Synthesis of bimetallic SnPt-nanoparticle catalysts for chemoselective hydrogenation of crotonaldehyde: Relationship between SnxPty alloy phase and catalytic performance

Article abstract of DOI:10.1016/j.cattod.2017.11.008

Bimetallic SnPt-nanoparticle (SnPt-NP) catalysts with several types of Sn_xPt_y alloy structures were prepared using a polyalcohol reduction process; the catalytic behavior of each Sn_xPt_y alloy phase toward the selective hydrogenation of unsaturated aldehydes to the corresponding unsaturated alcohols was elucidated. Atomic absorption spectroscopy (AAS) and transmission electron microscopy (TEM) indicate that SnPt-NP catalysts with various Sn/Pt atomic ratios can be successfully prepared by a polyalcohol reduction process using Pt(acac)₂ and Sn(AcO)₂ as the metal precursors. X-ray diffraction (XRD) results reveal that the Sn₁Pt₃, Sn₁Pt₁, and Sn₂Pt₁ alloy phases are formed through control of the Sn/Pt atomic ratio of the starting mixture during preparation. The Sn₁Pt₃ alloy phase enhanced the hydrogenation of both the C[dbnd]C and C[dbnd]O bonds during the selective hydrogenation of crotonaldehyde. When only Sn₁Pt₁ alloy phases (Sn/Pt = 1.40), accompanying with separated Sn phase, were formed in the SnPt-NP catalysts, the highest unsaturated alcohol (UOL) selectivity (71.5% at 37.6% conversion) was observed. The formation of the Sn₂Pt₁ alloy phase led to decreased UOL selectivity. We suggest that the Sn₁Pt₁ alloy phase is an effective bimetallic SnPt hydrogenation catalyst for the selective formation of unsaturated alcohols.

Full text of DOI:10.1016/j.cattod.2017.11.008

Accepted Manuscript
Title: Synthesis of Bimetallic SnPt-Nanoparticle Catalysts for
Chemoselective Hydrogenation of Crotonaldehyde:
Relationship between SnxPty Alloy Phase and Catalytic
Performance
Authors: Keita Taniya, Chih Hao Yu, Hiromu Takado, Taiki
Hara, Atsushi Okemoto, Takafumi Horie, Yuichi Ichihashi,
Shik Chi Tsang, Satoru Nishiyama
PII:
S0920-5861(17)30760-5
DOI:
Reference:
https://doi.org/10.1016/j.cattod.2017.11.008
CATTOD 11122
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Catalysis Today
Received date:
Revised date:
Accepted date:
30-5-2017
25-10-2017
5-11-2017
Please cite this article as: Keita Taniya, Chih Hao Yu, Hiromu Takado, Taiki
Hara, Atsushi Okemoto, Takafumi Horie, Yuichi Ichihashi, Shik Chi Tsang, Satoru
Nishiyama, Synthesis of Bimetallic SnPt-Nanoparticle Catalysts for Chemoselective
Hydrogenation of Crotonaldehyde: Relationship between SnxPty Alloy Phase and
Catalytic Performance, Catalysis Today https://doi.org/10.1016/j.cattod.2017.11.008
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Synthesis of Bimetallic SnPt-Nanoparticle Catalysts for Chemoselective Hydrogenation
of Crotonaldehyde: Relationship between SnxPty Alloy Phase and Catalytic Performance
Keita Taniyaa, b, *, Chih Hao Yuc, Hiromu Takadod, Taiki Harad, Atsushi Okemotoe,
Takafumi Horieb, d, Yuichi Ichihashib, d, Shik Chi Tsangc, Satoru Nishiyamad
a Organization for Advanced and Integrated Research, Kobe University, 1-1 Rokkodai,
Nada, Kobe, 657-8501, Japan
b
Center for Membrane and Film Technology, Graduate School of Engineering, Kobe
University, 1-1 Rokkodai, Nada, Kobe 657-8501, Japan
c Wolfson Catalysis Centre, Inorganic Chemistry Laboratory, The University of Oxford,
Oxford, OX1 3QR, UK
d Department of Chemical Science and Engineering, Graduate School of Engineering,
Kobe University, 1-1 Rokkodai, Nada, Kobe, 657-8501, Japan
e
Research Center for Chemical Process Technology, National Institute of Advanced
Industrial Science and Technology (AIST), 4-2-1 Nigatake, Miyagino, Sendai, 983-8551,
Japan
*TEL: +81-78-803-6546, FAX: +81-78-803-6179,
e-mail: taniya@platinum.kobe-u.ac.jp
1

