Structure, chemical composition, and reactivity correlations during the in situ oxidation of 2-propanol

Article abstract of DOI:10.1021/ja200178f

Unraveling the complex interaction between catalysts and reactants under operando conditions is a key step toward gaining fundamental insight in catalysis. We report the evolution of the structure and chemical composition of size-selected micellar Pt nanoparticles (～1 nm) supported on nanocrystalline γ-Al₂O₃ during the catalytic oxidation of 2-propanol using X-ray absorption fine-structure spectroscopy. Platinum oxides were found to be the active species for the partial oxidation of 2-propanol (<140 °C), while the complete oxidation (>140 °C) is initially catalyzed by oxygen-covered metallic Pt nanoparticles, which were found to regrow a thin surface oxide layer above 200 °C. The intermediate reaction regime, where the partial and complete oxidation pathways coexist, is characterized by the decomposition of the Pt oxide species due to the production of reducing intermediates and the blocking of O₂ adsorption sites on the nanoparticle surface. The high catalytic activity and low onset reaction temperature displayed by our small Pt particles for the oxidation of 2-propanol is attributed to the large amount of edge and corner sites available, which facilitate the formation of reactive surface oxides. Our findings highlight the decisive role of the nanoparticle structure and chemical state in oxidation catalytic reactions.

Full text of DOI:10.1021/ja200178f

ARTICLE
pubs.acs.org/JACS
Structure, Chemical Composition, And Reactivity Correlations
during the In Situ Oxidation of 2-Propanol
Kristof Paredis,† Luis K. Ono,† Simon Mostafa,† Long Li,‡ Zhongfan Zhang,‡ Judith C. Yang,‡,§
Laura Barrio,|| Anatoly I. Frenkel,^,* and Beatriz Roldan Cuenya†,*
†Department of Physics, University of Central Florida, Orlando, Florida 32816, United States
‡Department of Mechanical Engineering and Materials Science, University of Pittsburgh, Pittsburgh, Pennsylvania 15261, United States
§Department of Chemical and Petroleum Engineering, University of Pittsburgh, Pittsburgh, Pennsylvania 15261, United States
Instituto de Catꢀalisis y Petroleoquímica, CSIC, Madrid 28049
^Department of Physics, Yeshiva University, New York, New York 10016, United States
S Supporting Information
b
ABSTRACT: Unraveling the complex interaction between catalysts and reactants under
operando conditions is a key step toward gaining fundamental insight in catalysis. We report
the evolution of the structure and chemical composition of size-selected micellar Pt
nanoparticles (∼1 nm) supported on nanocrystalline γ-Al2O3 during the catalytic oxidation
of 2-propanol using X-ray absorption fine-structure spectroscopy. Platinum oxides were
found to be the active species for the partial oxidation of 2-propanol (<140 ꢀC), while the
complete oxidation (>140 ꢀC) is initially catalyzed by oxygen-covered metallic Pt
nanoparticles, which were found to regrow a thin surface oxide layer above 200 ꢀC. The
intermediate reaction regime, where the partial and complete oxidation pathways coexist, is characterized by the decomposition of
the Pt oxide species due to the production of reducing intermediates and the blocking of O2 adsorption sites on the nanoparticle
surface. The high catalytic activity and low onset reaction temperature displayed by our small Pt particles for the oxidation of
2-propanol is attributed to the large amount of edge and corner sites available, which facilitate the formation of reactive surface
oxides. Our findings highlight the decisive role of the nanoparticle structure and chemical state in oxidation catalytic reactions.
’ INTRODUCTION
oxidation of 2-propanol. Besides gaining fundamental under-
standing on structureꢀreactivity relationships, chemical pro-
cesses involving the oxidation of alcohols are of importance for
the industrial synthesis of fine chemicals as well as for their
subsequent removal from the waste streams.24,25
The rational design of the next-generation of catalysts requires
detailed knowledge of the correlation between structure, chemi-
cal composition, and reactivity. Despite Pt being the most
industrially relevant and widely investigated catalyst, its complex
interaction with common reactants such as oxygen still provides
many challenges to the scientific community.1 Gaining insight
into the nature and structure of the catalytically active Pt species
and their interaction with adsorbates under reaction conditions
is therefore key in understanding and controlling catalytic
activity.2ꢀ11 The downscaling of the catalysts to the nanometer
range further complicates the interpretation due to the possible
existence of crystallographically and chemically metastable
phases, the presence of different surface facets in nanoparticles
(NPs) with distinct size and shape, the interaction with the
support, and a variety of additional parameters.1,12ꢀ22 Therefore,
obtaining unambiguous results on the origin of a particular
catalytic performance requires nearly monodispersed NPs in
size, shape, and chemical composition.14
Despite intense research efforts, the mechanism behind the
oxidation of alcohols over noble metal catalysts is still under
debate. Several models have been proposed. The classical dehy-
drogenation involves two successive dehydrogenation steps of the
adsorbed alcohol. The oxygen is merely used to oxidize the
released hydrogen in order to vacate metallic sites.24,26ꢀ28
Another suggested pathway is the direct interaction of surface
oxygen species with the adsorbed alcohol (or its partially dehy-
drogenated intermediate).24,27,29 The third approach also favors
the dehydrogenation of the adsorbed alcohol, but is followed by
the oxidative removal of the reaction intermediates, rather than
the hydrogen.24,30 These results indicate that the role of oxygen
has not yet been uniquely determined. Furthermore, in the case
of Pt, the formation of oxidized compounds during oxidation
reactions and the resulting influence on the reaction kinetics isstill
heavily debated. Some studies report catalyst deactivation due to
The present study takes advantage of micellar encapsulation
methods for the synthesis of model nanocatalysts with well-
defined geometries.14,23 Such systems are used to address the
effect of the NP structure and oxidation state on the catalytic
activity and selectivity of Pt NPs supported on γ-Al2O3 for the
Received: January 7, 2011
Published: April 06, 2011
r
2011 American Chemical Society
6728
dx.doi.org/10.1021/ja200178f J. Am. Chem. Soc. 2011, 133, 6728–6735
|

Journal of the American Chemical Society
ARTICLE
oxide formation,26,27,31ꢀ33 while others attribute beneficial cata-
lytic properties to the platinum oxides.34ꢀ40 For instance,
the catalytic activity of Pt NPs for the oxidation of carbon
monoxide has been attributed to the formation of highly reactive
surface oxide phases generated under oxygen-rich reaction
conditions.38,41ꢀ43 Clearly, the formation of oxide species plays
an important role in oxidation reactions and it is therefore of great
importance for the identification of the structure and chemical
composition of the catalytically active phase.44ꢀ50
The method of X-ray absorption fine structure (XAFS)
spectroscopy, which allows element-specific structural and che-
mical analysis under operando conditions, has been used to
investigate the catalytic oxidation of 2-propanol. In particular, we
have evaluated the stability and reactivity of chemisorbed oxygen
species and Pt oxides during the oxidation of 2-propanol over
size-selected Pt NPs. Thanks to the well-defined NP morphology
and in situ real-time characterization, insight into the detailed
mechanisms guiding oxidation reactions will be gained.
