Chemistry Reference
In-Depth Information
Fig. 13 Mass spectra in the
range of Au
5
to Au
12
when
reacted with O
2
.The
upper
panel
shows the anionic
species (reacted at
100
C),
the lower the neutral (reacted
at
177
C). All
Au
n
(O
2
)
m
/0
species are
labelled with an asterisk.
Other peaks in the mass
spectrum relate to water and
other impurities. Neutral
clusters are ionised by 7.9 eV
photons from an F
2
laser
IR-MPD spectra these are given as two different traces for Au
8
O
2
. Accompanying
DFT calculations reveal that this is due to an adsorbate driven isomerisation
between two forms of the Au
8
O
2
cluster complex. Depending on the residence
time in the reaction channel, either the cluster complex corresponding to the
ground-state Au
8
structure (a 4-pointed star) or the excited-state structure (an -
edge-capped centred hexagon) prevails. It transpires that the complex of the edge-
capped centred hexagon structure is lower in energy than the 4-pointed star
complex but a significant barrier to the rearrangement exists (0.43 eV) hence
the possibility for kinetic trapping [
95
]. Further, both investigations find that this
structural change is accompanied with a transition from a
1
bound complex for
ʼ
2
complex in the edge-capped centred hexagon.
the 4-pointed star to a
ʼ
4.1.2 Neutrals
One of the suggested mechanisms for the catalytic activity of nano-dispersed gold
particles builds upon the observation that gold clusters are found to become
negatively charged when deposited over defect sites in oxide supports and, indeed,
such systems have been shown to be catalytically active [
1
,
2
]. This, in part, drove
the experimental and theoretical interest in the anionic gold cluster complexes with
oxygen. Neutral clusters, however, are much more likely to be present in a real
catalyst and thus it is interesting to investigate if and how neutral gold clusters
interact with molecular oxygen.
Figure
13
compares the mass spectra of the anionic gold cluster complexes and the
neutral cluster complexes when using 7.9 eV photons for ionisation. As was
