An investigation of the factors influencing the activity of Cu/CexZr1-xO2 for methanol synthesis via CO hydrogenation

Article abstract of DOI:10.1016/j.jcat.2006.05.002

An investigation was carried out to identify the effects of incorporating Ce into ZrO₂ on the catalytic activity and selectivity of Cu/Ce_xZr_1-xO₂ for the hydrogenation of CO to methanol. A series of Ce_xZr_1-xO₂ solid solutions was synthesized by forced hydrolysis at low pH. The resulting catalysts were characterized to determine the structure of the mixed oxide phase, the H₂ and CO adsorption capacities of the catalyst, and the reducibility of both oxidation states of both Cu and Ce. The methanol synthesis activity goes through a maximum at x = 0.5, and the activity of 3 wt% Cu/Ce_0.5Zr_0.5O₂ catalyst is four times higher than that of 3 wt% Cu/ZrO₂ when tested at total pressure of 3.0 MPa and temperatures between 473 and 523 K with a feed containing H₂ and CO (H₂/CO=3). The maximum in methanol synthesis activity is paralleled by a maximum in the hydrogen adsorption capacity of the catalyst, an effect attributed to the formation of Ce³⁺{single bond}O(H){single bond}Zr⁴⁺ species by dissociative adsorption of H₂ on particles of supported Cu followed by spillover of atomic H onto the oxide surface and reaction with Ce⁴⁺{single bond}O{single bond}Zr⁴⁺ centers. In situ infrared spectroscopy shows that formate and methoxide groups are the primary adspecies present on Cu/Ce_xZr_1-xO₂ during CO hydrogenation. The rate-limiting step for methanol synthesis is the elimination of methoxide species by reaction with Ce³⁺{single bond}O(H){single bond}Zr⁴⁺ species. The higher concentration of Ce³⁺{single bond}O(H){single bond}Zr⁴⁺ species on the oxide surface, together with the higher Bronsted acidity of these species, appears to be the primary cause of the four-fold higher activity of 3 wt% Cu/Ce_0.5Zr_0.5O₂ relative to 3 wt% Cu/ZrO₂.

Full text of DOI:10.1016/j.jcat.2006.05.002

Journal of Catalysis 241 (2006) 276–286
www.elsevier.com/locate/jcat
An investigation of the factors influencing the activity of
Cu/Ce Zr O for methanol synthesis via CO hydrogenation
x
1−x 2
∗
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 21 March 2006; revised 18 April 2006; accepted 5 May 2006
Available online 9 June 2006
Abstract
An investigation was carried out to identify the effects of incorporating Ce into ZrO on the catalytic activity and selectivity of Cu/CexZr
O
2
2
1−x
for the hydrogenation of CO to methanol. A series of CexZr
O solid solutions was synthesized by forced hydrolysis at low pH. The resulting
2
1−x
catalysts were characterized to determine the structure of the mixed oxide phase, the H and CO adsorption capacities of the catalyst, and the
2
reducibility of both oxidation states of both Cu and Ce. The methanol synthesis activity goes through a maximum at x = 0.5, and the activity
of 3 wt% Cu/Ce Zr O catalyst is four times higher than that of 3 wt% Cu/ZrO when tested at total pressure of 3.0 MPa and temperatures
0.5 0.5
2
2
between 473 and 523 K with a feed containing H and CO (H /CO = 3). The maximum in methanol synthesis activity is paralleled by a maximum
2
2
3+
4+
in the hydrogen adsorption capacity of the catalyst, an effect attributed to the formation of Ce –O(H)–Zr species by dissociative adsorption
of H on particles of supported Cu followed by spillover of atomic H onto the oxide surface and reaction with Ce –O–Zr centers. In situ
4+
4+
2
infrared spectroscopy shows that formate and methoxide groups are the primary adspecies present on Cu/CexZr
O during CO hydrogenation.
1−x
2
3+
4+
The rate-limiting step for methanol synthesis is the elimination of methoxide species by reaction with Ce –O(H)–Zr species. The higher
3+
4+
concentration of Ce –O(H)–Zr species on the oxide surface, together with the higher Brønsted acidity of these species, appears to be the
primary cause of the four-fold higher activity of 3 wt% Cu/Ce Zr O relative to 3 wt% Cu/ZrO .
0.5 0.5
2
2
©
2006 Elsevier Inc. All rights reserved.
Keywords: Methanol; Cu; Zirconia; ZrO ; Synthesis gas
2
1
. Introduction
Zirconia-supported copper catalysts exhibit a high activity
zirconia) was shown to be nearly an order of magnitude more
active for methanol synthesis than Cu/t-ZrO2 (t-ZrO2, tetrago-
nal zirconia) for equivalent zirconia surface areas and surface
concentrations of dispersed Cu [6]. This difference is attributed
primarily to the presence of oxygen vacancies on the surface
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 supported
Cu particles [4]. CO adsorbs preferentially on the surface of
zirconia to form formate species that then undergo hydrogena-
tion to produce methanol. The hydrogen atoms required for this
process are produced by dissociative adsorption of H2 on the
surface of the dispersed Cu and then spillover onto the zirconia
surface. It has also been shown that the phase of zirconia influ-
ences catalyst activity. Thus, Cu/m-ZrO2 (m-ZrO2, monoclinic
of m-ZrO , which facilitate the reaction of CO with neigh-
2
boring hydroxyl groups to generate formate species and pro-
vide additional sites for hydrogen activation and storage. These
properties result in higher CO adsorption capacities and higher
rates of elimination of methoxide species on Cu/m-ZrO2 rela-
tive to Cu/t-ZrO2. More recently, the effects of incorporating
Ce into ZrO2 on the catalytic performance of Cu/ZrO2 for the
hydrogenation of CO have been investigated [8]. The three-fold
higher rate of methanol synthesis on 1.2 wt% Cu/Ce0.3Zr0.7O2
relative to 1.2 wt% Cu/m-ZrO2 was attributed to the higher sur-
face concentration of H atoms on the former catalyst; however,
a clear picture of how Ce incorporation into ZrO2 increases
*
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.2006.05.002

K.A. Pokrovski, A.T. Bell / Journal of Catalysis 241 (2006) 276–286
277
the hydrogen adsorption capacity of the catalyst was not ad-
dressed.
with a Siemens D5000 diffractometer, which uses Cu-Kα radi-
ation and a graphite monochromator. Scans were made in the
◦
◦
The present study was undertaken to examine the influ-
ence of incorporating Ce into ZrO2 on the catalytic activ-
ity of Cu/CexZr1−xO2 for methanol synthesis from CO/H2.
CexZr1−xO2 mixed oxides were prepared by forced hydroly-
sis at low pH, and Cu was dispersed on the surface of these
materials by the deposition-precipitation technique. Catalysts
were characterized by X-ray diffraction (XRD), Raman spec-
troscopy, XANES, and temperature-programmed reduction
2θ range of 20 to 90 with a step size of 0.02 and a time/step
of 11 s. Raman spectra were recorded with a HoloLab 5000
Raman spectrometer (Kaiser Optical) at room temperature at
−
1
a resolution of 2 cm . The stimulating light source was a
Nd:YAG laser, the output of which was frequency-doubled to
532 nm. Laser power at the sample was approximately 20 mW.