Figure 1. (a) HAADF-STEM micrograph of micellar Pt NPs supported
on γ-Al2O3 taken after polymer removal and subsequent 2-propanol
oxidation. The corresponding diameter histogram is shown in (b).
Na2Pt(OH)6) using the FEFF6 code and fitted to the EXAFS spectra
with the Artemis package.54,55 First shell fitting was performed in r-space
after Fourier transformation of the k2-weighted data. The typical r-
ranges were 1.3ꢀ3.2 Å, whereas the k-ranges were typically 2.5ꢀ12 Åꢀ1
.
The best fit value for the passive electron reduction factor was obtained
from the Pt foil data, and was fixed to 0.86 for all the nanoparticle
analyses. The fit parameters and fit quality results are summarized in
Table 2 of the Supporting Information (SI). Additional representative
XANES and EXAFS data, together with corresponding fits, can also be
found in SI Figures 1ꢀ4.
’ EXPERIMENTAL SECTION
(a). Sample Preparation. Dissolving a nonpolar/polar diblock
copolymer (polystyrene-block-poly(2-vinylpyridine), PS-P2VP) in to-
luene results in the formation of inverse micelles which are loaded with a
metal precursor (H2PtCl6) to create encapsulated NPs. By varying the
polymer head length and adjusting the metal salt-polymer head ratio (L),
NPs with distinct sizes and shapes can be synthesized.23 The NPs are
deposited on commercial nanocrystalline (powder) γ-Al2O3 supports
(∼40 nm average grain size) by adding the support to the NP polymeric
solution. Subsequently, the solvent is evaporated and the polymer
removed by a 24 h annealing treatment at 375 ꢀC in an oxygen
atmosphere. The latter treatment results in the partial oxidation of the
NPs and the formation of core(Pt-metal)-shell(PtOx) NPs.23,44 For
strongly interacting NP/support systems (e.g., Pt/γ-Al2O3), annealing
small NP micelles with low metal loadings was found to lead to flat 2D-
like NPs.23 For this study, NPs encapsulated by PS(16000)-P2VP(3500)
with a metal/P2VP ratio L of 0.05 were prepared.14 Our annealing
pretreatment in O2 resulted in carbon and chlorine-free NPs as verified
by XPS.14,23 The Pt weight loading was 1%. For the high-angle annular
dark-field scanning transmission electron microscopy (HAADF-STEM)
measurements, the Pt/γ-Al2O3 powders were dissolved in ethanol, and a
drop of this solution was placed onto a Cu grid coated with an ultrathin
carbon film (Ted Pella, Inc.) and subsequently dried in air. Information
on the NP diameter and average size distribution was obtained by
measuring the full width at half-maximum of the HAADF intensity
profile across the NPs.
(b). Analysis of the Structure and Chemical Composition
(EXAFS, XANES). Extended X-ray absorption fine structure spectros-
copy (EXAFS) and near-edge spectroscopy (XANES) measurements
were conducted at beamline X18B of the National Synchrotron Light
Source (NSLS) at Brookhaven National Laboratory. The catalyst
(25 mg) was loaded into a reactor cell compatible with transmission
XAFS spectroscopy measurements, and interfaced to a quadrupole mass
spectrometer to evaluate the catalytic performance up to 240 ꢀC. Mass
flow controllers were used to feed a stream of ∼2% propanol and ∼20%
O2 balanced with He to a total flow of 25 mL/min. Prior to the reaction,
the Pt NPs were reduced for 1 h in H2 at 260 ꢀC.
’ RESULTS
(a). Structure and Morphology (TEM, EXAFS). Figure 1
shows a HAADF-STEM micrograph of the polymer-free γ-
Al2O3-supported Pt NPs (a), along with the corresponding
diameter histogram (b). The data were acquired after the
propanol oxidation reaction. A detailed size analysis revealed a
0.7 ( 0.2 nm average NP diameter and a 0.9 ( 0.3 nm volume-
weighted NP diameter. In a previous study by our group, the
shape of similarly prepared NPs was resolved via EXAFS, STEM
and cluster shape modeling after reduction in H2, yielding a two-
dimensional truncated octahedron as most representative
shape.14,22,23 A similar analysis was carried out here for the
NPs under investigation before exposure to the reactants, and an
analogous 2D-shape was obtained (see SI for details). A repre-
sentative image of this shape is presented as inset in Figure 1(b).
We took special care to estimate the degree of structural and
morphological homogeneity of nanoparticles within the sample,
which is a requirement when EXAFS results are interpreted in
terms of an average size and shape of a representative nanopar-
ticle. Although the vast majority of the NPs in this sample were
distributed within a narrow range of sizes below 1.3 nm (see
Figure 1), a small number of larger Pt particles (with a mean size
of ∼13 nm) were detected by TEM in some regions of the
sample. To evaluate the possible effect of these large particles on
the structural model deduced from the analysis of the combined
EXAFS and STEM data, as well as on the reaction mechanism, we
estimated the overall number density of the large particles by a
method described in the SI. Our results demonstrate that the
number of small particles significantly exceeds the number of
large particles in the sample by a factor of ca. 20 000, that they
occupy ∼85% of the total Pt volume of the sample, and that their
share of the total number of surface atoms, responsible for the
reactive behavior of the Pt NPs is ∼99%.
After acquisition (three scans were averaged at each temperature), the
XAFS data were imported into the ATHENA software and aligned using
as reference a simultaneously measured Pt foil. Subsequently, the
smooth isolated atom background was removed using the AUTOBK
algorithm.51ꢀ53 Theoretical models for the data analysis were calculated
for platinum and platinum oxide (based on the crystal structure of
EXAFS data acquired at room temperature after NP reduction
at 260 ꢀC in H2 before and after the 2-propanol oxidation reaction
6729
dx.doi.org/10.1021/ja200178f |J. Am. Chem. Soc. 2011, 133, 6728–6735

Journal of the American Chemical Society
ARTICLE
Figure 2. Conversion and selectivity data as a function of temperature
for the steady state oxidation of 2-propanol over micellar Pt NPs
supported on γ-Al2O3.
