Controlling hydrogenation of C=O and C=C bonds in cinnamaldehyde using silica supported Co-Pt and Cu-Pt bimetallic catalysts

Article abstract of DOI:10.1016/j.apcata.2012.01.019

Liquid phase hydrogenation of cinnamaldehyde was evaluated over SiO ₂ supported Co-Pt and Cu-Pt bimetallic and Co, Cu, Pt monometallic catalysts in a batch reactor. H₂-temperature-programmed reduction (H₂-TPR) was utilized to characterize the reduction behavior and pulse CO chemisorption measurement was performed to characterize the number of active sites of the catalysts. Transmission electron microscopy (TEM) analysis was used to characterize metallic particle size distribution and extended X-ray absorption structure (EXAFS) measurements were performed to verify the bimetallic bond formation. The reactor evaluation results show that Co-Pt and Cu-Pt bimetallic catalysts exhibit much higher hydrogenation activity than the corresponding monometallic catalysts, and Co-Pt shows much higher selectivity toward C=O bond hydrogenation than Cu-Pt. The trend of hydrogenation activity and selectivity is consistent with previous studies of the hydrogenation of unsaturated aldehydes on model bimetallic surfaces.

Full text of DOI:10.1016/j.apcata.2012.01.019

Applied Catalysis A: General 419–420 (2012) 126–132
Contents lists available at SciVerse ScienceDirect
Applied Catalysis A: General
journal homepage: www.elsevier.com/locate/apcata
Controlling hydrogenation of C O and C C bonds in cinnamaldehyde using silica
supported Co-Pt and Cu-Pt bimetallic catalysts
a
b
b
a
a,∗∗
Renyang Zheng , Marc D. Porosoff , Jacob L. Weiner , Shuliang Lu , Yuexiang Zhu
,
b,∗
Jingguang G. Chen
a
Beijing National Laboratory for Molecular Sciences, State Key Laboratory for Structural Chemistry of Unstable and Stable Species, College of Chemistry and Molecular Engineering,
Peking University, Beijing 100871, China
Department of Chemical Engineering, Center for Catalytic Science and Technology, University of Delaware, Newark, DE 19716, USA
b
a r t i c l e i n f o
a b s t r a c t
Article history:
Liquid phase hydrogenation of cinnamaldehyde was evaluated over SiO2 supported Co-Pt and Cu-
Pt bimetallic and Co, Cu, Pt monometallic catalysts in a batch reactor. H2-temperature-programmed
reduction (H2-TPR) was utilized to characterize the reduction behavior and pulse CO chemisorption mea-
surement was performed to characterize the number of active sites of the catalysts. Transmission electron
microscopy (TEM) analysis was used to characterize metallic particle size distribution and extended X-ray
absorption structure (EXAFS) measurements were performed to verify the bimetallic bond formation. The
reactor evaluation results show that Co-Pt and Cu-Pt bimetallic catalysts exhibit much higher hydrogena-
tion activity than the corresponding monometallic catalysts, and Co-Pt shows much higher selectivity
toward C O bond hydrogenation than Cu-Pt. The trend of hydrogenation activity and selectivity is consis-
tent with previous studies of the hydrogenation of unsaturated aldehydes on model bimetallic surfaces.
Received 7 November 2011
Received in revised form 9 January 2012
Accepted 15 January 2012
Available online 25 January 2012
Keywords:
