Oxidative steam reforming of methanol over CuZnAl(Zr)-oxide catalysts; a
new and efficient method for the production of CO-free hydrogen for fuel cells
S. Velu, K. Suzuki* and T. Osaki
Ceramics Technology Department, National Industrial Research Institute of Nagoya, 1-1 Hirate-cho, Kita-ku, Nagoya
462-8510, Japan. E-mail: ksuzuki3@nirin.go.jp
Received (in Cambridge, UK) 31st August 1999, Accepted 15th October 1999
Steam reforming of methanol in the presence of air over
CuZnAl(Zr)-oxide catalysts derived from hydrotalcite-like
hydroxycarbonate precursors offers CO-free hydrogen suita-
ble for fuel cells with about 100% methanol conversion at
around 230 °C.
CuZnAl(Zr)-hydroxycarbonate precursors with different
Cu:Zn:Al atomic ratios were synthesized by a coprecipitation
method at room temperature using a mixture of NaOH and
Na2CO3 as precipitants.5 The (Cu+Zn)/Al(Zr) atomic ratio in
the starting solution was varied from 2 to 4. The Cu surface
areas (SCu), Cu dispersion (D) and the Cu crystallite sizes (tCu
)
Hydrogen is forecast to become a major source of energy in the
future.1 Hydrogen fueled vehicles using fuel cells [e.g. polymer
electrolyte membrane fuel cell (PEMFC)] are under develop-
ment in an effort to reduce CO2 emissions which accelerate
global warming. In addition, fuel cell-powered vehicles, using
hydrogen fuel, do not emit any environmental pollutants, such
as NOx, SOx and hydrocarbons. For use in fuel cells for mobile
applications, hydrogen is typically produced through steam
reforming of methanol (SRM) [eqn. (1)] over CuZn-based
catalysts.2 Recently, partial oxidation of methanol (POM) [eqn.
(2)] has also been suggested as a route for hydrogen extraction
from methanol.3 Unfortunately, both of these reactions produce
considerable amounts of CO ( > 100 ppm) as a byproduct. For
application in PEMFCs, even traces of CO ( > 20 ppm) in the
reformed gas deteriorates a Pt electrode and the cell perform-
ance is dramatically lowered.4 Currently, second stage catalytic
reactors are being used to remove this CO by the water-gas-shift
reaction [eqn. (3)], CO oxidation or methane formation.
However, the addition of further steps for fuel gas refinement
would lower the total efficiency of the propulsion system.
Hence, in order to utilize hydrogen fuel for fuel cells, it is highly
desirable to develop a process that can produce hydrogen
without CO in the reformed gas.
were calculated by the TPR-N2O passivation experiment as
described in the literature.6 Catalytic test reactions were
performed in a packed-bed micro-reactor (4 mm i.d.) using 90
mg of the catalyst (particle size 0.30–0.355 mm) in the
temperature range 180 to 290 °C at atmospheric pressure.
Liquid methanol (for the POM reaction) or premixed water and
methanol with a H2O/CH3OH molar ratio of 1.3 (for the SRM
and OSRM reactions) was fed into the pre-heater at a rate of 1.6
or 2.5 cm3 h21 by means of a micro-feeder. Synthetic air (20.2
vol.% O2 in N2) at a rate of 10 to 20 cm3 min21 and Ar (carrier
gas, 43 cm3 min21) flows were adjusted by means of a mass
flow controller. The reaction products were analyzed on-line
using two gas chromatographs equipped with TC detectors and
porapak Q and molecular sieve 13 X columns. The performance
of the catalyst was evaluated after 25 h of on-stream
operation.
The physicochemical properties of the catalysts used in the
present study are summarized in Table 1. Initially, a systematic
study on the POM reaction was performed over a series of
CuZnAl-oxide catalysts without Zr (CZAZ-A through CZAZ-
D). Among them, the catalyst CZAZ-C exhibited the best
performance. Hence, this catalyst was chosen first to be
examined for its catalytic performance in the SRM and OSRM
reactions. Fig. 1 compares the catalytic performance of the
CZAZ-C catalyst in the POM, SRM and OSRM reactions. As
will be noted, the methanol conversion is lowest in the SRM
reaction at low temperatures and the conversion rate increases
rapidly with increasing reaction temperature. The conversion
reaches about 100% at around 290 °C where the selectivity of
CO is < 5 mol% (CO concentration ≈ 200 ppm). Interestingly,
when air was passed through during the SRM reaction, the
conversion approaches 100% at a lower temperature of about
230 °C, without any detectable CO in the reformed gas. Because
of the high methanol conversion of about 100%, the H2
production rate in the OSRM reaction increased by a factor of
two compared to that obtained from the SRM or POM reactions.
It is of great significance that CO only started forming above
260 °C in the SRM reaction, while it started forming above
CH3OH + H2O ) 3H2+CO2 DH = + 49.4 kJ mol21
CH3OH + 1/2O2 ) 2H2+CO2 DH = 2 192.2 kJ mol21 (2)
CO + H2O ) H2+CO2 DH = 2 39.4 kJ mol21
(3)
(1)
In the present study, we have performed the SRM reaction in
an oxidizing atmosphere (in the presence of air) over a series of
novel CuZnAl(Zr)-oxide catalysts obtained by calcining the
CuZnAl(Zr)-hydroxycarbonates containing hydrotalcite (HT)/
aurichalcite phases, at 450 °C for 5 h. We demonstrate here for
the first time that under the reaction conditions employed, a
combined steam reforming–partial oxidation of methanol,
termed herein after as “oxidative steam reforming of methanol
(OSRM)”, reaction takes place to produce CO-free hydrogen
with about 100% methanol conversion at 230 °C.
Table 1 Physicochemical properties of CuZnAl(Zr)-oxide catalysts
Metal composition of the precursors (atomic ratio)a
SBET
/
H2 consumptionb/
mmol g21
Catalyst
Cu
Zn
Al
Zr
m2 g21
Scu/m2 g21
Dcu (%)
tCu/Å
CZAZ-A
CZAZ-B
CZAZ-C
CZAZ-D
CZAZ-E
CZAZ-F
0.73
1.02
1.42
1.37
1.40
1.38
0.88
1.30
1.71
1.80
1.65
1.72
1.00
1.00
1.00
1.00
0.45
0.00
0.00
0.00
0.00
0.00
0.55
1.00
56
71
84
108
82
75
3.3
3.6
6.0
4.2
4.7
4.3
203
181
176
227
232
279
38.6
34.3
33.4
43.1
44.0
52.9
26
29
30
23
23
19
a Determined by X-ray fluorescence spectroscopy. b Hydrogen consumption estimated by the temperature programmed reduction (TPR) experiments.
Chem. Commun., 1999, 2341–2342
This journal is © The Royal Society of Chemistry 1999
2341
Velu
