Effect of dopants on the activity of Cu/M0.3Zr0.7O2 (M = Ce, Mn, and Pr) for CO hydrogenation to methanol

Article abstract of DOI:10.1016/j.jcat.2006.07.031

Previous investigations have shown that Cu/ZrO₂ is an active catalyst for the hydrogenation of CO to methanol and that both components of the active play an active role in the reaction mechanism. It has also been shown that the substitution of Ce for Zr into the ZrO₂ lattice results in significantly enhanced methanol synthesis activity. The present investigation was undertaken with the aim of understanding whether other substituents, such as Mn and Pr, could also enhance the activity of Cu/ZrO₂. Zirconia and Ce-, Mn-, and Pr-substituted zirconia were prepared by forced hydrolysis at low pH, starting from nitrates of each metal, and Cu was then dispersed onto the surface of the calcined oxide by deposition-precipitation. All catalysts were characterized by XRD, XANES, and temperature-programmed reduction in H₂. H₂ and CO chemisorption capacities were also measured. The area-based activity of 3 wt% Cu/M_0.3Zr_0.7O₂ decreased in the order 3 wt% Cu/Ce_0.3Zr_0.7O₂ > 3 wt% Cu/Pr_0.3Zr_0.7O₂ > 3 wt% Cu/Mn_0.3Zr_0.7O₂ > 3 wt% Cu/ZrO₂. Catalyst activity was found to correlate with H₂ adsorption capacity and the proportion of bridge-bonded hydroxyl groups. The importance of the latter species is ascribed to their higher Bronsted acidity, which contributes to the rapid release of methoxide groups formed on the oxide surface and the formation of methanol. Dopant cations that can participate in redox cycles (e.g., Ce and Mn) are desirable, because they can enhance the methanol synthesis activity of Cu/M_0.3Zr_0.7O₂ catalysts to a greater degree than cations that do not participate in redox cycles (e.g., Pr).

Full text of DOI:10.1016/j.jcat.2006.07.031

Journal of Catalysis 244 (2006) 43–51
www.elsevier.com/locate/jcat
Effect of dopants on the activity of Cu/M0.3Zr0.7O2 (M = Ce, Mn, and Pr)
for CO hydrogenation to methanol
Konstantin A. Pokrovski, Alexis T. Bell ∗
Chemical Sciences Division, Lawrence Berkeley National Laboratory and Department of Chemical Engineering, University of California,
Berkeley, CA 94720-1462, USA
Received 31 May 2006; revised 27 July 2006; accepted 31 July 2006
Available online 27 September 2006
Abstract
Previous investigations have shown that Cu/ZrO is an active catalyst for the hydrogenation of CO to methanol and that both components of
2
the active play an active role in the reaction mechanism. It has also been shown that the substitution of Ce for Zr into the ZrO lattice results
2
in significantly enhanced methanol synthesis activity. The present investigation was undertaken with the aim of understanding whether other
substituents, such as Mn and Pr, could also enhance the activity of Cu/ZrO . Zirconia and Ce-, Mn-, and Pr-substituted zirconia were prepared by
2
forced hydrolysis at low pH, starting from nitrates of each metal, and Cu was then dispersed onto the surface of the calcined oxide by deposition–
precipitation. All catalysts were characterized by XRD, XANES, and temperature-programmed reduction in H . H and CO chemisorption
2
2
capacities were also measured. The area-based activity of 3 wt% Cu/M Zr
O
decreased in the order 3 wt% Cu/Ce Zr
O > 3 wt%
0.3 0.7
2
0.3 0.7 2
Cu/Pr Zr
O
> 3 wt% Cu/Mn Zr
O > 3 wt% Cu/ZrO . Catalyst activity was found to correlate with H adsorption capacity and the
0.3 0.7
2
0.3 0.7 2 2 2
proportion of bridge-bonded hydroxyl groups. The importance of the latter species is ascribed to their higher Brønsted acidity, which contributes
to the rapid release of methoxide groups formed on the oxide surface and the formation of methanol. Dopant cations that can participate in redox
cycles (e.g., Ce and Mn) are desirable, because they can enhance the methanol synthesis activity of Cu/M Zr O catalysts to a greater degree
0.3 0.7
2
than cations that do not participate in redox cycles (e.g., Pr).
© 2006 Elsevier Inc. All rights reserved.
Keywords: Methanol; Cu; ZrO ; Synthesis gas
2
1. Introduction
on Cu/ZrO2. Consistent with the deductions drawn from these
mechanistic studies, it has also been shown that the phase of
zirconia and the surface density of the dispersed Cu influence
catalyst activity. Thus, Cu/m-ZrO2 (m-ZrO2—monoclinic zir-
conia) is found to be nearly an order of magnitude more active
for methanol synthesis than Cu/t-ZrO2 (t-ZrO2—tetragonal zir-
conia) for equivalent zirconia surface areas and surface concen-
trations of dispersed Cu, and, for a given phase of zirconia, the
methanol synthesis activity increases linearly with the surface
concentration of Cu [6].
Pokrovski et al. [8,9] recently showed that incorporation
of Ce into ZrO2 can increase the methanol synthesis activity
of the resulting catalyst above that measured for Cu/m-ZrO2.
