Catalytic oxidation of methanol on Pd metal and oxide clusters at near-ambient temperatures

Article abstract of DOI:10.1039/b707465d

Supported Pd clusters catalyze methanol oxidation to methyl formate with high turnover rates and >90% selectivity at near ambient temperatures (313 K). Metal clusters are much more reactive than PdO clusters and rates are inhibited by the reactant O₂. These data suggest that ensembles of Pd metal atoms on surfaces nearly saturated with chemisorbed oxygen are required for kinetically-relevant C-H bond activation in chemisorbed methoxide intermediates. Pd metal surfaces become more reactive with increasing metal particle size. The higher coordination of surface atoms on larger clusters leads to more weakly-bound chemisorbed species and to a larger number of Pd metal ensembles available during steady-state catalysis. Chemisorbed oxygen removes H-atoms formed in C-H bond activation steps and inhibits methoxide decomposition and CO₂ formation, two functions essential for the high turnover rates and methyl formate selectivities reported here. the Owner Societies.

Full text of DOI:10.1039/b707465d

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www.rsc.org/pccp | Physical Chemistry Chemical Physics
Catalytic oxidation of methanol on Pd metal and oxide clusters at
near-ambient temperatures
Janine Lichtenberger, Doohwan Lee and Enrique Iglesia*
Received 17th May 2007, Accepted 17th July 2007
First published as an Advance Article on the web 27th July 2007
DOI: 10.1039/b707465d
Supported Pd clusters catalyze methanol oxidation to methyl
formate with high turnover rates and 490% selectivity at near
ambient temperatures (313 K). Metal clusters are much more
reactive than PdO clusters and rates are inhibited by the reactant
3
(353 K). CH OH oxidation rates on these catalysts were
strongly influenced by the oxidation state of supported Pd
clusters. Methanol oxidation turnover rates decreased markedly
with decreasing Pd cluster size, as a result of a concomitant
increase in the binding energy of chemisorbed oxygen species,
which leads to lower concentrations of vacancies in chemi-
sorbed oxygen monolayers during steady-state catalysis.
O . These data suggest that ensembles of Pd metal atoms on
2
surfaces nearly saturated with chemisorbed oxygen are required
for kinetically-relevant C–H bond activation in chemisorbed
methoxide intermediates. Pd metal surfaces become more reactive
with increasing metal particle size. The higher coordination of
surface atoms on larger clusters leads to more weakly-bound
chemisorbed species and to a larger number of Pd metal ensem-
bles available during steady-state catalysis. Chemisorbed oxygen
removes H-atoms formed in C–H bond activation steps and
This remarkable reactivity of Pd-based clusters in the oxida-
tive dehydrogenation of methanol and ethanol provides signifi-
cant opportunities for the efficient use of these products as
precursors to more useful chemicals; specifically, they provide
direct routes from biomass-derived alcohols to methyl formate
and acetaldehyde. Methyl formate is used to produce formamide
22
inhibits methoxide decomposition and CO
2
formation, two func-
via aminolysis and formic acid via acid-catalyzed hydrolysis.
It also leads to precursors to ethylene glycol (HCOOCH
HCHOQHOCH COOCH ) via HCHO-coupling and to acetic
tions essential for the high turnover rates and methyl formate
selectivities reported here.
3
+
2
3
acid via intramolecular isomerization (HCOOCH Q
3
CH COOH) on metal complexes (Rh, Ir, Co, Ni, Ru, Pd) with
3
2
2,23
iodide co-catalysts or promoters.
Introduction
Methanol (CH
3 2
OH) reactions with O lead to formaldehyde
Results and discussion
(
HCHO), methyl formate (MF) and dimethoxymethane (DMM)
3,4
1
products, useful as chemical intermediates, on MoO
2
, V
3
2 5
O
Pd-containing catalysts were exposed to methanol–O reactants
2
5,6
7
and Mo–Sn oxides. Ru oxides catalyze CH
3
OH oxidation to
either immediately after treatment in 20% O –He flow at 373 K
2
MF with extraordinary rates and selectivities at near-ambient
temperatures (300–400 K). Pd-based catalysts are widely used
for oxidation of higher alcohols to aldehydes at low tempera-
(
denoted as ‘‘PdO’’ sample) or after subsequent treatment of
these samples in H at 373 K (denoted as ‘‘Pd’’ sample). These
two treatments led to markedly different CH OH oxidation
2
3
8–13
14
tures in liquid
seldom been used to oxidize smaller and less reactive alcohols
oC ). The oxidation of C –C alcohols was reported to occur
and supercritical media, but they have
rates, indicating that the Pd oxidation state strongly influences
kinetically-relevant steps.
