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species, which act as sites for O
2
activation. Oxygen clearly suppressed the
formation of coke. In the mechanism proposed, the electrophilic O species
facilitated O-H bond cleavage and metal-alcoholate formation, whereas Au
NPs in close proximity acted as sites for C-H cleavage.
74
The role of the size of NPs on catalytic behavior has also been addressed.
Zheng and Stucky
75
showed that silica-supported Au NPs with a nominal size
of 6 nm are more active than 3.5 and 8.2 nm particles in the gas-phase
oxidation of ethanol. Similar results were also presented by Rossi and co-
workers, at the World Congress on Oxidation Catalysis held in Berlin in
2001;
76
they showed that a high selectivity to the corresponding aldehyde
could be obtained in the gas-phase oxidation of various aliphatic alcohols,
77
and the highest activity was observed with Au particles larger than 10 nm.
Guan and Hensen
60
found a strong influence of the gold particle size on the
non-oxidative dehydrogenation of ethanol, with nanoparticles of about 6 nm
showing much higher activity than smaller or larger particles. This optimal
catalytic activity was attributed to the existence of a maximum density of
surface steps with a suitable geometry for the removal of b-H atoms from
adsorbed ethoxide. In the presence of oxygen, the generation of adsorbed
oxygen species accelerated the reaction rate and decreased selectivity be-
cause of the contribution of combustion. Notably, in this case the highest
intrinsic activity was shown with particles larger than about 7 nm: an effect
which was attributed to a higher density of strongly adsorbed oxygen ada-
toms, which are much scarcer on smaller gold particles.
Few papers have reported the reactivity of bimetallic particles. Guan and
Hensen
78
reported a system of Au-Ir metallic NPs of size 2-3 nm (where Au
retards the bulk oxidation and sintering of Ir in an oxidizing atmosphere
at temperatures higher than 350 1C). The NPs displayed enhanced activity
in ethanol oxidation to acetaldehyde, outperforming their monometallic
counterparts, but with slightly lower selectivity to C
2
compounds (acetalde-
hyde with traces of acetic acid) compared with Au NPs (92% versus 99% for
the latter). The model proposed involves intimate contact between Au sites
for dissociative ethanol adsorption and Ir sites, covered by O adatoms, which
catalyze C-H bond cleavage to yield acetaldehyde.
The catalytic and electrocatalytic activity of Pd alloyed with transition
metals such as Cu for ethanol oxidation was enhanced compared with Pd
alone.
79
In the gas-phase reaction, the nanoalloy Pd-Cu catalyst was shown
to suppress the formation of acetic acid; the effect was due both to a surface
enrichment of Pd in the alloy and to the oxophilicity of the base metal
Cu alloyed with Pd, as demonstrated by a significant positive shift of the
reduction potential of the oxygenated Pd species on the surface.
Table 8.1 lists some catalyst types based on Au and Ag which have been
reported in the literature for ethanol gas-phase oxidation; in most cases,
acetaldehyde was the main product of oxidation, but in some cases acetic
acid was also produced in remarkable yields. In these cases, formation of the
acid was shown at temperatures higher than 250 1C with catalysts which
started to be active in ethanol conversion in the range 150-200
d
n
4
r
4
n
g
|
3
.
1C.
52
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