The methanol synthesis activity of 3 wt% Cu/CexZr1−xO2 was
found to pass through a maximum at x ≈ 0.5 with an increase in
Ce content. The maximum in the area-based methanol synthesis
activity was paralleled by a maximum in the hydrogen adsorp-
Zirconia-supported copper catalysts exhibit high activity for
the hydrogenation of CO to methanol and can be used with or
without the presence of CO2 [1–7]. Mechanistic studies have
shown that the active centers for methanol synthesis occur on
the surface of the oxide, rather than the surface of the sup-
ported Cu particles [4]. CO adsorbs preferentially on the surface
of zirconia to form formate species, which then undergo hy-
drogenation to produce methanol. The hydrogen atoms needed
for this process are produced by dissociative adsorption of H2
on the surface of the dispersed Cu and then spillover onto the
zirconia surface. Therefore, both components of the catalyst
play an active role in the hydrogenation of CO to methanol
*
Corresponding author.
E-mail address: alexbell@berkeley.edu (A.T. Bell).
0021-9517/$ – see front matter © 2006 Elsevier Inc. All rights reserved.
doi:10.1016/j.jcat.2006.07.031

44
K.A. Pokrovski, A.T. Bell / Journal of Catalysis 244 (2006) 43–51
tion capacity. The authors attributed this latter effect to the for-
mation of Ce3+-O(H)-Zr4+ species by dissociative adsorption
of H2 on particles of supported Cu, followed by spillover of
atomic H onto the oxide surface and its reaction with Ce4+-O-
Zr4+ centers. The authors concluded that the higher concentra-
tion of Ce3+-O(H)-Zr4+ species on the oxide surface, together
with the higher Brønsted acidity of these species, appears to
be the primary cause for the fourfold-higher activity of 3 wt%
Cu/Ce0.5Zr0.5O2 relative to 3 wt% Cu/ZrO2.
Table 1
Effect of oxide composition on the surface properties of 3 wt% Cu/
M
Zr
O
0.3 0.7 2
Catalyst
S.A.
H
adsorption
CO adsorption
capacity
(µmol/m )
2
2
capacity
(µmol/m )
(m /g)
2
2
3 wt% Cu/ZrO
3 wt% Cu/Ce Zr
3 wt% Cu/Pr Zr
3 wt% Cu/Mn Zr
123
127
96
0.3
0.50
0.65
0.71
0.67
2
O
1.51
1.42
0.94
0.3 0.7
2
O
0.3 0.7
2
O
160
0.3 0.7
2
The present study was undertaken to examine the influence
of different dopants incorporated into ZrO2 on the catalytic ac-
tivity of 3 wt% Cu/M0.3Zr0.7O2 (M = Ce, Mn, and Pr) for
methanol synthesis from CO/H2. Mixed oxides with the stoi-
chiometry M0.3Zr0.7O2 were prepared by forced hydrolysis at
low pH, and Cu was dispersed on the surface of these materi-
als by deposition–precipitation. These catalysts were character-
ized by XRD, XANES, and temperature-programmed reduction
(TPR) in H2, and the H2 and CO adsorption capacities of each
catalyst were measured by temperature-programmed desorp-
tion (TPD). Steady-state catalytic performance measurements
were made, and transient-response, in situ infrared studies were
conducted to probe the reactivity of adsorbed species.
393 K under vacuum for >2 h. BET surface areas were calcu-
lated using a five-point isotherm. The surface areas measured
after calcination ranged from 96 to 160 m2/g and are reported
in Table 1. Nearly identical values were obtained after the dis-
persion of Cu.
Hydrogen TPR was carried out using 0.15 g of a calcined
sample that had been purged with He at 298 K for 30 min. The
flow was then switched from pure He to a 2% H2/He mixture
flowing at 60 cm3/min, and the temperature of the sample was
ramped at 20 K/min from 298 to 673 K. The consumption of
H2 was monitored using a mass spectrometer (Cirrus, Spectra
Products).
Cu K-edge, Mn K-edge, Ce LIII-edge, and Pr LIII-edge
XANES data were acquired at the Stanford Synchrotron Radi-
ation Laboratory (SSRL) on beamline 2–3. Each sample was
mixed with boron nitride, pressed into a rectangular pellet
(0.43 × 1.86 cm), and then placed in an in situ cell for trans-
mission experiments [10]. A sufficient quantity of each sample
was used to give a calculated absorbance of 2. Each sample
was calcined in 10% O2/He at 573 K for 2 h, then cooled to
298 K, purged with He, and evacuated to 10−6 Torr to remove
residual oxygen. A 2% H2/He mixture was then passed through
the cell at a flow rate of 60 cm3/min. In situ XANES data at
Cu K-edge, Mn K-edge, Ce LIII-edge, and Pr LIII-edge were
acquired while heating each sample in a flow of 2% H2/He
(60 cm3/min) from 298 to 573 K at 4 K/min. XANES analyses
were carried out using the Athena version 0.8.041 software [11,
12]. The energy was calibrated using the Cu K-edge of a Cu foil
(E0 = 8980 eV), the Mn K-edge of a Mn foil (E0 = 6539 eV),
the Ce LIII-edge of CeO2 (E0 = 5728 eV), and the Pr LIII-edge
of Pr(C2H3O2)3 (Pr3+ white line at 5954 eV). Pre-edge absorp-
tions due to the background and detector were subtracted using
a linear fit to the data in the range of −200 to −50 eV relative to
the sample edge energy (E0). Each spectrum was then normal-
ized by a constant determined by the average absorption in the
range of 100–300 eV relative to E0. The edge energy of each
sample and reference were taken at the first inflection point be-
yond any pre-edge peaks.
