2004 active cuzno and cuznoal2o3 catalysts prepared by homogeneous

8/10/2019 2004 Active CuZnO and CuZnOAl2O3 Catalysts Prepared by Homogeneous

1/5

Applied Catalysis A: General 263 (2004) 249253

Active Cu/ZnO and Cu/ZnO/Al2O3catalysts prepared by homogeneousprecipitation method in steam reforming of methanol

Tetsuya Shishido a,1, Yoshihiro Yamamoto b, Hiroyuki Morioka c,Ken Takaki b, Katsuomi Takehira b,

a Department of Chemistry, Tokyo Gakugei University, Nukui-Kita 4-1-1, Koganei, Tokyo 184-8501, Japanb Department of Chemistry and Chemical Engineering, Graduate School of Engineering, Hiroshima University,

Kagamiyama 1-4-1, Higashi-Hiroshima, Hiroshima 739-8527, Japanc Hiroshima Prefectural Institute of Industrial Science and Technology, Kagamiyama 3-10-32, Higashi-hiroshima, Hiroshima 739 -0046, Japan

Received 25 September 2003; received in revised form 9 December 2003; accepted 11 December 2003

Abstract

Cu/ZnO and Cu/ZnO/Al2O3 catalysts were prepared by homogeneous precipitation (HP) using urea hydrolysis and were applied for

hydrogen production by steam reforming of methanol. The catalysts showed higher activities than those prepared by coprecipitation (CP). It is

suggested that the good catalytic performances of HP-Cu/ZnO and HP-Cu/ZnO/Al2O3are due to both highly dispersed Cu metal species and

to high accessibility of the Cu metal species to methanol and steam. The homogeneous precipitation method by urea hydrolysis is preferable

for the preparation of active Cu/ZnO and Cu/ZnO/Al2O3 catalysts.

2003 Elsevier B.V. All rights reserved.

Keywords:Hydrogen; Homogeneous precipitation; Methanol; Cu/ZnO

1. Introduction

Hydrogen is forecasted to become a major source of

energy in the future. However, as the major impediment

to the wider use of hydrogen as an energy source are the

difficulties inherent in storage and distribution, one solu-

tion to these problems is the on-board hydrogen generation

from a suitable liquid fuel of high energy density. It is

well known that hydrogen can be obtained directly from

methanol according to three different processes: decompo-

sition (Eq. (1))[14], partial oxidation (Eq. (2)) [5,6] and

steam reforming (SRM:Eq. (3))[711]

CH3OH = CO+ 2H2, H= +22kJmol1 (1)

CH3OH+12 O2=CO2 + 2H2, H= 192.2kJmol

1

(2)

Corresponding author. Tel.: +81-824-24-7744;

fax: +81-824-24-7744.

E-mail addresses:[email protected] (T. Shishido),

[email protected] (K. Takehira).1 Tel.: +81-42-329-7494; fax: +81-42-329-7494.

CH3OH+H2O = CO2 + 3H2, H= +49.4kJmol1

(3)

Unfortunately, the decomposition and the partial oxidation

produce considerable amount of CO as a byproduct. In the

use of the reformed gas in a fuel cell, even a trace of CO

deteriorates the Pt electrode. On the other hand, the steam

reforming of methanol can produce H2/CO2 in the molar

ratio of 3/1 and no significant CO below 573 K. Thus, it is

likely that the steam reforming is the most suitable process

to produce hydrogen from on-board methanol for the fuel

cell for the automobile power source.

A large number of catalysts for the steam reforming

of methanol have been reported in the literature [711].

Among these, the majority interestingly use Cu/ZnO-based

system which has also been used in the reverse reaction, i.e.

methanol synthesis from synthesis gas. Since the catalytic

activity of the Cu/ZnO system itself is not high enough,

various modifications have been proposed, such as addition

of a suitable promoter[7]and combination with an effective

component[8,9]. In this paper, we report an effective prepa-

ration method of Cu/ZnO and Cu/ZnO/Al2O3 catalysts, i.e.

the homogeneous precipitation method by urea hydrolysis.

