Structure-Dependent Selective Hydrogenation of α,β-Unsaturated Aldehydes over Platinum Nanocrystals Decorated with Nickel

Article abstract of DOI:10.1002/cplu.201402109

The shape sensitivity of monometallic Pt and bimetallic PtˉNi nanocrystals in α,β-unsaturated aldehydes is studied by using a cubic shape enclosed by six {100} facets as well as an octahedral shape surrounded by eight {111} facets. Compared with monometallic Pt and bimetallic PtˉNi cubic/octahedral shapes, Pt₃Ni cubes enhanced the selective hydrogenation of the C=O double bond and suppressed the selective hydrogenation of the C=C double bond of the α,β-unsaturated aldehyde. The Pt, Pt₃Ni, or PtNi octahedral shape is an unfavorable structure for C=O hydrogenation and enables the activation of the whole conjugated system of the molecule, which leads to complete hydrogenation to form the saturated alcohol product. The synergistic effects of the surface structure and electronic properties of Pt or PtˉNi nanocrystals play a key role in controlling the selective hydrogenation of C=C and C=O bonds of α,β-unsaturated aldehydes.

Full text of DOI:10.1002/cplu.201402109

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DOI: 10.1002/cplu.201402109
Structure-Dependent Selective Hydrogenation of
a,b-Unsaturated Aldehydes over Platinum Nanocrystals
Decorated with Nickel
[a, b]
[a]
[a]
[a]
[a]
[a]
Zhi Jiang,
Yonghui Zhao, Lingzhao Kong, Ziyu Liu, Yan Zhu,* and Yuhan Sun*
The shape sensitivity of monometallic Pt and bimetallic PtꢀNi
nanocrystals in a,b-unsaturated aldehydes is studied by using
a cubic shape enclosed by six {100} facets as well as an octahe-
dral shape surrounded by eight {111} facets. Compared with
monometallic Pt and bimetallic PtꢀNi cubic/octahedral shapes,
drogenation of the C=O double bond by taking advantage of
the synergistic effects of metallic catalysts, in which bimetallic
catalysts with particular geometric and electronic structures
[16–20]
display strongly structure-dependent behavior.
In particu-
lar, Pt-based bimetallic nanocrystals such as pure Pt crystals ex-
hibit defined crystallographic planes and can be used to ex-
Pt Ni cubes enhanced the selective hydrogenation of the C=O
3
[21–26]
double bond and suppressed the selective hydrogenation of
plore the structure sensitivity of liquid-phase reactions.
the C=C double bond of the a,b-unsaturated aldehyde. The Pt,
Benzene hydrogenation was used as a example to demon-
strate that both cyclohexene and cyclohexane formed on cu-
boctahedral Pt nanoparticles, and only cyclohexane formed on
Pt Ni, or PtNi octahedral shape is an unfavorable structure for
3
C=O hydrogenation and enables the activation of the whole
conjugated system of the molecule, which leads to complete
hydrogenation to form the saturated alcohol product. The syn-
ergistic effects of the surface structure and electronic proper-
ties of Pt or PtꢀNi nanocrystals play a key role in controlling
the selective hydrogenation of C=C and C=O bonds of a,b-un-
saturated aldehydes.
[27]
cubic Pt nanoparticles.
In this study, monometallic Pt and bimetallic PtꢀNi (Pt atoms
replaced with Ni atoms) nanocrystals with a well-defined cubic
shape enclosed by six {100} facets as well as an octahedral
shape surrounded by eight {111} facets were synthesized.
Herein, the replacement of one atom of every four Pt atoms or
two Pt atoms with one Ni atom (referred to as Pt Ni and PtNi)
3
can be effectively controlled by the choice of ratios between
Pt and Ni precursors. Studies of the hydrogenation of an a,b-
unsaturated aldehyde (cinnamaldehyde) were undertaken on
the well-defined single-crystal surface of cubic and octahedral
shapes to establish the specific modes of adsorption and ach-
ieve a better understanding of the mechanism of selective hy-
drogenation. With respect to monometallic Pt and bimetallic
PtꢀNi cubic/octahedral shapes for selective hydrogenation of
Great effort has been made to control the selective hydrogena-
tion of C=O and C=C bonds of a,b-unsaturated aldehydes or
[
1–5]
ketones for valuable industrial products.