2. Experimental
2.1. Catalyst preparation
M0.3Zr0.7O2 (M = Ce, Mn, and Pr) was prepared as de-
scribed previously [8,9]. Zirconyl nitrate (ZrO(NO3)2·xH2O,
99.99%, Aldrich), cerium(III) nitrate (Ce(NO3)3·6H2O,
99.999%, Aldrich), manganese(II) nitrate (Mn(NO3)2·xH2O,
99.999%, Alfa Aesar), and praseodymium(III) nitrate (Pr-
(NO3)3·6H2O, 99.99%, Alfa Aesar) were used as precursors.
Appropriate amounts of metal salts were dissolved in deionized
water (0.5 M total metals basis) and boiled under reflux for
240 h. The final solutions had a pH < 1. NH4OH was added
dropwise to agglomerate the resulting fine particles and fa-
cilitate their filtration. The recovered precipitate was washed
with deionized water. The washed solid was then dried in air
overnight at 383 K. Each sample was then calcined at 873 K
in dry air flowing at a rate of 100 cm3/min. The temperature
was ramped from room temperature at a rate of 2 K/min to the
final temperature, which was maintained for 3 h. Copper was
then dispersed onto each support by the method of deposition–
precipitation [6] to obtain a series of 3 wt% Cu/M0.3Zr0.7O2
catalysts.
2.2. Catalyst characterization
The crystallographic phase of M0.3Zr0.7O2 was determined
by X-ray diffraction (XRD). XRD patterns were obtained with
a Siemens D5000 diffractometer using CuKα radiation and a
graphite monochromator. Scans were made in the 2θ range of
20◦–90◦ with a step size of 0.02◦ and a time/step of 11 s.
The BET surface area of each M0.3Zr0.7O2 support was de-
termined using an Autosorb 1 (Quantachrome Instruments) gas
adsorption system. Before each analysis, samples were dried at
H2 and CO adsorption capacities were determined using
TPD. In the case of H2, the sample was calcined and then re-
duced at 573 K in a 2% H2/He mixture flowing at 60 cm3/min.
The sample was then cooled in 2% H2/He to 298 K and
purged in He. Desorption was conducted by ramping the sam-
ple temperature at 20 K/min from 298 to 773 K in flowing
He (60 cm3/min) while monitoring the desorbing gas by mass
spectrometry. To determine the CO adsorption capacity, the

K.A. Pokrovski, A.T. Bell / Journal of Catalysis 244 (2006) 43–51
45
sample was calcined, reduced in 2% H2/He mixture flowing
at 60 cm3/min 573 K, cooled to 523 K, and flushed with He
(60 cm3/min) for 30 min. A 4.0% CO/He mixture was then
passed over the catalyst for 60 min at a flow rate of 60 cm3/min.
The sample was then cooled to 298 K in a 4.0% CO/He mixture
flowing at 60 cm3/min before it was purged for 30 min with He
(60 cm3/min) to remove any weakly adsorbed species. Desorp-
tion of adsorbed CO was carried out in a manner identical to
that used for adsorbed H2. Because CO and CO2 are desorbed
during this experiment, the total amount of adsorbed CO was
taken as the sum of both carbon-containing components.
2.3. Catalyst testing
Activity and selectivity measurements for CO hydrogenation
were carried out in a glass-lined stainless steel reactor; details of
the apparatus have been described previously [6]. Before test-
ing, each catalyst was calcined in a 10% O2/He mixture flowing
at 60 cm3/min. The sample was heated from room tempera-
ture to 573 K at a rate of 0.5 K/min and then maintained at
573 K for 2 h. The sample was then cooled to 323 K, swept
with He, and reduced in a 10% H2/He mixture flowing at the
rate of 60 cm3/min while the temperature was increased at a
rate of 2 K/min up to 573 K. The flow of 10% H2/He was main-
tained at 573 K for 1 h before switching to a flow of 100% H2
for an additional 1 h. Reactions were carried out with 0.15 g of
catalyst at a total pressure of 3.0 MPa. Total reactant gas flow
was 60 cm3/min (at STP) with a H2/CO ratio of 3/1. Two hours
were allowed to achieve steady state before product gas mix-
tures were analyzed. The temperature was then ramped to the
next highest temperature at a rate of 2 K/min, after which it
was again maintained for 2 h. Conversion and selectivity were
referenced to the consumption of CO, the limiting reactant.
Fig. 1. XRD patterns of M Zr O . Peaks marked as ꢀ are due to monoclinic
phase and as # are due to tetragonal phase.
0.3 0.7
2
tion pattern can be ascribed to tetragonal ZrO2 (t-ZrO2), with
only a trace of monoclinic ZrO2 (m-ZrO2) evident at 28.5◦.
The volume fraction of m-ZrO2 was estimated as ∼0.2 using
the following relationships [14]:
Vm = 1.311Xm/(1 + 0.311Xm)
and
(1)
ꢀ
ꢁ ꢂ ꢀ
ꢁ
¯
¯
Xm = Im(111) + Im(111)
Im(111) + Im(111) + It(111) ,
(2)
¯
where Im(111) and Im(111) are the line intensities of the (111)
¯
and (111) peaks for m-ZrO2 and It(111) is the intensity of the
(111) peak for t-ZrO2.
