Selective amplification of 34;O bond hydrogenation on Pt/TiO2: Catalytic reaction and sum-frequency generation vibrational spectroscopy studies of crotonaldehyde hydrogenation

Article abstract of DOI:10.1002/anie.201400081

The hydrogenation of crotonaldehyde in the presence of supported platinum nanoparticles was used to determine how the interaction between the metal particles and their support can control catalytic performance. Using gas-phase catalytic reaction studies and in situ sum-frequency generation vibrational spectroscopy (SFG) to study Pt/TiO₂ and Pt/SiO₂ catalysts, a unique reaction pathway was identified for Pt/TiO₂, which selectively produces alcohol products. The catalytic and spectroscopic data obtained for the Pt/SiO₂ catalyst shows that SiO₂ has no active role in this reaction. SFG spectra obtained for the Pt/TiO₂ catalyst indicate the presence of a crotyl-oxy surface intermediate. By adsorption through the aldehyde oxygen atom to an O-vacancy site on the TiO ₂ surface, the 34;O bond of crotonaldehyde is activated, by charge transfer, for hydrogenation. This intermediate reacts with spillover H provided by the Pt to produce crotyl alcohol. Surface activity: Sum-frequency generation vibrational spectroscopy was employed to determine the role of oxide supports in platinum-catalyzed crotonaldehyde hydrogenation. A unique species was observed on the Pt/TiO₂ catalysts, thus explaining the dramatic difference in catalytic activity and selectivity when compared to those of Pt/SiO₂.

Full text of DOI:10.1002/anie.201400081

Angewandte
Chemie
DOI: 10.1002/anie.201400081
Heterogeneous Catalysis
=
Selective Amplification of C O Bond Hydrogenation on Pt/TiO2:
Catalytic Reaction and Sum-Frequency Generation Vibrational Spec-
troscopy Studies of Crotonaldehyde Hydrogenation**
Griffin Kennedy, L. Robert Baker, and Gabor A. Somorjai*
Abstract: The hydrogenation of crotonaldehyde in the pres-
ence of supported platinum nanoparticles was used to deter-
mine how the interaction between the metal particles and their
support can control catalytic performance. Using gas-phase
catalytic reaction studies and in situ sum-frequency generation
vibrational spectroscopy (SFG) to study Pt/TiO2 and Pt/SiO2
catalysts, a unique reaction pathway was identified for Pt/TiO2,
which selectively produces alcohol products. The catalytic and
spectroscopic data obtained for the Pt/SiO2 catalyst shows that
SiO2 has no active role in this reaction. SFG spectra obtained
for the Pt/TiO2 catalyst indicate the presence of a crotyl-oxy
surface intermediate. By adsorption through the aldehyde
example, Ti4+ is reduced to Ti3+.[11,12] These electron-rich sites
interact with adsorbate molecules and activate them toward
a certain reaction pathway.[4] Studies of various metal oxide
films deposited on metal surfaces have shown that the oxide
[7,13]
=
plays a key role in activating C O bonds.
The activity of
=
catalysts for C O bond hydrogenation in these studies scaled
with the Lewis acidity of the oxide, thus indicating the critical
role charge transfer plays in the catalytic process. The
implication of charge transfer between the support and
adsorbate draws a comparison to acid-base catalysis in
which the generation of charged intermediates is the crucial
step in determining the activity and selectivity of a reac-
tion.[14,15] In this study it is proposed that intermediates are
generated at O-vacancy sites on the TiO2 surface of Pt/TiO2
catalysts and that these intermediates are active for the
=
oxygen atom to an O-vacancy site on the TiO2 surface, the C O
bond of crotonaldehyde is activated, by charge transfer, for
hydrogenation. This intermediate reacts with spillover H
provided by the Pt to produce crotyl alcohol.
=
hydrogenation of the C O group in crotonaldehyde.