0926-860X/$ see front matter 2003 Elsevier B.V. All rights reserved.

doi:10.1016/j.apcata.2003.12.018


2/5

250 T. Shishido et al. / Applied Catalysis A: General 263 (2004) 249253

We performed the SRM over Cu/ZnO and Cu/ZnO/Al2O3catalysts and found that the catalysts prepared by the homo-

geneous precipitation by urea hydrolysis are more highly

efficient and more stable for SRM than those prepared by

coprecipitation.

2. Experimental

2.1. Catalyst preparation

Cu/ZnO and Cu/ZnO/Al2O3 catalysts were prepared by

two methods, i.e. coprecipitation (CP) and homogeneous

precipitation method by urea hydrolysis (HP). CP-Cu/ZnO

and CP-Cu/ZnO/Al2O3 catalysts were prepared by copre-

cipitation, i.e., the aqueous solution of metal nitrates was

dropped into aqueous solution of Na2CO3with vigorous stir-

ring and the pH was adjusted at 7 with NaOH. The obtained

precipitate was dried at 353 K, followed by calcination at

573 K for 3 h. Both HP-Cu/ZnO and HP-Cu/ZnO/Al2O3cat-alysts were prepared by homogeneous precipitation by urea

hydrolysis [1214] as follows: urea was mixed into a so-

lution of metal nitrates at room temperature, and the urea

was hydrolyzed by heating the mixture at 363 K. During the

hydrolysis of urea, hydroxide ions are generated in the ho-

mogeneous solution (Eq. (4)).

CO(NH2)2 +H2O 2NH4++HCO3

+OH (4)

It is expected that the homogeneity of the precipitate will be

higher than that prepared by conventional coprecipitation,

since there is no gradient in concentration of precipitants

in the solution. The precipitates were then filtered, washedwith distilled water, dried in air at 353 K and finally calcined

at 573 K for 3 h.

2.2. Characterization

The structures of catalysts were characterized by XRD

(Rigaku RINT2550VHF). Inductively coupled plasma (ICP)

analyses were measured by Shimadzu ICPS-100V. BET

measurements were conducted using N2 at 77K with a

Micromeritics Flowsorb2300 instrument. The copper metal

surface areas were determined by N2O decomposition at

363 K as reported by Evans et al. [15],assuming a reaction

stoichiometry of two Cu atoms per oxygen atom and a Cu

surface density of 1.63 1019 Cu atom/m2. Scanning

electron micrographs (SEM) were obtained over on a JEOL

JSM6340F.

2.3. Catalytic reactions

The steam reforming and oxidative steam reforming of

methanol were carried out using a fixed-bed flow reactor at

atmospheric pressure using 200 mg of catalysts. The cata-

lysts were diluted with 200 mg of quartz granules. The ther-

mocouple was introduced from the top of the reactor, and

was placed at the center of the catalyst bed. The catalysts

were reduced at 623 K for 20 min in 14.3vol.% H2/N2 flow

(total flow rate: 35 ml-NTP min1). The feed gas composi-

tion was MeOH/H2O/N2 = 10/12/30 ml-NTP min1 in the

steam reforming. The products were analyzed by on-line

TCD gas chromatographs with Porapak-Q and Molecular

Sieve 5A columns.

3. Results and discussion

Fig. 1shows the XRD patterns of CP- and HP-Cu/ZnO

catalysts before (A) and after (B) calcination. Before the

calcination, both CP- and HP-Cu/ZnO catalysts clearly

showed the diffraction lines due to aurichalcite ((Cu,

Zn)5(CO3)2(OH)6, ICDD 17743), wherein the copper is

atomically mixed with zinc [16,17]. After the calcination,

auricalcite decomposed to CuO and ZnO phases. The width

of diffraction lines of CuO and ZnO phases in HP-Cu/ZnO

Intensity/

kcps

6055504540353025

2 / degree

Intensity/kcps

40353025201510

2 / degree

0.5

1.0

CuOZnO

(b)

(a)

(b)

(a)

Aurichalcite(A)

(B)

Fig. 1. XRD patterns of Cu/ZnO catalysts before (A) and after (B)

calcination. (a) CP-Cu/ZnO, (b) HP-Cu/ZnO.


