Steam reforming of methanol over Cu/ZrO2/CeO2 catalysts: A kinetic study

Article abstract of DOI:10.1016/j.jcat.2004.12.020

Steam reforming of methanol (SRM) was investigated over Cu/ZrO ₂/CeO₂ (CZC) catalysts prepared via a novel synthetic method based on coprecipitation and polymer templating. Structural characterization of the samples was performed by N₂ adsorption-desorption, N₂O decomposition, and X-ray diffraction. The variation of the Cu loading resulted in an increased Cu crystallite size and a decreased specific surface area of the active particles. Catalytic investigations were carried out in a fixed-bed reactor at 10⁵ Pa, with a CH₃OH:H₂O ratio of 1:1. The samples with Cu contents higher than 5% exhibited good long-term stabilities and low CO levels during continuous operation. The kinetic model suggested for the transformation involved the reverse water-gas shift (RWGS) and methanol decomposition (MD), in addition to the SRM reaction. Kinetic measurements were made in the temperature range of 503-573 K, and the experimental results could be well simulated. The highest methanol conversions and the lowest CO levels were observed in the temperature range of 523-543 K. The apparent activation energies for the individual reactions were found to depend on the Cu content of the catalyst. Since the influence of mass transport limitations on the kinetic data could be excluded, it was established that the variation of the Cu concentration in the precursor material altered the microstructure of the Cu particles and, accordingly, the active Cu surface, which resulted in the formation of significantly different catalysts.

Full text of DOI:10.1016/j.jcat.2004.12.020

Journal of Catalysis 230 (2005) 464–475
www.elsevier.com/locate/jcat
Steam reforming of methanol over Cu/ZrO2/CeO2 catalysts:
a kinetic study
A. Mastalir a,b, B. Frank b, A. Szizybalski a, H. Soerijanto a,b, A. Deshpande c, M. Niederberger c,
R. Schomäcker b, R. Schlögl a, T. Ressler a,∗
a
Fritz-Haber-Institut der MPG, Abteilung Anorganische Chemie, Faradayweg 4-6, 14195 Berlin, Germany
b
Institut für Chemie, TU Berlin, Sekr. TC-8, Straße des 17. Juni 124, 10623 Berlin, Germany
c
Max-Planck-Institut für Kolloid- und Grenzflächenforschung, Abteilung Kolloidchemie, MPI-KG 14424 Potsdam, Germany
Received 19 October 2004; revised 16 December 2004; accepted 17 December 2004
Abstract
Steam reforming of methanol (SRM) was investigated over Cu/ZrO /CeO (CZC) catalysts prepared via a novel synthetic method based
2
2
on coprecipitation and polymer templating. Structural characterization of the samples was performed by N adsorption–desorption, N O
2
2
decomposition, and X-ray diffraction. The variation of the Cu loading resulted in an increased Cu crystallite size and a decreased specific
5
surface area of the active particles. Catalytic investigations were carried out in a fixed-bed reactor at 10 Pa, with a CH OH:H O ratio of
3
2
1:1. The samples with Cu contents higher than 5% exhibited good long-term stabilities and low CO levels during continuous operation. The
kinetic model suggested for the transformation involved the reverse water–gas shift (RWGS) and methanol decomposition (MD), in addition
to the SRM reaction. Kinetic measurements were made in the temperature range of 503–573 K, and the experimental results could be well
simulated. The highest methanol conversions and the lowest CO levels were observed in the temperature range of 523–543 K. The apparent
activation energies for the individual reactions were found to depend on the Cu content of the catalyst. Since the influence of mass transport
limitations on the kinetic data could be excluded, it was established that the variation of the Cu concentration in the precursor material
altered the microstructure of the Cu particles and, accordingly, the active Cu surface, which resulted in the formation of significantly different
catalysts.
2005 Published by Elsevier Inc.
Keywords: Steam reforming of methanol; Copper; Zirconia; Ceria; N O chemisorption; Long-term stability; CO formation; Kinetic model; Reverse
2
water–gas shift reaction; Methanol decomposition; Activation energy
1. Introduction
venient operation, the absence of moving parts, and the low
emission of hazardous compounds [1,5]. The combustion of
hydrogen in a fuel cell is regarded as a clean process, releas-
ing energy and providing only water as an exhaust material
[4,7,8]. However, hydrogen is not a natural energy source;
its generation consumes a large amount of energy, either
from natural gas or via the electrolysis of water [4]. Further-
more, for a fuel-cell vehicle, the storage and the supply of
hydrogen, a volatile and explosive gas, impose mechanical
problems and safety hazards on a commercial level [1,4,8].
Several liquid fuel candidates have been discussed for
on-board reforming, including methanol, ethanol, gasoline,
and diesel [1], of which methanol is considered the most
In the past decade, considerable attention has been fo-
cused on the reduction of the significant emissions originat-
ing from mobile sources, such as internal combustion en-
gines [1–4]. For environmental reasons, the development of
proton-exchange membrane fuel cells (PEMFCs) has gained
in importance [5,6]. As compared with conventional heat en-
gines, several advantages of fuel cell application have been
established, including a higher efficiency and a more con-
*
Corresponding author. Fax: +49 30 8413 4405.
E-mail address: ressler@fhi-berlin.mpg.de (T. Ressler).
0021-9517/$ – see front matter
doi:10.1016/j.jcat.2004.12.020
2005 Published by Elsevier Inc.