The BET surface area of each Ce Zr O support was de-
termined using an Autosorb 1 (Quantachrome Instruments) gas
adsorption system. Prior to each analysis, samples were dried at
x
1−x
2
(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. These
studies suggest that an important contributor to the higher
methanol synthesis activity of Cu/CexZr1−xO2 is the pres-
3
93 K under vacuum for >2 h. BET surface areas were calcu-
lated using a five-point isotherm. After calcinations, the surface
areas ranged from 80 to 140 m /g, as reported in Table 1.
Within experimental error, identical BET areas were obtained
for the catalysts after Cu deposition.
Hydrogen TPR studies were conducted using 0.15 g of a
calcined sample, which had been purged with He at 298 K
for 30 min. The flow was then switched from pure He to a
2
the sample was ramped at 20 K/min from 298 to 673 K. The
consumption of H2 was monitored using a mass spectrometer
2
ence of Ce4 (O)Zr centers at the catalyst surface. It is pro-
+
4+
3+
4+
posed that hydrogen adsorbs at these centers as Ce (OH)Zr .
A maximum in the area-based activity of Cu/CexZr1−xO2 oc-
curs for x = 0.5, coinciding with the maximum H2 adsorption
capacity of the catalyst.
3
% H2/He mixture flowing at 60 cm /min, and temperature of
(
Cirrus, Spectra Products).
Cu K-edge and Ce LIII-edge XANES data were acquired
2
. Experimental
at the Stanford Synchrotron Radiation Laboratory (SSRL) on
beamline 2-3. Each sample was mixed with boron nitride and
pressed into a rectangular pellet (0.43 × 1.86 cm) and loaded
into an in situ cell for transmission experiments [9]. A suf-
ficient quantity of each sample was used to give a calculated
absorbance of 2.5. Each sample was calcined in 10% O2/He at
2
.1. Catalyst preparation
A series of CexZr1−xO2 (x = 0, 0.1, 0.3, 0.5, 0.7, and 1.0)
was prepared by boiling a 0.5 M (total metals basis) aque-
ous solution of zirconyl nitrate (ZrO(NO3)2 · xH2O, 99.99%,
Aldrich) and cerium(III) nitrate (Ce(NO3)3 · 6H2O, 99.999%,
Aldrich) under reflux for 240 h [6,8]. The final solutions had
a pH < 1. NH4OH was added dropwise to agglomerate the
resulting fine particles and facilitate their filtration. The recov-
ered precipitate was washed with deionized water. The washed
solid was then dried in air overnight at 383 K. Each sample
was calcined at 873 K in dry air flowing at 100 cm /min. The
temperature was ramped from room temperature at a rate of
2
Copper was then dispersed onto each support by deposition-
precipitation [6] to obtain a series of 3 wt% Cu/CexZr1−xZrO2
catalysts.
5
73 K for 2 h, then cooled to 298 K, purged with He, and evac-
−
6
uated to 10 Torr to remove residual oxygen. A 2% H2/He
mixture was then passed through the cell at a flow rate of
3
6
0 cm /min. In situ XANES data at the Ce LIII-edge were ac-
quired while the sample was heated from 298 K to 573 K at
3
3
4 K/min in a flow of 2% H /He (60 cm /min). Similar ex-
2
periments were performed to acquire data at the Cu K-edge.
XANES analyses were carried out using the Athena (version
K/min to the final temperature, which was maintained for 3 h.
0.8.041) software [10,11]. The energy was calibrated using the
Ce LIII-edge of CeO2 (E0 = 5728 eV) and the Cu K-edge of a
Cu foil (E0 = 8980 eV). Pre-edge absorptions due to the back-
ground 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 normalized by a
constant determined by the average absorption in the range of
100–300 eV relative to E0. The edge energy of each sample
2
.2. Catalyst characterization
The crystallographic phase of CexZr1−xO2 was determined
by XRD and Raman spectroscopy. XRD patterns were obtained
Table 1
Effect of oxide composition on the surface properties of 3 wt% Cu/CexZr
O
2
1−x
3
+
Catalyst
S.A.
H adsorption capacity
(µmol/m )
CO adsorption capacity
(µmol/m )
Ce /Ce-total
2
2
2
2
(m /g)
(%)
3
3
3
3
3
3
wt% Cu/ZrO
123
138
127
83
87
53.7
0.3
0.50
0.60
0.65
1.43
1.50
1.58
–
2
wt% Cu/Ce Zr
O
O
O
O
1.16
1.51
3.06
2.79
2.31
32.97
62.77
60.81
24.88
15.19
0.1 0.9
2
2
2
2
wt% Cu/Ce Zr
0.3 0.7
wt% Cu/Ce Zr
0.5 0.5
wt% Cu/Ce Zr
0.7 0.3
wt% Cu/CeO
2

278
K.A. Pokrovski, A.T. Bell / Journal of Catalysis 241 (2006) 276–286
and reference was taken at the first inflection point beyond any
pre-edge peaks.
2.4. Infrared spectroscopy studies
H2 and CO adsorption capacities and were determined us-
In situ transmission infrared spectroscopy experiments were
conducted using a low dead-volume infrared cell equipped with
CaF2 windows [12]. To remove any residual surface species be-
fore testing, each sample was calcined in a 10% O2/He mixture
flowing at 60 cm /min. The sample was heated from room tem-
perature to 523 K at a rate of 2 K/min and then there for 8 h.
ing TPD. For H2, the sample was calcined and then reduced at
3
5
73 K in a 2% H2/He mixture flowing at 60 cm /min. The sam-
ple was then cooled in 2% H2/He to 298 K and purged in He.
Desorption was conducted by ramping the sample temperature
3
3
at 20 K/min from 298 to 773 K in flowing He (60 cm /min)
while monitoring the desorbing gas using a mass spectrom-
eter. To determine the CO adsorption capacity, the sample
The sample was then cooled to 323 K, swept with He, and re-
duced in a 10% H2/He mixture flowing at 60 cm /min while
3
the temperature was increased at a rate of 2 K/min up to 523 K.
The flow of 10% H2/He was maintained at 523 K for 1 h be-
fore 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 hydrogenation experiments
were carried out at a total pressure of 0.5 MPa.
was calcined, then reduced in 2% H2/He mixture flowing at
3
6
0 cm /min 573 K, cooled to 523 K, and flushed with He
3
(60 cm /min) for 30 min. A 4.0% CO/He mixture was then
3
passed over the catalyst for 60 min at a flow rate of 60 cm /min.
The sample was then cooled to 298 K in a 4.0% CO/He mixture
3
flowing AT 60 cm /min before it was purged for 30 min with
3
He (60 cm /min) to remove any weakly adsorbed species. Des-
3
. Results
orption of adsorbed CO was carried out in a manner identical to
that used for adsorbed H2.
