Shape-selective enantioselective hydrogenation on Pt nanoparticles

Article abstract of DOI:10.1021/ja9043328

The structure sensitivity of enantioselective hydrogenations on chirally modified metals was investigated using Pt nanoparticles of different shapes. All three samples had an average particle size of 10 nm, but the fraction of dominantly cubic, cubooctahedral, and octahedral particles varied with decreasing {100} and increasing {111} faces in the same order. In the absence of chiral modifier the hydrogenation of ethyl pyruvate was independent of the shape of the Pt nanoparticles; variation of the specific reaction rates did not exceed the experimental error on all self-prepared catalysts and on a commercial Pt/Al₂O₃ used as reference. Addition of cinchonidine or quinine induced a significant rate enhancement by a factor of 4-15, and the rate was always higher with quinine. Also, 72-92% ees were achieved, and the reaction was shape selective: both the rate and the ee increased with increasing Pt{111}/Pt{100} ratio. A similar correlation in the hydrogenation of ketopantolactone confirmed that decarbonylation or aldol-type side reactions of ethyl pyruvate were not the reason for structure sensitivity. A combined catalytic and theoretical study revealed that the probable origin of the particle shape dependency of enantioselective hydrogenation is the adsorption behavior of the cinchona alkaloid. DFT studies of cinchonidine interacting with Pt(100) and Pt(111) terraces indicated a remarkably stronger interaction on the former crystallographic face by ca. 155 kJ/mol. The higher adsorption strength on Pt(100) was corroborated experimentally by the faster hydrogenation of the homoaromatic ring of the alkaloid, which fragment interacts the strongest with Pt during its adsorption. Thus, an ideal catalyst for the hydrogenation of activated ketones contains dominantly Pt{111} terraces, which crystallographic face is more active and affords higher enantioselectivity, combined with the higher stability of the modifier.

Full text of DOI:10.1021/ja9043328

Published on Web 07/30/2009
Shape-Selective Enantioselective Hydrogenation on Pt
Nanoparticles
Erik Schmidt, Angelo Vargas, Tamas Mallat, and Alfons Baiker*
Department of Chemistry and Applied Biosciences, ETH Z u¨ rich, H o¨ nggerberg, HCI,
CH-8093, Z u¨ rich, Switzerland
Received May 28, 2009; E-mail: baiker@chem.ethz.ch
Abstract: The structure sensitivity of enantioselective hydrogenations on chirally modified metals was
investigated using Pt nanoparticles of different shapes. All three samples had an average particle size of
1
{
0 nm, but the fraction of dominantly cubic, cubooctahedral, and octahedral particles varied with decreasing
100} and increasing {111} faces in the same order. In the absence of chiral modifier the hydrogenation of
ethyl pyruvate was independent of the shape of the Pt nanoparticles; variation of the specific reaction
rates did not exceed the experimental error on all self-prepared catalysts and on a commercial Pt/Al
2
O
3
used as reference. Addition of cinchonidine or quinine induced a significant rate enhancement by a factor
of 4-15, and the rate was always higher with quinine. Also, 72-92% ees were achieved, and the reaction
was shape selective: both the rate and the ee increased with increasing Pt{111}/Pt{100} ratio. A similar
correlation in the hydrogenation of ketopantolactone confirmed that decarbonylation or aldol-type side
reactions of ethyl pyruvate were not the reason for structure sensitivity. A combined catalytic and theoretical
study revealed that the probable origin of the particle shape dependency of enantioselective hydrogenation
is the adsorption behavior of the cinchona alkaloid. DFT studies of cinchonidine interacting with Pt(100)
and Pt(111) terraces indicated a remarkably stronger interaction on the former crystallographic face by ca.
155 kJ/mol. The higher adsorption strength on Pt(100) was corroborated experimentally by the faster
hydrogenation of the homoaromatic ring of the alkaloid, which fragment interacts the strongest with Pt
during its adsorption. Thus, an ideal catalyst for the hydrogenation of activated ketones contains dominantly
Pt{111} terraces, which crystallographic face is more active and affords higher enantioselectivity, combined
with the higher stability of the modifier.
1
. Introduction
There has been an impressive development in the past years
surface science studies on chirally modified metals helped to
gain a closer insight into the interaction of the substrate and
the chiral modifier with the metal surface and led to novel
in heterogeneous enantioselective catalysis. The simple strategy
of using a conventional (supported) metal catalyst in the
presence of a chiral compound that provides the suitable
stereochemical control led to high enantioselectivities in various
15-23
concepts for the origin of enantioselection.
Despite this progress, there is still a significant gap between
the catalytic-synthetic approach focusing on the development
of highly enantioselective catalyst systems and the theoretical
and surface science routes that apply idealized conditions to
allow a better understanding of the phenomena. An example is
1
-6
reactions.
Theoretical calculations of the adsorption of the
chiral modifier on model metal surfaces represent the first step
toward rationalization of the experimental observations.
7-14
The
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10.1021/ja9043328 CCC: $40.75  2009 American Chemical Society

Enantioselective Hydrogenation on Pt Nanoparticles
A R T I C L E S
the early observation of a remarkable selectivity enhancement
by a reductive treatment of supported Pt prior to the hydrogena-
enantioselectivity (97.6% ee) achieved in the same reaction using
tiny Pt nanoparticles with a narrow particle size distribution
(1.4 ( 0.2 nm).
2
4
42
tion of R-ketoesters. It was shown later that treatment of Pt/
Al in flowing hydrogen at elevated temperature doubled the
enantioselectivity, as compared to the treatment in air under
2
O
3
Here we report another approach to reduce the gap between
surface science and practical catalysis. We synthesized Pt
nanoparticles of different shapes but very similar size and tested
them in the enantioselective hydrogenation of an R-ketoester
and an R-ketolactone. The efficiency of this approach is well
documented in the study of the influence of surface morphology
of metal nanoparticles on the reaction rate and chemoselec-
2
5
otherwise identical conditions. The shift in enantioselectivity
was reversible by changing the gas atmosphere, and redispersion
of the Pt particles was evidenced by HRTEM. The effect of
heat treatment was attributed to adsorbate, i.e., hydrogen and
26
oxygen, induced restructuring, but no evidence for the change
in the surface morphology of Pt has been found. Another
4
3
tivity. The reactions include hydrogenation and dehydro-
27,28
4
4-46
46-51
efficient catalyst pretreatment method is ultrasonication,
and
genation,
oxidation and electrooxidation,
isomeriza-
5
2
53
its positive effect on the enantioselection is probably also linked
to changes in the morphology of the metal particles. In the
hydrogenation of 3,5-di(trifluoromethyl)acetophenone, the enan-
tioselectivity more than tripled after stirring the catalyst slurry
under nitrogen in the presence of cinchonidine or quinoline prior
tion, and oxygen reduction. Since our focus is on the role
of surface morphology in enantioselection, we choose a method
that provides relatively big particles of ∼10 nm with a narrow
5
4
particle size distribution. Thus, the effect of particle size can
be eliminated and the observed differences in enantioselectivity
can be unambiguously correlated with the surface morphology
of Pt.