(
4
1
4
2 3 2
Turnover rates on PdO/Al O (treated in 20% O –He at
on ‘‘giant palladium clusters’’ with Pd Phen (Oac) and
180
561
60
6
73 K for 2 h) increased gradually with time on stream (Fig. 1;
33 K). Reaction products were not detectable on these cata-
15–18
)60 composition
Pd561Phen60
O
60(PF
6
(phen = 1,10-phenan-
3
throline) with the formation of MF as the main product. MF
and HCHO were also reported as undesired incomplete oxida-
3 2
lysts at 313 K, even after contact with CH OH–O reactant
mixtures for 12 h, due to a very slow reaction rate. The gradual
19
tion products during methanol combustion on Pd- and
20,21
Pt-based
increase in CH OH oxidation rates on PdO samples at 333 K
3
catalysts at low temperatures (o400 K).
Here we report the highest turnover rates reported for
may indicate that PdO clusters reduce slowly during CH OH–
3
O reactions and that Pd metal surfaces catalyze kinetically-
2
À1
CH OH oxidation (up to 2.6 mol CH
3
3
OH s (surface Pd
3
relevant CH OH oxidation steps much more efficiently than
À1
atom) ) with very high MF selectivities (B90%) at near-
ambient temperatures (313 K). These materials also catalyzed
PdO surfaces, a conclusion confirmed by the marked increase in
reactivity observed when these catalysts were treated in H
2
to
À1
ethanol oxidation with high turnover rates (0.21 s ) and
form Pd metal clusters, as described below.
acetaldehyde selectivities (B85%) at low temperatures
Pd metal centers were proposed to act as active sites in liquid-
8
phase oxidation of cinnamyl and benzyl alcohols on Pd/
24
2
5
Department of Chemical Engineering, University of California at
Berkeley, Chemical Sciences Division, E.O. Lawrence Berkeley
National Laboratory, Berkeley, CA 94720. E-mail:
iglesia@berkeley.edu; Fax: +1 (1)510642 4778;
Tel: +1 (1)510 6429673
Al
reactive than Pd/Al O samples for the decomposition (in He)
2 3 2 3
O . Cordi and Falconer reported that PdO/Al O was less
2
3
and oxidation (in O ) of methoxide species formed via methanol
2
dissociation. These reactivity differences were attributed to the
4
902 | Phys. Chem. Chem. Phys., 2007, 9, 4902–4906
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Fig. 1 (a) Methanol conversion rates with time on stream over a
Fig. 2 (a) Selectivities with time on stream over 4 wt% PdO/Al
treated in 20% O –He at 675 K prior to reaction (conditions: 4 kPa
CH OH, 9 kPa O
.7 mg PdO/Al
diluted with 1 g of quartz)). (b) Selectivities with time on stream over a
wt% Pd/Al treated in 20% O –He at 675 K followed by treatment
in H at 373 K prior to reaction (conditions: 4 kPa CH OH, 9 kPa O
kPa N , balance He, 1 cm s total flow, 313 K, conversion: 6.6%,
catalyst dilution: 150 mg of a 1 : 150 Pd/Al : Al internally
diluted sample and 350 mg additional Al )).
2 3
O
4
wt% PdO/Al
2
O
3
catalyst (dispersion 0.24) treated in 20% O
OH, 9 kPa O , 1 kPa N
, conversion:
.02 to 1.9%, (the catalyst bed was diluted with 1 g of quartz)). (b)
Methanol conversion rates with time on stream over a 1 wt% Pd/Al
dispersion 0.43) treated in 20% O –He at 675 K followed by treatment
in H at 373 K prior to reaction (conditions: 4 kPa CH OH, 9 kPa O
kPa N , balance He, 1 cm s total flow, 313 K, conversion: 6.6%,
catalyst dilution: 150 mg of a 1 : 150 Pd/Al : Al internally-
diluted sample and 350 mg additional Al )).