A. Mastalir et al. / Journal of Catalysis 230 (2005) 464–475
465
favorable alternative [1,9]. Although mostly produced from
natural gas, methanol can also be obtained from renewable
sources, thus lowering the amount of carbon dioxide pro-
duced [5,9]. Moreover, via methanol synthesis from hydro-
gen and carbon dioxide, methanol can be employed as a
storage and transport medium for hydrogen. In addition to
its good availability and low boiling point, methanol pos-
sesses a high hydrogen/carbon ratio (4/1) and contains no
carbon–carbon bond, which considerably reduces the risk of
coke formation during its reactions [1,5,6].
Several studies have been reported in the literature on
the applications of Cu- and Pd-based catalysts for SRM
[1–6,8–18], of which Cu-containing catalysts are clearly
preferred because of their high activity and selectivity at
lower temperatures [1,12,19]. Although Cu catalysts are also
regarded as susceptible to thermal deactivation, their sin-
tering abilities may be considerably reduced by the addi-
tion of one or more oxide species, such as ZnO, Al2O3,
or Cr2O3 [20]. The most efficient catalysts for SRM, in-
cluding the traditional Cu/ZnO and Cu/ZnO/Al2O3 sys-
tems, have been investigated in detail [1,3,6,9,15,16,21–25].
The in situ characterization of Cu/ZnO under SRM acti-
vation and operating conditions revealed that the interac-
tion of the Cu and ZnO phases has a pronounced effect
on the catalytic activity [22,23]. For Cu/ZnO/Al2O3 cat-
alysts, high methanol conversions and low CO selectivi-
ties have been obtained [1,5,8,9,16] and kinetic analysis
has provided important information on the overall reac-
tion mechanism [5,6,16,21,25,26]. Compared with the con-
ventional ZnO- or Al2O3-supported Cu catalysts, ZrO2-
containing samples have shown increased activities and re-
duced CO levels for the SRM reaction [3,5,12,27–29]. The
promoting effect of ZrO2 has been attributed to an im-
proved reducibility of CuO, which tends to increase the
Cu dispersion [29]. ZrO2 has also been reported to pre-
vent the sintering of Cu crystallites under reaction condi-
tions [5,12,29] and thus may be regarded as a structural
stabilizer. Likewise, the application of CeO2, as either a
support material or a promoter, has been found to improve
the efficiency of Cu-based catalysts [11,30,31]. CeO2 has
been found to increase the thermal stability and the activ-
ity of Al2O3-supported Cu catalysts through a synergetic
effect and to favor the conversion of CO via the water–
gas shift reaction [30]. Moreover, CeO2 has a high oxygen
storage capacity [32,33], and the partially reduced CeO2
sites formed under reductive conditions produce mobile oxy-
gen, which tends to have a beneficial effect on the cat-
alytic performance [31,32]. For ZrO2–CeO2-supported cat-
alysts, the interaction between the oxide phases, leading
to the formation of a thermally stable solid solution, has
been reported to increase the mobility of oxygen in both
phases [32].
The most important processes for the production of hy-
drogen from methanol:
k
D
CH3OH(g) ꢀ 2H2 + CO, ꢀH◦ = 91 kJmol−1
;
(1)
k
−D
k
R
CH3OH(g) + H2O(g) ꢀ CO2 + 3H2,
k
−R
ꢀH◦ = 50 kJ mol−1
;
(2)
(3)
kO
CH3OH(g) + 0.5O2 ꢀ 2H2 + CO2,
k
−O
ꢀH◦ = −192 kJ mol−1
are decomposition (MD), steam reforming (SRM), and par-
tial oxidation (POM) [1]. The combined or oxidative reform-
ing of methanol (CRM or OSRM), a combination of (2) and
(3), has also been investigated [2,6,10,11]. The decomposi-
tion of methanol is a strongly endothermic reaction produc-
ing a high CO yield, which makes it rather unsuitable for
fuel-cell applications [1]. The steam reforming reaction is
also endothermic; however, it typically affords a substantial
H2 yield of 75%, while maintaining high carbon dioxide se-
lectivity (∼ 25%) [1,4,8]. The partial oxidation of methanol
is an exothermic reaction with a rapid start-up and a dynamic
response [4]. Nevertheless, the formation of hot spots in the
reactor may result in sintering of the Cu particles, which
tends to decrease the catalytic performance [8,11]. Further-
more, this reaction produces a considerably lower amount of
hydrogen than SRM [1,8,9].
As the highest hydrogen yield may be achieved by the
steam reforming of methanol [1], the latter reaction was in-
vestigated within the framework of the present study. The
major drawback of SRM is the formation of CO as a by-
product, which, even at a low concentration of 100 ppm,
decreases the fuel-cell performance by poisoning the Pt elec-
trode [2,6,12]. Currently, second-stage catalytic reactors are
being used to remove CO by the water–gas shift reaction,
oxidation, or methane formation [6,10]. However, this CO
clean-up unit is rather inconvenient, as it occupies a large
volume and decreases the efficiency of the fuel cell through
hydrogen consumption [8]. To eliminate the need for gas pu-
rification, high-performance catalysts are required that pro-
vide substantial methanol conversion and H2 selectivity, to-
gether with the lowest possible amount of CO [10,12]. The
complete absence of CO in the product gas may be difficult
to achieve under SRM conditions.
The aim of the current work was the structural and cat-
alytic investigation of novel Cu/ZrO2/CeO2 catalysts pre-
pared by coprecipitation. To increase the specific surface
area, highly porous polymer beads were employed as a tem-
plate for the preparation of the solid catalysts [34]. Samples
with different Cu loadings were examined for the steam
reforming of methanol; their catalytic properties, includ-
ing long-term stability and CO production, are discussed
here. Furthermore, the reaction mechanism was studied and
a kinetic analysis was undertaken to determine the rate
constants and the activation energies for the model reac-
tions.

