K. Taniya et al. / Catalysis Communications 14 (2011) 6–9
7
JEM-2000FX and a JOEL JEM2011, respectively. Energy dispersive X-
ray was performed for the SiO -encapsulated SnPt as synthesized by
2
using INCA (Oxford Instruments). X-ray powder diffraction patterns
of the catalysts were measured at room temperature on using the
Rigaku RINT-2000 with Cu–Kα radiation.
2.3. Catalytic test
Hydrogenation of crotonaldehyde was carried out in a 30 mL
autoclave. The reagent grade of crotonaldehyde was purchased by
Nacalai Tesque, Kyoto, Japan. Crotonaldehyde (0.2 mL) was diluted
with 4 mL of 2-methyl-2-butanol as a solvent and charged in a glass
tube with 20–25 mg of catalyst. The glass tube was inserted in
autoclave. The reaction solution was not in direct contact with the
inner surface of the autoclave, and was vigorously agitated by a
magnetic stirrer. Hydrogen was charged and discharged several times
in order to substitute the air in the autoclave with hydrogen. The
autoclave was heated up to 373 K in an oil bath in which the
temperature was controlled by a temperature regulator. The reaction
lasted for 20 h. Then, the autoclave was cooled to 273 K, and 0.2 mL of
cyclohexane as an internal standard was added to the solution. The
solution was centrifuged to separate catalysts, and the products were
analyzed by GC (GC-18A, Shimazu) equipped with G-300 column
(Chemicals Evaluation and Research Institute, Japan).
Scheme 1. Reaction pathway of crotonaldehyde hydrogenation.
nanoparticle formation step and (2) SiO
.197 g (0.5 mmol) of platinum (II) acetylacetonate and 0.118 g
0.5 mmol) of tin (II) acetate were dispersed in 20 mL of dioctylether.
2
coating step. In the first step,
0
(
Then, 0.32 mL of oleic acid, 0.34 mL of oleyl amine and 0.78 g of 1,2-
hexadecanediol were added to the above solution. The solution was
heated up to ca. 553 K with bubbling of N
2
, and then kept refluxing for
1
h. A color change (from yellow to auburn via orange) was observed
during heating up. After cooling to room temperature, the mixture,
which had a black color, was centrifuged in order to separate the bi-
metal nanoparticles from the solution. The supernatant was dis-
carded, and the obtained precipitates were washed three times by
ethanol. The samples after washing by ethanol were dried at room
temperature overnight and the SnPt nanoparticle (Sn/Pt=1.0) was
finally obtained. In the second step, to encapsulate the SnPt
nanoparticles in silica, 50 mmol of Igepal CO-520 (polyoxyethylene
5) nonylphenyl ether) was added to 450 mL of cyclohexane and the
mixture was subjected to an ultrasonic treatment for 10 min. Then,
0 mL of a SnPt solution prepared by redispersion of the SnPt solid in
cyclohexane (1 mg/mL) was added to the above solution. After the
mixture had been stirred for 2 h, 4.0 mL of ammonia solution (35%)
was added. Finally, 3.0 mL of tetraethyl orthosilicate was added, and
the mixture was stirred for 48 h to encourage hydrolysis and
condensation of the silica precursor. The resulting nanoparticles
were collected by using centrifugation, and washed by ethanol. These
3. Results and discussion
The XRD patterns of Pt/SiO
SnPt as synthesized are shown in Fig. 1. In the Pt/SiO
were observed around 39.5°, 46.5° and 67.5° (open circle) of 2θ, which
were assigned to the reduced Pt metal [15–17]. In the Sn-Pt/SiO , the
2
, Sn-Pt/SiO
2
and SiO
2
-encapsulated
2
, three peaks
(
2
peaks ascribed to the SnPt alloy phase, which the molar ratio Sn to Pt
is 1:1, appeared around 41.5°, 44.0° and 62.0° (open down triangle)
[18,19], and the peaks around 39.0°, 45.5° and 66.0° (open square)
were observed. The positions of latter peaks were close to the Pt metal
peaks, and these peaks appeared on the Sn containing catalysts.
Because these peaks were observed at the lower angles than the Pt
metal peaks, it was suggested that the Sn species would be
incorporated into the Pt phase, bringing about expanding the lattice
structure (of the Pt phase). Therefore, we identified these peaks with
5
samples are denoted as the SiO
Additionally, these samples were calcined in flowing air at 623 K for
h and reduced in flowing H at 573 K for 2 h under atmospheric
pressure. The samples after these treatments are referred to SiO
encapsulated SnPt. The SiO -encapsulated Pt catalysts were prepared
by the above method without tin (II) acetate.
2
-encapsulated SnPt as synthesized.
2
the Sn-doped Pt phase. As for the SiO -encapsulated SnPt as
2
2
synthesized, the peaks attributed to the SnPt alloy phase were mainly
observed. These results indicated that the uniform SnPt alloy phase
could be synthesized by polyol process.
2
-
2
To compare the catalytic performance, the supported Pt and Sn–Pt
bi-metallic catalyst was prepared by an impregnation method and a
co-impregnation method, respectively. Silica support used in this
study was Q-10 supplied from FUJI SILYSIA CHEMICAL LTD. It was
calcined in a flowing air at 773 K for 5 h prior to catalyst preparation.
The Pt/SiO
at 353 K with a solution prepared by diluting an aqueous solution of
PtCl ·6H O with ethanol (30 mL). The loading of Pt was 4 wt.%. The
Sn-Pt/SiO was prepared by impregnation of SiO support (1.0 g) at
53 K with a solution prepared by diluting an aqueous solution of
PtCl ·6H O and SnCl ·2H O with ethanol (30 mL). The loading of
2 2
was prepared by an impregnation of SiO support (1.0 g)
H
2
6
2
2
2
3
H
2
6
2
2
2
Pt was 4 wt.% of the support and the molar ratio of Sn/Pt was 1.0. Then
the samples were dried overnight at 393 K, calcined under air flow at
8
2
23 K for 2 h and then reduced under H flow for 2 h at 573 K.
2
.2. Characterization
Transmission electron microscopy (TEM) images of the Sn-Pt/SiO
and the SiO -encapsulated SnPt as synthesized were taken with a JEOL
2
2 2 2
Fig. 1. XRD patterns of each catalyst; (a) Pt/SiO , (b) Sn-Pt/SiO and (c) SiO -
encapsulated SnPt as synthesized.
2