17th march 2008nccr hydrogen - production prospects and challenges national centre for catalysis...

17th March 2008 NCCR

Hydrogen - Production Prospects and Challenges

National Centre for Catalysis Research (NCCR)

Indian Institute of Technology, Madras


HYDROGEN FUTURE: FACTS AND FALLACIES

M. Aulice Scibioh and B. Viswanathan, Bulletin of the Catalysis Society of India, vol.3, pp.72-81(2004)

• A transition to a ‘hydrogen economy’ is a sea change in our energy infrastructure and

is not to be taken lightly.• only 50% can reach the end user due to losses in electrolysis, hydrogen compression

and the fuel cell. • The rush into a hydrogen economy is neither supported by energy efficiency

arguments nor justified with respect to economy or ecology.• In fact, it appears that hydrogen will not play an important role in a sustainable

energy economy because the synthetic energy carrier cannot be more efficient than the energy from which it is made.

• Renewable electricity is better distributed by electrons than by hydrogen. Consequently, the hasty introduction of hydrogen as an energy carrier cannot be a stepping stone into a sustainable energy future. The opposite may be true.

• Because of the wastefulness of a hydrogen economy, the promotion of hydrogen may counteract all reasonable measures of energy conservation. Even worse, the forced transition to a hydrogen economy may prevent the establishment of a sustainable energy economy based on an intelligent use of precious renewable resources. .

17th March 2008 NCCROxidant ---- gaseous oxygen/air (In general, the oxygen needed by a fuel cell is supplied in the form of air)

FUEL

G0 kcal/mol

E0theoretical

(V)

E0max

(V)

Energy density

(kWh/kg)

Hydrogen (H2) -56.69 1.23 1.15 32.67

Methanol (CH3OH) -166.80 1.21 0.98 6.13

Ammonia(NH3) -80.80 1.17 0.62 5.52

Hydrazine(N2H4) -143.90 1.56 1.28 5.22

Formaldehyde(HCHO) -124.70 1.35 1.15 4.82

Carbon monoxide(CO) -61.60 1.33 1.22 2.04

Formic acid(HCOOH) -68.20 1.48 1.14 1.72

Methane(CH4) -195.50 1.06 0.58 -

Propane(C3H8) -503.20 1.08 0.65 -

Choice of fuel and oxidant Chemical & electro-chemical data on various fuels


Comparison of fuel properties

Properties Hydrogen (H2)

Methane (CH4)

Gasoline

(-CH2-)

Lower heating value(kWhKg-1) 33.33 13.9 12.4

Self ignition temperature (°C) 585 540 228-501

Flame temperature (°C ) 2.045 1.875 2.200

Ignition limits in air ( Vol %) 4-75 5.3-15 1.6-7.6

Minimal Ignition energy (mWs) 0.02 0.29 0.24

Flame propagation in air (ms-1) 2.65 0.4 0.4

Diffusion coefficient in air (cm2s-1) 0.61 0.16 0.05

Toxicity No No High

L. Schlapbach et al Nature, 414 (2001) 353.


Transition to hydrogen economy

Production

Storage

DistributionChoice limited

Metal HydrideMOF

Petrol dispensing station

http://img.sparknotes.com/figures/0/02480ae8fc1a41b131a3fdb5a698e9a3/lyticcell.gif


• Broad-based use of hydrogen as a fuel– Energy carrier analogous to electricity– Produced from variety of primaryenergy sources– Can serve all sectors of the economy: transportation, power, industry,buildings and residential– Replaces oil and natural gas as thepreferred end-use fuel – Makes renewable and nuclear energy “portable” (e.g. transportation needs)• Advantages:– Inexhaustible– Clean– Universally available to all countries

Transition to a “Hydrogen Economy” in Indian context

http://upload.wikimedia.org/wikipedia/commons/thumb/5/55/Emblem_of_India.svg/310px-Emblem_of_India.svg.png


Hydrogen Production Technologies

• Steam Reforming and partial oxidation

• Thermal

• Thermo-chemical (Fe-X2;S-I2)

• Electrolysis

• Electrochemical

• Photolysis

• Photochemical

• Photoelectrochemical(PEC PVC)