(see SI Figure 5), revealed the overall structural stability of the
NPs in this sample, since similar EXAFS spectra (within experi-
mental noise) were recorded in both instances, indicating the
lack of significant NP coarsening and/or structural reconstruc-
tion. This result is in agreement with the TEM data shown
in Figure 1, since the image was acquired after reactant
exposure.
(b). Catalytic Reactivity. Figure 2 summarizes the tempera-
ture dependence of the conversion of 2-propanol over our Pt
NPs under steady-state reaction conditions. The ∼50 ꢀC onset
reaction temperature(defined as the 50% conversion temperature)
of these 2D-shaped NPs is in accordance with our earlier
study14 and considerably lower than that of 3D NPs of similar
TEM diameter (>65 ꢀC, ref 14). This difference is attributed to
the larger number of undercoordinated active reaction sites
available on the NP surface of the bilayer 2D NPs.14 Our 2D Pt
catalysts reach a 100% conversion of 2-Propanol at 140 ꢀC. The
main reaction products observed are acetone, carbon dioxide,
and water, as expected from the partial and complete oxidation
of 2-propanol:
Figure 3. (a) Pt L3 XANES spectra recorded after polymer removal
(in O2 at 375 ꢀC, as-prepared), after reduction (in H2 at 260 ꢀC), and
during the oxidation of 2-propanol (under 2-propanol and O2 flow). (b)
Integrated XANES intensity (right axis) and absorption peak energy shift
relative to the reduced state of the same NPs (left axis) as a function of the
reaction temperature.
(i.e., before the onset of the alcohol conversion) shows an
increased white line intensity as compared to the fully reduced
spectrum, yet still lower than the original oxidic spectrum of the as-
prepared samples. This result points to a reduced extent of
oxidation. Interestingly, the white line intensity shows a decrease
at 120 ꢀC and a subsequent increase at 240 ꢀC, such effects will be
described in more detail in the Discussion section.
C3H7OH þ 1=2 O2 f C3H6O þ H2O
C3H7OH þ 9=2 O2 f 3 CO2 þ 4 H2O
ð1Þ
ð2Þ
The detailed evolution of the integrated white line intensity
during the reaction is quantitatively summarized in Figure 3(b).
Initially a slight increase of the intensity of the absorption peak is
observed, coinciding with the onset of alcohol conversion. With
increasing temperature, the intensity steadily decreases to reach a
minimum at 120ꢀ140 ꢀC, at which point the complete oxidation
is the dominating reaction pathway, and subsequently increases
again with temperature. Since the white line intensityis influenced
by NP/adsorbate charge transfer phenomena, more specifically,
by the electronegativity of the adsorbates, these variations point to
a change in adsorbate composition or coverage during the various
stages of the reaction.33,38,58,61,62 Size effects, charge transfer
phenomena between the NPs and the support, are also known
to affect the XANES region.63,64 Since the average NP size was
found to remain quasi constant throughout the reaction (see SI
Figure 5), and dramatic changes in the NP/support interaction
are not likely to occur within this relatively small temperature
regime, they are expected to have a minor influence. Therefore,
the changes in the XANES data displayed in Figure 3(b) are
dominated by NP/adsorbate interactions.65
Figure 2 also displays the product selectivity. Initially, high
selectivity (>97%) for acetone (reaction pathway 1) is observed,
but with increasing temperature the reaction gradually shifts to
the complete oxidation (reaction pathway 2), reaching a CO2
selectivity of more than 98%. In addition, above 140 ꢀC a small
fraction of propene (<2%) is observed, which originates from the
dehydration of 2-propanol on the γ-Al2O3 support.56,57
(c). In Situ Evolution of the Structure and Chemical
Composition of Pt NP Catalysts (XANES, EXAFS). Figure 3(a)
presents Pt L3 XANES spectra of micellar Pt NPs on γ-Al2O3
during various steady state stages of the reaction. As was
mentioned before, the removal of the encapsulating polymeric
ligands by the 24 h anneal in O2 results in coreꢀshell oxidized
NPs.23 Since the L3 transition probes the empty 5d states, oxidation
leads to a higher edge intensity (white line), as observed for the
partially oxidized as-prepared sample.58 By comparing the shape of
the XANES spectrum of the as-prepared NPs to literature
reports on Pt oxide powders (see SI Figure 1), we conclude
that PtO2 is the most likely species present as NP shell.54,59,60
A considerable decrease in the white line intensity was
observed upon reduction in H2. After the introduction of the
reactants (O2 and 2-propanol), the spectrum recorded at 40 ꢀC
Along with the integrated intensity, Figure 3(b) also presents
the energy shift of the maximum of the Pt L3 adsorption edge
relative to the absorption peak of the reduced particles. The shift
of the main edge to lower or higher energies is often attributed to
the transfer of electronic density to or from the X-ray absorbing
6730
dx.doi.org/10.1021/ja200178f |J. Am. Chem. Soc. 2011, 133, 6728–6735

Journal of the American Chemical Society
ARTICLE
Figure 5. (a) Dependence of the first nearest neighbor coordination
number on the reaction temperature during the oxidation of 2-propanol
over Pt NPs supported on γ-Al2O3: the PtꢀPt, PtꢀO(I), and PtꢀO(II)
contribution for the small 2D NPs obtained from the fitting of in situ
EXAFS spectra are shown. The dashed gray line indicates the PtꢀPt
coordination number after NP reduction in H2 and before reactant
exposure. (b) PtꢀPt and PtꢀO(I) bond distances as a function of
temperature. The dashed gray line indicates the PtꢀPt distance after NP
reduction in H2. Note the different y-scales in both graphs.
Figure 4. Fourier transformed Pt L3 edge spectra taken at 60 ꢀC
(squares), 140 ꢀC (triangles) and 240 ꢀC (circles) under reactant flow
(k2-weighted, 2.5< k <12 Åꢀ1). The spectra are vertically displaced for
clarity. The inset shows the magnitude of the data and the fit with the
corresponding contributions to the 60 ꢀC spectrum.
atom, respectively. Its interpretation in nanoscale metal clusters is
sometimes complicated by additional factors, namely, the increase
in the Fermi level energy as well as the increase in the electronic
screening, since both effects lead to blueand red shifts of the X-ray
absorption edge, respectively.66 Therefore, the shift of the absorp-
tion edge measured cannot be used by itself to determine the
direction of charge transfer. As demonstrated in Figure 3(b), the
energy shift follows a similar trend to the integrated intensity; an
initial increase with a maximum at 70 ꢀC, followed by a slow
decrease up to 160 ꢀC, and a slight increase at higher temperature.