3
.1. Material characterization of CexZr1−xO2 supports
XRD patterns of the CexZr1−xO2 materials are shown in
2
.3. Catalyst testing
Fig. 1a. The principal features seen in the diffraction pattern
Activity and selectivity measurements for CO hydrogena-
can be ascribed to tetragonal ZrO2 (t-ZrO2), with only a trace
tion were carried out in a glass-lined stainless-steel reactor
as described previously [6]. Before testing, each catalyst was
◦
of monoclinic ZrO2 (m-ZrO2) evident at 28.2 . The volume
fraction of m-ZrO2 was estimated to be ∼0.2 using the rela-
3
calcined in a 10% O2/He mixture flowing at 60 cm /min.
tionships [13]
The sample was heated from room temperature to 573 K at
1
.311Xm
0
3
1
.5 K/min and maintained at 573 K for 2 h, then cooled to
Vm =
1
+ 0.311Xm
23 K, and swept with He. Finally, the sample was reduced in a
3
0% H2/He mixture flowing at 60 cm /min while the tempera-
and
ture was increased at a rate of 2 K/min up to 573 K. The flow of
0% H2/He was maintained at 573 K for 1 h before switching to
¯
Im(111) + Im(111)
1
Xm =
,
¯
Im(111) + Im(111) + It(111)
m
¯
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
where I (111) and I (111¯) are the line intensities of the (111)
m
3
reactant gas flow was 60 cm /min (at STP) with a H2/CO ratio
and (111) peaks for m-ZrO and I (111) is the intensity of
2
t
of 3/1. Product gas mixtures were analyzed after 2 h on stream
at a given temperature. The temperature was then ramped to the
next highest temperature at a rate of 2 K/min and maintained
there for 2 h. Conversion and selectivity were referenced to the
consumption of CO, the limiting reactant.
the (111) peak for t-ZrO . The diffraction peaks shifted to
lower values of 2Θ on introduction of Ce into ZrO , consistent
2
2
with an increase in the lattice cell parameter on the substitu-
tion of Zr4 (radius 0.86 Å) by Ce cations (radius, 0.97 Å).
+
4+
Ce Zr O with x = 0.5 and 0.7 showed a small degree of
x
1−x
2
Fig. 1. XRD patterns (a) and Raman spectra (b) of CexZr
O .
2
1−x

K.A. Pokrovski, A.T. Bell / Journal of Catalysis 241 (2006) 276–286
279
phase segregation into cerium-rich and zirconium-rich phases,
can be assigned to the tetragonal phase [18–20]. Comparison
of the XRD patterns and Raman spectra for Ce0.5Zr0.5O2 and
Ce0.7Zr0.3O2 suggests that these materials contain two phases,
a cubic (cerium-rich) phase and a tetragonal (zirconium-rich)
phase. Raman spectrum of pure ceria exhibited a single peak at
◦
as evidenced by the appearance of XRD peaks at ∼28.7 and a
◦
shoulder at ∼30.0 , respectively [14].
Each material was also characterized by Raman spec-
troscopy. Fig. 1b shows Raman spectra of CexZr1−xO2 col-
lected at 298 K. Pure ZrO2 exhibited peaks at 153, 181, 270,
−
1
463 cm characteristic of the cubic phase [18–20].
−
1
−1
3
35, 385, 478, 615, and 640 cm . The peak at 188 cm is
characteristic of the monoclinic zirconia, whereas the peaks at
3.2. Characterization of Cu/CexZr1−xO2
−1
1
53 and 270 cm are due to the tetragonal phase [15–17].
Thus, Raman spectroscopy confirmed the presence of tetrago-
nal ZrO2, but also revealed the existence of a significant amount
of monoclinic ZrO2. However, neither XRD nor visible Raman
spectroscopy could provide insight into the relative location
of the m- and t-ZrO2 phases, because both techniques sam-
ple the particles of ZrO2 uniformly. Work by Li and Li [17]
using XRD, visible Raman spectroscopy, and UV–Raman spec-
troscopy has demonstrated that the transformation of t-ZrO2
to m-ZrO2 occurs initially at the surface of the ZrO2 parti-
cles. m-ZrO2 could be detected at lower calcination tempera-
tures by UV–Raman spectroscopy compared with either visible
Raman spectroscopy or XRD, due to the higher surface sen-
sitivity of UV–Raman spectroscopy relative to visible Raman
spectroscopy. Therefore, the difference in the identity of the
predominant phase of ZrO2 determined using XRD and Raman
spectroscopy is very likely due to the presence of a layer of m-
ZrO2 on the surface of bulk t-ZrO2 particles. Further support
for this picture was obtained from infrared spectra of the O–H
stretching region, as discussed below.
Fig. 2 presents TPR profiles of 3 wt% Cu/CexZr1−xO2. The
3 wt% Cu/ZrO2 exhibited a principle peak centered at 455
K, attributed to the reduction of highly dispersed CuO [23,
24]. The amount of H2 consumed was slightly greater than
the value corresponding to the complete reduction of CuO
(H2/CuO ≈ 1.1), in good agreement with previous reports
[6]. The 3 wt% Cu/CexZr1−xO2 (x > 0) samples exhibited
a significantly greater-than-expected H2 consumption for the
4+
reduction of CuO, suggesting that some of the Ce under-
went reduction to Ce3 . The appearance and interpretation of
the observed TPR spectra are similar to those reported for
CuO/Ce0.44Zr0.56O2 [25].
+
In situ XANES experiments were conducted to determine
the degree of Cu and Ce reduction. As shown in Fig. 3a, Cu
K-edge XANES demonstrates that the reduction of Cu was
complete after the samples were heated in 2% H2/He at 573 K
for 1 h. Ce LIII-edge XANES spectra collected after the H2
reduction of 3 wt% Cu/CexZr1−xO2 are presented in Fig. 3b.
Comparing these spectra with those of Ce2(SO4)3 and CeO2
shows that both Ce4 and Ce
+
3+
were present after reduc-
The Raman spectrum of Ce0.1Zr0.9O2 (Fig. 1b) exhibited
−
1
peaks at 149, 265, 319, 468, and 633 cm , attributed to the
tetragonal phase of zirconia [15–17]. The XRD pattern also
confirms the tetragonal structure of Ce0.1Zr0.9O2 (Fig. 1a). The
position of the 111 XRD peak for Ce0.1Zr0.9O2 shifted to a
tion. The distribution between these two oxidation states of
3
+
Ce was calculated using a linear combination fitting of Ce
and Ce4 standards in Athena [10,11]; the results are given in
+
4+
3+
Table 1. The maximum of reduction of Ce to Ce was ob-
served for 3 wt% Cu/Ce0.3Zr0.7O2 and 3 wt% Cu/Ce0.5Zr0.5O2.
