Selective production of hydrogen from partial oxidation of methanol
over silver catalysts at low temperatures
Liuye Mo,ab Xiaoming Zhengb and Chuin-Tih Yeh*a
a Department of Chemistry, National Tsinghua University, Hsinchu, Taiwan. E-mail: ctyeh@mx.nthu.edu.tw;
Fax: +886-3-5726047
b Institute of Catalysis, Zhejiang University (Xixi campus), Hangzhou, 310028, China
Received (in Cambridge, UK) 3rd February 2004, Accepted 14th April 2004
First published as an Advance Article on the web 18th May 2004
Hydrogen can be effectively and selectively produced from the
partial oxidation of methanol over Ag/CeO2–ZnO catalyst at
low temperatures (Tr < 200 °C).
nMeOH = 0.5) of 100 ml min21 flow with 12.2 mol% CH3OH
(metered by a liquid pump and preheated to about 100 °C), 6.1
mol% O2 and 81.7 mol% Ar (controlled by mass flow controller)
was catalyzed by 0.1 g catalyst. Reaction products were analyzed
by a TCD-GC equipped with columns of Porapak Q and Molecular
Sieve 5A.
Table 1 also displays POM catalytic performance from the
prepared catalysts at 160 °C. H2, H2O, CO and CO2 were major
products detected. Dimethyl ether (DME) was an additional by-
product found from catalysis over Ag/Ce20Al. The support of silver
catalysts significantly affected their CMeOH and SH2 in POM. Ag/
ZnO showed a higher SH2 and SCO2 than those of Ag/Ce20Al and
Ag/CeO2. However, Ag/Ce20Zn exhibited the highest methanol
conversion (CMeOH = 92%) and SH2 (97%) among the studied
catalysts. Interestingly, the performance of Ag/CeO2 increased
significantly on physically mixing with ZnO (the mixing catalyst of
Ag/CeO2 + ZnO). In order to exclude a possible dilution effect of
ZnO in the Ag/CeO2 + ZnO, a physical mixture of Ag/CeO2 and
quartz was also tried but showed poor catalytic performance.
Evidently, a synergistic effect on CMeOH and SH2 exists in Ag/
CexZnO.
Fig. 1 demonstrates effect of the Ce loading (x) on the catalytic
performance of Ag/CexZn catalysts. Both CMeOH and SH2 increased
with x, but they reached a maximum when x = 20. The selectivity
of CO2 (SCO2 ~ 94%) was not affected by the loading of Ce.
Fig. 2 presents the effect of the reaction temperature on the
catalytic performance of Ag/Ce20Zn. SCO2 decreased on increasing
the temperature. This trend may be attributed to the endothermic
nature of the reversed water gas shift reaction [RGWSR, eqn.
(3)].
Fuel cell technology is promising for efficient conversion of
chemical energy into electrical energy with negligible emission of
pollutant.1 There are many types of fuel cell under development.
Among them, the proton exchange membrane fuel cell (PEMFC)
can operate at the lowest temperature ~ 90 °C and may be applied
to mobile cars, cell phones and notebook computers. However, the
application is hindered by technical difficulties in storage, trans-
portation and distribution of hydrogen fuel. The difficulties may be
eliminated by an on-site catalytic reactor that produces hydrogen
from liquid fuels. Methanol is a promising fuel because of its ease
of handling, low cost, and abundant feedstock.2
Hydrogen may be produced directly from methanol according to
a number of different processes such as steam reforming [SR, eqn.
(1)],3–4 partial oxidation [POM, eqn. (2)]5–9 or oxidative steam
reforming (OSR).10–11
CH3OH + H2O ? 3H2 + CO2, DH0 = +49.4 kJ mol21
(1)
CH3OH+1/2O2 ? 2H2 + CO2, DH0 = 2192.2 kJ mol21
(2)
The endothermic SR is well developed with copper based catalysts
at reaction temperatures of Tr
> 230 °C Comparably, the
exothermic POM uses oxygen (air) instead of water as oxidant and
does not require an external heat supply.