Since the two effects, the change in the white line intensity and the
shift of the absorption edge, correlate with each other in terms of
the direction of the charge transfer, they can be interpreted self-
consistently in terms of the same origin.
(<100 ꢀC), the PtꢀPt coordination number significantly de-
creases, while the average PtꢀPt bond length slightly increases
(2.76 ( 0.02 Å). Furthermore, two additional PtꢀO contribu-
tions are identified. First, a short distance PtꢀO(I) pair typical of
platinum oxide or chemisorbed oxygen is found at 2.03 ( 0.01 Å.
Although the presence of chemisorbed oxygen cannot be ex-
cluded, the accompanying decrease of the PtꢀPt contribution
clearly points to the formation and stabilization of a surface oxide.
This conclusion is based on the property of EXAFS coordination
numbers to be related not only to the number of nearest
neighboring bonds, but also to the total number of absorbing
atoms. Thus, if two phases are present in the sample, one metallic
with PtꢀPt bonds, and the other oxide without PtꢀPt bonds, the
PtꢀPt coordination number measured in that sample will de-
crease, as is observed in our case. However, since various Pt oxides
(PtO, Pt3O4, PtO2) exhibit similar PtꢀO distances and coex-
istence of different phases might occur, the EXAFS data cannot be
used to uniquely assign this contribution to a specific oxide
species.35,67 However, it is clear that the stable PtO2 compounds
initially present on the as-prepared samples (annealed in O2 for 24 h)
are removed by the in situ annealing treatment in H2 prior to the
reaction, since the PtO2 characteristic features do not reappear after
exposure to 2-propanol and oxygen (see SI Figure 1). Second, a long
distance PtꢀO contribution, PtꢀO(II), is observed at 2.53 ( 0.05
Å, which agrees well with the values previously reported for alcohols
chemisorbed on Pt, in particular, for 2-propanol on Pt(111).68ꢀ71
The appearance of the PtꢀO(I) contribution is an evidence of
the partially oxidized state of the NPs in this temperature regime
(<120 ꢀC), which is in agreement with our XANES results. The
PtꢀO(II) contribution demonstrates the direct adsorption of
2-propanol on Pt, indicating that not all available sites are
occupied by oxygen. Despite the oxidation, the EXAFS data still
clearly show a PtꢀPt contribution from a metallic phase (see
r-range of 2.2 to 3 Å in Figure 4), implying the presence of a
metallic NP core. The XANES spectra of our Pt NPs acquired at
room temperature after a prolonged annealing treatment in O2
(24 h) and those measured after in situ reduction and subsequent
exposure to the reactants (O2 þ 2-propanol) can be found in SI
Figure 6. The strong decrease in the white line intensity in the
Figure 4 shows Fourier transformed k2-weighted Pt L3 EXAFS
data acquired in situ at different temperatures (60 ꢀC, 140 ꢀC,
and 240 ꢀC) under reaction conditions, that is, under a simulta-
neous flow of 2-propanol and O2. These spectra were fitted with
various contributions resulting from Pt atoms in different
chemical environments: (1) PtꢀO(I) due to a chemisorbed O
layer or the presence of an oxide, (2) PtꢀO(II) due to the
presence of adsorbed 2-propanol on Pt, and (3) PtꢀPt for the
metallic Pt component. The different fitting contributions are
illustrated in the inset of Figure 4 for the 60 ꢀC spectrum and
allow the determination of the respective first nearest neighbor
coordination numbers (NPtꢀPt, NPtꢀO(I), NPtꢀO(II)) and dis-
tances (dPtꢀPt, dPtꢀO(I), and dPtꢀO(II)). The fits of the 140 and
240 ꢀC data can be found in SI Figure 3. The coordination
numbers and the corresponding distances are presented as a
function of temperature in Figure 5(a) and (b), respectively. The
distance of the PtꢀO(II) component, 2.53 ( 0.05 Å, was found
to be nearly constant in the fits and is therefore not shown in
Figure 5(b). The horizontal dashed lines refer to the coordina-
tion number and distance after NP reduction in H2 (spectrum
taken at room temperature), which was the starting point before
exposure to the reactants.
The fittingprocedure yields a PtꢀPt coordinationNPtꢀPt of 7.1
( 0.5, with a PtꢀPt distance of 2.74 ( 0.01 Å for the reduced NPs
measured in H2. Upon initial reactant exposure (O2 and
2-propanol) and in the low temperature reaction regime
6731
dx.doi.org/10.1021/ja200178f |J. Am. Chem. Soc. 2011, 133, 6728–6735

Journal of the American Chemical Society
ARTICLE
Figure 6. Schematic representation of the catalyst structure in the as-prepared state, after reduction, and during various stages of the reaction. Reactants
and products are indicated above the illustrations. Possible alternative mechanisms involving a homogeneous PtOx shell (right) and a 2-phase catalyst
composed by PtOx patches coexisting with metallic Pt regions (left) are separated by the dashed lines.
sample exposed to the reactants as compared to that acquired
before NP reduction demonstrates that our NPs are only partially
oxidized under reaction conditions.
principle facilitate NP mobility) for extended periods of time at
higher temperatures (375 ꢀC) than the maximum reaction
temperature (240 ꢀC). Therefore, if NP mobility (and possibly
coarsening) were born to occur, they would likely have taken
place during our 24 h pretreatment in O2.
Our in situ EXAFS and mass spectrometry data revealed the
enhanced activity (low onset reaction temperature) and tem-
perature-dependent selectivity of our micellar Pt NPs for the
partial and total oxidation of 2-propanol. In addition, insight into
the evolution of the oxidation state of the active nanocatalysts
under different reaction conditions could be extracted.
