] G.J. Stiegel, M. Ramezan, International Journal of Coal Geology, 65 (2006) 173-

190.

[2] M. Conte, A. Iacobazzi, M. Ronchetti, R. Vellone, Journal of Power Sources, 100

(2001) 171-187.

[3] I. Dincer, International Journal of Hydrogen Energy, 27 (2002) 265-285.

[4] N.Z. Muradov, International Journal of Hydrogen Energy, 18 (1993) 211-215.

[5] Q. Wang, X. Chen, A.N. Jha, H. Rogers, Renewable and Sustainable Energy

Reviews, 30 (2014) 1-28.

[6] J. Dufour, D.P. Serrano, J.L. Gálvez, A. González, E. Soria, J.L.G. Fierro,

International Journal of Hydrogen Energy, 37 (2012) 1173-1183.

[7] H.F. Abbas, W.M.A. Wan Daud, International Journal of Hydrogen Energy, 35

(2010) 1160-1190.

[8] S. Sushil, A.M. Alabdulrahman, M. Balakrishnan, V.S. Batra, R.A. Blackley, J.

Clapp, J.S.J. Hargreaves, A. Monaghan, I.D. Pulford, J.L. Rico, W. Zhou, Journal of

Hazardous Materials, 180 (2010) 409-418.

[9] T.V. Reshetenko, L.B. Avdeeva, Z.R. Ismagilov, A.L. Chuvilin, V.B. Fenelonov,

Catalysis Today, 102-103 (2005) 115-120.

[10] I. González, J.C. De Jesus, C.U. de Navarro, M. García, Catalysis Today, 149

(2010) 352-357.

[11] G. Karagiannakis, C. Kokkofitis, S. Zisekas, M. Stoukides, Catalysis Today, 104

(2005) 219-224.

[12] G. Karagiannakis, S. Zisekas, C. Kokkofitis, M. Stoukides, Applied Catalysis A:

General, 301 (2006) 265-271.


101

[13] X. Dong, Z. Liu, Y. He, W. Jin, N. Xu, Journal of Membrane Science, 331 (2009)

109-116.

[14] K.-S. Han, J.-H. Kim, H.-K. Kim, K.-T. Hwang, International Journal of

Hydrogen Energy, 38 (2013) 16133-16139.

[15] A. Caravaca, A. de Lucas-Consuegra, V.J. Ferreira, J.L. Figueiredo, J.L. Faria,

J.L. Valverde, F. Dorado, Applied Catalysis B: Environmental, 142–143 (2013) 298-

306.

[16] I. Garagounis, V. Kyriakou, C. Anagnostou, V. Bourganis, I. Papachristou, M.

Stoukides, Industrial and Engineering Chemistry Research, 50 (2011) 431-472.

[17] M. Stoukides, Research on Chemical Intermediates, 32 (2006) 187-204.

[18] M. Stoukides, Catalysis Reviews - Science and Engineering, 42 (2000) 1-70.

[19] C. Kokkofitis, M. Ouzounidou, A. Skodra, M. Stoukides, Solid State Ionics, 178

(2007) 507-513.

[20] T. Horita, N. Sakai, T. Kawada, H. Yokokawa, M. Dokiya, Journal of the

Electrochemical Society, 142 (1995) 2621-2624.

[21] S. Giddey, A. Kulkarni, C. Munnings, S.P.S. Badwal, Energy, 68 (2014) 538-547.

[22] M. Dudek, M. Sitarz, P. Tomczyk, Journal of Solid State Electrochemistry,

(2014).

[23] J. Martinez-Frias, A.Q. Pham, S.M. Aceves, International Journal of Hydrogen

Energy, 28 (2003) 483-490.

[24] W. Wang, J.M. Vohs, R.J. Gorte, Topics in Catalysis, 46 (2007) 380-385.

[25] A.C. Lee, R.E. Mitchell, T.M. Gür, Solid State Ionics, 192 (2011) 607-610.

[26] C. Jiménez-Borja, A. de Lucas-Consuegra, F. Sapountzi, F. Dorado, A.

Katsaounis, J.L. Valverde, Chemical Physics Letters, 519–520 (2012) 89-92.



[28] R.T.K. Baker, Carbon, 27 (1989) 315-323.

[29] M.D. Salazar-Villalpando, International Journal of Hydrogen Energy, 37 (2012)

2121-2128.

Chapter 2

102

[30] O.O. James, B. Chowdhury, M.A. Mesubi, S. Maity, RSC Advances, 2 (2012)

7347-7366.


Consuegra, J.L. Valverde, S. Souentie, C.G. Vayenas, D. Tsiplakides, S. Balomenou,


[32] S. Seimanides, P. Tsiakaras, X.E. Verykios, C.G. Vayenas, Applied Catalysis, 68

(1991) 41-53.

[33] G.L. Semin, V.D. Belyaev, A.K. Demin, V.A. Sobyanin, Applied Catalysis A:

General, 181 (1999) 131-137.

[34] V.A. Sobyanin, V.D. Belyaev, V.V. Gal'vita, Catalysis Today, 42 (1998) 337-

340.

3.1. Introduction

3.2. Methodology

3.3. Process description

3.3.1. Catalytic steam reforming of

ethanol-water process

3.3.2. Electrochemical reforming of

ethanol-water process

3.4. Process simulation and energetic

evaluation

3.5. Conclusions

3.6. References

CHAPTER 3:

Electrochemical Reforming vs. Catalytic

Reforming of Ethanol: A Process Energy

Analysis for Hydrogen Production

INTRODUCTION

Catalytic steam reforming of ethanol

PROCESS DESCRIPTION

RESULTS

Hydrogen can be currently produced by a number of processes such as natural gas or biogas reforming, gasification of coal and biomass, water electrolysis,

photoelectrolysys and biological process. Traditionally, large-scale production of hydrogen is mainly based on the methane reforming process, however, this pathway

has a non-renewable nature as methane is mainly obtained from natural gas. In this sense, there is a growing interest in the search for effective alternatives to

produce hydrogen from renewable sources. In this regard, ethanol is very attractive because of its relatively high hydrogen content, broad availability, non-toxicity,

secure storage and handling. In addition, it can be obtained from the fermentation of biomass

Electrochemical reforming of ethanol

Energy consumption

Hydrogen yield and energy consumption

-Electrical energy consumption and process energy (electrical and

thermal energy) consumption were computed in the electrochemical

reforming process. In this case, the main energy requirement was the

electrical energy one.

- The electrochemical reforming process seemed to be less intensive in

feed stock material and energy consumption for H2 production than the

process based on the catalytic conventional route. In addition, the

calculated energy consumption of the overall electrochemical reforming

process was lower than that required by a traditional water electrolyzer

stack.

-For the catalytic steam reforming process electrical and thermal energy

consumption was considered. The highest energy consumption corresponded

to the stream e-3, which provided the energy required to heat the outlet

stream from the heat exchanger network to the reforming reactor

temperature (T = 800 ºC).

Catalytic steam reforming of ethanol is and endothermic process that requires external heat input and other process of hydrogen purification such as WGS and

COPROX reaction:

C2H5OH + 3H2O 2CO2 + 6H2 (∆H = 173.1 kJ mol-1) A novel process based on ethanol electro-oxidation (electrochemical reforming of ethanol) has recently attracted great interest since it allows the simultaneous

production of pure hydrogen and accomplished its separation in a single step:

C2H5OH + xH2O Cderived products + yH+ + ye- 2H+ + 2e- H2

The aims of this study is to compare the energetic performance of two technologies for renewable H2 production via reforming of ethanol-water mixtures:

electrochemical and catalytic ones. The energetic analysis was carried out by simulating these processes with Aspen HYSYS (Aspen Tech V.7.1). Finally, a

comparative study of the two processes was performed in order to evaluate the yield and energy consumption in the production of hydrogen.

Water

P-01

Ethanol

P-02

C-01

C-03HE-01 HE-02 HE-03 HE-04 HE-05

C-02

C-04 C-05 C-06 C-07 C-08HE-06 H-01

C-10

SR

B-01

C-13 HWGS

C-14 C-15

LWGS

C-16

C-17

COPROX

C-20

C-21

HE-07

C-22

C-23 C-24

H-03

SP-01K-02

K-01C-18

e-1

e-2

e-3

e-4

e-8

e-7

e-5

WATER GAS SHIFT

C-26

C-25Hydrogen

C-09

C-11

C-12

C-27

C-30

C-29

C-28C-32

C-31

H-02

C-19

e-6

P-01

P-02

H-01

Ethanol

Water

C-02

C-01

C-03

C-11

SP-01

C-08

A C

HE-01

e1

e-2

e-3

e-6

e-4

e-5

PEM

CELL

Hydrogen

C-05C-04 C-06

C-07

C-10

P-03C-09

e-1 e-2 e-3 e-4 e-5 e-6 e-7 e-8

0

8

16

60

80

100

120

140

En

ergy

/ k

J m

ol-1 H

2

e-1 e-2 e-3 e-4 e-5 e-6

0.0

0.3

0.6

0.9

1.2

30

60

90

120

150

180

210

En

ergy

/ k

J m

ol-1 H

2

e-1 e-2 e-80.00

0.01

0.02

0.03

0.04

0.05

0.06

En

ergy

/ k

J m

ol-1 H

2

e-1 e-20.00

0.03

0.06

0.09

0.12

0.15

0.18

0.21

En

ergy

/ k

J m

ol-1

H2

0.00

0.01

0.02

0.03

0.04

Catalytic steam reforming Electrochemical reforming

Yie

ld /

kg

H2

kg

-1

C2H

5O

H

0

10

20

30

En

erg

y c

on

sum

pti

on

/ k

W h

kg

-1

H2

Catalytic steam reforming of ethanol

Electrochemical reforming of ethanol

CHAPTER 3. ELECTROCHEMICAL REFORMING VS.

CATALYTIC REFORMING OF ETHANOL: A PROCESS

ENERGY ANALYSIS FOR HYDROGEN PRODUCTION

107

Abstract

n the last years the process of electrochemical reforming of alcohols

performed in low temperature Proton Exchange Membrane (PEM)

reactors have shown to be an interesting way of producing pure

hydrogen in a single reaction step. In addition, this technology allows to

produce hydrogen with lower electrical energy requirements compare to

other related technologies such as water electrolysis. This chapter reports

an energetic analysis of the hydrogen production via catalytic steam and

electrochemical ethanol reforming processes. For both systems, a complete

flow diagram process was proposed and simulated by Aspen HYSYS

according to literature data. Besides hydrogen, other byproducts such as

acetaldehyde (electrochemical reforming) and ethylene and methane

(catalytic reforming) were also considered. The energy requirement of the

different process units was calculated according to the operating

parameters. Material balances revealed electrochemical reforming to

present higher hydrogen yields (0.0436 vs. 0.0304 kg H2 / kg C2H5OH of the

classical catalytic reforming). In addition to its higher simplicity,

simulation results showed a lower energy consumption in the H2 production

by the electrochemical approach (29.2 vs. 32.70 kWh per Kg of H2). These

results demonstrated the interest of the electrochemical reforming of ethanol

to obtain high purity hydrogen in a single reaction/separation step, thereby

representing an interesting alternative to classical catalytic reforming.

I

Chapter 3

108

3.1. Introduction

As previously mentioned in previous chapters of this memory, hydrogen

can be currently produced by a number of processes such as natural gas or

biogas reforming [1], gasification of coal and biomass [2, 3], water

electrolysis [4], photoelectrolysis [5] and biological processes [6].

Traditionally, large-scale production of hydrogen is mainly based on the

methane reforming process. However, this pathway has a non-renewable

nature as methane is mainly obtained from natural gas. Furthermore,

together with hydrogen, other carbon derived products such as carbon

monoxide and carbon dioxide are formed as side products. In this sense,

there is a growing interest in the search for effective alternatives to

produce hydrogen from renewable sources. In this regard, ethanol is very

attractive because of its relatively high hydrogen content, broad

availability, non-toxicity, secure storage and handling. In addition, it can

be obtained from the fermentation of biomass [7].

The catalytic steam reforming of ethanol is an endothermic process

that requires external heat input which could be supplied from external

sources in order to maintain the system at a steady reaction temperature.

C2H5OH + 3H2O 2CO2 + 6H2 (∆H = 173.1 kJ mol-1) (3.1)

Heat can also be supplied externally by the combustion of part of the feed,

by burning combustible off gases or by a combination of both processes.

Additionally, the ethanol-steam mixture is catalytically converted to

carbon monoxide according to the following reaction:

C2H5OH + H2O 2CO + 4H2 (∆H = 298.5 kJ mol-1) (3.2)

Electrochemical reforming vs. Catalytic reforming of ethanol

109

Hydrogen production via catalytic reforming of ethanol involves two

more additional steps aimed at reducing the concentration of CO below the

stringent levels required by hydrogen fuel cells operating downstream: the

water gas shift reaction (WGS, reaction 3.3) and CO preferential oxidation

reaction (COPROX, reaction 3.4 and reaction 3.5). WGS (reaction 3.3) is an

important step in which CO is generally oxidized to CO2 in excess steam:

CO + H2O ↔ CO2 + H2 (∆H = -41 kJ mol-1) (3.3)

In the COPROX process, the following reactions occurs in the gas

phase:

CO + ½ O2 CO2 (∆H = -238 kJ mol-1) (3.4)

H2 + ½ O2 H2O (∆H = -242 kJ mol-1) (3.5)

The final step is the purification of the hydrogen from the gas stream

exiting the COPROX process, which can be accomplished using several

techniques. The most common methods are: pressure-swing-adsorption

(PSA) to separate CO2 followed by the condensation of the remaining H2O,

PSA to separate H2, and membrane separation of H2 [8]. Consequently,

production of high purity hydrogen by catalytic ethanol reforming is a

complicated process including several reaction and separation steps. A

novel process based on ethanol electrooxidation (also called electrochemical

reforming of ethanol) has recently attracted great interest since it allows

the simultaneous production of pure hydrogen and accomplished its

separation in a single step. In addition, the separation in a single-step of

the reaction product from the reaction chamber shifts the reaction

equilibrium to the right and thus, higher yields are achieved. This process

is based on a low temperature Proton Exchange Membrane (PEM) reactor

Chapter 3

110

configuration, consisting on a membrane electrode assembly (MEA) formed

by an Anode/membrane/Cathode, which allows simultaneous production

and separation of hydrogen. Consequently, this technology allows to

produce pure hydrogen in a quick and convenient way. In addition, this

process can be used to store electrical energy via H2 production [9]. Recent

studies have shown very promising results when the electrolysis of water-

alcohols mixtures proceeds. Thus, methanol [10-13], ethanol and bio-

ethanol [9, 14], glycerol [15, 16] and ethylene glycol [17] have been

successfully tested at lab scale. When compared to water electrolysis, the

electrochemical reforming of organics can be carried out at significantly

lower voltages, thus leading to a reduction of electricity consumptions. In

the electrochemical reforming process, an ethanol-water mixture is

supplied to the anode cell. The electro-oxidation of ethanol is carried out by

applying an electrical power, turning into the production of protons on the

anode catalyst-electrode according to the following reaction:

C2H5OH + xH2O Cderived products + yH+ + ye- (3.6)

The produced protons are selectively transported through the PEM

membrane to the cathode compartment, which leads to the production of

hydrogen:

2H+ + 2e- H2 (3.7)

In the previous chapter (Chapter 2) it was studied the simultaneous

production/separation of H2 and C2 hydrocarbons in a double chamber

SOEC configuration. It allowed to obtain pure hydrogen in the inner

chamber by the water electrolysis at the high reaction temperatures

necessary for the oxidative coupling reaction in the outer chamber. Then,

following the topic of the pure hydrogen production, the aims of this


111

chapter is to compare the energetic performance of two technologies for

renewable hydrogen production via reforming of ethanol-water mixtures:

electrochemical and catalytic ones. The performance of both processes was

evaluated by material and energy balances performed using conditions

reported in the literature. The energetic analysis was carried out by

simulating these processes with Aspen HYSYS (Aspen Tech V.7.1).

Finally, a comparative study of the two processes was performed in order

to evaluate the yield and energy consumption in the production of

hydrogen.

3.2. Methodology

The simulation of the catalytic steam reforming and electrochemical

reforming of ethanol-water mixtures processes was performed under

stationary conditions using the flowsheeting simulator Aspen HYSYS

(AspenTech V.7.1). Peng-Robinson equation was used to calculate the

thermodynamic properties of each flow stream. This equation of state is

widely used in reforming processes of ethanol and thus, it was used in this

chapter for comparison purposes [17, 18]. The component list was

restricted to C2H5OH, H2O, H2, CO, C2H4O, CO2, CH4, O2, N2 and C2H4 for

the catalytic steam reforming [21-24] and to C2H5OH, H2O, H2 and C2H4O

for the electrochemical reforming as experimentally confirmed [16]. The

reaction conditions for the catalytic steam reforming of ethanol were taken

from literature data for a Pt/Al2O3 catalyst (1 wt.% metal loading) [19]. In

the case of the electrochemical reforming of ethanol, catalysts based on Pt-

Ru (40 wt.% Pt-20 wt.% Ru) and Pt (20 wt.% Pt), both supported on carbon,

were used. The metal loading was of 1.5 mg·cm-2and 0.5 mg·cm-2 for the Pt-

Ru and Pt catalysts, respectively. As reported in literature, these metals

have been typically used as the anode (Pt-Ru) and cathode (Pt) electrodes

Chapter 3

112

in electrochemical reforming reactors [19]. The operating conditions and

the polarization curves for the process simulation were also taken from

literature [9, 17, 19-22].

The following assumptions were considered:

- Air, water and ethanol were injected into the process at room conditions

(T = 25 ºC and P = 1 atm).

- The composition of atmospheric air was fixed at 21 % O2 and 79 % N2.

- 10 % of excess air was used in the combustion furnace to heat the

reforming reactor unit.

- 10 % of excess air was used in the COPROX reactor unit for CO

oxidation.

- The adiabatic efficiency of pumps and compressor was considered to be

80 %.

- An average pressure drop of 9.807 kPa was considered for the heat

exchangers, heaters and splitter.

- The reforming reactor and PEM reactor units were simulated as a

conversion reactor using reported data.

- WGS and COPROX reactor were simulated as equilibrium reactors.

3.3. Process description

3.3.1. Catalytic steam reforming of ethanol-water process

Figure 3.1 shows the flow diagram of the catalytic steam reforming

process. The hydrogen production process began by pumping liquid water

(stream Water) and liquid ethanol (stream Ethanol). Both feedstocks were

separately fed at reference environmental conditions, i.e. 25 ºC and 1 atm

with a water/ethanol molar ratio= 6/1.


113

Wa

ter

P-0

1

Eth

an

ol

P-0

2

C-0

1

C-0

3H

E-0

1H

E-0

2H

E-0

3H

E-0

4H

E-0

5

C-0

2

C-0

4C

-05

C-0

6C

-07

C-0

8H

E-0

6H

-01

C-1

0

SR

B-0

1

C-1

3H

WG

S

C-1

4C

-15

LW

GS

C-1

6

C-1

7

CO

PR

OX

C-2

0

C-2

1

HE

-07

C-2

2

C-2

3C

-24

H-0

3

SP

-01

K-0

2

K-0

1C

-18

e-1

e-2

e-3

e-4

e-8

e-7

e-5

WA

TE

R G

AS

SH

IFT

C-2

6

C-2

5H

yd

rogen

C-0

9

C-1

1

C-1

2

C-2

7

C-3

0

C-2

9

C-2

8C

-32

C-3

1

H-0

2

C-1

9

e-6

Fig

ure

3.1

. F

low

dia

gra

m o

f th

e c

ata

lyti

c ste

am

refo

rmin

g p

roce

ss o

f eth

an

ol.

Chapter 3

114

The streams were later pumped by a centrifugal adiabatic pump to a

pressure of 2.5 atm and subsequently mixed (stream C-03). This pressure

was high enough to deal with the successive pressure drops originated in

downstream units. The water/ethanol mixture was then heated (stream C-

10) before entering the reformer reactor. The required heat was provided

by a heat exchanger network (from HE-01 to HE-06). The final

temperature of this stream (T = 800 ºC) was finally reached with heater H-

01 [22]. The hydrogen production in the ethanol reformer process was

carried out at T = 800 ºC and P = 1.8 atm with a molar flow of 736.6 mol h-1

for water and 122.7 mol h-1 for ethanol. The global conversion of ethanol in

this process was fixed at 60 % [19]. This conversion was the sum of all the

reactions taking place in the catalytic reforming reactor. The reformer

reactor (SR) was equipped with a furnace which provides the required heat

to perform the endothermic reforming reaction. 30 % of the outlet stream

coming from the reformer reactor was subsequently fed to the combustor

(B-01), which was modeled as an equilibrium reactor. This value was

optimized according to the energy consumption through different

simulations (not shown here). The minimum amount of supplied energy for

H2 production was 32.70 kWh per kg of H2. Then, stream C-27 was burnt

with 10 % of excess of air (Stream C-28) [23]. Next, the stream coming

from the reformer, which contained mainly H2 and CO, was conducted to

the water-gas shift (WGS) reactor, where a large amount of CO is

converted to CO2 and H2 (reaction 3.3). Water gas shift reaction is

considered to be reversible. At low temperatures, the reaction equilibrium

shifts to the right and favors the formation of H2 and CO2. On the other

hand, at high temperatures, the equilibrium shifts to the left, limiting the

complete conversion of CO [24]. As usual, this reaction was carried out in

two steps. This way, a high temperature shift reactor (HTS), which


115

operates at 400 ºC, and a low temperature shift reactor (LTS), which

operates at 250 ºC, were considered [20]. The remaining CO coming from

the latter reactor was oxidized with 10 % excess of air [20] (reaction 3.4) in

a COPROX reactor unit at 170 ºC. As expected, hydrogen was also oxidized

(reaction 3.5). The lack of inlet air in the COPROX reactor (operating at

170 ºC [21]) produce an incomplete conversion of CO, i.e. the CO

concentration did not reach the specified value (20 ppm). On the contrary,

an excess of air can produce an excessive oxidation of hydrogen, leading to

a decrease of its yield [25, 26]. In addition, an excess of air increases the

electric energy consumption in the compressor [23]. The temperature of the

reactor clearly influenced the performance of both reactions. A strong

temperature increase due to the high exothermicity of both reactions

involves a loss of selectivity towards CO oxidation since its activation

energy is lower than that of H2 oxidation.

In addition, low reaction temperatures reduce the possibility of

formation of NOx (produced from the reaction of nitrogen and oxygen gases

in the air during combustion).

Finally, the hydrogen was separated by using a membrane process (SP-

01) at 300 ºC [8], which was modeled as a black box (split unit) with a

prescribed membrane effectiveness releasing a pure hydrogen stream

(Stream C-30). The membrane effectiveness was defined as the molar

percentage of the inlet hydrogen that was transferred to the product

stream [8]. In practice, a real membrane should be capable of producing

hydrogen with purity higher than 99% (Stream Hydrogen). The different

material and process energy are shown in Table 3.1.

Chapter 3

116

Table 3.1. Description of the material and process energy for the catalytic

steam reforming of ethanol-water mixtures.

Material

streams Description

Water Water input stream (room conditions)

Ethanol Ethanol input stream (room conditions)

C-01 Water stream pressurized at 2.5 atm

C-02 Ethanol stream pressurized at 2.5 atm

C-03 – C-08 Water/ethanol mixture input at heat exchanger network (HE-01 to HE-06)

C-09 Pre-heated water/ethanol mixture

C-10 Feed stream to the reforming reactor (SR)

C-11 Output stream to the reforming reactor (SR)

C-12 70 % outlet stream from the reformer reactor to heat exchanger (HE-02)

C-13 Feed stream to the high temperature water gas shift reactor (HWGS)

C-14 Output stream to the high temperature water gas shift reactor (HWGS)

C-15 Feed stream to the low temperature water gas shift reactor (LWGS)

C-16 Output stream to the low temperature water gas shift reactor (LWGS)

C-17 Feed stream to the CO preferential oxidation reactor (COPROX)

C-18 Air input stream (room conditions)

C-19 Air stream pressurized at 1.5 atm

C-20 Air input steam to the CO preferential oxidation reactor (COPROX)

C-21 Output stream from CO preferential oxidation reactor (COPROX)

C-22 Output stream from heat exchanger (HE-01) to heat exchanger (HE-07)

C-23 Output stream from heat exchanger (HE-07) to heater (H-03)

C-24 Input stream from the membrane process (SP-01)

C-25 Output stream from the membrane process (SP-01) to heat exchanger (HE-07)

C-26 Cool output stream from heat exchanger (HE-07)

C-27 30 % outlet stream from the reforming reactor to burner (B-01)

C-28 Air input stream (room conditions)

C-29 Air stream pressurized at 1.8

C-30 Output stream from the burner (B-01)

C-31 Cool output stream from heat exchanger (HE-06)

C-32 Hydrogen stream input at heat exchanger (HE-04)

Hydrogen Pure hydrogen outlet

Process

energy Description

e-01 Pump (P-01) electrical stream

e-02 Pump (P-02) electrical stream

e-03 Energy required to heat stream C-09

e-04 Reforming reactor energy stream

e-05 Compressor (K-01) electrical stream

e-06 CO preferential oxidation reactor (COPROX) energy stream


e-08 Membrane process (SP-01) energy stream


117

P-0

1

P-0

2

H-0

1

Eth

an

ol

Wa

ter

C-0

2

C-0

1

C-0

3

C-1

1

SP

-01

C-0

8

A C

HE

-01

e1

e-2

e-3

e-6

e-4

e-5

PE

M

CE

LLH

yd

rogen

C-0

5C

-04

C-0

6

C-0

7

C-1

0 P-0

3C

-09

3.3.2. Electrochemical reforming of ethanol-water process

For the hydrogen production via electrochemical reforming of ethanol,

an integrated process was proposed according to a previous work focused

on the electrochemical reforming of ethylene glycol [17] (Figure 3.2).

