Angewandte
Chemie
Well-defined hybrid metal nanoparticles with core–shell
structures have attracted enormous attention because of their
unique optic,[5–7] magnetic,[8] electronic,[9,10] and catalytic[11–19]
properties that cannot be obtained in monometallic nano-
particles. In the catalytic field of core–shell bimetals, the core
metals can affect the catalytic active metal species in the shell,
thus giving rise to improvements in their activities and
selectivities for organic reactions. However, the core metals
cannot act directly as the active species because the shell
covers the core metals entirely, and the access of reactants to
the core metals is prevented. Furthermore, the core metals
reported to date are often relatively large (> 20 nm in
diameter), which is generally not suitable for catalytically
active species. The solutions to these above issues will
generate new approaches toward novel applications of
core–shell catalysts.
Herein, we present a new concept for the rational design
of a core–shell catalyst that has active metal nanoparticles in
the core and an oxide support with nanospaces in the shell.
We successfully synthesized a novel core–shell AgNPs–CeO2
nanocomposite (AgNPs@CeO2) involving core AgNPs 10 nm
in diameter and a shell assembled with spherical CeO2 NPs
3–5 nm in diameter. The shell has nanospaces between the
CeO2 NPs that permit the access of reactants to the active
center of the AgNPs. The AgNPs@CeO2 catalyst showed
complete chemoselectivity for the reduction of both nitro-
styrenes to aminostyrenes and epoxides to alkenes by using
Figure 3. Electron micrographs of AgNPs@CeO2. a) SEM image of
AgNPs@CeO2. b) HRTEM image of the single AgNPs@CeO2 nano-
composite. c) The line-scan STEM-EDS across the AgNPs@CeO2
nanocomposite (Ag: red squares, Ce: green squares). The circled areas
correspond to the spherical CeO2 NPs.
micelle technique with the redox reaction between silver(I)
and cerium(III) is crucial in forming the nanocomposite;
much larger AgNPs@CeO2 with a 100 nm diameter was
obtained when the reverse micelle method was not used.[23]
The catalytic activity of AgNPs@CeO2 was evaluated in
the reduction of 3-nitrostyrene (1) using H2. Chemoselective
reduction of the nitro moiety of a molecule that contains an
=
H2 while retaining the reducible C C bonds. The advantages
=
of the core–shell structure are shown in the remarkable
improvement of the chemoselectivity compared with AgNPs
supported on CeO2 (AgNPs/CeO2). Moreover, AgNPs@CeO2
was easily separable from the reaction mixture and was
reusable without loss of catalytic activity or selectivity.
easily reduced C C bond is a challenge. The functionalized
products obtained are useful intermediates.[24,25] AgNPs@-
CeO2 exhibited complete chemoselectivity toward the reduc-
tion of the nitro group of 1, and afforded the desired
3-aminostyrene product (2) in 98% yield without reduction
=
AgNPs@CeO2 was synthesized by the combination of the
reverse micelle technique and the redox reaction between
silver(I) and cerium(III).[20] CeO2 has advantages as a BM
because of its basicity and the facile control of its shape and
size.[21–23] The scanning electron microscopy (SEM) image of
AgNPs@CeO2 showed uniform spherical nanoparticles 30 nm
in diameter (Figure 3a). Transmission electron microscopy
(TEM) showed the two areas of an electron-dense core 10 nm
in diameter and an electron-poor shell 8 nm thick (Figure 3b).
Energy-dispersive X-ray spectroscopy (EDS) analysis clearly
demonstrated that the nanocomposite was composed of an
Ag core and CeO2 shell (Figure 3c). Close inspection of the
HRTEM image showed that the spherical CeO2 NPs about
3–5 nm in diameter assembled to form the shell (Figure 3c
and Figure 1S in the Supporting Information). The zero-
valent state of Ag in AgNPs@CeO2 was confirmed by the Ag
K-edge X-ray absorption near-edge structure (XANES)
analysis (Figure 2S in the Supporting Information). The
basic nature of AgNPs@CeO2 were calculated by CO2
adsorption analysis, which showed that 165 mmolgÀ1 CO2
was adsorbed. These results conclusively show that a uniform
core–shell AgNPs–CeO2 nanocomposite can be synthesized.
The core AgNPs 10 nm in diameter were covered with the
shell assembled of spherical basic CeO2 NPs. The nanospaces
between the spherical CeO2 NPs enable the access of
reactants to the AgNPs core. The combination of the reverse
of the C C double bond to 3-ethylaniline (3) or 3,3’-
divinylazobenzene (4) by dehydrative condensation (Fig-
ure 4a). Interestingly, the C C bond of the desired product of
2 was intact, even after complete conversion of 1. The
complete chemoselectivity cannot be obtained by the conven-
tional catalysts where the hydrogenation of the C C bonds of
the products is unavoidable and results in a decrease in the
selectivity with increasing reaction time.
AgNPs similar in size to AgNPs@CeO2 were supported on
CeO2 (AgNPs/CeO2) by using the impregnation method, and
were employed in the reduction under similar reaction
conditions. AgNPs/CeO2 had low chemoselectivity toward 2
with the formation of undesired products of 3 and 4, where 2
and 4 were gradually hydrogenated to 3 and 2, respectively
(Figure 4b). The conversion of 4 to 2 was confirmed in a
separate experiment in which 4 was employed as a starting
material under similar reaction conditions. These results
clearly show that the core–shell structure of AgNPs@CeO2 is
suitable for the complete chemoselective reduction of the
=
=
=
nitro functionality while retaining the C C bond. The
efficiency of the core–shell structure was also demonstrated
in the reduction of nitrobenzene in the presence of styrene
(Scheme 1). Notably, nitrobenzene was efficiently converted
to aniline while styrene was not reduced at all. This result was
in sharp contrast with that obtained using AgNPs/CeO2,
where both nitrobenzene and styrene were hydrogenated.
Angew. Chem. Int. Ed. 2012, 51, 136 –139
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
137