29
to the reaction. Catalyst restructuring due to reactive adsorption
of the heteroaromatic compounds was confirmed by electron
microscopy, but no correlation between surface morphology and
enantioselectivity could be derived.
2. Experimental Section
30
The structure sensitivity of heterogeneous catalytic reactions
is commonly investigated by varying the size of the metal
2
.1. Materials. Pt colloids with a controlled shape have been
3
1-34
synthesized by reduction of an aqueous solution of K PtCl (Aldrich,
9.9+%) with H (PanGas, 5.0), according to a method described
2
4
particles in the range 1-20 nm.
The biggest changes of
9
2
54
proportions between the high- and low-coordinated surface sites
occur between 1 and 5 nm. There are numerous reports on
the structure sensitivity of hydrogenations on chirally modified
in the literature. Different Pt particle shape distributions were
achieved by varying the concentration of the colloid stabilizer
3
5
poly(acrylic acid) (PAA, Aldrich, M ) 2100). The preparation
W
1,4,36-38
Pt-group metals and Ni,
For example, it was earlier described that in the hydrogenation
of pyruvate esters on cinchonidine-(CD)-modified Pt/Al the
rate and enantioselectivity increased with increasing Pt particle
but the results are contradictory.
of the samples Pt-1, Pt-2, and Pt-3 was conducted at Pt:PAA molar
ratios of 1:1, 1:3, and 1:5, respectively.
2
O
3
2 2 2
For the preparation of Pt-1/SiO , Pt-2/SiO , and Pt-3/SiO , a
suspension of SiO (Degussa, Aerosil 200) in water was prepared
2
3
9
size up to 3-4 nm. A feasible explanation for the poor
enantioselectivity characteristic of metal particles below 2 nm
by sonification and added to the Pt colloidal solution. After stirring
for 1 h the colloids were deposited onto silica by dropwise
acidification until pH ) 2 (0.1 M HCl). The samples containing 5
wt % Pt were filtered off, washed with water (10 times) to remove
excess PAA, and dried overnight at 60 °C and 10 mbar. Silica was
chosen as the support because of the superior resolution of the TEM
4
0,41
is a geometric effect:
the small particles cannot accom-
modate the bulky modifier-substrate complex that occupies
9
2
0-25 surface atoms, and the nonmodified reaction leads to a
racemic product. But this concept is in contrast to the excellent
pictures, compared to Al
was divided into two fractions and deposited onto SiO
Degussa, Aluminum oxide C) to allow a direct comparison to the
commercial catalyst Pt/Al (Engelhard 4759), which was chosen
2
O
3
. One colloidal solution (sample Pt-1)
2
and Al
2 3
O
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A R T I C L E S
Schmidt et al.
as reference because it is the best known and mostly used catalyst
in the enantioselective hydrogenation of ketones.
magnetic stirrer (750 rpm). The pressure was controlled by a
constant-volume constant-pressure system (B u¨ chi BPC 9901).
5
6
Before the catalytic experiments, ethyl pyruvate (Acros, 98%)
was freshly distilled and toluene (Fluka, 99.7%) was dried over an
activated molecular sieve. Ketopantolactone (Roche, 99%), acetic
acid (Acros, 99.8%), cinchonidine (CD, Fluka, >98% alkaloid), and
quinine (QN, Fluka, >99% alkaloid) were used as received.
According to a procedure described in the literature, 5 mL of
toluene containing 0.68 µmol of CD was mixed with 21 ( 1 mg
of catalyst (5 wt.% Pt) before addition of 0.92 mmol of ketopan-
tolactone. The reactions were carried out at 70 bar and 264 K for
30 min to ensure full conversion. The reaction rate was determined
at ambient pressure (at 273 K in toluene using 10 mg of catalyst
and 0.46 mmol of substrate) in the reactor used for the hydrogena-
tion of ethyl pyruvate. Analysis and calculations were made
analogous to those of ethyl pyruvate hydrogenation.
2
.2. Methods. For TEM investigations, the material was de-
posited onto a holey carbon foil supported on a copper grid. The
measurements were performed with a CM30ST microscope (FEI;
LaB
6
cathode, operated at 300 kV, point resolution ∼2 Å). Scanning
transmission electron microscopic (STEM) images were recorded
with a high-angle annular dark field (HAADF) detector at a Tecnai
F30 microscope (FEI; field emission gun, operated at 300 kV, point
resolution ∼2 Å). HAADF-STEM images reveal the metal particles
with bright contrast (Z contrast). Energy dispersive X-ray spec-
troscopy (EDS) was used to distinguish between the metal particles
and the support. An EDAX system attached to the Tecnai F30
microscope was used for the EDS experiments.
Hydrogenation of the aromatic ring system of CD was carried
out under conditions identical to those used for hydrogenation of
ethyl pyruvate but in the absence of the substrate. First, the reaction
mixture was stirred under nitrogen for 5 min to allow the adsorption
of the modifier on the catalyst surface. The reaction was started by
switching the atmosphere to hydrogen. Samples of the reaction
mixture were taken via syringe, the catalyst was filtered off, and
the filtrate was diluted with acetic acid 5-fold before UV analysis
on a Cary 400 spectrometer with a 10 mm path length quartz
cuvette. The concentration of the modifier in solution used for
estimating its conversion was determined with a calibration curve.
2.4. Computational Methods. All calculations have been
performed with the CP2K program package suite of programs aimed
at performing efficient electronic structure calculations and mo-
The Pt content of the catalysts was determined with atomic
absorption spectroscopy (AAS) on a Varian SpectrAA 220 FS
spectrometer equipped with an air/C H burner and a D -lamp for
2 2 2
background correction. The samples were dissolved in a HF/aqua
regia mixture overnight.
X-ray photoelectron spectroscopic (XPS) analysis of the Pt
5
7
lecular dynamics at different levels of theory. The electronic
structure was modeled by means of Density Functional Theory
nanoparticles deposited onto SiO
Heraeus LHS11 MCD instrument using Mg K
2
was performed on a Leybold
(1253.6 eV)
R
(
DFT), where the Kohn-Sham orbitals were expanded using the
radiation. The sample was pressed into a sample holder, evacuated
58,59
in a load lock to 10-6 mbar, and transferred to the analysis chamber
Gaussian and Plane Wave basis set formalism (GPW).
Within
-
9
the GPW formalism, the wave function is described using Gaussian
type orbitals (GTOs), with an auxiliary plane wave (PW) basis set
expansion of the charge density. The GTOs have been expanded
in terms of triple-ꢀ valence with double polarization (TZV2P) basis
sets for H and second row elements and a triple-ꢀ valence (TZV)
basis set with very confined functions (smallest exponent 0.1) for
Pt. For the auxiliary PW basis set the energy cutoff has been set at
(
typical pressure <10 mbar). The peaks were energy-shifted to
the binding energy of Si 2p for SiO (103.4 eV) to correct for the
2
charging of the material. The surface composition of the catalysts
was determined from the peak areas of Pt 4f, Si 2p, O 1s, Cl 2s,
and C 1s, which were computed after subtraction of the Shirley-
type background by empirically derived cross section factors.