3/5

T. Shishido et al. / Applied Catalysis A: General 263 (2004) 249253 251

Table 1

Activity and selectivity for steam reforming of methanol over Cu/ZnO and Cu/ZnO/Al2O3 catalystsa

Catalystb Cu/(Cu + Zn)c Reaction

temperature (K)

Conv. of

MeOH (%)

Selectivity (%) Rate of H2production

(cm3 min1 g1)CO2 CO

CP-Cu/ZnO 0.3 473 12.1 100.0 0.0 18.2

523 46.4 99.6 0.4 69.6

573 84.9 95.2 4.8 127.4

HP-Cu/ZnO 0.3 473 32.4 100.0 0.0 48.6

523 89.2 97.3 2.7 133.8

573 100.0 95.6 4.4 150.0

0.5 473 37.5 100.0 0.0 56.3

523 94.2 99.6 0.4 141.3

573 100.0 94.3 5.7 150.0

0.7 473 27.0 100.0 0.0 40.5

523 85.6 99.3 0.7 128.4

573 100.0 95.4 4.6 150.0

HP-Cu/ZnO/Al2O3d 0.5 473 47.4 100.0 0.0 71.1

523 97.3 99.0 1.0 146.0

573 100.0 92.7 7.3 150.0

a

MeOH/H2O/N2 = 10/12/30 ml-NTP min1

.b CP: co-precipitation, HP: homogeneous precipitation using urea hydrolysis.c Metal composition was calculated from the amount of reagents.d Cu/Zn/Al = 45/45/10 mol%, metal composition was calculated from the amount of reagents.

was larger than those in CP-Cu/ZnO, indicating that the

particle size of CuO and ZnO in HP-Cu/ZnO is smaller

than that in CP-Cu/ZnO. After the reduction, CuO was re-

duced to Cu metal, whereas the ZnO phase still remained.

The XRD patterns of CP- and HP-Cu/ZnO/Al2O3 cata-

lysts before calcination also showed the diffraction lines

due to aurichalcite. After the calcination, both CuO and

ZnO phases were observed. On the contrary, no diffrac-

tion lines of any Al compounds, such as Al2O3, CuAl2O4and ZnAl2O4 were observed, suggesting that Al species

were highly dispersed in the HP-Cu/ZnO/Al2O3. After the

reduction, CuO was reduced to Cu metal.

The catalytic activities of the Cu/ZnO and Cu/ZnO/Al2O3catalysts prepared by CP- and HP-methods for the steam

reforming of methanol are summarized in Table 1. In all

the reactions, the effluent gas contained H2and CO2as the

major component, together with a small amount of CO. No

other products, such as dimethyl ether, methyl formate and

Table 2

Physical properties and TOF values for steam reforming of methanol over Cu/ZnO and Cu/ZnO/Al2O3 catalysts

Catalyst Cu/(Cu + Zn)a Conv. (%)b SBETc (m2 g-cat1) SCu0

d (m2 g-cat1) TOFe (102 s1)

CP-Cu/ZnO 0.3 12.1 40.7 16.0 2.78

HP-Cu/ZnO 0.3 32.4 107.9 41.0 2.90

0.5 37.5 76.4 41.6 3.30

0.7 27.0 62.5 23.7 4.17

HP-Cu/ZnO/Al2O3 0.5 47.4 97.5 47.0 3.70

a Metal composition was calculated from the amount of reagents.b Reaction temperature 473 K.c BET surface area.d Cu metal surface area was determined by N2O decomposition method[15].e Hydrogen molecules produced per surface Cu atom per second at 473 K.

methane, could be obtained over any of the catalysts tested.

The methanol conversion and hydrogen production rate in-

creased with increasing the reaction temperature. The phys-

ical properties and TOF values are shown inTable 2.The

TOF values were calculated from the surface area of metal-

lic Cu and the production rate of hydrogen. The actual Cu

loadings which are measured by ICP analyses are consistent

with the Cu loadings estimated from the amount of reagents.