Introduction of 30 at% of Ce into the ZrO2 lattice resulted
in a shift of the diffraction peaks to lower 2Θ values. The 111
diffraction peak shifted from 30.3◦ for pure ZrO2 to 29.5◦ for
Ce0.3Zr0.7O2. As noted previously [8], detailed analysis of the
data obtained from XRD and Raman spectroscopy indicates
that Ce0.3Zr0.7O2 crystallized in the tꢀꢀ phase. The XRD pro-
file of Pr0.3Zr0.7O2 shown in Fig. 1 closely resembles that of
Ce0.3Zr0.7O2, suggesting that introduction of Pr into zirconia
lattice also stabilized the tꢀꢀ phase of zirconia. Mn0.3Zr0.7O2
crystallized as a mixture of monoclinic and tetragonal phases
(Fig. 1). The volume fraction of the monoclinic phase was esti-
mated to be ∼0.3 using Eq. (1). The XRD peaks of manganese-
doped zirconia corresponding to the monoclinic phase (marked
with a ꢀ in Fig. 1) are not shifted relative to those for pure
m-ZrO2, whereas the XRD peaks due to the tetragonal phase
(marked with a # in Fig. 1) are shifted to higher values of 2Θ.
These observations suggest that substitution of Zr4+ cations by
the smaller Mn3+ cations occurred only in the tetragonal phase.
This conclusion is consistent with the results of Occhiuzzi et
al., who showed that the tetragonal and cubic modifications of
ZrO2 stabilize the Mn2+ (3d5) and Mn3+ (3d4), respectively,
and that monoclinic zirconia stabilizes Mn4+ (3d3) [15].
2.4. Infrared spectroscopy studies
In situ transmission infrared spectroscopy experiments were
conducted using a low dead-volume infrared cell equipped with
CaF2 windows [13]. To remove any residual surface species
before testing, each sample was calcined in a 10% O2/He mix-
ture flowing at 60 cm3/min. The sample was heated from room
temperature to 523 K at 2 K/min and then maintained at 523
K for 8 h. The sample was then cooled to 323 K, swept with
He, and reduced in a 10% H2/He mixture flowing at the rate
of 60 cm3/min while the temperature was increased at the rate
of 2 K/min up to 523 K. The flow of 10% H2/He was main-
tained at 523 K for 1 h before switching to a flow of 100% H2
for an additional 1 h. The sample was then flushed with He for
1 h before sample testing. Carbon monoxide adsorption and hy-
drogenation experiments were carried out at a total pressure of
0.5 MPa.
3. Results
3.1. Material characterization of M0.3Zr0.7O2 supports
3.2. Characterization of Cu/M0.3Zr0.7O2
XRD patterns of the M0.3Zr0.7O2 materials are shown in
Fig. 1. For ZrO2, the principal peak seen at 30.3◦ in the diffrac-
TPR profiles of 3 wt% Cu/M0.3Zr0.7O2 are presented in
Fig. 2. The 3 wt% Cu/ZrO2 and 3 wt% Cu/Pr0.3Zr0.7O2 ex-

46
K.A. Pokrovski, A.T. Bell / Journal of Catalysis 244 (2006) 43–51
Fig. 3. Infrared spectra of the hydroxyl group stretching region taken for 3
wt% Cu/M Zr following calcination and reduction. Spectra referenced
to empty cell in He.
Fig. 2. H -TPR spectra for 3 wt% Cu/M Zr O . Heating rate = 20 K/min;
2
0.3 0.7 2
O
3
0.3 0.7
2
2% H /He flow rate = 60 cm /min.
2
hibited principal peaks centered at 450 K, attributed to the re-
duction of highly dispersed CuO [16,17]. The amount of H2
consumed was slightly greater than that corresponding to the
complete reduction of CuO (H2/CuO ∼ 1.1). This observation
is in a good agreement with previous reports [6,8,9]. The 3 wt%
Cu/M0.3Zr0.7O2 (M = Ce and Mn) samples exhibited signifi-
cantly greater consumption of H2 than expected for the reduc-
tion of CuO, suggesting that some of the Ce and Mn cations
undergoes reduction.
In situ XANES experiments were conducted to determine
the degree of Cu reduction and the oxidation states of Ce, Mn,
and Pr in 3 wt% Cu/M0.3Zr0.7O2. The spectra obtained for these
samples and for relevant standards are given in Supplementary
material. As shown previously [9], Cu K-edge XANES demon-
strates that the reduction of Cu was complete after the samples
were heated in 2% H2/He at 573 K for 1 h. In situ TPR XANES
experiments at the Pr LIII-edge indicated that praseodymium in
3 wt% Cu/Pr0.3Zr0.7O2 was present in the 3+ state and did not
change its oxidation state on reduction in H2 at 573 K. Ce LIII-
edge XANES spectra collected after the H2-reduction of 3 wt%
Cu/Ce0.3Zr0.7O2 revealed that ∼63% of the Ce4+ cations were
reduced to Ce3+ by H2 at 573 K [9]. Mn K-edge XANES of
calcined 3 wt% Cu/Mn0.3Zr0.7O2 showed that manganese was
incorporated into tetragonal zirconia matrix as Mn3+, which
is consistent with the results of Occhiuzzi et al. [15]. Heating
the 3 wt% Cu/Mn0.3Zr0.7O2 sample to 573 K in 2% H2/He
resulted in the partial reduction of Mn3+ to Mn2+ (∼33% of
Mn2+).
either bi-bridging or tri-bridging groups [18–20]. The peak po-
sitions for ZrO2 (3666 and 3731 cm−1) more closely resemble
those observed for m-ZrO2 (3668 and 3729 cm−1), than those
of t-ZrO2 (3660 and 3738 cm−1) [6]. Similarly to m-ZrO2, the
ZrO2 sample exhibited a higher relative concentration of the
low frequency band (at 3666 cm−1), suggesting that the ZrO2
particles contain a significant fraction of m-ZrO2 at their sur-
face. Introduction of guest cations into zirconia increased the
relative concentration of bridging hydroxyl groups (b-OH) as
seen in Fig. 3. This observation is consistent with our previ-
ous studies of CexZr1−xO2 mixed oxides [8,9]. The position
of the OH absorption band due to bridging hydroxyl groups
shifted from 3666 cm−1 for 3 wt% Cu/ZrO2 to 3670 cm−1
for 3 wt% Cu/Ce0.3Zr0.7O2 and 3 wt% Cu/Pr0.3Zr0.7O2, and to
3674 cm−1 for 3 wt% Cu/Mn0.3Zr0.7O2, indicating the partici-
pation of guest cations in the bonding of bridging OH groups.