2
–He at
2
3
À1
6
75 K prior to reaction (conditions: 4 kPa CH
3
2
2
,
3
2
, 1 kPa N , balance He, 1 cm s total flow, 333 K,
O , conversion: 0.02 to 1.9%, (the catalyst bed was
2 3
2
3
À1
2 3
balance He, 1 cm s total flow, 333 K, 3.7 mg PdO/Al O
3
0
2 3
O
1
O
2 3
2
(
2
2
3
2
,
3
À1
2
3
2
,
1
2
3
À1
1
2
(
2
O
3
2 3
O
(
O
2 3
2 3
O
2 3
O
2 3
O
slow extraction of lattice oxygen in the case of PdO. Thin PdO
though O is a stoichiometric reactant, specifically required for
2
2
6
films (3–5 monolayers) deposited on Au substrates did not
activate CH OH up to 623 K, because of their low reactivity in
CH OH dissociation and their resistance to reduction. In
contrast, Pd metal films catalyzed methanol oxidation at
25 K, a temperature B200 K higher, however, than we report
here on impregnated Pd/Al catalysts.
the oxidative removal of H-atoms, abstracted during methoxide
3
decomposition, as H
decomposition reactions, as CO
unselective sequential dehydrogenation of methoxide intermedi-
ates to H and CO. This latter role of chemisorbed oxygen is
consistent with that proposed earlier based on methoxide
2
O (and of any CO, formed in side
3
2
) and for the suppression of
5
2
2 3
O
29
The evolution of methanol oxidation reactivity with time on
stream, reported here for PdO clusters at much lower tempera-
tures, is consistent with the effectiveness of reduced Pd surfaces
in the oxidative dehydrogenation of alcohols. MF was the only
product detected during methanol–O reactions on PdO/Al O
decomposition studies on single crystal surfaces.
Supported Pd metal clusters gave very high MF selectivities
(B90%) and only small amounts of CO (Fig. 2). MF, CO , and
2
2
H O were the only products detected; CO and HCHO products
2
were not detected (detection limits: 0.5 and 3%, at B10%
conversion). The absence of HCHO indicates that its reaction
2
2
3
throughout these experiments (14 h; Fig. 2).
Pd/Al
in H at 373 K for 1 h) gave much higher reaction rates at
13 K than unreduced samples (Fig. 1). Turnover rates were
2
O
3
samples (prepared from PdO/Al
2
O
3
by treatment
with chemisorbed methoxide or its oxidation to CO
2
is fast. The
2
absence of CO and H among products is not surprising,
2
3
because, if formed in surface reactions, they would desorb from
Pd surfaces only at temperatures well above those required for
B40 times higher than on PdO samples; rates remained
constant throughout these experiments (15 h). These trends
are markedly different from the gradual increase in rate
30
methanol oxidation (4330 K for H ; 4480 K for CO ).
2
Table 1 summarizes methanol oxidation turnover rates (nor-
malized by surface metal atoms) and selectivities on supported
Pd, Pt and Ru clusters. Selectivities are compared at different
conversions because of the wide range reported in previous
studies, as well as the high reactivity of our materials, which
lead to severe catalyst temperature gradients at conversions
3 2
observed on PdO clusters placed in contact with CH OH–O
reactants (Fig. 1). We conclude from these data that the
Pd metal clusters remained active and that they did not oxidize
during the reaction at these temperatures. We also con-
clude that exposed Pd atoms present within chemisorbed
oxygen layers are involved in rate-determining steps, as also
inferred in the liquid-phase oxidation of higher alcohols on
2 3
above 10%. Pd/Al O catalysts give rates similar to those on
previously reported catalysts, but at 20–80 K lower tempera-
tures and with higher methyl formate selectivities. Methanol
oxidation gave low MF yields (o13%) at 340 K on 0.01 wt%
8
,24
Pd catalysts
and the catalytic combustion of CH
4
on
2
7
PdO surfaces.
