Chemistry Reference
In-Depth Information
complexation will become thermodynamically less favourable. At Au
21
, the magic
shell closing of a 20 e
system dramatically lowers the IE, making the complex
formation favourable once again.
Whilst the differences in the reactivity pattern of neutral and anionic gold
clusters are essentially due to the cluster's charge, more subtle variations in charge
density can be realised by electron-donating or electron-accepting ligands. One
example of how the reactivity can be modified in this way is the activity of partially
hydrogen-covered cationic gold clusters towards oxygen. As has been previously
mentioned, the gold cations are thought to be unreactive towards molecular oxygen
with the exception of Au
10
+
[
13
]. Upon binding H
2,
however, all of the even-sized
clusters (Au
2
+
,Au
4
+
and Au
6
+
) are found to now bind one O
2
molecule [
101
]. This
cooperative effect is based on molecular hydrogen ligands effectively acting as
electron donors, i.e. increasing electron density at the gold, and thereby enabling
the activation of O
2
via single electron transfer as discussed above.
These combined investigations reveal that oxygen activation, at least in the gas
phase, is dependent upon the ability of the gold cluster to transfer an electron into
the
*
HOMO of the oxygen molecule. Thus only certain clusters are able to
activate oxygen upon complexation. The extent of activation is comparable for
all sizes and charge states, as the observed vibrational frequencies are similar for all
the species presented. Lastly, the dynamic nature of the system is often very
important for the observed reactivity, with the neutral clusters undergoing,
sometimes dramatic, rearrangements in response to oxygen adsorption.
ˀ
4.2 Carbon Monoxide
The prototypical reaction for modelling oxidative reactions with gold nanoparticles
is the oxidation of carbon monoxide. Historically this reaction is of great signifi-
cance as it was the first one observed to be catalysed by gold nanoparticles and
spawned the entire research field [
79
,
80
]. The reaction serves as a useful model for
other oxidative reactions as well as being of industrial interest in its own right,
for example, to selectively remove CO from process gases (e.g. CO from H
2
gas)
or as a scrubbing agent in water-based fuel cells or car catalytic converters.
The binding of carbon monoxide to transition metals is a well-known phenom-
enon, typically described by the Blyholder model [
102
]. The Blyholder model
assumes a two-component bonding interaction; one component is electron donation
from the CO HOMO, which is of
-bonding character, into an empty, correctly
symmetry adapted orbital of the metal centre. This occurs concurrently with back-
donation from filled metallic d-orbitals into the empty
˃
* LUMO of the CO (the
second component). From such a binding model, one can clearly see that metal-CO
binding will be favoured for metallic centres which contain a partially filled set of
d-orbitals. For gold clusters, this is not the case as the d-orbitals are completely
filled and this is reflected in the weak binding of CO to gold surfaces [
103
] and
indeed the rarity of gold-carbonyl complexes in both the solid and solution phases.
Similar to molecular oxygen, the binding to the metal centre involves orbitals
which are of C-O anti-bonding character and so IR spectroscopy serves as an
ˀ
