Gold-Based Catalysts - Transition Metal Catalysis in Aerobic Alcohol Oxidation - page 137

Furthermore, alloying gold with a second metal (Pt, Pd and Cu) has been
found to enhance the catalytic performance of gold in numerous oxidation
reactions due to a synergistic effect among the metals. 18,19,42,46-48,72-77 This
has also resulted in particularly stable catalysts with respect to the mono-
metallic ones, as reported in the following sections.
A thorough catalyst characterization requires a concert of analytical
techniques. Inductively coupled plasma atomic emission spectroscopy (ICP-
AES) is employed to determine the real metal content, while a combination
of X-ray powder diffraction (XRPD) and transmission electron microscopy
(TEM) provides the average value and statistical distribution of metal par-
ticle diameters. Scanning transmission electron microscopy-high angle
annular dark field (STEM-HAADF) and scanning transmission electron
microscopy-X ray energy-dispersive spectroscopy (STEM-XEDS) represent
the advanced evolution of such techniques. The elemental composition of
the surface (1-10 nm depth) and the electronic state of each element can be
determined by X-ray photoelectron spectroscopy (XPS). The presence of
different types of gold species can be detected by the temperature-
programmed reduction (TPR) technique, because different reduction tem-
peratures are a clear indication for different metal species on an identical
supporting material or for diverse interactions of the same species with
different materials.

d n 4 r 4 n g | 3

5.3 Selective Oxidation of Alcohols by Gold-Based
Catalysis
The wide distribution, easy availability from renewable sources and em-
ployment of their derivatives as valuable chemicals make alcohols strategic
substrates to be converted into the corresponding aldehydes, ketones or
acids. Finding proper catalytic systems able to facilitate their selective oxi-
dation under sustainable conditions is what both academia and industry are
aiming for. This has led to a competition among platinum, palladium, ru-
thenium, copper and gold catalysts, which represent the most promising
candidates for substituting the traditional polluting methods. Here, atten-
tion is focused on gold compared with other metals, trying to offer a general
picture of how properly tuning various parameters can boost the catalytic
activity while addressing the selectivity towards a given product.
The effective aerobic oxidation of alcohols to carbonylic and carboxylic
compounds employing gold began at the end of the 1990s (Scheme 5.1). 5
The discovery of gold as a powerful catalyst opened the way to more
and more sophisticated catalytic systems, where gold, alone or alloyed with
other metals, is supported on a wide range of materials from different car-
bons and oxides to the ultimate carbon nanotubes. 11,12,59 Furthermore, one
can choose whether to perform the oxidation by homogeneous or hetero-
geneous catalysis, in the gaseous or liquid phase, with ordinary solvents,
supercritical CO 2 or solventless, while varying the pressure, temperature and

.