Hydrogenation
Cinnamaldehyde
Bimetallic Catalysts
Co-Pt
Cu-Pt
©
2012 Elsevier B.V. All rights reserved.
1
. Introduction
In order to better understand the hydrogenation pathways of
␣,␤-unsaturated aldehydes on bimetallic catalysts, fundamental
Selective hydrogenation of ␣,␤-unsaturated aldehydes to the
studies on Pt single crystal and Pt-based bimetallic surfaces have
been performed by adding a 3d-metal, such as Fe, Co, Ni and Cu, to
Pt(1 1 1) [18,19]. The 3d-Pt bimetallic catalysts often show proper-
ties that differ distinctly from those of their parent metals, offering
the opportunity to obtain novel catalysts with enhanced activity
and/or selectivity [20–22]. For example, it has been demonstrated
under ultra-high vacuum (UHV) conditions that a Pt-terminated
Pt-Co-Pt(1 1 1) surface, which represents a subsurface bimetallic
structure with Pt on the top-most surface layer and Co residing
in the subsurface region, promoted the selective hydrogenation of
acrolein (CH2 CH CH O) toward the corresponding unsaturated
alcohol [19]. The combined experimental and DFT results suggested
corresponding allylic alcohols has received considerable atten-
tion because of its industrial applications [1–3]. The competing
hydrogenation pathways of ␣,␤-unsaturated aldehydes can occur
through the C C bond to produce the saturated aldehydes and
the carbonyl group to produce unsaturated alcohols. Hydrogena-
tion of the C C bond is thermodynamically more favorable, and
hence research efforts have been directed at improving the selectiv-
ity to the more desirable unsaturated alcohols [4]. Many attempts
have been made to promote the selective hydrogenation of ␣,␤-
unsaturated aldehydes by taking advantage of the synergistic
effects of bimetallic catalysts [5–17], such as Co-Pt [5–8], Fe-Pt [9],
Ru-Pt [10,11], and Sn-Pt [12]. The catalytic activity and selectivity
are influenced by several factors, including reaction conditions and
operation mode (e.g. gas or liquid phase hydrogenation), electronic
and geometric structures of the metal catalysts, type of catalyst
supports, and catalyst preparation and activation procedures.
that the presence of weakly adsorbed acrolein through a di
␴ C O
configuration appeared to be responsible for this desired hydro-
genation pathway [19].
More recently, these surface science results have been extended
to ␥-Al O3 supported Co-Pt bimetallic catalysts, which exhibit sig-
2
nificantly higher activity than monometallic Co and Pt catalysts
for the hydrogenation of C O bonds in two probe molecules, ace-
tone and acetaldehyde [23]. In the current work we extended
the hydrogenation of monocarbonyl compounds to that of
␣,␤-unsaturated aldehydes on silica supported Co-Pt bimetallic
catalysts. In addition, Cu-Pt catalysts were also evaluated due
∗
Corresponding author. Tel.: +1 302 831 0642; fax: +1 302 831 1048.
Corresponding author.
E-mail addresses: zhuyx@pku.edu.cn (Y. Zhu), jgchen@udel.edu (J.G. Chen).
∗
∗
0
926-860X/$ – see front matter © 2012 Elsevier B.V. All rights reserved.
doi:10.1016/j.apcata.2012.01.019

R. Zheng et al. / Applied Catalysis A: General 419–420 (2012) 126–132
127
to its selectivity in the partial hydrogenation of butadiene [24]
and 1,4-cyclohexadiene [25]. Several characterization methods,
including H -temperature-programmed reduction (H -TPR), pulse
2.2.4. Extended X-ray absorption fine structure (EXAFS)
EXAFS experiments were performed on the X18B beamline at
the National Synchrotron Light Source (NSLS), Brookhaven National
Laboratory. The sample preparation procedures have been previ-
ously reported [27]. The catalysts were reduced under a diluted
2
2
CO chemisorption, transmission electron microscopy (TEM) and
extended X-ray absorption fine structure (EXAFS), were employed
in order to understand the catalytic behavior and bimetallic effects.
Because of the toxicity of acrolein, cinnamaldehyde (CMD) was
used as the probe molecule in the current work to identify the
selective hydrogenation pathways on the bimetallic catalysts. From
the point of view of practical applications, the corresponding
semi-hydrogenated product, cinnamyl alcohol (CML), is utilized as
components in pharmaceuticals, fragrances, and perfumes [4,26].
−
1
hydrogen flow (5% H2 in He, 40 mL min ) while heating to 623 K
−
1
at a rate of 10 K min . The samples were held at 623 K for 1 h
−
1
and then allowed to cool to room temperature at 10 K min
under a diluted H2 flow. The Pt LIII-edge EXAFS spectra were
then collected at room temperature using a double crystal Si(1 1 1)
monochromator. EXAFS measurements were also collected from
a Pt foil in order to calibrate the edge energies of the catalyst
samples.