Fig

ure

3.2

. . F

low

dia

gra

m o

f th

e e

lect

roch

em

ical

refo

rmin

g p

roce

ss o

f eth

an

ol.

Chapter 3

118

Aspen HYSYS cannot directly simulate an electrochemical reforming

PEM reactor unit but it could be modeled as a sequence of a conversion

reactor and a split separation unit, where the produced hydrogen was

completely separated from the reaction mixture. Table 3.2 summarizes the

description of the different material and process energy included in the

process.

Table 3.2. Description of the material and process energy for the

electrochemical reforming of ethanol-water mixtures.

The operating conditions of this process were optimized in a previous

work devoted to the electrochemical reforming of ethylene glycol [17].

Water and ethanol were mixed at 2 atm to form a 1 ml/min flow with a

concentration 6 M ethanol. Then, the mixture was preheated by a heat

exchanger (HE-01) using the sub-product stream C-10. This stream

Material streams Description

Ethanol Ethanol input (room conditions)

Water Water input (room conditions)

C-01 Ethanol stream pressurized at 2 atm

C-02 Water stream pressurized at 2 atm

C-03 Water/ethanol mixture input at heat exchanger (HE-01)

C-04 Pre-heated water/ethanol mixture

C-05 Water/ethanol mixture feed stream

C-06 PEM cell input stream

C-07 Liquid PEM cell output stream

C-08 Recirculation water/ethanol stream

C-09 Stream C-08 (adjusted pressure)

C-10 Vapor stream required to heat stream C-03

C-11 Cool output from heat exchanger (HE-01)

Hydrogen Pure hydrogen PEM cell output

Process energy Description

e-01 Pump (P-01) electrical energy



e-04 Electrical energy applied to the system

e-05 Splitter energy stream



119

contained water and acetaldehyde produced by the electrochemical

reforming of ethanol according to the following overall reaction:

C2H5OH C2H4O + H2 (3. 8)

The resulting stream (C-04) was then mixed with the recycle stream

which was rich in ethanol (C-09). The stream (C-05) was heated to 90 ºC

(operating temperature of the PEM electrochemical reforming unit). The

electrochemical reforming of ethanol-water mixtures occurred at the anode

of the PEM (A), leading to the production of acetaldehyde (3.8) (C-07) and

pure hydrogen in the cathode (C) (3.7). Stream C-07 also contained an

important amount of unreacted ethanol, due to the low efficiency of the

process. Consequently, this stream (C-07) was separated in a splitter unit

(S-01) and then pumped and recycled (C-09) to the PEM electrochemical

reforming cell. The splitter unit (S-01) allowed to calculate, in a theoretical

way, the energy required for the separation of the unreacted ethanol.

3.4. Process simulation and energetic evaluation

Table 3.3 illustrates the main operation parameters used in the

simulation of both processes [9, 17, 19-22]. Firstly, the temperature of the

reformer reactor was established from a previous study of the catalytic

ethanol steam reforming. According to Khila et al. [20], the highest

hydrogen concentration was obtained at a temperature of 800 ºC.

Temperatures beyond 800 ºC would increase the heat required, thus

reducing the process efficiency. Thus, a steam reforming reactor

temperature of 800 ºC was selected. According to Liguras et al. [19], the

overall ethanol conversion for a reforming process carried out at 800 ºC

with a Pt catalyst is 60% under the explored reaction conditions.

Chapter 3

120

Ta

ble

3

.3.

Base

-ca

se e

xp

eri

men

tal

para

mete

rs f

or

cata

lyti

c st

eam

refo

rmin

g a

nd

ele

ctro

chem

ical

refo

rmin

g

Ca

taly

tic s

tea

m r

efo

rm

ing

pro

ce

ss

Refe

ren

ce

[21

, 2

4]

[22

]

[22

]

[23

]

Ele

ctr

och

em

ica

l refo

rm

ing

pro

cess

Refe

ren

ce

[11

,19

]

T /

ºC

80

0

40

0

25

0

17

0

T /

ºC

90

Ca

taly

st

Pt

Eq

uil

ibri

um

rea

ctor

Eq

uil

ibri

um

rea

ctor

Eq

uil

ibri

um

rea

ctor

Ca

taly

st

An

od

e P

t-R

u/C

Ca

thod

e P

t/C

Sele

ctiv

ity

/ %

14

28

15

13

30

Con

vers

ion

/

%

8.4

0

16

.80

9.0

0

7.8

0

18

.00

Con

vers

ion

/

%

2.0

5

Rea

ctio

n

C2H

5O

H +

3H

2O

→ 2

CO

2 +

6H

2

(3

.1)

C2H

5O

H +

H

2O

→ 2

CO

+

4H

2

(3

.2)

C2H

5O

H +

2H

2 →

2C

H4

+ H

2 O

(3.9

)

C2H

5O

H

→ C

2H

4 +

H

2 O

(3.1

0)

C2H

5O

H

→ C

2H

4O

+

H2

(3

.8)

CO

+ H

2O

→

CO

2 +

H

2

(

3.3

)

CO

+ H

2O

→

CO

2 +

H

2

(

3.3

)

CO

+ ½

O2 →

CO

2

(3

.4)

H2 +

½ O

2 →

H2O

(3

.5)

Rea

ctio

n

C2H

5O

H

→ C

2H

4O

+

H2

(3

.8)

Un

it

Ste

am

refo

rmin

g

rea

ctor

(SR

)

Hig

h t

em

pera

ture

wa

ter

gas s

hif

t

rea

ctor

(HW

GS

)

Low

tem

pera

ture

wa

ter

gas s

hif

t

rea

ctor

(LW

GS

)

CO

pre

fere

nti

al

oxid

ati

on

rea

ctor

(CO

PR

OX

)

Un

it

PE

M c

ell


121

In this chapter, the following side reactions in the reforming reactor

were also considered:

C2H5OH + H2O 2CO + 4H2 (3.2)

C2H5OH + 2H2 2CH4 + H2O (3.9)

C2H5OH C2H4 + H2O (3.10)

C2H5OH C2H4O + H2 (3.8)

The conversion of each reaction were established from the selectivity

values reported by Liguras et al. [19] (Table 3.3). 80 % of the CO was

converted in the high temperature water gas shift reactor (HTS) whereas

12 % of the remaining CO was converted in the lower temperature water

gas shift one (LTS). The gas leaving the WGS reactors contained about 1 %

of CO due to thermodynamics limitations [20]. Therefore, a second step for

removing CO was necessary. In the COPROX reactor, the remaining CO

was oxidized to CO2 at 170 ºC with a 10 % excess air (reaction 3.4). Giunta

el al., [21] analyzed different temperature control schemes for the

COPROX reaction and found that the most suitable temperature in terms

of selectivity towards CO was 170 ºC. However, hydrogen is partially

oxidized at these conditions (reaction 3.5) [23]. Reaction 3.5 was

thermodynamically favored because the hydrogen concentration in the

mixture was clearly higher than that of CO. For this reason, the catalyst

used should be highly selective to the total oxidation of CO (reaction 3.4).

The product stream was mainly composed by hydrogen, containing less

than 20 ppm of CO. The high purity of this stream made it suitable to be

used in PEM fuel cells [21]. The final step is the separation of the

hydrogen from the gas stream exiting the COPROX reactor, which is

mostly constituted by H2, H2O, C2H5O, CO2 and air, which can be

accomplished through a membrane. Membrane processes for hydrogen

Chapter 3

122

separation is a promising technology that is receiving high attention in the

last years, producing a high purity hydrogen stream (>99 %). The

membrane operated at 300 ºC and pure hydrogen was obtained at

atmospheric pressure [8]. Table 5.4 summarizes the values of the main

variables of the catalytic steam reforming process. It can be observed that

the hydrogen production rate was 85.09 mol h-1; i.e., 0.0236 mol s-1.

Additionally, several byproducts with industrial applications such as

ethylene, acetaldehyde and methane were also obtained. Acetaldehyde is

mainly used as a starting material in the synthesis of acetic acid, n-butyl

alcohol, ethyl acetate, and other chemical compounds. Ethylene is the raw

material used in the manufacture of polymers and methane can be used as

fuel and raw material for synthesizing other compounds.

As abovementioned, the operating conditions of the electrochemical

reforming unit were obtained from a previous work [9, 17]. The ethanol-

water mixture (6 M ethanol concentration) was fed at 90 ºC with a flow

rate of 1 ml min-1 (stream C-06). The electrical energy applied to the PEM

cell (e4) was set from the current-potential curves (current = 0.4 A and Vcell

= 0.9 V) [9, 17]. These parameters were directly related to the hydrogen

produced in the cathodic side of the PEM cell by the Faraday law, i.e. rH2 =

I/nF, where rH2 is the hydrogen production rate (mol s-1), n is the number of

transferred electrons, and F is the Faraday constant. This theoretical

hydrogen production rate value was experimentally confirmed by de

Lucas-Consuegra et al. [17]. As described by Caravaca et al. [14],

acetaldehyde is one of main byproducts from the electrochemical reforming

of ethanol. For this reason, acetaldehyde was assumed to be the unique by-

product of the reaction (reaction 3.8).


123

Table 3.4. Molar and energy balances of the catalytic steam reforming process.

F / mol h-1

Stream T

/ ºC

P

/ atm H2O C2H5OH H2 CO2 CH4 CO C2H4O C2H4 Air

Water 25 1 736.6 - - - - - - - -

Ethanol 25 1 - 122.7 - - - - - - -

C-01 25 2.5 736.6 - - - - - - - -

C-02 25 2.5 - 122.7 - - - - - - -

C-03 25 2.5 736.6 122.7 - - - - - - -

C-04 118 2.4 736.6 122.7 - - - - - - -

C-05 120 2.3 736.6 122.7 - - - - - - -

C-06 153 2.2 736.6 122.7 - - - - - - -

C-07 162 2.1 736.6 122.7 - - - - - - -

C-08 219 2.0 736.6 122.7 - - - - - - -

C-09 553 1.9 736.6 122.7 - - - - - - -

C-10 800 1.8 736.6 122.7 - - - - - - -

C-11 800 1.8 720.2 49.1 120.3 22.6 29.7 19.4 25.5 12.2 -

C-12 800 1.8 504.1 34.4 71.6 15.8 20.8 13.6 17.9 8.6 -

C-13 400 1.7 504.1 34.4 71.6 15.8 20.8 13.6 17.9 8.6 -

C-14 400 1.7 491.0 34.4 84.8 29 20.8 0.4 17.9 8.6 -

C-15 250 1.6 491.0 34.4 84.8 29 20.8 0.4 17.9 8.6 -

C-16 250 1.6 490.6 34.4 85.1 29.3 20.8 0.1 17.9 8.6 -

C-17 170 1.5 490.6 34.4 85.1 29.3 20.8 0.1 17.9 8.6 -

C-18 25 1 - - - - - - - - 502.6

C-19 73 1.5 - - - - - - - - 502.6

C-20 170 1.5 - - - - - - - - 502.6

C-21 170 1.5 490.7 34.4 85.1 29.4 20.8 - 17.9 8.6 502.6

C-22 45 1.4 490.7 34.4 85.1 29.4 20.8 - 17.9 8.6 502.6

C-23 257 1.3 490.7 34.4 85.1 29.4 20.8 - 17.9 8.6 502.6

C-24 300 1.2 490.7 34.4 85.1 29.4 20.8 - 17.9 8.6 502.6

C-25 300 1 490.7 34.4 - 29.4 20.8 - 17.9 8.6 502.6

C-26 45 1 490.7 34.4 - 29.4 20.8 - 17.9 8.6 502.6

C-27 800 1.8 216.1 14.7 30.7 6.8 8.9 5.8 7.7 3.7 -

C-28 25 1 - - - - - - - - 215.3

C-29 98 1.8 - - - - - - - - 215.3

C-30 906 1.8 233.9 14.7 30.7 15.7 - 5.8 7.7 3.7 197.5

C-31 230 1.7 233.9 14.7 30.7 15.7 - 5.8 7.7 3.7 197.5

C-32 300 1.1 - - 85.1 - - - - - -

Hydrogen 163 1 - - 85.1 - - - - - -

Table 5.5 shows the value of the overall molar balances of the

electrochemical reforming unit. Under these conditions the ethanol

Chapter 3

124

conversion obtained in the simulation process was 2.05 % for a hydrogen

production rate of 7.462·10-3 mol h-1 i.e. 2.072·10-6 mol s-1.

Table 3.5. Molar and energy balances of the simulation of the electrochemical

reforming process.

F / mol h-1

Stream T / ºC P / atm H2O C2H5OH H2 C2H4O

Water 25 1.0 0.648 - - -

Ethanol 25 1.0 - 0.007 - -

C-01 25 2.0 - 0.007 - -

C-02 25 2.0 0.648 - - -

C-03 25 2.0 0.648 0.007 - -

C-04 84 1.8 0.648 0.007 - -

C-05 88 1.8 2.160 0.363 - -

C-06 90 1.6 2.160 0.363 - -

C-07 90 1.3 2.160 0.355 - 0.007

C-08 90 1.3 1.512 0.355 - -

C-09 90 1.8 1.512 0.355 - -

C-10 90 1.3 0.648 - - 0.007

C-11 30 1.1 0.648 - - 0.007

Hydrogen 90 1 - - 0.007 -

Figure 3.3 summarizes the value of the overall energy consumption

per mol of hydrogen produced in the different units of the process of steam

reforming (a) and electrochemical reforming (b). It can be observed that

the highest energy input for the catalytic steam reforming was the energy

stream e-3 (unit H-01), which provided the additional energy to heat the

inlet stream (C-03) at the reforming reactor temperature (T = 800 ºC).

Energy stream e-4 accounted for the second highest energy consumption in

the process, which was used for providing the necessary energy to carry

out the endothermic reforming reaction. This result is in agreement with a

previous work of Tippawan et al., [27].


125

e-1 e-2 e-3 e-4 e-5 e-6 e-7 e-8

0

8

16

60

80

100

120

140

En

erg

y /

kJ

mol-1

H2

a)

b)

e-1 e-2 e-3 e-4 e-5 e-6

0.0

0.3

0.6

0.9

1.2

30

60

90

120

150

180

210

e-1 e-2 e-80.00

0.01

0.02

0.03

0.04

0.05

0.06

Ener

gy / k

J m

ol-1 H

2

En

erg

y /

kJ

mol-1

H2

e-1 e-20.00

0.03

0.06

0.09

0.12

0.15

0.18

0.21

En

erg

y / k

J m

ol-

1 H

2

Figure 3.3. Energy consumption of the different units of the (a) catalytic

steam reforming and (b) electrochemical reforming of ethanol-water mixtures.

Chapter 3

126

On the other hand, the lower energy consumption corresponded to the

energy required for pumping the feed streams (e-01 and e-02). In the

electrochemical reforming process, two kinds of energy streams can be

distinguished: the process energy (e-1, e-2, e-3, e-5, e-6), which includes all

the energy required in the process except the electrical energy needed for

the electrochemical reforming reaction and electrical ones (e-4). In this

case, the highest energy input corresponded to the electrical energy

consumption in the PEM cell (e-4 = 173.6 kJ mol-1 H2). This energy

consumption was similar to that recently reported (116.3 kJ mol-1 H2) for

the electrochemical reforming of ethylene glycol [17]. Both values were

obtained from experimental potential and current values, i.e. P = V·I (W).

This electrical energy value is more than twice the thermal energy

required for heating stream C-05 (e-3) and several orders of magnitude

higher than those required for the pumping of streams and the separation

of the unreacted ethanol.

Figure 3.4 displays for each process a comparison between the amount

of produced H2 in kg of hydrogen per kg of ethanol and the energy balance

in kW h per kg of hydrogen. For comparative purposes, the yield of

hydrogen production was referred to the amount of consumed ethanol

(Figure 3.4a). Regarding the catalytic steam reforming of ethanol, the

value obtained in this chapter (0.0301 kg H2 kg-1 C2H5OH) was similar to

that reported by Khila et al., [20] (0.0347 kg H2 kg-1 C2H5OH). Regarding

the case of the electrochemical reforming of ethanol, the obtained value

(0.0434 kg H2 kg-1 C2H5OH) was in the same range than those obtained by

de Lucas-Consuegra et al., [17] for the case of the electrochemical

reforming of ethylene glycol (0.0598 kg H2 kg-1 C2H6O2). From the feed

stock consumption point of view, it was observed that the electrochemical


127

reforming process was slightly more productive that the catalytic one. On

the other hand, the overall energy consumption for both processes was also

calculated (Figure 3.4b). It can be seen that the electrochemical reforming

process consumed less energy per kg of hydrogen produced, (29.2 kWh/kg

of H2) than the catalytic one (32.70 kWh/kg of H2). The higher energy

efficiency of the electrochemical reforming process for H2 production, if

compared to that of the catalytic one, can be related to the simplicity of

this process, which also operates at higher temperatures and pressures

and does not require of further purification and separation steps (e.g.

water gas shift reactors, COPROX reactor, etc). It should be mentioned

that a lower consumption energy value (12.18 kWh per kg of H2) have been

previously reported for the ethanol steam reforming process [22]. In the

present chapter, experimental catalytic results were used in the

simulation of the reforming unit, in contrast with reference [22], where the

authors assumed that the reforming reactor operated at equilibrium

conditions. Concerning the electrochemical reforming process, a value of

17.14 kWh per kg of H2 was recently reported for the electrochemical

reforming of ethylene glycol [17]. On the other hand, a value of 21.53 kWh

per kg of H2 was also reported for a methanol electrolyzer stack [28] where

the electrical energy was the only requirement considered. Unlike this

work where the electrical energy requirement (29.2 kWh/kg of H2) was set

from the deactivated current-potential curves, i.e. when the system

reaches the steady state value, references [17] and [28] set this

requirement from fresh MEA. In addition, it should be mentioned that the

electrical energy consumption value found here is lower than the amount

of energy required by a commercial water electrolyzer stacks, which are in

the range of 53.4 – 70.1 kWh per kg of H2 produced [24, 29].

Chapter 3

128

0.00

0.01

0.02

0.03

0.04

b)

Catalytic steam reforming Electrochemical reforming

Yie

ld /

kg

H2

kg

-1

C2H

5O

H

a)

0

10

20

30

En

erg

y c

on

sum

pti

on

/ k

W h

kg

-1

H2

Figure 3.4. Comparison of (a) H2 yield and (b) energy consumption in

catalytic steam reforming and electrochemical reforming processes.

In this sense, the US Department of Energy (DOE) has pointed out

that the electrical energy input to the electrolyzer stack should drop to 43

kWh per kg of H2 in 2020 [30]. Even considering the DOE output energy

for ethanol of 7.4 kWh per kg of H2 [31], the value of the required total

energy for the electrochemical reforming of ethanol make this technology

to be competitive. The use of ethanol is certainly promising due to its

possible renewable production (fermentation of biomass or steam

reforming of cellulosic materials) with reasonably low energy cost [32].

Ethanol allows electrolysis at potential < 1 V if compared to that of water

(1.23 V), leading to electrical power savings [14, 32].

Apart that the electrochemical reforming process seems to be more simple

and competitive that the most widely studied catalytic steam reforming,

the former process allows to produce H2 of high purity quickly and


129

conveniently (single reaction and a separation step at mild operating

conditions).

3.5. Conclusions

The following conclusions could be drawn from this chapter:

The electrochemical reforming process allowed to obtain pure hydrogen

in a single step while the catalytic process required additional steps of

purification and separation leading to a more complicated H2 production

plant.

For the catalytic steam reforming process electrical and thermal energy

consumption was considered. The highest energy consumption

corresponded to the stream e-3, which provided the energy required to heat

the outlet stream from the heat exchanger network to the reforming

reactor temperature (T = 800 ºC).

Electrical energy consumption and process energy (electrical and

thermal energy) consumption were computed in the electrochemical

reforming process. In this case, the main energy requirement was the

electrical energy one.

The electrochemical reforming process seemed to be less intensive in

feed stock material and energy consumption for H2 production than the

process based on the catalytic conventional route. In addition, the

calculated energy consumption of the overall electrochemical reforming

process was lower than that required by a traditional water electrolyzer

stack.

Chapter 3

130

3.6. References

[1] G. Iaquaniello, F. Giacobbe, B. Morico, S. Cosenza, A. Farace, International

Journal of Hydrogen Energy, 33 (2008) 6595-6601.

[2] F. Li, L. Zeng, L.-S. Fan, Industrial & Engineering Chemistry Research, 49 (2010)

11018-11028.

[3] R.C. Saxena, D. Seal, S. Kumar, H.B. Goyal, Renewable and Sustainable Energy

Reviews, 12 (2008) 1909-1927.

[4] W. Doenitz, R. Schmidberger, E. Steinheil, R. Streicher, International Journal of

Hydrogen Energy, 5 (1980) 55-63.

[5] Q. Huang, Q. Li, X. Xiao, Journal of Physical Chemistry C, 118 (2014) 2306-

2311.

[6] D. Das, T.N. Veziroǧlu, International Journal of Hydrogen Energy, 26 (2001) 13-

28.

[7] M. Ni, D.Y.C. Leung, M.K.H. Leung, International Journal of Hydrogen Energy,

32 (2007) 3238-3247.

[8] A.P. Simpson, A.E. Lutz, International Journal of Hydrogen Energy, 32 (2007)

4811-4820.

[9] A. Caravaca, A. De Lucas-Consuegra, A.B. Calcerrada, J. Lobato, J.L. Valverde,

F. Dorado, Applied Catalysis B: Environmental, 134-135 (2013) 302-309.

[10] G. Sasikumar, A. Muthumeenal, S.S. Pethaiah, N. Nachiappan, R. Balaji,

International Journal of Hydrogen Energy, 33 (2008) 5905-5910.

[11] C.R. Cloutier, D.P. Wilkinson, International Journal of Hydrogen Energy, 35

(2010) 3967-3984.

[12] T. Take, K. Tsurutani, M. Umeda, Journal of Power Sources, 164 (2007) 9-16.

[13] Z. Hu, M. Wu, Z. Wei, S. Song, P.K. Shen, Journal of Power Sources, 166 (2007)

458-461.

[14] A. Caravaca, F.M. Sapountzi, A. De Lucas-Consuegra, C. Molina-Mora, F.

Dorado, J.L. Valverde, Int. J. Hydrogen Energy, 37 (2012) 9504-9513.


131

[15] A.T. Marshall, R.G. Haverkamp, International Journal of Hydrogen Energy, 33

(2008) 4649-4654.

[16] S. Kongjao, S. Damronglerd, M. Hunsom, J Appl Electrochem, 41 (2011) 215-

222.

[17] A. de Lucas-Consuegra, A.B. Calcerrada, A.R. de la Osa, J.L. Valverde, Fuel

Processing Technology, 127 (2014) 13-19.

[18] S.M. Mousavi Ehteshami, S.H. Chan, Energy Technology & Policy, 1 (2014) 15-

22.

[19] D.K. Liguras, D.I. Kondarides, X.E. Verykios, Applied Catalysis B:

Environmental, 43 (2003) 345-354.

[20] Z. Khila, N. Hajjaji, M.-N. Pons, V. Renaudin, A. Houas, Fuel Processing

Technology, 112 (2013) 19-27.

[21] P. Giunta, M. Moreno, F. Mariño, N. Amadeo, M. Lobarde, Chemical

Engineering & Technology, 35 (2012) 1055-1063.

[22] D. Montané, E. Bolshak, S. Abelló, Chemical Engineering Journal, 175 (2011)

519-533.