The difference in
ꢀ
1
bond energy—715 kJmol for the energy of the C=O double
ꢀ
1
bond and 615 kJmol for the C=C double bond—makes the
hydrogenation of the C=O bond more difficult than hydroge-
nation of the C=C bond, yet the resulting unsaturated alcohol
products from C=O hydrogenation are valuable intermediates
the a,b-unsaturated aldehyde, Pt Ni cubes with six (100) surfa-
3
[
6–9]
for the production of perfumes and flavorings.
Selective hy-
ces enhanced the selectivity of C=O bond hydrogenation and
suppressed the selectivity of C=C bond hydrogenation, which
could be attributed to the modified electronic structure and
drogenation of a,b-unsaturated aldehydes/ketones is often
used as a structure-dependent reaction to investigate the ca-
pability of the hydrogenation of C=O and C=C bonds over
model catalysts. As we already know, the catalytic activity and
selectivity are affected by several factors, including catalyst
preparation and activation procedures, the geometric structure
steric effect on Pt Ni cubes through doping of Ni atoms. How-
3
ever, the octahedron with the closed-packed (111) surfaces, re-
gardless of whether it was Pt, Pt Ni, or PtNi, was an unfavora-
3
ble structure for C=O hydrogenation, as it induces a steric hin-
drance toward the accommodation of the C=O group and ena-
bles the activation of the entire conjugated system of the mol-
ecule, thus leading to complete hydrogenation to form the
saturated alcohol product. Thus, surface, composition, and
structure are essential for achieving synergy in terms of catalyt-
ic activation of the C=C and C=O bonds of a,b-unsaturated al-
dehydes/ketones.
[10–15]
of catalysts, as well as the electronic structure of catalysts.
Therefore, many attempts have been made to promote the hy-
[
a] Z. Jiang, Dr. Y. Zhao, Dr. L. Kong, Dr. Z. Liu, Prof. Y. Zhu, Prof. Y. Sun
CAS Key Laboratory of Low-Carbon Conversion Science and
Engineering, Shanghai Advanced Research Institute
Chinese Academy of Sciences, No. 100 Haike Road
Pudong District, Shanghai 201210 (P. R. China)
Fax: (+86)21-20350867
Typical transmission electron microscopy (TEM) images of
E-mail: zhuy@sari.ac.cn
Pt Ni shapes are shown in Figure 1. The cubic nanocrystals are
3
sunyh@sari.ac.cn
less than 10 nm in size and exclusively bound by six {100}
facets, as indicated by the continuous lattice fringes with inter-
planar spacings of 0.185 nm on the basis of high-resolution
[
b] Z. Jiang
University of Chinese Academy of Sciences
Beijing 100049 (P. R. China)
(
HR) TEM analysis (Figure 1b). Energy-dispersive spectrometry
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/cplu.201402109.
(EDS) revealed the coexistence of Pt and Ni in the cubes at an
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Figure 2. TEM images of (a) PtNi cubes, (b) PtNi octahedrons, (c) Pt cubes,
3
Figure 1. (a) TEM and (b) HRTEM images of Pt Ni cubes. (c) TEM and
and (d) Pt octahedrons. Insets show the atom ratios of Pt and Ni from EDS.
(d) HRTEM images of Pt
3
Ni octahedrons. Insets show the atom ratios of Pt
and Ni from EDS.
atomic ratio of about 3:1 (Figure S1a in the Supporting Infor-
mation). Figure 1c,d shows that the octahedral nanocrystals
are uniform in their narrow size distribution (about 5 nm) and
exclusively enclosed by eight equivalent {111} facets, which
was determined by the crystal lattice fringes being 0.214 nm
apart in the HRTEM image (Figure 1d). The atomic ratio of Pt
and Ni in the octahedron is about 3:1, as indicated by EDS
were replaced with Ni atoms, such as half of the Pt atoms
being replaced with Ni atoms to form PtNi cubes, however, hy-
drogenation of the C=O bond was suppressed and the unsatu-
rated alcohol was a minor product, as shown in Figure 3e.