The EXAFS spectrum was analyzed using the IFFEFIT 1.2.11 data
analysis package (Athena, Artemis, Atoms, and FEFF6) [28,29]. Local
structural information was obtained by using Artemis to fit each
data set with theoretical standards generated by FEFF6 in R-space
2
. Experimental methods
2.1. Catalyst preparation
[
30]. For each catalyst the Pt-Pt and the Pt-3d (3d = Co or Cu) contri-
A slurry-based impregnation method was used to prepare the
.0 wt%Co–1.7 wt%Pt/SiO2 and 5.4 wt%Cu–1.7 wt%Pt/SiO2 bimetal-
lic catalysts, with a Co:Pt (or Cu:Pt) metal atomic ratio of 10:1.
The corresponding monometallic Co, Cu and Pt catalysts were also
prepared to serve as control samples.
butions to the theoretical EXAFS were taken into account in fitting
the data for the bimetallic catalysts, while only Pt-Pt contributions
were included in fitting the monometallic Pt catalyst. The theo-
retical Pt-Pt photoelectron amplitudes and phases were calculated
for the bulk Pt fcc structure. The Pt-3d contributions were calcu-
lated using the same Pt fcc structure with the exception that the
Pt atoms in the first nearest-neighbor shell were replaced with Co
5
The silica gel was first calcined at 1073 K for 3 h to obtain the
thermally stable silica support, with a BET specific surface area of
2
−1 −1
3
3
70 m g and a total pore volume of 0.84 cm g . The metal pre-
2
0
or Cu atoms. The passive electron reduction factor (S ) was found
cursors were dissolved in an excess of deionized water (15 mL H O
2
to be 0.80 from fitting of the Pt-foil data, and this value was fixed
through the fitting of all the catalysts. The seven parameters used
in the fitting procedure were the correction to the edge energy, the
coordination numbers of the Pt-Pt and Pt-3d bonds, corrections to
their model interatomic distances, and the mean square deviations
in interatomic distances (EXAFS Debye–Waller factors).
per gram of catalyst) and then impregnated into the silica support.
The solution was sonicated for 1 h, then dried at 373 K for 24 h, and
finally calcined at 623 K for 2 h. The metal precursors used in this
study were Co(NO ) ·6H O, Cu(NO ) ·3H O, and Pt(NH ) (NO )
3
2
2
3
2
2
3
4
3 2
(
A.R.).
2
.2. Catalyst characterization
.2.1. H -temperature-programmed reduction (H -TPR)
2.3. Catalytic evaluation
2
2
2
TPR experiments were performed to determine the reduction
The catalytic hydrogenation reaction was performed in a stain-
behavior of the catalysts. The experiments were carried out in
a U-shaped tubular quartz reactor heated by an electric furnace.
For each measurement, 0.040 g of calcined catalyst (40–80 mesh)
less steel stirred autoclave with a volume of 100 mL. In a typical
experiment, 0.100 g catalyst was reduced in a U-shaped quartz tube
−
1
−1
under a H2 (20 mL min ) and N2 (20 mL min ) mixture at 623 K
(723 K for monometallic Co catalyst) for 1 h and then cooled to room
temperature. To prevent air contact of the freshly reduced cata-
lyst powder, about 3 mL isopropanol (solvent) was dropped into
the tube to immerse the catalyst under the H2/N2 mixture. Then
the isopropanol immersed catalyst was transferred carefully to the
autoclave, including 26 mL isopropanol and 3 mL CMD. The total
volume of reactant and solvent was 27 mL. After the autoclave was
was exposed to a reducing gas consisting of 5.0 vol% H in Ar
2
−1
(
1
10 mL min ) with a temperature ramp from room temperature to
073 K at 10 K min . The amount of hydrogen consumption was
−1
detected by a thermal conductivity detector (TCD).
2.2.2. CO chemisorption
Pulse CO chemisorption was performed using a Micromeritics
sealed, H was charged four times to replace air and the initial pres-
AutoChem II 2920 to determine the number of active sites on the
surfaces of the reduced catalysts. Approximately 0.05 g of the cata-
2
sure of H2 was 0.5 MPa. The autoclave was heated to the reaction
temperature of 353 K in 0.5 h and H2 was charged to a final pres-
sure of 4.0 MPa. The hydrogenation reaction began by starting the
stirring mechanism. After the desired reaction time, the stirring
was cut off, and the autoclave was quickly cooled to room tem-
perature and depressurized, and about 0.3 mL liquid samples were
collected periodically. The liquid samples were analyzed on a gas
chromatography (GC) equipped with a flame ionization detector
−1
lyst was reduced by 30 mL min 10 vol.% H2 in He at 623 K (723 K
for monometallic Co catalyst) for 1.5 h, and then held for 2 h in
He. After cooling in He, gas pulses consisting of 5.0 vol.% CO in He
−1
were injected at 50 mL min at 308 K and the signal was monitored
using a TCD.