[23] P. Giunta, C. Mosquera, N. Amadeo, M. Laborde, Journal of Power Sources, 164

(2007) 336-343.

[24] A. Haryanto, S. Fernando, N. Murali, S. Adhikari, Energy & Fuels, 19 (2005)

2098-2106.

[25] F. Mariño, C. Descorme, D. Duprez, Applied Catalysis B: Environmental, 54

(2004) 59-66.

[26] F. Mariño, C. Descorme, D. Duprez, Applied Catalysis B: Environmental, 58

(2005) 175-183.

[27] P. Tippawan, A. Arpornwichanop, Bioresource Technology, 157 (2014) 231-239.

[28] S.P. Sethu, S. Gangadharan, S.H. Chan, U. Stimming, Journal of Power Sources,

254 (2014) 161-167.

[29] V. Bambagioni, M. Bevilacqua, C. Bianchini, J. Filippi, A. Lavacchi, A.

Marchionni, F. Vizza, P.K. Shen, ChemSusChem, 3 (2010) 851-855.

Chapter 3

132

[30] Development and demonstration Plan of the US Department of Energy, in: D.o.

Energy (Ed.), Fuel Cell Technology Office Multi-Year Reserach, 2011.

[31] H. Shapouri, Energy Balance of the Corn-Ethanol Industry, in: USDA (Ed.),

2008.

[32] Y.X. Chen, A. Lavacchi, H.A. Miller, M. Bevilacqua, J. Filippi, M. Innocenti, A.

Marchionni, W. Oberhauser, L. Wang, F. Vizza, Nature Communications, 5 (2014).

4.1. Introduction

4.2. Experimental


and EPOC parameters


electrolyte cell



4.3.1. Influence of the preparation

method of the catalyst film

4.3.2. Kinetic study and

electrochemical promotion experiments

4.4. Conclusions

4.5. References

CHAPTER 4:

Electrochemical Promotion of Ni with

Alkali Ions in the CO2 Hydrogenation

Toward CO and CH4

INTRODUCTION


TPR analysis

RESULTS

H2/CO2 = 30 % /15 %, N2 balance, FT = 6 NL h-

1, T = 240 ºC

Hydrogenation of CO2 can be considered as one of the most important chemical conversion reactions not only

for the effective decrease of the overall CO2 emissions but also for the production of many possible renewable

fuels (hydrocarbons or alcohols). In the hydrogenation of CO2 process, depending on both the employed

catalytic system (metal/support) and on the reaction conditions, different products can be obtained including

CO, CH4, formic acid, formaldehyde, methanol or higher alcohols.

The reaction of hydrogenation of CO2 can be summarized according to the following scheme:

xCO2 + (2x – z + y/2)H2 → CxHyOz + (2x – z)H2O

Hence, the control of the catalyst selectivity toward different hydrogenation products in highly required. In

this sense the activity and selectivity can be modified by electrically polarizing the catalyst-electrode via the

effect of the electrochemical promotion of catalysis (EPOC).

Single chamber solid electrolyte cell reactor

- Final electrical resistance 0.3 Ω at 350 ºC

- Reduction to metallic Ni

Influence of the applied potential Effect of the temperature

H2/CO2 = 30 % /15 %, N2 balance, FT = 6 NL h-1

Effect of the applied potential and the H2 feed concentration

-The reaction rate of CO2 and production rate of CO

and CH4 were enhanced by increasing

temperatures

CO → electrophilic behaviour

CH4 → electrophobic behaviour

In this study, three different king of Ni-based catalysts were prepared on K-βAl2O3 solid electrolyte by combining the annealing of an organometallic paste and

the addition of a catalyst powder. The different catalysts films were tested in the CO2 hydrogenation reaction under electrochemical promotion by K+ ions, and

were characterized by XRD and SEM.

Outlet Inlet


Catalyst-working

Electrode (WE)

Reactor cap Cooling

Quartz

tube

Au wires

Alumina tube

with 4 bores

H2

e-

Kd+

Kd+

Kd+Kd+

d-

d-

d- d-

I < 0

e-

Ni

CO2

CO2

CO2

CO2

H2

Kd+

Kd+

Kd+

d-

d-

d-

(K-β-Al2O3)K+ K+K+K+

0

50

100

150

200

250

300

350

400

0 30 60 90 120 150 180 210

0

50

100

150

200

250

300

350

Tem

per

ature

/ º

C

0

20

40

60

80

100

120

140

Res

ista

nce

/

Tem

per

ature

/ º

C

Time / min

H2

consu

mpti

on /

a.u.

K-βAl2O3

Ni

Au

Ω

280

320

360

Tem

per

atu

re /

ºC

140 150 160 1700

5

10

15

20

Time / min

Res

ista

nce

/

SEM analysis

50 µm

b3) b4)

100 µm

100 µm 50 µm

a1) a2)

b1) b2)

c2)

Al Ni

50 µm

c1)

100 µm

0.0

0.3

0.6

0.9

1.2

0.0

0.2

0.4

0.6

0.8

1.0

0.00

0.05

0.10

0.15

0.20

0.25

0.30

0.35

0 20 40 60 80 100 120 140 160 180 200

-1000

-800

-600

-400

-200

0

200

400

600

800

UW

R /

V U

WR /

V

GNA NA N

UW

R /

V

(rC

O2/ m

ol C

O2·m

ol-1 N

i·s-1

)x10

3

a)

-2

-1

0

1

2

b)

(rC

O/ m

ol C

O·m

ol-1 N

i·s-1

)x10

3

-2

-1

0

1

2

c)

(rC

H4/ m

ol C

H4·m

ol-1 N

i·s-1

)x10

3

-2

-1

0

1

2

Current

Cur

rent

/ A

Time / min

d)

-3

-2

-1

0

1

2

UWR

UW

R/ V

-NA showed the higher catalytic activity

0

10

20

30

40

50

-2 -1 0 1 20

5

10

15

0

20

40

60

b)

(rC

O /

mol

CO

· m

ol-1 N

i·s-1

)x105

T = 240 ºC

T = 270 ºC

T = 300 ºC

c) 100 Nml min-1

UWR

/V

(rC

H4 /

mol

CH

4· m

ol-1 N

i·s-1

)x105

H2 / CO

2 = 30% / 1.5 %

Increasing

(rC

O2 /

mol

CO

2· m

ol-1 N

i·s-1

)x105

a)

0

10

20

30

40

50

60

-2 -1 0 1 2

40

50

60

70

80

90

T = 240 ºC

H2/CO

2 = 10

H2/CO

2 = 20

SC

H4

/%

H2/CO

2 = 2

H2/CO

2 = 6

SC

O/%

UWR / V

T = 240 ºC, FT = 6 NL h-1

2ºC/min, H2= 3 % (N2 balance) FT = 6 NL h-1

CHAPTER 4. ELECTROCHEMICAL PROMOTION

OF Ni WITH ALKALI IONS IN THE CO2

HYDROGENATION TOWARD CO AND CH4

137

Abstract

hree different kind of Ni-based catalysts were prepared on a K-

βAl2O3 solid electrolyte by combining the annealing of an

organometallic paste and the addition of a catalyst powder. The

different catalysts films were tested in the CO2 hydrogenation reaction

under electrochemical promotion by K+ ions, and were characterized by

XRD and SEM. The catalyst film derived from the addition of an α-Al2O3

powder to the Ni catalyst ink presented the highest catalytic activity as a

result of the increase in Ni catalyst film porosity. The influence of the

applied potential and other operation variables were evaluated on the Ni

catalytic activity and selectivity. Hence, the CO production rate was

enhanced either by decreasing the applied potential (with the consequent

supplied of K+ ions to the catalyst surface) or by increasing the CO2 (electron

acceptor) feed concentration. On the other hand, CH4 production rate was

favoured at positive potentials (removing K+ from the catalyst surface) or by

increasing the H2 (electron donor) feed concentration. The global CO2

consumption rate increased upon negative polarization in all experiments

and the Electrochemical Promotion of Catalysis (EPOC) effect showed to be

reversible and reproducible. Hence, the electrochemical promotion

phenomena demonstrated to be a very useful technique to in-situ modify

and control the catalytic activity and selectivity of a non-noble metal such

as Ni for the production of CH4 or syngas via CO2 valorisation.

T

Chapter 4

138

4.1. Introduction

As seen in previous chapters (Chapter 2 and Chapter 3) hydrogen

production free of CO2 emissions is a priority goal for the scientific

community. However, there are still many processes that emit CO2 causing

an overall increase in global warming. Therefore, different strategies are

being developed to mitigate the global warming and climate change, some

of them are focused on the separation, storage and utilization of the CO2.

In this sense, hydrogenation of CO2 can be considered as one of the most

important chemical conversion reactions not only for the effective decrease

of the overall CO2 emissions but also for the production of many possible

renewable fuels (hydrocarbons or alcohols) [1-3]. However, CO2 is a

thermodynamically stable compound and thus requires high activation

energy for its transformation into other chemicals [4, 5]. Although

numerous organic syntheses involve CO2 as the feedstock, only a few have

reached industrial commercialization, for instance, the production of urea

and its derivatives, salicylic acid, and carbonates [1, 5]. Most studies on

catalytic hydrogenation of CO2 have been performed using metal catalysts

(e.g. Pt, Rh, Pd, Ru, Cu, Fe, Co and Ni) supported on different oxides (e.g.

Nb2O3, ZrO2, Al2O3, SiO2, and MgO) [6-10]. Depending on both the

employed catalytic system (metal/support) and on the reaction conditions,

different products can be obtained including CO, CH4, formic acid,

formaldehyde, dimethyl ether, methanol or higher alcohols [1, 4-6, 11, 12].

These catalytic reactions can be summarized according to the following

scheme:

xCO2 + (2x – z + y/2)H2 → CxHyOz + (2x – z)H2O (4.1)

Electrochemical Promotion of Ni with Alkali Ions in the CO2 Hydrogenation Toward CO and CH4

139

For instance, the reverse water gas shift (RWGS, eq. 3.2) and CO2

methanation (eq.3.3) reactions can be described as follows:

CO2 + H2 → CO + H2O (4.2)

CO2 + 4H2 → CH4 + 2H2O (4.3)

Hence, the control of the catalyst selectivity toward the different

hydrogenation products is highly required. In this sense, alkali promotion

has been described to be an effective approach to improve the activity

and/or selectivity of different metal catalysts [13]. For instance, the

promoter effect of potassium in the CO2 hydrogenation reaction has been

previously studied over conventional heterogeneous catalysts [14, 15].

Both the activity and the selectivity of a metal or metal oxide catalyst film

deposited on an ionic conductor can also be modified, in a significant,

controlled and reversible manner, by electrically polarizing the catalyst-

electrode via the effect of electrochemical promotion of catalysis (EPOC)

also known as NEMCA effect (non-faradaic electrochemical modification of

catalytic activity). This phenomenon was discovered by Stoukides and

Vayenas [16] and is based on the controlled migration of promoting ions,

e.g., O2-, Na+, K+ or H+ ions, from an electroactive support, such as β-Al2O3

(a Na+ or K+ conductor), YSZ (yttrium-stabilized-zirconia, an O2- conductor)

or CZI (CaZr0.9In0.1O3-α, a H+ conductor), to the catalytic metal/gas

interface. The EPOC phenomenon has been applied on a wide variety of

catalysts and in a large number of important industrial and environmental

catalytic reactions [17]. In particular, the electrochemical promotion has

been shown to enhance the catalyst activity and selectivity in the

hydrogenation of CO2 [2, 3, 6, 11, 12, 18-24]. Hence, the promotional effect

may allow to in-situ control the catalytic behavior of the system for syn-gas

Chapter 4

140

or CH4 production via CO2 hydrogenation. Most EPOC studies on CO2

hydrogenation (as shown on Table 4.1) have been performed with a YSZ

solid electrolyte and a noble metal catalyst such as Pt, Pd, Ru and Rh [2, 3,

6, 11, 18, 20, 21]. Nevertheless, only a few studies have been carried out by

using cationic solid electrolytes [3, 12, 19, 21-23] or non-noble metal

catalysts (Ni or Cu) [2, 11, 12, 20, 22]. However, for practical use, it is clear

that the development of low-cost and highly efficient catalyst is desired. In

this sense, we focused on the developing an efficient catalyst film of

suitable performance for the CO2 hydrogenation reaction via

electrochemical promotion by K+ ions. Hence, K-βAl2O3 was selected as the

solid electrolyte, and different catalyst film configurations were proposed

based on Ni catalyst: i) Ni catalyst film prepared by deposition of an

organometallic paste, ii) Ni catalyst film prepared by deposition of an

slurry made by mixing a Ni metal paste and α-Al2O3 powder and iii) and Ni

particles dispersed on Al2O3 powder and deposited on the solid electrolyte

by an slurry with Au paste. By this latter case, the possibility of exploring

the electro-promotional effect on dispersed metal nanoparticles was also

examined. Hence three different electrochemical catalysts (Ni/K-βAl2O3/Au,

Ni-α-Al2O3/K-βAl2O3/Au and Au-Ni(30%)-α-Al2O3/K-βAl2O3/Au) were

studied on the basis of different characterization techniques and catalytic

activity measurements under electrochemical promotion conditions.

Additionally, for the selected catalytic system a kinetic study under EPOC

conditions was carried out, as well as the possibility of controlling the Ni

catalytic activity and selectivity toward the different obtained products

(syn-gas or methane) via EPOC.


141

Ta

ble

4.1

. E

PO

C s

tud

ies

of

the C

O2 h

yd

rogen

ati

on

react

ion

usi

ng d

iffe

ren

t ca

taly

sts

an

d s

oli

d e

lect

roly

tes

Refe

ren

ce

[2]

[3]

[18

]

[11

]

[6]

[19

]

EP

OC

beh

avio

ur

Invert

ed

volc

an

o

Invert

ed

volc

an

o

Ele

ctro

ph

ob

ic

(CH

4)/

Ele

ctro

ph

ibic

(CO

)

Ele

ctro

ph

ibic

Ele

ctro

ph

ob

ic

(CH

4)/

Ele

ctro

ph

ibic

(CO

)

Ele

ctro

ph

ob

ic

(CH

4)/

Ele

ctro

ph

ibic

(CO

). T

he

oth

ers

vary

wit

h e

ract

ion

con

dit

ion

s

Ma

in

rea

ctio

n

pro

du

cts

CO

, C

H4

C2H

4

CO

CO

, C

H4

CO

, C

H4

CO

, C

H4

CO

, C

H4,

CH

3O

H,

C2H

5O

H,

C2+

C3

FT

/ N

Lh

-1

60

2.3

-18

0.9

-3

6

12

90-5

22

H2

/ %

5.6

22.7

63

5.6

30

47.5

-76

CO

2

/ %

1

72.8

5.5

1

3

19-4

7.5

T

/ ºC

220

-380

533

-605

346

-477

200

-440

200

-300

400

Ca

taly

st f

ilm

pre

para

ion

tecn

iqu

e

Sp

utt

eri

ng

Org

an

om

eta

llic

pa

ste

Org

an

om

eta

llic

pa

ste

Imp

regn

ati

on

Imp

regn

ati

on

Dip

-coa

tin

g

Soli

d

ele

ctro

lyte

YS

Z

YS

Z,

Na

-βA

l 2O

3

YS

Z

YS

Z

YS

Z

K-β

Al 2

O3

Ca

taly

st

Rh

, P

t,C

u

Pd

Rh

Ni,

Ru

Ru

Pt

Chapter 4

142

Ta

ble

4.1

. (C

on

t.).

EP

OC

stu

die

s of

the C

O2 h

yd

rogen

ati

on

react

ion

usi

ng d

iffe

ren

t ca

taly

sts

an

d s

oli

d e

lect

roly

tes

Refe

ren

ce

[12

]

[20

]

[21

]

[22

]

[23

]

[24

]

EP

OC

beh

avio

ur

It v

ari

es

wit

h

rea

ctio

n

con

dii

tion

s

It v

ari

es

wit

h

rea

ctio

n

con

dii

tion

s

Ele

ctro

ph

obic

(CH

4)/

Ele

ctro

ph

ibic

(CO

)

Ele

ctro

ph

ibic

Ele

ctro

ph

obic

(CH

4)/

Ele

ctro

ph

ibic

(CO

)

Ele

ctro

ph

ibic

Ma

in

rea

ctio

n

pro

du

cts

CH

3O

H,

C2H

5O

H,

C2H

6O

CO

, C

H4,

CH

3O

H,C

2

H6,

C3H

6,

C2H

6O

CO

, C

H4

CO

CO

, C

H4

CO

FT

/ N

Lh

-1

90

-52

2

90

6

12

-1.8

6

1.2

H2

/ %

63

.3-7

6

47

.5-7

6

1-1

5

0.3

-4.9

30

1-1

0

CO

2

/ %

19

-31

19

-47

.5

0.0

25

-2

0.6

-

2.2

4

3

1-1

0

T

/ ºC

20

0-4

00

22

5-4

50

20

0-3

40

55

0-7

50

28

0-4

20

65

0-8

00

Ca

taly

st f

ilm

pre

para

ion

tecn

iqu

e

Ele

ctro

less

dep

osi

tion

Org

an

om

eta

llic

pa

ste /

Ele

ctro

less

Imp

regn

ati

on

Pd

Org

an

om

eta

llic

pa

ste/

Cu

pow

der

Imp

regn

ati

on

Org

an

om

eta

llic

pa

ste

Soli

d

ele

ctro

lyte

K-β

Al 2

O3

YS

Z

YS

Z,

K-β

Al 2

O3

SZ

Y

K-β

Al 2

O3

YS

Z

Ca

taly

st

Cu

Pt,

Ni,

Pd

Ru

Cu

Ru

Pt


143

4.2. Experimental

4.2.1. Catalytic activity measurements and EPOC parameters

The experimental setup was described in detail in a previous chapter

(Chapter 1). The reaction gases (Praxair, Inc.) were certified standards

(99.999 % purity) of CO2, H2 and N2, the latter being used as the carrier

gas, and the gas flow rates were controlled by a set of mass flowmeters

(Brooks 5850 E and 5850 S). The catalytic experiments were carried out at

atmospheric pressure with an overall gas flow rate, FT, ranging from 1.2 to

12 NL h-1, at three temperatures (T = 240, 270, and 300 ºC) and a feed

composition ranging from 3 to 30 % for H2 and from 1.5 to 10 % for CO2 (N2

balance).

Reactant and product gases were on-line analyzed by using a micro

gas-chromatograph (Varian CP-4900) as described in details in Chapter 1.

The detected reaction products were: CO, CH4 and H2O. The error in the

carbon atom balance did not exceed 5 %. In order to carry out the

electrochemical promotion (EPOC) experiments, the three electrodes

(working, counter and reference) were connected to a potentiostat-

galvanostat Voltalab PGZ 301 (Radiometer Analytical) using gold wires.

The inertness of both the gold counter/reference electrodes and the α-Al2O3

powder and Au-α-Al2O3 film was checked via blank experiments performed

under the studied reaction conditions.

The CO2 conversion and the CO and CH4 selectivities were calculated as

follows:

𝐶𝑂2 𝑐𝑜𝑛𝑣𝑒𝑟𝑠𝑖𝑜𝑛 / % =𝐹0

𝐶𝑂2−𝐹𝐶𝑂2

𝐹0𝐶𝑂2

𝑥100 (eq. 4.1)

𝐶𝑂 𝑠𝑒𝑙𝑒𝑐𝑡𝑖𝑣𝑖𝑡𝑦 / % =𝐹𝐶𝑂

𝐹0𝐶𝑂2−𝐹𝐶𝑂2

𝑥100 (eq. 4.2)

𝐶𝐻4 𝑠𝑒𝑙𝑒𝑐𝑡𝑖𝑣𝑖𝑡𝑦 / % =𝐹𝐶𝐻4

𝐹0𝐶𝑂2−𝐹𝐶𝑂2

𝑥100 (eq. 4.3)

Chapter 4

144

where F0i and Fi are the molar flow rates of the i species at the inlet and at

the outlet of the reactor, respectively. On the other hand, the magnitude of

the electropromotional effect was quantified by two parameters commonly

used in this kind of studies:

- The rate enhancement ratio of each compound (ρi), defined by the

following equation:

𝜌i = 𝑟i

𝑟i,0 (eq. 4.4)

where ri and ri,0 are the promoted (UWR < 2 V) and reference state (UWR = 2

V) catalytic production rates, respectively, of the corresponding compound.

- The promotion index (PIK+), calculated by the following equation:

𝑃𝐼𝐾+ =

∆𝑟

𝑟0

𝜃𝐾+ (eq. 4.5)

where ∆r = r – r0 is the K-induced change in catalytic reaction rate and θK+

is the potassium coverage calculated from the integration of the current (I)

vs. time (t) curves via the Faraday law (eq. 4.6):

𝜃𝐾+ = ∫|I|dt

nF𝑁𝐺

t

0 (eq. 4.6)

where n is the potassium ion charge, i.e., +1, F is the Faraday constant

(96484.6 C), and NG is the active mol of the Ni catalysts, was calculated in

each case by means of the total amount of deposited Ni and the particle

diameter and dispersion values estimated from the (111) from XRD

analysis peak via Scherrer equation.


145


Each electrochemical catalyst was based on a 19-mm-diameter, 1-mm-

thick K-βAl2O3 (Ionotec) pellet solid electrolyte. In each electrochemical

catalyst, the Au counter and reference electrodes were firstly deposited on

one side of the electrolyte by annealing a gold organometallic paste (Fuel

Cell Materials ref. 231001) at 800 ºC for 2 h (heating ramp of 5 ºC/min).

Then, three Ni-based catalyst were prepared by different preparation

methods. In the first sample (Ni/K-βAl2O3/Au, denoted as “N”), a coating of

a commercial Ni ink (Fuel Cell Material ref. 233001) was applied on the

electrolyte followed by calcination at 800 ºC for 2 h (heating ramp of 5

ºC/min). In the second sample (Ni-α-Al2O3/K-βAl2O3/Au, denoted as “NA”),

the catalyst film was prepared by mixing 40 mg of the Ni ink with 20 mg of

a commercial α-Al2O3, (Alfa Aesar) powder, and some ethylene glycol to

achieve an slurry with suitable viscosity. Then, the slurry deposited on the

K-βAl2O3 pellet was subjected to the same heat treatment as that

underwent by the previous electrode (800 ºC). In the third sample (Au-

Ni(30%)-α-Al2O3/K-βAl2O3/Au, denoted as “GNA”), 40 mg of the Au ink and

ethylene glycol were mixed with 20 mg of α-Al2O3 powder which was

previously impregnated with Ni (30 % in weight) through a conventional

impregnation method. The α-Al2O3 powder was impregnated with a

Ni(NO3)2·6H2O (Panreac) precursor water solution in a glass vessel under

vacuum at 90 ºC to yield 30 % weight of Ni. After drying over night at 120

ºC, the resulting powder was calcined at 450 ºC (heating ramp of 5ºC / min)

for 1 h. The slurry of 30% Ni/αAl2O3 + Au ink was deposited on the solid

electrolyte and annealed at 800 ºC for 2 h. In this way, the catalytic

contribution of the Ni dispersed particles on alumina support was isolated

for its catalytic evaluation due to the negligible activity of the Au particles

Chapter 4

146

coming from the ink. Table 4.2 summarizes the description and

nomenclature of each electrochemical catalyst along with the total Ni

weight, particle size and active surface area of all the catalyst electrodes.

Ta

ble

4.2

. S

um

mary

of

the d

iffe

ren

t N

i base

d e

lect

roch

em

ical

cata

lyst

NG /

Act

ive

mol

of

Ni

9.2

8·1

0-6

3.4

3·1

0-6

2.4

5·1

0-6

dm

eta

l aft

er

react

ion

/

nm

39.9

37.5

35.2

Tota

l N

i

weig

ht

/ m

g

17.8

0

6.1

7

4.1

3

Pow

der

ad

dit

ion

to

the s

lurr

y

-

αA

l 2O

3

30.7

3 %

Ni-

αA

l 2O

3

Meta

l

past

e

Ni

Ni

Ni

Desc

rip

tion

Ni-

K-β

Al 2

O3 /A

u

Ni-

αA

l 2O

3 /K

-βA

l 2O

3 /A

u

Au

-Ni(

30%

)-αA

l 2O

3 /K

-βA

l 2O

3 /A

u

Nom

en

clatu

re

N

NA

GN

A


147

The active surface area of the catalysts, NG, was calculated in each case

by means of the total amount of deposited mol of metal and the particle

diameter and dispersion values were estimated from the (111) peak via

Scherrer equation. It can be observed that the lower the amount of Ni

used, the lower the particle size. Additionally, blank experiments were

carried out on a pure gold electrode (Au-K-βAl2O3/Au) and a gold paste

electrode mixed with a non-impregnated alumina powder, (Au-α-Al2O3/K-

βAl2O3/Au) to confirm its negligible activity for the CO2 hydrogenation

reaction.


