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With monometallic Pt as the catalyst, the hydrogenation
preferentially occurred on the C=C bond and the selectivity to
desired COL was <12% throughout the reaction (Figure 5c).
HCAL was preferentially produced as the primary intermediate
and further hydrogenated to HCOL (the Supporting Informa-
tion, Figure S6). The addition of Sn exerted a great influence
on the reaction pathway. As shown in Figure 5b, the selectivity
to COL, which was measured at about 60% conversion of CAL
O)-di-s(C=C) structure (the Supporting Information, Fig-
ure S13a) with an adsorption energy of À1.14 eV. For the ad-
2
sorption of C=C bond on the Pt (111) surface, both the h -di-
s(C=C) configuration (the Supporting Information, Fig-
2
ure S13b) and the h –p(C=C) configuration (Figure S13c) were
investigated, and the adsorption energies were À0.90 and
À0.60 eV, respectively. The atop adsorption for C=O (the Sup-
porting Information, Figure S13d) is much weaker, having an
adsorption energy of À0.55 eV. All these adsorption energies
(Table 1) are in good agreement with the calculated values re-
(
the conversion of Pt Sn was 7.5% as an exception), in-
30 70
creased from 6 (Pt) to 62 (Pt Sn ), 77 (Pt Sn ), and >90
7
5
25
60
40
[
22]
(
Pt Sn , Pt Sn , and Pt Sn ), respectively. The product distri-
ported by Yang et al. From these adsorption energies, we
can see that the C=C bond activation is in preference to the
C=O bond activation on the Pt(111). This is in consistent with
our observation that the hydrogenation preferentially occurred
on the C=C bond on monometallic Pt.
50
50
40
60
30
70
butions (the Supporting Information, Figure S6) demonstrated
that the reaction proceeded through preferential hydrogena-
tion of C=O bond for bimetallic PtÀSn NPs. The selectivity de-
livered by Pt Sn and Pt Sn NPs declined rapidly, leaving
75
25
60
40
HCOL as the final product. In contrast, Pt Sn , Pt Sn , and
5
0
50
40
60
Pt Sn NPs maintained their selectivity at the initial level
30
70
(
above 90%) with prolonged reaction time.
6
Table 1. Calculated adsorption of CAL on different surfaces: Pt (111), Pt /
SnO2Àx (110), and Pt
6
/SnO2Àx (110) with one Ovac
.
The efficiency of a reaction system is determined by a combi-
nation of conversion and selectivity. A higher conversion
means minimum recycling of the reactant, and a higher selec-
tivity means fewer purification steps. In this case, Pt Sn is
[a]
Surface
E
ads [eV]
[22]
Pt(111) Pt(111)
Pt /SnO
6
2Àx
6 2Àx
Pt /SnO (110)
with one Ovac
(110)
50
50
4
the most efficient catalyst among all those investigated. It ach-
ieved >99% conversion of CAL and 92% selectivity to COL at
ambient conditions. With this highly efficient catalyst in hand,
we then set out to investigate its stability. As shown in Fig-
ure 5d, both the conversion of CAL and the selectivity to COL
remained stable in the first five cycles of reuse. Starting from
the sixth cycle, the conversion of CAL gradually decreased,
whereas the selectivity to COL was maintained above 90%. An
h -di-s(C=O)-di-s(C=C) À1.14 À1.11
N/A
N/A
atop adsorption on Pt À0.90 À1.07
À0.67
À1.01
À1.02
À0.51
À0.93
À1.10
À1.05
À1.13
2
h -di-s(C=C)
À0.60 À0.58
À0.55 À0.46
2
h -p(C=C)
atop adsorption on Sn N/A
N/A
[
a] N/A=No adsorption.
EDX study indicated that the composition of Pt Sn NPs after
We then built a catalyst model, namely, the Pt /SnO (110)
6 2Àx
5
0
50
ten cycles of reuse remained almost the same as those of the
fresh ones (the Supporting Information, Figure S11). The de-
creased catalytic activity could be attributed to the mass loss
during catalyst recovery, and aggregation of the used catalyst
surface, according to the coordination number of the EXAFS
data of Pt Sn alloy, to investigate the adsorption of CAL. Ex-
50
50
perimentally, the fitted coordination numbers for PtÀSn and
PtÀPt in Pt Sn alloy are 1.7 and 5.2, respectively (the Sup-
50
50
(
the Supporting Information, Figure S12) could also lead to the
porting Information, Table S2). As show in Figure S14a and
b (the Supporting Information), the model was constructed by
decline of the active surface.
It should be noted that separated Pt and tin oxides (the
Supporting Information, Figure S9) with the same composition
as Pt Sn NPs exhibited an increased CAL conversion rate but
placing six Pt atoms on the SnO surface, taking the place of 5
2
O atoms and 1 Sn atom. In this catalyst model, the average co-
ordination number of PtÀSn is 1.6, in reasonable agreement
with the experimental value. The coordination number of the
Pt atom in the center of all six Pt atoms is 5. The adsorption of
CAL on this catalytic model was then investigated. For the ad-
sorption of C=O bond on Pt atom, the most stable atop ad-
sorption configuration (the Supporting Information, Fig-
ure S14c) has an adsorption energy of À0.67 eV. It is also pos-
sible that the C=O bond is adsorbed onto the Sn atoms. For
50
50
a decreased COL selectivity (the Supporting Information, Fig-
ure S10). This control experiment suggested that the close con-
tact of Pt sites with tin oxides might be a requirement for the
production of desired COL. A similar result was observed in
[
21]
a previous study.
DFT calculations and the reaction mechanism
2
the h -di-s(C=C) adsorption on Pt atom (the Supporting Infor-
For the selective hydrogenation of a,b-unsaturated aldehyde,
the adsorption of CAL on the catalyst is the first step. By com-
paring the calculated adsorption energies of C=O and C=C
bond through density functional theory, we could make an es-
timation about the activation priority of these two kinds of
mation, Figure S14d), the adsorption energy is À1.01 eV. The
2
most stable CAL adsorption configuration is the h -p(C=C) ad-
sorption (the Supporting Information, Figure S14e), with an ad-
sorption energy of À1.02 eV. The most stable adsorption of the
C=O bond on the Sn atom (the Supporting Information, Fig-
ure S14 f) is À0.51 eV. The adsorption of C=C bond on Sn
atoms is not feasible. From the result above, we can see that
the activation of C=O did not preferentially occur on these
[
22]
double bonds.
First, we investigated the adsorption of CAL on Pt (111) sur-
face as a benchmark for our calculation. The most stable con-
4
figuration for CAL adsorption on Pt (111) surface is a h -di-s(C= two kinds of surfaces.
Chem. Eur. J. 2015, 21, 12034 – 12041
12038
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