The measured negative effect in O
1
9
2
on methanol oxidation
2 3
Pd/g-Al O (42% dispersion). HCHO yields increased with
rates is consistent with the involvement of vacancies in kineti-
cally-relevant steps. A positive reaction order was measured
2
temperature (maximum yield, 11% at 480 K); CO was the
predominant product (B67% yield; 340 K) at all conditions in
these studies. The same authors observed MF (maximum yield
2
with respect to methanol. These inhibition effects prevail even
8
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3 2
Table 1 CH OH oxidation turnover rates (per surface atom) and product selectivities on alumina-supported PdO, Pd, Pt and RuO samples
CH OH conversion turnover
3
Selectivity
rate to MF, HCHO and
DMM/mol (g-atom Msurf-h)
conversion (%)
À1
/
Catalysts
Metal (wt%)
Temperature/K
HCHO MF
DMM
CO
2
a
Pd/Al
PdO /Al
Pd/Al
Pt–Al
2
O
3
1.0
4.0
0.01
0.054
4.4
313
333
340
330
393
3
8460 / 6.6
234 / 1.6
4158 / 80
4575 / 93
70 / 20
—
—
6
1
11.6
90
100
16
60
30.1
—
—
—
—
10
—
78
39
1.0
b
x
2
O
3
a,19
2
O
3
2
1
2
O
3
7
RuO /Al
2
2
O
3
57.4
a
À1
4
kPa CH
3
OH, 9 kPa O , 1 kPa N
2
2
, balance He, 1 cm s ; ‘‘Pd’’ refers to samples treated in H
2
at 373 K for 1 h; ‘‘PdO
x
’’ refers to samples treated in
OH–O reactants.
b
0% O –He at 673 K for 2 h. Rates on supported PdO
2
2
x
clusters represent values measured after ca. 10 h of exposure to the CH
3
2
6
0% at 330 K), HCHO, and CO during methanol oxidation on
2
MF and CO selectivities were not influenced by Pd disper-
2
2
.054 wt% Pt–Al O (71% dispersion) at modest tempera-
1
0
sion (Fig. 4). MF synthesis turnover rates (extrapolated to zero
residence time and conversion), however, increased markedly as
Pd dispersion decreased, indicating that surfaces on large Pd
clusters are much more reactive than on smaller ones. Similar
2
3
tures (320–400 K) (Table 1). Similar MF selectivities (B50%
at 333 K and 60% conversion) were reported on 1 wt% Pt–
2
0
À1
Al
O
2 3
with low MF synthesis rates of 0.017 mol g (atom
À1 À1
Pdtotal
)
s
. Liquid phase methanol oxidation on ‘‘giant Pd
4
trends were reported previously for CH -oxidation on PdO
2
7
clusters’’ (Pd561Phen60(Oac)180; phen = 1,10-phenanthroline)
clusters at low temperatures. This latter reaction is limited by
C–H activation on vacancy-oxygen (*–O*) site pairs and Pd
dispersion effects were attributed to a smaller number of
vacancies as the binding energy of chemisorbed oxygen in-
creased with decreasing ensemble size as previously shown for
À1
gave methyl formate formation rates (0.005 mol g (atom
À1 À1
15
at 293 K) much lower than reported here (0.564
Pdtotal
)
À1
s
À1 À1
mol g (atom Pdtotal
)
s
at 313 K). These large organome-
selectivities of 78 and 22%,
tallic Pd clusters gave MF and CO
2
3
2
33,34
respectively, at low methanol conversions (1.25% after 1 h in
batch reactor). Table 1 also shows rates and selectivities on
supported RuOx catalysts, which also gave high combined
selectivities to MF, DMM (dimethoxymethane), and HCHO
at low temperatures and were recently reported as the most
Pd and Pt
clusters. This trend reflects, in turn, the higher
coordinative unsaturation of Pd surface atoms exposed on
smaller clusters. The similar trends observed for CH OH
3
oxidation in the present study are also consistent with the
requirement for reduced Pd surfaces, possibly vacancies on
clusters densely covered by chemisorbed oxygen. The availabil-
ity of such vacancies would decrease with increasing Pd metal
dispersion as a result of the stronger binding of oxygen on
coordinatively unsaturated metal atoms prevalent on small
clusters. The absence of dispersion effects on selectivity suggests
7
active among metal oxide catalysts. Methanol oxidation turn-
over rates on these RuO
orders of magnitude lower at 80 K higher temperatures as
compared to the Pd/Al catalyst used in this study.
Fig. 3 shows turnover rates (per surface Pd atom) (313 K, 4
kPa CH OH, 45 kPa O ) and selectivities on 1 wt% Pd/Al
catalyst (43% dispersion) as a function of methanol conversion
varied by changing the reactant space velocity). CH OH
x
catalysts are approximately two
2 3
O
3
2
O
2 3
2
that MF and CO form via the same surface intermediates, the
number, but not the reactive properties, of which depends on
cluster size.