• Biological

• Bio-chemical, photobiological

Various ways of production of hydrogen

Technology awaited

CO2emissions

Water

Principle of PEC


STATUS OF THERMOCHEMICAL CYCLES


POTENTIAL THERMOCHEMICAL CYCLES


Photocatalysis

Conventional redox

reactionOxidizing agent should have

more positive potentialPhotocatalysis -

simultaneous oxidation and

reduction The redox couple capable of

promoting both the reactions

can act as photocatalystMetals, Semiconductors and

Insulators

reaction assisted by photons

catalyst

10

Metals

VB

CB

VB

CB

VB

CBH+/H2

H2O/O2

Insulators

SC

E

Metals: No band gap,Only reduction or oxidation, Depends on the band position Insulators: High band gap, High energy requirement

Material selection for PEC or Photocatalysis


H+/H2

H2O/O2

OR Type – Oxidation and Reduction

R Type – Reduction

O Type – Oxidation

X Type – None

0.00V

1.23V

SELECTION OF MATERIALS

CdS, ZnS, ZnO undergo photo corrosion Activity decrease as the time increasesS deposition on the catalyst surface reduce the light absorption ability of catalyst.

Among them TiO2 is widely usedThough ZnO, CdS, ZnS and WO3 have wide band gap they undergo photocorrosion

17th March 2008 NCCR Various recombination process on the photo-excited semiconductor surface and inside the

bulk.

Coupling involves mixing small band gap semiconductor with a higher band gap oneSmaller band gap semiconductor absorbs in visible region and transfers excitons to the other semiconductorRecombination in the small band gap semiconductor reduced

SOME CONCEPTUAL DESIGNS


Selection Criterion• Ionic solids as the ionicity of the M-O

bond increases, the top of the valence band (mainly contributed by the p- orbitals of oxide ions) becomes less and less positive (since the binding energy of the p orbitals will be decreased due to negative charge on the oxide ions) and the bottom of the conduction band will be stabilized to higher binding energy values due to the positive charge on the metal ions which is not favourable for the hydrogen reduction reaction.

• More ionic the M-O bond of the semiconductor is, the less suitable the material is for the photo-catalytic splitting of water. The bond polarity can be estimated from the expression

• Percentage ionic character (%) =

100)1( 4

)( 2

BA

e

Semiconductor M-O Percentage ionic character

TiO2

SrTiO3

Fe2 O3

ZnOWO3

CdSCdSe

LaRhO3

LaRuO3

PbOZnTeZnAsZnSeZnSGaPCuSe

BaTiO3

MoS2

FeTiO3

KTaO3

MnTiO3

SnO2

Bi2O3

Ti-OTi-O-Sr

Fe-OZn-OW-OCd-SCd-S

La-O-RhLa-O-Ru

Pb-OZn-TeZn-AsZn-SeZn-SGa-PCu-Se

Ba-O-TiMo-S

Fe-O-TiK-O-Ti

Mn-O-TiSn-OBi-O

59.568.547.355.557.517.616.553.053.526.5 5.06.8

18.419.53.5

10.070.84.3

53.572.759.042.239.6


Some Governing Principles• The oxide semiconductors though - suitable

for the photo-catalytic water splitting reaction in terms of the band gap value which is greater than the water decomposition potential of 1.23 V.

• Most of these semiconductors have bond character more than 50-60 % and hence modulating them will only lead to increased ionic character and hence the photo-catalytic efficiency of the system may not be increased as per the postulates developed

• Therefore from the model developed in this presentation the following postulates have been evolved.

• Hence we need stoichiometrically both oxidation and reduction for the water splitting and this reaction will not be achieved by one of the trapping agents namely that is used for electrons or holes. Even if one were to use the trapping agents for both holes and electrons, the relative positions of the edge of the valence band and bottom of the conducting band may not be adjusted in such a way to promote both the reactions simultaneously

The photo-catalytic semiconductors are often used with addition of metals or with other hole trapping agents so that the life time of the excitons created can be increased. In this mode, the positions of the energy bands of the semiconductor and that of the metal overlap appropriately and hence the alteration can be either way and also in this sense only the electrons are trapped at the metal sites and only reduction reaction is enhanced

Normally the semiconductors used in photo-catalytic processes are substituted in the cationic positions so as to alter the band gap value. Even though it may be suitable for using the available solar radiation in the low energy region, it is not possible to use semiconductors whose band gap is less than 1.23 V and any thing higher than this may be favourable if both the valence band is depressed and the conduction band is destabilized with respect to the unsubstituted system. Since this situation is not obtainable in many of the available semiconductors by substitution at the cationic positions, this method has not also been successful.


Some Possibilities

In addition the dissolution potential of the substituted systems may be more favourbale than the water oxidation reaction and hence this will be the preferred path way. These substituted systems or even the bare semiconductors which favour the dissolution reaction will undergo only preferential photo-corrosion and hence cannot be exploited for photo-catalytic pathway. In this case ZnO is a typical example.