466
A. Mastalir et al. / Journal of Catalysis 230 (2005) 464–475
2. Experimental
2.1. Catalyst preparation
The Cu/ZrO2/CeO2 (CZC) catalysts were synthesized
from metal sols, prepared from the appropriate amounts of
the mixed precursors (NH4)2Ce(NO3)6, ZrO(NO3)2 · H2O,
and Cu(NO3)2 · 2.5H2O (all Aldrich products of > 99% pu-
rity) dissolved in 40 ml of distilled water. The total metal
content of the resulting sol was 5 × 10−3 mol. We coprecip-
itated the metal sols as metal hydroxides by increasing the
pH above 10 through the addition of a NaOH solution. The
resulting precipitate was repeatedly washed with distilled
water until a pH of 7 was obtained, and then suspended in
a mixture of 5 ml of distilled water and a calculated amount
of 90% HNO3. Depending on the sample composition, the
HNO3/metal molar ratio was varied between 1/1 and 1.5/1.
Ultrasonic treatment of the suspension for 45–60 min re-
sulted in the formation of a transparent sol [35].
Templating was carried out with XAD-16 nonfunction-
alized polystyrene beads (a Sigma product), with a specific
surface area of 800 m2 g−1 and an average pore diameter of
10 nm. The wet beads (1.5 ml) and the metal sol (0.005 mol)
were mixed together and then heated in an oven at 333 K for
2 days to ensure solvent evaporation. The beads were sub-
sequently dried at 373 K for another day and then washed
with distilled water to remove any excess deposit from their
surface. After drying at 333 K for 1 day, the samples were
calcined in an oven at 773 K in a N2 stream of 100 ml/min
for 2.5 h and then for an additional 8 h in air. Upon cal-
cination gray–green materials were obtained (see Fig. 1).
Because of the mean particle diameter (500 µm) and the
regular character of the polymer beads, the resulting CZC
samples could be conveniently applied as catalysts in fixed-
bed reactors.
Fig. 1. Scanning electron micrograph of a Cu/ZrO /CeO catalyst (CZC5)
after calcination.
2
2
Table 1
Characteristic data of the CZC catalysts
0
0
Sample Cu
(%) (%) (%) (m g
4.4 44.6 51.0 96
CZC15 12.1 44.3 43.6 102
Zr
Ce
BET area
d
p
Cu area Cu area
−1
−1
2
−1
2
2
)
(nm) (m g
)
(m g
)
cat
Cu
CZC5
9.4 0.9
9.9 1.8
9.8 1.8
10.0 1.5
51.4
25.8
12.2
7.6
CZC25 23.9 37.6 38.5
CZC35 31.0 32.7 36.3
94
83
Four representative CZC samples were synthesized by
the above method, with Cu contents of 5, 15, 25, and 35%
(denoted CZC5, CZC15, CZC25 and CZC35, respectively),
to be characterized and tested for the steam reforming of
methanol. A ZrO2/CeO2 molar ratio of 1/1 for the support
materials was maintained in each case.
2375 BET apparatus equipped with a Vacprep 061 degasser.
To remove traces of water and impurities from the surface,
all samples were degassed before measurement at 13 Pa
and 393 K for 12 h. The specific surface areas were calcu-
lated from the BET equation, and the average pore diameters
(dp) were obtained by the BJH method from the desorption
branches of the adsorption isotherms.
2.2. Characterization
2.2.1. X-ray fluorescence analysis
2.2.3. N2O decomposition
The specific Cu0 surface areas of the samples were
measured by N2O decomposition, with the reactive frontal
chromatography (RFC) method introduced by Chinchen et
al. [36].
The results of the elementary analysis of the CZC sam-
ples, performed with a Seiko (SEA 2010) instrument, are
shown in Table 1. The amount of Cu detected in the samples
was found to be only slightly lower than that expected from
the preparation conditions, and the ratio ZrO2/CeO2 = 1/1
proved to be well adjusted.
The method is based on the oxidation of the exposed Cu0
surface atoms of a reduced sample when the feed is switched
from an inert gas stream to a mixture containing N2O as an
oxidizing species. The Cu0 surface area can then be deter-
mined from the amount of N2O consumed in the reaction
2.2.2. N2 adsorption–desorption
The specific surface areas of the CZC samples were deter-
mined from N2 adsorption–desorption data obtained at the
temperature of liquid N2 (77 K), by using a Micromeritics
2Cu + N2O → Cu2O + N2.
(4)

A. Mastalir et al. / Journal of Catalysis 230 (2005) 464–475
467
Before measurements, the CZC samples were activated
in a stream of MeOH/H2O (2/1) with a flow rate of
36 cm3 min−1 at 523 K. To diminish the amount of adsorbed
molecules on the Cu surface, the samples were subsequently
purged in pure He (50 cm3 min−1) at 523 K for 1 h and then
cooled to 313 K, where the measurements were performed
in a 0.5% N2O/He stream of 15 cm3min−1. The catalysts
were diluted with boron nitride to provide a sample bed with
a height of ca. 25 mm, with a thermocouple positioned di-
rectly inside the powder bed. The material was placed on a
quartz frit in a tubular quartz reactor.
For the RFC method described by Chinchen et al., the
area under the ion current trace (m/e = 28 for N2) has
been used directly to determine the Cu0 surface area. For
the current experiments, however, the same MS signal ob-
tained from the amount of N2 produced for the CZC samples
was found to be insufficient for evaluation. Therefore, the
amount of N2 formed during decomposition was indirectly
estimated from the retarded evolution of N2O (m/e = 44),
as compared with blank experiments performed with boron
nitride. Earlier investigations for Cu/ZnO samples revealed
a good agreement between the Cu0 surface areas determined
by N2O RFC and those obtained from the Cu crystallite size
[23,37]. We calculated the Cu0 surface area, assuming a sto-
ichiometry of N2O/Cu = 0.5 and a value of 1.47 × 1019 Cu
atoms/m2 for the surface density [5,29,36].
reactor by a stainless-steel grid. For flow conditioning, in-
ert Pyrex beads (d = 500 µm) were placed below and on top
of the catalyst bed. The reactant mixture of MeOH (HPLC
grade, 99.9% purity) and distilled water, with a MeOH/H2O
molar ratio of 1/1, was introduced into the reactor with a
Dionex HPLC P 580 pump. Both reactants were degassed
before use at 20 kPa. For time-on-stream measurements, the
flow rate for a single channel was 0.07 cm3 min−1, whereas
for kinetic investigations, it was varied between 3.33×10−3
and 0.67 cm3 min−1. Kinetic measurements were made with
fresh catalysts to eliminate aging effects, and experimen-
tal data were collected after 5 days on stream. To achieve
steady-state conditions, experimental data at low flow rates
were collected 60 min after the flow rate had been adjusted.
Before measurement, the catalysts were activated in situ in a
MeOH/H2O = 1/1 stream of 0.07 cm3 min−1 at 523 K for
16 h [15,16,25].
The reaction products, consisting of effluent gases (H2
and CO2 as main products and a minor amount of CO),
together with unreacted methanol and water, were passed
through a condenser at 268 K, which removed most of the
liquid from the product gas stream. Complete removal of
the condensed reactant mixture was achieved with an addi-
tional condenser at 253 K. The liquid composition was ana-
lyzed with an Intersmat gas chromatograph (GC), equipped
with a thermal conductivity detector (TCD), operated with
a 50 m × 0.53 mm CP-Wax column at 363 K. The com-
position of the dry product gas mixture was determined
on-line, with a Varian 3800 GC with a TCD. Helium was
applied as a carrier gas, and separation was achieved with
a 25 m × 0.53 mm CarboPLOT P7 column at 304 K. For
quantitative analysis of the product gas stream, a calibra-
tion gas mixture, containing 0.5% CO, 4.5% N2, 25% CO2,
and 70% H2 (a Messer Griesheim product), was used. Un-
der the present experimental conditions, no evidence for the
formation of the by-products methane, methyl formate, or
dimethyl ether was found [27,39].
2.2.4. X-ray diffraction
Ex situ X-ray diffraction (XRD) measurements were per-
formed with a STOE STADI P transmission diffractome-
ter (Cu-Kα radiation, Ge monochromator) equipped with a
position-sensitive detector (PSD) (internal resolution 2Θ =
0.01◦). The CuO crystallite diameters were calculated from
the Scherrer equation [38]. The full width at half-maximum
(FWHM) was determined by fitting a Lorentzian profile
function to the CuO (111) and (200) diffraction peaks at
2Θ = 38.371◦ and 38.923◦, respectively.
2.3. Catalytic test reaction
3. Results and discussion
The steam reforming of methanol was studied in a fixed-
bed tubular reactor (stainless steel, i.d. = 10 mm) with the
use of a three-channel set-up, which ensured that three cat-
alysts could be investigated at the same time, under exactly
the same conditions. To achieve an efficient heat transfer,
the reactors were placed in an aluminum heating block. The
temperature of the reactor was regulated by PID control of
the cartridge heaters situated inside the aluminum block.
Time-on-stream investigations were performed at 523 K, and
the mass of catalyst was varied between 0.100 and 0.770 g,
depending on the Cu content, to keep the amount of CuO
constant for all samples (0.0225 g). For kinetic measure-
ments, the mass of catalyst was increased by a factor of 2.6,
corresponding to a CuO content of 0.0584 g for each sample,
to achieve full conversion in the temperature range investi-
gated (503–573 K). The catalyst was supported inside the
3.1. Catalyst characterization and long-term stability
The results obtained for the CZC samples from N2
adsorption–desorption and N2O decomposition are summa-
rized in Table 1.
The BET surface areas obtained for the CZC samples
were higher than those reported for ZnO-supported Cu cat-
alysts [39] and of an order similar to those published for
Cu/Al2O3 samples by Cheng et al. [19]. A decreasing BET
surface area with increasing Cu loading [19] was observed
only for the samples with Cu contents exceeding 5%. As
seen in Table 1, the values for the average pore diameter
were very close for each sample (9.35–10.0 nm). Although
the Cu0-specific surface areas for the CZC materials proved
to be lower than expected from the internal surface areas and