At reaction temperatures above 120 ꢀC, the PtꢀPt contribu-
tion becomes dominant again and the corresponding coordina-
tion number returns to the value obtained for the reduced NPs
(NPtꢀPt ≈ 7.2). This result indicates the decomposition of the
platinum oxide species and the return of the NPs to the metallic
state. The disappearance of the oxide is also reflected in a
decrease of the short PtꢀO(I) coordination number (from ∼2
to ∼1). However, a minor PtꢀO(I) content still remains and
together with the metallic state of the particle, this suggests the
presence of chemisorbed oxygen on the NP surface. The
presence of O on the Pt surface is also reflected by the larger
intensity of the XANES white line measured at 120 ꢀC as
compared to that of the reduced NPs, Figure 3(a). Interestingly,
the PtꢀO(II) contribution vanishes above 100 ꢀC, when 100%
conversion of 2-propanol is achieved. Parallel to the intensity
increase in XANES at temperatures above 200 ꢀC, we also
observe a small decrease in PtꢀPt coordination and correspond-
ing increase in PtꢀO(I) coordination.
Figure 6 schematically displays the proposed reaction mechan-
ism. As demonstrated by the XANES and EXAFS data, the
initially reduced metallic particles immediately partially oxidize
upon exposure to the reactant flow (O2 and 2-propanol) at room
temperature. This effect is evidenced by the decrease in the
number of PtꢀPt bonds and the concomitant increase in the
PtꢀO(I) bonding. The decrease of the PtꢀPt coordination could
also be due to an adsorbate-induced change in the NP shape, with
flatter NPs giving rise to lower coordination numbers. However, a
shape change alone cannot cause such a large reduction in PtꢀPt
coordination numbers as observed here. Moreover, such effect
should also be ruled out here, since the change in the first nearest
neighbor coordination number is observed upon reactant expo-
sure, and O2 is known to cause NP rounding.72,73 The latter effect
should give rise to an increase in PtꢀPt coordination, contrary to
the present observation.76 The presence of chemisorbed oxygen
in combination with the oxide cannot be excluded. However, the
XANES spectra in Figure 3(a) only show the partial oxidation of
the NPs, and PtꢀPt bonding is still observed for both catalysts via
EXAFS, reflecting the presence of a metallic NP core. EXAFS also
reveals the presence of adsorbed 2-propanol on the NP surface,
which indicates that not all Pt sites at the surface are occupied by
oxygen. With the onset of 2-propanol conversion (between 40
and 50 ꢀC), the intensity of the XANES white line increases
slightly, Figure 3(b), which can be attributed to oxygen atoms
occupying surface sites that become available after the desorption
of 2-propanol as acetone. Here, 2-propanol and oxygen are not
competing for active sites, as varying the O2 to propanol ratio
between 3 and 205 was not found to influence the measured
conversion (see SI Figure 7).
’ DISCUSSION
Although the overall structural stability of our Pt nanocatalysts
was evidenced via EXAFS and TEM (NP diameter), small
changes in the NP shape during the reaction (e.g., rounding of
corners) cannot be ruled out, since they could fall within the
error margins of the coordination numbers obtained at different
stages of the reaction. In fact, nanoparticle rounding has been
observed in environmental TEM studies of Au and Pt NPs under
oxygen.72ꢀ75 A complete 2D to 3D shape transformation should
however not have taken place in our sample, since such structural
transformation would give rise to a clear increase in the co-
ordination numbers, which was not observed here. It should be
considered that the NP/support interface might also play a key
role in the stabilization of the flat and small (<1 nm) NPs initially
available on our sample (after H2 reduction). In addition, the
possible mobility of our NPs in the course of the reaction has not
been considered. This assumption is based on the fact that our
NPs were pretreated in O2 (an environment which should in
6732
dx.doi.org/10.1021/ja200178f |J. Am. Chem. Soc. 2011, 133, 6728–6735

Journal of the American Chemical Society
ARTICLE
Consequently, based on the evolution of the XANES white
line intensity in the temperature regime where the partial
oxidation of 2-propanol to acetone takes place and on the
corresponding EXAFS data, we can conclude that PtOx com-
pounds are the catalytically active phase. The large density of
step, edge and corner sites in our small 2D (flat) Pt NPs is likely
to facilitate the dissociation of O2 and the subsequent formation
of surface oxides for high oxygen coverages. The improved
reactivity of metal surfaces with either chemisorbed oxygen or
surface oxides has been previously reported.48 For example,
Wang et al.37 observed that one-dimensional PtO2 stripes formed
at Pt steps react more easily with CO than chemisorbed O on Pt.
Although PtO2 was shown to be the preferential oxide formed on
1 nm sized nanoparticles supported on γ-Al2O3, oxide structures
with intermediate stoichiometry such as Pt3O4 and PtO are also
reported to be thermodynamically stable showing enhanced
chemical reactivity for CO oxidation and methanol decomposi-
tion, respectively.21,35,44,71,77 Alayon et al.38 also observed the
higher activity of defective Pt oxide species on partially oxidized
Pt/Al2O3 catalysts for the oxidation of CO as compared to
metallic Pt. In the former study, dynamic changes in the structure
of the catalysts were observed, with different contents of PtOx
species present under distinct reaction conditions. Furthermore,
Smith and co-workers observed that oxygen strongly bonded to
kink sites on a Pt surface facilitated dehydrogenation reactions.78
Since the dehydrogenation is the key step in the partial oxidation
of 2-propanol, we believe that the high density of low coordi-
nated sites available on our small 2D-shaped particles and their
unique interaction with oxygen are the key features responsible
for the catalytic activity of our micellar Pt NPs.
However, even though based on our EXAFS and XANES data
it is observed that the catalysts active for the selective partial
oxidation of 2-propanol are Pt NPs with a metallic core and an
oxide shell. The specific structure of this oxide (PtO, PtO2 or
other PtOx) cannot be concluded. Since EXAFS does not
provide information on the uniformity of the oxide shell, we
cannot exclude the presence of metallic Pt at the NP’s surface.
Therefore, in addition to a homogeneous oxide layer, a nonuni-
form PtOx layer is also proposed in Figure 6 (left side of the
dashed lines). Interestingly, a decrease in the degree of NP
oxidation is not observed during the selective partial oxidation of
Propanol (50ꢀ70 ꢀC), suggesting a Mars-van Krevelen process,
where the oxygen in the surface oxide lattice, which is consumed
by hydrogen released in the dehydrogenation of the 2-propanol,
is subsequently replenished by dissociated molecular oxygen
from the gas phase. Again, the specific NP structure (small size
and 2D shape) is here held responsible for its enhanced reactivity
with O2 at relatively low temperatures.
oxidation pathway is preferred. Since PtOx species are generally
stable up to 230 ꢀC in vacuum, these results illustrate that the
decomposition of PtOx is governed by the reaction process, and
not by the increasing temperature.13,60 In summary, the active
catalysts for the complete oxidation of 2-propanol are metallic Pt
NPs with chemisorbed oxygen adsorbed on their surfaces coex-
isting with carbonaceous intermediates.