Although the content of copper in CP-Cu/ZnO (Cu/Zn =

30/70) was equal to that in HP-Cu/ZnO (Cu/Zn = 30/70),

the conversion of methanol and the production rate of hydro-

gen on CP-Cu/ZnO were lower than those on HP-Cu/ZnO,

indicating that the preparation method influences the cat-

alyticactivity. TheHP-Cu/ZnOshowedhighervalues of both

BET and Cu metal surface areas as well as the higher activity

than those of CP-Cu/ZnO. SEM observation of HP-Cu/ZnO

(Cu/Zn = 30/70) revealed the presence of spherical fine

particles (ca. 10m in diameter) as shown inFig. 2at a


4/5

252 T. Shishido et al. / Applied Catalysis A: General 263 (2004) 249253

Fig. 2. SEM images (5000) of the HP-Cu/ZnO (a) and the CP-Cu/ZnO (b).

magnification of 5000. Judging from the SEM observation

inFig. 2(a), one can conclude that the spherical fine par-

ticles consist an assembly of small thin plates and possess

a developed porous structure. Although some aggregates of

sphericalfine particles were observed when the magnifica-

tion was low (not shown), every particle still showed a quite

spherical morphology. On the other hand, CP-Cu/ZnO af-

forded rather massive precipitation (Fig. 2(b)).

Interestingly, although both CP- and HP-Cu/ZnO have

the same crystal structure of auricalcite in the XRD ob-

servations, the obtained morphologies were quite different,

resulting in the significant difference between BET results

and Cu metal surface areas. On the contrary, the TOF value

on HP-Cu/ZnO was almost identical to that on CP-Cu/ZnO.

This result suggests that the property of active site is little

affected by the preparation method for Cu/ZnO catalyst. A


5/5

T. Shishido et al. / Applied Catalysis A: General 263 (2004) 249253 253

50

60

70

80

90

100

0 5 10 15 20 25

Time on stream /h

Conversiono

fmethanol/%

Fig. 3. Time course of the steam reforming of methanol over the Cu/ZnO

and the Cu/ZnO/Al2O3 catalyst. () HP-Cu/ZnO/Al2O3 (45/45/10), ()

HP-Cu/ZnO (30/70), () CP-Cu/ZnO/Al2O3 (45/45/10) and () Com-

mercial Cu/ZnO/Al2O3. Reaction temperature, 523K; catalyst, 0.2g;

MeOH/H2O/N2 = 10/12/30 ml-NTP min1.

well-mixed copper-zinc hydroxide may be formed by the

homogeneous precipitation. Thus, it is likely that highly dis-

persed Cu metal particles were formed after the reduction,

and this highly dispersed Cu metal species may show the

high accessibility to methanol and steam. These are reasons

why the high activity for the steam reforming of methanol

was obtained over HP-Cu/ZnO even at low temperature, as

shown inTable 1.Here, we point out that the homogeneous

precipitation method by urea hydrolysis is a hopeful candi-

date as the preparation method of active Cu/ZnO catalystsfor hydrogen production via steam reforming of methanol.

An increase in the Cu content in HP-Cu/ZnO catalyst up

to 50 mol% (Cu/Zn =50/50) resulted in increases in the

conversion of methanol, the production rate of hydrogen

and Cu metal surface area. When the Cu content exceeded

50 mol%, methanol conversion, the formation rate of hydro-

gen and the Cu metal surface area all gradually decreased

with increasing Cu content. These results strongly suggest

that the Cu metal surface area, viz., the dispersion of Cu

metal species, affects the catalytic activity.

HP-Cu/ZnO/Al2O3 (Cu/Zn/Al = 45/45/10) showed

higher activity than that on HP-Cu/ZnO (Cu/Zn =50/50)

as well as higher values of both BET and Cu metal

surface area. This indicates that the aggregation of Cu

species was inhibited by the addition of Al species to

Cu/ZnO system. The TOF value of HP-Cu/ZnO/Al2O3(Cu/Zn/Al = 45/45/10) is larger than that of HP-Cu/ZnO

