interactions [10–14], have achieved high activity and selectivity in fine chemical
synthesis under mild conditions. Accordingly, this review provides an overview of
metal NP heterogeneous catalysts developed for redox reactions using H 2 or O 2
under liquid-phase conditions. As many reviews and reports on heterogeneous
catalysts for hydrogenation [15–17] and aerobic oxidation [18–21] have been
published, herein we have focused on state-of-the-art metal NP catalysts, which
overcome the limitations of conventional catalysts in selective redox reactions.
2 Metal NP-Catalyzed Selective Reductions Using H 2
Selective hydrogenations are important methods in fine chemical synthesis. However, the chemoselective hydrogenation of targeted functional groups in the presence
of other reducible groups is difficult. In particular, chemoselective hydrogenation in
the presence of easily reducible alkene groups represents a significant challenging.
To date, various approaches using metal NP catalysts to achieve high
chemoselectivity have been proposed through the design of metal size, surface,
shape, and components. In this section, rationally designed and developed novel
metal NP catalysts for the chemoselective hydrogenation of nitro, carbonyl, and
alkyne moieties while retaining easily reducible alkene groups are discussed.
2.1 Design of Core–Shell NPs
The hydrogenation of aromatic nitro compounds bearing other reducible groups is
important for the direct synthesis of functionalized aniline derivatives, which are
useful feedstocks in the chemical industry. In 2005, Corma et al. discovered that a
Au/TiO 2 catalyst promoted the chemoselective hydrogenation of 3-nitrostyrene to
give 3-aminostyrene with 98.5% conversion and 95.9% selectivity [22]. Since this
pioneering report, many catalysts have been developed for the chemoselective
hydrogenation of functionalized aromatic nitro compounds [23]. However, these
catalysts suffer from overhydrogenation of aminostyrene to ethylaniline at high
conversion levels. This is due to the chemoselectivity often being derived from
preferential adsorption of the nitro moiety over the alkene moiety, resulting in alkene
hydrogenation occurring prior to nitro hydrogenation at low nitro compound concentrations. Therefore, a new catalyst design strategy for the chemoselective hydrogenation of nitro compounds is needed as an alternative to the preferential adsorption
method.
Au or Ag NPs and basic metal oxide support are known to cooperatively
dissociate H 2 into polar hydrogen species H
δ+ and H
δ, which straddle the interfacial
perimeter sites [24, 25]. These polar hydrogen species show favorable reactivity with
nitro groups compared to that with alkene groups [26]. Mitsudome et al. developed a
novel core–shell nanostructured catalyst, Ag@CeO 2 , for the chemoselective
Metal Nanoparticles for Redox Reactions
51
synthesis under mild conditions. Accordingly, this review provides an overview of
metal NP heterogeneous catalysts developed for redox reactions using H 2 or O 2
under liquid-phase conditions. As many reviews and reports on heterogeneous
catalysts for hydrogenation [15–17] and aerobic oxidation [18–21] have been
published, herein we have focused on state-of-the-art metal NP catalysts, which
overcome the limitations of conventional catalysts in selective redox reactions.
2 Metal NP-Catalyzed Selective Reductions Using H 2
Selective hydrogenations are important methods in fine chemical synthesis. However, the chemoselective hydrogenation of targeted functional groups in the presence
of other reducible groups is difficult. In particular, chemoselective hydrogenation in
the presence of easily reducible alkene groups represents a significant challenging.
To date, various approaches using metal NP catalysts to achieve high
chemoselectivity have been proposed through the design of metal size, surface,
shape, and components. In this section, rationally designed and developed novel
metal NP catalysts for the chemoselective hydrogenation of nitro, carbonyl, and
alkyne moieties while retaining easily reducible alkene groups are discussed.
2.1 Design of Core–Shell NPs
The hydrogenation of aromatic nitro compounds bearing other reducible groups is
important for the direct synthesis of functionalized aniline derivatives, which are
useful feedstocks in the chemical industry. In 2005, Corma et al. discovered that a
Au/TiO 2 catalyst promoted the chemoselective hydrogenation of 3-nitrostyrene to
give 3-aminostyrene with 98.5% conversion and 95.9% selectivity [22]. Since this
pioneering report, many catalysts have been developed for the chemoselective
hydrogenation of functionalized aromatic nitro compounds [23]. However, these
catalysts suffer from overhydrogenation of aminostyrene to ethylaniline at high
conversion levels. This is due to the chemoselectivity often being derived from
preferential adsorption of the nitro moiety over the alkene moiety, resulting in alkene
hydrogenation occurring prior to nitro hydrogenation at low nitro compound concentrations. Therefore, a new catalyst design strategy for the chemoselective hydrogenation of nitro compounds is needed as an alternative to the preferential adsorption
method.
Au or Ag NPs and basic metal oxide support are known to cooperatively
dissociate H 2 into polar hydrogen species H
δ+ and H
δ, which straddle the interfacial
perimeter sites [24, 25]. These polar hydrogen species show favorable reactivity with
nitro groups compared to that with alkene groups [26]. Mitsudome et al. developed a
novel core–shell nanostructured catalyst, Ag@CeO 2 , for the chemoselective
Metal Nanoparticles for Redox Reactions
51