• Therefore it is deduced that the systems which has ionic bond character of about 20-30% with suitable positions of the valence and conduction band edges may be appropriate for the water splitting reaction.

• This rationalization has given one a handle to select the appropriate systems for examining as photo-catalysts for water splitting reaction

Very low value of the ionic character also is not suitable since these semiconductors do not have the necessary band gap value of 1.23 V. - the search for utilizing lower end of the visible region is not possible for direct water splitting reaction. If one were to use visible region of the spectrum, then only one of the photo-redox reactions in water splitting may be preferentially promoted and probably this accounts for the frequent observation that non-stiochiometric amounts of oxygen and hydrogen were evolved in the photo-assisted splitting of water

There are some other aspects of photo-catalysts on which some remarks may be appropriate.

Though they have been derived from the solid state point of view like flat band potential , band bending, Fermi level pinning, these parameters also can be understood in terms of the bond character and the redox chemical aspects by which the water splitting reaction is dealt.


Some Other Opportunities

TYPICAL PHOTOCATALYTIC PROCESS

Photodecomposition of water

Photo-catalytic formation of fuel

Photo-catalysis in pollution abatement

Photo-catalysis route for chemicals

(G.Maghesh, B.Viswanathan, R.P.Viswanath & T.K Varadarajan, PEPEEF, Research Signpost (2007)pp.321-357.)

ENGINEERING THE SEMICONDUCTOR ELECTRONIC STRUCTURES

without deterioration of the stability

should increase charge transfer processes at the interface

should improvements in the efficiency


Modifications and opportunites

• various conceptual principles have been incorporated into typical TiO2 system so as to make this system responsive to longer wavelength radiations. These efforts can be classified as follows:

• Dye sensitization• Surface modification of the semiconductor

to improve the stability • Multi layer systems (coupled

semiconductors)• Doping of wide band gap semiconductors

like TiO2 by nitrogen, carbon and Sulphur • New semiconductors with metal 3d valence

band instead of Oxide 2p contribution • Sensitization by doping.• All these attempts some kind sensitization

and hence the route of charge transfer has been extended and hence the efficiency could not be increased considerably. Success appears to be eluding the researchers.

• THE AVAILABLE OPPORTUNITIESWhat modifications?

• Identifying and designing new semiconductor materials with considerable conversion efficiency and stability

• Constructing multilayer systems or using sensitizing dyes - increase absorption of solar radiation

• Formulating multi-junction systems or coupled systems - optimize and utilize the possible regions of solar radiation

• Developing nanosize systems - efficiently dissociate water


The opportunities

• The opportunities that are obviously available as such now include the following:-Identifying and designing new semi-conductor materials with considerable conversion efficiency and stability– Constructing multilayer systems or using sensitizing

dyes so as to increase absorption of solar radiation.– Formulating multi-junction systems or coupled systems

so as to optimize and utilize the possible regions of solar radiation.

– Developing catalytic systems which can efficiently dissociate water.

18


Opportunities evolved• Deposition techniques have been considerably perfected and

hence can be exploited in various other applications like in thin film technology especially for various devices and sensory applications.

• The knowledge of the defect chemistry has been considerably improved and developed.

• Optical collectors, mirrors and all optical analysis capability have increased which can be exploited in many other future optical devices.

• The understanding of the electronic structure of materials has been advanced and this has helped to our background in materials chemistry.

• Many electrodes have been developed, which can be a useful for all other kinds of electrochemical devices.

19


Limited success – Why?The main reasons for this limited success in all these directions are due

to:• The electronic structure of the semiconductor controls the reaction

and engineering these electronic structures without deterioration of the stability of the resulting system appears to be a difficult proposition.

• The most obvious thermodynamic barriers to the reaction and the thermodynamic balances that can be achieved in these processes give little scope for remarkable improvements in the efficiency of the systems as they have been conceived and operated. Totally new formulations which can still satisfy the existing thermodynamic barriers have to be devised.

• The charge transfer processes at the interface, even though a well studied subject in electrochemistry has to be understood more explicitly, in terms of interfacial energetics as well as kinetics. Till such an explicit knowledge is available, designing systems will have to be based on trial and error rather than based on sound logical scientific reasoning.

20


• Nanocrystalline (mainly oxides like TiO2, ZnO, SnO and Nb2O5 or chalcogenides like CdSe) mesoscopic semiconductor materials with high internal surface area If a dye were to be adsorbed as a monolayer, enough can be retained on a given area of the electrode so as to absorb the entire incident light.