468
A. Mastalir et al. / Journal of Catalysis 230 (2005) 464–475
Fig. 3. Methanol conversions during SRM of four CZC catalysts as a
Fig. 2. X-ray diffraction patterns of four CZC catalysts with a nominal cop-
per concentration of 5% (CZC5), 15% (CZC15), 25% (CZC25), and 35%
5
function of time-on-stream (m
= 0.0225 g, T = 523 K, p = 10 Pa,
CuO
3
−1
w = 0.07 cm min , MeOH/H O = 1/1).
2
(CZC35) with a constant ZrO /CeO ratio of about 1. Arrows indicate CuO
2
2
phase.
ples is determined not only by the Cu surface area, but also
by the particular microstructure of the Cu particles, which
may be similar to the microstrain in Cu/ZnO catalysts [23].
The catalytic activities of all of the samples were found to di-
minish considerably for a period of 5 days, although to a dif-
ferent extent, which displayed no systematic variation with
the Cu content. The conversions determined after 5 days ap-
proached a constant value, except for that of CZC5, which
exhibited a further decrease. After an initial period of deac-
tivation, the CZC samples with Cu contents exceeding 5%
displayed good long-term stabilities, and, hence, these sam-
ples may be suitable for extended operation. The relatively
poor thermal stability of CZC5 may be related to the small
Cu particle size, in light of the fact that small particles tend
to be more susceptible to thermal sintering under reaction
conditions [40], even at temperatures lower than 523 K [41].
In addition to the CZC samples, a copper-free CeO2/ZrO2
sample was prepared with the same procedure as described
above. This material did not exhibit any detectable activity
in the steam reforming of methanol under the reaction con-
ditions employed. Hence, the differences observed for the
CZC catalysts in the steam reforming of methanol can be at-
tributed mainly to the bulk and surface properties of the Cu
phase in these materials.
The CO partial pressures determined during the above
measurements are depicted in Fig. 4. As mentioned above,
the sample masses were adjusted to provide the same Cu
content. For this reason, the mass of CZC5 considerably ex-
ceeded those of the other samples, and this may account for
the substantially higher amount of CO detected for CZC5
than for the samples with higher Cu contents. Moreover,
following a pronounced decrease for about 5 days, the CO
levels were stabilized and did not exhibit any significant dif-
ference. This finding is consistent with the results of N2O
decomposition, indicating that the active Cu contents of
these samples were very similar. It may be concluded from
Figs. 3 and 4 that an increase in the Cu content from 5 to
15% has a beneficial effect on the catalytic performance of
the Cu contents [29], the values exhibited a trend similar to
that of the BET results. The highest Cu0 surface area per
gram of copper was obtained for CZC5. An increasing Cu
content resulted in a decreasing Cu0 surface area roughly in
agreement with an increasing Cu particle size. The net de-
crease in the Cu0 specific surface area with increasing Cu
content, observed for CZC15, CZC25, and CZC35, is in
correlation with earlier results reported in the literature for
supported Cu catalysts [5,19].
The XRD patterns of the CZC samples displayed in Fig. 2
revealed that the characteristic CuO peaks could be distin-
guished only for the samples with the highest Cu contents.
The absence of CuO signals for CZC5 and CZC15 may be
attributed in part to their lower Cu loading. Furthermore,
the Cu crystallite size for the above samples may be too
small to be detected by XRD, as related to a pronounced line
broadening [5]. The broadening of the peaks also renders it
difficult to establish whether the Zr, Ce, and O atoms are
arranged in a single mixed phase or in separate phases. For
the same reason, the corresponding crystal structures (cu-
bic and/or tetragonal) cannot be reliably determined. The
Cu crystallite sizes obtained from the diffraction peaks for
CZC25 and CZC35 were 12.4 and 15.3 nm, respectively.
The slight increase in the particle size observed for the latter
sample is in line with the decrease in the Cu0-specific sur-
face area obtained from N2O decomposition. Further struc-
tural data on the Cu/CeO2/ZrO2 catalysts obtained under
reaction conditions and corresponding structure-activity re-
lationships will be presented elsewhere.
To make a comparison between the long-term stabilities
of the CZC catalysts, the methanol conversions obtained dur-
ing a continuous operation of 16 days under standard condi-
tions were plotted as a function of time on stream (Fig. 3).
It can be seen that no linear correlation exists between the
catalytic activity depicted in Fig. 3 and the Cu surface area
(Table 1). Evidently, the catalytic activity of the CZC sam-