The presence of the colloid stabilizer PAA on the Pt surface
was confirmed by CO adsorption followed by DRIFTS on a Bruker
Equinox 55 spectrometer, equipped with a liquid-nitrogen-cooled
MCT detector. Samples were dried in flowing Ar (PanGas, 6.0; 20
mL/min) for 1 h before adsorption of CO (10% in He, 20 mL/min)
for 90 min, followed by purging with Ar before the measurement.
3
20 Ry. The interaction of the valence electrons with frozen atomic
cores has been described Via the use of norm-conserving, dual-
6
0
space type pseudopotentials (PP). For second row elements a PP
that includes the first shell electrons has been used, while for Pt
the PP included the 5s and 5p semicore electrons. The exchange
and correlation term has been modeled using the Perdew-Burke-
2.3. Catalytic Hydrogenation. Hydrogenation of ethyl pyruvate
6
1
Ernzerhof (PBE) functional.
was carried out in a magnetically stirred 50 mL glass reactor at
5
5
2
91 K under flowing hydrogen at ambient pressure. The dried
The metal surfaces exposing the (111) and the (100) crystal-
lographic planes have been modeled using a slab approach, where
the semi-infinite bulk is approximated by a finite number of atomic
layers, parallel to the desired crystallographic plane. The Pt(111)
surface supercell was modeled by an orthorhombic cell constituted
by four layers of (6 × 6) atoms per layer, piled along the z axis,
for a total of 144 atoms (Figure 1a). The dimensions of the cell
along the directions parallel to the surface are 16.82 and 14.56 Å.
The Pt(100) surface supercell was modeled by an orthorhombic
cell constituted by four layers of (4 × 4) atoms per layer, piled
along the z axis, for a total of 128 atoms (Figure 1b). The resulting
slab is separated from its periodic images in the direction
perpendicular to the surface by interposing an adequate amount of
empty space (more than 15 Å). The metal lattice constant was
chosen according to the optimal theoretical value found for bulk
catalyst (21 ( 1 mg, 5 wt.% Pt) was stirred (750 rpm) in 9 mL of
acetic acid for 5 min under flowing nitrogen followed by flowing
hydrogen for 5 min. CD or QN as chiral modifier (3.4 µmol) and
n-tetradecane as internal standard were injected in 1 mL of an acetic
2
acid solution, followed by further stirring for 5 min under H . For
experiments in the absence of modifier, 1 mL of neat acetic acid
was used instead of the modifier solution. The reaction was started
by injecting the substrate (0.33 mL, 4.0 mmol). Samples of the
reaction mixture were filtered and diluted with ethyl acetate before
GC analysis (Thermo Finnigan Trace 2000, CP-Chirasil-Dex CB
capillary column, FID detector). The enantioselective reactions
needed 3-10 h to reach full conversion of ethyl pyruvate. Both
CD and QN afford the (R) enantiomer in excess. The TOFs were
calculated from the linear part of the conversion-time plot
6
2
(
(
conversions below 80%) based on the fraction of surface Pt atoms
dispersion) as determined by TEM. The ee’s were determined at
Pt (3.965 Å). The geometries have been optimized using the
full conversion. The slow reactions in the absence of a chiral
modifier were stopped after 4 h, at 10-12% conversion.
Hydrogenation of ketopantolactone was performed in a 50 mL
stainless steel reactor equipped with a glass-liner, PTFE cover and
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2360 J. AM. CHEM. SOC. 9 VOL. 131, NO. 34, 2009

Enantioselective Hydrogenation on Pt Nanoparticles
A R T I C L E S
Figure 1. Pt(111) and Pt(100) supercells used for adsorption studies.
Figure 2. Typical TEM pictures of PAA-stabilized platinum colloids Pt-1, Pt-2, and Pt-3 in aqueous solution (top series) and the corresponding silica-
supported samples Pt-1/SiO , Pt-2/SiO , and Pt-3/SiO
2
2
2
.
Broyden-Fletcher-Goldfarb-Shanno minimization algorithm
Pt(100) surface. Note that the energy of CD is not present since it
cancels in the summation. The Surface Open(4) conformation of
adsorbed CD was chosen since it resulted in the most likely
geometry for the formation of surface docking structures, as shown
6
3-67
(
BFGS)
until the atomic displacements were lower than 3 ×
-
3
-4
1
0
bohr and the forces were lower than 4.5 × 10 Ha/bohr. For
both cells the geometry optimizations were run by keeping the first
layer of the slabs fixed, while all other degrees of freedom (top
three metal layers and adsorbate molecule) were set free to optimize.
The problem of the adsorption energy of CD on platinum has been
1
3
elsewhere.
3. Results and Discussion
1
3,68,69
addressed elsewhere,
while in this work we have focused
on the differences between adsorption energies of CD on Pt(111)
and Pt(100), which has been calculated as follows:
3.1. Characterization of Model Catalysts. Variation of the
Pt:PAA ratio in the colloid synthesis leads to different particle
shape distributions, as illustrated by the TEM pictures in Figure
En{CD-on-Pt(111)} - En{Pt(111)} - En{CD-on-Pt(100)}
En{ (
+
2. The average diameter of 10 nm is very similar for all samples,
) En(ADS)
Pt 100)}
but the preferred shape varies from mainly cubic and truncated
cubic particles in Pt-1 over mainly cubooctahedral (Pt-2) to
truncated octahedral and octahedral particles (Pt-3), in good
where En{CD-on-Pt(111)} was the calculated energy of the system
composed by CD adsorbed on Pt(111), En{CD-on-Pt(100)} was the
calculated energy of the system composed by CD adsorbed on
Pt(100), En{Pt(111)} and En{Pt(100)} were the calculated energies of
the Pt(111) and Pt(100) surfaces, and finally ∆En(ADS) was the
calculated difference between the adsorption of CD on Pt(111) and
4
6,54,70
agreement with literature data.
It has been shown by
54
46
HRTEM, CO adsorption microcalorimetry and electrochemi-
4
6
cal methods for Pt colloids prepared by this method that the
presence of cubic particles is related to a high fraction of
Pt{100} faces, whereas hexagonal particles such as tetrahedrons
and octahedrons expose preferentially Pt{111} terraces.
The average particle size and the shape distribution are well
maintained during deposition of the colloids onto silica (Table
(
(
(
(
(
(
63) Liu, D.; Nocedal, J. Siam. J. Sci. Stat. Comp. 1989, 10, 1–17.
64) Nocedal, J. Math. Comput. 1980, 35, 773–782.
65) Shanno, D.; Kettler, P. Math. Comput. 1970, 24, 657–664.
66) Goldfarb, D. Math. Comput. 1970, 24, 23–26.
