Chemistry Reference
In-Depth Information
Fig. 20 Mechanism for the
catalytic oxidation of CO by
O
2
at Au
2
. Reprinted from
Bernhardt et al. [
130
].
Copyright 2005. With
permission from Elsevier
[
17
,
126
]. Such species may be formed upon transferring one of the O atoms
supplied by O
2
to CO, i.e. resulting in its oxidation, and release of CO
2
.
For Au
2
the reaction kinetics have been studied both experimentally and theoret-
ically in great detail [
17
,
128
,
129
] providing the realisation of a catalytic cycle for
CO oxidation (Fig.
20
). The cycle begins with formation of the oxygen complex
which then reacts with a single CO molecule. Two possible structures of this
intermediate are discussed, either a carbonate or a peroxyformate species. For the
carbonate species of Au
2
, this then undergoes an Eley-Rideal reaction step with a
second CO to evolve two CO
2
molecules and regenerate the starting Au
2
. The
alternative peroxyformate species also undergoes an Eley-Rideal reaction step to
form a stabilised complex which then fragments to regenerate the bare gold cluster
and evolve two CO
2
molecules. A similar mechanism has been suggested for
neutral Au
2
but involving Au
2
(CO)
2
as a catalytically active species [
62
].
The reactions between the neutral carbonyl clusters and O
2
have also been
investigated under multiple collision conditions using a fast flow reactor
[
114
]. The reaction is monitored by observing the loss of carbonyl-complex signal
when oxygen is present in the reaction cell. In order to exclude the effects of losses
from scattering, the observed depletion is compared with and normalised to the
reduction in signal for a reaction cell filled with a partial pressure of N
2
, which is
assumed to be unreactive and have a scattering cross section similar to that of O
2
.
As with other studies of the neutral clusters, the data is hampered by the high IEs of
the gold clusters and as such only the odd-sized clusters are measurable. For these
species, all of the observed carbonyl complexes react with O
2
at 300 K with the
cluster complexes Au
3
(CO)
2
and Au
5
(CO)
4
being local maxima in the reactivity
which reduces at the largest cluster sizes investigated (Au
9
(CO)
6
). Unfortunately
there is no clear electronic or structural argument to rationalise this observed
variation in reactivity.