A. Mastalir et al. / Journal of Catalysis 230 (2005) 464–475
469
Fig. 5. Methanol conversions during SRM as a function of the reactant
Fig. 4. CO formation during SRM of four CZC catalysts as a func-
5
flow rate, determined for various sieve fractions of CZC15 (m = 0.5500 g,
tion of time-on-stream (m
= 0.0225 g, T = 523 K, p = 10 Pa, w =
CuO
5
3
−1
T = 523 K, p = 10 Pa, MeOH/H O = 1/1).
0.07 cm min , MeOH/H O = 1/1).
2
2
CZC through improvement of the long-term stability and
suppression of the CO production. Although the effect of
a further increase in the Cu loading proved to be less im-
portant for the stability of the catalyst, it resulted in lower
CO levels. Nevertheless, the CO production is also depen-
dent on the conversion of methanol [29], as the activity order
of the CZC samples (Fig. 3) is in correlation with the order
of the CO levels throughout the entire time-on-stream in-
terval (Fig. 4). Accordingly, under the present experimental
conditions, the samples with increased Cu contents may be
regarded as more efficient catalysts for SRM, with respect to
the formation of CO in particular.
Inasmuch as the CZC catalysts are porous materials and
the methanol conversions in Fig. 3 displayed no systematic
variation with either the Cu loading or the Cu0 surface area,
the effect of mass transport limitations on the catalytic per-
formance was also investigated. It was previously observed
that the CZC samples were not uniform in size. Thus, for
each material, three fractions with different particle diam-
eters could be obtained by sieving. Moreover, a Cu/CeO2
sample containing 25% Cu, prepared by the same method,
was found to deteriorate slightly during SRM, as a small
amount of the original catalyst beads (3.3%) were trans-
formed into a fine powder after reaction. On the other hand,
the Zr-containing beads of the CZC samples remained essen-
tially unchanged after catalytic investigations, which con-
firms that ZrO2 is an important structural stabilizer [12,29].
The methanol conversions obtained for the different sieve
fractions of CZC15 under the same conditions are plotted in
Fig. 5 as a function of the reactant flow rate.
Fig. 6. CO partial pressures determined for various sieve fractions of
CZC15 during SRM under standard conditions (m = 0.5500 g, T = 523 K,
5
p = 10 Pa, MeOH/H O = 1/1).
2
partial pressure on the methanol conversion, as depicted in
Fig. 6.
According to Fig. 6, the CO levels for the three fractions
at the same conversions were very similar, and thus all values
could be fitted with the same exponential function. It may
therefore be concluded that intraparticle transport phenom-
ena exerted no considerable influence on the CO formation.
Hence, because mass transport limitations can be excluded,
a reliable kinetic model was suggested for the steam reform-
ing of methanol. We determined the kinetic parameters by
fitting the model to the experimental results obtained for the
CZC catalysts.
When the flow rate was increased, the conversions for
the various sieve fractions declined to the same extent. At a
given flow rate, the values obtained decreased slightly with
an increasing particle diameter; however, the differences de-
tected were not significant with respect to mass transport
limitation. This is corroborated by the dependence of the CO
3.2. Kinetic model
The individual reactions to be included in the kinetic
model of the methanol steam reforming process are still
under debate. Earlier studies have suggested that the kinet-
ics could be sufficiently described with the use of only one