Upon increasing the reaction temperature to 240 ꢀC, the
increase in the XANES white line and the PtꢀO(I) coordination
number were observed. These trends might be attributed to a
faster and increased desorption of carbonaceous species from the
NP surface, leading to a smaller effective coverage. As a result,
more surface sites become available for the dissociation of oxygen
and subsequent Pt oxide formation. However, the catalytic
activity did not decrease upon the formation of this thin oxide
layer, as can be seen in Figure 2. Consequently, in this high
temperature regime of the complete oxidation, the active cata-
lysts consist of Pt NPs with a very thin surface oxide layer, likely
inhomogeneous in nature with exposed metallic Pt species.
No decrease in the catalytic activity of the Pt NPs was observed
under our experimental conditions (O2-rich flow) during an
extended period of time (26 h), indicating that carbon monoxide
intermediates are effectively oxidized to CO2 in the temperature
regime corresponding to the total oxidation of propanol. As a
result, our small 2D-shaped NPs are not only good catalysts in
terms of their low onset reaction temperature, but also with
respect to their resistance to poisoning.
Summarizing, our in situ investigation of the structure and
chemical composition of nanosized Pt catalysts at work
(operando conditions) provides valuable insight on the funda-
mental mechanisms that govern the catalytic chemistry of
oxidation reactions. In particular, it highlights the important role
played by surface oxides and chemisorbed oxygen species.
’ CONCLUSIONS
The evolution of the structure and chemical composition of
small two-dimensional Pt NPs supported on γ-Al2O3 has been
followed in situ during the oxidation of 2-propanol. It is shown
that the catalysts undergo significant chemical and structural
changes in the course of the reaction. Platinum oxide species
were found to be present on the NP surface during the partial
oxidation of Propanol to acetone, and suggested to play an active
role in the reactivity observed. Due to the large amount of step,
edge and corner sites available on our small NPs, the dissociation
of O2 and subsequent formation of surface oxides appears to be
favorable, resulting in a low reaction onset temperature
(∼50 ꢀC). In the complete oxidation regime, the active catalysts
are initially metallic NPs with chemisorbed oxygen species on
their surface. However, with increasing temperature within the
complete oxidation reaction regime, catalyst reoxidation was
observed. Such effect might be assigned to a higher availability
of reactive sites (step/kink/corner atoms) able to dissociate O2
upon desorption of intermediate passivating carbonaceous spe-
cies. The decomposition of PtOx in the intermediate reaction
regime, where partial and total oxidation processes coexist, is
attributed to the reducing effect of intermediate carbonaceous
species available at the onset of the complete oxidation, as well as
to the competition of CO and O2 for adsorption sites on the NP
surface.
Above 70 ꢀC, a decrease of the XANES intensity, Figure 3(b),
along with a decrease in PtꢀO coordination, Figure 5(a), is
observed. These changes reflect the partial decomposition of the
surface oxide. Interestingly, the onset of the Pt oxide decom-
position runs parallel to the decrease in the reaction selectivity for
acetone. The disappearance of the PtOx species and return to the
metallic state observed by EXAFS at 120 ꢀC, Figure 5(a),
coincides with the minimum in the XANES white line intensity,
and with the transition from partial to selective complete
oxidation. Our data suggest that the activation of the CꢀC bond
(scission) and the subsequent presence of intermediate carbo-
naceous compounds, such as CO, on the NP surface are
responsible for the reduction of the NPs.33 In fact, the metallic
state is reinstated at the temperature where the complete
Altogether, our research provides valuable insight into the
mechanisms underlying the oxidation of 2-propanol over Pt NPs,
6733
dx.doi.org/10.1021/ja200178f |J. Am. Chem. Soc. 2011, 133, 6728–6735

Journal of the American Chemical Society
ARTICLE
featuring marked structureꢀreactivity correlations, and empha-
sizing the decisive role played by the NP structure (size and
shape) on the formation and stabilization of surface oxides, which
have been shown to be directly involved in the present catalytic
oxidation process.
(13) Ono, L. K.; Yuan, B.; Heinrich, H.; Roldan Cuenya, B. J. Phys.
Chem. C 2010, 114, 22119.
(14) Mostafa, S.; Behafarid, F.; Croy, J. R.; Ono, L. K.; Li, L.; Yang, J.;
Frenkel, A. I.; Roldan Cuenya, B. J. Am. Chem. Soc. 2010, 132, 15714.
(15) Karim, A. M.; Prasad, V.; Mpourmpakis, G.; Lonergan, W. W.;
Frenkel, A. I.; Chen, J. G.; Vlachos, D. G. J. Am. Chem. Soc. 2009,
131, 12230.
(16) Croy, J. R.; Mostafa, S.; Liu, J.; Sohn, Y.; Heinrich, H.; Roldan
Cuenya, B. Catal. Lett. 2007, 119, 209.
(17) Narayanan, R.; El-Sayed, M. A. J. Phys. Chem. B 2005,
109, 12663.
(18) Oudenhuijzen, M. K.; van Bokhoven, J. A.; Ramaker, D. E.;
Koningsberger, D. C. J. Phys. Chem. B 2004, 108, 20247.
(19) Ono, L. K.; Roldan Cuenya, B. Catal. Lett. 2007, 113, 86.
(20) Croy, J. R.; Mostafa, S.; Liu, J.; Sohn, Y.-h.; Roldan Cuenya, B.
Catal. Lett. 2007, 118, 1.
(21) Croy, J. R.; Mostafa, S.; Hickman, L.; Heinrich, H.; Roldan
Cuenya, B. Appl. Catal., A 2008, 350, 207.
(22) Mostafa, S.; Croy, J. R.; Heinrich, H.; Roldan Cuenya, B. Appl.
Catal., A 2009, 366, 353.
(23) Roldan Cuenya, B.; Croy, J. R.; Mostafa, S.; Behafarid, F.; Li, L.;
Zhang, Z.; Yang, J. C.; Wang, Q.; Frenkel, A. I. J. Am. Chem. Soc. 2010,
132, 8747.