Graphical abstract
Highlight
 Effect of SnxPty alloy phase for crotonaldehyde hydrogenation is investigated.
 SnPt-NP catalysts can be prepared by a polyalcohol reduction process.
 Sn1Pt3, Sn1Pt1 and Sn2Pt1 alloy phases were formed by changing Sn/Pt atomic
ratio.
 Sn1Pt1 alloy is the most effective for selective formation of unsaturated alcohols.
2

Abstract
Bimetallic SnPt-nanoparticle (SnPt-NP) catalysts with several types of SnxPty alloy
structures were prepared using a polyalcohol reduction process; the catalytic behavior of
each SnxPty alloy phase toward the selective hydrogenation of unsaturated aldehydes to
the corresponding unsaturated alcohols was elucidated.
Atomic absorption
spectroscopy (AAS) and transmission electron microscopy (TEM) indicate that SnPt-NP
catalysts with various Sn/Pt atomic ratios can be successfully prepared by a polyalcohol
reduction process using Pt(acac)2 and Sn(AcO)2 as the metal precursors. X-ray
diffraction (XRD) results reveal that the Sn1Pt3, Sn1Pt1, and Sn2Pt1 alloy phases are
formed through control of the Sn/Pt atomic ratio of the starting mixture during preparation.
The Sn1Pt3 alloy phase enhanced the hydrogenation of both the C=C and C=O bonds
during the selective hydrogenation of crotonaldehyde. When only Sn1Pt1 alloy phases
(Sn/Pt = 1.40), accompanying with separated Sn phase, were formed in the SnPt-NP
catalysts, the highest unsaturated alcohol (UOL) selectivity (71.5% at 37.6% conversion)
was observed. The formation of the Sn2Pt1 alloy phase led to decreased UOLselectivity.
We suggest that the Sn1Pt1 alloy phase is an effective bimetallic SnPt hydrogenation
catalyst for the selective formation of unsaturated alcohols.
Keywords: SnPt bimetallic catalysts, Nano particles, Polyalcohol reduction process,
Selective hydrogenation, Unsaturated aldehyde
3

1. Introduction
Addition of Sn to Pt catalysts has been investigated in several different catalytic reaction
systems [1-18]. These include the dehydrogenation of light alkanes [1-3], reforming of
hydrocarbons [4-6], oxidation of carbon monoxide [7, 8], and selective hydrogenation of
unsaturated carbonyl compounds [9-22].
During the selective hydrogenation of unsaturated carbonyl compounds, the addition of
Sn to the hydrogenation catalyst has been reported to improve selectivity toward the
corresponding unsaturated alcohols [9-22]. We have also investigated the selective
hydrogenation of unsaturated aldehydes over bimetallic Sn-Pt and Sn-Rh catalysts [21-
24].
The catalytic performance of bimetallic catalysts has been reported to depend on the
Sn/Pt atomic ratio, precursors employed, and preparation procedures [9-22]. Even
though the role of Sn is not yet completely understood, two effects have been proposed
to explain the observed enhancement of catalytic selectivity by adding Sn. The carbonyl
group is activated by an ionic Sn species (Sn2+ or Sn4+) through interactions with the
oxygen atom of the carbonyl group; consequently, the hydrogenation of the C=O bond is
facilitated [9-12]. Based on IR studies, we proposed that Sn plays an important role not
only in the suppression of interactions between the C=C bond and the group 14 metal,
but also in the selective activation of the carbonyl group [24]. The Pt ensembles are
diluted by metallic Sn and are accompanied by the formation of SnxPty alloys. Pt
ensembles preferentially activate the C=C bonds of unsaturated aldehydes, catalyzing the
formation of the undesired saturated aldehydes and alcohols. Dilution of the Pt
4