The different electrochemical catalysts were firstly reduced by an in-

situ temperature programmed reduction experiment, (TPR) under a H2

stream of 3 % (N2 balanced) with an overall flow rate of 6 NL·h-1 under

temperature programmed conditions (2ºC / min) from room temperature to

350 ºC. During the TPR, the H2 consumption was continuously monitored

by a thermal conductivity detector, as well as the in-plane Ni film surface

electrical resistance measured by a digital multimeter between two points

separated by 1 cm.

After reduction, the three Ni-based catalyst were characterized by X-

Ray Diffraction (XRD) analysis with a Philips PW-1710 instrument using

Ni-filtered Cu Kα radiation (λ = 1.5404 Å). Diffractograms were compared

with the JCPDS-ICDD references. All the catalysts were also characterized

after the catalytic activity experiments to evaluate their stability under

the studied reaction conditions. The morphology of the different catalyst

films was also evaluated by using a Quanta 250 scan electron microscopy

(SEM). This instrument is connected to an EDAX Apollo X (AMETEK),

which analyses the chemical compositions of the samples via X-Ray

Chapter 4

148

analyses (EDAX). The Ni metal loading in the impregnated α-Al2O3 was

determined by atomic absorption spectrophotometry, using a SPECTRA

220FS analyser. As shown on Table 4.2 the total amount of Ni on the

powder catalyst was 30.73 % in weight.


4.3.1. Influence of the preparation method of the catalyst film

Figure 4.1 shows the variation of the surface electrical resistance of the

Ni catalyst film and the H2 consumption rate during the temperature

programmed reaction experiment (TPR) for sample N. During the TPR

experiments, no significant differences were appreciated between the two

electrochemical catalyst based on the Ni catalyst film (N, and NA). For

sample N, it can be observed that at the beginning of the experiment, the

as-deposited Ni catalyst film showed a very high value of the electrical

resistance (1.25 x 108 Ω). It seems to indicate that the catalyst preparation

method (involving calcination at high temperature under air atmosphere)

led to a nickel film on its oxidised state (as will be shown later by XRD).

However, during the TPR experiment the electrical resistance decreased

and finally stabilized at only 0.3 Ω after 1 hour at 350 ºC indicating the

complete reduction of the catalyst film to metallic Nickel (as will be shown

later by XRD), in agreement with previous works with Ni catalysts [25-27].

This reduction process can be confirmed by the H2 consumption peak

which started at around 250 ºC. The initial decrease in the electrical

resistance of the Ni catalyst film with temperature (25-125 ºC) with no H2

consumption could be attributed to the semiconducting properties of the

NiO [28]. On the other hand, the appearance of a local maximum value

around 300 ºC could be explained by the partial formation of different


149

K-βAl2O3

Ni

Au

Ω

Ni

β-Al2O3

Au

280

320

360

Tem

pera

ture

/ º

C

140 150 160 1700

5

10

15

20

Time / min

Resi

stan

ce /M

0

50

100

150

200

250

300

350

400

0 30 60 90 120 150 180 2100

50

100

150

200

250

300

350

Tem

pera

ture

/ º

C

0

20

40

60

80

100

120

140

Resi

stan

ce /

M

Tem

pera

ture

/ º

C

Time / min H

2 c

on

sum

pti

on

/a

.u.

a)

b)

intermediate Ni oxides of lower electrical conductivity. However, the

surface composition is unknown in the course of the TPR experiment.

Figure 4.1. Variation of Ni surface electrical resistance (a) and H2

consumption rate (b) during the TPR experiment for catalyst N. Inset of (a) shows

the final variation of Ni electrical resistance and temperature with time. Heating

rate of 2 ºC/min under a 3 % H2 stream (N2 balance).

Figure 4.2 shows the crystalline structure of the different Ni-based

catalysts, namely N (Figure 4.2.a), NA (Figure 4.2.b) and GNA (Figure

4.2.c), analysed by XRD both after the in-situ TPR up to 350 ºC (fresh

Chapter 4

150

20 30 40 50 60 70 80 90

-Al2O

3

b)

Inte

nsi

ty /

a.u

.

2º

Fresh

catalyst

Used

catalyst

-Al2O

3

Ni

(11

1)

(11

1)

(20

0)

(20

0)

(22

0)

(22

0)

NA

20 30 40 50 60 70 80 90

-Al2O

3

(00

6)

NiO2

(22

2)

(31

1)

(22

0)(20

0)

NiOBefore

reduction

a)

Inte

nsi

ty /

a.u

.

2/ º

Fresh

catalyst

Used

catalyst(1

11

)

Ni

(20

0)

(22

0)

(11

1)

(20

0)

(22

0)

N

20 30 40 50 60 70 80 90

2/ º

Inte

nsit

y / a

. u

.

Used

catalyst

c)

Fresh

catalyst

(111)

(220)

Au

Ni

(100)

(22

0)

GNA

(20

0)

(200)

-Al2O

3

-Al2O

3

catalyst, top spectra) and after exposure to all the studied reaction

conditions (used catalyst, bottom spectrum). For the case of sample N, the

XRD analysis before reduction (TPR) is also included in Figure 4.2a.

Figure 4.2. XRD analysis patterns of catalysts (a) N, (b) NA and (c) GNA before

and after exposure to reaction conditions. For catalyst N (a), the XRD analysis

patterns before reduction is also included.


151

In this latter case it can be observed that the Ni catalyst film was mainly on

its oxidized state (NiO and NiO2, JCPDS, 78-0643 and JCPDS, 85-1977,

respectively), which is in agreement with the electrical resistance

measurements during TPR of Figure 4.1. However, the Ni catalyst films were

completely reduced during the TPR experiments since no patterns of nickel

oxides appeared in any of the fresh catalysts (after reduction and before

activity measurements). In all cases, the main diffraction peaks (111),

(200) and (220) which appeared at 2θ = 44.5º, 51.8º and 76.4º, respectively,

were associated with metallic nickel and exhibited a face-centered cubic

(FCC) crystalline structure (JCPDS, 87-0712). Diffraction peaks related to

the K-βAl2O3 solid electrolyte (JCPDS, 02-0921) were found in all the

spectra, and samples NA and GNA (Figures 4.2b and 2c, respectively) also

exhibited the peaks corresponding to α-Al2O3 (JCPDS, 01-1296). In

addition, reflections appearing at 38º, 44º, 64º and 82 º in Figure 4.2c

corresponded, respectively, to the (111), (200), (220) and (222) planes of

metallic gold (JCPDS, 01-1172) in sample GNA. Remarkably, the reduced

state of the Ni catalysts remained during catalytic experiments due to the

presence of H2 in the feed. Additionally, the intensity of the Ni XRD peaks

(111, 200, and 220) in used samples of catalyst N and NA (Figure 2.a and

2.b, respectively) slightly increased in relation to those obtained in the

fresh samples, as observed in post-reaction XRD spectra (denoted in all the

cases as used catalyst). However, the particle size of Ni estimated by

Scherrer equation before and after the experiments was practically the

same in all cases. This denoted the stability of the catalyst under

experimental conditions, also supported by the good reproducibility

observed in the catalytic performance under every application of the

reference estate (+2V), as will be shown later.

Chapter 4

152

As can be observed, a Ni particle size of 39.9 nm was estimated in

catalyst N (Table 4.2), which decreased in catalysts NA and GNA. These

values were of the same order than those reported in other EPOC studies

with Ni catalysts prepared by different techniques [11, 20] and should

imply a high catalytic activity [1, 19]. Aside from the approximate

character of this calculation, the addition of the alumina powder to the Ni

and Au pastes slightly decreased the metal particle size and very likely

increased the Ni particles dispersion. On the other hand, a very slight

increase in the particle size was observed in all cases after time on stream

(particle size before reaction of 38.9, 36.5 and 34.7 for catalysts N, NA and

GNA, respectively), which denoted a minimal thermal sintering effect

under the studied reaction conditions.

SEM micrographs and EDAX analysis of certain regions of the different

Ni catalyst-electrodes obtained after exposure to reaction conditions are

shown in Figure 4.3. It can be observed that all samples showed to be

porous, which facilitated the reactants and products diffusion. However, it

can also be observed that the catalyst preparation method (by the addition

of a powder to the ink) markedly influenced its surface composition and

morphology. From Figures 4.3.a1 and 4.3.a2 (sample N), the obtained Ni

film seems to resemble a typical homogeneous foam structure with big Ni

agglomerates ranging from 6.0 to 7.0 µm [25, 29]. However, two

components can be clearly distinguished in sample NA (Figures 4.3.b1 and

3.3.b2); Ni agglomerates with similar morphology than those present in

sample N (Figure 4.3.a2) and larger size particles with a smooth surface

which corresponded with the α-Al2O3 powder.


153

50 µm

b3) b4)

100 µm

100 µm 50 µm

a1) a2)

b1) b2)

c2)

Al Ni

50 µm

c1)

100 µm

Figure 4.3. SEM micrographs of catalysts (a) N, (b) NA and (c) GNA after

exposure to reaction conditions. EDAX mapping is also included for catalysts NA.

These particles of α-Al2O3 were round shaped with a typical size

distribution ranging between 30 and 65 µm. Moreover, an elemental

Chapter 4

154

mapping of Figure 4.3.b2 was performed as shown in Figure 4.3.b3 and

3.3.b4 corresponding to Al and Ni, respectively. It was clear that the

previously mentioned largest particles were composed of Al (in green,

Figure 4.3.b3) and O (not shown), thus corresponding with the α-Al2O3

powder, while the smallest agglomerates mainly contained Ni (in blue,

Figure 4.3.b4). It can also be observed that sample NA exhibited a larger

roughness (porosity) and a higher number of valleys and holes in

comparison with sample N due to the presence of the heterogeneous

particles of α-Al2O3. Hence, it is very likely that the incorporation of α-

Al2O3 powder allowed to re-disperse the Ni particles coming from the ink,

leading to an increase of the catalytic surface area as already shown in a

previous study with Pt ink [30] and demonstrated by an increase in the

metal dispersion (3%, 3.4% and 3.5% for catalyst, N, NA and GNA,

respectively. Finally, sample GNA (Figures 4.3.c1 and 4.3.c2) showed a

similar porous structure than sample NA formed by the Au particles

coming from the Au ink and the α-Al2O3 powder.

In order to select the most suitable Ni catalyst preparation method for

CO2 hydrogenation via EPOC, all samples (N, NA and GNA) were tested

under certain reaction conditions and potentials (UWR) ranging from +2 to -

2 V. Figure 4.4 shows the dynamic response of the reaction rates for CO2

consumption, CO and CH4 formation for each catalyst (N, NA and GNA) and

the electrical current obtained upon the catalyst potential variation for catalyst

N. During the measurement of transient currents, no significant differences

were appreciated between the three electrochemical catalyst (N, NA and

GNA).

All reported rate values were normalized by the total mol of deposited

Ni. These experiments were carried out at 240 ºC with a feed composition


155

of H2/CO2 = 30 %/1.5 % (N2 balance) and an overall flow rate of 6 NL h-1.

The electropromotional behavior was the same as that of samples N and

NA (no electro-promotional effect was observed for sample GNA). Firstly,

the application of a positive potential UWR = + 2 V (reference state) at the

beginning and at the end of each experiment (as well as before each

negative polarization) ensured the removal of K+ ions from the catalyst

surface, which had previously migrated to the catalyst surface by thermal

migration [3, 31]. Thus, as shown in Figure 4.4.d, under an application of a

constant voltage, the generated current decreased with time to values close

to zero indicating that a steady state coverage of potassium was achieved

after certain time. In this way, a reference state was obtained and the

reversibility of the electropromoted effect was checked. At the reference

state, CO and CH4 were already produced by means of the reverse water-

gas shift (eq. 2) and CO2 methanation (eq. 3) reactions, respectively. Then,

the subsequent decrease in the applied catalyst potential increased the

amount of K+ ions that migrated electrochemically from the K-βAl2O3

pellet to the catalyst surface (promoted state), according to the obtained

negative current (Figure 4.4.d). As a consequence, the overall reaction rate

of CO2 increased as well as the production rate of CO while the production

rate of CH4 decreased for the case of samples N and NA. According to

previous EPOC studies on CO2 hydrogenation [6, 18, 19, 21], the RWGS

reaction is electrophilic, i.e., the CO production rate increases with

decreasing potential (with increasing K+ coverage). At the same time, an

electrophobic behaviour for the methanation reaction was typically

observed, i.e., the CH4 production rate increased with increasing potential

and decreasing the amount of K+ promoter supplied.

Chapter 4

156

0.0

0.3

0.6

0.9

1.2

0.0

0.2

0.4

0.6

0.8

1.0

0 20 40 60 80 100 120 140 160 180 200

0.00

0.05

0.10

0.15

0.20

0.25

0.30

0.35

UW

R /

V U

WR /

V

GNA NA N

UW

R /

V

(rC

O2

/ m

ol C

O2·m

ol-1

Ni·

s-1)x

10

3

a)

-2

-1

0

1

2

b)

(rC

O/

mol C

O·m

ol-1

Ni·

s-1)x

10

3

-2

-1

0

1

2

Time / min

c)

(rC

H4

/ m

ol C

H4·m

ol-1

Ni·

s-1)x

10

3

-2

-1

0

1

2

Figure 4.4. Influence of the applied potential vs. time on the reaction rate values

of (a) CO2 consumption, (b) CO production and (c) CH4 production with the

different catalysts (N, NA and GNA) and obtained electric current (d) vs. time to

step changes in the applied catalyst potential UWR for catalyst N. Reaction

conditions: H2/CO2 = 30 %/1.5 % (N2 balance), FT = 6 NL·h-1, T = 240 ºC.


157

The obtained results with samples N and NA were consistent with the

general rules of chemical and electrochemical promotion [17]. In these two

samples the migration of K+ onto the catalyst-working electrode would

weaken the nickel chemical bond with the electron donor adsorbates

(hydrogen) and strengthen that with the electron acceptors (CO2) [2, 3, 6,

18, 24]. The increase in the binding strength of CO2 on the Ni surface

would favour the dissociative adsorption of CO2 through RWGS reaction

rather than CO2 methanation. Hence, the presence of K+ ions on the

catalyst-film led to a strong promotional effect in the CO2 hydrogenation

mechanism toward CO formation. This effect has also been reported in

several previous studies on chemical and electrochemical promotion with

K+ ions on different metal catalysts [12, 19, 21, 32, 33]. Moreover, it should

be noted that for both samples: N and NA, after each polarization at +2 V

the electropromotional effect showed to be fully reversible and reproducible

in terms of catalytic activity. It demonstrates the great advantage of the

EPOC effect compared to the conventional chemical promotion by

providing full control and optimization of the promoted state of the

catalyst in the course of the reaction [13].

On the other hand, it is interesting to note that sample NA showed the

higher catalytic activity in terms of reaction rate normalized per amount of

deposited Ni for all the applied catalyst potentials between +2 and -2 V.

This catalytic enhancement derived from the powder addition can be

explained considering the increase in the Ni ink porosity, as mentioned

above. Catalyst NA was globally found to be the most active catalyst.

Regarding the catalytic results obtained with catalyst GNA, it must be

taken into account that both the pure gold electrode and the gold paste

mixed with non-impregnated alumina powder showed to be completely

inactive under all the studied reaction conditions (and negligible electro-

Chapter 4

158

promotional effect). Hence, the residual catalytic activity of catalyst GNA

can be attributed to the presence of Ni particles impregnated on the

powder since negligible activity was observed on a Au film prepared with

inert α-Al2O3 prepared for blank experiments (not shown). Hence, the

obtained results in Figure 4.4 would confirm the absence of any electro-

promotional effect related to Ni particles dispersed on the α-Al2O3. It can

be explained considering the absence of conduction pathways of K+ ions

from the solid electrolyte to achieve the Ni particles dispersed on the α-

Al2O3 support. Summarizing, Ni metal particles provided by the deposited

metal ink were the active catalyst (not the impregnated Ni particles) which

were also electrochemically promoted for the CO2 hydrogenation reaction.

According to all these previous results, all the electrochemical promotions

experiments in the next section were performed with catalyst NA.

4.3.2. Kinetic study and electrochemical promotion experiments

Figure 4.5 shows the effect of gas flow rate, FT, on the CO2 reaction rate,

CO2 conversion and selectivity to CO and CH4 at the reference state, UWR =

2 V, and different feed compositions. The reference catalytic rate initially

increased with flow rate and reached a plateau at high values (FT > 75

Nml·min-1). This indicates the absence of any mass transfer limitation

phenomena above this flow rate. Hence, all the catalytic experiments

hereafter were carried out at 100 Nml·min-1. As expected, the CO2

conversion values decreased continuously upon increasing the flow rate.

Very interestingly, the CH4 selectivity also followed a negative trend.


159

20

25

30

35

40

45

50

55

60

0 20 40 60 80 100 120 140 160 180 200 220

3

4

5

6

7 H2 / CO2 = 10

H2 / CO2 = 20

H2 / CO2 = 2

H2 / CO2 = 6

C

H4 s

ele

ctiv

ity /

%

20

25

30

35

40

45

50

55

60

T = 240 ºC

CO

sele

ctiv

ity /

%

a)

b)( r C

O2 /m

ol

CO

2·m

ol-1

Ni

· s-1

) x

105

FT / NmL min-1

0.0

0.5

1.0

1.5

2.0

2.5

CO

2 c

on

vers

ion

/ %

Figure 4.5. Effect of the overall gas flow rate on the (a) CO2 consumption rate and

CO2 conversion, (b) CH4 and CO selectivity under +2 V polarization for catalyst

NA. Reaction conditions: H2/CO2 = 30 %/1.5 % (N2 balance), T = 240 ºC.

This seems to suggest that CH4 was produced, to a large extent, from

the CO hydrogenation (eq. 10), as a consecutive step of the RWGS reaction,

rather that from the direct CO2 methanation reaction (4.3).

CO + 3H2 → CH4 + H2O (4.4)

In fact, concerning the kinetics of the reactions, reaction rate of CO2

methanation is generally considered to be slower than that of CO one [34].

Furthermore, it can also be observed that an increase in the hydrogen

Chapter 4

160

concentration led to higher CO2 conversion and CH4 selectivity, which will

be better discussed below.

Figures 4.6 and 4.7 show the steady state effect of applied potential on

reaction rates of CO2, CO and CH4 at 240 ºC under different H2 feed

composition, H2 = 3 – 30 %, (Figure 4.6) and CO2 feed composition, CO2 =

1.5 – 10 % (Figure 4.7). In all cases, a cathodic (negative) polarization

increased the reaction rate of the CO production (electrophilic behaviour)

while an anodic (positive) polarization enhanced the reaction rate of CH4

formation (electrophobic behaviour). Since the potential-induced change in

CO production rate was more pronounced for all the experiments than that

in CH4 one, the overall reaction rate of CO2 was found to increase at

negative polarization (overally electrophilic behaviour). Besides, very

useful information on the reaction kinetics at the reference state can be

drawn from these two figures, which can be then correlated with the

electrochemical promotion behaviours observed in these experiments. In

the absence of K+ promoter (positive polarizations), the rate of CO

formation exhibited a zero to negative order with respect to H2

concentration (Figure 4.6) and a marked positive order with respect to the

CO2 concentration (Figure 4.7). On the contrary, the rate of CH4 formation

showed a positive order with respect to H2 and a negative to zero order

with respect to CO2. As a direct consequence, the CO2 reaction rate was

positive order with respect to both H2 and CO2.


161

4

6

8

10

12

2

4

6

8

10

-2 -1 0 1 20.0

0.5

1.0

1.5

2.0

2.5

3.0

H2 = 3 %

H2 = 9 %

H2 = 15 %

H2 = 30 %

(rC

O2

/ m

ol

CO

2·

mol-1

Ni

·s-1

)x1

05

c)

b)

(rC

O /

mol

CO

· m

ol-1

Ni

·s-1

)x1

05

Increasing K+

T = 240 ºC

100 Nml min-1

(rC

H4

/ m

ol

CH

4·

mol-1

Ni

·s-1

)x1

05

UWR / V

CO2 = 1.5 %a)

Figure 4.6. Effect of the H2 concentration on the reaction rate values of (a) CO2

consumption, (b) CO production and (c) CH4 production under different applied

potentials (UWR) for catalyst NA. Reaction conditions: CO2 = 1.5 % (N2 balance), FT

= 6 NL·h-1, T = 240 ºC.

Chapter 4

162

0

5

10

15

20

-2 -1 0 1 20

1

2

3

5

10

15

20

25

c)

b)

(rC

O /

mol

CO

· m

ol-1

Ni·

s-1)x

10

5

CO2 = 1.5 %

CO2 = 3 %

CO2 = 5 %

CO2 = 10 %

UWR / V

(rC

H4

/ m

ol

CH

4·

mol-1

Ni·

s-1)x

10

5

H2 = 30 %

T = 240 ºC

100 NmL min-1

(rC

O2

/ m

ol

CO

2·

mol-1

Ni·

s-1)x

10

5

a)

Increasing K+

Figure 4.7. Effect of the CO2 concentration on the reaction rate values of (a) CO2

consumption, (b) CO production and (c) CH4 production under different applied

potentials (UWR) for catalyst NA. Reaction conditions: H2 = 30 % (N2 balance), FT =

6 NL·h-1, T = 240 ºC.


163

Similar trends were observed in other EPOC studies on CO2

hydrogenation [6, 18]. Hence, in view of these results, one could easily

explain the EPOC behaviour found upon by varying the catalyst potential.

As mentioned above, CO production through RWGS was favoured by

increasing CO2 feed concentration, which is equivalent to increase the CO2

chemisorption strength on the Ni surface via alkali electrochemical

promotion. On the other hand, since methanation reaction under reference

state was already inhibited by increasing the CO2 feed concentration, it is

expected that the potassium backspillover at negative potentials and the

consequent strengthening of the CO2 chemisorption also derived in a

decreasing of the methane formation rate. Hence, RWGS is an electrophilic

reaction, i.e., it is positive order in the electron acceptor reactant (CO2) and

negative or zero order in the electron donor reactant (H2), while CO2

methanation is an electrophobic reaction, i.e., it is zero-to negative order in

the CO2 and positive order in the H2 [18, 21]. Then, it is clear that the

EPOC phenomena allows to in-situ control the competitive chemisorption

of the different adsorbates on the catalyst surface, which may allow to

control the selectivity requirements as will be shown later.

Figure 4.8 depicts the effect of temperature on the reaction rates of CO2,

CO and CH4 with a feed composition of H2/CO2 = 30 %/1.5 % (N2 balance).

At the reference state (UWR = 2 V) and electropromoted (UWR < 2 V)

conditions, it can be observed that the reaction rate of CO2, as well as also

the production rate of CO and CH4, were enhanced at increasing

temperatures. A similar positive effect of temperature was also observed in

other EPOC studies with different metal catalysts [11, 18, 22, 24].

Chapter 4

164

0

10

20

30

40

50

-2 -1 0 1 20

5

10

15

0

20

40

60

b)

(rC

O /

mol

CO

· m

ol-1

Ni·

s-1)x

10

5

T = 240 ºC

T = 270 ºC

T = 300 ºC

c) 100 Nml min-1

UWR

/V

(rC

H4 /

mol

CH

4·

mol-1

Ni·

s-1)x

10

5

H2 / CO

2 = 30% / 1.5 %

Increasing

(rC

O2 /

mol

CO

2·

mol-1

Ni·

s-1)x

10

5

a)

Figure 4.8. Effect of the temperature on the reaction rate values of (a) CO2

consumption, (b) CO production and (c) CH4 production rate under different

applied potentials (UWR) for catalyst NA. Reaction conditions: H2/CO2 = 30 %/1.5 %

(N2 balance), FT = 6 NL·h-1.


165

At all temperatures, the same EPOC behavior of the RWGS and

methanation reactions was found (previously discussed at 240 ºC), i.e.;

electrophilic CO and electrophobic CH4 productions, respectively.

Moreover, it is important to note that the Ni catalyst was activated via

electrochemical promotion in such a way that a very similar production

rate of CO was achieved under promoted conditions at 240 and 270 ºC as

observed in the absence of promoter at 270 and 300 ºC, respectively.