2.2.3. Transmission electron microscopy (TEM)
(FID).
TEM analysis was performed on pre-reduced catalysts using a
JEOL 2010F equipped with a Schottky field emission gun operated
at 200 keV, with an ultra-high resolution pole piece providing a
point resolution of 1.9 A˚ . Imaging analysis was performed in the
3. Results
scanning mode (STEM) using a 12 nm camera length and a 1.0 nm
diameter nanoprobe. TEM samples were prepared by grinding and
suspending reduced catalysts in ethanol, and then a few droplets
of this solution were placed onto a carbon-coated copper grid. The
grids were allowed to fully dry before loading the samples into the
TEM.
3.1. H -TPR
2
The reduction behavior of the calcined catalysts by TPR is shown
in Fig. 1. The Co/SiO shows two main reduction peaks close to each
other with temperature maxima at about 590 K and 630 K. The first
peak is assigned to the reduction of Co O4 to CoO, and the second
2
3

1
28
R. Zheng et al. / Applied Catalysis A: General 419–420 (2012) 126–132
Table 2
6
30 K
5
90 K
60 K
Results of Pt LIII-edge EXAFS fitting.
Co/SiO2
Catalyst
N(Pt-Pt)
N(Pt-3d)
R(Pt-Pt) ( A˚ )
Pt/SiO2
Co-Pt/SiO2
Cu-Pt/SiO2
9.1 ± 0.5
4.2 ± 1.0
3.7 ± 0.5
3.0 ± 0.7
8.1 ± 0.6
–
5
2.75 ± 0.01
2.72 ± 0.01
2.56 ± 0.01
0.006 ± 0.001
0.006 ± 0.001
2.68 ± 0.01
2.60 ± 0.01
0.006 ± 0.001
0.009 ± 0.001
4
00 K
R(Pt-3d) ( A˚ )
–
2
˚
2
␴
(Pt-Pt) (A)
0.006 ± 0.001
CoPt/SiO2
2
␴ (Pt-3d) ( A˚ )
2
–
6
10 K
x 0.2 Cu/SiO2
3.2. CO chemisorption measurements
The CO uptake values are listed in Table 1. The CO uptake for
6
00 K
600
−
1
the Co-Pt/SiO2 bimetallic catalyst is 29.4 ␮mol g , higher than the
sum of the two corresponding monometallic catalysts. However,
the CO uptake value for the Cu-Pt/SiO2 is less than the sum of the
two monometallic catalysts. The CO uptake values on Cu/SiO2 is
x 0.2 CuPt/SiO2
400
500
700
800
900
1000
−
1
less than 2 ␮mol g , similar as the adsorption of CO on SiO2 sup-
port itself [36], suggesting that the adsorption of CO on Cu/SiO2 is
negligible at 308 K.
Temperature / K
Fig. 1. TPR profiles of the calcined Co-Pt and Cu-Pt bimetallic and corresponding
monometallic catalysts supported on silica.
3.3. TEM
one to the subsequent reduction of CoO to metallic Co [31,32]. The
detection of the tail or shoulder reduction peak around 700–800 K
suggests the presence of surface Co species with different degree of
interaction with the support, possibly due to the reduction of cobalt
silicate species formed during the TPR experiments by reaction of
highly dispersed CoO with the silica support. As shown in Fig. 1,
the Co-Pt/SiO2 bimetallic catalysts can be reduced at a much lower
High angle annular dark field (HAADF) TEM images and parti-
cle size distributions are shown in Fig. 2. Particle size distributions
were calculated by measuring particle diameters in several differ-
ent images for each catalyst using the software ImageJ and are
summarized in Table 1. The dominant particle size in each cata-
lyst appears to be the smaller particles ranging in diameter from
approximately 1 nm to 4 nm. The average diameter is 2.0 nm for
temperature than Co/SiO . The two main reduction peaks of Co-
2
Co-Pt/SiO , and 2.6 nm for Cu-Pt/SiO2 and Pt/SiO2.