67) Broyden, C. Math. Comput. 1970, 24, 365–382.
68) Santarossa, G.; Iannuzzi, M.; Vargas, A.; Baiker, A. Chem. Phys.
Chem. 2008, 9, 401–413.
(70) Inaba, M.; Ando, M.; Hatanaka, A.; Nomoto, A.; Matsuzawa, K.;
Tasaka, A.; Kinumoto, T.; Iriyama, Y.; Ogumi, Z. Electrochim. Acta
2006, 52, 1632–1638.
(
69) Vargas, A.; Baiker, A. J. Catal. 2007, 247, 387.
J. AM. CHEM. SOC. 9 VOL. 131, NO. 34, 2009 12361

A R T I C L E S
Schmidt et al.
Table 1. Characteristics of the Supported Pt Nanoparticles and the
Scheme 1. Hydrogenation of Ethyl Pyruvate in the Presence of
Cinchona Alkaloids
Reference Catalyst Pt/Al
2
O
3
dominant
shape
Pt{111}/ metal
a
Pt{100} dispersion
sample
Pt, wt %
dav, nm
Pt-1/SiO
2
4.6 ( 0.5 9.9 ( 1.2
cubic
0.23
n.d.
0.13
b
b
b
b
Pt-1/Al
2
O
3
4.5 ( 0.6 n.d.
n.d.
cubooctahedral 1.05
2.44
n.d.
Pt-2/SiO
Pt-3/SiO
2
4.5 ( 0.4 9.8 ( 1.0
0.13
0.14
0.32
2
4.7 ( 0.5 10.2 ( 1.0 octahedral
b
Pt/Al
2
O
3
5.8 ( 0.3 3.2 ( 1.2
spherical
n.d.
(
reference)
a
Atomic ratio of surface Pt atoms to the total number of Pt atoms.
Not determined.
b
1
). A detailed evaluation based on analyzing approximately 600
Table 2. Hydrogenation of Ethyl Pyruvate: Initial Rates (TOFs) and
Enantiomeric Excesses (ee’s) to (R)-Lactate at Full Conversion
3
5,71
to 1000 particles for each sample
can be found in the
Supporting Information. The most important characteristic is
the {111}/{100} ratio of exposed Pt crystal faces as estimated
chiral
modifier
ee,
%
TOF,
-1
rate
enhancement
catalyst
h
by TEM; it increases from Pt-1/SiO
2 2
over Pt-2/SiO to Pt-3/
Pt/Al
(reference)
2
O
3
-
CD
QN
-
80
85
156
1200
1480
-
7.7
9.5
SiO (see Table 1). Note that not only the fraction of terrace
2
sites such as Pt{111} and Pt{100} changes, but also the amount
and type of low coordinated sites (e.g., edge and corner sites,
see Supporting Information) are slightly different. Their propor-
tion to the total number of surface atoms is, however, rather
small due to the high average particle diameter. The average
Pt-1/SiO
100}
2
-
CD
QN
-
72
77
126
530
850
-
4.2
6.7
{
Pt-1/Al
100}
2
O
3
-
CD
QN
-
71
76
143
570
840
-
4.0
5.9
2 3
particle size of the reference Pt/Al O is considerably smaller,
{
and the particle shape is nearly spherical. Thus, both Pt{100}
and Pt{111} terraces are present, but due to the lower particle
size, the amount of low coordinated sites is significantly higher
Pt-2/SiO2
{100}+{111}
-
CD
QN
-
78
83
147
730
1380
-
5.0
9.4
2
9
compared to the self-prepared catalysts.
Characterization of the supported catalysts by XPS revealed
the absence of any impurity except a significant amount of
organic residue, which is attributed to the colloid stabilizer PAA
adsorbed strongly on the catalyst surface. The amount of carbon
was identical in all self-prepared samples. The peak positions
for Si 2p (103.4 eV), O 1s (532.6-532.8 eV), Pt 4f7/2 (70.5-70.7
eV), and C 1s (284.7-285 eV) are in agreement with literature
Pt-3/SiO
{111}
2
-
CD
QN
-
86
92
127
1350
1890
-
10.6
14.8
ketones, as no significant poisoning effect under reaction
conditions, in the liquid phase, was observed (see the next
section).
0
72
2
data for SiO , Pt , and poly(acrylic acid) (PAA).
3
.2. Enantioselective Hydrogenation of Activated Ketones.
The presence of residual PAA molecules on the Pt surface
was further elucidated by DRIFT spectroscopy of CO adsorp-
tion. The spectra obtained after CO adsorption and removal of
gas phase CO by purging with Ar were compared to the
The performance of the supported Pt nanoparticles was inves-
tigated in the enantioselective hydrogenation of activated
ketones. The results related to the hydrogenation of ethyl
pyruvate (Scheme 1) are collected in Table 2.
2 3
corresponding spectrum obtained on the reference Pt/Al O . The
This reaction is known to be disturbed by side reactions when
exceptionally low intensity of the signal related to atop
adsorption of CO on Pt in the case of all self-prepared samples
indicates that the Pt surface is efficiently blocked by the colloidal
stabilizer, in agreement with recent reports on the strong
7
7
inappropriate conditions are chosen. Aldol condensation
catalyzed by the Pt surface, the amine modifier, and the basic
sites on the alumina support leads to high molecular weight
byproducts that partially block the active sites and distort the
results. To suppress the side reactions it is important to use acetic
acid as solvent, maintain a high surface hydrogen concentration
during reaction (i.e., to avoid hydrogen “starvation” conditions),
and add the pyruvate directly before starting the reaction.
In Table 2 the reaction rates are characterized by the TOFs,
related to the total number of surface Pt atoms. The observed
TOFs for the unmodified reaction leading to racemic products
are very similar on all four self-prepared catalysts, despite their
different Pt particle shapes. All values fall in a relatively narrow
5
2,73-75
adsorption of the colloid stabilizer.
This interpretation
is underlined by the unusually low frequency of this CO signal
-
1
(
2064-2065 cm ) corresponding to the formation of isolated
76
CO islands typical for extremely low surface coverage. Note
that removal of PAA is possible by oxidative and reductive
catalyst pretreatments at elevated temperature, but no such
treatment was necessary before asymmetric hydrogenation of
(
(
71) VanHardeveld, R.; VanMontfoort, A. Surf. Sci. 1966, 4, 396–430.
72) Wagner, C. D. ; Naumkin, A. V. ; Kraut-Vass, A.; Allison, J. W.;
Powell, C. J.; Rumble, J. J. R. NIST X-ray Photoelectron Spectroscopy
Database (http://srdata.nist.gov/xps/).
-1
range of 136.5 ( 10.5 h , where the deviations do not exceed
the expected standard deviation of determining the specific
reaction rate ((5-10%). Clearly, pyruvate hydrogenation on
Pt is a structure insensitive reaction.
(
(
(
(
73) Bratlie, K. M.; Lee, H.; Komvopoulos, K.; Yang, P.; Somorjai, G. A.