ensemble with metallic Sn sites hinders the hydrogenation of the C=C bond and inhibits
the readsorption of the unsaturated alcohol; this suppressed further hydrogenation of the
unsaturated alcohol to the corresponding saturated alcohol [9-11, 14].
Several
researchers have investigated supported bimetallic SnPt catalysts using inert supports
such as carbon or silica in order to remove any possible strong interactions involving the
metal support [9, 10, 16]. Tin species on SnPt catalyst systems have been reported to be
present in oxidized states, which include SnOx species, and/or metallic states that form
SnxPty alloy particles on the supported bimetallic SnPt catalysts. Claus [19] and
Marinelli et al. [20] independently reported that C=O bonds are more easily activated
over oxidized Sn species than the SnPt alloy phase formed in Sn-Pt/SiO2. Coloma et al.
[9, 10] suggested that the Sn1Pt1 alloy phase also suppresses the hydrogenation of the
C=C double bond.
When supported bimetallic SnPt catalysts are prepared by conventional methods, such
as the co-impregnation method, several SnxPty-alloy and Sn-oxide compositions are
formed concurrently. The presence of several Sn sites of differing composition on these
bimetallic SnPt catalysts makes it difficult to clarify which Sn sites determines the
catalytic performance for the formation of unsaturated alcohols. Therefore, it is
important to prepare bimetallic nanoparticles with “uniform composition and structure.”
Recently, methods for the preparation of unsupported monometallic and bimetallic
nanoparticle (NP) have been investigated. For example, NPs composed of Co [25], Pt
[26], CoPt [26-28], FePt [29, 30], and SnPt [31-34] have been successfully prepared by
several procedures. If bimetallic SnPt NPs with various Sn/Pt atomic ratios can be
5

prepared, then these unsupported catalysts can be applied in studies aimed at solving the
above-mentioned problems, namely the interference by interactions involving the metal
support.
In order to remove the effect of the metal support, the preparation of unsupported
bimetallic SnPt-nanoparticle (SnPt-NP) catalysts was attempted using a polyalcohol
reduction process. The Sn/Pt atomic ratios introduced in the prepared SnPt-NP catalysts
during the polyalcohol reduction process were varied to form the several types of SnxPty
alloys. To find an effective bimetallic SnPt structure and composition, the bulk
structures of the SnPt-NP catalysts obtained in this study were examined by XRD
measurements. We show that these NPs not only catalyze the reduction of unsaturated
aldehydes, but also selectively produce crotyl alcohol during the hydrogenation of
crotonaldehyde.
2. Experimental
2.1. Catalyst preparation
2.1.1. Pt-NP and SnPt-NP catalysts
Unsupported Pt-NP and SnPt-NP catalysts were prepared by the polyalcohol reduction
process in the liquid phase [26, 27, 32]. A three-neck round bottom flask was charged
with a mixture of platinum(II) acetylacetonate (0.197 g), tin(II) acetate (0-0.177 g), oleic
acid (0.32 mL), oleyl amine (0.34 mL), and 1,2-hexadecanediol (0.78 g) in dioctyl ether
(20 mL). The Sn/Pt atomic ratio was controlled to be between 0 and 1.5 by varying the
amount of introduced tin(II) acetate. N2 was bubbled through the mixture while it was
6

heated to ~553 K using a heating mantle, followed by heating at reflux for 1 h. After
cooling to room temperature, the mixture, which was originally yellow (without Sn
precursor) or auburn (with Sn precursor) in color and had changed to black during heating,
was centrifuged to separate the bimetallic nanoparticles. The supernatant was discarded,
and the obtained precipitate was washed three times with ethanol (the total amount = 60
mL). The sample was dried at room temperature for 2 h in a vacuum desiccator to finally
obtain Pt nanoparticle (Pt-NP) or SnPt nanoparticle (SnPt-NP) catalysts.
2.1.2. SiO2-supported bimetallic Sn-Pt catalysts (Sn-Pt/SiO2) prepared by the co-
impregnation method
The Sn-Pt/SiO2 catalysts were prepared by co-impregnating 1-2 g of a SiO2 support at
353 K with an ethanolic solution (30 mL) of H2PtCl6·6H2O and SnCl2·2H2O. The Pt
loading on the support was 4 wt% and the atomic ratio of Sn/Pt was between 0 and 2.0.
The samples were dried overnight at 393 K in a static oven, then calcined at 823 K for 2
h under a flow of air, followed by reduction at 573 K for 2 h under a flow H2.
2.2. Characterization
2.2.1. Atomic Absorption Spectroscopy (AAS)
The compositions of Pt and Sn in the Pt-NP and the SnPt-NP catalysts were determined
by Atomic Absorption Spectroscopy (HITACHI, Z-2000). Prior to each measurement,
10 mg of the sample was dissolved in 10 mL of aqua regia at 353 K.
7

