Gas Phase Formation, Structure and Reactivity of Gold Cluster Ions - Gold Clusters, Colloids and Nanoparticles - page 179

h i

He Au n L

ðÞ þ= ðÞ þ=

Au þ=
n

þ L ! Au n L ! ð 14 Þ

Table 8 lists the gas-phase binding energies of gold cluster ions towards a range
of substrates [ 261 , 262 ]. The motivation of many of these studies was to provide a
firm thermochemical understanding of key steps associated with possible catalytic
cycles for the processing of substrates. Thus the reader is directed to related studies
on ion-molecule reactions (Sect. 4.1 ) and catalytic cycles (Sect. 4.3 ).
An examination of the data in Table 8 reveals some interesting trends:

1. The thermochemistry associated with room temperature CO adsorption on
isolated gold cluster cations exhibits a pronounced cluster size effect for the
adsorption energies for the first CO molecule binding to the gold cluster ions.
The binding energies decrease with increasing cluster size from 1.09 eV for
n 26. Exceptions were local maxima of between

0.73 and 0.75, which were found for n

¼ 6 to less than 0.65 eV or n ¼

30, 31, and 48, 49. The atom-by-atom
variations were suggested to arise from different binding sites on the gold
clusters, consistent with DFT calculations on the smallest cluster sizes n

¼

¼ 3-9

20.
2. Not surprisingly, the binding energies also depend on the nature of the substrate.
Thus CO binds more strongly than CH 4 to the gold cluster cations.
3. The magnitude of the cluster charge is important. Thus CO binds more strongly
to the cluster cations than the cluster anions.

and n ¼

The combination of experiment and DFT calculations provides valuable insight
into how binding energies change upon sequential addition of CO and how this
influences the geometry of the gold cluster core. A case in point is the binding of
CO to the homo and hetero, M gold and silver cluster cations, M 5 x M 0 x + (where
M

Ag) [ 267 ]. At room temperature, Au 5 + rapidly absorbed four CO
molecules, with absorption of a fifth CO only occurring at temperatures lower than
250 K. This is consistent with the results of DFT calculations (Fig. 25 ), which
reveal that the 'bow-tie' structure for the bare Au 5 + cluster can readily bind four CO
molecules at each of the four corner atoms. Binding of a fifth CO requires the gold
cluster core to undergo a rearrangement to a structure described as a 'side-capped
tetrahedron'. Related structural transitions were determined for the mixed gold-
silver clusters Au 3 Ag 2 + ,Au 2 Ag 3 + and AuAg 4 + .

Au and M 0 ¼

¼

4 Reactivity of Gold Cluster Ions

The gas-phase reactions of gold cluster anions and cations with single and multiple
neutral substrates have been widely studied over the past 2 decades. Much of this
work has been inspired by Haruta's discovery of CO oxidation by gold clusters