’ ASSOCIATED CONTENT
S
Supporting Information. (i) A description of the meth-
b
od employed to determine the average NP shape as well as to
estimate the degree of structural and morphological homogene-
ity of the NPs, (ii) additional details on the fit parameters and
results extracted from EXAFS modeling, (iii) XAFS spectra of the
NP sample during different stages of the 2-propanol oxidation
together with reference data from bulk metallic Pt and PtO2,
(iv) representative EXAFS data of the NP sample under different
reaction conditions in k-space and r-space (including fits),
(v) data of the conversion of 2-propanol as a function of the
O2 to 2-propanol ratio. This material is available free of charge via
the Internet at http://pubs.acs.org.
(24) Mallat, T.; Baiker, A. Chem. Rev. 2004, 104, 3037.
(25) Derwent, R. G.; Pearson, J. K. Environ. Technol. 1997, 18, 1029.
(26) Markusse, A. P.; Kuster, B. F. M.; Koningsberger, D. C.; Marin,
G. B. Catal. Lett. 1998, 55, 141.
(27) Nicoletti, J. W.; Whitesides, G. M. J. Phys. Chem. 1989, 93, 759.
(28) Schauermann, S.; Hoffmann, J.; Johanek, V.; Hartmann, J.;
Libuda, J. Phys. Chem. Chem. Phys. 2002, 4, 3909.
(29) Augustine, R. L.; Doyle, L. K. J. Catal. 1993, 141, 58.
(30) Keresszegi, C.; B€urgi, T.; Mallat, T.; Baiker, A. J. Catal. 2002,
211, 244.
’ AUTHOR INFORMATION
Corresponding Author
roldan@physics.ucf.edu; anatoly.frenkel@yu.edu
’ ACKNOWLEDGMENT
We are grateful to Dr. J. R. Croy, E. Zhou, and C. Frye (UCF)
for assistance in sample preparation, to Dr. Relja Vasic (BNL) for
his help during some of the XAFS measurements, and to F.
Behafarid for NP shape modeling. Reactivity studies were
supported by DE-FG02-08ER15995. Electron microscopy and
synchrotron studies were funded by DOE BES (DE-FG02-
03ER15476, DE-FG02-05ER15688). NFCF at the University
of Pittsburgh is acknowledged for the use of JEM 2100F. Use of
NSLS was supported by DOE (DE-AC02-98CH10866). B.R.C.
wouldliketothankProf. H. J. Freundforkindlyhostinghersabbatical
research stay at the Fritz-Haber-Institute (Berlin, Germany), where
part of this manuscript was written.
(31) Hartmann, N.; Imbihl, R.; Vogel, W. Catal. Lett. 1994, 28, 373.
(32) Olsson, L.; Fridell, E. J. Catal. 2002, 210, 340.
€
(33) Carlsson, P.-A.; Osterlund, L.; Thorm€ahlen, P.; Palmqvist, A.;
Fridell, E.; Jansson, J.; Skoglundh, M. J. Catal. 2004, 226, 422.
(34) Dam, V. A. T.; de Bruijn, F. A. J. Electrochem. Soc. 2007,
154, B494.
(35) Seriani, N.; Jin, Z.; Pompe, W.; Ciacchi, L. C. Phys. Rev. B 2007,
76, 155421.
(36) Chen, M. S.; Cai, Y.; Yan, Z.; Gath, K. K.; Axnanda, S.;
Goodman, D. W. Surf. Sci. 2007, 601, 5326.
(37) Wang, J. G.; Li, W. X.; Borg, M.; Gustafson, J.; Mikkelsen, A.
Pedersen, T. M.; Lundgren, E.; Weissenrieder, J.; Klikovits, J.; Schmid, M.;
Hammer, B.; Andersen, J. N. Phys. Rev. Lett. 2005, 95, 256102.
(38) Alayon, E. M. C.; Singh, J.; Nachtegaal, M.; Harfouche, M.;
van Bokhoven, J. A. J. Catal. 2009, 263, 228.
’ REFERENCES
(1) Roldan Cuenya, B. Thin Solid Films 2010, 518, 3127.
(2) Bultel, L.; Roux, C.; Siebert, E.; Vernoux, P.; Gaillard, F. Solid
State Ionics 2004, 166, 183.
(39) Andras, S.; Michael, A. H.; Yates, J. T., Jr. J. Chem. Phys. 1992,
96, 6191.
(40) John, L. G.; Edward, B. K. J. Chem. Phys. 1983, 78, 963.
(41) Ackermann, M. D.; Pedersen, T. M.; Hendriksen, B. L. M.;
Robach, O.; Bobaru, S. C.; Popa, I.; Quiros, C.; Kim, H.; Hammer, B.;
Ferrer, S.; Frenken, J. W. M. Phys. Rev. Lett. 2002, 89, 046101.
(42) Mallens, E. P. J.; Hoebink, J. H. B. J.; Marin, G. B. Catal. Lett.
1995, 33, 291.
(3) Vernoux, P.; Gaillard, F.; Bultel, L.; Siebert, E.; Primet, M.
J. Catal. 2002, 208, 412.
(4) Vayenas, C. G.; Bebelis, S.; Ladas, S. Nature 1990, 343, 625.
(5) Singh, J.; Alayon, E.; Tromp, M.; Safonova, O.; Glatzel, P.;
Nachtegaal, M.; Frahm, R.; van Bokhoven, J. Angew. Chem. 2008,
120, 9400.
(43) Ackermann, M. D.; Pedersen, T. M.; Hendriksen, B. L. M.;
Robach, O.; Bobaru, S. C.; Popa, I.; Quiros, C.; Kim, H.; Hammer, B.;
Ferrer, S.; Frenken, J. W. M. Phys. Rev. Lett. 2005, 95, 255505.
(44) Croy, J. R.; Mostafa, S.; Heinrich, H.; Roldan Cuenya, B. Catal.
Lett. 2009, 131, 21.
(6) Singh, J.; Nachtegaal, M.; Alayon, E. M. C.; Stotzel, J.;
van Bokhoven, J. A. Chem. Cat. Chem. 2010, 2, 653.
(7) Singh, J.; van Bokhoven, J. A. Catal. Today 2010, 155, 199.
(8) Xu, Y.; Shelton, W. A.; Schneider, W. F. J. Phys. Chem. A 2006,
110, 5839.
(45) Kulkarni, D.; Wachs, I. E. Appl. Catal., A 2002, 237, 121.
(46) Hawkins, J. M.; Weaver, J. F.; Asthagiri, A. Phys. Rev. B 2009,
79, 125434.
(47) Gu, Z.; Balbuena, P. B. J. Phys. Chem. C 2007, 111, 9877.
