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because of (i) its ability to activate O
2
at low temperature, (ii) the compara-
tively weak adsorption of the partial oxidation products shown, compared
with Pt and Pd, which saves the product from consecutive overoxidation
reactions,
45
and (iii) its low tendency to activate C-H bonds.
46
The same
material (but with residual Ag in the dealloyed material) catalyzed the oxi-
dation of ethanol to acetaldehyde or ethyl acetate (at 160 1C, with a selectivity
of 48% and 32% ethanol conversion) or into acetic acid (at 240 1C, with
selectivity 68% and conversion over 70%), as the prevailing compound, de-
pending on the reaction temperature, O
2
: ethanol molar feed ratio and
amount of residual Ag in the dealloyed material.
47
On the other hand, the
role of Ag has also been demonstrated by Ding et al.,
48
who reported 82%
ethanol conversion with 94% selectivity to acetaldehyde, using nanoporous
Ag at 250 1C. Surprisingly, the catalyst showed high selectivity to aldehydes
from various primary alcohols, except methanol, which gave methylformate
as the prevailing compound. This was attributed to the fact that the b-H
elimination of alkoxy species in methanol is more dicult than with higher
alcohols; therefore, the methoxy species accumulated on the catalyst surface
further react with the absorbed formaldehyde to yield the dimeric
compound.
The nature of the support may have a profound influence on the reactivity
of NPs; the support itself may also show catalytic properties, as in the case of
systems described by Takei et al.
49,50
For example, a very high yield of
acetaldehyde was shown using Au NPs supported over acidic MoO
3
(94%,
with 5% ethylene at 240 1C, catalyst with 1% Au loading) or weakly basic
La
2
O
3
. Oxidation to acetic acid took place principally over Au NPs deposited
on n-type metal oxides such as ZnO (44% yield of acetaldehyde, 46% of acetic
acid, 6% of ethyl acetate, at 220 1C) and V
2
O
5
, whereas complete oxidation
was preferentially obtained over p-type semiconducting metal oxides such as
MnO
2
and Co
3
O
4
. Even though many of these metal oxides proved active in
ethanol oxidation, the presence of Au enhanced the activity, and the
selectivity was also greatly affected by the type of metal oxide support. In the
mechanism proposed by the authors, following the generation of the ethoxy
species over either basic or acidic metal oxide or n-type semiconducting
metal oxide supports, the former reacts with O
- generated at the Au NP
surface or at the interface between Au and the support - to produce acet-
aldehyde. The presence of an excessive concentration of activated O species
on the surface of the p-type metal oxide support was the reason for the high
catalytic activity shown and the combustion of the C
2
species, with prevailing
formation of carbon oxides.
Evidence for the role of electrophilic, highly reactive O species were also
reported by Sobolevet et al.,
51-53
who showed that TiO
2
-supported Au NPs,
approximately 2 nm in size, gave rise to 'double-peak' catalytic activity
during the gas-phase oxidation of ethanol, which was not observed for NPs
supported over either Al
2
O
3
or SiO
2
. The low-temperature peak, at 120 1C,
was attributed to the formation of a specific active oxygen form on the
Au/TiO
2
surface, generated under mild reaction conditions. The prevailing
d
n
4
r
4
n
g
|
3
.
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