• Since the particle sizes involved are small, there is no significant local electric field and hence the photo-response is mainly contributed by the charge transfer with the redox couple.

• Two factors essentially contribute to the photo-voltage observed, namely, the contact between the nano crystalline oxide and the back contact of these materials as well as the Fermi level shift of the semiconductor as a result of electron injection from the semiconductor.

21


Another aspect of the nano crystalline state is the alteration of the band gap to larger values as compared to the bulk material which may facilitate both the oxidation/reduction reactions that cannot normally proceed on bulk semiconductors. The response of a single crystal anatase can be compared with that of the meso-porous TiO2 film sensitized by ruthenium complex (cis RuL2 (SCN)2, where L is 2-2’bipyridyl-4-4’dicarboxlate). The incident photon to current conversion efficiency (IPCE) is only 0.13% at 530 nm ( the absorption maximum for the sensitizer) for the single crystal electrode while in the nano crystalline state the value is 88% showing nearly 600-700 times higher value.

22

This increase is due to better light harvesting capacity of the dye sensitized nano crystalline material but also due to mesoscpic film texture favouring photo-generation and collection of charge carriers . It is clear therefore that the nano crystalline state in combination with suitable sensitization is one another alternative which is worth investigating.


New Opportunities1. New semi-conducting materials with conversion efficiencies and

stability have been identified. These are not only simple oxides, sulphides but also multi-component oxides based on perovskites and spinels.

2. Multilayer configurations have been proposed for absorption of different wavelength regions. In these systems the control of the thickness of each layer has been mainly focused on.

23

3. Sensitization by dyes and other anchored molecular species has been suggested as an alternative to extend the wavelength region of absorption.

4. The coupled systems, thus giving rise to multi-junctions is another approach which is being pursued in recent times with some success

5. Activation of semiconductors by suitable catalysts for water decomposition has always fascinated scientists and this has resulted in various metal or metal oxide (catalysts) loaded semi conductors being used as photo-anodes


New opportunities (Contd)• Recently a combinatorial electrochemical synthesis and

characterization route has been considered for developing tungsten based mixed metal oxides and this has thrown open yet another opportunity to quickly screen and evaluate the performances of a variety of systems and to evolve suitable composition-function relationships which can be used to predict appropriate compositions for the desired manifestations of the functions.

• It has been shown that each of these concepts, though has its own merits and innovations, has not yielded the desired levels of efficiency. The main reason for this failure appears to be that it is still not yet possible to modulate the electronic structure of the semiconductor in the required directions as well as control the electron transfer process in the desired direction.

24


Where are we?

LIMITED SUCCESS – WHY?

THE OPPORTUNITIES EVOLVED

• Difficulties on controlling the semi-conductor electronic structure without deterioration of the stability

• Little scope on the thermodynamic barriers and the thermodynamic balances for remarkable improvements in the efficiency

• Incomplete understanding in the interfacial energetic as well as in the kinetics

Deposition techniques -thin film technology, for various devices and sensory applications. Knowledge of the defect chemistry has been considerably improved and developed. Optical collectors, mirrors and all optical analysis capability have increased Understanding of the electronic structure of materials Many electrodes have been developed- useful for all other kinds of electro-chemical devices.


AMOUNT OF HYDROGEN EVOLVED BY CdS PHOTOCATALYST

26


TEM IMAGE OF CdS NANOPARTICLES

CatalystParticle Size

(nm)

Surfacearea (m2/g)

Rate of hydrogenproduction ( moles /h)

CdS - Y 8.8 36 102

CdS - Z 6 46 68

CdS - 11 26 67

CdS - Bulk 23 14 45

CdS-Z

CdS-

100 nm100 nm

CdS-Z

27


SCANNING ELECTRON MICROGRAPHS

28

CdS-Z CdS-Y

CdS- CdS- bulk


Effect of Metal loading on SC

Electrons are transferred to metal surface Reduction of H+ ions takes place at the metal surface The holes move into the other side of semiconductor The oxidation takes place at the semiconductor surface

Pt, Pd & Rh show higher activity. High reduction potential. Hydrogen over voltage is less for Pt, Pd & Rh

PHOTOCATALYSIS ON Pt/TiO2 INTERFACE Effect of metals on hydrogen evolution rate

Pt

Pd

Rh

Au

Cu

Ag

Ni

Fe

Ru

3 %


Activity of the catalyst is directly proportional to work function of the metal and M-H bond strength.