Recently, the POM reaction has been studied over copper5–7 and
palladium based8,9 catalysts. Nevertheless, a Tr > 230 °C is still
required and the hydrogen selectivity is still low (SH2 < 70% at
n
O2/nMeOH = 0.5). In our laboratory, Au/ZnO catalyst was used for
POM reaction with low CO contamination and high SH2 ( ~ 95%).12
But a Tr > 230 °C is still needed. It is highly desirable to operate
the POM reaction at low temperatures if it is to be coupled to a fuel
cell. Herein, we report Ag/CeO2–ZnO catalyst which not only
shows excellent POM performance at low Tr but also exhibits high
CO2 + H2 Ô CO + H2O, DH0 = 41 kJ mol21
(3)
Keq of RGWSR decreases with the temperature and a high SCO2
(therefore a low SCO) is therefore expected from the equilibrium at
low temperatures. Both CMeOH and SH2 in Fig. 2 increased with the
reaction temperature and exceeded 98% at 180 °C.
Fig. 3 shows the effect of Ce on the stabilities of Ag/CexZn
catalysts. Ag/ZnO catalyst deactivated slowly at the initial stage
SH2
Supported catalysts of Ag/ZnO, Ag/CeO2–ZnO and Ag/CeO2–
Al2O3 were prepared by the deposition precipitation method.12
.
A
solution of 0.15 M AgNO3 (or a mixture solution of 0.15 M AgNO3
and 0.40 M Ce(NO3)3·6H2O) was precipitated to powders of ZnO
(d ~ 180 nm, from Merk) or fumed alumina (from CABOT)
suspended in deionized water. Acidity of the suspended solution
was adjusted to pH ~ 9.0 with 0.10 M NaOH. After 2 h of stirring
at room temperature, the suspension was filtered and washed with
deionized water. The resulting precipitate was then dried at 100 °C
for 24 h. Another supported catalyst of Ag/CeO2 was prepared by
a similar procedure without addition of ZnO or alumina. Supported
catalysts of Ag/CeO2–ZnO and Ag/CeO2–Al2O3 were designated
as Ag/CexZn and Ag/CexAl (x represents nominal loading of Ce in
wt.% of catalysts), respectively. The nominal loading of Ag was 5
wt.% in all of the prepared catalysts. True loadings detected by ICP-
mass are listed in Table 1. Mixing samples of “Ag/CeO2+ZnO” and
“Ag/CeO2 + quartz” were prepared by physically mixing the Ag/
CeO2 with ZnO powder or quartz sand at a ratio of 1 : 2. These
mixtures were ground into fine powders and then pressure-moulded
into granules. They were further crushed, sieved to 60–80 mesh,
and reduced for 1 h at 200 °C in a hydrogen flow. The POM activity
Table 1 Physical characterization and catalytic performance of different
catalysts at 160 °C
Ag
Ce
b
c
c
d
Catalyst
(wt.%) (wt.%) d (nm)a
CMeOH SH2 SCO2 SDME
Ag
Ce
Ag/ZnO
Ag/CeO2
4.1
3.9
5.0
4.8
~
0
~
20
18
~
30(48)
10
11
~
8
7
74
69
87
83 95
65 58
17 85
97 94
88 98
24 89
0
0
20
0
0
0
Ag/Ce20Al
Ag/Ce20Zn
Ag/CeO2+ZnO
Ag/CeO2+quartz ~
CeO2/ZnO
10(13) 6 92
10
10
~
8
8
6
78
54
0
~
20
0
0
0
0
a Estimated by XRD using the Scherrer equation and the values in
b
parentheses are used catalysts’. CMeOH
=
(nMeOH, in 2 nMeOH,out)/
nMeOH, in 3 100%. c SH2 = nH2/2(nMeOH, in 2 nMeOH,out) 3 100%; SCO2
CO2/(nMeOH, in 2 nMeOH,out) 3 100%; SDME = 2nDME/(nMeOH, in
=
d
n
2
nMeOH,out) 3 100%.
was tested in a fixed bed reactor (4 mm in id). A reactant gas (nO2
/
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C h e m . C o m m u n . , 2 0 0 4 , 1 4 2 6 – 1 4 2 7
T h i s j o u r n a l i s © T h e R o y a l S o c i e t y o f C h e m i s t r y 2 0 0 4
Mo, Liuye