Hence, the application of an electric potential which corresponded with a

current of the order of few µA may cause a similar modification in the

catalytic performance as an increase in temperature of 30 ºC. It

demonstrates the great practical interest of the EPOC phenomenon in

view of energy savings by activating metal catalyst at lower temperatures

[13, 35, 36]. Finally, the apparent activation energy calculated via

Arrhenius plot under reference state (UWR = 2 V) was 74.26 kJ·mol-1 for

CO2 consumption. This value is in the same range as those reported in

other works. For instance, an activation energy of 40-80 kJ·mol-1 was

reported for the hydrogenation of CO2 over Ni catalyst [11]. Peebles et al.

obtained activation energies of 88.7 kJ mol-1 and 72.8-82.4 kJ mol-2 for the

methanation and dissociation of CO2 on Ni (100) for producing CH4 and

CO, respectively [37]. On the other hand, an activation energy of 62.14 kJ

mol-1 under electro-promoted conditions (-2 V) for CO2 consumptions was

obtained. The activation energy was reduced from 74.26 kJ·mol-1 (reference

state) to 62.14 kJ mol-1 (promotion conditions), which demonstrates the

positive effect of K+ ions on the kinetics of the process as already concluded

in other previous works of potassium-electropromotion [38, 39].

Chapter 4

166

0.8

1.2

1.6

2.0

2.4

1.0

1.5

2.0

2.5

3.0

3.5

-2 -1 0 1 2

0.2

0.4

0.6

0.8

1.0PI

K+=-6.78

PIK

+=98.38

PI+

K=286.14

PIK

+=206.21

PIK

+=30.75

PI+

K=129.15

a)

CO

2

T = 240 ºC

T = 270 ºC

T = 300 ºC

PIK

+=105.13

b)

C

OIncreasing

PIK

+=-63.68

PIK

+=-80.68

c)

C

H4

UWR

/ V

Figure 4.9 shows the variation of the reaction rate enhancement ratio

(ρi), calculated for CO2, CO and CH4 through eq. 4.4, with the applied

potential, under a feed composition of H2/CO2 = 30% / 1.5% (N2 balance)

and three different reaction temperatures.

Figure 4.9. Effect of the applied potential (UWR) on the rate enhancement ratio

(ρi) of (a) CO2, (b) CO and (c) CH4 at different temperatures (T = 240 ºC, 270 ºC

and 300 ºC), with H2/CO2 = 30 %/1.5 % (N2 balance) and FT = 6 NL·h-1 for catalyst

NA. The promotional index (PIK+) values are also depicted for an applied potential

of -0.5 V. Data obtained from the experimental results shown in Fig. 4.7.


167

The corresponding promotional indexes (PIK+, eq. 3.8) are also indicated

for an applied potential of -0.5 V. At all explored temperatures, a clear

electrophilic EPOC effect can be observed in the global CO2 reaction and

the CO production (ρ > 1, PIK+ > 0), due to the promotional effect K+ ions

promoters previously explained at negative potentials. The electrophobic

behavior of the CH4 formation rate was also verified (ρ < 1, PIK+ < 0).

Hence, CO showed the highest promotion index values as well as the most

pronounced increase in the reaction rate with respect to the reference

state. The higher the reaction temperature, the lower the value of the rate

enhancement ratios is, which was associated to the increase in the

reference reaction rates (r0). Regarding the promotional index, the

maximum values were obtained at 270 ºC, since it depended on the relative

increase of the catalytic rates and potassium coverages. This fact could be

attributed to the increase in the ionic conductivity of the solid electrolyte,

and hence of the potassium coverage at fixed potential, with the

temperature.

Finally, the influence of the applied potential on the steady-state

variation of the selectivity toward CO and CH4 at different temperatures

(T = 240, 270 and 300 ºC) with different feed composition (H2/CO2 = 2-20) is

shown in Figure 4.10. At all temperatures, the decrease in the applied

potential led to a strong increase in the CO selectivity (Figures 4.10.a2, b2

and c2) and a decrease of the CH4 selectivity (Figures 4.10.a1, b1 and c1).

For instance, with a H2/CO2 ratio of 20, the selectivity of the Nickel

catalyst toward CO production was enhanced up to more than 95 % with a

potassium coverage, θK+, between 0.019 and 0.066, depending on the

reaction temperature.

Chapter 4

168

0

10

20

30

40

50

-2 -1 0 1 2

60

70

80

90

100

T = 300 ºC

H2/CO

2 = 10

H2/CO

2 = 20

SC

H4

/%

H2/CO

2 = 2

H2/CO

2 = 6

SC

O/%

UWR / V

C1)

C2)0

10

20

30

40

50

-2 -1 0 1 2

50

60

70

80

90

100

T = 270 ºC

H2/CO

2 = 10

H2/CO

2 = 20

SC

H4

/%

H2/CO

2 = 2

H2/CO

2 = 6

SC

O/%

UWR / V

b1)

b2)0

10

20

30

40

50

60

-2 -1 0 1 2

40

50

60

70

80

90

T = 240 ºC

H2/CO

2 = 10

H2/CO

2 = 20

SC

H4

/%

H2/CO

2 = 2

H2/CO

2 = 6

SC

O/%

UWR / V

a1)

a2)

Figure 4.10. Effect of the applied potential (UWR) and H2/CO2 ratio feed

concentration on the selectivity of CH4 and CO, at (a) 240 ºC, (b) 270 ºC and (c) 300

ºC for catalyst NA. Reactions conditions: FT = 6 NL·h-1.

As reported in previous EPOC studies on CO2 hydrogenation [12, 19,

21, 23], the activity and selectivity of other metallic catalysts were also in-

situ modified by pumping alkali ions. A value of 75 % in methane

selectivity at T = 300 ºC (H2/CO2 = 7) was reported by Theleritis et al., [21].

In the present work, a value of 50 % was obtained at the reference state.

Although the selectivity value obtained in this study was lower, the main

advantage to be highlighted here is the use of a non-noble metal catalyst

(Ni). Moreover, CH4 production rate was increased against CO production

at higher H2 concentrations, in good agreement with the previous

discussion and other CO2 hydrogenation studies [6, 19, 22, 40]. The

decrease in temperature also enhanced the methanation selectivity, as

reported in other works related to conventional chemical promotion [40]

and electrochemical promotion [6, 11, 21]. Depending on both the applied


169

potential and the reactions conditions, one can control the Ni catalytic

activity and the selectivity toward CO and CH4 by means of the controlled

migration of K+ ions from a solid electrolyte.

4.4 Conclusions

Different Ni catalysts films were prepared on K-βAl2O3 by combining

the organometallic paste deposition and the addition of a powder, and

tested in the CO2 hydrogenation reaction. The addition of the α-Al2O3

powder to the Ni ink resulted in a slightly increase of the film porosity as

shown in SEM images which turned into a higher catalytic activity.

The Ni catalyst films were stable under the studied reaction conditions.

The Ni catalyst remained in their reduced state during the catalytic tests

and the Ni particle size was stable as confirmed by XRD analysis. This

suggested that the possible thermal sintering effect was negligible, leading

to a reversible EPOC behaviour between the different potentiostatic

transitions.

Both the activity and selectivity of the catalysts were in-situ modified by

the controlled electrochemical migration of K+ ions from the solid

electrolyte (electro-active catalyst support). The reverse water-gas shift

and methanation reactions presented electrophilic and electrophobic EPOC

behaviours, respectively, since negative polarizations promoted the

production of CO decreasing the CH4 production rate.

In good agreement with the obtained catalytic results and with the

rules of chemical and electrochemical promotion, the kinetics experiments

confirmed that CO production rate presented a positive order with respect

Chapter 4

170

to the electron acceptor reactant (CO2) and a negative to zero order with

respect to the electron donor (H2).

The Ni catalytic selectivity can be strongly modified and control by the

application of an electric potential. Depending on the reaction conditions,

CO and CH4 selectivity was enhanced up to more than 95 and 45 %

respectively via EPOC. Hence, one could in-situ control the preferential

formation of syngas or CH4 production rate via CO2 hydrogenation, which

may be of significant practical importance.

4.5. References

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5.1. Introduction

5.2. Experimental


5.2.2. Preparation of the

electrochemical catalyst



5.3.1. Characterization of the Cu

cathodic-catalyst and deposited

electrodes

5.3.2. Electrocatalytic experiments for

CO2 conversion

5.4. Conclusions

5.5. References

CHAPTER 5:

Gas Phase Electrocatalytic Conversion

of CO2 on Cu Carbon-based Catalyst-

Electrodes Toward Fuels

INTRODUCTION


RESULTS

T = 90 ºC, FCO2,cathode = 0.5 NmL·min-1,

FCO2,anode = 6 NmL·min-1.

The shift from fossil fuel based economy towards a renewable energy one is a central strategy for achieving

sustainability and energy efficiency in the chemical industry. Meanwhile, reducing CO2 emissions is the key to proceed

effectively in this direction.

In this sense, convert CO2 into useful low-carbon fuels seems to be attractive and promising solution due to the energy

demand. One of the most interesting technology is the electrocatalytic conversion of CO2

The reaction of CO2 conversion can be summarized according to the following scheme:

xCO2 + 2(2x – z + y/2)H+ + 2(2x – z + y/2)e- → CxHyOz + (2x – z)H2O

This process is based on a low temperature Proton Exchange Membrane (PEM) reactor configuration, consisting on a

membrane electrode assemble (MEA) formed by an Anode/Membrane/Cathode.

Electrochemical reactor

Influence of the applied current Effect of the temperature

H2/CO2 = 30 % /15 %, N2 balance, FT = 6 NL h-1

Then main aim of this study was to carry out the electroreduction of CO2 in gas-phase by using the H+ produced from steam electrolysis in order to convert CO2

into high added-value compounds for an industrial and environmental point.

TEM analysis

0

5

10

15

20

25

30

35

40

Cu-CNFCu-AC

I = -10 mA

I = -20 mA

I = -30 mA

(rC

O2

m

ol·

h-1·m

g-1C

u)·

10

2

Cu-G

Influence of the applied current

T = 90 ºC, FCO2,cathode = 0.5 NmL·min-1,


-The increase in the applied current led to a rise in

the CO2 consumption

- Catalytic activity: Cu-AC > Cu-CNF > Cu-G

Cu-G Cu-AC CuCNF

0

5

10

15

20

25

30

35

40

45

50

55

60

(rC

O2

m

ol·

h-1·m

g-1C

u)·

10

2

T = 80 ºC

T = 90 ºC

I = -20 mA, FCO2,cathode = 0.5 NmL·min-1,


-The reaction rate of CO2 increased

with increasing the temperature

Effect of the selectivity

Cu-CNF Cu-AC Cu-G

Cu-CNF Cu-AC Cu-G

Cu

c)

CNF

I = -10 mA I = -20 mA I = -30 mA

0

10

20

30

40

50

60

70

80

90

100c)b)

Acetaldehyde

Methyl formate

CH4

Sel

ecti

vit

y /

%

Methanol

Acetone

a)

I = -10 mA I = -20 mA I = -30 mA

0

10

20

30

40

50

60

70

80

90

100 Methanol

Acetone Acetaldehyde

Methyl formate

Sel

ecti

vit

y /

%

CH4

I = -10 mA I = -20 mA I = -30 mA

0

10

20

30

40

50

60

70

80

90

100

Methyl formate

2-Propanol

n-Propanol

Methanol

Ethanol CO

Acetaldehyde

Sel

ecti

vit

y /

%

CH4

CHAPTER 5. GAS PHASE ELECTROCATALYTIC

CONVERSION OF CO2 ON Cu CARBON-BASED

CATALYST-ELECTRODES TOWARD FUELS

CO2 feeding N2 purge

O.C.C. I = -20 mA O.C.C.

0 50 100 150 200 250 300 350 400

0

2

4

6

8

10

12

14

16

18

20

22

0 50 100 150 200 250 300 350

0

20

40

60

80

100

120

140

160

0 50 100 150 200 250 300 350

0

2

4

6

8

10

12

14

16

18

20

22

CH4

Acetaldehyde

Methyl formate

Acetone

Methanol

(r /

mm

ol·

h-1·m

g-1C

u)·

10

3

Time / min

a)

CH4

Acetaldehyde

Methyl formate

Acetone

Methanol

(r /

mm

ol·

h-1·m

g-1C

u)·

10

3

Time / min

b)

(r /

mm

ol·

h-1·m

g-1C

u)·

10

3

Time / min

CH4

CO

Acetaldehyde

Methyl formate

Methyl acetate

Methanol

Ethanol

2-propanol

n-Propanol

c)

e-e-I < 0

H+

MEA

(Membrane Electrode Assembly)

Cathodic Catalyst

Cu-G, Cu-AC, Cu-CNFAnodic Catalyst

IrO2

Membrane

Sterion®

Furnace

ThermocoupleAu wire Au wire

25 % H2O (N2)

Vent

CO2

GC analysis

CO2 + H+ + e- → Products H2O → 2H+ + 2e- + ½ O2

H+

H+

Gas phase electrocatalytic conversion of CO2 to syn-fuels on Cu based catalyst-electrodes

177

Abstract

A novel electrocatalytic system based on a low temperature proton

exchange membrane (Sterion®) was developed for the gas phase

electrocatalytic conversion of CO2. This configuration allows the

introduction of renewable energy in the chemical production chain via fuels

production from direct CO2 electro-reduction at atmospheric pressure and

low temperatures (below 90 ºC). For that purpose, three different Membrane

Electrode Assemblies (MEAs) based on three different Cu based cathodic-

catalyst were prepared and characterized: Cu-G/Sterion/IrO2, Cu-

AC/Sterion/IrO2 and Cu-CNF/Sterion/IrO2; graphite (G), activated

carbon (AC) and carbon nanofibers (CNF). Thus, H2O was fed and

electrolyzed on the IrO2 anode of the cell, thereby supplying H+ across the

membrane to react with CO2 in the cathodic-catalyst and leading to the

production of a mixture of syn-fuels (syn-gas, methanol, acetaldehyde,

methane…). Remarkably, the nature of the cathodic-catalyst carbon support

had a strong influence on the electrocatalytic activity of the system, being

the surface area of the carbon support the most important parameter.

Hence, the Cu-AC-based cathodic-catalyst showed the highest CO2

electrocatalytic activity, due to the highest surface area of the AC support

and the larger metal dispersion of the Cu particles leading to acetaldehyde

and methanol as the main reaction products. Besides, the lower

conductivity of the AC support the Cu-AC cathodic catalyst also required

the lowest energy consumption for the electrocatalytic conversion of CO2.

A

Chapter 5

178

5.1. Introduction

As seen in previous chapter (Chapter 4), two main technologies have

been proposed to reduce CO2 emissions: (i) capture and geological

sequestration of CO2 [1] and (ii) conversion into useful low-chain carbon

fuels [2]. Sequestration still has certain barriers that make it unaffordable

from an industrial point of view, such as the high cost of CO2 capture,

separation, purification and transportation. On the other hand, conversion

into fuels seems to be a more attractive and promising solution that can

meet the growing energy demands. The chemical conversion of CO2 can be

effectively performed via hydrogenation reactions [3-5]. This conversion can

be achieved by chemical [6, 7], photocatalytic [8], electrocatalytic [4, 9-12],

biological [13] and reforming [14]. Among them, the electrochemical

pathway has been recognized as an efficient way to convert CO2 to energy-

rich products. The process possesses several advantages, namely: (i)

control of the process by electrode potentials and reaction temperature; (ii)

the supporting electrolytes can be fully recycled so that the overall

chemical consumption can be minimized to simply water or wastewater;

(iii) the electrochemical reaction systems are compact, modular, on-

demand, and easy for scale-up applications; (iv) the electricity used to

drive the process can be potentially obtained from a renewable source; (v)

no external H2 is required for the CO2 reduction process as H+ are in situ

generated within the process. Hence, the valorization of the CO2 molecule

by electrochemical reduction has attracted worldwide interest due to its

potential environmental and economic benefits [2, 15-17]. This technology,

when coupled to a renewable energy source such as solar and wind, could

generate carbon neutral fuels or high added-value chemicals that are

conventionally derived from petroleum at a competitive price. As a matter


179

of fact, the electrochemical reduction of CO2 using a liquid electrolyte,

either aqueous or organic, is being actively investigated in literature [18,

19]. However, the main drawback of these processes is the recovery of the

reaction product from the liquid electrolyte as the energy required to

separate the products is higher than the energy stored in the produced

molecules [10]. In this sense, the gas phase electrocatalytic conversion of

CO2 to liquid fuels allows easy product separation since there are no

problems of solubility of CO2 as in the case of liquid phase/electrolytes and

no needs to recover the products from a liquid phase. Thus, gas-phase

electroreduction of CO2, mainly developed by the group of Prof. Centi [4, 9-

12], represents a valuable opportunity to incorporate renewable energy

into the value chain of chemical industries. In this regard, the obtained

products contain a higher energy density and are easier to transport and

store.

The gas phase electrocatalytic conversion of CO2 is based on the use of a

low temperature Proton Exchange Membrane (PEM) reactor configuration,

consisting on a membrane electrode assembly (MEA) formed by an

Anode/Membrane/Cathode system. Water is electrolyzed at the anode of

the cell leading to the formation of O2 and H+ which are electrochemically

supplied across the membrane to the cathodic-catalyst where they react

with the adsorbed CO2, leading to the formation of different molecules

according to the following general reaction:

xCO2 + 2(2x – z + y/2)H+ + 2(2x – z + y/2)e- → CxHyOz + (2x – z)H2O (5.1)

Hence, the influence of the cathodic-catalyst may have strong

importance on the resultant electrocatalytic activity of the system.

Chapter 5

180

In contrast to previous studies [4, 9-12], this work reports for the first

time a systematic study based on three different Cu cathodic-catalysts

prepared on three different carbon supports: graphite (G), activated carbon

(AC) and carbon nanofibers (CNF). The use of carbon materials have

proven to be the best catalyst supports for such applications due to their

specific properties, such as acid and base resistance, porosity, conductivity

and the possibility of recovering the metals by combustion of the supports

[20]. As a result, carbon materials have been used as conductive substrates

for metal nanoparticles in electrocatalysis for the conversion of CO2 [4, 10,

11]. The use of carbon-based electrocatalysts, e.g. similar to those used in

PEM fuel cells, is critical to obtain good performances and control the

selectivity in CO2 conversion. In this sense, carbon support plays multiples

roles in these types of systems by allowing a good dispersion of metal

nanoparticles and especially facilitates the effectiveness of electrons and

protons transport due to a better distribution of the metal nanoparticles [4].

Hence, in this work the role of different carbon supports as well as the

influence of the applied current and reaction temperature on the catalytic

activity and selectivity of the gas phase electrocatalytic conversion of CO2

have been studied.

5.2. Experimental


The catalytic activity measurements were carried out in an

experimental set-up as shown in Figure 5.1. The electrocatalytic

experiments were carried out at atmospheric pressure with an overall gas

flow rate of 0.5 NmL min-1 of CO2 for the cathode and 6 NmL min-1 for the

anodic stream (60 % H2O/N2), at different temperatures (T = 80 ºC and 90


181

FIC

FIC

FIC

CO2

N2

N2

TIC

Vent

Water

saturator

Reactor

Potentiostat-

Galvanostat

micro GC

Temperature

Controller

Vent

Flow

controller

Furnace

ºC, optimum values for the operation of the Sterion® membrane). Reactant

and products from the cathodic chamber of the cell were analyzed by using

a double channel gas chromatograph (Bruker 450-GC) equipped with

Hayesep Q-Molsieve 13X consecutive columns and flame ionization

detectors. The detected reaction products were a mixture of syn-fuels: syn-

gas (H2, CO), CH4, CO, methanol, acetaldehyde, acetone, methyl formate,

ethanol, 2-propanol and n-propanol. The error in the carbon atom balance

did not exceed 5 %. A potentiostat/galvanostat (Voltalab 21, Radiometer

Analytical) was used to supply a constant current (-10 to -30 mA) between

the electrodes which were connected using gold wires. The potentiostat-

galvanostat was also used to perform galvanostatic voltammetry

measurements under different reaction conditions.

Figure 5.1. Scheme of the experimental set-up for the CO2 electrochemical

conversion.

Chapter 5

182

e-e-I < 0

H+

MEA

(Membrane Electrode Assembly)

Cathodic Catalyst

Cu-G, Cu-AC, Cu-CNFAnodic Catalyst

IrO2

Membrane

Sterion®

Furnace

ThermocoupleAu wire Au wire

25 % H2O (N2)

Vent

CO2

GC analysis

CO2 + H+ + e- → Products H2O → 2H+ + 2e- + ½ O2

H+

H+

The electrocatalytic experiments of CO2 electro-reduction were carried

out in a lab-scale continuous electrocatalytic reactor operating at

atmospheric pressure. Figure 5.2 shows a schematic drawn of the

electrocatalytic reactor.

The cell reactor was made of two quartz tubes that act as cathodic and

anodic compartments, and included two inlets, for CO2 and H2O/N2,

respectively, and two outlets streams. The system was heated with a

furnace up to 90 ºC connected to a K-type thermocouple and a temperature

control system.

Figure 5.2. Schematic drawn of the electrochemical reactor used for the CO2

electrochemical conversion.

Water was introduced into the anode side of the cell by flowing N2

through a saturator in order to achieve liquid/vapor equilibrium. The

water content in the anodic chamber of reaction mixture (25 % H2O/N2)

was controlled by the vapor pressure of water at the temperature of the

saturator (65 ºC). All lines placed downstream from the saturator were

heated above 100 ºC to prevent condensation. In this side, the electrolysis

of water was produced with the IrO2 electrode in order to produce protons


183

across the Sterion® membrane. Furthermore, the fed water stream was

also used to hydrate the Sterion® membrane and keep its proton

conductivity properties [4]. The cathodic part of the cell operates in contact

with a gas flow of pure CO2 (Praxair, Inc. certified standards 99.999 %

purity). Both gas flow rates (N2 for the anode and CO2 for the cathode)

were controlled by a set of mass flowmeters (Brooks 5850 E and 5850 S).

The selectivity towards each compound was calculated by the following

equation:

𝑋𝑖 𝑠𝑒𝑙𝑒𝑐𝑡𝑖𝑣𝑖𝑡𝑦 / % = 𝐹𝑋𝑖

𝐹𝐶𝑂2−0 𝐹𝐶𝑂2

𝑥 100 (5.2)


Copper catalysts supported on G, AC and functionalized CNF were used

as cathode materials of the electrocatalytic reactor. Iridium (IV) oxide

(IrO2) was used as the anode.

Commercial graphite (Aldrich), commercial active carbon (Panreac) and

synthesized functionalized carbon nanofibers were used as starting

materials. Carbon nanofibers were prepared by the catalytic chemical

vapor deposition method (CVD) in a fixed-bed reactor at atmospheric

pressure. The synthesis was conducted over a Ni/SiO2 (10%, w/w) catalyst

at 600 ºC to obtain fishbone type carbon nanofibers, employing ethylene as

the carbon source and hydrogen as the carrier gas (C2H4/H2, 4/1, v/v, 900

Ncm3 min-1). The carbon deposit obtained was demineralized using HF

(48 %, v/v) in order to remove the parent catalyst particles and to avoid

any residual Ni effect in later catalysts preparation/characterization steps.

Chapter 5

184

The material was dried for 12 h at 383 K in air to remove water prior to

characterization. Further details regarding the CNF synthesis are given in

a previous work [21]. The functionalization of the CNFs was performed by

an oxidative treatment in HNO3 to introduce oxygen functionalities on the

carbon surface. Metal nanoparticles were later deposited on the graphite,

active carbon and functionalized carbon nanofibers by the impregnation

method. The different supports (G, AC and CNF) were placed in a glass

vessel and kept under vacuum at room temperature for 2 h to remove

water and other compounds adsorbed on the structure. A known volume of

ethanolic solution of Cu(NO3)2·3H2O (Panreac) (the minimum amount

required to wet the solid) was then poured over the sample. After 2 h, the

solvent was removed by evaporation under vacuum at 90 ºC in a rotary

evaporator. The catalysts were dried at 120 ºC overnight, calcined in N2

atmosphere at 350 ºC using a heating ramp of 5ºC/min and kept at that

temperature for 4 h. Finally, they were reduced in H2 at 350ºC for 2 h

(heating rate 5 ºC· min-1). The total load of metal was around 50 wt.%.