These results are consistent with findings of previous studies
of CexZr1−xO2 [26–28] showing that the introduction of zir-
◦
lower value of 2Θ (30.1 ) relative to that for pure t-ZrO2
◦
30.3 ). Thus, the introduction of 10 at% cerium into zirconia
(
clearly stabilized the tetragonal phase. Increasing the cerium
◦
4+
content to 30 at% shifted the 111 XRD peak to 29.5 . The broad
conium into ceria increased the reducibility of Ce due to
diffraction peaks for Ce0.3Zr0.7O2 make it impossible to defin-
itively distinguish between tetragonal and cubic phases of this
material. The Raman spectrum of Ce0.3Zr0.7O2, presented in
increased oxygen mobility for oxides with intermediate Ce/Zr
ratios.
−
1
Fig. 1b, shows a principle peak at 469 cm and two smaller
−
1
−1
peaks at 320 and 620 cm . The peak at 469 cm is character-
istic of a cubic fluorite structure [18–20]. However, the presence
of peaks at 320 and 620 cm (due to tetragonal distortion of
oxygen sub lattice) and the absence of the characteristic tetrag-
−
1
−
1
onal peak at ∼270 cm indicate that Ce0.3Zr0.7O2 is in the
ꢀ
ꢀ
t phase [20]. This phase is intermediate between the tetrago-
nal and cubic phases, in which the oxygen atoms are displaced
from the positions of these atoms in an ideal fluorite struc-
ꢀ
ꢀ
ture. Although the t phase is commonly reported to exist at
Ce compositions between 65 and 80 at% [20], other authors
have reported the generation of this phase at lower Ce content
for small particles [21,22].
The Raman spectra of Ce0.5Zr0.5O2 and Ce0.7Zr0.3O2 exhib-
−
1
ited a principle peak at 467 cm and smaller peaks at 254, 315,
−1
−1
and 625 cm . The peak at 465 cm is characteristic of a cubic
fluorite structure, whereas the peaks at 254, 315, and 625 cm
Fig. 2. H -TPR spectra for 3 wt% Cu/Ce Zr
O . Heating rate, 20 K/min;
2
2
x
1−x
−
1
3
2
% H /He flow rate, 60 cm /min.
2

280
K.A. Pokrovski, A.T. Bell / Journal of Catalysis 241 (2006) 276–286
Fig. 3. XANES spectra of 3 wt% Cu/Ce Zr O at (a) Cu K-edge and (b) Ce LIII-edge reduction in 2% H /He at 573 K for 1 h.
0.3 0.7
2
2
Fig. 4. Infrared spectra of the hydroxyl group stretching region taken for 3 wt%
Cu/CexZr following calcination and reduction. Spectra referenced to
empty cell in He.
Fig. 5. Infrared spectra of the C–O stretching region of surface methoxide
O
1−x
2
groups taken in 0.5% CH OH/He flow at 523 K. Spectra referenced to the sam-
3
ples under He flow at 523 K.
The infrared spectra of the O–H stretching region obtained
after reduction at 523 K are shown in Fig. 4. Spectra were
referenced 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 and CeO2 [29–31]. Surface
hydroxyl groups on ZrO2 and CeO2 are commonly assigned
based on the number of coordinating cations, with the higher-
frequency species representing terminal groups and the lower-
frequency species representing either bi- or tri-bridging groups
of bridging hydroxyl groups, and these species became dom-
inant for Ce content >30 at%. The OH stretching band due to
3+
the bridging hydroxyl group (IIB) bonded to Ce cations is
−
1
evident at 3647 cm in the infrared spectra of mixed oxides
[30,31].
The nature of surface species formed on adsorption of
methanol was characterized using in situ infrared spectroscopy.
Fig. 5 shows spectra obtained on reduced 3 wt% Cu/Ce Zr O2
x
1−x
obtained at 523 K during sample exposure to a flow of 0.5%
CH OH/He at a total pressure of 0.50 MPa. The peaks at 1150–
3
−
1
−1
[
29–31]. The peak positions for ZrO2 (3666 and 3731 cm
)
1160 and 1112–1120 cm are assigned to C–O stretching
more closely resemble those observed for m-ZrO2 (3668 and
vibrations of terminal methoxide species (t-OCH ) bonded to
3
−1
−1
4+
3+
3
729 cm ) than those of t-ZrO2 (3660 and 3738 cm ) [6].
Zr and Ce cations, respectively [32,33]. The absence of a
−
1
Similar to m-ZrO2, the ZrO2 sample exhibited a higher relative
band at 1105 cm for terminal methoxide species bonded to
−
1
4+ 4+
Ce cations [32,33] suggests that all of the Ce present at
concentration of the low-frequency band (at 3666 cm ), sug-
gesting that the ZrO2 particles contain a significant fraction of
m-ZrO2 at their surface. The infrared spectrum of reduced CeO2
exhibited three absorption bands in the OH stretching region
at 3710 (terminal, I), 3684 (bridged, IIA), 3655 (bi-bridged,
3
+
the surface of the 3 wt% Cu/Ce Zr O was reduced to Ce
x
1−x
2
during reduction of the samples in H at 523 K before being ex-
2
posed to methanol. As the cerium concentration in the support
−
1
increased, the relative intensity of peak at 1155 cm decreased
−1
−1
IIB), and 3560 cm (tri-briged, III) [30,31]. The introduc-
tion of Ce into zirconia increased the relative concentration
and the intensity of the peak at 1115 cm increased. The broad
band centered at 1050–1075 cm is assigned to C–O stretch-
−
1

K.A. Pokrovski, A.T. Bell / Journal of Catalysis 241 (2006) 276–286
281
Fig. 6. Effect of temperature on the selective conversion and overall selectivity of CO to methanol during CO hydrogenation: catalyst mass, 0.15 g; P = 3.0 MPa;
3
H /CO = 3; total flow rate, 60 cm /min.
2
Table 2
Effect of oxide composition on the activity of 3 wt% Cu/CexZr
O for CO hydrogenation
2
1−x
Catalyst
Conversion (selectivity) at 523 K
Productivity
Productivity
−
1
−1
−2 −1
m h )
(%)
(g
g
h
)
(mg
CH3OH cat
CH3OH
3
3
3
3
3
3
wt% Cu/ZrO
1.73 (94.23)
3.12 (95.24)
5.30 (97.57)
4.61 (97.02)
2.54 (96.80)
0.57 (79.78)
0.136
0.245
0.416
0.362
0.199
0.025
1.10
1.78
3.28
4.36
2.29
0.46
2
wt% Cu/Ce Zr
O
O
O
O
0.1 0.9
2
2
2
2
wt% Cu/Ce Zr
0.3 0.7
wt% Cu/Ce Zr
0.5 0.5
wt% Cu/Ce Zr
0.7 0.3
wt% Cu/CeO
2
3
Catalyst mass, 0.15 g; T = 523 K; P = 3.0 MPa; H /CO = 3; total flow rate, 60 cm /min.