Nano Lett. 2007, 7, 3097–3101.
74) Liu, D.; Gao, J.; Murphy, C. J.; Williams, C. T. J. Phys. Chem. B
Importantly, no significant difference in reaction rates was
observed by replacing the silica support with alumina (compare
2
004, 108, 12911–12916.
75) Crump, C. J.; Gilbertson, J. D.; Chandler, B. D. Top. Catal. 2008, 49,
33–240.
76) Krebs, H.-J.; L u¨ th, H. Appl. Phys. 1977, 14, 337–342.
2
(77) von Arx, M.; Mallat, T.; Baiker, A. Top. Catal. 2002, 19, 75–87.
1
2362 J. AM. CHEM. SOC. 9 VOL. 131, NO. 34, 2009

Enantioselective Hydrogenation on Pt Nanoparticles
A R T I C L E S
2 2 3
Pt-1/SiO and Pt-1/Al O ). In addition, the ee’s are also very
similar; the deviations do not exceed the estimated error of the
analysis ((0.5%). This is an indication that the support effect
is minor and the self-prepared silica-supported catalysts can be
compared to the widely used reference catalyst Pt/Al
In the absence of a chiral modifier, the reference catalyst Pt/
is only slightly more active than the self-prepared
2 3
O .
2 3
Al O
catalysts. The small difference indicates that the residual colloid
stabilizer that diminished the adsorption of CO on Pt at the solid/
gas interface has only a minor poisoning effect at the solid/
liquid interface during pyruvate hydrogenation. This astonishing
observation may be explained by the efficient competition at
the solid/liquid interface between the colloid stabilizer and the
74
reaction components: the solvent, the substrate, and hydrogen.
In particular, the effect of acetic acid solvent seems to be
important due to its chemical similarity to the polycarboxylic
acid type stabilizer. Thus, investigation of the enantioselective
hydrogenation of ethyl pyruvate on the as-prepared catalysts is
possible without any forceful pretreatment, which could result
in particle agglomeration and restructuring of the Pt surface.
Addition of the chiral modifier induced a considerable rate
78
enhancement (“ligand acceleration”) compared to the unmodi-
fied reaction (Table 2). The rate enhancement varies in the range
4-15 and strongly depends on the crystallographic face exposed
on Pt and also on the modifier structure. The rate acceleration
correlates well with the Pt particle shape: it increases with the
increasing Pt{111}/Pt{100} ratio on the self-prepared catalysts.
The Pt/Al
enhancement. This catalyst exposes both Pt{100} and Pt{111}
faces similar to Pt-2/SiO but contains a higher fraction of low
2 3
O reference catalyst is characterized by medium rate
2
coordinated Pt surface atoms. This comparison suggests that
the relative amount of low coordinated Pt sites has a negligible
effect on the catalyst performance; however, a reliable prediction
of the possible structure sensitive behavior of the asymmetric
hydrogenation on very small Pt clusters is impossible. The
observed rate enhancement in the presence of CD is in good
Figure 3. Time dependence of conversion (filled symbols) and ee (empty
symbols) in the hydrogenation of ethyl pyruvate on Pt/Al O reference
2 3
7
9
catalyst (top) and Pt-2/SiO (bottom). Symbols: circles for the unmodified
2
agreement with literature values.
reaction, squares in the presence of CD, and triangles in the presence of
The rate enhancement and enantioselectivity, both induced
by addition of the chiral modifier, are clearly correlated; i.e.,
catalysts with a higher Pt{111}/Pt{100} ratio are more reactive
and afford higher ee’s. In the best case scenario, over 90%
QN.
conversion the enantioselectivity remains practically constant
for both catalysts. The behavior of Pt-1/SiO
2 2
and Pt-3/SiO was
ee was achieved on Pt-3/SiO
2
which catalyst exposes the highest
very similar to that of Pt-2/SiO (see Supporting Information).
2
relative amount of Pt{111} and shows the strongest rate
acceleration. Note that a positive correlation between the
reaction rate and enantioselectivity has frequently been observed
for supported Pt catalysts, and it is attributed to the common
mechanistic origin of the two phenomena.
In Table 2 the ee’s determined at full conversion are given.
Variation of the enantioselectivity with time (and conversion)
The changes in the enantioselectivity in the early stage of the
hydrogenation of pyruvate esters and other activated ketones
are commonly related to changes in the surface composition of
the reactants and to the removal of some impurities. The
significantly stronger initial transient period in reactions over
the silica-supported Pt nanoparticles compared to that on the
1
,2
1
,80,81
2 3
reference Pt/Al O indicates that partial removal of the polymeric
is illustrated in Figure 3 using Pt/Al
2
O
3
and Pt-2/SiO
2
catalysts.
stabilizer from the surface of Pt nanoparticles plays an important
role. It is expected that the presence of strongly adsorbed foreign
species disturbs the modified reaction more than that of the
unmodified (racemic) hydrogenation, since the former requires
a much bigger ensemble of Pt surface sites. Thus, (partial)
removal of the stabilizer by the competing adsorption of
hydrogen, substrate, modifier, and solvent should lead to
enhanced enantioselectivity.
These two catalysts behaved similarly in many respects, but
there is a remarkable difference in the initial transient period
until ∼20-30% conversion. In case of the reference catalyst
2 3
Pt/Al O variation of the ee with conversion is small, while there
is a rapid selectivity enhancement in the early stage of the
reaction over the silica-supported Pt nanoparticles. Above 30%
The second test reaction was the hydrogenation of ketopan-
tolactone in the presence of CD (Table 3). An advantage of
this substrate is the absence of an H atom in R-position to the
keto-carbonyl group, the structural difference of which reduces
the number of possible side reactions (aldol condensation),
compared to the hydrogenation of ethyl pyruvate. Moreover,
(
(
78) Garland, M.; Blaser, H. U. J. Am. Chem. Soc. 1990, 112, 7048.
79) Blaser, H. U.; Jalett, H. P.; Monti, D. M.; Reber, J. F.; Wehrli, J. T.
Stud. Surf. Sci. Catal. 1988, 41, 153.
(80) Blaser, H. U.; Jalett, H. P.; M u¨ ller, M.; Studer, M. Catal. Today 1997,
3
7, 441–463.
(
81) Sz o¨ ll o¨ si, G.; Cser e´ nyi, S.; F u¨ l o¨ p, F.; Bart o´ k, M. J. Catal. 2008, 260,
2
45–253.
J. AM. CHEM. SOC. 9 VOL. 131, NO. 34, 2009 12363

A R T I C L E S
Schmidt et al.
Table 3. Hydrogenation of Ketopantolactone: Initial Rates (TOFs)
and Enantiomeric Excesses (ee’s) at Full Conversion
a
chiral
modifier
TOF,
h
rate
enhancement
ee,
%
ee,
%
-
1
catalyst
Pt/Al
2
O
3
-
CD
250
500
-
2.0
-
45
-
76
(
reference)
Pt-1/SiO
100}
2
2
-
140
445
-
-
-
{
CD
3.2
47
72
Pt-2/SiO
-
CD
145
610
-
4.2
-
53
-
78
{
100}+{111}
Pt-3/SiO
111}
2
-
CD
125
630
-
5.0
-
61
-
81
{
a
Measured at 70 bar and 264 K.
ketopantolactone shows no significant tendency to decarbony-
8
2
lation on Pt.