470
A. Mastalir et al. / Journal of Catalysis 230 (2005) 464–475
or two of the possible overall reactions, under the assump-
tion that the others were at equilibrium or their rates were
negligible. Initially, SRM was supposed to proceed by the
formation of CO and H2, followed by the water–gas shift
(WGS) reaction [15,42]. The formation of CO2 by the direct
reaction of methanol and steam has also been proposed [43].
Methanol decomposition was involved in the reaction mech-
anism in certain studies [14,42,44], whereas in other cases,
it was regarded as insignificant [16,21].
An adequate kinetic model for the steam reforming of
methanol on a commercial Cu/ZnO/Al2O3 catalyst, which
includes methanol decomposition [Eq. (1)] and the WGS re-
action [Eq. (5)], in addition to the SRM process [Eq. (2)],
has recently been suggested by Peppley et al. [25]
reactants and products can be expressed as follows:
XXw
pCO
pCO
=
p,
(6)
(7)
2 + 2X
X(1 − Xw)
2 + 2X
=
p,
2
X(2 + Xw)
2 + 2X
p
=
p,
(8)
H2
1 − X
pMeOH
=
p,
(9)
2 + 2X
1 − (1 − Xw)X
2 + 2X
pH O
=
p.
(10)
2
The partial pressures can be obtained from Eqs. (6)–
(10), given that p = 105 Pa, and Xw can be calculated from
k
W
CO + H2O(g) ꢀ CO2 + H2, ꢀH◦ = −41 kJ mol−1. (5)
Raoult’s law as
ϕCO
k
−W
Further investigations of SRM revealed that the above model
may also apply for other catalysts, including Zr- or Ce-
containing systems [28–30]. For the present study, it was
found that a kinetic evaluation that included MD in the reac-
tion scheme afforded a better fit to the experimental data. Ac-
cordingly, evaluation of the experimental data obtained for
the CZC samples was based on Peppley’s model. The kinetic
model suggested by Takahashi et al., involving a methyl for-
mate intermediate [45], may be regarded as less suitable for
evaluation, given that no methyl formate production was ob-
served under the present experimental conditions. In fact,
methyl formate is more likely to be formed when the reac-
tion is conducted in an excess of methanol [3].
Xw =
,
(11)
ϕCO + ϕ
CO2
where ϕCO and ϕCO are the volume fractions for CO and
2
CO2, respectively, which can be determined from the corre-
sponding GC peak areas after the calibration gas mixture has
been analyzed.
The rate equations for the model reactions [Eqs. (1), (2),
and (5)] can be described as follows:
m2
2
m3
CO2
rSRM = kRpMmeOHpH O − k−Rp pH3m
,
(12)
(13)
(14)
1
4
2
rMD = kDpm
,
5
MeOH
m6
m8 m9
rRWGS = k−Wp pm − kWpCOpH O
.
7
H2
CO2
2
CO formation has generally been observed at high
methanol conversions and long contact times, indicating that
CO is a secondary product, formed by the reverse water–gas
shift (RWGS) reaction [3,6,28]. For the CZC samples, ki-
netic analysis suggested that the amount of CO produced by
MD was considerably lower than that formed by the RWGS
reaction, and therefore we simplified the reaction scheme
by regarding MD as an irreversible reaction. Furthermore,
because of the low Hüttig temperature of Cu [46], as re-
flected in its relatively low melting point (1356 K) [5,6],
Cu-based catalysts tend to undergo thermal deactivation at
temperatures exceeding 573–623 K [5,6,47]. To eliminate
aging effects, kinetic measurements for the CZC samples
were carried out in the temperature range of 503–573 K.
According to the above considerations, the reaction
scheme utilized for methanol steam reforming on CZC cata-
lysts included the MD, SRM, and RWGS reactions [Eqs. (1),
(2), and (5)]. Because SRM is the sum of the other two re-
actions, the above processes are not independent [26]. We
calculated the partial pressures of the reactants and prod-
ucts with the assumption that the contribution of MD to
the conversion was negligible. If the SRM reaction for an
equimolar mixture of methanol and water takes place with
a conversion of X, then the total mole number after SRM is
n = n0(1 + X). If we assume that the RWGS reaction pro-
ceeds with a conversion of Xw, the partial pressures of the
Accordingly, the differential equations for the single
components (obtained with the Maple V software, release
4.00c) are given below:
∂pMeOH
∂t
∂p
2
= −(rSRM + rMD
)
,
(15)
1 + X
p
H2O
H2O
= −(rSRM + rRWGS) − (rSRM + rMD
)
, (16)
∂t
∂pCO
∂t
pMeOH,0
pCO
= (rMD + rRWGS) − (rSRM + rMD
)
,
(17)
(18)
pMeOH,0
∂pCO
∂t
pCO
2
2
= (rSRM − rRWGS) − (rSRM + rMD
)
,
pMeOH,0
∂pH
2
= (3rSRM + 2rMD − rRWGS
)
∂t
pH
2
− (rSRM + rMD
)
.
(19)
pMeOH,0
The increased mole numbers during SRM (4 mol of prod-
ucts formed from 2 mol of reactants) were taken into account
via expression of the conversion X by the partial pressures
of methanol as
pMeOH,0 − pMeOH,t
X =
.
(20)
pMeOH,0 + pMeOH,t
Likewise, the contact time τ was determined by con-
sidering the increase in volume in the catalyst bed during

A. Mastalir et al. / Journal of Catalysis 230 (2005) 464–475
471
Table 2
Reaction orders for the components involved in the kinetic model
SRM
RWGS
MD
Component
Reaction order
Component
Reaction order
Component
MeOH
Reaction order
MeOH
0.6
0.4
1.0
1.0
CO
1.0
1.0
1.0
1.0
1.3
2
H O
H
2
2
CO
CO
H O
2
H
2
2
conversion. We calculated this by integrating the progress of
conversion X(ꢁ) as a function of the reactor length ꢁ inside
the catalyst bed
ꢁ
ꢁ
ꢀ
ꢀ
1
A
1
τ =
τ(ꢁ)dꢁ =
dꢁ,
(21)
ꢁ
w0
1 + X(ꢁ)
0
0
where A is the cross section of the reactor and w0 is the
initial flow rate of the reactants. Furthermore, X(ꢁ) may be
sufficiently well approximated by an exponential function,
X(ꢁ) = 1 − e−ωꢁ
,
(22)
and the parameter ω can be obtained from the values of the
final conversion Xf and the final length of the catalyst bed ꢁf
Fig. 7. Partial pressures of the components in the reaction mixture dur-
ln(1 − Xf)
ω = −
.
(23)
ing SRM on CZC15; experimental data and fitting results: m = 0.6726 g,
ꢁf
5
T = 503 K, p = 10 Pa, MeOH/H O = 1/1.
2
The kinetic evaluation was performed with the Berke-
ley Madonna 8.0.1 software. A Runge-Kutta method was
used to solve the differential equations [Eqs. (15)–(19)],
and the experimental data were fitted by a least-squares
method. We determined the rate constants for the model re-
actions by fitting the simulation to the experimental data
by varying the reaction orders until a good agreement was
obtained, and this procedure was repeated until an optimal
fit was achieved. According to the previous results of Pur-
nama et al., the total reaction order for SRM (m1 + m2)
was 1, whereas the individual reaction orders determined
experimentally for methanol and water were 0.6 and 0.4, re-
spectively [21]. The application of the above reaction orders
ensured an optimal fit for the current experimental data. For
the components involved in the RWGS reaction, the reaction
orders were set to 1 [21], in correlation with those published
by Choi and Stenger in their detailed study on the kinetics of
the WGS reaction [48], including rate expressions derived
from various reaction mechanisms [49]. The rate constants
for both the RWGS and the MD reactions were considerably
smaller than that of SRM, as revealed by the low CO produc-
tion, and thus the reaction orders of the components implied
in RWGS and MD had no appreciable effect on the fitting
results. The reaction orders are summarized in Table 2.
The partial pressures for the components of the product
stream were determined as a function of the contact time at
temperatures of 503, 523, 543, and 573 K for all catalysts.
The results obtained for CZC15 are displayed in Figs. 7–10.
No significant differences were observed between the re-
spective data for the other CZC samples.
Fig. 8. Partial pressures of the components in the reaction mixture dur-
ing SRM on CZC15; experimental data and fitting results: m = 0.6726 g,
5
T = 523 K, p = 10 Pa, MeOH/H O = 1/1.
2
It can be seen from Figs. 7–10 that the partial pressures
of the reactants methanol and water decreased to the same
extent with increasing contact times at all temperatures. The
most pronounced decrease can be observed at 573 K, where
complete transformation occurred at τ < 1 s. Meanwhile,
the partial pressures of the main reaction products H2 and
CO2 increased steadily during reaction and could be fitted
with saturation curves, for which the limits for H2 and CO2
production proved to be 70–75 and 24 kPa, respectively. This
implies that the amount of H2 gained in the reaction at 573 K