The nature of surface species formed on adsorption of
methanol was characterized by in situ IR spectroscopy. Fig. 4
shows spectra obtained on reduced 3 wt% Cu/M0.3Zr0.7O2
taken at 523 K during sample exposure to a flow of 0.5%
CH3OH/He at a total pressure of 0.50 MPa. The peaks at
1150–1160 cm−1 are assigned to C–O stretching vibrations of
terminal methoxide species (t-OCH3) bonded to Zr4+, and ab-
sorption bands at 1118 and 1120 cm−1 are assigned to t-OCH3
adsorbed on Pr3+ and Ce3+ cations, respectively [8,9,21,22].
The absence of a band at 1105 cm−1 for terminal methoxide
species bonded to Ce4+ cations [8,9,21,22] suggests that all of
the Ce4+ present at the surface of the 3 wt% Cu/Ce0.3Zr0.7O2
was reduced to Ce3+ during reduction of the samples in H2 at
523 K before exposure to methanol. The broad band centered at
1050–1075 cm−1 (Fig. 4) is assigned to C–O stretching vibra-
tions of bridge-bonded methoxide species (b-OCH3) [8,9,21,
22]. The C–O stretching bands of surface methoxide species ad-
sorbed on manganese cations were not identified because their
positions overlapped with the C–O stretching frequencies of
bridge-bonded methoxide species adsorbed on ZrO2 [23]. The
position of the peak associated with b-OCH3 blue-shifted with
The IR spectra of the O–H stretching region obtained after
reduction at 523 K are shown in Fig. 3. Spectra were refer-
enced to the empty cell filled with He. Peak intensities were
normalized to account for slight differences in the weights of
each sample. The positions of these bands are similar to those
reported previously for ZrO2 [18–20]. Surface hydroxyl groups
on ZrO2 are commonly assigned based on the number of coor-
dinating cations, with the higher-frequency species representing
terminal groups and the lower-frequency species representing

K.A. Pokrovski, A.T. Bell / Journal of Catalysis 244 (2006) 43–51
47
Fig. 4. Infrared spectra of the C–O stretching region of surface methoxide
groups on 3 wt% Cu/M Zr taken in 0.5% CH OH/He flow at 523 K.
O
0.3 0.7
2
3
Spectra referenced to the samples under He flow at 523 K.
introduction of dopants into the ZrO2 lattice, indicating the par-
ticipation of guest cations in methoxide bonding (Fig. 4). The
decreased intensity of IR bands in the OH stretching region on
adsorption of methanol indicates that surface methoxide species
were formed by the interaction of gas-phase methanol with sur-
face OH groups. The increase in the relative concentration of
bridged CH3O species formed on adsorption of methanol on
3 wt% Cu/M0.3Zr0.7O2 is consistent with the increase in the
concentration of bridged hydroxyl groups (Figs. 3 and 4). The
degree of blue-shifting of the C–O stretching frequencies of
b-OCH3 is consistent with the degree of blue-shifting of the
nd H stretching frequencies of bridging hydroxyl groups being
the greatest for the 3 wt% Cu/Mn0.3Zr0.7O2 samples (Figs. 3
and 4).
Fig. 5. Effect of temperature on the selective conversion and overall selectivity
of CO to methanol during CO hydrogenation on 3 wt% Cu/M Zr O : cata-
0.3 0.7
2
3
lyst mass = 0.15 g; P = 3.0 MPa; H /CO = 3; total flow rate = 60 cm /min.
The H2 and CO adsorption capacities of 3 wt% Cu/M0.3
-
2
Zr0.7O2 determined from TPD spectra are given in Table 1. The
introduction of guest cations into the ZrO2 lattice increased the
amount of CO adsorbed per unit area on 3 wt% Cu/M0.3Zr0.7O2
by ∼30%; however, the CO adsorption capacity was almost in-
dependent of the type of dopant used (Table 1). In contrast,
the amount of H2 adsorbed was greatly affected by the oxide
composition. Desorption of H2 was observed in the temper-
ature range 320–680 K and was not accompanied by the re-
lease of water, except for 3 wt% Cu/Mn0.3Zr0.7O2, for which a
small quantity of desorbed water was observed at temperatures
above 680 K. This observation suggests that hydrogen desorp-
tion occurs via the decomposition of surface hydroxyl groups
formed on reduction in H2. The maximum H2 adsorption ca-
pacity was observed for 3 wt% Cu/Ce0.3Zr0.7O2, followed by 3
wt% Cu/Pr0.3Zr0.7O2 and 3 wt% Cu/Mn0.3Zr0.7O2.
methanol selectivity. The only major byproduct observed was
methane. Because the measured conversion levels are far be-
low those expected for equilibrium at the conditions used, the
observed rates of methanol formation are not influenced by
the rates of methanol decomposition. The steady-state activi-
ties of 3 wt% Cu/M0.3Zr0.7O2 for methanol synthesis are given
in Table 2. The methanol productivity and selectivity of 3 wt%
Cu/M0.3Zr0.7O2 is strongly affected by the catalyst composi-
tion. The most active catalyst based on catalyst surface area is
3 wt% Cu/Ce0.3Zr0.7O2, followed by Mn- and Pr-doped zirco-
nia based catalysts in that order.