2.2.2. High Resolution Transmission Electron Microscopy (HR-TEM)
HR-TEM images of the Pt-NP and the SnPt-NP (Sn/Pt = 1.40) were acquired on a JOEL
JEM2011 instrument operating at 200 kV.
2.2.3. X-ray diffraction (XRD)
X-ray powder diffraction patterns of the catalysts were acquired at room temperature
using a Rigaku RINT-2000 instrument equipped with a Cu-Kα radiation source. The X-
ray tube operated at 40 kV and 40 mA. The diffraction patterns were recorded over the
20-60° 2θ range at a rate of 0.5 °/min.
2.3. Chemoselective hydrogenation
The hydrogenation of crotonaldehyde was carried out in a stainless steel autoclave (30
mL). A glass tube was charged with crotonaldehyde (0.2 mL) in 2-methyl-2-butanol (4
mL) and 10 mg of NP catalysts was added. The glass tube was inserted into an autoclave
such that the reactant solution was not in direct contact with the inner surface of the
autoclave. The autoclave was charged and discharged with hydrogen several times in
order to replace the air in the autoclave with hydrogen, after which it was charged with
2.0 MPa (gauge) of H2. The autoclave was heated to 373 K in an oil bath in which the
temperature was controlled by a temperature regulator, and the solution was vigorously
agitated using a magnetic stirrer for 0.5 h. The autoclave was then cooled to 273 K to
quench the reaction, and cyclohexane (0.2 mL) was added as an internal standard. The
mixture was centrifuged to separate the catalyst, and the products were analyzed by gas
8

chromatography (GC) (GC-18A, Shimazu) with a G-300 column (Chemicals Evaluation
and Research Institute, Japan; column dimensions: 40 m, 1.2 mm i.d., 1.0 μm film
thickness).
As for the hydrogenation of crotonaldehyde over the Sn-Pt/SiO2 catalysts prepared by
the co-impregnation method, the reaction was carried out at 373 K under 2.0 MPa (gauge)
of H2 for 1 h with 25 mg of catalysts.
3. Results and discussion
3.1. The Sn and Pt content of the SnPt-NP catalysts
The Pt nanoparticles and Pt-based bimetallic nanoparticles were previously prepared
using a polyalcohol reduction process [26, 27, 32], and we applied this method to the
preparation of the SnPt nanoparticles in this study. Table 1 lists the Pt and Sn
compositions, and the Sn/Pt atomic ratios in each NP as determined by AAS. The
amounts of Sn in the SnPt-NP catalysts increased, while the levels of Pt decreased with
increasing amounts of Sn introduced during preparation. Each nanoparticle (Pt-NP and
SnPt-NP) included ~30 wt% of unidentified compounds that could not be observed by
AAS. These probably arise from the capping agents (oleyl amine and oleic acid) that
stabilize the NPs, and the ethanol used for NP washing during preparation. The Sn/Pt
atomic ratio in the various SnPt-NP catalysts as a function of the Sn/Pt atomic ratio of the
starting mixture is shown in Fig. 1. As shown in Fig. 1, the Sn/Pt atomic ratios of the
catalysts determined by AAS are almost identical to the Sn/Pt atomic ratios introduced
during preparation. In the following sections, the Sn/Pt atomic ratios of the NP catalysts
9