The catalyst inks for the preparation of each electrode were prepared by

mixing appropriate amounts of the catalysts, IrO2 commercial catalysts

powders (Alfa Aesar, 99 %), Cu-graphite powder, Cu-activated-carbon

powder and Cu-carbon nanofibers powder with a Nafion solution (5 wt.%,

Aldrich chemistry, Nafion® 117 solution) and isopropanol (Sigma Aldrich)

with a binder/solvent volume ratio of 0.04. The selection of IrO2 as the

anode for the three explored MEAs has been done according to its unique

and superior ability for water oxidation reaction in conventional PEM

electrolyzers [22]. Then, the different inks were deposited on Carbon paper

(Fuel Cell Earth) substrates at 65 ºC until a metal loading of 0.5 mg·cm-2

for the anode was obtained after drying. For the cathodic catalyst, the final


185

loading was measured by atomic absorption spectrophotometry obtaining

0.37, 0.20 and 0.38 mg·cm-2 for the Cu-G, Cu-AC and Cu-CNF cathodic

catalysts, respectively. The geometric surface area of both electrodes was

12.56 cm2 (4 cm of circular diameter electrode). A proton conducting

Sterion® membrane of 185 µm thickness (supplied by Hydrogen works)

was used as the electrolyte (H+ conductor material). Prior to use, the

Sterion® membrane was treated by successive immersion at 100 ºC for 2 h

in H2O2 in order to remove organic impurities, in H2SO4 for activation and

in deionized water to remove traces of solutions. Finally, in order to

prepare the membrane electrode assembly (MEA), the membrane was

sandwiched between a couple of electrodes. Then, the whole system was

hot-pressed using a press (GRASEBY SPEAC) at 120 ºC and a pressure of

1 metric ton for 3 min.


Cu based electrodes were characterized before reaction tests by X-Ray

Diffraction (XRD) analysis with a Philips PW-1710 instrument, using Ni-

filtered Cu Kα radiation (λ = 1.5404 Å). The samples were scanned at a rate

of 0.02º·step-1 over the range 20º ≤ 2θ ≤ 90º (scan time 2 s・step-1) and the

diffractograms were compared with the JCPDS-ICDD references.

Cu metal loading, on the cathodic powdered catalyst and per area of

deposited electrode was determined by atomic absorption

spectrophotometry, using a SPECTRA 220FS analyzer. The sample (ca. 0.5

g) was treated in 2 mL HCl, 3 mL HF and 2 mL H2O2 followed by

microwave digestion (523 K).

Temperature programmed reduction (TPR) experiments were conducted

for the different cathodic powder catalysts in a commercial Micromeritics

Chapter 5

186

AutoChem 2950 HP unit equipped with a TCD detector. Samples (ca.

0.15 g) were loaded into a U-shaped tube and ramped from room

temperature up to 900 ºC (10 ºC min−1), using a reducing gas mixture of

17.5% v/v H2/Ar (60 cm3 min−1).

Transmission electron microscopy (TEM) analyses for the powder

cathodic catalysts were conducted on a JEOL JEM-4000EX unit with an

accelerating voltage of 400 kV. Samples were prepared by ultrasonic

dispersion in acetone with a drop of the resulting suspension evaporated

onto a holey carbon-supported grid.

Finally, surface area/porosity measurements of powder catalysts were

conducted using a Micromeritics ASAP 2010 for activated carbon and a

QUADRASORB 3SI sorptometer apparatus for graphite and carbon

nanofibers. In both cases, N2 was used as the sorbate at -193 ºC. The

samples were outgased at 180 ºC under vacuum (5x10-3 Torr) for 12 h prior

to the analysis. Specific surface areas were determined by the multi-point

BET method and Langmuir. The microporosity of the materials was

evaluated by Howath-Kawazoe (HK) method and the mesoporosity was

calculated by the Barret-Joyner-Halenda (BJH) method.


5.3.1. Characterization of the Cu carbon-based cathodic catalyst

The three different cathodic-catalysts were characterized by N2

adsorption, atomic absorption spectrophotometry, temperature-

programmed reduction (TPR), transmission electron microscopy (TEM)

and X-ray diffraction (XRD). Physicochemical properties of the supports,

catalyst and fresh electrodes are shown in Table 5.1.


187

AC showed high values of Langmuir area and total pore volume, as

previously described in literature [23] (Table 5.1). In this case, the N2

adsorption/desorption isotherm (not shown) can be described as a

combination of types I and IV isotherms (IUPAC classification). On the

other hand, graphite (G) was characterized by a low surface area and

limited porosity showing a type IV adsorption/desorption isotherm (not

shown), with a very small volume of adsorbed N2 (Table 5.1). This fact is in

good agreement with the low porosity of these materials [23]. Finally,

carbon nanofibers (CNF) showed a type IV N2 adsorption/desorption

isotherm (not show). In this case, a mesoporous nature was observed and a

BET surface area in the range commonly observed for these materials was

obtained (10-300 m2 g-1) [24] (Table 5.1).

After the metal introduction, an important decrease of BET and

Langmuir surface area and pore volume took place in all the cases, which

can be attributed to the partial pore blockage by metal particles [25]. On

the other hand, the total amount of Cu measured by the atomic absorption

spectrophotometry was closed to 50 % in three synthesized catalysts

powders.

Chapter 5

188

Ta

ble

5.1

. P

hysi

coch

em

ica

l p

rop

ert

ies

of

the s

up

port

s, c

ata

lyst

s, a

nd

fre

sh

ele

ctro

des.

P

arti

cle

siz

e

fro

m T

EM

/

nm

- 96

- 40

- 74

Ele

ctr

od

e

me

tal

we

igh

t

/ m

g C

u c

m-2

-

0.3

7

-

0.2

0

-

0.3

8

Po

wd

er m

eta

l

loa

din

g / w

t.%

51.4

6

-

53.9

7

-

51.0

5

-

TP

R-T

ma

x

/ ºC

-

198

-

207

-

202

To

tal

po

re

vo

lum

e / c

m3 g

-1

0.0

68

0.0

46

0.3

73

0.2

11

0.5

55

0.2

33

Su

rfa

ce

are

a

/ m

2g

-1

10

7

906

836

95

47

Sa

mp

le

G

Cu

-G

AC

Cu

-AC

CN

F

Cu

-CN

F


189

100 200 300 400 500 600 700 800

TC

D S

ign

al

(a.u

.)

Temperature / ºC

Cu-G

Cu-AC

Cu-CNF

TPR profiles associated with each catalyst are given in Figure 5.3. For

the three catalysts the maximum reduction temperature was obtained at

around 200 ºC (Tmax), which was characterized by a sharp peak. Tmax

obtained for each catalyst, associated with the first hydrogen consumption

peak, are also given in Table 5.1. On other hand, an additional shoulder

centered at around 250 ºC was observed for the cathodic-catalysts Cu-G

and Cu-AC [26].

Figure 5.3. TPR profiles of the fresh catalyst.

This bimodal distribution of H2-TPR was described by Kargol et al. [27].

The first temperature maximum is due to the reduction of CuO and partial

reduction of Cu(II) ions to Cu(I). The presence of this compound has been

proved by the XRD technique on the powder catalyst (not shown here). The

second maximum corresponds to the reduction of Cu(I) species strongly

interacting with the support. On the other hand, the Cu-CNF cathodic-

Chapter 5

190

catalyst suggested one-step process of reduction. This process is ascribed to

the direct reduction of CuO to metallic copper [27]. Finally, a third peak

(appearing at temperature range between 520 and 600 ºC) associated to

the carbon support gasification was also observed for cathodic-catalyst Cu-

CNF [24]. According to the obtained results, 350 ºC was chosen as a

suitable reduction temperature to ensure the metal activation without

affecting the surface properties of the supports.

Transmission electron microscopy (TEM) was used to determine the metal

particle size and characteristics of the three prepared Cu-based cathodic-

catalysts. Representative TEM micrographs are shown in Figure 5.4 ((a) Cu-G

powder, (b) Cu-AC powder and (c) Cu-CNF powder). In the present work, the

mean Cu particle size, evaluated as the surface-are weighted diameter (𝑠) was

calculated according to:

𝑠 =∑ 𝑛𝑖𝑑𝑖

3𝑖

𝑛𝑖𝑑𝑖2 (5.3)

where ni represents the number of particles of diameter di.

The estimated particle size for the different catalyst is listed in Table

5.1. TEM micrographs for cathodic-catalysts, Cu-G (Figure 5.4.a) and Cu-

CNF (Figure 3.c) showed a high particle size value (96 and 74 nm,

respectively) and hence a lower dispersion of Cu particles is expected with

respect to the Cu-AC powder. For this latter case (Figure 5.4.b), the

corresponding TEM micrographs showed a smaller particle size (40 nm)

and consequently, a higher dispersion of Cu particles on the AC support.


191

Cu

Cu

Cu

a)

b)

c)

CNF

Figure 5.4. Representative TEM images of the powders of (a) Cu-G, (b) Cu-AC

and (c) Cu-CNF.

This observation is related to the high values of surface area and total

pore volume obtained for the activated carbon support as discussed above.

It is worth noting that for the case of cathodic-catalyst based on CNF

Chapter 5

192

(Figure 5.4.c), the carbon nanofibers can also be observed in the TEM

images.

Figure 5.5 shows the XRD analysis of the resultant Cu-based electrodes:

Cu-G (a) Cu-AC (b) and Cu-CNF (c) (after deposition of the Cu cathodic-

catalysts powders on the carbon paper substrates). The inset of Figure

5.5.a, 5.5.b and 5.5.c shows the magnification of XRD patterns. All samples

showed two peaks around 2θ = 25º and 55º, which could be associated with

the presence of the carbon paper substrate used as the current collector

and gas diffusion layer. This fact was corroborated by the XRD spectrum of

this material (not shown here). The main diffraction Cu peaks (111), (200),

(220) and (331) appeared in the three cases at 2θ = 43.3º, 50.4º, 74.1º and

90 º, respectively. These peaks are associated with metallic copper and

exhibited a face-centered cubic (FCC) crystalline structure (JCPDS, 85-

1326) (Inset of Figure 5.5.a, 5.5.b and 5.5.c). Additionally, the presence of

small CuO peaks (111) and (223) could be detected at 2θ = 32.5º and 86.6º,

respectively (JCPDS, 78-2076). However, the intensity of these peaks is

very low which indicates that the copper is almost completely reduced and

it has not been oxidized during the catalyst ink deposition on the carbon

paper support.


193

20 30 40 50 60 70 80 90

20 30 40 50 60 70 80 90

CuO

Inte

nsi

ty / a

.u.

2 / º

C

Cu

(111)

(200)

(220)

(331)

Inte

nsi

ty /

a.u

.

2 / º

b)

20 30 40 50 60 70 80 90

20 30 40 50 60 70 80 90

(331)

(220)(2

00)

(111)

Inte

nsi

ty / a

.u.

2 / º

CuO

C

Cu

Inte

nsi

ty /

a.u

.

2 / º

a)

20 30 40 50 60 70 80 90

20 30 40 50 60 70 80 90

CuO

Inte

nsit

y /

a.u

.

2 / º

C

Cu

(11

1)

(20

0)

(22

0)

(33

1)

Inte

nsi

ty /

a.u

.

2 / º

c)

Figure 5.5. XRD analysis patterns of cathodic-catalysts on carbon paper

substrates: (a) Cu-G, (b) Cu-AC and (c) Cu-CNF. Insets in (a), (b) and (c) show the

magnification of XRD patterns of cathodic-catalysts Cu-G, Cu-AC and Cu-CNF,

respectively.

Chapter 5

194

Prior to the catalytic activity measurements, the three different MEAs

(Cu-G/Sterion/IrO2 (Figure 5.6.a), Cu-AC/Sterion/IrO2 (Figure 5.6.b) and

Cu-CNF/Sterion/IrO2 (Figure 5.6.c) were in-situ characterized by a

galvanostatic voltammetry under two different reaction atmospheres fed to

the cathodic chamber: under presence of CO2 (FCO2, cathode = 0.5 NmL·min-1,

FH2O, anode = 6 NmL·min-1) and without feeding CO2 to the cell (FCO2, cathode =

0 NmL·min-1, FH2O, anode = 6 NmL·min-1) at 90 ºC.

In this latter case, N2 was fed to the cathodic side in order to purge the cell.

The potential (UWC) variation was recorded with the applied current (I)

between 0 and -20 mA with a scan rate of 80 µA s-1

. It can be observed that for

the three MEAs without feeding CO2 to the cathode, water electrolysis began

between -1.2 and -1.6 V [28], according to the following electrochemical

reaction:

H2O → H+ + ½ O2 + e

- (5.4)

An increase in the applied current led to higher negative potential values

and hence to an increase in the protons production rate. At the cathodic side,

hydrogen can be obtained due to the combination of protons that were

transported through the protonic membrane and the electrons transferred from

the external circuit:

2H+ + 2e

- H2 (5.5)


195

-3.0 -2.8 -2.6 -2.4 -2.2 -2.0 -1.8 -1.6 -1.4 -1.2 -1.0

-25

-20

-15

-10

-5

0

5

-3.0 -2.8 -2.6 -2.4 -2.2 -2.0 -1.8 -1.6 -1.4 -1.2 -1.0

-25

-20

-15

-10

-5

0

5

-3.0 -2.8 -2.6 -2.4 -2.2 -2.0 -1.8 -1.6 -1.4 -1.2 -1.0

-25

-20

-15

-10

-5

0

5

FCO

2,cathode

= 0.5 Nml·min-1 / F

H2O,anode

= 6 Nml·min-1

FCO

2,cathode

= 0 Nml·min-1 / F

H2O,anode

= 6 Nml·min-1

Cu

rren

t /

mA

Potential / V

a)

FCO

2,cathode

= 0.5 Nml·min-1 / F

H2O,anode

= 6 Nml·min-1

FCO

2,cathode

= 0 Nml·min-1 / F

H2O,anode

= 6 Nml·min-1

Cu

rren

t /

mA

Potential / V

b)

FCO

2,cathode

= 0.5 Nml·min-1 / F

H2O,anode

= 6 Nml·min-1

FCO

2,cathode

= 0 Nml·min-1 / F

H2O,anode

= 6 Nml·min-1

Cu

rren

t /

mA

Potential / V

c)

Figure 5.6. Influence of the reaction atmosphere on the current-potential

curves obtained during a galvanostatic voltammetry for cathodic-catalysts on

carbon paper substrates: (a) Cu-G, (b) Cu-AC and (c) Cu-CNF. Conditions:

temperature = 90 ºC, sweep rate = 80 µA·s-1.

Chapter 5

196

On the other hand, it can be observed that for the three MEAs, a higher

negative current values was obtained at fixed potential when CO2 was fed

to the cathode of the electrochemical system (FCO2, cathode = 0.5 Nml·min-1 /

FH2O,anode = 6 Nml·min-1). It demonstrates that the CO2 present in the gas

phase took part in the electrocatalytic process by its further adsorption

and reaction on the cathodic-catalyst with the electrochemically supplied

H+. In fact the presence of CO2 decreased the potential of the electrolysis

cell (for the same current), then acting as a depolarizating agent for the

electrochemical cathodic reaction [29]. The use of different molecules as

depolarizating agents has been studied for the electrolytic production of H2

at high temperatures in Solid Oxide Electrolysers. In this regard, the use

of CH4 [30, 31], CO [32] and C [29, 30] allowed to strongly decrease the

required electrical power input for the electrolysis process via reaction

with the O2- ions transported across the Anionic Conductor Electrolyte. As

can be observed in the Figure 5.6, the highest difference in the

voltammetry experiments performed with and without CO2 (highest

depolarization effect) occurred with Cu-AC cathodic-catalyst. This fact can

be attributed to the higher number of Cu active sites due to its higher

dispersion on the high surface area AC support exposed to the gas phase.

It probably facilitates the CO2 adsorption leading to faster reaction

kinetics with H+, increasing the current at fixed potential. Then, one may

expect a higher electrocatalytic activity of the cathodic-catalyst Cu-AC for

the CO2 electro-reduction (as will be shown later). Additionally, it can be

observed that cathodic-catalyst Cu-AC achieved the higher potential range

during voltammetry to reach the applied currents (up to -20 mA). It is due

to the lower electrical conductivity (around 250 Ω-1·m-1) of the AC support

vs. G and CNF which increase the overall electrical resistance of the MEA.


197

Hence, the lowest potential range was obtained with cathodic-catalyst Cu-

G, due to its highest electrical conductivity (around 1600 Ω-1·m-1) among

the carbonaceous supports. Finally, carbon nanofibers exhibited an

intermediate electrical conductivity value (around 900 Ω-1·m-1), leading to

intermediate vales of the current-potential curves.

5.3.2. Electrocatalytic experiments for CO2 conversion

Figure 5.7 shows the time-on-stream evolution of the different

measured products rates for a constant applied current of -20 mA at 90 ºC

for the three different cathodes under study at Temperature = 90 ºC,

FCO2,cathode = 0.5 NmL·min-1, FH2O,anode = 6 NmL·min-1. Initially, under open

circuit conditions (O.C.C, no current application), no products were

obtained at any case. Then, a constant current of -20 mA was applied for

approximately 300 min under the same reaction atmosphere. During this

current imposition step, hydrogen (not shown here), and different products

such as methanol, acetaldehyde, acetone, methane, methyl formate, carbon

monoxide, methyl acetate, ethanol, 2-propanol and n-propanol were

obtained via CO2 electro-reduction (reactions 5.6-5.15).

Most of these products have already been identified in similar previous

studies of electrocatalytic conversion of CO2 [4, 9-12] using Fe, Co, Cu and Pt

as cathodic-catalyst over CNT according to the following electrochemical

reactions.

CO2 + 6H+ + 6e- → CH3OH + H2O (5.6)

2CO2 + 10H+ + 10e- → CH3CHO + 3H2O (5.7)

3CO2 + 16H+ + 16e- → CH3COCH3 + 5H2O (5.8)

Chapter 5

198

CO2 + 8H+ + 8e- → CH4 + 2H2O (5.9)

2CO2 + 8H+ + 8e- → HCOOCH3 + 2H2O (5.10)

CO2 + 2H+ + 2e- → CO + H2O (5.11)

3CO2 + 14H+ + 14e- → CH3COOCH3 + 4H2O (5.12)

2CO2 + 12H+ + 12e- → CH3CH2OH + 3H2O (5.13)

3CO2 + 18H+ + 18e- → CH3CH(OH)CH3 + 5H2O (5.14)

3CO2 + 18H+ + 18e- → CH3CH2CH2OH + 5H2O (5.15)

Finally, in all the experiments, the cathodic side of the cell was purged

with N2 (30 Nml·min-1) under open circuit conditions in order to remove all

the products for subsequent reaction experiments.

In first place it should be mentioned that the electrochemical nature of

the different obtained products vs. the catalytic route (CO2 hydrogenation

via previous H2 evolution reaction) was confirmed with further

experiments performed under open circuit conditions by co-feeding CO2

and H2 at the same temperature to the cathode (not shown here). Under

these explored reaction conditions, no reaction products were detected

confirming the electrochemical nature of the products observed on Figure

5.7. It is further supported by the modification of the current-potential

curves previously shown under presence of CO2 during voltammetry

experiments which confirms CO2 adsorption and further reaction with H+.

This is in good agreement with previous works of catalytic CO2

hydrogenation on Cu-based catalyst which showed that temperatures

typically above 250 ºC are required [33, 34] for the process.


199

CO2 feeding N2 purge

O.C.C. I = -20 mA O.C.C.

0 50 100 150 200 250 300 350 400

0

2

4

6

8

10

12

14

16

18

20

22

0 50 100 150 200 250 300 350

0

20

40

60

80

100

120

140

160

0 50 100 150 200 250 300 350

0

2

4

6

8

10

12

14

16

18

20

22

CH4

Acetaldehyde

Methyl formate

Acetone

Methanol

(r /

mm

ol·

h-1·m

g-1C

u)·

10

3

Time / min

a)

CH4

Acetaldehyde

Methyl formate

Acetone

Methanol

(r /

mm

ol·

h-1·m

g-1C

u)·

10

3

Time / min

b)

(r /

mm

ol·

h-1·m

g-1C

u)·

10

3

Time / min

CH4

CO

Acetaldehyde

Methyl formate

Methyl acetate

Methanol

Ethanol

2-propanol

n-Propanol

c)

Figure 5.7. Time-on-stream evolution of different products for a constant current

of -20 mA. Cathodic-catalysts on carbon paper substrates: (a) Cu-G, (b) Cu-AC

and (c) Cu-CNF. Conditions: temperature = 90 ºC, FCO2,cathode = 0.5 NmL·min-1,

FH2O,anode = 6 NmL·min-1

Chapter 5

200

The coupling of the steam electrolysis process in the proposed

configuration allows to directly supplying H+ to the cathodic catalyst, of

higher reactivity, which allows working at lower reaction temperatures vs.

catalytic CO2 hydrogenation processes.

On the other hand it can be observed that among the three investigated

cathodic-catalyst, the Cu-AC system showed the higher electrocatalytic

activity (around five times higher) in comparison with the other two

cathodic-catalysts: Cu-G and Cu-CNF. This observation is in good

agreement with the galvanostatic voltammetry curves previously shown

which demonstrates the higher depolarization effect caused by CO2 on the

MEA based on Cu-AC. This higher electrocatalytic activity could be related

to the higher surface area of the AC support leading to a higher metal

dispersion of the Cu particles, which is a key factor for CO2 adsorption and

further reaction via the electrochemical reactions (5.6-5.10). It should be

mentioned that on these experiments the effect of the conductivity of the

different carbon supports it is not affecting the electrocatalytic activity of

the system since the three MEAS are compared under the same current

application (-20 mA), i.e., under the same H+ supplied rate to the cathode

(r=I/(nF) = 1.036·10-7 mol s-1). However, is clear that a high potential will

be required to achieve the current of -20 mA for the case of the Cu-AC

cathodic catalyst (-2.72 V) vs. the Cu-G and Cu-CNF (-1.54 V and -2.11 V

respectively), therefore leading to a higher electrical energy consumption.

Concerning the different obtained products it can be observed that

methanol was the main reaction product for the case of the Cu-G cathodic-

catalyst while acetaldehyde was the main one for the case of Cu-AC and

Cu-CNF systems. This observation can be explained according to previous

studies of catalytic CO2 hydrogenation that have shown that Cu particles


201

a) b)

0

5

10

15

20

25

30

35

40

Cu-CNFCu-AC

I = -10 mA

I = -20 mA

I = -30 mA

(rC

O2

/mm

ol·

h-1·m

g-1C

u)·

10

2

Cu-G

T = 90 ºC

Cu-G Cu-AC CuCNF0

5

10

15

20

25

30

35

40

45

50

55

60

(rC

O2

/mm

ol·

h-1·m

g-1C

u)·

10

2

T = 80 ºC

T = 90 ºCI = -20 mA

of higher size are more selective for methanol production rather than for

acetaldehyde [35, 36]. On the other it should be mentioned that a wider

variety of reaction products with higher number of carbon atoms was

obtained for the case of Cu-CNF cathodic-catalyst, which could be related

to the presence of the functional oxygen groups in the CNF support. As

reported by Genovese et al, [4], the nature of functional groups on the

carbon surface when CNF is used as a catalyst support has a significant

influence on determining the possibility to form > C1 products from CO2.

Figure 5.8.a and 5.8.b shows the effect of the applied current and

reaction temperature, respectively, in the normalized CO2 electrocatalytic

steady state reaction rate (per mg of deposited Cu) after 300 min of

polarization at each current.

Figure 5.8. (a) Effect of the current at 90 ºC and (b) temperature (T = 80 and 90

ºC) at I = -20 mA on the steady state CO2 consumption rate for Cu/Sterion/IrO2

and Cu-C/Sterion/IrO2 electrodes. Conditions: FCO2,cathode = 0.5 NmL·min-1,

FH2O,anode = 6 NmL·min-1.

In agreement with the previous experiment, it can be observed that for

all the explored reaction conditions (applied currents and temperatures),

the electrocatalytic activity of the cathodic-catalyst Cu-AC is higher than

Chapter 5

202

that of Cu-G and Cu-CNF. Additionally, it can be observed that the higher

the applied current, the higher the CO2 reaction rate, attributed to the

higher amount of H+ ions electrochemically supplied. On the other hand,

the higher the reaction temperature, the higher the electrocatalytic

activity of the system. This fact can be attributed to the enhanced kinetics

of electrochemical reactions at higher reaction temperature [37]. However,

90 ºC was the highest explored temperature, which ensures the stability

and conductivity of the protonic membrane under suitable humidity

conditions.

The influence of the applied current (I = -10, -20 and -30 mA) on the

steady state variation of the selectivity of the different products was

studied at 90 ºC (Figure 5.9) after the polarization at each current for 300

min: for cathodic-catalysts Cu-G, Cu-AC and Cu-CNF, Figures 5.9.a, 5.9.b

and 5.9.c, respectively.

Figure 5.9. Effect of the applied current on the steady state selectivity toward the

different products on Cu cathodic-catalysts: (a) Cu-G, (b) Cu-AC and (c) Cu-CNF.

Conditions: Temperature = 90 ºC, FCO2,cathode = 0.5 NmL·min-1, FH2O,anode = 6

NmL·min-1.