3.4. IR spectroscopy studies
The nature of surface species and the dynamics of CO ad-
sorption and hydrogenation were studied using in situ IR spec-
troscopy. Fig. 6 shows spectra obtained during CO adsorption
on 3 wt% Cu/M0.3Zr0.7O2 catalysts previously reduced in H2
at 523 K. Spectra were collected at 523 K after the catalyst
had been exposed to a flow of 15% CO/He at a total pres-
sure of 0.50 MPa for 1 h. For the 3 wt% Cu/ZrO2 sample, the
3.3. Catalytic performance of Cu/M0.3Zr0.7O2
The effects of reaction temperature on the activity and se-
lectivity of the 3 wt% Cu/M0.3Zr0.7O2 catalysts are shown in
Fig. 5. The conversion of CO to methanol increased over the
temperature range 473–523 K, accompanied by a decrease in

48
K.A. Pokrovski, A.T. Bell / Journal of Catalysis 244 (2006) 43–51
Table 2
Effect of oxide composition on the activity of 3 wt% Cu/M Zr
O for CO hydrogenation
2
0.3 0.7
Catalyst
Conversion
(selectivity) at 523 K (%)
Productivity
Productivity
−1 −1
−2 −1
h )
(g-CH OH g-cat
3
h
)
(mg-CH OH m
3
3 wt% Cu/ZrO
1.73 (94.23)
5.30 (97.57)
2.62 (96.15)
3.85 (97.37)
0.136
0.416
0.206
0.302
1.10
3.28
2.15
1.89
2
3 wt% Cu/Ce Zr
O
2
0.3 0.7
3 wt% Cu/Pr Zr
O
0.3 0.7
2
3 wt% Cu/Mn Zr
O
2
0.3 0.7
3
Note. Catalyst mass = 0.15 g; T = 523 K; P = 3.0 MPa; H /CO = 3; total flow rate = 60 cm /min
2
3
Fig. 6. Infrared spectra taken for 3 wt% Cu/M Zr
O at 523 K in 0.05 MPa CO and 0.45 MPa He flowing at a total rate of 60 cm /min. Spectra referenced to
2
0.3 0.7
3 wt% Cu/Ce Zr
O under 0.50 MPa He flow at 523 K.
2
x
1−x
bands observed at 1563, 1386, and 1369 cm−1 are attributable
to the υas(OCO), δ(CH), and υs(OCO) modes, respectively, of
b-HCOO-Zr [21,24–26]. Accompanying features for b-HCOO-
to a H2/CO mixture at 523 K. The band at 1147 cm−1 is as-
signed to the υ(CO) mode of t-CH3O species adsorbed on Zr4+
.
Peaks corresponding to C–O stretching vibrations of b-CH3O
species were present at 1040–1060 cm−1. The blue-shifting of
the υ(CO) band for b-CH3O species occurring on introduction
of Ce, Pr, and Mn was similar to that observed in the spectra
of adsorbed methanol (Fig. 5) and suggests that some CH3O
species were bonded to both Zr4+ and guest cations. The inten-
sities of the bands for b-HCOO appearing at 1580 and 1375
cm−1 decreased relative to that observed on adsorption of
CO (Fig. 6). In the C–H stretching region, bands for CH3O
species were present at 2926–2920 and 2822–2815 cm−1. The
shoulders at 2970 and 2880 cm−1 are attributable to b-HCOO
species. Similar to the intensity of the peak at 1580 cm−1, the
intensities of the peaks at 2970 and 2880 cm−1 decreased for
mixed-metal oxides. The δ(CH) feature for CH3O was evident
at 1446 cm−1 but was of very weak intensity [21,24–26]. The
Zr in the CH stretching region occur at 2969 and 2888 cm−1
,
characteristic of [υas(OCO)+δ(CH)] and υs(CH), respectively
[21,24–26]. Weak features at 2934 and 2832 cm−1 attributed
to CH3O-Zr [21,24–26] appear even in the absence of gas-
phase H2. The weak bands at 1155 and 1040 cm−1 are assigned
to C–O stretching vibrations of terminal (t-OCH3) and bridged
(b-OCH3) methoxide species on ZrO2, respectively [21,24–26].
The shoulder located at approximately 1320 cm−1 is assigned
to b-CO23−-Zr species [21,24–26]. Peaks at 1440–1420 cm−1
can be attributed to various carbonate and carboxylate species
on the surface of ZrO2 [21,24–26].
The IR spectra of CO adsorbed on 3 wt% Cu/M0.3Zr0.7O2
are qualitatively similar to those for 3 wt% Cu/ZrO2. The ab-
sorption band attributed to υas(OCO) blue-shifted with the in-
troduction of guest cations into the zirconia lattice. The relative
concentration of methoxide species features (bands at 1155,
1050, 2935, and 2831 cm−1) also increased with the introduc-
tion of dopants into ZrO2. The presence of Ce3+ and Pr3+ in
the catalyst increased the intensities of broad features between
∼1550–1450 and 1350–1250 cm−1, which are indicative of
various carbonate and carboxylate species [21,24–27].
broad features between ∼1550–1450 and 1350–1250 cm−1
,
attributed to various carbonate and carboxylate species, were
similar to those observed during the adsorption of CO (Fig. 6).