Interestingly, octahedral nanocrystals including Pt, Pt Ni, and
3
PtNi achieved different results to the cubic nanocrystals. As
shown in Figure 3b,d,f, hydrogenation of both C=O and C=C
bonds was suppressed over Pt and PtꢀNi octahedrons, and sa-
turated alcohol from complete hydrogenation was a major
product, although saturated aldehyde was formed during the
initial reaction and then disappeared after 5 hours of reaction.
The selective hydrogenation of C=O and C=C bonds was not
found over the (111) surface of the close-packed octahedral
structure, which indicated that the selective hydrogenation of
C=O and C=C bonds is independent of the compositions of
PtꢀNi with octahedral nanocrystals.
(Figure S1b). TEM images of monometallic Pt cubes and octa-
hedrons as well as bimetallic PtNi cubes and octahedrons are
also shown in Figure 2. The insets in Figure 2a,b show the
atomic ratios of Pt and Ni in cubes (1:1) and in the octahedron
(1:1), respectively.
The comparisons of the catalytic activity of monometallic Pt
and bimetallic PtꢀNi (Pt Ni and PtNi) nanocrystals for the selec-
3
tive hydrogenation of a,b-unsaturated aldehydes (cinnamalde-
hyde) are shown in Figure 3. Although Pt, PtNi, and Pt Ni
3
cubes and octahedrons achieved high catalytic activity for the
hydrogenation of unsaturated aldehyde, they have different
abilities when it comes to selective hydrogenation of C=O and
C=C bonds. In terms of cubic nanocrystals, the saturated alde-
hyde from C=C bond hydrogenation was found before
To examine the electronic properties of the catalytically
active Pt Ni cubes, monometallic Pt and bimetallic PtꢀNi (Pt Ni
3
3
and PtNi) cubes/octahedrons were studied by means of high-
resolution X-ray photoelectron spectroscopy (XPS). As shown
in Figure 4a, the Pt 4f7/2 and Pt 4f5/2 binding energies for the
monometallic Pt cubes are 70.9 and 74.2 eV, respectively. Shifts
10 hours of reaction and then transformed to the complete hy-
drogenation product after 10 hours of reaction. The phenom-
in binding energies were found for both Pt Ni and PtNi cubes.
3
enon can be also obtained over Pt (Figure 3a), Pt Ni (Figure 3c),
The Pt 4f7/2 and Pt 4f5/2 binding energies for Pt Ni cubes were
3
3
and PtNi cubes (Figure 3e). Nevertheless, the selectivity to-
wards the unsaturated alcohol from C=O bond hydrogenation
was different with cubes. Hydrogenation of only the C=O
double bond over monometallic Pt cubes was not achieved.
When one-quarter of the Pt atoms were replaced with Ni
0.4 eV higher than the value that is characteristic of Pt cubes.
Pt 4f7/2 and Pt 4f5/2 binding energies of PtNi cubes shifted to
the higher binding energies such as 71.4 and 74.7 eV relative
to pure Pt cubes. For the octahedral nanocrystals, as shown in
Figure 4b, Pt 4f7/2 and Pt 4f5/2 binding energies for Pt Ni and
3
atoms to form Pt Ni cubes, the selectivity of C=O bond hydro-
PtNi octahedrons were almost consistent with those of pure Pt
octahedrons (Pt 4f7/2 and Pt 4f5/2 are 71.0 and 74.1 eV, respec-
tively). These results reflect the fact that the electronic proper-
3
genation was higher than that of C=C bond hydrogenation
after 3 hours of reaction (see Figure 3c). When more Pt atoms
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cubes is not higher than that
over Pt Ni cubes. It suggests
3
that the electronic structure can
account for the better selectivity
of C=O bond hydrogenation on
PtꢀNi cubes than that on PtꢀNi
octahedrons, and yet it does not
manage to explain why Pt Ni
3
cubes favor the activation of the
C=O group more than PtNi
cubes.