2
ing vibrations of bridge-bonded methoxide species (b-OCH3)
decomposition of surface hydroxyl groups formed on reduction
in H2, as shown below:
−
1
[
3
32,33]. The position of this peak shifted from 1045 cm for
−
1
wt% Cu/ZrO2 to 1072 cm
for 3 wt% Cu/Ce0.7Zr0.3O2.
H
This trend suggests that with increasing Ce content, the frac-
tion of Ce3 in the pairs and triads of metal cations involved
in bridge-bonded methoxide species increased. The decreased
intensity of the infrared bands in the OH stretching region
on adsorption of methanol indicates that surface methoxide
species were formed by interaction of gas-phase methanol
with surface OH groups. The increased relative concentration
+
O
O
1
3+
4+
3+
4+ + H
ꢀ
Ce
Zr
Ce
Zr
2
2
3
.3. Catalytic performance of Cu/CexZr1−xO2
of bridged CH O species with increased Ce content is con-
sistent with the increased concentration of bridged hydroxyl
groups (Figs. 4 and 5). The spectrum of methanol adsorbed
3
The effects of reaction temperature on the activity and se-
lectivity of the 3 wt% Cu/CexZr1−xO2 catalysts are illustrated
in Fig. 6. The conversion of CO to methanol increased over
the temperature range of 473–523 K, accompanied by de-
creased methanol selectivity. The only major byproduct ob-
served was methane. Because the measured conversion levels
were far below those expected for equilibrium at the condi-
tions used here, the observed rates of methanol formation were
not influenced by the rates of methanol decomposition. The
steady-state activities of 3 wt% Cu/Ce Zr O for methanol
on 3 wt% Cu/CeO reduced at 523 K exhibited C–O stretch-
2
ing modes at 1112 (terminal, I), 1072 (bi-bridged, IIA), 1061
−
1
(
bi-bridged, IIB), and 1030 cm (tribriged, III). The peak po-
sitions are consistent with the values reported by Binet and
Daturi [33].
2
The H and CO adsorption capacities of 3 wt% Cu/Cex
Zr1−xO2 determined from TPD spectra are listed in Table 1.
x
1−x
2
The amount of CO adsorbed per unit area increased monotoni-
cally with increasing cerium content. In contrast, the amount of
H2 adsorbed goes through the maximum at Ce/Zr = 1. Desorp-
tion of H2 was observed in the temperature range of 320–680
K and was not accompanied by the release of water. The latter
observation suggests that hydrogen desorption occurred via the
synthesis are presented in Table 2. The methanol productiv-
ity of 3 wt% Cu/Ce Zr O reached a maximum versus Ce
x
1−x
2
content. The most active catalyst based on catalyst weight
−
1
−1
h
was 3 wt% Cu/Ce0.3Zr0.7O (0.42 g
2
CH3OH cat
g
), but the
most active catalyst based on catalyst surface area was 3 wt%
−
2
−1
h ).
Cu/Ce0.5Zr0.5O (4.36 mg
2
CH3OH
m

282
K.A. Pokrovski, A.T. Bell / Journal of Catalysis 241 (2006) 276–286
3
Fig. 7. Infrared spectra taken for 3 wt% Cu/CexZr
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
1−x
3
wt% Cu/CexZr
O under 0.50 MPa He flow at 523 K.
2
1−x
and 1350–1250 cm 1 indicative of various carbonate and car-
−
3
.4. Infrared spectroscopy studies
boxylate species [36].
The nature of surface species and the dynamics of CO ad-
After CO adsorption for 1 h, H was introduced into the
2
sorption and hydrogenation were studied using in situ infrared
spectroscopy. Fig. 7 shows spectra obtained during CO adsorp-
tion on 3 wt% Cu/CexZr1−xO2 catalysts previously reduced in
H2 at 523 K. Spectra were collected at 523 K after the cat-
alyst had been exposed to a flow of 15% CO/He at a total
pressure of 0.50 MPa for 1 h. For the 3 wt% Cu/ZrO2 sam-
flowing 15% CO/He mixture (total pressure, 0.50 MPa) so as
to achieve a H2/CO ratio of 3/1. Fig. 8 shows spectra obtained
on 3 wt% Cu/CexZr1−xO2 at steady state after 6 h of expo-
−1
sure to a H2/CO mixture at 523 K. The band at 1147 cm
is assigned to the ν(CO) mode of t-CH3O species adsorbed on
4+
Zr . Peaks corresponding to C–O stretching vibrations of b-
−
1
ple, the bands observed at 1563, 1386, and 1369 cm are
attributable to the νas(OCO), δ(CH), and νs(OCO) modes, re-
spectively, of b-HCOO–Zr [32–35]. Accompanying features
for b-HCOO–Zr in the CH stretching region occurred at
−1
CH3O species were present at 1040–1064 cm . The blue shift
of the ν(CO) band for b-CH3O species with increasing cerium
content was similar to that observed in the spectra of adsorbed
methanol (Fig. 6), suggesting that some CH3O species were
−
1
2
[
2
969 and 2888 cm , characteristic of (νas(OCO) + δ(CH))
4+
3+
bonded to both Zr and Ce cations. The intensities of the
32–35] and νs(CH) [32–35], respectively. Weak features at
−1
bands for b-HCOO appearing at 1576, 1386, and 1366 cm de-
−1
934 and 2832 cm attributed to CH3O–Zr [32–35] appeared
creased relative to those observed on adsorption of CO (Fig. 7).
In the C–H stretching region, bands for CH3O species were
even in the absence of gas phase H2. Weak bands at 1155
−1
and 1040 cm are assigned to C–O stretching vibrations of
terminal (t-OCH3) and bridged (b-OCH3) methoxide species
on ZrO2, respectively; the shoulder located at approximately
−
1
present at 2926–2920 and 2822 cm . The shoulders at 2970
−
1
and 2880 cm are attributable to b-HCOO species. Similar to
the peak at 1576 cm , the intensities of the peaks at 2970 and
880 cm decreased with increasing cerium content. The band
at 2796 cm attributed to methoxy species adsorbed on Ce
cations increased in intensity with increasing cerium content.
−
1
− 2−
1
1
320 cm is assigned to b-CO3 –Zr species [32–35]. The
−
1
2
−
1
peaks at 1440–1420 cm can be attributed to various carbon-
ate and carboxylate species on the surface of ZrO2 [32–35].
Formate species adsorbed on 3 wt% Cu/CeO2 sample are char-
acterized by absorption bands at 2950 (νas(OCO) + δ(CH)),
−
1
3+
−1
The δ(CH) feature for CH3O was evident at 1446 cm but
−
852 νs(CH), 1575 νas(OCO), 1380 (δ(CH)), and 1369 cm
1
was very weak in intensity [32–35]. The broad features between
2
−
1
∼
1550–1450 and 1350–1250 cm attributed to various car-
(
νs(OCO)) [32–35]. These bands are attributed to the bidentate
3+
bonate and carboxylate species were similar to those observed
during the adsorption of CO (Fig. 7). A red shift of the peak
at 1070–1061 cm (observed during methanol adsorption on
formate species adsorbed on Ce cations. Broad features at
−
1
∼
1550–1450 and 1350–1250 cm in the infrared spectrum of
−
1
CO adsorbed on the 3 wt% Cu/CeO2 sample are attributed to
various carbonate and carboxylate species [32–35]
−
1
reduced 3 wt% Cu/CeO2) to 1053 cm and the appearance of
−
1
the band at 1090 cm observed during CO hydrogenation sug-
The infrared spectra of CO adsorbed on 3 wt% Cu/Cex
Zr1−xO2 were qualitatively similar to those for 3 wt% Cu/ZrO2
and 3 wt% Cu/CeO2. With increasing Ce content, the shoul-
gests that Ce4 species were formed on reduction of formate to
+
methoxide species [32–35].