Fig. 8 compares the dynamics of (a) CHOO and (b) CH3O
consumption on 3 wt% Cu/M0.3Zr0.7O2. Transient-response
spectra were obtained by replacing CO in the 3/1 H2/CO feed
with He while maintaining the total pressure at 0.50 MPa.
The integrated peak area in the region of 1600–1500 cm−1
for HCOO species, and the integrated peak area in the re-
gion of 1200–1000 cm−1 for CH3O species were normalized
to the value observed at the beginning of the transient. The
After CO adsorption for 1 h, H2 was introduced into the
flowing 15% CO/He mixture (total pressure = 0.50 MPa) so as
to achieve a H2/CO ratio of 3/1. Fig. 7 shows spectra obtained
on 3 wt% Cu/M0.3Zr0.7O2 at steady-state after 6 h of exposure

K.A. Pokrovski, A.T. Bell / Journal of Catalysis 244 (2006) 43–51
49
3
Fig. 7. Infrared spectra taken for 3 wt% Cu/M Zr
O at 523 K in 0.05 MPa CO, 0.15 MPa H , and 0.30 MPa He flowing at a total rate of 60 cm /min. Spectra
2 2
0.3 0.7
referenced to 3 wt% Cu/Ce Zr
O under 0.50 MPa He flow at 523 K.
2
x
1−x
Table 3
Effect of oxide composition on the apparent rate constant of 3 wt% Cu/
Zr for CO hydrogenation
M
O
2
0.3 0.7
−3
Catalyst
k
C
app
CH O
k
× 10
app
3
−1
(a.u.)
(min
)
3 wt% Cu/ZrO
5.8
18.1
10.2
7.4
1.1
2
3 wt% Cu/Ce Zr
O
3.67
1.91
2.04
0.3 0.7
2
3 wt% Cu/Pr Zr
O
0.3 0.7
2
3 wt% Cu/Mn Zr
O
2
0.3 0.7
Note. T = 523 K; 0.15 MPa H , 0.05 MPa CO, P = 0.5 MPa; total flow rate
tot
2
3
= 60 cm /min. Values of k
C
are normalized to the value of 1.1 for
app
CH O
3
3 wt% Cu/ZrO .
2
rate of CHOO hydrogenation to methoxide was much higher
than the rate of methoxide elimination regardless of catalyst
composition, indicating that the rate-determining step did not
change with the catalyst composition. This observation is in
good agreement with previous reports [7–9]. The apparent first-
order rate constant for the removal of methoxide species (kapp),
determined from the initial portion of the transient, is presented
in Table 3. Here kapp depends on the catalyst composition in a
manner similar to that seen for the steady-state activity per unit
surface area given in Table 2. However, it is important to note
that the integrated band intensity for methoxide species (termi-
nal and bridging) was little affected by the introduction of Ce
and Pr into ZrO2, even though the distribution between linear
and bridging methoxide species changed for doped catalysts.
However, both types of methoxide species appear to react at the
same rate, likely due to rapid interconversion between bridging
and terminal forms, as reported previously [8,9]. The value of
kapp for 3 wt% Cu/Mn0.3Zr0.7O2 was only slightly higher than
that for 3 wt% Cu/ZrO2 (7.4 vs 5.8 min−1), but the integrated
band intensity for methoxide species was ∼1.5 times higher on
3 wt% Cu/Mn0.3Zr0.7O2 catalyst.
(a)
(b)
Fig. 8. Peak areas of (a) CHOO and (b) CH O features for 3 wt% Cu/
4. Discussion
−
3
M
Zr
O at 523 K after switching feed from 0.05 MPa CO, 0.15 MPa H ,
0.3 0.7 2 2
The results of the present study are closely related to those
reported by Pokrovski et al. [8,9] for methanol synthesis on
3 wt% Cu/CexZr1−xO2. In that study, it was shown that the
and 0.30 MPa He to 0.15 MPa H and 0.35 MPa He flowing at a total rate of
60 cm /min. Areas normalized to the values observed at the beginning of the
transient.
2
3

50
K.A. Pokrovski, A.T. Bell / Journal of Catalysis 244 (2006) 43–51
removal as methanol on Cu/CexZr1−xO2. It was also noted
that under CO hydrogenation conditions, some of the surface
Ce3+ cations were oxidized to Ce4+ cations, suggesting that
the ability of Ce to participate in a redox cycle is important for
methanol synthesis (Scheme 1 in Ref. [9]).
The present study has shown that the substitution of Zr
cations by Mn or Pr cations enhanced the H2 adsorption ca-
pacity of 3 wt% Cu/M0.3Zr0.7O2 in a manner similar to that
observed for the substitution of Ce cations (see Table 1). The
IR spectra presented in Fig. 3 also show that the substitution
of Ce and Pr cations and, to a lesser extent, Mn cations en-
hances the fraction of bridge-bonded hydroxyl groups present
on the oxide surface. Because bridge-bonded hydroxyl groups
are more Brønsted acidic than terminal hydroxyl groups [9,
28], the introduction of dopants with lower valences than Zr4+
[i.e., Ce3+, Pr3+, and Mn3+ (or Mn2+)] increases the Brønsted
acidity of zirconia. It is notable that whereas Pr3+ and Ce3+ in-
creased the H2 adsorption capacity of the catalyst and the frac-
tion of bridge-bonded hydroxyl groups to very similar degrees,
the presence of Ce3+ cations at the catalyst surface enhanced
the methanol synthesis activity of 3 wt% Cu/M0.3Zr0.7O2 to a
significantly higher degree than the presence of Pr3+ cations.