Despite of the considerably different reaction conditions, the
positive correlation between the increasing {111}/{100} ratio
of exposed Pt crystal faces and the reaction characteristics (rate
enhancement and enantioselectivity) is valid also in this reaction.
The rate of the unmodified reaction on all three homemade
catalysts is very similar, the deviation does not exceed the
-1
expected error (TOF ) 135 ( 10 h ). Hence, also in this case
the hydrogenation of the CdO bond on the unmodified Pt
surface is structure insensitive, and addition of CD induces
significant, structure-specific rate acceleration and enantiose-
lectivity. Overall, the observed ee’s are somewhat lower than
Figure 4. CD adsorbed on Pt(111) (a) and on Pt(100) (b).
Table 4. Bond Lengths (in Å) of the Quinoline Moiety (Anchoring
Group) in the Adsorbed CD on Pt(111) and Pt(100) Surfaces
a
(
Figure 4a and b, Respectively)
5
5,56
the best values reported for both substrates,
because the
N(1′)C(2′) C(2′)C(3′) C(3′)C(4′) C(4′)C(9′) C(9′)C(10′) C(9′)C(5′)
beneficial catalyst prereduction under elevated temperature could
not be used here to avoid changes in the Pt nanoparticle shape.
Pt(111)
Pt(100)
1.371
1.374
1.431
1.468
1.487
1.441
1.505
1.489
1.476
1.487
1.491
1.460
Note that the performance of Pt-3/SiO
Pt{111}/Pt{100} ratio is superior to the as-received reference
Pt/Al under both conditions applied.
.3. Adsorption of Cinchonidine on Pt(111) and Pt(100)
2
containing the highest
C(5′)C(6′) C(6′)C(7′) C(7′)C(8′) C(8′)C(10′) C(10′)N(1′) average
2
O
3
Pt(111)
Pt(100)
1.471
1.472
1.432
1.490
1.468
1.467
1.485
1.459
1.403
1.459
1.456
1.454
3
1
3,69,83
Surfaces. It has been shown elsewhere
that due to its
conformational complexity CD can have several adsorption
modes on a platinum surface. Both experimental and theoretical
evidences support the view that the Surface OPEN(4) (SO(4))
conformation is the most populated (in terms of fractional
a
Atom numbering refers to Scheme 1.
Table 5. Bond Distances (in Å) between the Atoms of the
Adsorbed Quinoline (Anchoring Group) and Pt Atoms, for CD
Adsorption on Pt(111) and Pt(100) Surfaces (Figure 4a and b,
13
a
coverage) after adsorption. In the present study we are mainly
interested in the difference between the adsorption energy of
CD on Pt(111) and Pt(100) surfaces. Figure 4 shows CD
adsorbed SO(4) on Pt(111) and Pt(100) (Cartesian coordinates
of both adsorbed structures are available as Supporting Informa-
tion). At the level of theory used, CD is more strongly adsorbed
on Pt(100) than on Pt(111) by ca. 155 kJ/mol. This is reflected
by the larger amount of charge transfer from the metal to the
Respectively)
N(1′)Pt
C(2′)Pt
C(3′)Pt
C(4′)Pt
C(9′)Pt
C(5′)Pt
Pt(111)
Pt(100)
2.231
2.397
2.332
2.161
2.184
2.157
2.174
2.234
2.191
2.159
2.163
2.176
C(6′)Pt
C(7′)Pt
C(8′)Pt
C(10′)Pt
average
Pt(111)
Pt(100)
2.214
2.147
2.212
2.157
2.159
2.165
2.205
2.193
2.207
2.195
6
8
84
adsorbed molecule. Lowdin charge analysis shows in fact
that back-donation from Pt(100) to the adsorbate is larger than
that from Pt(111) by ca. 0.2 e.
a
Atom numbering refers to Scheme 1.
1
4
remarkably distorted. In particular, bonds are elongated and
the planarity of the aromatic system is lost (rehybridization).
Structural parameters that characterize the chemisorptions of
CD on Pt have been summarized in Tables 4-6. Table 4 gives
a list of the bond lengths of quinoline (anchoring group) upon
adsorption on Pt(111) and Pt(100). Table 6 gives a list of the
deviation from planarity of the C-H and C-C bonds of
In relation to structural parameters, it has already been shown
that upon adsorption of CD the quinoline moiety becomes
(
82) Bonalumi, N.; B u¨ rgi, T.; Baiker, A. J. Am. Chem. Soc. 2003, 125,
1
3342–13343.
(
(
83) Vargas, A.; Baiker, A. J. Catal. 2006, 239, 220–226.
84) Lowdin, P.-O. J. Chem. Phys. 1950, 18, 365–375.
1
2364 J. AM. CHEM. SOC. 9 VOL. 131, NO. 34, 2009

Enantioselective Hydrogenation on Pt Nanoparticles
A R T I C L E S
Table 6. Rehybridization of C-H and C-C Bonds of Quinoline for
Al
O
were less active (see support information), and the
2
3
CD Adsorption on Pt(111) and Pt(100) Surfaces (Figure 4a and b,
a
quinoline ring was barely converted on Pt-3/SiO
right). After 5 h the concentration of CD in solution was higher
than that in the hydrogenation on Pt-1/SiO after only 30 min.
2
(Figure 5,
Respectively)
C(2′)H
C(3′)H
C(4′)C(9)
C(5′)H
C(6′)H
C(7′)H
2
Pt(111)
Pt(100)
11
29.9
23.8
26.8
47
21.9
42.9
16.2
18.9
41.1
18.3
41.0
The main hydrogenation product observed in the presence
of acetic acid is 5′,6′,7′,8′,10,11-hexahydrocinchonidine via
hydrogenation of the homoaromatic ring of the quinoline moiety
85,86
C(8′)H
average
and the vinyl group.
nm as can be seen in Figure 4 for Pt-1/SiO
This species gives a UV signal at ∼270
2
. Significant amounts
Pt(111)
Pt(100)
43.8
21.2
29.4
28.3
of 1′,2′,3′,4′,10,11-hexahydrocinchonidine (broad feature at 295
8
6
nm), formed via hydrogenation of the heteroaromatic ring of
quinoline, and that of the fully saturated CD cannot be excluded
a
Atom numbering refers to Scheme 1. The values (degrees) indicate
the deviation of dihedral angles from planarity.