I = -10 mA I = -20 mA I = -30 mA

0

10

20

30

40

50

60

70

80

90

100c)b)

Acetaldehyde

Methyl formate

CH4

Sele

ctiv

ity /

%

Methanol

Acetone

a)

I = -10 mA I = -20 mA I = -30 mA

0

10

20

30

40

50

60

70

80

90

100 Methanol

Acetone Acetaldehyde

Methyl formate

Sele

ctiv

ity /

%

CH4

I = -10 mA I = -20 mA I = -30 mA

0

10

20

30

40

50

60

70

80

90

100

Methyl formate

2-Propanol

n-Propanol

Methanol

Ethanol CO

Acetaldehyde

Sel

ecti

vit

y /

%

CH4


203

As shown before, methanol was the main obtained product on cathodic-

catalyst Cu-G. In this case, the selectivity to methanol varied between 60

% under I = -10 mA and 75 % for an applied current of I = -30 mA. On the

other hand for the cathodic-catalysts Cu-AC and Cu-CNF, acetaldehyde

was the main reaction product with selectivity values around 60 %. It can

be observed that for the case of Cu-G and Cu-AC (Figure 5.9.a and 5.9.b)

an increase in the applied cathodic current (from -10 mA to -30 mA) led to

an increase in the methanol selectivity at the expense of a decrease in the

acetaldehyde. In this case is clear that at higher supplied rate of protons at

higher intensity values the selectivity shift to lighter and more saturated

compounds in agreement with previous studies [38, 39], i.e. toward

methanol production (reaction 5.6). However, for the Cu-CNF cathodic-

catalyst, the variation of product selectivity with the applied current does

not seem to follow a clear trend, probably due to the high number of

obtained products obtained and to the complex number of reactions that

occurred on the system.

In order to finally address, the best electrocatalytic system in terms of

activity but also energy consumption, Table 5.2 shows a comparison

between the three different MEAs at T = 90 ºC and I = -30 mA for the

overall energy consumption for CO2 conversion (kWh mol-1 CO2) as well as

energy consumption for the production of methanol (kWh mol-1 CH3OH) and

acetaldehyde (kWh mol-1 CH3CHO).

Chapter 5

204

Table 5.2. Comparison of energy consumption for different membrane

electrode assemblies in the electrocatalytic conversion of CO2 at 90 ºC and I

= -30 mA

Sample kWh mol-1CO2

kWh mol-1CH3OH kWh mol-1

CH3CHO

Cu-G 199.8 260.6 2574.7

Cu-AC 92.4 197.9 185.9

Cu-CNF 189.1 23785.8 274.2

Even though, the AC support has the lower electrical conductivity value

(as shown on the voltammetry curves of Figure 5.6) its higher activity at

the same current conditions shown on Figures 5.7 and Figures 5.8 led to

the lower energy consumption values for CO2 conversion and methanol and

acetaldehyde production. Hence, the MEA based on the Cu-AC cathodic-

catalyst consumed less energy per kg of methanol produced and per kg of

acetaldehyde produced than the other two MEAs based on Cu-G and Cu-

CNF cathodic-catalyst. For each system the lower energy consumption is

evidently obtained with the most selective product: methanol for the case

of Cu-G cathodic catalyst and acetaldehyde for the case of Cu-AC and Cu-

CNF. These results allow to conclude that among the three different

explored cathodic catalyst, the higher activity of the Cu-AC cathodic

catalyst due to the higher dispersion of Cu particles on the high surface

area AC support is the most important parameter besides its lower

electrical conductivity. However, is clear that suitable values of both

parameters: activity and conductivity should be found for the design of

efficient cathodic catalyst for the CO2 electrochemical conversion. Then the

use of novel supports (e.g. reduced graphene powder, carbon black,

molybdenum-carbide-derived carbon…) of high surface area and high

electrical conductivity may decrease the overall energy consumption values

reported on Table 5.2 in view of the practical application of this


205

configuration for renewable production of solar fuels via electrochemical

conversion of CO2.

5.4. Conclusions

Three different Cu-based cathodic-catalysts (metal loading closed to 50

%) supported on Graphite (G), Activated Carbon (AC) and Carbon

Nanofibers (CNF) have been prepared via impregnation technique and

characterized by different techniques.

Among them, the Cu-AC-based cathodic-catalyst showed the highest

CO2 electrocatalytic activity under all the explored reaction conditions, due

to the highest surface area of the AC support and the larger metal

dispersion of the Cu particles. Methanol was the main reaction product for

the case of the Cu-G cathodic-catalyst while acetaldehyde was the main

one for the case of Cu-AC and Cu-CNF systems which can be attributed to

the higher size of Cu particle sizes in these two latter cases more selective

for methanol formation.

Concerning the product selectivity variation with the current, for the

cased of Cu-G and Cu-AC cathodic catalyst, an increase in the applied

cathodic current led to an increase in the methanol selectivity at the

expense of a decrease in the acetaldehyde. In this case is clear that at

higher supplied rate of protons at higher intensity values the selectivity

shift to lighter and more saturated compounds. However, for the Cu-CNF

cathodic-catalyst, the variation of product selectivity with the applied

current does not seem to follow a clear trend, probably due to the high

number of obtained products obtained and to the complex number of

reactions that occurred on the system.

Chapter 5

206

Besides the lower conductivity of the AC support, the energy

consumption values for CO2 conversion and methanol and acetaldehyde

production was achieved with the MEA based on Cu-AC cathodic catalyst.

Therefore, the higher activity of the Cu-AC cathodic catalyst due to the

higher dispersion of Cu particles on the high surface area AC support is

the most important parameter besides its lower electrical conductivity

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209

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6.1. General Conclusions

6.2. Recommendations

CHAPTER 6:

General Conclusions

and Recommendations

Chapter 6

213

The present work has been aimed at the study of novel electrocatalytic

systems for energetic and environmental applications. This chapter lists

the main conclusions derived from the research performed in this Doctoral

Thesis. In addition, some recommendations are suggested to be taken into

account in further studies.

6.1. Conclusions

The results obtained from the present research work support the

following main conclusions:

o The developed Pt/YSZ/Pt solid electrolyte cell allowed the

production of syngas of flexible H2/CO ratios from ethanol-water

streams by the application of different polarizations. In this way,

the proposed configuration may be of great interest especially for

biorefinery applications where H2, syngas and electricity may be

produced from bioethanol.

o The proposed double chamber Pt-YSZporous/YSZ/Pt solid electrolyte

cell allowed to simultaneously produce H2 and C2 hydrocarbons

from methane. In this regard, the carbon generated in the methane

decomposition step served as a depolarizating agent in the steam

electrolysis process decreasing the electrical energy consumption.

o The study reported in Chapter 3 evaluated the energetic analysis

for the hydrogen production via catalytic steam and electrochemical

ethanol reforming processes by Aspen HYSYS simulation. The

highest yield in the hydrogen production and the lowest energy

consumption were obtained in the electrochemical reforming of

General Conclusions and Recommendations

214

ethanol. These results demonstrated the potential of this process for

obtaining high purity hydrogen in a single reaction/separation step.

o The Ni-αAl2O3/K-βAl2O3/Au solid electrolyte cell can be

electrochemically promoted for the reverse water gas shift reaction

demonstrating the occurrence of the EPOC phenomenon.

Additionally, this system allowed the possibility of control the Ni

catalytic activity and selectivity toward CO and CH4 by means of

the controlled migration of K+ ions from a solid electrolyte. Thus, it

may have significant importance for the practical use of

electrochemistry to activate non noble metal catalyst for the CO2

hydrogenation process.

o The developed Cu-G/Sterion/IrO2, Cu-AC/Sterion/IrO2 and Cu-

CNF/Sterion/IrO2 MEAs allowed, without feeding H2, obtaining a

wide range of compounds such as methanol, acetaldehyde and

methane via electrocatalytic reduction of CO2. These results

demonstrate the interest of these PEM reactors configurations to

obtain high added-value compounds from an industrial and

environmental point of view.

6.2. Recommendations

The following proposals can be stated in order to complete and extend

this research work:

o To investigate the behaviour of non-noble metals in the steam

electrolysis process and partial oxidation of ethanol as well as the

simultaneous production/separation of H2 and C2 hydrocarbons.

This way, the economy of this process could be improved in view of

its practical application.

Chapter 6

215

o To perform an exergetic analysis to select the optimum operating

parameters for the catalytic steam reforming and the

electrochemical reforming of ethanol, allowing to evaluate the

location, cause and true magnitude of energy wastes and losses in

the system.

o To use YSZ as a solid electrolyte in order to improve the activity

towards CH4 production in the CO2 hydrogenation via EPOC

phenomenon.

o To evaluate the impact of different catalysts and preparation

techniques on the performance of the proposed PEM cell

configuration in the CO2 valorisation process. Additionally, the

temperature of the reaction process could be increased by using a

Sterion® membrane doped with H3PO4.

List of Publications and

Conferences

List of publications and conferences

219

Publications

1. Coupling Catalysis and Electrocatalysis for Hydrogen Production

in a Solid Electrolite Membrane Reactor. A. de Lucas-Consuegra,

N. Gutiérrez-Guerra, A. Caravaca, J.C. Serrano-Ruiz, J. L.

Valverde. Applied Catalysis A: General, 483, 25-30 (2014).

2. Electrochemical Reforming vs. Catalytic Reforming of Ethanol: A

process Energy Analysis for Hydrogen Production. N. Gutiérrez-

Guerra, M. Jiménez-Vázquez, J.C. Serrano-Ruiz, J. L. Valverde,

A. de Lucas-Consuegra. Chemical Engineering and

Processing: Process Intensification, 95, 9-16 (2015).

3. Direct Production of flexible H2/CO Synthesis Gas in a Solid

Electrolyte Membrane Reactor. A. de Lucas-Consuegra, N.

Gutiérrez-Guerra, J.L. Endrino, J.C. Serrano-Ruiz, J. L.

Valverde. Journal of Solid State Electrochemistry. DOI:

10.1007/s10008-015-2922-8.

4. Electrochemical activation of Ni catalyst with potassium ionic

conductors for CO2 hydrogenation. N. Gutiérrez-Guerra, J.

González-Cobos, J.C. Serrano-Ruiz, J. L. Valverde, A. de Lucas-

Consuegra. Topics in Catalysis. In press.

5. A gas phase electrocatalytic conversion of CO2 to liquid fuels at

lower temperature using different carbon-based support. N.

Gutiérrez-Guerra, L. Moreno-López, J.C. Serrano-Ruiz, J. L.

Valverde, A. de Lucas-Consuegra. Submitted to Applied Catalysis

B.

Patents

1. Procedimiento de Obtención de Gas de Síntesis. Antonio de Lucas

Consuegra, Nuria Gutiérrez Guerra, Jesús González Cobos, Carmen

Jiménez Borja, José Luis Valverde Palomino, P201330975, 28

Junio 2013.

2. Procedimiento de Obtención de Metanol a partir de CO2 y Sistema

Electroquímico para Realizarlo. Juan Carlos Serrano Ruíz, Antonio

de Lucas Consuegra, Nuria Gutiérrez-Guerra, José Luis Valverde

Palomino. Submitted

List of publications and conferences

220

Conferences

Oral presentations in congress:

1. Conversión electrocatalítica de CO2 en compuestos de interés

industrial. N. Gutiérrez-Guerra, C. Marchante, J.C. Serrano-

Ruíz, J.L. Valverde, A. de Lucas-Consuegra. JJ. II. SECAT’

2014. I Encuentro de Jóvenes Investigadores de la SECAT,

Málaga (España), Junio 2014.

2. Promoción electroquímica de catalizadores de Ni para la

hidrogenación de CO2. N. Gutiérrez-Guerra, J. González-Cobos, J.C.

Serrano-Ruiz, J. L. Valverde, A. de Lucas-Consuegra. SECAT’

2015, Barcelona (España), Julio 2015.

Poster presentations in congress:

1. Nuevas perspectivas de la electrocatálisis en fase gas. A. de Lucas-

Consuegra, N. Gutiérrez, C. Jiménez-Borja, J. González-Cobos,

J.L. Endrino, J.L. Valverde. SECAT’13. Sevilla (España), Junio,

2013.

2. Análisis energético y exergético de los procesos de reformado

catalítico y electro-reformado de etanol para la producción de

hidrógeno. N. Gutiérrez, J.L. Valverde, J.C. Serrano-Ruíz, A. de

Lucas-Consuegra. I WORKSHOP EN INGENIERÍA QUÍMICA

(FEIQ), Ciudad Real (España), Noviembre 2013

3. Electrochemical Regeneration of Pt catalyst for hydrogen

production in a solid electrolyte membrane reactor. A. de Lucas-

Consuegra, N. Gutiérrez-Guerra, A. Caravaca, J.C. Serrano-Ruíz,

J.L. Valverde. ISE 2014, Laussane (Switzerland), Septiembre

2014.

4. Regeneración electroquímica para la producción de hidrógeno en

un reactor de membrana. . N. Gutiérrez, J.L. Valverde, J.C.

Serrano-Ruíz, A. de Lucas-Consuegra. IV Jornadas Doctorales.

Cuenca (España), Octubre, 2014.

5. Electrochemical modification of Ni catalyst with alkali ionic

conductors for CO2 hydrogenation. N. Gutiérrez-Guerra, J.

González-Cobos, J.C. Serrano-Ruiz, J. L. Valverde, A. de Lucas-

Consuegra. ISE 2015, Taipei, (Taiwan), October 2015.

Anexo

i

Anexo

ii

Anexo

iii

Anexo

iv

Anexo

v

Anexo

vi

Anexo

vii

Procedimiento de obtención de gas de síntesis

DESCRIPCIÓN

La presente invención se refiere a un procedimiento de obtención de gas de

síntesis (H2/CO) de ratio controlable mediante un proceso catalítico y

electroquímico que emplea una celda electroquímica formada por 5

electrolitos sólidos conductores aniónicos o catiónicos. El control del ratio

H2/CO se lleva a cabo en una única etapa bajo condiciones constantes de

operación, es decir, a temperatura constante de la celda electroquímica y

condiciones constantes de composición y concentración de la corriente de

entrada. 10

Por tanto, la presente invención se engloba en el campo técnico de la

producción de gas de síntesis y para su utilización en la industria

petroquímica o en la producción de combustibles.

15

ESTADO DE LA TÉCNICA ANTERIOR

El gas de síntesis (mezcla de H2/CO) es conocido por tener una gran

variedad de aplicaciones en la industria petroquímica. Por ejemplo, el gas

de síntesis puede ser empleado en la producción de amoníaco o metanol. 20

Además, el gas de síntesis se puede utilizar como producto intermedio en

la producción de gasolinas sintéticas, para su uso como combustible o

lubricante a través de la síntesis de Fischer-Tropsch. Para estas

aplicaciones el ratio H2/CO requerido es típicamente de 2. Sin embargo,

existen otros procesos dentro de la industria petroquímica como son los 25

procesos de oxosíntesis que requieren ratios de H2/CO menores,

Anexo

viii

comprendidos entre 1 y 2, o incluso monóxido de carbono (CO) puro, como

ocurre en los procesos de carbonilación. Por otro lado, en la industria

petroquímica también existen muchos procesos donde se requiere

hidrógeno (H2) de alta pureza, tales procesos son por ejemplo reacciones de

hidrogenación, interesando en este caso obtener un ratio H2/CO mayor de 5

2, lo más alto posible.

El gas de síntesis se obtiene generalmente a nivel industrial mediante

procesos catalíticos de reformado o de oxidación parcial de hidrocarburos,

principalmente a partir de metano (EP0168892 A2). Éste tipo de procesos 10

permite obtener un ratio H2/CO fijo y típicamente de 3. Para obtener un

ratio H2/CO distinto son necesarias etapas adicionales de purificación,

separación y conversión como por ejemplo: reacciones de desplazamiento

del agua en estado gaseoso (denominadas en inglés water gas shift),

procesos de adsorción a presión u oxidación preferencial de CO. Estas 15

etapas adicionales, previas al proceso de síntesis, implican una mayor

complejidad del proceso así como mayores costes de producción del

producto final.

Otra posibilidad conocida de variar el ratio H2/CO de forma controlada se 20

realiza mediante la adición controlada de oxígeno (O2) puro a la atmósfera

donde se lleva a cabo la reacción de síntesis [Cao, Y. et al Energ. Fuel.

2008, 22, 1720-1730], donde se produce una oxidación parcial o reformado

autotérmico que modifica la concentración de CO producido. La adición de

O2 puro en este tipo de procesos implica etapas previas y adicionales de 25

separación del mismo del nitrógeno (N2) del aire que implica una mayor

complejidad del proceso, al añadir más etapas al proceso. Por otro lado, se

puede controlar el ratio H2/CO en estos procesos ajustando las condiciones

Anexo

ix

de operación tales como la temperatura a la que se lleva a cabo la síntesis o

la relación entre el hidrocarburo de partida y el O2 añadido. La complejidad

y los costes de estos procesos son altos porque las temperaturas utilizadas

suelen ser altas, mayores de 1000 ºC y se requieren de reactores de dos

entradas de gases y dos salidas de gases para poder trabajar en doble 5

atmósfera [US47993904].

Por tanto, para superar todos los problemas técnicos mencionados es

necesario desarrollar un nuevo proceso de obtención de gas de síntesis de

ratio controlable de H2/CO. 10

DESCRIPCION DE LA INVENCIÓN

La presente invención se refiere a un procedimiento de obtención de gas de

síntesis (H2/CO) de ratio controlable mediante un proceso catalítico y

electroquímico que emplea una celda electroquímica formada por 15

electrolitos sólidos conductores iónicos, aniónicos o catiónicos. El control

del ratio H2/CO se lleva a cabo bajo condiciones constantes de operación, es

decir, a temperatura constante de la celda electroquímica y condiciones

constantes de composición y concentración de la corriente de entrada.

20

En la presente invención la corriente de entrada se selecciona de entre una

corriente gaseosa de hidrocarburos ligeros junto con una corriente de vapor

de agua, o una corriente gaseosa que contiene al menos un alcohol (C1-C3).

Por “hidrocarburos ligeros” se entiende a aquellos compuestos químicos 25

orgánicos formados únicamente de hidrógeno y carbono (C1-C4), incluyendo

al gas natural.

Anexo

x

El gas natural es un gas combustible que proviene de formaciones

geológicas, por lo que constituye una fuente de energía no renovable.

Además de metano, el gas natural puede contener dióxido de carbono,

etano, propano, butano y nitrógeno, entre otros gases.

5

Por tanto, en la presente invención, los hidrocarburos ligeros se

seleccionan de la lista que comprende metano, etano, propano, butano, gas

natural o cualquiera de sus combinaciones.

En el caso de que el conductor electrolito sólido sea un material conductor 10

aniónico, por ejemplo conductor de iones oxígeno (O2-), en la presente

invención éste comprende al menos un electrodo selectivo a la electrólisis

del agua y al menos un contraelectrodo selectivo a la reacción de reformado

y a la oxidación parcial de la corriente de entrada de la celda

electroquímica. 15

Por tanto, en la presente invención, cuando se emplean conductores

aniónicos, la adición de corrientes gaseosas de hidrocarburos

humidificadas o de corrientes gaseosas alcohólicas, junto con o sin una

corriente de vapor de agua, va a permitir que además del gas de síntesis 20

obtenido por reformado convencional catalítico en el catalizador

electroquímico se produzcan procesos adicionales electrocatalíticos que

permitan controlar el ratio H2/CO final bajo condiciones constantes, es

decir, a temperatura constante de la celda electroquímica y condiciones

constantes de composición y concentración de la corriente de entrada. 25

Estos procesos adicionales son principalmente el proceso de electrólisis de

vapor de agua (H2O --> H2 + O2-) que permite producir una mayor cantidad

de H2 y la oxidación electroquímica y catalítica del hidrocarburo o alcohol

que no haya reaccionado y del CO producido a partir de los iones O2- y las

Anexo

xi

moléculas de O2, ambos generados en el proceso de electrólisis anterior. De

este modo el ajuste final del ratio H2/CO se lleva a cabo en una sola etapa.

El H2 adicional producido en el proceso de electrólisis así como la oxidación

de parte del CO producido a dióxido de carbono (CO2) permite modificar

considerablemente el ratio del gas de síntesis. 5

Por tanto, cuando la celda electroquímica se encuentra a una temperatura

de entre 300 ºC y 980 ºC, y bajo unas condiciones constantes de

composición y concentración de la corriente de entrada, tiene lugar el

proceso catalítico de reformado sobre el contraelectrodo selectivo a este 10

proceso. Adicionalmente, bajo la aplicación de corriente eléctrica ocurre la

reacción de electrólisis con la consecuente producción de H2.

Simultáneamente los iones O2- generados en la reacción electroquímica son

transportados por el electrolito sólido conductor hasta el contraelectrodo

que actúa como catalizador de la oxidación electrocatalítica de la corriente 15

de entrada y del CO con la consecuente producción de gas de síntesis

(H2/CO). Además parte del CO, puede ser oxidado a CO2 por oxidación

electroquímica, lo que permite un control neto del ratio H2/CO del gas de

síntesis producido al controlar la velocidad de cada uno de los procesos con

la intensidad eléctrica. Es la intensidad de voltaje aplicada la que permite 20

controlar la velocidad electroquímica de los procesos mencionados.

En el caso de que el conductor electrolito sólido sea un material conductor

catiónico, por ejemplo conductor de iones sodio Na+ y potasio K+, en la

presente invención éste comprende un electrodo catalizador selectivo a 25

proceso de reformado de la corriente de entrada de la celda electroquímica

y un contraelectrodo metálico. Esta configuración permite promocionar por

vía electroquímica el proceso de reformado de la corriente de entrada,

Anexo

xii

mediante el envío de iones promotores desde el material conductor

catiónico al electrodo selectivo del proceso de reformado.

De este modo mediante el conocido fenómeno de promoción electroquímica

de catalizadores heterogéneos o efecto NEMCA (del acrónimo inglés Non 5

Faradaic Electrochemical Modification of Catalitic Activity), la presencia

de los iones electropositivos por ejemplo de iones Na+ y K+ en el electrodo

catalizador favorece la quimisorción de moléculas electronegativas como es

el agua frente al hidrocarburo o el alcohol que forman la corriente de

entrada. En la presente invención, la adsorción controlable de la corriente 10

de entrada se realiza mediante la variación del potencial eléctrico que

varía el contenido de promotor enviado al electrodo catalizador

permitiendo controlar el grado de descomposición de la corriente de

entrada y el grado del proceso de reformado, con ello el ratio de H2/CO del

gas de síntesis resultante obtenido. 15

En el caso de utilizar, en la presente invención un conductor electrolito

sólido catiónico (conductor Na+ o K+), el ajuste final del ratio H2/CO se

produce por el envío de promotores iónicos (Na+ o K+) mediante corriente

eléctrica al electrodo catalizador que modifican la adsorción del agua en los 20

centros activos y con ello la cinética del proceso catalítico. Esto ocurre bajo

una concentración constante de la corriente de entrada y una temperatura

específica de operación de la celda electroquímica.

Por tanto, las principales ventajas con respecto a las técnicas 25

convencionales de reformado es, primeramente, que no es necesaria la

incorporación de una corriente de O2 puro en la celda electroquímica ya que

éste se produce in situ durante el proceso de electrólisis, evitándose de este

Anexo

xiii

modo etapas previas adicionales como por ejemplo 20 la separación previa

adicional de N2 del aire.

En la presente invención, además del gas de síntesis obtenido por

reformado convencional catalítico en el catalizador electroquímico se 5

produzcan procesos adicionales electrocatalíticos que permitan controlar el

ratio H2/CO, es decir, en una 25 única etapa se produce gas de síntesis de

ratio H2/CO controlable.

Además, la celda electroquímica que se puede utilizar es sencilla, no 10

requiere de complejas cámaras de separación de atmósferas. En la

presente invención, el control del ratio H2/CO se lleva a cabo mediante una

variación del voltaje aplicado a los electrodos que permite el control de los

procesos electroquímicos. Las condiciones de operación son constantes, la

síntesis se realiza a una temperatura constante y a una temperatura baja 15

en comparación con las utilizadas en las técnicas convencionales.

La corriente de entrada no sólo se limita a hidrocarburos ligeros gaseosos

húmedos, también se pueden utilizar corrientes gaseosas de alcoholes

puras o humedecidas. 20

Por tanto, un primer aspecto de la invención se refiere a un procedimiento

para producir gas de síntesis, de ratio H2/CO controlable, que comprende el

paso de una corriente de entrada seleccionada de entre una corriente

gaseosa de hidrocarburos ligeros y una corriente de vapor de agua, o una 25

corriente gaseosa que contiene al menos un alcohol (C1-C3) a una celda

electroquímica que se encuentra a una temperatura de entre 300ºC y

980ºC, caracterizado porque dicha celda electroquímica contiene un

Anexo

xiv

conductor electrolito sólido iónico al que se le aplica un potencial de entre -

3 y +3 voltios.

