COMMUNICATION
DOI: 10.1002/chem.201204160
Core–Shell AgNP@CeO2 Nanocomposite Catalyst for Highly
Chemoselective Reductions of Unsaturated Aldehydes
Takato Mitsudome,[a] Motoshi Matoba,[a] Tomoo Mizugaki,[a] Koichiro Jitsukawa,[a] and
Kiyotomi Kaneda*[a, b]
In supported metal-nanoparticle (NP) catalysts, a strong
metal–support interaction (SMSI) is one of the most impor-
tant factors that influence the catalytic performance. Sup-
ports that affect the catalytic properties of metal NPs can be
considered to be “macroligands” for the active metal NPs,
and the ligand effect can be tuned by screening the supports,
altering their compositions, and controlling various parame-
ters during their preparation. However, metal NPs are often
located on the surface of the supports, and the ligand effect
can only take place at the interface between the bottom of
the metal NPs and the surface of the supports. This implies
that many sites remain on the metal NP surface with the po-
tential to yield the ligand effect through interaction with the
support. Thus, increasing such ligand interactions between
metal NPs and supports will lead to superior catalytic per-
formances over conventionally supported metal NPs.
nanogaps among adjacent CeO2 NPs in the shell enabled
the reactants to access the active Ag species in the core.
Maximizing the interaction between AgNPs and basic sites
of the CeO2 efficiently induced the heterolytic cleavage of
H2. The resulting Ag–hydride and proton species at the in-
terface preferred polar functional groups over C=C bonds
(Scheme 1).
As a new strategy for increasing the number of metal–
support interaction sites, an attractive conceptual design is
core–shell nanocomposite catalysts consisting active metal
NPs in the core and supports in the shell.[1] The core–shell
structure would be ideal for exploiting the ligand effect due
to its maximized metal NP–support interface area. However,
covering active core species with a shell makes it difficult
for reactants to diffuse into the core, and thus the applica-
tions of core–shell catalysts to organic transformations
under liquid-phase conditions are limited.[2]
We have recently reported a successful synthesis of a
core–shell AgNP–CeO2 nanocomposite (AgNP@CeO2) con-
sisting of AgNPs with a 10 nm diameter in the core and self-
assembled, spherical CeO2 NPs with a 3 nm diameter in the
shell.[3] AgNP@CeO2 could catalyze highly chemoselective
reductions of both nitrostyrenes to aminostyrenes and epox-
ides to alkenes, while maintaining the C=C bonds. The
Scheme 1. Chemoselective reductions using AgNP@CeO2.
In our continued development of unique core–shell nano-
composite catalysts, we report herein that AgNP@CeO2
showed excellent selectivities for the chemoselective reduc-
tion of unsaturated aldehydes with H2 to the corresponding
unsaturated alcohols, which often serve as important inter-
mediates in fragrances and pharmaceuticals.[4] Furthermore,
AgNP@CeO2 was successfully dispersed on CeO2 (AgNP@
CeO2-D), while maintaining its core-shell structure, which
led to a significant enhancement in the catalytic activity of
AgNP@CeO2. The developed AgNP@CeO2-D catalyst af-
forded superior selectivity to various conventional support-
ed metal NPs in chemoselective reductions. AgNP@CeO2-D
was also separable from the reaction mixture and reusable
with the retention of its activity and selectivity.
AgNP@CeO2 was synthesized according to our previous
report.[3] The initial mole ratio of Ag to Ce (Ag/Ce) in the
preparation was 1.5. The catalytic activity of AgNP@CeO2
was examined in the hydrogenation of citral (1) as a model
reaction in tetrahydrofuran (THF) under a pressurized H2
atmosphere at 1508C. The reaction proceeded efficiently to
afford a mixture of the allylic alcohols geraniol and nerol
(2) in 96% selectivity at complete conversion, accompanied
by small amounts of citronellal (3) and citronellol (4)
(Table 1, entry 1).[5] Neither unsupported AgNPs nor CeO2
alone showed any catalytic activity (entries 12 and 13), con-
[a] Dr. T. Mitsudome, M. Matoba, Dr. T. Mizugaki,
Prof. Dr. K. Jitsukawa, Prof. Dr. K. Kaneda
Department of Materials Engineering Science
Graduate School of Engineering Science, Osaka University
1-3, Machikaneyama, Toyonaka, Osaka 560-8531 (Japan)
Fax : (+81)6-6850-6260
[b] Prof. Dr. K. Kaneda
Research Center for Solar Energy Chemistry Osaka University
1-3, Machikaneyama, Toyonaka, Osaka 560-8531 (Japan)
Supporting information for this article is available on the WWW
firming the necessity of CeO2 as a macroACTHNUGTRENNUGliCAHUTNGTRENgNUG and for AgNPs.
Chem. Eur. J. 2013, 19, 5255 – 5258
ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
5255