2
by these experiments. On Pt-3/SiO no corresponding signal of
a partially hydrogenated quinoline moiety was observed, in good
agreement with the persistency of the CD signal.
quinoline (rehybridization). Note that the stronger adsorption
of CD on Pt(100) is reflected by the smaller average distance
between the atoms of the anchoring group and the Pt atoms of
the surface. In fact, this results in 2.21 Å for CD on Pt(111)
and 2.19 Å for CD on Pt(100) (Table 5). Other average
parameters, namely internal bond lengths (Table 4) and rehy-
bridization (Table 6), do not change significantly.
A closer examination of Tables 4-6 reveals that major
changes occur at the level of the C(6′) and C(7′) carbon atoms,
on the homoaromatic ring of quinoline, when passing from
adsorption on Pt(111) to adsorption on Pt(100). This site of the
molecule upon adsorption on Pt(100) is closer to the metal,
strongly rehybridized, and the bond length C(6′)-C(7′) is
remarkably elongated. This is due to the adsorption topology,
itself conditioned by the surface topology, and indicates that
the homoaromatic ring of quinoline undergoes the most remark-
able structural changes on the strongest adsorption site.
The study reveals an unprecedented high stability toward
hydrogenation of the quinoline anchoring moiety of the alkaloid
on the Pt{111} face. The reactivity toward aromatic hydrogena-
tion increases with the increasing Pt{100}/Pt{111} ratio of
exposed crystal faces.
For comparison, structure sensitivity in the hydrogenation of
benzene on supported Pt nanoparticles of different shapes has
7
3
recently been reported. While on catalysts containing almost
exclusively Pt particles of cubic shape, i.e., on Pt{100}, the
fully hydrogenated cyclohexane was the only product, in the
presence of Pt{111} and Pt{100} (on samples containing
cubooctahedral particles) both cyclohexene and cyclohexane
were observed. Unfortunately, no data on the hydrogenation of
quinoline and its derivatives are available yet.
It should be emphasized that the theoretical calculations
discussed previously (Tables 4-6) show that upon adsorption
on a Pt(100) surface the homoaromatic ring of the quinoline
moiety undergoes a remarkably larger distortion and deviation
from aromaticity as compared to the same adsorption on Pt(111).
The C(6′)-C(7′) bond of the anchoring moiety is especially
elongated and rehybridized on Pt(100), and from a theoretical
point of view it should be especially sensitive to saturation. This
expectation has been confirmed with the above-noted faster
saturation of the homoaromatic moiety of the quinoline fragment
of CD on the Pt{100} rich catalyst. The correlation between
the adsorption strength of aromatics and the hydrogenation rate
on Pt can probably be attributed to the weakening of the
aromaticity, and therefore of the aromatic stability, that is
directly proportional to the strength of the interaction of the
hydrocarbon to the metal.
3
.4. Hydrogenation of Cinchonidine Studied by UV Spec-
troscopy. Adsorption of cinchona alkaloids on the metal surface
via the quinoline ring, the so-called “anchoring moiety”, is a
key element of all mechanistic models developed for ketone
1
hydrogenation on Pt. The enantioselectivity is affected by both
the strength and the mode of adsorption: flat (π-bonded) or tilted
(
N-lone pair bound). The former adsorption mode is assumed
to be more favorable to achieve high ee’s. An indirect way to
characterize the adsorption strength of the alkaloid under
reaction conditions is to investigate the rate of saturation of the
8
5
quinoline ring. Note that the vinyl group of the alkaloid in
the 10-11 position is hydrogenated much faster, but this
function is not involved in the enantioselection and this reaction
will not be discussed here.
Hydrogenation of CD on all three silica-supported Pt colloids
was followed with UV spectroscopy. The reaction conditions
were identical to those of pyruvate hydrogenation at ambient
pressure, except for the absence of the substrate. The rate of
saturation of the aromatic ring of CD is illustrated in Figure 5
Note that hydrogenation of the cinchona alkaloid modifier
has only minor importance in practical catalysis, since saturation
of the aromatic ring of the alkaloid weakens the adsorption on
Pt, the converted molecule desorbs from the surface, and another
1,2
CD molecule replaces it. Hence, the slower the target reaction
relative to the transformation of CD, the higher excess of the
alkaloid is necessary to modify the Pt surface and avoid any
significant change in the enantiomeric excess with time (or
conversion). The plots in Figure 3 demonstrate that the
experimental conditions (the concentration of the alkaloid
modifier) were properly chosen, since the ee did not drop at
high conversion, which would be an indication of an insufficient
coverage of Pt by the modifier.
for Pt-1/SiO
highest ratio of Pt{111}/Pt{100} exposed faces, respectively.
The corresponding UV spectra of the hydrogenation of CD
over Pt-1/SiO and Pt-3/SiO are shown in the Supporting
2 2
and Pt-3/SiO , which possess the lowest and
(
2
2
Information.) The sharp signal at 315 nm and the broader feature
at ∼305 nm are specific for the extended aromatic system of
8
6
quinoline. The decreasing intensity of the signal at 315 nm
with time is attributed to the conversion of the aromatic ring of
CD. This reaction is efficiently catalyzed by Pt-1/SiO
, left side); almost no CD is left after 5 h. Pt-2/SiO
2
(Figure
and Pt/
3
.5. Origin of Structure Sensitivity. In the unmodified
5
2
hydrogenation of ethyl pyruvate (Table 2) similar TOFs in the
-
1
range 126-156 h were observed on all four self-prepared
catalysts possessing the same Pt particle size (ca. 10 nm) and
dominantly cubic, cubooctahedral, or octahedral shapes, and on
(
(
85) Huck, W.-R.; B u¨ rgi, T.; Mallat, T.; Baiker, A. J. Catal. 2003, 216,
76–287.
86) Hoxha, F.; Mallat, T.; Baiker, A. J. Catal. 2007, 248, 11–19.
2
J. AM. CHEM. SOC. 9 VOL. 131, NO. 34, 2009 12365

A R T I C L E S
Schmidt et al.
Figure 5. Hydrogenation of the aromatic ring of CD followed by UV spectroscopy on Pt-1/SiO
2
and Pt-3/SiO
2
. Curve (a) is obtained after adsorption on
the catalyst for 5 min under nitrogen; hydrogenation for 30 (b), 60 (c), 120 (d), 210 (e), and 300 min (f).
a commercial reference catalyst with smaller spherical Pt
particles (3.2 nm). Evidently, hydrogenation of the keto-carbonyl
group of ethyl pyruvate is structure insensitive: the reaction rate
depends neither on the preferred Pt particle shape nor on the
average particle size in the investigated range.
Introduction of the chiral modifiers CD and QN turned the
reaction to structure sensitive, and on all samples remarkably
higher rates, by a factor of 4-15, were achieved compared to
the unmodified reaction. Additionally, there is an unambiguous
positive correlation between the Pt{111}/Pt{100} ratio of
the resulting high molecular weight byproducts may have a
larger influence on the enantioselection than that of the Bi or S
adatoms.