2
Pt/SiO2 decrease to about 400 K and 560 K, respectively, and the
Co-Pt/SiO2 catalysts become completely reduced at temperatures
below 623 K.
3.4. EXAFS
On the other hand, Cu/SiO2 and Cu-Pt/SiO2 exhibit narrow
and intense reduction peaks at 610 K and 600 K, respectively. The
peak temperatures and profile shapes are consistent with previous
work of TPR of pure CuO [33] and Cu/SiO2 [34]. The TPR pro-
files indicate that all the catalysts, except monometallic Co, were
Fig. 3(a) shows the Pt LIII-edge X-ray absorption near-edge
structure (XANES) spectra of the three catalysts before and after
reduction, along with the spectra of the Pt foil for reference. Exam-
ining the Pt LIII-edge XANES reveals information regarding the
oxidation state of the samples [37]. The XANES in Fig. 3(a) shows
that before reduction in H2 the catalysts exhibit large white-line
peaks that disappear after reduction. All of the Pt LIII-edge spectra
after reduction are similar to the Pt foil, indicating that the Pt has
been fully reduced in the bimetallic catalysts.
completely reduced at 623 K. Previous work [35], by the H -TPR
2
of the reference sample of mechanical mixture of 20 wt%CuO and
8
0 wt%SiO , has shown that the calculated degree of reduction of
2
1
0.0 wt%Co–1.2 wt%Pt/SiO2 was 94%, indicating a complete reduc-
tion of the bimetallic catalysts. In addition, the catalytic evaluation
was performed for Co/SiO2 reduced at 623 K, 723 K, and 823 K,
with the 723 K sample showing the highest activity. The Co cat-
alyst should not be completely reduced at 623 K from the TPR of
Fig. 3(b) presents the Fourier transformed (R-space) experimen-
tal data after reduction and the fits obtained using FEFF6 [30]. The
results from the fitting procedure are summarized in Table 2. In
Co/SiO , whereas the Co metal particles would agglomerate when
the bulk phase, Co and Pt have first nearest neighbor distances of
2.506 A˚ (Co-Co) and 2.774 A˚ (Pt-Pt). In the case of Pt-3d (3d = Co,
Cu) bimetallic catalysts the first nearest neighbor distance should
be an intermediate value between the two monometallic distances.
As shown in Table 2, the Pt-Co distance falls between the bulk Co-Co
and Pt-Pt distances, confirming the formation of Pt Co bimetallic
2
reduced at 823 K, both of which should lead to a lower activity.
Therefore, the Co/SiO2 catalyst was reduced at 723 K in the current
work.
bonds. A similar trend is observed for Cu-Pt/SiO . Also, good fits to
Table 1
2
CO chemisorption results and particle size distributions from TEM measurements.
the Pt LIII-edge data could only be obtained by including both Pt-Pt
and Pt-3d contributions in the model, which strongly suggests that
bimetallic bonds are present in the bimetallic nanoparticles.
In addition to the Pt-3d interatomic distances, the Pt-3d coor-
dination numbers can also be used to identify the presence of
bimetallic bonds and measure the extent of bimetallic formation
in the catalyst particles. As listed in Table 2, the Co-Pt and Cu-Pt
coordination numbers are 3.7 and 8.1, respectively, confirming the
presence of bimetallic bonds. The comparison also suggests that
the extent of bimetallic formation is less in Co-Pt/SiO2 than in Cu-
Pt/SiO2.
Catalyst
CO uptake
Particle size distribution statistics
−
1
(
␮mol g
)
Median
diameter
Average
diameter
(nm)
Standard
deviation in
diameter (nm)
(
nm)
Co-Pt/SiO2
Cu-Pt/SiO2
Pt/SiO2
29.4
8.8
16.7
3.8
1.9
2.5
2.5
–
2.0
2.6
2.6
–
0.5
0.5
0.7
–
Co/SiO2
Cu/SiO2
1.1
–
–
–

R. Zheng et al. / Applied Catalysis A: General 419–420 (2012) 126–132
129
Fig. 2. TEM images and particle size distributions for (a) Pt/SiO2, (b) Co-Pt/SiO2 and (c) Cu-Pt/SiO2.