−
1
ders at 2951 and 2854 cm , assigned to bidentate formate
The relative rates of consumption of formate and methox-
ide species were evaluated by switching from a CO/H2 mixture
to one containing only H2. Fig. 9a shows spectra of 3 wt%
Cu/Ce0.3Zr0.7O2 collected at the beginning of the transient ex-
periment. The intensities of bands corresponding to b-HCOO
and CH3O species decreased after the feed was switched from
species adsorbed on Ce3 cations [25], increased in intensity.
+
The relative concentration of methoxide species features (bands
−
1
at 1155, 1050, 2935, and 2831 cm ) also increased with in-
creasing cerium content. The presence of Ce in the catalyst in-
creased the intensities of broad features between ∼1550–1450

K.A. Pokrovski, A.T. Bell / Journal of Catalysis 241 (2006) 276–286
283
3
Fig. 8. Infrared spectra taken for 3 wt% Cu/CexZr
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
1−x
referenced to 3 wt% Cu/CexZr
O under 0.50 MPa He flow at 523 K.
2
1−x
Fig. 9. (a) Infrared spectra taken for 3 wt% Cu/Ce Zr
and 0.35 MPa He flowing at a total rate of 60 cm /min. (b) Peak areas of b-HCOO and CH O features for 3 wt% Cu/Ce Zr
O at 523 K after switching feed from 0.05 MPa CO, 0.15 MPa H , and 0.30 MPa He to 0.15 MPa H
2 2
0
.3 0.7
2
3
−
−
O at 523 K after switching
0.3 0.7 2
3
3
feed from 0.05 MPa CO, 0.15 MPa H , 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
2
2
observed at the beginning of the transient.
a H2/CO/He mixture to a H2/He mixture. The evolution of
peak areas of formate and methoxide species is presented in
Fig. 9b. Peak areas for both b-HCOO and CH3O were normal-
ized to the value observed at the beginning of the transient.
In agreement with previous reports [7,8], the intensity of b-
HCOO band decreased more rapidly from the intensities of
the bands for methoxide species. These observations are con-
sistent with the results of Bell and coworkers [7,8] and in-
dicate that the rate-limiting step of methanol synthesis from
H2/CO was the elimination of surface methoxide species. To
investigate the effect of cerium content on the rate of surface
methoxide elimination, transient response experiments were
performed for all 3 wt% Cu/CexZr1−xO2 catalysts studied in
this work.
Table 3
Effect of oxide composition on the apparent rate constant of 3 wt% Cu/
CexZr
O for CO hydrogenation
2
1
−x
−
3
Catalyst
kapp × 10
kapp × C
CH3O
−
1
)
(
min
(a.u.)
3
3
3
3
wt% Cu/ZrO
wt% Cu/Ce Zr
wt% Cu/Ce Zr
wt% Cu/Ce Zr
5.8
6.5
1.1
2
O
O
O
1.61
3.67
4.165
2.55
0.35
0
.1 0.9
2
2
2
18.1
22.9
12.2
3.8
0.3 0.7
0.5 0.5
3 wt% Cu/Ce0.7Zr0.3O2
wt% Cu/CeO
3
2
T = 523 K; 0.15 MPa H , 0.05 MPa CO, Ptot = 0.5 MPa; total flow rate,
2
3
6
3
0 cm /min. Values of kapp × C
are normalized to the value of 1.1 for
CH O
3
wt% Cu/ZrO .
2
Fig. 10 compares the dynamics of CHOO and CH3O con-
sumption on 3 wt% Cu/CexZr1−xO2. Transient-response spec-
tra were obtained by replacing the 15% CO/He in the 3/1
H2/CO flow with He while maintaining the total pressure at
normalized to the value observed at the beginning of the tran-
sient. With increasing Ce content, the rates of CHOO and CH3O
consumption increased up to a maximum for x = 0.5, and then
decreased. The rate of CHOO hydrogenation to methoxide was
much higher then the rate of methoxide elimination regardless
of catalyst composition, indicating that the rate-determining
−
1
0
.50 MPa. The integrated peak areas at 1600–1500 cm for
−
1
HCOO species and 1200–1000 cm for CH3O species were

284
K.A. Pokrovski, A.T. Bell / Journal of Catalysis 241 (2006) 276–286
Fig. 10. Peak areas of (a) formate and (b) methoxide features for 3 wt% Cu/CexZr
O at 523 K after switching feed from 0.05 MPa CO, 0.15 MPa H , and
2 2
1−x
3
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
step did not change with catalyst composition. The apparent
first-order rate constant for the removal of methoxide species,
kapp, determined from the initial portion of the transient is pre-
sented in Table 3. Here too, it is observed that kapp passed
through a maximum for x = 0.5 in a manner similar to the
steady-state activity per unit surface area given in Table 2. It is
important to note, however, that the integrated band intensity for
methoxide species (terminal and bridging) was little affected by
the Ce content of the catalyst, even though the distribution be-
tween linear and bridging methoxide species changed with Ce
content. However, as shown in Fig. 9, both types of methoxide
species appeared to react at the same rate, likely due to a rapid
interconversion between the bridging and terminal forms.
