Recent Developments in Gold Nanomaterial Catalysts for Oxidation Reaction through Green and Sustainable Routes - Advanced Materials for Agriculture, Food and Environmental Safety - page 199

• charge transfer between the support and gold; and

• the perimeter interface between gold and the oxide support.

Supported gold represents a second generation of catalysts for alcohol
and carbohydrate oxidation: they show a great improvement in selectiv-
ity and stability and thus represent challenging substitutes for platinum
group metals. From a commercial point of view, gold is more suitable
than Pt, Pd, etc., and involves many organic reactions. h e kinetic
aspects of gold catalysis are that the turnover frequency is strongly
related to the size of metallic gold particles morphology and the metal-
support interaction. Considering other fundamental peculiarities, such
as biocompatibility, availability and easy recovery, gold appears to be
an exciting catalyst for sustainable processes based on the use of clean
reagents, particularly O 2 , ot en in aqueous solution or in the absence of
solvent, under mild conditions.

8.1.1 Quantum Size Ef ects

Dif erent literature reviews and experimental results, have observed that
the catalytic activity of gold nanoparticles supported on dif erent mate-
rials is strongly dependent on the particle size of gold and nature of the
support material [12, 15, 16-21] and type of reactions [22, 16-21]. h is
was indicated by the valence band spectra measured by ultraviolet photo-
electron spectroscopy (UPS) [23]. As the gold size diminished, the peak at
2 eV in the UPS spectra disappeared. A similar ef ect could be observed
in the core level X-ray photoelectron spectra, indicated by a shit in the
binding energies towards the higher energies. It is important to note that
by ascribing the dependence of the catalytic activity on the Au particle
size to a higher density of active sites (corner atoms), the turnover fre-
quency (per corner atom) does not solely depend on the particle size; it
is the same in both small and large particles. h is is fundamentally dif er-
ent from the quantum size ef ect [24-27], which ascribes the activity of
small Au particles to a particle-size-dependent turnover frequency, aris-
ing from changes in electronic structure as the particle size decreases. To
prepare highly active gold catalysts, it is crucial to minimize the size of the
gold particles to diameters >10 nm and to deposit them as hemispherical
particles, which maximize the perimeter lengths. Scheme 8.1 shows there
are several preparation methods, e.g., impregnation, co-precipitation,
deposition-precipitation, etc. Recently, Haruta et al. [28, 29] demonstrated
solid grinding as an ei cient tool for preparation of highly active gold
nanoclusters on oxide support.