are assumed to be those determined by AAS, as shown in Table 1.
3.2. Morphologies of the SnPt-NP catalysts
HR-TEM images of the Pt-NP and SnPt-NP (Sn/Pt = 1.40) prepared by polyalcohol
reduction are displayed in Fig. 2. Nanoparticles with polyhedral structures are observed
for the Pt-NP catalysts in Fig. 2a, and the diameters of these nanoparticles were in the 10-
20 nm range. As for the SnPt-NP catalysts (Sn/Pt = 1.40) (Fig. 2b), smaller particles
with diameters of 2-10 nm were obtained. The addition of Sn brought about a decrease
in particle size compared to the Pt-NP catalysts, although the same amounts of Pt
precursor, oleyl amine, oleic acid, and 1,2-hexadecanediol were employed during
preparation. The initial Sn content has been reported to affect the particle size
distribution of SnPt-NP; one is synthesized by a polyalcohol reduction method similar to
our procedure except for Sn precursor using Sn(acac)2 [32], and another is fabricated by
contacting Pt(dba)2 (dba = dibenzylidene acetone) and (n-Bu)3SnH in THF under H2 at
room temperature without the use of stabilizing ligands [34]. A similar observation was
observed during the synthesis of CoPt3 nanoparticles, prepared by a polyalcohol reduction
method via simultaneous reduction of Pt(acac)2 and thermodecomposition of Co2(CO)8
in the presence of 1-adamantanecarboxylic acid and hexadecylamine as stabilizing agents
[28]. This behavior was attributed to the apparent presence of a second metal species.
Nevertheless, the effect of second metals on the size of bimetallic NPs is currently under
investigation.
10

3.3. Composition of the SnxPty alloys in various SnPt-NP catalysts of differing Sn/Pt
atomic ratios
Figure 3 displays the XRD patterns of the Pt-NP, and SnPt-NP catalysts of varying Sn/Pt
atomic ratios. The peaks at 2θ values of around 39.5 and 46.5° are assigned to reduced
Pt metal [27, 35, 36]. The peaks ascribed to the Sn1Pt1 alloy phase, in which the molar
ratio of Sn to Pt is 1:1, appeared at 2θ values of around 25.0, 30.0, 41.5, and 44.0° [37,
38], and the peaks attributed to the Sn2Pt1 alloy phase, in which the molar ratio of Sn to
Pt is 2:1, can be observed at 2θ values of around 24.0, 40.0, 47.0, and 57.5° [39].
In case of the Pt-NP catalysts, the observed peaks are characteristic of face-centered-
cubic (fcc) crystalline Pt. Upon the addition of a small amount of Sn (Sn/Pt = 0.48),
these diffraction peaks were observed to shift to slightly lower 2θ diffraction angles than
the corresponding Pt-NP peaks. The peaks at 2θ values of 38.8 and 44.6° are assigned
to the Sn1Pt3 alloy phase [40]. This result suggests that the lattice has undergone
expansion resulting from the insertion of the larger Sn atoms into the fcc Pt unit structure
in order to form the Sn1Pt3 alloy phase. The Sn1Pt1 alloy phase appeared at Sn/Pt atomic
ratios between 0.71 and 0.92. In this ratio range, the Sn1Pt3 and Sn1Pt1 alloy phases
were observed to coexist. The intensities of the peaks corresponding to the Sn1Pt3 phase
were observed to decrease, while those of the Sn1Pt1 phase increased with increasing Sn
content. The Sn1Pt1 alloy phase was the only phase observed at Sn/Pt atomic ratios
between 1.40 and 1.51. The addition of further Sn (Sn/Pt = 2.19) brought about the
formation of the Sn2Pt1 alloy phase.
The SnPt-NP catalysts with 1.51 and 2.19 of Sn/Pt atomic ratios showed Sn1Pt1 and
11

Sn2Pt1 alloy phases, respectively. They are higher than 1 and 2 of stoichiometric Sn/Pt
atomic ratios of Sn1Pt1 and Sn2Pt1 alloy phases, respectively. The excess amount of Sn
would exist as Sn metal and/or Sn oxides nanoparticles which are separated from SnPt
bimetallic nanoparticles.
3.4. Relationship between the SnxPty alloy composition and the catalytic performance of
the SnPt-NP catalysts for selective hydrogenation of crotonaldehyde
The reaction pathway for the hydrogenation of crotonaldehyde is shown in Scheme 1.
The hydrogenation of the C=O bond in crotonaldehyde gives crotyl alcohol (UOL, target
molecule). The hydrogenation of the C=C bond in crotonaldehyde results in the
production of butyraldehyde (SAL), while the sequential hydrogenation of UOL and SAL
gives 1-butanol (SOL). These molecules were the only detected products in this study.
Figure 5 depicts the catalytic activity and products distribution for hydrogenation of
crotonaldehyde over the Pt-NP and SnPt-NP catalysts depend on the Sn/Pt atomic ratio
of the various NPs. SAL was mainly formed over the monometallic Pt-NP catalyst.
The conversion was significantly enhanced by the addition of a small amount of Sn (Sn/Pt
= 0.48), after which it gradually decreased with increasing Sn content. The SAL
selectivity gradually decreased until the Sn/Pt ratio reached 1.40, after which it gradually
increased between Sn/Pt atomic ratios of 1.51 and 2.19. The maximum SOL selectivity
was 28.2%, which was observed at a Sn/Pt ratio of 0.48, and this selectivity was almost
constant over the 0.71-2.19 Sn/Pt ratio range. The selectivity for UOL gradually
improved as the Sn/Pt ratio reached 1.40; the addition of further Sn decreased the UOL
12