472
A. Mastalir et al. / Journal of Catalysis 230 (2005) 464–475
Fig. 11. Methanol conversions during SRM of CZC15 as a function of con-
Fig. 9. Partial pressures of the components in the reaction mixture dur-
5
tact time at different reaction temperatures (m = 0.6726 g, p = 10 Pa,
ing SRM on CZC15, experimental data and fitting results: m = 0.6726 g,
5
MeOH/H O = 1/1).
2
T = 543 K, p = 10 Pa, MeOH/H O = 1/1.
2
gated, the amount of CO typically varied between 0.06%
and 0.4%, only exceeding that at 573 K, at contact times
longer than 0.4 s. Unlike the main reaction products, the for-
mation of CO can be described with S-shaped curves at each
temperature, which may be regarded as further evidence that
CO is a secondary product in the reaction [6,21]. As shown
in Figs. 7–10, the partial pressures of CO could be fitted at
all temperatures and contact times when MD was included
in the reaction scheme. Nevertheless, it should be noted that
the significance of MD can be observed only at low con-
tact times, and therefore CO production is suggested to oc-
cur predominantly by the RWGS reaction. The conversions
obtained for CZC15 at different temperatures, plotted as a
function of the contact time, are displayed in Fig. 11.
The complete transformation of methanol could be achie-
ved at all reaction temperatures, except for 503 K, where
the contact time corresponding to full conversion was de-
termined by extrapolation (τ ∼ 10 s). It can be seen that
an increase in the reaction temperature resulted in a marked
decrease in the contact time corresponding to the same con-
version level. Accordingly, the full conversion of methanol
at 573 K was observed at a contact time of 0.92 s. Although
an increase in the reaction temperature proved to be benefi-
cial in terms of the catalytic activity, as suggested by Fig. 11,
it also resulted in the formation of an increased amount of
CO in the product mixture (see Fig. 12). Furthermore, a pro-
longed exposure of the catalyst to 573 K was found to result
in a moderate deactivation, which may be due to sintering
of the active Cu particles [6,47]. For the other CZC samples,
similar observations have been made. Likewise, Lindström
and Pettersson reported that sintering of Cu particles of a
Cu/ZnO/Al2O3 catalyst subjected to 613 K resulted in a loss
of Cu surface area [50]. Hence, the optimal reaction tem-
perature range for the steam reforming of methanol on CZC
catalysts is 523–543 K, at which high methanol conversions
can be achieved while a low CO level is maintained.
Fig. 10. Partial pressures of the components in the reaction mixture dur-
ing SRM on CZC15; experimental data and fitting results: m = 0.6726 g,
5
T = 573 K, p = 10 Pa, MeOH/H O = 1/1.
2
attains the theoretical maximum of 75%, corresponding to
the thermodynamic equilibrium state [4], and approaches it
reasonably well at lower temperatures. The differences in the
partial pressures of H2 and CO2 at various contact times con-
firm that the amount of H2 formed in the reaction was 3 times
higher than that of CO2, irrespective of the reaction temper-
ature and the conversion level. With increasing contact time,
CO2 formation was less affected than H2 production, and the
contact time required to reach the saturation value decreased
with the reaction temperature to an appreciable extent.
Compared with a commercial Cu/ZnO/Al2O3 cata-
lyst [21], the H2 partial pressures obtained for the CZC sam-
ples were somewhat lower (74–85% of those reported for
the commercial sample between 503 and 573 K, respec-
tively) at short contact times. Nevertheless, the saturation
values observed for the CZC catalysts at long contact times
approached 75 kPa at all temperatures.
The minor amount of CO detected in the reaction mix-
ture was found to increase with both the contact time and
the reaction temperature. In the temperature range investi-
For the CO partial pressures, we fitted the experimental
data by setting the final values, calculated at long contact

A. Mastalir et al. / Journal of Catalysis 230 (2005) 464–475
473
Table 3
Reaction rate constants determined for the SRM, RWGS and MD reactions on the CZC catalysts at different reaction temperatures
a,d −1 −1
b,d
−1 −1 −1
c,d
−0.3 −1 −1
Sample
k
(s
g
)
k
(bar
s
g
)
k
D
(bar
s
g
)
R
−W
503 K
523 K
543 K
573 K
503 K
523 K
543 K
573 K
503 K
523 K
543 K
573 K
CZC5
2.92
22.58
20.25
26.67
10.71
50.67
55.17
61.33
34.40
117.33
79.95
68.44
268.02
146.37
165.69
0.04
0.27
0.22
0.31
0.11
0.60
0.95
0.58
0.30
1.28
1.35
1.54
1.12
3.55
3.06
2.31
0.04
0.09
0.14
0.36
0.13
0.36
0.38
0.96
0.66
1.02
1.13
1.54
2.12
2.89
4.50
3.44
CZC15
CZC25
CZC35
89.00
a
Reaction rate constant of SRM.
Reaction rate constant of RWGS.
Reaction rate constant of MD.
b
c
d
All values were referred to a CuO content of 1.0 g.
Fig. 13. Arrhenius plots for the SRM, RWGS and MD reactions determined
for CZC15.
Fig. 12. CO partial pressures during SRM of CZC15 as a function of con-
5
tact time at different reaction temperatures (m = 0.6726 g, p = 10 Pa,
MeOH/H O = 1/1).
2
tion of the reaction orders indicated in Table 2, are listed in
Table 3. For a better comparison, all values were referred to
a CuO content of 1.0 g.
times from the thermodynamical equilibrium constant of the
RWGS reaction, as displayed in Fig. 12. For comparison, we
also determined these values experimentally by investigating
a large amount of catalyst (m = 3.55 g) at a low flow rate of
0.01 cm3 min−1 to ensure an extended contact time for the
reaction. The CO partial pressures obtained from repeated
runs at different temperatures were in good agreement with
the theoretical values. It can be observed in Fig. 12 that the
experimental CO levels are located at the low end of the the-
oretical curves and, hence, are far below those predicted by
equilibrium calculations at all temperatures. It follows that
the limiting CO partial pressures could be attained only at
contact times considerably longer than those obtained under
the present experimental conditions (see also Figs. 7–10).
Accordingly, it is reasonable to assume that the kinetics
of the CO formation is more complex than that described
within the framework of this study [21]. Nevertheless, the
kinetic model employed provides a good approximation for
the experimental data obtained for all components in the
contact time region where complete methanol conversion
occurs, which may be of major interest for industrial appli-
cations.
For the most active catalyst (CZC35) investigated at the
highest reaction temperature (573 K), the Thiele modulus
[51] was calculated to be 0.594, from which the mass trans-
fer factor (the actual reaction rate as referred to the ideal
reaction rate without transport limitation) was 0.977, indicat-
ing that the mass transport limitation was indeed negligible
and thus the reaction was kinetically controlled. Given that
the reaction rates for the other CZC samples were found
to be typically lower, in particular at temperatures below
573 K, the effect of intraparticle transport phenomena on the
kinetic parameters can be safely excluded.
The apparent activation energies were determined from
the slopes of the Arrhenius plots
EA
ln(k) = ln(k0) −
.
(24)
RT
A typical illustration of the Arrhenius parameters for the
model reactions on CZC15 is displayed in Fig. 13. It may
readily be observed from the slopes that the activation ener-
gies of SRM and RWGS are comparable and considerably
lower than that of MD. The values of the activation energies
for the SRM, RWGS, and MD reactions, determined for all
of the CZC samples, are listed in Table 4.
The reaction rate constants (k) for the individual reac-
tions, obtained from the simulations performed by applica-