Table 3
3.5. Catalytic evaluation
Performance of the catalysts in terms of product selectivity at 20% conversion of
cinnamaldehyde.
The hydrogenation of CMD involves the parallel and consecu-
tive reduction of two functional groups, C C and C O bonds. The
products obtained were CML, hydrocinnamaldehyde (HCMD), and
−
1
−1
)
Time required Productselectivity(%) S(C O)/(C C)a
h)
Catalyst
k (h
g
(
3
-phenylpropanol (PPL). In the current work no other side products
CML HCMD PPL
were detected. Hence, the reaction pathways involved are believed
to be those shown in Scheme 1.
The conversion of CMD and generation of products as functions
of reaction time are displayed in Fig. 4. The conversion of CMD
Co-Pt/SiO2 1.74
1.4
5.6
8.8
76
52
70
9.5
42
29
15
3.74
Co/SiO2
Pt/SiO2
0.40
0.28
5.9 1.23
1.7 2.34
a
The ratio of C O bond hydrogenated (CML + PPL) to C C bond hydrogenated
after 6 h on Co-Pt/SiO bimetallic catalysts is up to 65%, three times
(HCMD + PPL).
2
higher than that on the corresponding monometallic catalysts.
As the reaction time increases, the selectivity to CML decreases
slightly. In contrast, Co/SiO2 shows poor performance in terms of
both activity and product selectivity, yielding nearly equal amounts
of the semi-hydrogenated products, CML and HCMD.
bimetallic formation on the hydrogenation selectivity. Typically the
selectivity is compared at a similar conversion [9]. Therefore, the
time required for 20% conversion of CMD was estimated from Fig. 2,
and then used to obtain the product selectivity and the ratio of
To make a more quantitative comparison, the hydrogenation
rate was quantified by fitting a first-order rate equation to the con-
sumption of CMD, as listed in Table 3. The hydrogenation rate over
C
C
O to C C hydrogenated, as listed in Table 3. The ratio of C O to
C hydrogenation is around 1 over Co/SiO , suggesting poor selec-
2
tivity. In comparison, the ratio over Co-Pt/SiO2 is 3.74. Comparing
−1
−1
Co-Pt/SiO2 is 1.74 h
g , much higher than the sum of that over
with Pt/SiO , the Co-Pt/SiO2 shows higher selectivity to CML and
2
monometallic Co/SiO2 and Pt/SiO . Because Co-Pt/SiO2 showed
2
lower selectivity to HCMD, indicating that Co-Pt/SiO2 selectively
hydrogenates the C O bond rather than the C C bond.
The performances of the Co-Pt and Cu-Pt catalysts in
terms of conversion and selectivity after 2 h reaction time
are compared in Table 4. The Co-Pt/SiO2 and Cu-Pt/SiO2
much higher hydrogenation activity than Cu-Pt/SiO , we chose Co-
2
Pt as the model bimetallic system to demonstrate the effect of
OH
O
H2
Table 4
(CMD)
(CML)
Performance of the catalysts in terms of cinnamaldehyde conversion and product
selectivity after 2 h reaction time.
H2
H
2
Catalyst
Conversion (%)
Product selectivity (%)
S(C O)/(C C)
CML
HCMD
PPL
O
OH
Co-Pt/SiO2
Co/SiO2
Pt/SiO2
Cu-Pt/SiO2
Cu/SiO2
28.6
7.9
4.8
10.7
0
78
58
62
35
–
8.8
38
36
60
–
14
4.4
2.3
5.2
–
4.02
1.45
1.68
0.61
–
H2
(HCMD)
(PPL)
Scheme 1. Proposed schemes of cinnamaldehyde hydrogenation.

1
30
R. Zheng et al. / Applied Catalysis A: General 419–420 (2012) 126–132
2
Fig. 3. (a) Normalized absorption of Pt LIII-edge XANES spectra of the three catalysts and a Pt foil before and after reduction; (b) Fourier transformed (magnitude) k -weighted
EXAFS function (ꢀ(k)) of Pt LIII-edge after reduction data (dashed lines) and first shell fits (solid lines).