(Cu/Zn = 50/50). This result suggests that the property

of the active site is also influenced by the presence of Al

species. Moreover, the stability of the Cu/ZnO catalyst was

also improved by the addition of Al.Fig. 3shows the time

course of the steam reforming of methanol at 523 K. The

activity of the HP-Cu/ZnO catalyst was low even at the

beginning of the reaction and declined during the reaction

for 24 h. On the other hand, the HP-Cu/ZnO/Al2O3showed

high enough and stable enough activity compared to the

HP-Cu/ZnO. Ross et al. reported that the addition of Al

on Cu/ZnO increased the sustainability, resulting in no no-

ticeable deactivation at any temperature during the steam

reforming reaction[7].Furthermore, the HP-Cu/ZnO/Al2O3

showed higher activity and stability than CP-Cu/ZnO/Al2O3(Cu/Zn/Al = 45/45/10) and a commercial Cu/ZnO/Al2O3catalyst (Cu/Zn/Al = 42/46/12) in this reaction.

As a conclusion, the homogeneous precipitation method

by urea hydrolysis is preferable for the preparation of the

active Cu/ZnO and Cu/ZnO/Al2O3 catalysts for hydrogen

production by steam reforming of methanol (SRM). The

high activities of the catalysts prepared by the homogeneous

precipitation method may be due to the highly dispersed

Cu metal particles and the high accessibility of Cu metal

particles to methanol and steam. The activity and stability

were improved by the addition of Al species to Cu/ZnO

system.

Acknowledgements

The authors sincerely appreciate thefinancial support by

the Hiroshima Industrial Technology Organization. The au-

thors also thank Mrs. E. Tanabe and M. Honda for their kind

support with the SEM and ICP experiments.

References

[1] R. Shiozaki, T. Hayakawa, Y. Liu, T. Ishii, M. Kumagai, S.Hamakawa, K. Suzuki, T. Itoh, T. Shishido, K. Takehira, Catal. Lett.

58 (1999) 131.

[2] T. Shishido, H. Sameshima, K. Takehira, Top. Catal. 22 (2003) 261.

[3] Y. Matsumura, K. Tanaka, N. Tode, T. Yazawa, M. Haruta, J. Mol.

Catal. A 152 (2000) 157.

[4] W. Cheng, C. Shiau, T.H. Liu, H.L. Tung, J. Lu, C.C. Hsu, Appl.

Catal. A 170 (1998) 215.

[5] S. Velu, K. Suzuki, T. Osaki, Catal. Lett. 62 (1999) 159.

[6] M.L. Cubeiro, J.L.G. Fiero, Appl. Catal. A 168 (1998) 307.

[7] J.B. Breen, J.R.H. Ross, Catal. Today 511 (1999) 521.

[8] S. Velu, K. Suzuki, T. Osaki, Chem. Commun., (1999) 2341.

[9] G. Shen, S. Fujita, S. Matsumoto, N. Takezawa, J. Mol. Catal. A:

Chem. 124 (1997) 123.

[10] C.J. Jiang, D.L. Trimm, M.S. Wainwright, Appl. Catal. A 97 (1993)

145.[11] B.A. Peppley, J.C. Amphlett, L.M. Kearns, R.F. Mann, Appl. Catal.

A 179 (1999) 521.

[12] R.J. Candal, A.E. Regazzoni, M.A. Blesa, J. Mater. Chem. 2 (1992)

657.

[13] G.J. de A.A. Soler-Illia, R.J. Candal, A.E. Regazzoni, M.A. Blesa,

Chem. Mater. 9 (1997) 184.

[14] H. Morioka, H. Tagaya, K. Karasu, J. Kadokawa, K. Chibaa, J. Solid

State Chem. 117 (1995) 337.

[15] J.W. Evans, M.S. Wainwrightt, A.J. Bridgewater, D.J. Young, Appl.

Catal. 7 (1983) 75.

[16] J.M. Charnock, P.F. Schfield, C.M.B. Henderson, G. Cressy, B.A.

Cressey, Mineral. Mag. 60 (1990) 887.

[17] M.M. Harding, B.M. Kariuki, R. Cernik, G. Cressey, Acta Cryst.

B50 (1994) 673.

2004 active cuzno and cuznoal2o3 catalysts prepared by homogeneous

Documents