474
A. Mastalir et al. / Journal of Catalysis 230 (2005) 464–475
Table 4
Apparent activation energies for the SRM, RWGS and MD reactions on the
CZC catalysts
−1
−1
−1
−1
Reaction E (kJ mol ) E (kJ mol ) E (kJ mol ) E (kJ mol
)
A
A
A
A
CZC5
CZC15
CZC25
CZC35
SRM
RWGS 115.5
MD 142.6
109.2
85.4
88.2
118.7
66.1
85.7
104.9
60.7
69.3
75.5
Fig. 15. Apparent activation energies of the model reactions (SRM, RWGS,
and MD) as a function of the Cu content of four CZC catalysts.
served in the activation energy of MD, indicating that at
lower Cu loading the contribution of MD to CO formation
diminishes. Takeguchi et al. have reported a similar trend for
the SRM reaction on two Cu/ZnO catalysts [53], although
a potential mass transport limitation was not taken into ac-
count, and thus their studies cannot be directly compared
with the results presented here. Conversely, in the current
study, the well-known effect of mass transport limitation on
the kinetic studies, resulting in a reduced apparent activa-
tion energy, can be excluded. Therefore, it is concluded that
the variation of the Cu concentration in the CeO2/ZrO2 pre-
cursor material gave rise to the formation of CZC samples
with significantly different catalytic behavior. Whereas the
chemical complexity in the various CZC catalysts remains
the same, the chemical composition adjusted in the prepa-
ration procedure results in a significantly altered structural
complexity of the materials. These catalysts with vastly dif-
ferent active surfaces exhibit a catalytic behavior that is not
simply correlated with the accessible surface area. Disper-
sion, Cu particle size, and Cu support interactions have a
pronounced influence on the microstructure of the Cu par-
ticles and the effect of the microstructure on the active Cu
surface. Eventually, researchers must elucidate both the Cu
surface area and the microstructural properties of the Cu par-
ticles to fully understand the significantly different catalytic
activities and the kinetics described above.
Fig. 14. Arrhenius representation of the reaction rate constants obtained for
the SRM reaction on the CZC catalysts.
According to Table 4, no significant differences between
the activation energies of the SRM and RWGS reactions can
be observed, except for CZC25. For the latter sample, the
activation energy of the RWGS reaction was considerably
higher than that for SRM, which may account for the partic-
ularly low CO level observed for this catalyst (see Fig. 4).
On the other hand, the activation energies for MD proved to
be considerably higher than those for SRM and RWGS, and
therefore MD may be regarded as the rate-limiting step for
CO formation at short contact times. As mentioned above,
the significance of MD in the overall reaction mechanism
decreases dramatically at contact times exceeding 0.3–1 s,
depending on the reaction temperature. Accordingly, the en-
hanced CO levels obtained at longer contact times can be
attributed to the predominance of the RWGS reaction, which
requires a considerably lower activation energy.
For the main reaction (SRM), the apparent activation en-
ergies proved to be relatively close for all samples, apart
from CZC5, for which the highest value was obtained. For
comparison, the Arrhenius plots for all of the CZC samples
are depicted in Fig. 14. For CZC5 and CZC15, the appar-
ent activation energies are in good agreement with those re-
ported in the literature for Cu/ZnO/Al2O3 catalysts, whereas
the values for CZC25 and CZC35 are lower [6,16,21,52].
For all of the model reactions (SRM, RWGS, and MD),
the activation energies determined exhibit a linear corre-
lation with the Cu content, as shown in Fig. 15. With an
increasing amount of Cu, a systematic decrease was found
for each reaction. The most pronounced decrease can be ob-
4. Conclusions
Novel Cu/ZrO2/CeO2 materials prepared by coprecipi-
tation and polymer templating were investigated as cata-
lysts in the steam reforming of methanol. Catalytic mea-
surements were performed under continuous operation in a
fixed-bed reactor at atmospheric pressure. Time-on-stream
experiments indicated that after an initial period of deac-
tivation, the conversions for the samples with Cu contents
exceeding 5% were stabilized, and thus these samples could
be suitable for prolonged catalytic applications. An increase

A. Mastalir et al. / Journal of Catalysis 230 (2005) 464–475
475
in the Cu content from 5 to 15% was found to improve the
long-term stability and suppress the CO production consid-
erably, whereas the effect of a further enhancement was less
significant. With respect to a reduced formation of CO dur-
ing SRM, the samples with increased Cu loading proved to
be more efficient catalysts.
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overall transformation included the SRM, MD, and RWGS
reactions. The experimental data obtained in the range of
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