To improve the selectivity of
C=O bond hydrogenation and
suppress the selectivity of C=C
bond hydrogenation, the hydro-
genation of C=C step is vital.
Density functional theory (DFT)
calculations were performed to
reveal the origin of the higher
reactivity of the cubic shape
than octahedral-shaped Pt Ni in
3
suppressing the selectivity of C=
C
bond
hydrogenation.
Pt Ni(100) and Pt Ni(111) surfaces
3
3
were modeled by using 3ꢁ3
and 2ꢁ2 super cells with three
layers of thickness. A vacuum
region of 13 ꢂ was considered
to avoid electronic interaction
between slabs. In the calcula-
tions, the bottom layer of metal
atoms was fixed and the top
two layers and the adsorbates
were relaxed. The adsorption
energy was defined as Eads
=
Eadsorbate/slabꢀE
ꢀEslab,
in
adsorbate
which Eads is the calculated ad-
sorption energy, Eadsorbate/slab is the
energy of the adsorbed system,
Eadsorbate is the energy of adsor-
bate in the gas phase, and Eslab is
3 3
Figure 3. Catalytic performance of (a) Pt cubes, (b) Pt octahedrons, (c) Pt Ni cubes, (d) Pt Ni octahedrons, (e) PtNi
cubes, and (f) PtNi octahedrons for the selective hydrogenation of cinnamaldehyde, respectively.
ties of bimetallic PtꢀNi cubes have been influenced through Ni
the surface energy. The most stable configurations of C H -CH-
6 5
atoms replacing the Pt atoms of Pt cubes, whereas the charges
CH -CH OH on the Pt Ni(100) and Pt Ni(111) surfaces obtained
2 2 3 3
are not redistributed in Pt Ni and PtNi octahedrons even
from our DFT calculations are shown in Figure 5a,b, and the
corresponding binding energies are ꢀ3.81 and ꢀ2.18 eV, re-
spectively. At the same time, the DFT results of H and C H -
3
though the Pt atoms of Pt octahedrons are replaced by Ni
atoms. Therefore, the electron transfer from the Ni species to
the Pt sites evidenced in XPS analysis should explain the pro-
motional effect of Ni in the performance of Pt cubes in terms
of the selectivity of C=O and C=C hydrogenation. The high
electron density of Pt weakens the strength of the Ptꢀ(C=C)
bonding and decreases the activity of the C=C bond through
an increase in the repulsive four-electron interaction, and it in-
creases the probability of C=O bond activity attributed to the
6
5
CH=CH-CH OH and selected geometric parameters are shown
2
in Table S1 of the Supporting Information. The adsorption
energy indicates higher binding energy on the Pt Ni(100) sur-
3
face than on the Pt Ni(111) surface, which is due to the more
3
open surface structure of the (100) surface than the (111) sur-
face. After obtaining the adsorbed structures of the intermedi-
ates, we have further calculated the elementary step related to
the formation of C H -CH-CH -CH OH. Figure 6 shows the path-
[
3,21,28]
strength of the Pt(5d)ꢀCO(2p*) bonding interactions.
It is
6
5
2
2
noted that Pt 4f7 and Pt 4f electron binding energies for
way to produce C H -CH-CH -CH OH on the Pt Ni(100) surface
/2
5/2
6
5
2
2
3
PtNi cubes were 0.1 eV higher than that of Pt Ni cubes, but
(in red) and Pt Ni(111) surface (in black). The located initial
3
3
the selectivity towards C=O bond hydrogenation over PtNi
states (IS) and transition states (TS) are shown in Figures S2
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Figure 6. Comparison of the calculated energy profiles for C
CH -CH-CH -CH OH on the Pt Ni(100) surface (in red) and
OH+H!C
Pt Ni(111) surface (in black).
