Gold-Based Catalysts - Transition Metal Catalysis in Aerobic Alcohol Oxidation - page 142

obtain core-shell structures besides alloys, a sequential reduction of the
metal combinations was carried out to allow the formation of core-shell
structures, whereas the simultaneous reduction of both metals led to
homogeneous alloys. The materials were then characterized using XPS and
aberration-corrected STEM. The supported nanoparticles were tested in the
oxidation of benzyl alcohol (and also toluene) using tert-butyl hydroperoxide
at 80 1C, in order to elucidate any potential structure-activity relationships.
Amongst the Au-Pt catalyst variants, the supported nanoalloys prepared
using simultaneous reduction gave the best catalytic performance. However,
the Pt shell -Au core /C catalyst, prepared by the sequential addition of Pt to Au
'seeds', displayed an improved catalytic activity in comparison with the TiO 2 -
supported analog. In contrast, both the Au shell -Pt core variant (prepared by the
sequential addition of Au to the Pt 'seeds') and the Au-Pt variant (prepared
by simultaneous metal reduction) allowed a higher catalytic activity to be
achieved for TiO 2 -supported nanoparticles. As far as the product distribution
is concerned, when Au-Pd catalysts were used a significant amount of
toluene was formed, whereas when the Pd component of the alloy was re-
placed with Pt, toluene was not detected and the main products were ben-
zaldehyde and reaction products derived from the aldehyde. Furthermore,
the catalytic performance of the Au-Pt and Au shell -Pt core materials is very
different when supported on carbon and TiO 2 , since mainly benzoic acid is
formed by the TiO 2 -supported Au-Pt nanoparticles. In contrast, in the case
of Pt shell -Au core nanoparticles, both TiO 2 and carbon led to benzaldehyde as
the major oxidation product. Hence the importance of the alloy composition
and morphology,

d n 4 r 4 n g | 3

in addition to the support identity, is dramatically

highlighted. 58
Interestingly, around the same time, Wang's group prepared innovative
monometallic Au, Pd and Pt catalysts supported on a hybrid of natural DNA
and MMT (montmorillonite), showing higher activity and selectivity for the
oxidation of various primary alcohols than DNA-templated nanoparticles,
but also with comparable performance to the bimetallic catalysts reported
in the literature. 52 These organic-inorganic hybrids are made up of Na-
montmorillonite, which is a type of naturally occurring clay, and the natural
DNA from fish sperm as the template of metal nanoparticles. Depending
on these catalytic systems, highly ecient formation of the corresponding
aldehydes, carboxylic acids and esters was obtained. Both oxidative self-
esterification and cross-esterification of various alcohols could be smoothly
performed. Although bimetallic nanoparticles as active catalysts for selective
oxidation of primary alcohols to the aldehydes, carboxylic acids and esters have
been reported, this is probably the first example of such transformations being
easily catalyzed by monometallic nanoparticles. Furthermore, taking advan-
tage of the water-soluble reversibility of these catalysts, all the transformations
could be carried out effectively in water under mild conditions and the cata-
lysts were recovered by a simple phase separation process and reused. 52
A key point in the gold-catalyzed conversion of alcohols in the liquid phase
is the role of alkali. Only rare cases of conversion were observed in the

.