catalysts show different hydrogenation activity and selectivity. In
It is also well established that an increase of metal particle
size favors the selectivity to CML; the effect is particularly pro-
nounced with particle larger than 2–3 nm [9,38,39]. This trend can
be attributed to a steric effect whereby the planar CMD molecule
cannot adsorb parallel to a flat metal surface because of the steric
repulsion of the aromatic ring. By tilting away from the metal sur-
general, the catalytic activity follows the order of Co-Pt/SiO > Cu-
2
Pt/SiO > Co/SiO > Pt/SiO , while the selectivity to CML shows the
2
2
2
trend of Co-Pt/SiO > Pt/SiO > Co/SiO > Cu-Pt/SiO . The conversion
2
2
2
2
of CMD over Cu-Pt/SiO2 is higher than that over Pt/SiO , and
2
the Cu-Pt/SiO2 prefers to hydrogenate the C C bond compared
with Pt/SiO . The Cu/SiO2 shows no hydrogenation activity for the
face, the CMD molecule facilitates end-on adsorption and the C C
2
CMD.
bond is protected and hence the selectivity to CML is improved.
However, the metal particle size alone cannot account for the dif-
ferent selectivity over Co-Pt and Cu-Pt bimetallic catalysts. As listed
in Tables 1 and 4, the Co-Pt/SiO2 catalyst, with metal particle size
4
. Discussion
smaller than Pt/SiO , shows higher selectivity to CML. Furthermore,
2
The results presented above demonstrate that Co-Pt and Cu-
the Cu-Pt/SiO2 and Pt/SiO2 catalysts, possessing a similar metal par-
ticle size of 2.6 nm, show a significantly different selectivity trend.
Therefore, the slight difference of the metallic particle size can-
not account for the different selectivity trend of the two bimetallic
catalysts.
As supported by the CO chemisorption measurements, the
increase in the number of active sites on the Co-Pt/SiO2 should
partially contribute to the enhanced hydrogenation activity as com-
pared to the monometallic catalysts. However, the surface active
sites alone cannot explain the higher hydrogenation activity on
the Cu-Pt/SiO2, which has a lower CO uptake value than Pt/SiO2.
Therefore, it is necessary to understand the roles of Co and Cu on
the activity/selectivity from the point of electronic modification, as
discussed below.
Pt bimetallic catalysts exhibit much higher hydrogenation activity
than the corresponding monometallic catalysts, with Co-Pt show-
ing much higher selectivity toward C O bond hydrogenation and
Cu-Pt being more highly selective toward C C bond hydrogenation.
As indicated from the EXAFS results in Table 2, a significant frac-
tion of the Pt atoms in the bimetallic catalysts are surrounded by
Co or Cu atoms to form Co-Pt or Cu-Pt bimetallic bonds. In addi-
tion, the bimetallic catalysts can be reduced more easily than the
monometallic Co or Cu catalysts, as demonstrated by the H -TPR
2
results in Fig. 1. The formation of the Co-Pt and Cu-Pt bimetallic
catalysts should be responsible for the enhanced hydrogenation
activity as compared to that of the corresponding monometallic
catalysts.

R. Zheng et al. / Applied Catalysis A: General 419–420 (2012) 126–132
131
1
00
and di ␴ C O bonding configurations, with the latter favoring
the C O hydrogenation pathway [19]. Recent studies by FTIR spec-
troscopy of adsorbed CO and XPS on supported bimetallic catalysts
have shown that after reduction, the surface of Co-Pt and Cu-Pt
catalysts are composed of primarily Pt atoms [27,40], suggesting
that the bimetallic nanoparticles may more closely resemble the
Pt-terminated configurations from surface science studies [19]. As
described in detail previously [19,22], the presence of subsurface
Co and Cu modifies the electronic properties of surface Pt, leading to
the low temperature hydrogenation of the C O bond of unsaturated
oxygenates. Furthermore, DFT calculations [41] and experimental
verification [42] show that the Pt-terminated Pt-Co structure is pre-
ferred over Co-terminated Co Pt Pt in the presence of adsorbed
hydrogen. Therefore, in the current work one can conclude that the
8
0
0
0
0
(
a) Co-Pt/SiO2
Conversion
6
4
2
Selectivity to CML
Selectivity to HCMD
Selectivity to PPL
enhanced hydrogenation activity observed over Co-Pt/SiO and Cu-
Pt/SiO2 could be due to the presence of Pt-terminated bimetallic
structures.