6 5
H -CH=CH-
2
6
H
5
2
2
3
3
O-to-surface distance (3.18 ꢂ) on the Pt Ni(100) surface. As in
3
the case of the Pt Ni(100) surface, during the reaction, the ad-
3
sorption mode results in a longer distance between H and
C H -CH=CH-CH OH (3.03 versus 2.64 ꢂ) in the IS and results in
6
5
2
a further increase in the activation barrier to produce C H -CH-
6
5
CH -CH OH relative to the Pt Ni(111) surface. Thus, our DFT re-
2
2
3
sults suggest that the cubic shape could achieve higher selec-
tivity of C=O bond hydrogenation than octahedrally shaped
Pt Ni.
3
In summary, the influence of replacing Pt atoms with Ni
atoms on cubes is more significant in the selectivity for a,b-un-
saturated aldehyde hydrogenation than that on octahedrons.
Both monometallic Pt and bimetallic PtꢀNi octahedrons gave
similar results for cinnamaldehyde hydrogenation. For cubes,
selectivity towards C=O bond hydrogenation is enhanced as Pt
atoms on cubes are replaced with Ni atoms, and replacing
3
Figure 4. XPS Pt 4f spectra of (a) cubes and (b) octahedrons of Pt, Pt Ni, and
PtNi.
and S3 of the Supporting Information, respectively. The higher
hydrogenation barrier on the (100) plane might be caused by
the different adsorption configuration of C H -CH=CH-CH OH.
one-quarter of the Pt atoms with Ni atoms (Pt Ni cubes)
6
5
2
3
On the Pt Ni(100) surface, the CH OH group is bonded with
achieved higher selectivity toward C=O hydrogenation than re-
placing half of the Pt atoms with Ni atoms (PtNi cubes). Thus,
the synergistic effects of the surface structure and electronic
properties of nanocrystals are es-
3
2
the surface with an O-to-surface distance of about 2.08 ꢂ,
whereas CH OH does not bond with the surface with a longer
2
sential in achieving selective hy-
drogenation of the C=C and C=
O bonds of a,b-unsaturated al-
dehyde/ketone. Overall, this
study might raise interesting
possibilities for the development
of decorated, doped, or replaced
nanostructures with well-defined
crystal surfaces in heterogene-
ous catalysis.
Experimental Section
Synthesis of Pt Ni cubes
3
In a typical synthesis, under argon
flow, platinum acetylacetonate
[
Pt(acac)2] (20.0 mg, 0.05 mmol),
nickel acetylacetonate [Ni(acac)2]
10.0 mg, 0.039 mmol), oleylamine
(
Figure 5. Top and side views of calculated adsorption structures of C
b) Pt Ni(111) surfaces.
6
H
5
-CH-CH
2
-CH
2
OH on (a) Pt
3
Ni(100) and
(9 mL), and oleic acid (1 mL) were
mixed in a 25 mL three-necked,
(
3
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round-bottomed flask with a magnetic stirrer. The flask was im-
mersed in an oil bath at 1308C, and the reaction mixture turned
into a transparent yellowish solution. The flask was transferred to
a second oil bath at 2108C under carbon monoxide gas and was
injected with methanol (50 mL). The typical flow rate of CO gas was
set at 280 mLmin , and the reaction time was 20 min. The prod-
uct was precipitated by ethanol and then redispersed in cyclohex-
ane.
Acknowledgements
This study was supported by the National Natural Science Foun-
dation of China (21273151) and the Shanghai Pujiang Program
(13J1407700).
ꢀ1
Keywords: bimetallic catalysts
hydrogenation · nickel · platinum
·
electronic structure
·
PtNi cubes were prepared from [Pt(acac) ] and [Ni(acac) ] as the
2
2
precursors (the mole ratio of Pt and Ni precursors was 1:0.4) under
otherwise identical experimental conditions. Pt cubes were pre-
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The hydrogenation reaction was begun by turning on the stirring
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Characterization
TEM and HRTEM images were recorded with a JEOL JEM-2100 elec-
tron microscope. XPS experiments were carried out with an RBD
upgraded PHI-5000C ESCA system (Perkin–Elmer) with MgKa (hn=
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253.6 eV) or AlKa radiation (hn=1486.6 eV). The catalytic products
Received: April 14, 2014
were analyzed with a Shimadzu GC-2014 instrument equipped
with a flame ionization detector.
Revised: June 17, 2014
Published online on July 17, 2014
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