Crotonaldehyde hydrogenation (Scheme 1) and hydro-
genation of other unsaturated aldehydes have been studied
extensively because these reactions have three distinct,
competing reaction pathways: decarbonylation to produce
E
nhancement of a metal catalystꢀs reactivity and selectivity
by a seemingly inert metal oxide support has been docu-
mented in the literature for numerous gas-phase reactions,
such as methanol oxidation,[1] CO oxidation,[2] partial hydro-
genation of organic molecules,[3–6] CO and CO2 hydrogena-
tion,[7] and hydroformylation,[8] on a variety of metals and
supporting oxide systems. In some cases, by combining
a metal with a metal oxide support the activity of the
resulting system can far surpass what would be expected from
a simple combination of the two materials. Among a number
of explanations for these enhancements, depending on the
reaction and catalyst materials utilized, the proposed mech-
anism of particular interest to this work involves a direct
interaction between a reactant molecule and the oxide
support, and the spillover of a second reactant from the
metal.[9,10] In a reducing atmosphere at elevated temperatures,
and in the presence of Pt, metal oxides undergo changes in the
surface oxidation state by the formation of O vacancies. For
=
CO and propylene, hydrogenation of the C C bond to yield
Scheme 1. Reaction scheme showing all product pathways observed in
the course of reaction studies. Two sets of arrows represent the activity
for each pathway for Pt/SiO2 and Pt/TiO2. Equal sized arrows indicate
no influence of support. Different length arrows represent relative
difference between supports for that pathway.
[*] G. Kennedy, Dr. L. R. Baker, Prof. G. A. Somorjai
Department of Chemistry, University of California
Berkeley, CA 94720 (USA)
=
the saturated aldehyde, and the hydrogenation of the C O
bond to yield the unsaturated alcohol.[5,16–18] Selective hydro-
and
Materials Sciences and Chemical Sciences Divisions
Lawrence Berkeley National Laboratory
Berkeley, CA 94720 (USA)
genation of the carbonyl bond in such systems without
=
hydrogenation of the C C bond is of great interest since
unsaturated alcohols are useful chemical intermediates for
E-mail: somorjai@berkeley.edu
pharmaceuticals and fragrances. However, even though the
[**] This work was funded by the Director, Office of Science, Office of
Basic Energy Sciences, Division of Chemical Sciences, Geological,
and Biosciences of the U.S. Department of Energy under Contract
No. DE-AC02-05CH11231.
À1
=
hydrogenation of the C O bond has a 40 kJmol lower
activation barrier,[19] selectively producing the unsaturated
alcohol at high rates can be a challenge since hydrogenation
=
of the C C bond is thermodynamically favored by
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201400081.
35 kJmolÀ1.[20] Previous work by Kliewer et al.[19] on croton-
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> aldehyde hydrogenation using platinum (111) and (100)
single crystals shows that the dominant pathways at the
=
temperature used in the present work are C C bond hydro-
genation to form butyraldehyde, and decarbonylation to form
propylene, regardless of the crystal face. Based on these
=
results, a C O bond activating support could drastically alter
the activity and selectivity of crotonaldehyde hydrogenation
on Pt.
In this study Pt nanoparticle films, deposited onto SiO2
and TiO2 substrates, serve as model catalysts for the hydro-
genation of crotonaldehyde (Figure 1). This reaction is well
Figure 2. TOFs for the four observed products are reported for both
Pt/SiO2 and Pt/TiO2 catalysts. Similar rates of propylene production
are observed for both catalysts. Butanol and crotyl alcohol production
is only observed on Pt/TiO2 catalysts.
each observed product for Pt/SiO2 at 1208C. Only two
products are produced by this catalyst under these reaction
conditions: the decarbonylation product propylene (32% of
=
total products), and the C C bond hydrogenation product
=
butyraldehyde (68% of total products). Neither of the C O
hydrogenation products, 1-butanol and crotyl alcohol, are
detected. These results are similar to those obtained by
Kliewer et al.[19] when using Pt (111) and (100) single crystals:
propylene and butyraldehyde were also the two dominant
products at 1208C along with the combined alcohol products
making up less than 5% of all products on both crystal faces.
It is therefore confirmed that SiO2 has no impact on the
catalytic reactions occurring on the Pt nanoparticle surface.
In contrast, the Pt/TiO2 catalyst shows dramatically
different performance (Figure 2). Under the same reaction
conditions for crotonaldehyde hydrogenation, the Pt/TiO2
catalysts showed a three- to fourfold increase in total activity,
mainly from high rates of crotyl alcohol and butanol
formation. The combined TOF toward the alcohol products
is greater than that for either the propylene or butyraldehyde.
