From atoms to crystallites: adsorption on oxide-supported metal particles

The properties of metal clusters and nanoparticles are attracting more and more attention in fundamental and applied research. The application of such structures in heterogeneous catalysis is a particularly important area. Nevertheless, there is only limited fundamental knowledge about the dependence of adsorption behaviour and catalytic activity on particle size. In this article, we describe an approach to tackle such questions on the basis of model systems which are prepared by metal vapour deposition under ultrahigh vacuum conditions. A thin alumina film grown on a metal substrate is used as support. This has the advantage that scanning tunneling microscopy and photoelectron spectroscopy can be applied without any charging problems. Firstly, structural data covering palladium, rhodium and iridium deposits demonstrate that a wide spectrum of particle sizes and morphologies may be obtained by taking advantage of kinetically controlled nucleation and growth processes. Subsequently, we discuss the adsorption of simple molecules on the particles, such as CO and ethene. These studies, which have been carried out using infrared and photoemission spectroscopy, illustrate possible variations in the adsorption and reaction behaviour as a function of particle size. Aspects which will be considered include: adsorption at facets and defects, size-dependent variation of adsorption sites, formation of surface complexes, decomposition of molecules as well as co-adsorption phenomena.