(48) Hendriksen, B. L. M.; Ackermann, M. D.; van Rijn, R.; Stoltz, D.;
Popa, I.; Balmes, O.; Resta, A.; Wermeille, D.; Felici, R.; Ferrer, S.;
FrenkenJoost, W. M. Nat Chem. 2010, 2, 730.
(9) Ellinger, C.; Stierle, A.; Robinson, I. K.; Nefedov, A.; Dosch, H.
J. Phys.: Condens. Matter 2008, 20, 184013.
(10) Tao, F.; Salmeron, M. Science 2011, 331, 171.
(11) Pozdnyakova, O.; Teschner, D.; Wootsch, A.; Krohnert, J.;
Steinhauer, B.; Sauer, H.; Toth, L.; Jentoft, F. C.; Knop-Gericke, A.;
Paal, Z.; Schlogl, R. J. Catal. 2006, 237, 1.
(12) Narayanan, R.; El-Sayed, M. A. Nano Lett. 2004, 4, 1343.
6734
dx.doi.org/10.1021/ja200178f |J. Am. Chem. Soc. 2011, 133, 6728–6735

Journal of the American Chemical Society
ARTICLE
(49) Mainardi, D. S.; Calvo, S. R.; Jansen, A. P. J.; Lukkien, J. J.;
Balbuena, P. B. Chem. Phys. Lett. 2003, 382, 553.
(50) Gu, Z.; Balbuena, P. B. J. Phys. Chem. C 2007, 111, 17388.
(51) Ravel, B.; Newville, M. J. Synchrotron Rad. 2005, 12, 537.
(52) Newville, M. J. Synchrotron Rad. 2001, 8, 322.
(53) Newville, M.; Livins, P.; Yacoby, Y.; Rehr, J. J.; Stern, E. A. Phys.
Rev. B 1993, 47, 14126.
(54) Setthapun, W.; Williams, W. D.; Kim, S. M.; Feng, H.; Elam, J. W.;
Rabuffetti, F. A.; Poeppelmeier, K. R.; Stair, P. C.; Stach, E. A.; Ribeiro,
F. H.; Miller, J. T.; Marshall, C. L. J. Phys. Chem. C 2010, 114, 9758.
(55) Zabinsky, S. I.; Rehr, J. J.; Ankudinov, A.; Albers, R. C.; Eller, M. J.
Phys. Rev. B 1995, 52, 2995.
(56) Zhang, J. H.; Zhou, X. L.; Wang, J. A. J. Mol. Catal. A 2006,
247, 222.
(57) Burgos, N.; Paulis, M.; Mirari Antxustegi, M.; Montes, M. Appl.
Catal., B 2002, 38, 251.
(58) Yoshida, H.; Nonoyama, S.; Yazawa, Y.; Hattori, T. Phys. Scr.
2005, 2005, 813.
(59) Christensen, S. T.; Elam, J. W.; Rabuffetti, F. A.; Ma, Q.;
Weigand, S. J.; Lee, B.; Seifert, S.; Stair, P. C.; Poeppelmeier, K. R.;
Hersam, M. C.; Bedzyk, M. J. Small 2009, 5, 750.
(60) Kolobov, A. V.; Wilhelm, F.; Rogalev, A.; Shima, T.; Tominaga, J.
Appl. Phys. Lett. 2005, 86, 121909.
(61) Guo, N.; Fingland, B. R.; Williams, W. D.; Kispersky, V. F.; Jelic, J.;
Delgass, W. N.; Ribeiro, F. H.; Meyer, R. J.; Miller, J. T. Phys. Chem. Chem.
Phys. 2010, 12, 5678.
(62) Ramaker, D. E.; Koningsberger, D. C. Phys. Chem. Chem. Phys.
2010, 12, 5514.
(63) Ramaker, D. E.; van Dorssen, G. E.; Mojet, B. L.; Koningsberger,
D. C. Top. Catal. 2000, 10, 157.
(64) Boyanov, B. I.; Morrison, T. I. J. Phys. Chem. 1996, 100, 16318.
(65) Safonova, O. V.; Tromp, M.; van Bokhoven, J. A.; de Groot,
F. M. F.; Evans, J.; Glatzel, P. J. Phys. Chem. B 2006, 110, 16162.
(66) Sanchez, S. I.; Menard, L. D.; Bram, A.; Kang, J. H.; Small, M. W.;
Nuzzo, R. G.; Frenkel, A. I. J. Am. Chem. Soc. 2009, 131, 7040.
(67) Jacob, T.; Muller, R. P.; Goddard, W. A. J. Phys. Chem. B 2003,
107, 9465.
(68) Alcala, R.; Greeley, J.; Mavrikakis, M.; Dumesic, J. A. J. Chem.
Phys. 2002, 116, 8973.
(69) Tarmyshov, K. B.; Muller-Plathe, F. J. Chem. Phys. 2007,
126, 074702.
(70) Greeley, J.; Mavrikakis, M. J. Am. Chem. Soc. 2002, 124, 7193.
(71) Seriani, N.; Pompe, W.; Ciacchi, L. C. J. Phys. Chem. B 2006,
110, 14860.
(72) Giorgio, S.; Cabie, M.; Henry, C. R. Gold Bull. 2008, 41, 167.
(73) Giorgio, S.; Sao Joao, S.; Nitsche, S.; Chaudenson, D.; Sitja, G.;
Henry, C. R. Ultramicroscopy 2006, 106, 503.
(74) Kishita, K.; Sakai, H.; Tanaka, H.; Saka, H.; Kuroda, K.;
Sakamoto, M.; Watabe, A.; Kamino, T. J. Electron Microsc. 2009, 58, 331.
(75) Ramachandran, A. S.; Anderson, S. L.; Datye, A. K. Ultramicro-
scopy 1993, 51, 282.
(76) Cabiꢀe, M.; Giorgio, S.; Henry, C. R.; Axet, M. R.; Philippot, K.;
Chaudret, B. J. Phys. Chem. C 2010, 114, 2160.
(77) Wang, C.-B.; Lin, H.-K.; Hsu, S.-N.; Huang, T.-H.; Chiu, H.-C.
J. Mol. Catal. A 2002, 188, 201.
(78) Smith, C. E.; Biberian, J. P.; Somorjai, G. A. J. Catal. 1979,
57, 426.
6735
dx.doi.org/10.1021/ja200178f |J. Am. Chem. Soc. 2011, 133, 6728–6735