PHOTOCATALYTIC HYDROGEN EVOLUTION OVER METAL LOADED CdS NANOPARTICLES

30


MetalRedox

potential(E0)

Metal- hydrogen bond energy (K cal mol-1)

Work function

(eV)

Hydrogen evolution rate*(µmol h-1 0.1g-1)

PtPdRhRu

1.1880.9510.7580.455

62.864.565.166.6

5.655.124.984.71

60014411454

HYDROGEN PRODUCTION ACTIVITY OF METAL LOADED CdS PREPARED FROM H-ZSM-5

*1 wt% metal loaded on CdS-Z sample. The reaction data is presented after 6 h under reaction condition.

M. Sathish, B. Viswanathan, R. P. Viswanath Int. J. Hydrogen Energy ()31


EFFECT OF SUPPORT ON THE CdS PHOTOCATLYTIC ACTIVITY

2, 5,10 and 20 wt % CdS on support - by dry impregnation method

MgO support has higher photocatalytic activity - favourable band position

Alumina & Magnesia supports enhance photocatalytic activity

32


Pb2+/ ZnS

Absorption at 530nm (calcinations at 623-673K)

Formation of extra energy levels between the band gap by Pb 6s orbital

Low activity at 873K is due to PbS formation on the surface (Zinc blende to wurtzite)

(a) 573 K, (b) 623 K, (c) 673 K, (d) 773 K, and (e) 873K Band structure of ZnS doped with Pb.

Eg

I. Tsuji, et al J. Photochem. Photobiol. A. Chem 622 (2003) 1 33


250 ml of 5 mM Na2S solution

250 ml of 1 mM Cd(NO3)2

Rate of addition 20 ml / h

Ultrasonic waves

= 20 kHz

The resulting precipitate was washed with distilled water

until the filtrate was free from S2- ions

PREPARATION OF MESOPOROUS CdS NANOPARTICLE

BY ULTRASONIC MEDIATED PRECIPITATION

34


The specific surface area and pore volume are 94 m2/g and 0.157 cm3/g respectively

The adsorption - desorption isotherm – Type IV (mesoporous nature)

Mesopores are in the range of 30 to 80 Å size

The maximum pore volume is contributed by 45 Å size pores

N2 ADSORPTION - DESORPTION ISOTHERM

35


XRD pattern of as-prepared CdS -U shows the presence of cubic phase

The observed “d” values are 1.75, 2.04 and 3.32 Å corresponding to the (3 1 1) (2 2 0) and (1 1 1) planes respectively - cubic

The peak broadening shows the formation of nanoparticles

The particle size is calculated using Debye Scherrer Equation

The average particle size of as- prepared CdS is 3.5 nm

X- RAY DIFFRACTION PATTERN

M. Sathish and R. P. Viswanath Mater. Res. Bull (Communicated)36


The growth of fine spongy particles of CdS-U is observed on the surface of the CdS-U

The CdS-bulk surface is found with large outgrowth of CdS particles

The fine mesoporous CdS particles are in the nanosize range

The dispersed and agglomerated forms are clearly observed for the as-prepared CdS-U

CdS-U CdS - Bulk

TEM SEM

ELECTRON MICROGRAPHS

37

CdS-U

100 nm


Metal CdS-U CdS-Z CdS

bulk

-

Rh

Pd

Pt

73

320

726

1415

68

114

144

600

45

102

109

275

1 wt % Metal loaded CdS – U is 2-3 times more active than

the CdS-Z

PHOTOCATALYTIC HYDROGEN PRODUCTION

Na2S and Na2SO3 mixture used as sacrificial agent

Amount of hydrogen (µM/0.1 g)

38


Where are we?

LIMITED SUCCESS – WHY?

THE OPPORTUNITIES EVOLVED

• Difficulties on controlling the semi-conductor electronic structure without deterioration of the stability

• Little scope on the thermodynamic barriers and the thermodynamic balances for remarkable improvements in the efficiency

• Incomplete understanding in the interfacial energetic as well as in the kinetics

Deposition techniques -thin film technology, for various devices and sensory applications. Knowledge of the defect chemistry has been considerably improved and developed. Optical collectors, mirrors and all optical analysis capability have increased Understanding of the electronic structure of materials Many electrodes have been developed- useful for all other kinds of electro-chemical devices.


Thank you all foryour kind attention

17th march 2008nccr hydrogen - production prospects and challenges national centre for catalysis...

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