Chemistry Reference
In-Depth Information
2.3 Charge-Mediated Growth of Metallic Chains on Oxide Thin Films . . . ............. 99
2.4 Development of Two-Dimensional Metal Islands on Oxide Thin Films . . . . . . . . . . . 105
2.5 Metal Growth on Doped Oxide Materials . . . ......................................... 110
3 CO Adsorption on Supported Au Particles . . ............................................... 116
3.1 CO Adsorption on Au on Thin MgO/Ag Films . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 117
3.2 CO Adsorption on Au on Ultrathin Alumina and Iron Oxide Films . . . . . . . . . . . . . . . . 122
4 Nucleation and Bonding of Au on Chemically Modified Oxide Surfaces . . . ............. 124
4.1 Gold Nucleation at Hydroxyl Groups . . .............................................. 125
4.2 Surface Science Approach to Supported Au Catalyst Preparation . . . . . . . . . . . . . . . . . . 129
5 Synopsis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 132
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 132
1
Introduction
In 1984 Michael Mingos published two papers important for the present review
[
1
,
2
]. One paper was published in Account of Chemical Research in which he
summarized the state of affairs concerning the prediction of structures of cluster
compounds based on their total number of valence electrons. He and Ken Wade
pioneered in the early 1970s [
3
] what has been called the Polyhedral Skeletal
Electron Pair Theory, an extension of the VSEPR (valence shell electron pair
repulsion theory) [
4
] of molecular compounds. This led then to setting up of
electron counting rules, which connect the number of available valence electrons
with stability criteria for particular cluster geometries. The second paper was
published in Gold Bulletin entitled “Gold cluster compounds: are they metals in
miniature?” The results of his thoughts about surface-clusters analogies may be
summarized with two quotes from his paper: “The writer's personal view is that
such analogies bear the same deficiencies as attempting to relate benzene to
graphite and strained hydrocarbons such as cubane, C
8
H
8
, to diamond” and
“These results suggest that those scientists studying the catalytic properties of
crystallites of gold on inert supports should recognize that the clusters may change
their geometries as a result of introducing substrates and changes in environment.”
The latter, in particular, is an important observation that many researchers in
catalysis, even today, have not taken into consideration, seriously. It is one of the
messages that this paper wants to convey: the flexibility of nanoparticles to respond
to the environment, for example, the support, is the source for their reactivity. By
controlling the environment and using the flexibility of nanoparticles one may
change not only morphologies and structures but also charge states of active
components. For the latter, the metal-oxide interface plays an important role and
thus cannot be taken into consideration by the single crystal approach, so successful
in the past, culminating in the 2007 Nobel Prize for Gerhard Ertl [
5
]. While solid-
state theory has provided fundamental insights into processes and intermediates on
single crystal surfaces using density functional calculations with periodic bound-
aries these approaches have to be substantially modified to allow for successful
treatment of nanoparticles of relevant size on an oxide support. In this respect, also,