We have hypothesized that the superior methanol synthesis ac-
tivity achieved by doping zirconia with Ce cations may be a
consequence of the ability of such cations to undergo a re-
dox cycle between Ce4+ and Ce3+ during the course of the
catalytic cycle, which could contribute to the stabilization of
methoxide species on the catalyst surface [9,29]. Because Pr3+
does not participate in a redox cycle, the principal advantage
of introducing such cations into the lattice of zirconia is to sta-
bilize the adsorption of hydrogen in the form of Brønsted acid
protons. The substitution of Mn3+ into the lattice of zirconia
increased the H2 adsorption capacity of the oxide and the frac-
tion of bridge-bonded hydroxyl groups on the oxide surface,
but to a lesser degree than that achieved using Ce3+ or Pr3+
cations. The ability of Mn cations to undergo redox between
Mn3+ and Mn2+ is likely the reason why the methanol synthe-
sis activity of 3 wt% Cu/Mn0.3Zr0.7O2 was comparable to that
of 3 wt% Cu/Pr0.3Zr0.7O2. Thus, it appears that attaining a sig-
nificant increase in the methanol synthesis activity of 3 wt%
Cu/M0.3Zr0.7O2 requires that Mn have a valence below that of
Zr4+ and be capable of participating in a redox cycle.
Fig. 9. Productivity vs the product of k
and surface methoxide concentration.
app
Values of k
C
are normalized to the value of 1.1 for 3 wt% Cu/ZrO .
CH O 2
app
3
rate of methanol synthesis was determined by the rate at which
methoxide species adsorbed on the oxide surface underwent
reaction with Brønsted acidic protons. This conclusion was sup-
ported by the observation of a linear correlation between the
steady-state rate of methanol formation and the rate of methox-
ide hydrogenation expressed as kappCCH O. In this relation-
3
ship, CCH O is the concentration of methoxide species present
3
at the start of the transient (see, e.g., Fig. 8b), which is es-
timated from the area of the IR bands for methoxide species
observed in the range of 1200–1000 cm−1. As shown in Fig. 9,
the data for 3 wt% Cu/ZrO2, 3 wt% Cu/Ce0.3Zr0.7O2, 3 wt%
Cu/Pr0.3Zr0.7O2, and 3 wt% Cu/Mn0.3Zr0.7O2 all lay along a
single line, suggesting that the rate-limiting step was the same
for all four catalysts. Notable both here and in Table 3 is the
fact that the substitution of 30% of the Zr cations in ZrO2 by
Ce, Mn, or Pr cations enhances the methanol synthesis activity
of the catalyst; however, the degree of enhancement depends on
the nature of the cation introduced into the zirconia lattice. The
issue then is to explain how the rate of methanol synthesis is en-
hanced by the introduction of dopant cations, and why the level
of enhancement depends on the nature of the dopant cation.
In our study of 3 wt% Cu/CexZr1−xO2, we noted that the
highest methanol activity was observed for x = 0.3–0.5, and
that under reaction conditions, most of the Ce4+ cations present
at the surface of the catalyst were reduced to Ce3+ [9]. The
maximum in the rate of methanol synthesis coincides with a
maximum hydrogen adsorption capacity, which is attributed to
maximum formation of Ce3+-O(H)-Zr4+ species. These Brøn-
sted acid sites are produced by dissociative adsorption of H2
on the particles of supported Cu and subsequent spillover of
atomic H onto the oxide surface and followed by reaction with
Ce4+-O-Zr4+ centers. It was proposed that the higher Brønsted
acidity of Ce3+-O(H)-Zr4+ species relative to Zr4+-O(H)-Zr4+
species and the higher absolute concentration of the former
species were responsible for the increased rate of formate hy-
drogenation to methoxide and the higher rate of methoxide
5. Conclusion
The substitution of trivalent cations into the lattice zirconia
led to an increase in the area-based methanol synthesis activity
of 3 wt% Cu/M0.3Zr0.7O2 (M = Ce, Mn, Pr). The increase in
methanol synthesis activity was paralleled by an increase in the
hydrogen adsorption capacity of the catalyst and an increase in
the fraction of Brønsted acidic bridging OH groups. The abil-
ity of the substituted cations to participate in a redox cycle
under reaction conditions (e.g., Ce4+/Ce3+ and Mn3+/Mn2+
)
further contributes to increased methanol synthesis activity. The
highest methanol synthesis activity was observed for 3 wt%
Cu/Ce0.3Zr0.7O2 and is attributed to the high hydrogen adsorp-
tion of this catalyst along with the high acidity of the atomic

K.A. Pokrovski, A.T. Bell / Journal of Catalysis 244 (2006) 43–51
51
hydrogen present in the form of bridging hydroxyl groups, as
well as the ability of the Ce cations to cycle between Ce4+ and
Ce3+
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Figures showing Cu K-edge XANES spectra for 3 wt%
Cu/M0.3Zr0.7O2 taken after H2 reduction, Ce LIII-edge XANES
spectra of 3 wt% Cu/Ce0.3Zr0.7O2 taken before and after H2 re-
duction, Pr LIII-edge XANES spectra of 3 wt% Cu/Pr0.3Zr0.7O2
taken before and after H2 reduction, and Mn K-edge XANES
spectra of 3 wt% Cu/Mn0.3Zr0.7O2 taken before and after H2
reduction can be accessed free of charge at http://elsevier.org/
jcat/.
Please visit DOI: 10.1016/j.jcat.2006.07.031.
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