through a maximum at x = 0.5. The Ce LIII-edge XANES data
also indicate that the extent of reduction of Ce4 to Ce also
passed through a maximum for x = 0.3–0.5, but infrared spec-
tra of adsorbed methanol (Fig. 4) suggest that the Ce cations
present on the surface of the reduced catalyst were exclusively
+
3+
3+
Ce . This observation, together with the absence of water
formation on the desorption of adsorbed H , led to the pro-
2
posal (see above) that hydrogen was adsorbed on the oxide
3
+
4+
surface as Ce –O(H)–Zr and that these species were con-
verted to Ce4 –O–Zr on H desorption. Because no evidence
+
4+
2
for hydridic H was found by infrared spectroscopy, it appears
that the increased adsorption of hydrogen on the surface of
Cu/Ce Zr O was due not to the heterolytic dissociation of
x
1−x
2
H2 as proposed by Pokrovski et al. [8], but rather to the migra-
tion of H atoms spilled over from the surface of the dispersed
4
. Discussion
4
+
4+
Cu particles and their reaction with Ce –O–Zr pairs to form
3+
4+
Ce –O(H)–Zr . Assuming a uniform mixing of Ce and Zr in
the oxide phase, it thus seems reasonable to conclude that the
The results of the present study are fully consistent with
those reported by Pokrovski et al. [8] comparing the activ-
ity and selectivity of 1.2 wt% Cu/Ce0.3Zr0.7O2 and 1.2 wt%
Cu/ZrO2. In that study, the ratio of surface area-based activi-
ties for the two catalysts was 3 times greater in favor of the
Ce-containing catalyst, and the ratio of values of kapp was 2.4
for the Ce-containing catalyst. Based on an analysis of the
transient-response spectra, Pokrovski et al. concluded that hy-
drogenation of methoxide species adsorbed on the oxide surface
was the rate-determining step. Because the surface concentra-
tion of methoxide species was only 1.3 times greater for the
Ce-containing catalyst, the higher activity of this catalyst was
attributed to its higher adsorbed hydrogen content. Consistent
with this reasoning, the measured ratio of hydrogen adsorption
capacities at 523 K was reported to be 3.3 in favor of the Ce-
containing catalyst. Although no physical reason for the higher
adsorption capacity of the Ce-containing catalyst was proposed,
it was suggested that this effect may have been related to the
ability of Ce–O bonds at the oxide surface to stabilize H atoms
highest concentration of Ce3 –O(H)–Zr , and hence the high-
est level of H2 adsorption, occurred when x = 0.5, in agreement
with previous observations. We also note that hydrogen can be
+
4+
stored on the surface of ZrO2 in the form of Zr4 –O(H)–Zr
+
4+
.
Such structures are likely to form only in the vicinity of O anion
4+
defects in the surface, because Zr does not undergo reduction
to Zr3 . A further point of note is that protons present as Ce
+
3+
–
4+
O(H)–Zr would be expected to be more Brønsted-acidic, and
hence more reactive toward anionic species, such as formates
and methoxides, compared with protons present as Zr –O(H)–
4+
4+
Zr
.
Comparing the spectra of CH3O species present on reduced
catalysts (Fig. 5) and those present during CO hydrogenation
(Fig. 8) suggests that under our reaction conditions, some of the
surface Ce3 cations were oxidized to Ce . Based on the fore-
going considerations, we propose that the effect of Ce cations
on the activity of 3 wt% Cu/CexZr1−xO2 for methanol synthe-
sis can be interpreted in terms of the reaction sequence shown
in Scheme 1. H2 adsorbs on the surface of reduced Cu and
spills over onto the surface of CexZr1−xO2, where it reacts with
+
4+
δ−
δ+
as H and H species [8,37,38].
The results of the present investigation demonstrate that the
effects of Ce content on the methanol synthesis activity and
the hydrogen adsorption capacity of Cu/CexZr1−xO2 were not
monotonic but in fact varied in a similar fashion, both passing
4+
4+
3+
4+
Ce –O–Zr centers to form Ce –O(H)–Zr species. Pre-
vious studies have shown that Cu enhances the rate of H/D ex-

K.A. Pokrovski, A.T. Bell / Journal of Catalysis 241 (2006) 276–286
285
change of hydroxyl groups on the surface of Cu/ZrO2 and that
Cu/Ce0.3Zr0.7O2 has a higher adsorption capacity than Cu/ZrO2
[
than those present as Zr4 –O(H)–Zr , which are known to
+ 4+
3+
4+
be amphoteric, or those present as Ce –O(H)–Ce [40]. The
8,39]. As noted earlier, Ce3 –O(H)–Zr species are believed
+
4+
3+
4+
higher Brønsted acidity of Ce –O(H)–Zr species and the
higher absolute concentration of such species is hypothesized
to be responsible for the increased rate of formate species hy-
drogenation to methoxide species (Fig. 10a) and the higher rate
of methoxide removal as methanol (Fig. 10b). Thus, the incor-
to be responsible for the increased H2 adsorption capacity on
CexZr1−xO2 when x is ꢁ0.5 (Table 1; Fig. 12). When x is
4
+ 4+
>
0.5, the fraction of Ce –O–Zr centers decreases as the
4+
4+
fraction of Ce –O–Ce centers increases. The protons asso-
ciated with Ce3 –O(H)–Zr are more highly Brønsted-acidic
+
4+
poration of Ce cations into the framework of ZrO enhances the
2
adsorption of CO and to a greater extent the adsorption of H2.
The higher concentration of bridging hydroxyl species on the
surface of CexZr1−xO2 and, in particular, the higher Brønsted
acidity of Ce3 –O(H)–Zr species are believed to be the pri-
mary origins of the higher activity of 3 wt% Cu/CexZr1−xO2
seen in this study. Scheme 1 also provides a rational explanation
for the observed maximum in the area-based methanol activity
when x = 0.5.
+
4+
The relationship between the steady-state rate of methanol
synthesis and the transient-response data can now be examined.
Assuming that the steady-state rate of methanol formation,
rCH OH, is equal to the rate of methoxide elimination from the
3
surface of CexZr1−xO2, rCH OH can be expressed as kappCCH O
Scheme 1. Proposed reaction scheme of CO hydrogenation to methanol on
3
3
(
Table 3), where CCH O is the surface concentration of methox-
Cu/CexZr
O .
2
3
1−x
ide species at the start of the transient. The relative magnitude
of CCH O was estimated based on the area of IR bands in the
3
−
1
1
200–1000 cm range of surface methoxide species. Fig. 11
shows that rCH OH is linearly related to kappCCH O, in agree-
3
3
ment with the aforementioned assumption. The value of kapp is
not constant, however; as shown in Table 3, it passes through
a maximum as x increases from 0 to 1.0. One explanation for
the variation in kapp is that it is due to the influence of x on
the surface concentration of Brønsted acid protons of the form
3+
4+
Ce –O(H)–Zr . The surface concentration of such protons
is estimated by assuming that kapp = kintCOH, where kint is the
intrinsic rate coefficient for the reaction of Brønsted acid pro-
tons with methoxide groups and COH is the concentration of
Brønsted acid protons on the surface of CexZr1−xO2. If kint
is independent of x, then kapp should vary with x as a conse-
quence of the dependence of COH on x. This hypothesis cannot
be tested directly, however, because COH was not measured.
However, Fig. 12 does show that kapp and the H2 adsorption
capacity of 3 wt% Cu/CexZr1−xO2 depend on x in a very sim-
Fig. 11. Productivity vs. the product of kapp and surface methoxide concentra-
tion. Values of kapp × C
are normalized to the value of 1.1 for 3 wt%
CH O
3
Cu/ZrO
2
Fig. 12. Effect of cerium doping into ZrO on (a) the first order apparent rate constant, kapp, and (b) hydrogen storage capacity.
2

286
K.A. Pokrovski, A.T. Bell / Journal of Catalysis 241 (2006) 276–286
ilar manner. Thus, it is reasonable to conclude that the surface
concentration of Brønsted acid protons increases in a manner
similar to the overall H2 adsorption capacity of the catalyst.