The observed trend of the higher rate and enantioselectivity
on catalysts exposing higher Pt{111}/Pt{100} ratios is confirmed
in the hydrogenation of another activated ketone, ketopanto-
lactone, under different conditions (Table 3). In ketopantolac-
tone, there is no H atom in R-position to the keto group and no
significant side reactions could be detected by ATR-IR spec-
8
2
troscopy. This difference supports our interpretation that the
structure sensitivity is an intrinsic feature of the enantioselective
hydrogenation of R-ketoesters and lactones on Pt and not the
exposed crystal faces (Pt-3/SiO
2
> Pt-2/SiO
2
≈ Pt/Al
2 3
O > Pt-
1
/SiO ) and the rate and enantioselectivity of the reaction. In
2
8
8
contrast, the fraction of Pt defect sites (coordination number
below 8) seems to be unimportant.
result of undetected side reactions.
In the presence of CD and QN, the structure insensitive
hydrogenation of ketones becomes structure sensitive. Further
investigation revealed that hydrogenation of the aromatic ring
of the chiral modifier is also structure sensitive (Figure 5). Under
the conditions applied, almost no transformation of the ho-
moaromatic ring of the quinoline fragment of CD was observed
on the catalyst containing the highest Pt{111}/Pt{100} ratio (Pt-
Recently, there has been some debate in the literature on the
origin of rate enhancement and its possible correlation with the
8
7,88
enantioselection.
Note that our observation of a clear
correlation between the rate acceleration and the enantioselec-
tivity seems to be in agreement with recent catalytic and surface
8
1,89
science studies.
The role of steps and {100} terrace sites on Pt in pyruvate
hydrogenation was previously investigated by the preferential
deposition of Bi and S adatoms onto the metal surface of Pt/
3
2
/SiO ) and the rate increased with the increase in the fraction
of the Pt{100} face exposed.
DFT calculations showed that the probable origin of structure
sensitivity is the different adsorption strength, and the conse-
quent major distortion from aromaticity (and therefore bond
weakening), of the alkaloid on the Pt(100) with respect to the
Pt(111) surfaces (Figure 4, Tables 4-6). The major consequence
of this different adsorption is an especially pronounced elonga-
tion of the C(6′)-C(7′) bond, as well as strong rehybridization
at this site of the molecule (homoaromatic moiety of the
quinoline).
90
graphite. Based on the combined electrochemical and catalytic
study, it was proposed that the enantioselection would be more
efficient in the vicinity of step sites than those located on
terraces. It is questionable, however, whether the specific
adsorption of single Bi or S atoms is a reliable model for the
adsorption of the bulky CD-pyruvate ester complex, the
1-4,17
formation of which is necessary to induce enantioselection.
As estimated from theoretical calculations, an active site
ensemble of approximately 20-25 Pt atoms is necessary to
9
Hydrogenation experiments show that exactly this part of the
molecule is sensitive to saturation on Pt(100) rich catalysts.
Although enantioselectivity is a more complex process than the
adsorption behavior of the chiral modifier, it has been previously
shown that saturation of the anchoring moiety of a chiral
modifier leads to desorption of the modifier and to lower
accommodate the interacting complex. In addition, a serious
catalyst deactivation and poor enantioselectivities were observed
under high pressure conditions that were probably due to
91
hydrogenolysis of the solvent dichloromethane. Hydrochloric
acid catalyzes the hydrolysis of pyruvate esters, and it is a good
catalyst for the dimerization and oligomerization of activated
ketones. Blocking of a dominant fraction of Pt surface sites by
92
enantioselectivity. In other words, an efficient chiral modifica-
tion requires the formation of chiral sites which are stable under
hydrogenation conditions, and Pt{100} rich catalysts are
revealed to be less apt to stably accommodate modifiers based
on quinoline as anchoring unit. That is, the stronger interaction
(
(
(
87) Toukoniitty, E.; Murzin, D. Y. J. Catal. 2007, 251, 244–245.
88) Mallat, T.; Baiker, A. J. Catal. 2007, 251, 246–248.
89) Lavoie, S.; Lalibert, M.-A.; Mahieu, G.; Demers-Carpentier, V.;
McBreen, P. J. Am. Chem. Soc. 2007, 129, 11668–11669.
(90) Jenkins, D. J.; Alabdulrahman, A. M. S.; Attard, G. A.; Griffin, K. G.;
Johnston, P.; Wells, P. B. J. Catal. 2005, 234, 230–239.
(92) Baiker, A.; Blaser, H. U. In Handbook of Heterogeneous Catalysis;
Ertl, G., Kn o¨ zinger, H., Weitkamp, J., Eds.; Wiley-VCH: Weinheim,
Germany, 1997; Vol. 5, p 2422.
(
91) Meier, D. M.; Ferri, D.; Mallat, T.; Baiker, A. J. Catal. 2007, 248,
6
8–76.
1
2366 J. AM. CHEM. SOC. 9 VOL. 131, NO. 34, 2009

Enantioselective Hydrogenation on Pt Nanoparticles
A R T I C L E S
of the quinoline moiety of CD with the Pt(100) face compared
to Pt(111) reduces its stability against hydrogenation and
promotes fast deterioration of chirally active sites.
theoretical enantio-differentiating ability of ideal Pt(100) and
Pt(111) single crystal surfaces are expected to be considerably
bigger than those observed here with supported Pt nanoparticles.
The results provide a scientific basis for the design of highly
enantioselective, active, and stable enantioselective catalysts.
4
. Summary
The present study reveals for the first time that enantiose-
Acknowledgment. The authors thank W. Kleist for the XPS
analysis and F. Krumeich for the electron microscopic measure-
ments performed at the EMEZ. Financial support by the Swiss
National Foundation is gratefully acknowledged.
lective hydrogenation on chirally modified Pt nanoparticles is
shape selective: both the reaction rate and the enantioselectivity
increase with the increase in the Pt{111} to Pt{100} ratio. The
origin of structure sensitivity is traced to the different adsorption
behavior of the alkaloid modifier on these crystallographic faces,
which in turn affects the interaction with the substrate in the
activated complex during hydrogen uptake. In addition, the
present work provides unambiguous experimental evidence for
the phenomenon of “ligand acceleration” in heterogeneous
catalysis and its dependence on the crystallographic face of Pt.
It has to be emphasized that due to the individual particle
shape distributions not exclusively Pt{100} and Pt{111} faces
Supporting Information Available: Particle size and shape
distributions of samples Pt-1/SiO , Pt-2/SiO , and Pt-3/SiO ;
2 2 2
conversion vs time plots (including ee) of the ambient pressure
hydrogenation of ethyl pyruvate; Cartesian coordinates of
optimized geometry of CD on Pt surfaces; UV spectra of CD
hydrogenation on Pt/Al O and Pt-2/SiO ; DRIFT spectra of
2 3 2
CO adsorption on the catalysts. This material is available free
of charge via the Internet at http://pubs.acs.org.
2 2
are exposed in Pt-1/SiO and Pt-3/SiO , respectively. Each
sample contains terraces of both types and additional low
coordination sites. Therefore, the differences between the
JA9043328
J. AM. CHEM. SOC. 9 VOL. 131, NO. 34, 2009 12367