0
2
0
1
2
3
4
5
6
Time / h
1
00
Additionally, both the hydrogenation activity and selectivity
toward the C O bond in CMD are much higher in Co-Pt/SiO2 than
(
b) Co/SiO2
over Cu-Pt/SiO . This is consistent with surface science studies by
2
Conversion
Selectivity to CML
Selectivity to HCMD
Selectivity to PPL
8
0
0
0
0
Murillo et al. [19] on acrolein hydrogenation, which shows that
the activity and selectivity toward C O bond hydrogenation on
Pt Co Pt(1 1 1) is higher than that on Pt Cu Pt(1 1 1).
As reported by Gallezot and Richard [4], the selectivity of Pt
toward C O bond hydrogenation would be promoted with elec-
tropositive metal atoms. The electropositive metal acts as an
electron–donor ligand that increases the electron density on Pt,
6
4
2
thus decreasing the binding energies, particularly that of the C
C
bond, favoring the hydrogenation of the C O bond with respect
to the C C bond. This effect has been studied in detail theoret-
ically by Delbecq and Sautet [43] on a model bimetallic surface
Fe20Pt80(1 1 1), where the Fe atoms were located in the second
layer. Calculations showed that the electropositive metal promot-
ers acted as an electron-donating ligand, increasing the electron
density on the surface platinum atoms. It is possible that such
0
0
1
2
3
4
5
6
Time / h
1
00
(
c) Pt/SiO2
Conversion
effect also plays a role in the selective hydrogenation of the C O
bond of CMD over the Co-Pt and Cu-Pt bimetallic catalysts. More
detailed DFT calculations would be needed to further understand
the mechanisms for the selective hydrogenation of CMD.
8
0
0
0
0
Selectivity to CML
Selectivity to HCMD
Selectivity to PPL
6
4
2
5. Conclusions
The current work extends the hydrogenation of monocarbonyl
compounds to that of ␣,␤-unsaturated aldehydes on silica sup-
ported Co-Pt and Cu-Pt bimetallic catalysts. Results from H -TPR
2
studies show that the presence of Pt facilitates the reduction of
3
d metals (Co or Cu). Pulse CO chemisorption measurements show
that Co-Pt possesses significantly higher CO chemisorption capacity
compared to monometallic catalysts. TEM and EXAFS measure-
ments provide additional information on the metallic particle size
distribution and the formation of bimetallic bonds. The reactor
studies show that Co-Pt and Cu-Pt bimetallic catalysts exhibit sig-
nificantly higher hydrogenation activity than the corresponding
monometallic catalysts, and Co-Pt shows much higher selectiv-
ity toward C O bond hydrogenation than Cu-Pt, consistent with
previous surface science studies on the C C and C O bond hydro-
genation of acrolein. The current work demonstrates a similar trend
between model surfaces and supported catalysts, suggesting the
possibility of using the relevant surface science results to design
bimetallic catalysts for selective hydrogenation.
0
0
1
2
3
4
5
6
7
8
9
10 11 12
Time / h
Fig. 4. Conversion of cinnamaldehyde (CMD) and generation of products as a func-
tion of reaction time over (a) Co-Pt, (b) Co and (c) Pt catalysts. Reaction conditions:
53 K, 3 mL CMD, 27 mL isopropanol (solvent), 4 MPa H2, catalyst amount: 100 mg.
3
Previous work shows bimetallic surfaces that weakly bind
atomic hydrogen and alkenes display greater hydrogenation activ-
ity [22]. This weak interaction between the surface and the
adsorbates allows for novel, low-temperature hydrogenation to
occur. Recently, Murillo et al. performed DFT and experimental
studies to investigate the trend in C C and C O bond hydrogena-
tion of acrolein, CH2 CH CH O, on different bimetallic surface
structures [19]. Selective hydrogenation of the C O bond to pro-
duce 2-propenol (CH2 CH CH2 OH) occurs on Pt Co Pt(1 1 1)
Acknowledgements
This work was supported by the United States Department of
Energy, Office of Basic Energy Sciences (DE-FG02-00ER15104). The
as a result of the comparable binding strengths of the di
␴ C C

1
32
R. Zheng et al. / Applied Catalysis A: General 419–420 (2012) 126–132
authors from the Peking University acknowledge support from
the Major State Basic Research Development Program (grant no.
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