(
3
oxidation turnover rates decreased with increasing CH OH
3
conversion as a result of reactant depletion combined with a
strong inhibition by water co-products. Such inhibition effects
3
1
were also reported for methanol oxidation on Fe–Mo oxides.
Selectivities were not influenced by space velocity, indicating
that MF and CO are primary products and that MF does not
oxidize to CO in secondary reactions at the conditions required
for CH OH oxidation. MF can form via: (1) condensation of
x
x
3
adsorbed methoxides with HCHO to form methoxymethanol
intermediates (CH OCH OH) that then dehydrogenate to MF;
3
2
(
2) esterification of formic acid (HCOOH) intermediates formed
by HCHO oxidation; or (3) HCHO dimerization via Tischenko-
2
type reactions (Scheme 1). Adsorbed HCHO can also decom-
pose to chemisorbed CO* and H* before desorption, as ob-
served on Pd(111) surfaces, or react with chemisorbed O* to
2
9
form surface formate species (HCOO*). CO* oxidation and
HCOO* decomposition then lead to CO and remove otherwise
2
stranded strongly-adsorbed intermediates (Scheme 1). The high
MF selectivities reported here indicate that fast reactions
effectively scavenge HCHO to form MF, which is much less
reactive than HCHO in subsequent oxidation reactions that
form CO2.
Fig. 3 Methanol conversion turnover rates and product selectivities as
a function of methanol conversion changed by varying residence time,
1
2 3 3 2
wt% Pd/Al O , 4 kPa CH OH, 45 kPa O , balance He, 313 K,
catalyst dilution: 300 mg of a 1 : 150 Pd/Al O : Al O internally diluted
2
3
2 3
2 3
sample and 200 mg additional Al O .
4
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Pd(111) surfaces in the presence of chemisorbed oxygen led to
acetic acid desorption only above 420 K, because adsorbed
acetate precursors were stabilized against decomposition by
3
8
chemisorbed oxygen.
Table 2 compares ethanol oxidation rates on the Pd/Al O
2
3
catalysts used here with those reported previously on RuO /
2
7
39
.
SnO
2
and V
2
O
5
/TiO –SiO
2
2
x
Supported RuO clusters were
recently reported as the most active catalysts for ethanol and
7
methanol oxidation. Reaction rates are B10 times higher on
RuO
catalysts reported here give turnover rates B10 times greater
than on RuO clusters at B40 K lower reaction temperature,
x 2 5 2 3
clusters than on supported V O catalysts. The Pd/Al O
Scheme 1 Proposed reaction pathways (* represents adsorbed species).
x
with 490% selectivity to desired acetaldehyde and ethyl acetate
products.
Experimental
Supported Pd catalysts were prepared by incipient wetness
2 3
impregnation of g-Al O (Alcoa, HiQ31, BET surface area:
2
À1
3
À1
) with aqueous
2
80 m
Pd(NO
perature. Impregnated powders were treated in ambient air at
98 K overnight and then in flowing dry air (Praxair, zero-
g
, pore volume: 0.497 cm
g
3
)
2
Á 2H O solutions (Aldrich, 99.9%) at ambient tem-
2
3
3
À1 À1 À1
grade, 0.7 cm g
holding for 2 h. Al O was treated in flowing dry air at 873 K
s ) by heating to 673 K at 0.17 K s and
2
3
for 5 h before impregnation with aqueous Pd solutions.
Pd dispersion was measured using O chemisorption at 308 K
in a volumetric unit (Quantachrome, Autosorb-1) after treating
samples in H (Praxair, UHP) at 373 K for 1 h and evacuating
2
Fig. 4 Primary MF synthesis turnover rates and selectivities as a
function of Pd dispersion measured by O chemisorption, 4 kPa
CH OH, 45 kPa O , balance He, 313 K, 0.005% wt Pd irrespective of
2
2
at 373 K for 1 h to remove chemisorbed hydrogen. Saturation
uptakes were measured by extrapolating isotherms to zero
pressure and used to estimate the number of exposed Pd metal
atoms by assuming one chemisorbed oxygen atom per exposed
3
2
the Pd content in the starting material.
4
0
Pd atom. Metal dispersions were 0.10–0.43, corresponding
Supported Pd clusters also catalyze the selective oxidation of
ethanol to acetaldehyde with extraordinary rates and very high
selectivity to acetaldehyde and ethyl acetate at low temperatures
to clusters B2–10 nm in diameter for the Pd/Al O samples
used here.
