Chemistry Reference
In-Depth Information
4 Gold Cluster Complexes
Knowledge of the bare gold cluster structures is a vital first step towards under-
standing the origins of their chemistry at the nanoscale. More direct insights into the
function of the clusters in a reactive environment, however, can be obtained by
studying complexes of these clusters with small ligands of catalytic relevance.
From an experimental point of view, the focus is often either on investigating the
kinetics of complex formation (or transformation) or on structural (spectroscopic)
characterisation. To date complexes of gold clusters with a variety of ligands have
been investigated but, in the interests of space, we limit the present discussion to
two of the most studied: O
2
and CO. Additionally, the reaction kinetics have been
the subject of two comparatively recent reviews [
21
,
78
], which highlight the roles
of gold and binary silver-gold clusters and their reactions with small molecules.
As such in the present work, we shall place the emphasis on the spectroscopic
characterisation of gold cluster complexes.
4.1 Molecular Oxygen
Perhaps the most famous property of gold nanoparticles is their ability to catalyse
low-temperature oxidations using molecular oxygen as a feedstock [
79
-
83
]. These
reactions are fascinating given that the typical mechanism for metal-catalysed
oxidation reactions, the Mars-van Krevelen mechanism [
84
], relies on the forma-
tion of oxide ions, O
2
, a process which is known to be unfavourable for gold
surfaces. Clearly, nanoscale materials may show a different chemistry, and indeed
small gold oxide clusters, i.e. species containing dissociated O
2
, can be produced
[
85
-
89
]. Their formation, however, relies on the activation of molecular oxygen in
the plasma plume formed during laser ablation, a highly energetic process quite
unlike that found in a real catalyst. Nevertheless, the question still remains: How
does nanoscale gold react with and activate molecular oxygen?
4.1.1 Anions
The majority of the experimental investigations into gold cluster complexes with
molecular oxygen have focused on the anions. Charged systems are experimentally
easier to study, as previously mentioned, and the cations are less reactive towards
oxygen, with the exception of the decamer (Au
10
+
)[
13
].
For the anions, only the even-sized clusters, Au
2n
, show appreciable reactivity
with O
2
[
13
,
90
,
91
]. This has been attributed to the alternating open/closed shell of the
gold clusters arising from the 5
d
10
6
s
1
electron configuration of the Au atom [
90
]. This
alternating shell structure means the even-sized anions (which have open shells)
typically show lower electron-binding energies (BE) than their neighbouring clusters.
This oscillation in the BE of the clusters has been correlated with the energy for the
association reaction Au
n
+O
2
!
Au
n
O
2
as shown in Fig.
10
. The only significant
deviation from this correlation occurs at Au
16
, which shows an anomalously high BE