[9] R.E. Jentoft, S.E. Deutsch, B.C. Gates, Rev. Sci. Instrum. 67 (1996) 211.
10] M. Newville, J. Synchrotron Rad. 8 (2001) 322.
11] B. Ravel, M. Newville, J. Synchrotron Rad. 12 (2005) 537.
12] R.F. Hicks, C.S. Kellner, B.J. Savatsky, W.C. Hecker, A.T. Bell, J. Ca-
tal. 71 (1981) 216.
[
[
[
5
. Conclusions
[
[
13] H. Toraya, M. Yashmura, S. Somiya, J. Am. Ceram. Soc. 67 (1984) C–119.
14] G. Colon, M. Pijolat, F. Valdivieso, H. Vidal, J. Kašpar, E. Finocchio,
M. Daturi, C. Binet, J.C. Lavalley, R.T. Baker, S. Bernal, J. Chem. Soc.,
Faraday Trans. 94 (1998) 3717.
The progressive substitution of Ce into the lattice zirconia
leads to an increase in the area-based methanol synthesis ac-
tivity of 3 wt% Cu/CexZr1−xO2, which passes through a maxi-
mum for x = 0.5. The maximum in methanol synthesis activity
is paralleled by a maximum in the hydrogen adsorption capac-
ity of the catalyst. This latter effect is attributed to the formation
[
[
15] C. Schild, A. Wokaun, R.A. Koeppel, A. Baiker, J. Catal. 130 (1991) 657.
16] M. Li, Z. Feng, G. Xiong, P. Ying, Q. Xin, C. Li, J. Phys. Chem. B 105
(2001) 8107.
[17] C. Li, M. Li, J. Raman Spectrosc. 33 (2002) 301.
[
[
[
[
[
[
18] A. Trovarelli, F. Zamar, J. Lorca, C. Leitenburg, G. Dolcetti, J.T. Kiss,
3
+
4+
J. Catal. 169 (1997) 490.
19] V.S. Escribano, E.F. López, M. Panizza, C. Resini, J.M.G. Amores, G.
Busca, Solid State Sci. 5 (2003) 1369.
20] M. Yashima, H. Arashi, M. Kakihana, M. Yoshimura, J. Am. Ceram.
Soc. 77 (1994) 1067.
21] P. Fornasiero, G. Balducci, R. Di Monte, J. Kašpar, V. Sergo, G. Gubitosa,
A. Ferrero, M. Graziani, J. Catal. 164 (1996) 173.
22] S. Enzo, F. Delogu, R. Frattini, A. Primavera, A. Trovarelli, J. Mater.
Res. 15 (2000) 1538.
23] M. Shimokawabe, H. Asakawa, N. Takezawa, Appl. Catal. 59 (1990) 45.
of Ce –O(H)–Zr species by dissociative adsorption of H2
on particles of supported Cu, followed by spillover of atomic H
4+
4+
onto the oxide surface and reaction with Ce –O–Zr centers.
The higher concentration of Ce3 –O(H)–Zr species on the ox-
ide surface, together with the higher Brønsted acidity of these
species, appears to be the primary cause of the fourfold-higher
activity of 3 wt% Cu/Ce0.5Zr0.5O2 relative to 3 wt% Cu/ZrO2.
+
4
Acknowledgments
[24] R. Zhou, T. Yu, X. Jiang, F. Chen, X. Zheng, Appl. Surf. Sci. 148 (1999)
63.
2
[
25] M. Boaro, M. Vicaro, C. de Leitenburg, G. Dolcetti, A. Trovarelli, Catal.
Today 77 (2003) 407.
This work was supported by the Director, Office of Basic En-
ergy Sciences, Chemical Sciences Division of the US Depart-
ment of Energy under contract DE-AC02-05CH11231. Portions
of this research were carried out at the Stanford Synchrotron
Radiation Laboratory (SSRL), a national user facility operated
by Stanford University on behalf of the US Department of En-
ergy, Office of Basic Energy Sciences. The SSRL Structural
Molecular Biology Program is supported by the Department of
Energy, Office of Biological and Environmental Research, and
by the National Institutes of Health, National Center for Re-
search Resources, Biomedical Technology Program.
[26] A. Trovarelli, G. Dolcetti, C. de Leitenburg, J. Kašpar, P. Finetti, A. San-
toni, J. Chem. Soc., Faraday Trans. 88 (1992) 1311.
[27] C. de Leitenburg, A. Trovarelli, J. Kašpar, J. Catal. 166 (1997) 98.
[28] M. Boaro, M. Vicaro, C. de Leitenburg, G. Dolcetti, A. Trovarelli, Catal.
Today 77 (2003) 407.
[
[
[
29] J. Livage, Catal. Today 41 (1998) 3.
30] C. Binet, M. Daturi, J.C. Lavalley, Catal. Today 50 (1999) 207.
31] C. Binet, M. Daturi, J.C. Lavalley, J. Phys. Chem. 98 (1994) 6392.
[32] A. Badri, C. Binet, J.C. Lavalley, J. Chem. Soc., Faraday Trans. 93 (1992)
159.
1
[33] C. Binet, M. Daturi, Catal. Today 70 (2001) 155.
[34] M.Y. He, J.G. Ekerdt, J. Catal. 87 (1984) 381.
[35] D. Bianchi, T. Chafik, M. Khalfallah, S.J. Teichner, Appl. Catal. A:
Gen. 105 (1993) 223.
References
[36] C. Li, Y. Sakata, T. Arai, K. Domen, K. Maruya, T. Onishi, J. Chem. Soc.,
[
[
[
[
[
[
[
[
1] B. Denise, R.P.A. Sneeden, Appl. Catal. 28 (1986) 235.
2] Y. Sun, P.A. Sermon, J. Chem. Soc. Commun. (1993) 1242.
3] Y. Sun, P.A. Sermon, Catal. Lett. 29 (1994) 361.
4] I.A. Fisher, H.C. Woo, A.T. Bell, Catal. Lett. 44 (1997) 11.
5] Y.W. Suh, S.H. Moon, H.K. Rhee, Catal. Today 63 (2000) 447.
6] M.D. Rhodes, A.T. Bell, J. Catal. 233 (2005) 198.
Faraday Trans. I 85 (1989) 1451.
[37] C. Lamonier, A. Ponchel, A. D’Huysser, L. Jalowiecki-Duhamel, Catal.
Today 50 (1999) 247.
[38] J.L.G. Fierro, J. Soria, J. Sanz, J.M. Rojo, J. Solid State Chem. 66 (1987)
154.
[39] K.D. Jung, A.T. Bell, J. Catal. 193 (2000) 207.
[40] K. Tanabe, Solid Acids and Bases; Their Catalytic Properties, Academic
Press, New York, 1970.
7] M.D. Rhodes, K.A. Pokrovski, A.T. Bell, J. Catal. 233 (2005) 210.
8] K.A. Pokrovski, M.D. Rhodes, A.T. Bell, J. Catal. 235 (2005) 368.