2
3
Methanol and ethanol oxidation rates and selectivities were
measured using a packed-bed quartz microreactor (1 cm inner
diameter). Catalyst samples (0.5–4 mg) were diluted either with
quartz or with alumina to prevent temperature gradients and
used as 75–125 mm particles (pressed into wafers at 41 MPa,
crushed and sieved). The catalyst bed height of the diluted
samples was at least 1 cm in all runs. Samples were treated in
(
353 K) (Table 2). No CO was detected among the reaction
x
products. Ethyl acetate can form via: (1) acetic acid esterifica-
tion with ethanol, (2) oxidative condensation of acetaldehyde
3
5
with ethanol, or (3) condensation of acetaldehyde. Acetic acid
was not detected, but both acetaldehyde and acetic acid have
2 4
been reported as products of ethanol oxidation on Pd/MgAl O
3
6
37
.
3
and Pt/MgAl O
2 4
at 453 K and also on Pt/Al
O
2 3
At the
20% O (Praxair, 99.999%)/He (Praxair, UHP) flow (1.67 cm
2
À1 À1
lower temperatures of our experiments, any acetic acid formed
may not desorb, except perhaps via condensation reactions that
form ethyl acetate. Acetic acid desorption/decomposition on
s ) by heating to 673 K at 0.083 K s and holding for 2 h.
After this treatment, the samples were either used directly in
methanol oxidation (‘‘PdO sample’’) or treated in H (Praxair)
2
Table 2 CH CH
3
2
OH oxidation rates and product selectivities on supported Pd, RuO
2
and V O
2 5
clusters
CH CH OH
3
2
Selectivity
CH
3
CH OH
2
conversion
rate/mol
(g-atom Mtotal-h)
conversion (%)
conversion
turnover rate/mol
À1
Metal
(wt%) Temperature/K (g-atom Msurf-h)
/
Ethyl
ether
Ethyl
acetate
À1
Catalysts
Acetaldehyde
Acetal
a
Pd/Al
RuO
2
O
3
4.0
4.1
3.56
353
393
373
747
76.3
—
179 / 2.3
16.4 / 10–15
0.12 / —
À1
84.5
93.4
92.4
—
—
2.7
13
2.0
—
2.5
4.6
4.9
a,7
2
/SnO
/TiO –SiO
2
b,39
V O
2 5
2
2
a
3
, balance He, 1 cm s
À1
b
3
3
OH (liquid), 0.086 cm s
À1 À1
3
39
4
kPa CH CH
3
2
OH, 9 kPa O , 1 kPa N
2
2
.
0.73 cm h CH
3
CH
2
O (gas), 0.37 cm s He (gas).
2
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3
À1
(
0.5 cm
s
) at 373 K for 1 h by heating from ambient
À1
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1
2 H. L. Wu, Q. H. Zhang and Y. Wang, Adv. Synth. Catal., 2005, 347,
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2
5
1
of the Pd dispersion measured from O chemisorption uptakes
2
1
3 D. I. Enache, J. K. Edwards, P. Landon, B. Solsona-Espriu, A. F.
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carbon basis as the percentage of the carbon in the CH
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3
OH
14 M. Caravati, D. M. Meier, J. D. Grunwaldt and A. Baiker, J. Catal.,
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H
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5 Y. V. Lastovyak, S. L. Gladii, P. I. Pasichnyk, M. K. Starchevskii,
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2 3
O
1
1
6 M. K. Starchevskii, S. L. Gladii, Y. V. Lastovyak, P. I. Pasichnyk,
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2 3
O
1
Pd content in the starting material (by mixing, pressing into
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1
pellets, crushing and sieving as described above). These samples
À1
996, 37, 383–390.
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2
6
75 K and holding for 2 h and then cooled to ambient
À1
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2
0 P. Chantaravitoon, S. Chavadej and J. Schwank, Chem. Eng. J.,
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2
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2
2
2
2
2 3 2
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5 N. E. Quaranta, J. Soria, V. Cortes Corberan and J. L. G. Fierro, J.
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2
3
because their coordinatively unsaturated surfaces bind chemi-
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36 C. H. Christensen, B. Jorgensen, J. Rass-Hansen, K. Egeblad, R.
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The authors acknowledge the financial support and technical
guidance from BP as part of the Methane Conversion Co-
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906 | Phys. Chem. Chem. Phys., 2007, 9, 4902–4906
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