identity of the cluster upon deposition. Upon impact a redistribution of the total
energy of the cluster takes place. In the deposition process this energy consists of
the cluster’s kinetic and internal energy, the binding energy between cluster and
substrate and an eventual Coulomb energy between the cluster ion and an induced
image charge (metal) or surface charge density (dielectric) on the support. The
amount of this energy is decisive for the degree of melting, disordering, fragmentation, implantation or rebounding of the cluster. The cluster–surface and Coulomb interactions are given intrinsically by the chosen cluster and support materials, whereas the internal and kinetic energies can be changed to a large extent by
the experimental conditions. ‘Softlanding’ is defined as a collision outcome allowing plastic deformation of the cluster but not fragmentation and implantation into
the substrate [56]. Molecular dynamic studies [57–59] have shown that cluster implantation occurs at about 1 eV atom
À1 kinetic energy of the cluster, regardless of
the cluster–substrate system. Consequently, this value represents the upper kinetic
energy limit for softlanding conditions. Softlanding of size-selected Si 30 and Si 39
clusters on Ag(111) was observed in recent low-temperature STM experiments
performed at kinetic energy well below the softlanding limit and without rare
buffer layers, i.e. under the same experimental conditions as the experiments
shown below [60]. Finally, to prepare cluster-assembled nanocatalysts, less than
0.5% of a ML of size-selected clusters (1 ML ¼ 2 Â 10
15 clusters cm
À2 ) are deposited at 90 K, in order to land them isolated on the surface and to prevent agglomeration of the cluster on the surface. Under these experimental conditions,
the landing and the migration and trapping of the clusters on the surface was investigated with Monte Carlo simulations [47]. These simple calculations show that
the preparation of samples with monodispersed clusters is possible only when a
small number of clusters is deposited. In order to prepare clusters on identical adsorption sites it is therefore essential to use support materials with low defect
densities for systems with a strong cluster–support interaction or use surfaces with
high defect densities when the cluster–support interaction is weak. The deposition
of the clusters on defects is additionally favored by the recently discovered ‘‘funnel’’
effect [10]; ab initio calculations revealed that clusters are attracted, e.g. by F centers, well before landing on the surface.
Changing catalytic efficiency atom-by-atom: oxidation of carbon monoxide on pure
and strontium-doped gold nanocatalysts In the first example we focus on gold
nanocatalysts, which led recently to most remarkable and rather surprising findings. More specifically, low adsorption energies and high activation barriers for
dissociation of small molecules on extended gold surfaces are responsible for the
nobleness of gold as a bulk material [61]. Nanosize particles of gold, however, show
high and rather unusual catalytic reactivity [62, 63]. While the novel heterogeneous
catalysis by nanosize gold aggregates supported on oxides is of great significance,
the processes underlying the catalytic activity of gold in reduced dimensions and
the reaction mechanisms are not yet understood and still represent an enigma in
catalysis. In the following these issues are addressed theoretically and experimentally through investigations of the low temperature combustion of CO on size17.3 Chemical Reactions and Catalytic Processes on Free and Supported Clusters 567
energy of the cluster takes place. In the deposition process this energy consists of
the cluster’s kinetic and internal energy, the binding energy between cluster and
substrate and an eventual Coulomb energy between the cluster ion and an induced
image charge (metal) or surface charge density (dielectric) on the support. The
amount of this energy is decisive for the degree of melting, disordering, fragmentation, implantation or rebounding of the cluster. The cluster–surface and Coulomb interactions are given intrinsically by the chosen cluster and support materials, whereas the internal and kinetic energies can be changed to a large extent by
the experimental conditions. ‘Softlanding’ is defined as a collision outcome allowing plastic deformation of the cluster but not fragmentation and implantation into
the substrate [56]. Molecular dynamic studies [57–59] have shown that cluster implantation occurs at about 1 eV atom
À1 kinetic energy of the cluster, regardless of
the cluster–substrate system. Consequently, this value represents the upper kinetic
energy limit for softlanding conditions. Softlanding of size-selected Si 30 and Si 39
clusters on Ag(111) was observed in recent low-temperature STM experiments
performed at kinetic energy well below the softlanding limit and without rare
buffer layers, i.e. under the same experimental conditions as the experiments
shown below [60]. Finally, to prepare cluster-assembled nanocatalysts, less than
0.5% of a ML of size-selected clusters (1 ML ¼ 2 Â 10
15 clusters cm
À2 ) are deposited at 90 K, in order to land them isolated on the surface and to prevent agglomeration of the cluster on the surface. Under these experimental conditions,
the landing and the migration and trapping of the clusters on the surface was investigated with Monte Carlo simulations [47]. These simple calculations show that
the preparation of samples with monodispersed clusters is possible only when a
small number of clusters is deposited. In order to prepare clusters on identical adsorption sites it is therefore essential to use support materials with low defect
densities for systems with a strong cluster–support interaction or use surfaces with
high defect densities when the cluster–support interaction is weak. The deposition
of the clusters on defects is additionally favored by the recently discovered ‘‘funnel’’
effect [10]; ab initio calculations revealed that clusters are attracted, e.g. by F centers, well before landing on the surface.
Changing catalytic efficiency atom-by-atom: oxidation of carbon monoxide on pure
and strontium-doped gold nanocatalysts In the first example we focus on gold
nanocatalysts, which led recently to most remarkable and rather surprising findings. More specifically, low adsorption energies and high activation barriers for
dissociation of small molecules on extended gold surfaces are responsible for the
nobleness of gold as a bulk material [61]. Nanosize particles of gold, however, show
high and rather unusual catalytic reactivity [62, 63]. While the novel heterogeneous
catalysis by nanosize gold aggregates supported on oxides is of great significance,
the processes underlying the catalytic activity of gold in reduced dimensions and
the reaction mechanisms are not yet understood and still represent an enigma in
catalysis. In the following these issues are addressed theoretically and experimentally through investigations of the low temperature combustion of CO on size17.3 Chemical Reactions and Catalytic Processes on Free and Supported Clusters 567