selectivity. The highest UOL selectivity obtained was 71.5% at 37.6% conversion over
the SnPt-NP (Sn/Pt = 1.40) catalyst.
It is well known that the conversion level also affects the selectivity of UOL, because
the hydrogenation of crotonaldehyde is the successive reaction. Figure 6 shows the
selectivity of UOL as a function of crotonaldehyde conversion for hydrogenation of
crotonaldehyde over the SnPt-NP catalysts of varying Sn/Pt atomic ratios. The clear
correlation between the conversion and the UOL selectivity, higher selectivity at only
lower conversions, was not observed. These results strongly suggest that the UOL
selectivity depends on the SnxPty alloy species in the SnPt-NPs catalysts.
Figure 7 shows the relationship between C=C and C=O bond hydrogenation and the
Sn/Pt atomic ratio. The total amount of hydrogenated crotonaldehyde increased through
the addition of a small amount of Sn (Sn/Pt = 0.48). Further addition of Sn resulted in
a gradual decrease in the total amount of hydrogenation. As for the amounts of C=C
hydrogenation products (SAL and SOL), their maximum conversions were at Sn/Pt =
0.48. On the other hand, the amount of C=O hydrogenation gradually increased as the
Sn/Pt atomic ratio approached 0.92. These results indicate that increases in UOL
selectivity are not only due to inhibition of the C=C hydrogenation reaction, but are also
due to the acceleration of the C=O hydrogenation reaction over the bimetallic SnPt-NP
catalysts. It should be noted that the addition of a small amount of Sn enhances the
hydrogenation of both C=C bonds and carbonyl groups.
The reaction model for the hydrogenation of crotonaldehyde over the bimetallic SnPt
NPs is shown in Scheme 2. With respect to the SnPt-NP catalysts with a Sn/Pt atomic
13

ratio of 0.48, the Sn1Pt3 alloy phase was the only observed phase by XRD (Fig. 4) and
the total amount of crotonaldehyde hydrogenation was enhanced (Fig. 7). At this atomic
ratio, the Sn1Pt3 phase facilitates hydrogenation on its own, presumably because of
enhanced H2 activation and/or enhanced C=C and/or C=O adsorption. At Sn/Pt atomic
ratios between 0.71 and 1.50, the intensities of the XRD peaks corresponding to the Sn1Pt3
alloy gradually decrease and those of the Sn1Pt1 alloy phase gradually increase (Fig. 4).
The phase transfer from Sn1Pt3 to Sn1Pt1 appears to correlate with catalytic performance;
crotonaldehyde conversion gradually decreases but the selectivity for the formation of
crotyl alcohol gradually increases as the intensities of the Sn1Pt1-alloy phase XRD peaks
increase (Fig. 5). We suggest that the Sn1Pt1 alloy phase possesses higher UOL
formation selectivity and lower catalytic H2-activation activity in comparison with the
Sn1Pt3 alloy phase. When Sn/Pt = 2.19, the XRD pattern indicates that the Sn2Pt1 alloy
phase is mainly formed (Fig. 4). SAL is mainly formed during crotonaldehyde
hydrogenation over the Sn2Pt1 alloy phase. Liberková et al. [39] also reported that UOL
selectivity decreased upon formation of the Sn2Pt1 alloy phase. We reported that the
isomerization of crotyl alcohol to butyraldehyde proceeds over Rh-Sn/SiO2 catalysts [23].
In order to determine whether the decrease in UOL selectivity in the presence of the
Sn2Pt1 alloy phase is due to the isomerization of UOL to SAL, or hydrogenation of the
C=C bond, the hydrogenation of UOL was carried out for 2 h under 2.0 MPa of H2 in the
presence of SnPt-NP (Sn/Pt = 2.19) catalysts at 373 K; no formation of SAL was observed
under these conditions, and a 6.8% yield of SOL was obtained. These results confirm
that UOL does not isomerize to SAL, and that C=C bond hydrogenation takes place over
14