The selectivity for the alcohol products is zero for Pt/SiO2, but
is 52% for Pt/TiO2. Most importantly this change in
selectivity is not a result of the loss of activity toward the
other pathways, but is due to additional activity, thus
indicating the presence of a new catalytic active site when
Pt and TiO2 are brought into atomic contact, a site which
selectively produces alcohol products. The production of
butanol is believed to be from a concerted reaction of both
Figure 1. Schematic of samples prepared for catalytic tests (A) and
SFG studies (B). The metal oxide films are deposited on the substrate
(SiO2 wafer or Quartz window) by electron beam evaporation. Pt
nanoparticles are deposited onto the supporting material by Lang-
muir–Blodgett deposition. All samples are subsequently treated with
UV irradiation in air to remove the capping ligands from the Pt
nanoparticles.
suited to probe how the SiO2 and TiO2 supports impact, or do
not impact, the selectivity and activity for the various
products. By coupling catalytic reaction studies with sum-
frequency generation (SFG) vibrational spectroscopy,
a highly surface-sensitive technique, we gain information at
the molecular level about the surface species present on our
catalysts under the reaction conditions,[21–23] and can correlate
those species to the products formed with each catalyst. All
turnover frequencies (TOFs) reported here are normalized to
the number of Pt surface sites, as determined by ethylene
hydrogenation, to account for any differences in catalyst size
and metal loading. Full experimental details are given in
Supporting Information.
A Pt/SiO2 catalyst is used as a reference state under the
assumption that SiO2 has no active role in the catalysis.
Inspection of our kinetic results and comparison to previous
work, in which crotonaldehyde hydrogenation was carried out
on Pt single crystals, give strong evidence that this assumption
is valid. Catalytic tests were carried out in a batch-mode gas-
phase reactor, and the crotonaldehyde conversion was kept
below 15%. Figure 2 shows the TOF for the formation of
=
=
the C O and C C bonds of species adsorbed at the interface
between TiO2 and Pt, the alternative being that butanol is
formed as a secondary product from the hydrogenation of
butyraldehyde or crotyl alcohol. In our case butanol is
observed very early on and the rate of its formation does
not depend on the concentration of either of the primary
products, thus supporting a concerted mechanism on Pt/TiO2.
Additionally if one were to assume a pseudo-first-order rate
equation for the formation of the primary products (butyr-
aldehyde and crotyl alcohol) from crotonaldehyde and for the
formation of butanol from these primary products (as
determined in Figure 2 in the Supporting Information), the
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amount of butanol produced as a secondary product would be
below the detection limit of our apparatus. This data gives
additional support to the conclusion that butanol is formed by
a concerted process. Further catalytic tests were performed
using the primary products (crotyl alcohol and butyralde-
hyde) as reactants and are presented in the Supporting
Information. These tests help to further elucidate the
mechanism we propose.
increases the number of O-vacancy sites. The nonresonant
component of the SFG spectra is largely the result of free-
carrier motion at the surface of a sample. As more O vacan-
cies are formed on the TiO2 surface the free-carrier concen-
tration increases, as evidenced by increased surface conduc-
tivity,[25] and subsequently the nonresonant component of the
sum-frequency signal is larger. This increased signal serves as
a marker for the chemical and electronic structure of the
oxide surface and supports our hypothesis that O vacancies
are the active catalytic sites. The increased nonresonant
contribution can serve to elevate the baseline level in SFG
spectra and also enhance the intensity of resonant features
because of the relationship between the nonresonant and
resonant components of c(2) and the SFG intensity. The
resonant features observed are out of
To better understand the mechanism by which TiO2
=
enhances the rate of C O bond hydrogenation SFG vibra-
tional spectroscopy is employed to determine surface reaction
intermediates. Figure 3 shows the SFG spectra for Pt/SiO2 and
Pt/TiO2 under hydrogenation reaction conditions for croton-
aldehyde. The proposed adsorption geometries and reaction
phase with this nonresonant back-
ground and thus appear as negative
peaks.
The reported spectrum in Fig-
ure 3B is remarkably similar to the
same region in the gas-phase IR
spectrum for crotyl alcohol, and is
indicative of a crotyl-oxy intermedi-
ate bound to the TiO2 surface through
the aldehyde oxygen atom. The fea-
tures at 2880 and 2930 cmÀ1 are
assigned to the CH3 symmetric and
asymmetric stretching modes, respec-
tively.[26] The stretch at 2965 cmÀ1 is
assigned to the CH2 asymmetric
stretching mode of the O-bound
carbon atom.[27] Comparison of the
Pt/TiO2 spectrum with that of TiO2
alone (see Figure 3 in the Supporting
Information) shows that the aldehyde
stretch disappears and a CH2 stretch
appears in the presence of Pt, thus
indicating hydrogenation of the car-
Figure 3. SFG spectra for A) Pt/SiO2 and B) Pt/TiO2 under reaction conditions of 1 Torr crotonalde-
hyde, 100 Torr hydrogen, and 669 Torr argon at a catalyst temperature of 1208C. Assignment of
these spectra, explained in the text, yields the accompanying surface intermediates.