Por “celda electroquímica” se entiende en la presente invención un

dispositivo capaz de transformar una corriente eléctrica en una reacción 5

química de oxidación-reducción que no tiene lugar de modo espontáneo. La

celda electroquímica también se refiere en la presente invención a un

reactor electroquímico adecuado para su uso a nivel industrial en cualquier

configuración conocida por cualquier experto en la materia, como por

ejemplo, un reactor electroquímico con configuración tubular o de tipo 10

monolítico.

Preferiblemente la corriente de entrada no contiene O2 puro.

Preferiblemente, la corriente de entrada está diluida en una corriente de 15

gas inerte, donde el gas inerte se selecciona de la lista que comprende

nitrógeno (N2), helio (He), neón (Ne), argón (Ar), kriptón (Kr) y xenón (Xe).

Preferiblemente la corriente de gas inerte es de N2. La corriente de entrada

se puede diluir hasta en un 98% en volumen en dicha corriente de gas

inerte. 20

En una realización preferida, la celda electroquímica se encuentra a una

temperatura 30 de entre 500ºC y 900ºC.

En otra realización preferida, el potencial aplicado es de entre -2,5 y +2,5 25

voltios. Más preferiblemente, de entre -2 y +2 voltios.

Preferiblemente, en la presente invención los hidrocarburos ligeros

gaseosos se seleccionan de la lista que comprende metano, etano, propano,

Anexo

xv

butano, gas natural o cualquiera de sus combinaciones. En una realización

más preferida el hidrocarburo ligero es una combinación de hidrocarburos

ligeros que comprende al menos metano. En otra realización preferida, el

hidrocarburo ligero es gas natural.

5

La proporción de hidrocarburo ligero y vapor de agua depende del

hidrocarburo utilizado, por ejemplo, en el caso de que el hidrocarburo sea

metano, la proporción preferida será de aproximadamente 1:3.

Preferiblemente, en la presente invención el alcohol se selecciona de la 10

lista que comprende metanol, etanol, propanol o cualquiera de sus

combinaciones. Más preferiblemente, el alcohol es metanol o etanol.

En la presente invención, los alcoholes pueden ser bioalcoholes obtenidos

por la acción de un microorganismo o por algún otro procedimiento 15

biotecnológico. Por ejemplo, se pueden utilizar alcoholes procedentes de

corrientes residuales de alcoholeras con elevada graduación, de hasta 90ºC.

En otra realización preferida, al paso de la corriente gaseosa que contiene

al menos un alcohol (C1-C3) además se añade una corriente de vapor de 20

agua.

Las corrientes de entrada gaseosas de hidrocarburos o de alcohol y vapor

de agua se pueden mezclar antes de pasar a la celda electroquímica o

pueden pasar sin 25 mezclarse previamente. 25

Preferiblemente las corrientes gaseosas de hidrocarburo o alcohol y la

corriente de vapor de agua se mezclan antes de pasar a la celda

electroquímica.

Anexo

xvi

Por otro lado, en una realización preferida, el conductor electrolito sólido

iónico es un 30 conductor aniónico que conduce iones oxígeno (O2-).

En otra realización preferida, el conductor aniónico comprende un

electrolito sólido que se selecciona de entre óxido de zirconio, óxidos de 5

titanio, óxido de itrio estabilizado con óxido de zirconio, óxido de zirconio

estabilizado con calcio, perovskitas con conductividad mixta o cualquiera

de sus combinaciones.

Además, preferiblemente, el conductor aniónico comprende al menos un

electrodo selectivo a la electrólisis del agua y al menos un contraelectrodo 10

selectivo a la reacción de reformado y al proceso de oxidación parcial de la

corriente de entrada.

En otra realización preferida, el electrodo selectivo a la electrólisis del

agua es de platino (Pt). 15

Preferiblemente, el contraelectrodo selectivo a la reacción de reformado de

la corriente de entrada se selecciona de entre níquel (Ni), platino (Pt),

paladio (Pd) o cualquiera de sus combinaciones.

En otra realización preferida, la celda electroquímica que contiene un 20

conductor aniónico como el descrito anteriormente se encuentra a una

temperatura de entre 700 y 900 ºC cuando la corriente de entrada es una

corriente gaseosa de hidrocarburos ligeros y una corriente de vapor de

agua.

25

Anexo

xvii

En otra realización preferida, la celda electroquímica que contiene un

conductor aniónico como el descrito anteriormente se encuentra a una


corriente gaseosa que contiene al menos un alcohol (C1-C3).

5

Por otro lado, en otra realización preferida, el conductor electrolito sólido

iónico es un conductor catiónico. La corriente de entrada en este caso

también está formada por una corriente gaseosa de hidrocarburos ligeros y

una corriente de vapor de agua, o una corriente gaseosa que contiene al

menos un alcohol (C1-C3). Sin embargo, en el caso de que se quiera utilizar 10

el conductor electrolito iónico catiónico, la corriente gaseosa que contiene al

menos un alcohol (C1-C3) debe contener también una 30 corriente de vapor

de agua.

Preferiblemente, el conductor catiónico conduce iones sodio Na+ o potasio 15

K+.

En otra realización preferida, el electrolito catiónico se selecciona de entre

Na-β-Al2O3, K-β-Al2O3, NASICON, LISICON o cualquiera de sus

combinaciones.

20

En otra realización preferida, el conductor catiónico además comprende al

menos un electrodo metálico selectivo al proceso de reformado de la

corriente de entrada y al menos un contraelectrodo metálico.

Preferiblemente, el electrodo metálico selectivo al proceso de reformado de

la corriente de entrada es de platino (Pt). 25

Preferiblemente, el contraelectrodo metálico es de oro (Au).

Anexo

xviii

En otra realización preferida, la celda electroquímica, que contiene un

conductor catiónico como el descrito anteriormente, se encuentra a una


corriente gaseosa de hidrocarburos 15 ligeros y una corriente de vapor de

agua. 5

En otra realización preferida, la celda electroquímica, que contiene un

conductor catiónico como el descrito anteriormente, se encuentra a una


corriente gaseosa que contiene al 20 menos un alcohol (C1-C3) y que añade 10

una corriente de vapor de agua.

La fuente de voltaje utilizada para aplicar el voltaje mencionado

anteriormente a la celda electroquímica puede ser una fuente convencional

con procedencia de energía fósil o nuclear o una fuente renovable que 15

utiliza por ejemplo, energías hidráulica, solar, eólica, geotérmica, marina

y/o biomasa. Por lo que, por último, en una realización preferida, el

procedimiento utiliza una fuente convencional o renovable para la

aplicación del potencial. Más referiblemente, el procedimiento utiliza una

fuente convencional. 20

A lo largo de la descripción y las reivindicaciones la palabra "comprende" y

sus variantes no pretenden excluir otras características técnicas, aditivos,

componentes o pasos. Para los expertos en la materia, otros objetos,

ventajas y características de la invención se desprenderán en parte de la 25

descripción y en parte de la práctica de la invención. Los siguientes

ejemplos y figuras se proporcionan a modo de ilustración, y no se pretende

que sean limitativos de la presente invención.

Anexo

xix

BREVE DESCRIPCIÓN DE LAS FIGURAS

FIG. 1. Representación de una celda electroquímica que utiliza un

conductor electrolito sólido iónico, que puede ser tanto aniónico como

catiónico. 5

FIG. 2. Representación esquemática de un conductor electrolito sólido que

actúa como conductor aniónico que comprende un electrolito sólido

aniónico, un electrodo selectivo a la electrólisis del agua y un

contraelectrodo selectivo a la reacción de reformado y a la oxidación parcial 10

de la corriente de entrada.

FIG. 3. Variación del ratio H2/CO en función del potencial aplicado para

diferentes temperaturas de reacción. Condiciones: [CH4] = 1%, [H2O] = 3%,

[N2] = 96%, F = 100 ml/min, con conductor aniónico. 15

FIG. 4. Gráfica dinámica donde se muestras los distintos ratios H2/CO

obtenidos en función del potencial aplicado en un determinado periodo de

tiempo.

20

FIG. 5. Representación esquemática de un conductor electrolito sólido que

actúa como conductor catiónico que comprende un electrolito sólido

catiónico, un electrodo metálico selectivo al proceso de reformado de la

corriente de entrada y un contraelectrodo metálico.

25


diferentes temperaturas de reacción. Condiciones: [CH4] = 1%, [H2O] = 3%,

[N2] = 96%, F = 100 ml/min, con conductor catiónico.

Anexo

xx


diferentes temperaturas de reacción. Condiciones: [CH3OH] = 0,7%, [H2O]

= 2%, [N2] = 97,3%, F = 100 ml/min, con conductor aniónico.

EJEMPLO 5

FIG. 1. muestra la configuración de una celda electroquímica de

laboratorio que utiliza conductores electrolitos sólidos iónicos, tanto

aniónicos (9) como catiónicos (10). Esta celda consta de los siguientes

elementos: 10

(1) Electrodo de trabajo

(2) Contralectrodo

(3) Salida de gases

(4) Tapa metálica

(5) Serpentín de refrigeración 15

(5) Tubo de alúmina perforado

(5) Hilos de oro

(5) Tubo de cuarzo

(9) Conductor electrolito sólido aniónico

(10) Conductor electrolito sólido catiónico 20

(11) Entrada de gases

En la celda electroquímica que se muestra en FIG. 1., los conductores

electrolitos sólidos aniónico (9) y catiónico (10) están dentro del tubo de

cuarzo (8) que limita la zona donde se produce el gas de síntesis de ratio

H2/CO controlado y los procesos adicionales electrocatalíticos en fase 25

Anexo

xxi

gaseosa. Este tubo de cuarzo (8) está cerrado por una tapa metálica (4) que

acopla un tubo de alúmina perforado (6). El electrodo de trabajo (1) y el

contraelectrodo (2) se conectan con los conductores electrolitos sólidos

aniónico (9) o catiónico (10) a través de este tubo de alúmina perforado (6),

el potencial se aplica a estos electrodos (1) y (2). Las distintas 5

temperaturas que se alcanzan en la celda electroquímica se consiguen

mediante el serpentín de refrigeración (5) acoplada a la tapa metálica (4) y

mediante hilos de oro (7) que actúan como conductores térmicos y que se

introducen dentro de la celda a través del tubo de alúmina. La corriente de

entrada se introduce por la entrada de gases (11) y el gas de síntesis 10

producido se recoge a través de la salida de gases (3).

Ejemplo 1:

En este primer ejemplo, para producir gas de síntesis de ratio controlable 15

se utilizó una celda electroquímica como la que se representa en FIG. 1.

que comprende un conductor electrolito sólido aniónico como el que se

muestra en FIG. 2.

El conductor electrolito sólido aniónico representado en FIG. 2. consta de 20

los siguientes elementos:

(12) Contraelectrodo selectivo al proceso de reformado y oxidación parcial

de la corriente de entrada (por ejemplo CH4 → CH2, CO, CO2) es platino

(Pt) 25

(13) Electrodo de trabajo selectivo a la electrólisis del agua (H2O + 2e- →

H2) es platino (Pt)

Anexo

xxii

(14) Electrolito sólido aniónico tipo YSZ (óxido de ytrio estabilizado con

óxido de zirconio)

FIG. 2. muestra una representación esquemática de un conductor aniónico

que comprende un electrolito sólido aniónico que actúa como conductor tipo 5

YSZ de iones O2- y que además comprende un electrodo de trabajo selectivo

a la electrólisis del agua (13) y un contraelectrodo selectivo a la reacción de

reformado y oxidación parcial 20 de la corriente de entrada (12).

Ambos electrodos son conectados a una fuente de alimentación que permite 10

la aplicación de la intensidad eléctrica al sistema y que, por tanto, permite

el control de la composición del gas de síntesis en condiciones fijas de

operación (temperatura de operación de la celda electroquímica) y de

reacción (composición y concentración de la corriente de entrada de la

celda electroquímica). 15

El acoplamiento del proceso de electrólisis al proceso catalítico de

reformado permite llevar a cabo la producción adicional de H2 así como la

oxidación parcial del hidrocarburo (y por tanto el ajuste del ratio) sin

necesidad de alimentar oxígeno puro al reactor electroquímico (evitando 20

etapas previas de separación del mismo del aire). El O2 es generado in-situ

en el propio proceso lo que permite la presencia de reacciones secundarias

de oxidación total y parcial.

Para llevar a cabo la producción de gas de síntesis de ratio variable 25

utilizando esta celda electroquímica y el electrolito sólido aniónico descrito,

se introdujo una corriente de entrada compuesta por metano [CH4] = 1 %,

Anexo

xxiii

vapor de agua [H2O] = 3 % y nitrógeno como gas inerte [N2] = 96%, siendo

el caudal de esta corriente de F = 100 ml/min.

En FIG. 3. se puede observar cómo se puede variar el ratio H2/CO obtenido

en función del potencial y las diferentes temperaturas de reacción 5

aplicadas. Esta figura demuestra que la variación del voltaje permite el

control del ratio H2/CO. Los valores del ratio H2/CO obtenidos en esta celda

electroquímica que comprende un conductor electrolito sólido aniónico

oscilan de entre 1,5 y 11 para un rango de temperaturas de entre 750 ºC y

800 ºC. 10

Por otro lado FIG. 4. muestra los ratios de H2, CO y CO2 frente al tiempo

obtenidos usando la configuración de la celda electroquímica descrita en

este ejemplo. La palabra OCP se refiere al estado de circuito abierto en el

que se encuentra la celda electroquímica cuando no se aplica ningún 15

potencial. Durante los periodos de tiempo durante los cuales la celda

electroquímica está en circuito abierto, el ratio H2/CO se mantiene

prácticamente constante. En los periodos de tiempo donde se aplica un

potencial de 2,5 voltios se observa una respuesta inmediata, menor de 5

minutos, que se corresponde con el aumento drástico del ratio H2/CO en 20

función del voltaje aplicado.

Ejemplo 2:

En este ejemplo se utiliza una celda electroquímica como la que se 25

representa en FIG. 1. con un conductor electrolito sólido catiónico como el

que se muestra en FIG. 5. para producir gas de síntesis.

Anexo

xxiv

El electrolito sólido catiónico representado en la figura 5 consta de los

siguientes elementos:

(15) Electrodo de trabajo de platino selectivo al proceso de reformado de la

corriente de entrada (por ejemplo CH4 + H2O → H2, CO, CO2) 5

(16) Contraelectrodo de Au

(17) Electrolito sólido catiónico tipo Na-β-Al2O3

FIG. 5. muestra una representación esquemática de un electrolito sólido

catiónico (17) que actúa como conductor tipo Na-β-Al2O3 de iones Na+ y que

comprende un electrodo selectivo al proceso de reformado de la corriente de 10

entrada (15) y un contraelectrodo de Au (16).

Al igual que en el ejemplo 1, los electrodos son conectados a una fuente de

alimentación que permite la aplicación de la intensidad eléctrica al sistema

y que, por tanto, permite el control de la composición del gas de síntesis en 15

condiciones fijas de operación (temperatura de operación de la celda

electroquímica) y de reacción (composición y concentración de la corriente

de entrada de la celda electroquímica).

Para llevar a cabo la producción de gas de síntesis de ratio variable 20

utilizando la celda electroquímica y el electrolito sólido aniónico descrito,

se introdujo una corriente de entrada compuesta por metano [CH4] = 1%,

vapor de agua [H2O]= 3% y nitrógeno como gas inerte [N2] = 96%, siendo el

caudal de esta corriente de F = 100 ml/min.

25

El control de la cantidad de iones sodio Na+ promotores enviados al

electrodo catalizador se lleva a cabo mediante la aplicación controlada de

Anexo

xxv

corriente eléctrica que permite controlar la adsorción de las especies que

participan en el proceso catalítico y por ende el ratio del gas de síntesis

producido.

En FIG. 6. se muestra la variación del ratio H2/CO en función del potencial 5

aplicado para diferentes temperaturas de reacción a partir de una

corriente humidificada de metano. Esta figura demuestra que la variación

del voltaje permite el control del ratio H2/CO. Los valores del ratio H2/CO

obtenidos en esta celda electroquímica que comprende un conductor

electrolito sólido catiónico oscilan de entre valores de 6 y 30 para un rango 10

de temperaturas de entre 450 ºC y 550 ºC.

Ejemplo 3:

En este ejemplo se utiliza una celda electroquímica como la que se 15

representa en FIG. 1. con el conductor electrolito sólido aniónico que se

describe en el ejemplo 1 y se representa en FIG. 2 para producir gas de

síntesis.

Para llevar a cabo la producción de gas de síntesis de ratio controlable 20

utilizando esta celda electroquímica y el electrolito sólido aniónico descrito,

se introdujo una corriente de entrada compuesta por metanol [CH3OH] =

0,7 %, vapor de agua [H2O] = 2 % y nitrógeno como gas inerte [N2] = 97,3%,

siendo el caudal de esta corriente de F = 100 ml/min.

25

En FIG. 7. se puede observar cómo se puede variar el ratio H2/CO obtenido

en función del potencial y las diferentes temperaturas de reacción

Anexo

xxvi

aplicadas. Esta figura demuestra que la variación del voltaje permite el

control del ratio H2/CO. Los valores del ratio H2/CO obtenidos en esta celda

electroquímica que comprende un conductor electrolito sólido aniónico

oscilan de entre 2,4 y 9,13 para un rango de temperaturas de entre 500 ºC

y 600 ºC. 5

Los ejemplos 1 a 3 proporcionados a modo de ilustración no pretenden ser

limitativos de la presente invención. Aunque se refieran a una celda

electroquímica tamaño laboratorio, esta celda podría ser sustituida por

configuraciones tubulares o configuraciones de tipo reactor monolítico 10

(denominado en inglés Monolithic Electro20 promoted reactor) a escala

industrial.

REIVINDICACIONES

1. Un procedimiento para producir gas de síntesis, de ratio H2/CO 15

controlable, que comprende el paso de una corriente de entrada

seleccionada de entre una corriente gaseosa de hidrocarburos ligeros y

una corriente de vapor de agua, o una corriente gaseosa que contiene al

menos un alcohol (C1-C3) a una celda electroquímica que se encuentra a

una temperatura de entre 300ºC y 980ºC, caracterizado porque dicha 20

celda electroquímica contiene un conductor electrolito sólido iónico al

que se le aplica un potencial de entre -3 y +3 voltios.

2. El procedimiento, según la reivindicación 1, donde la corriente de

entrada está diluida en una corriente de gas inerte seleccionado de la 25

lista que comprende nitrógeno, helio, neón, argón, kriptón y xenón.

Anexo

xxvii

3. El procedimiento, según cualquiera de las reivindicaciones 1 o 2, donde

la celda electroquímica se encuentra a una temperatura de entre 500ºC

y 900ºC.

4. El procedimiento, según cualquiera de las reivindicaciones 1 a 3, donde 5

el potencial aplicado es de entre -2,5 y +2,5 voltios.

5. El procedimiento, según la reivindicación 4, donde el potencial aplicado

es de entre -2 y +2 voltios.

6. El procedimiento, según cualquiera de las reivindicaciones 1 a 5, donde 10

los 25 hidrocarburos ligeros gaseosos se seleccionan de la lista que

comprende metano, etano, propano, butano, gas natural o cualquiera de

sus combinaciones.

7. El procedimiento, según la reivindicación 6, donde el hidrocarburo ligero

es una combinación de hidrocarburos ligeros que comprende al menos 15

metano o es gas natural.

8. El procedimiento, según cualquiera de las reivindicaciones 1 a 5, donde

el alcohol se selecciona de la lista que comprende metanol, etanol,

propanol o cualquiera de sus combinaciones. 20

9. El procedimiento, según la reivindicación 8, donde el alcohol es metanol

o etanol.

Anexo

xxviii

10. El procedimiento, según cualquiera de las reivindicaciones 1 a 5, 8 o 9,

donde al paso de la corriente gaseosa que contiene al menos un alcohol

(C1-C3) además se añade una corriente de vapor de agua.

11. El procedimiento, según cualquiera de las reivindicaciones 1 a 10,

donde las corrientes gaseosas de hidrocarburo o alcohol y la corriente de 5

vapor de agua se 10 mezclan antes de pasar a la celda electroquímica.


donde el conductor electrolito sólido iónico es un conductor aniónico que

conduce iones oxígeno.

10

13. El procedimiento, según la reivindicación 12, donde el conductor

aniónico comprende un electrolito sólido que se selecciona de entre óxido

de zirconio, óxidos de titanio, óxido de itrio estabilizado con óxido de

zirconio, óxido de zirconio estabilizado con calcio, perovskitas con

conductividad mixta o cualquiera de sus combinaciones. 15

14. El procedimiento, según cualquiera de las reivindicaciones 12 o 13,

donde el conductor aniónico comprende al menos un electrodo selectivo

a la electrólisis del agua y al menos un contraelectrodo selectivo a la

reacción de reformado y a la 25 oxidación parcial de la corriente de

entrada. 20

15. El procedimiento, según la reivindicación 14, donde el electrodo

selectivo a la electrólisis del agua es de platino.

16. El procedimiento según cualquiera de las reivindicaciones 14 o 15,

donde el contraelectrodo catalítico poroso selectivo a la reacción de

Anexo

xxix

reformado y la oxidación parcial de la corriente de entrada se selecciona

de entre níquel, platino, paladio o cualquiera de sus combinaciones.


donde la celda electroquímica se encuentra a una temperatura de entre 5

700 y 900 ºC cuando la corriente de entrada es una corriente gaseosa de

hidrocarburos ligeros y una corriente de vapor de agua.


donde la celda electroquímica se encuentra a una temperatura de entre

500 y 750 ºC cuando la corriente de entrada es una corriente gaseosa 10

que contiene al menos un alcohol (C1-C3).


donde el conductor electrolito sólido iónico es un conductor catiónico.

20. El procedimiento, según la reivindicación 19, donde el conductor 15

catiónico conduceiones sodio Na+ o potasio K+.


donde el conductor catiónico comprende un electrolito sólido que se

selecciona de entre Naβ-Al2O3, K-β-Al2O3, NASICON, LISICON o

cualquiera de sus combinaciones. 20


donde el conductor catiónico además comprende al menos un electrodo

Anexo

xxx

metálico selectivo al proceso de reformado de la corriente de entrada y

al menos un contraelectrodo metálico.

23. El procedimiento, según la reivindicación 22, donde el electrodo

metálico selectivo al proceso de reformado de la corriente de entrada es 5

de platino.


donde el 30 contraelectrodo metálico es de oro.


donde la celda electroquímica se encuentra a una temperatura de entre 10

700 y 900 ºC cuando la corriente de entrada es una corriente gaseosa de

hidrocarburos ligeros y una corriente de vapor de agua.


donde la celda electroquímica se encuentra a una temperatura de entre

500 y 750 ºC cuando la corriente de entrada es una corriente gaseosa 15

que contiene al menos un alcohol (C1-C3) y una corriente de vapor de

agua.


donde se utiliza 10 una fuente convencional o renovable para la

aplicación del potencial. 20

Anexo

xxxi

DIBUJOS

FIG. 1

Anexo

xxxii

FIG. 2

FIG. 3

Anexo

xxxiii

FIG. 4

0 10 20 30 40 50 60 70 80 90 100 110

0.0

0.5

1.0

1.5

2.0

H2/CO = 20

H2/CO = 40

I= +2,5 V OCPI= -2.5 VOCP rH

2

rCO

rCO2

r H2, r C

O, r C

O2 (

mol/

s m

g P

t) 1

0-8

Time (min)

OCP

H2/CO = 1

Anexo

xxxiv

FIG. 5

FIG. 6

Anexo

xxxv

FIG. 7

Anexo

xxxvi

RESUMEN

Procedimiento de obtención de gas de síntesis

5

La presente invención se refiere a un procedimiento de gas de síntesis

(H2/CO) de ratio controlable mediante un proceso catalítico y electroquímio

que emplea una celda electroquímica formada por electrolitos sólidos

conductores iónicos, aniónicos o catiónicos. El control del ratio H2/Co se

lleva a cabo en una única etapa bajo condiciones constantes de operación, 10

es decir, a temperatura constante de la celda electroquímica y condiciones

constantes de composición y concentración de la corriente de entrada. En

la presente invención la corriente de entrada se selecciona de entre una

corriente de hidrocarburos ligeros y una corriente de vapor de agua, o una

corriente gaseosa que contiene al menos un alcohol (C1-C3). 15

Anexo

xxxvii

nuevas aplicaciones electrocatalÍticas para

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