the Sn2Pt1 alloy phase. As for the structure of the Sn2Pt1 alloy phase, Pt ensembles are
unlikely to exist because the amount of Sn present in this alloy is higher than that in the
Sn1Pt1 alloy. The crystal structure of the Sn2Pt1 alloy phase has been ascribed as cubic
[41]. Even though the arrangements of Sn and Pt in the Sn2Pt1 alloy are not yet
understood, at least one of the lattice planes in the Sn2Pt1 alloy phase probably contains
Pt ensemble sites that can activate hydrogen and C=C bonds. We suggest that the Sn2Pt1
alloy structure clearly causes UOL selectivity to decrease, although the reaction
mechanism of hydrogenation (the C=C activation and the H2 activation) over the Sn2Pt1
alloy phase is not well understood.
The SnPt-NP catalysts (Sn/Pt = 1.51) exhibited mainly Sn1Pt1 alloy phases, but the UOL
selectivity was lower in comparison with the material produced when Sn/Pt = 1.40,
implying that a small amount of the Sn2Pt1 alloy phase formed on these SnPt-NP catalysts.
These results indicate that the intrinsic ability of the Sn1Pt1 alloy phase to form UOLs
during the hydrogenation of unsaturated aldehydes is higher than other bimetallic SnPt
alloy phases.
In order to compare the catalytic performance of the SnPt-NP catalysts with those of the
Sn-Pt/SiO2 catalysts prepared using the co-impregnation method, the dependence of Sn/Pt
atomic ratios in the Sn-Pt/SiO2 on the catalytic activity and the products distribution for
crotonaldehyde hydrogenation in Fig. 8. The conversion decreased with increasing Sn
content. The selectivity for UOL gradually improved with increasing Sn/Pt atomic
ratios. The SOL selectivity was almost constant independent of Sn/Pt atomic ratios.
The highest UOL selectivity obtained was 48.8% at 2.7% conversion over the Sn-Pt/SiO2
15

(Sn/Pt = 2.0) catalyst. As shown in Fig. 4, the Sn1Pt3 and Sn1Pt1 alloy phases were formed
at the Sn/Pt atomic ratio between 1.5 and 2.0. The increase of UOL selectivity is
suggested to be due to the formation of these alloy species over the Sn-Pt/SiO2 catalysts.
Figure 9 shows the selectivity of UOL as a function of crotonaldehyde conversion for
hydrogenation of crotonaldehyde over the Sn-Pt/SiO2 catalysts of varying Sn/Pt atomic
ratios. Notably, the UOL selectivity markedly decreased from 33.7% (Sn/Pt = 1.0) to
9.5% (Sn/Pt = 0.5), even though the conversion only increased from 30.9% to 37.4%,
respectively. The selectivity of UOL is considered to monotonically vary in accordance
with the conversion level when the reaction proceeds over the same catalytic active
species. This result also suggests that the selectivity of UOL is strongly affected by the
surface species. It is implied that the SnxPty (probably Sn1Pt3) alloy phases are formed
as small particles and/or thin layers on the surface of Sn-Pt/SiO2 catalysts with 1.0 of
Sn/Pt atomic ratio although the peaks attributed to these alloy species cannot be
confirmed in XRD measurement (Fig. 4).
Figure 10 shows the relationship between C=C and C=O bond hydrogenation and the
Sn/Pt atomic ratio over the Sn-Pt/SiO2 catalysts. The total amount of hydrogenation and
the amounts of C=C hydrogenation products decreased with increasing Sn content. The
amount of C=O hydrogenation decreased until the Sn/Pt atomic ratio approached 0.5,
increased at the Sn/Pt atomic ratio between 0.5 and 1.0, and then decreased over the 1.0-
2.0 Sn/Pt ratio range. At the Sn/Pt atomic ratios higher than 1.0 on the Sn-Pt/SiO2
catalysts, the Sn1Pt3 and Sn1Pt1 alloy phases were observed in XRD measurement (Fig.
4). These results strongly suggest that the catalytic selectivity over the Sn-Pt/SiO2
16