intermediates are also shown. The Pt/SiO2 spectrum shows
two features at 2860 and 2915 cmÀ1, which are consistent with
previous work from Kliewer et al. on Pt single-crystal
surfaces.[19] The peak at 2915 cmÀ1 is assigned to a CH3 stretch
and the peak at 2860 cmÀ1 is assigned to an aldehyde CH
stretch. It has been shown previously that aldehyde stretches,
usually near 2800 cmÀ1, can be shifted into this range when
adsorbed onto a surface.[24] This assignment is in agreement
with the proposed h2 adsorption mode, which is believed to be
the intermediate responsible for the formation of butyralde-
hyde. The clear similarities between the SFG spectra taken on
the Pt/SiO2 and Pt (111) single crystal further support the
claim that the SiO2 support plays no active role in the
hydrogenation of crotonaldehyde on Pt.
The spectrum of Pt/TiO2 (Figure 3B) shows several
intense negative features at 2880, 2930, and 2965 cmÀ1. Both
the strength and sign of these peaks are due to the increased
nonresonant component of the signal from Pt/TiO2 in the
presence of hydrogen. Hydrogen spillover from Pt to TiO2
serves to reduce the surface of the TiO2 to TiO2–x, and in turn
makes it more metallic. Additionally, this surface reduction
bonyl carbon atom by H spillover from the Pt to the TiO2. The
result of this initial hydrogenation step is a crotyl-oxy surface
intermediate (Figure 3B), which is the precursor to the crotyl
alcohol product.
As stated previously, it is believed that the Ti3+ atoms at
O-vacancy sites bind strongly to the aldehyde oxygen atom
and activate the aldehyde carbon atom for hydrogenation by
a charge-transfer process, thus drawing an analogy to acid-
base catalysis. The charge-transfer process described previ-
ously,[4] wherein the excess electron at the Ti3+ site forms
À
a covalent Ti O bond with the aldehyde oxygen atom, results
=
in a charge on the aldehyde carbon atom as the C O bond
À
becomes a C O s bond. This charged carbon atom is highly
reactive and quickly reacts with the atomic hydrogen present
on the surface. In the previous case of furfural, the adsorption
of the aldehyde group at this Ti3+ site resulted in an energy
decrease of the system by 1.35 eV as determined by DFT
calculations. Addition of the hydrogen at the carbonyl carbon
atom lowered the energy of the system by an additional
0.2 eV. The adsorption and initial single hydrogen addition
are believed to be very fast and result in a fairly stable surface
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species, which we observe by SFG vibrational spectroscopy.
The subsequent hydrogen addition at the aldehyde oxygen
À
atom to form an O H bond and adsorption of a new reactant
molecule at this site would likely provide the energy required
to desorb the molecule as a product. In light of this and the
reaction data previously presented, which clearly demon-
strate that the change in selectivity of the Pt/TiO2 catalyst is
due to additional activity toward the alcohol products relative
to that of Pt/SiO2, we are confident that the SFG spectrum
observed for Pt/TiO2 is that of a reactive surface species.
It has been shown for crotonaldehyde hydrogenation that
the support for Pt particles has a substantial impact on the
selectivity and activity of a catalyst. Pt particles on SiO2 show
few, if any, differences in both their catalytic selectivity and
observed surface intermediates when compared to Pt single
crystals, thus confirming that SiO2 plays no active role in the
catalytic reaction. When Pt nanoparticles are supported on
TiO2, substantial changes in the catalytic activity and selec-
tivity are observed along with accompanying changes to the
SFG spectrum. The Pt/TiO2 catalyst has nearly four times the
activity of that of Pt/SiO2 and is over 50% selective toward
the alcohol products. SFG vibrational spectroscopy was
utilized to probe the molecular species present on the surface
of these catalysts under the reaction conditions. The spectrum
observed for Pt/SiO2 is remarkably similar to that obtained
for a Pt (111) single crystal under the same reaction
conditions, and leads to the same assignment of a species
which is an intermediate for the formation of butyraldehyde.
Analysis of the SFG spectra for TiO2 with Pt leads to the
conclusion that crotonaldehyde binds to the TiO2 surface
through O-vacancy sites, thus activating the molecule for
hydrogen addition at the aldehyde carbon atom. A crotyl-oxy
species is observed on the TiO2 surface, and is the inter-
mediate for the formation of crotyl alcohol.
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Received: January 4, 2014
Published online: February 19, 2014
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Keywords: alcohols · heterogeneous catalysis · hydrogenation ·
platinum · vibrational spectroscopy
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