catalysts prepared using the conventional co-impregnation catalysts also depends on the
formed SnxPty alloy species.
The liquid phase hydrogenation of crotonaldehyde over the SnPt bimetallic catalysts
was summarized in Table 2. The selectivity of UOL over the SnPt/SiO2 catalysts
prepared by the successive modification with Sn to Pt/SiO2 catalysts (Entries 1 and 2)
was higher than that prepared by the co-impregnation method (Entries 4 and 5). The
XRD measurements of these catalysts have not been carried out in these papers. The
preparation methods would affect the formation of SnxPty alloy phases in the SnPt
bimetallic catalysts, bringing about the difference of catalytic performance for the
hydrogenation of crotonaldehyde.
Catalytic activity (i.e. conversion of crotonaldehyde) was different between the SnPt-
NP catalysts and the Sn-Pt/SiO2 catalysts, which was due to the amount of surface Pt.
The presence of capping agents (oleyl amine and oleic acid) in the SnPt-NP catalysts
makes it difficult to estimate the amount of surface Pt by means of a conventional H2 and
CO chemisorption procedure, because the capping agents themselves probably affects the
adsorption of H2 and CO, and they are decomposed during the H2 pretreatment at 573 K
to sinter the nanoparticles. Therefore, we have not obtained the accurate data on the
amount of surface Pt in this study yet. Further investigation must be necessary.
4. Conclusions
The selective hydrogenation of crotonaldehyde over SnPt nanoparticles prepared using
a polyalcohol reduction process in the liquid phase was investigated. Key observations
17

are provided below.
1. SnPt NPs with various Sn/Pt atomic ratios can be successfully prepared by a
polyalcohol reduction process using Pt(acac)2 and Sn(AcO)2 as precursors. Sn1Pt3, Sn1Pt1,
and Sn2Pt1 alloy phases were formed by changing the atomic ratio of Sn to Pt introduced
during preparation.
2. The relationship between catalytic behavior for selective hydrogenation of
crotonaldehyde and the alloy structure was also investigated. The Sn1Pt3 alloy phase was
observed to bring about a significant enhancement in conversion and C=O hydrogenation.
When only the Sn1Pt1 alloy phase was formed in the SnPt NPs (Sn/Pt = 1.40), the highest
UOL selectivity (71.5% at a conversion of 37.6%) was obtained. The formation of the
Sn2Pt1 alloy phase led to a decrease in UOL selectivity. We strongly suggest that the
Sn1Pt1 alloy is the most effective SnxPty phase for the selective formation of unsaturated
alcohols.
Acknowledgements
The authors thank Jianjiang Mao of Fudan University in China for acquiring the TEM
images.
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22

Fig. 1 Sn/Pt atomic ratio in the nanoparticles determined by AAS as a function of Sn/Pt
atomic ratio in starting mixture during preparation. The dashed line of unit slope indicates
where the Sn/Pt atomic ratio of the NP is equal to the Sn/Pt atomic ratio in the starting
mixture.
23

Fig. 2 TEM images of (a) Pt-NP and (b) SnPt-NP (Sn/Pt=1.40) catalysts
24

Fig. 3 XRD patterns of Pt-NP and SnPt-NP catalysts: Sn/Pt= 0 (a), 0.48 (b), 0.71 (c),
0.92 (d), 1.40 (e), 1.51 (f) and 2.19 (g).
25

Fig. 4 XRD patterns of Pt/SiO2 and Sn-Pt/SiO2 prepared by the conventional co-
impregnation method: Sn/Pt= 0 (a), 0.2 (b), 0.5 (c), 1.0 (d), 1.5 (e), and 2.0 (f).
26

Fig. 5 Dependence of Sn/Pt atomic ratio for crotonaldehyde hydrogenation over the
SnPt-NP catalysts: (a) conversion and (b) selectivity of products.
27

Fig. 6 Relationship between the conversion of crotonaldehyde and the selectivity of
crotylalcohol for the hydrogenation of crotonaldehyde over the SnPt-NP catalysts. The
number described near each plot indicates the Sn/Pt atomic ratio in the SnPt-NP
catalysts.
28

Fig. 7 Dependence of Sn/Pt atomic ratio on the amount of hydrogenation of the C=C
bond and C=O bond.
29