Surface studies of supported model catalysts

Metal particles grown by vapour deposition on clean and well-defined oxide surfaces are used as model catalysts. These new model catalysts allow, unlike metal single crystals, a study of size and support effects in heterogeneous catalysis. The structure, the electronic properties and the reactivity of these supported model catalysts have been studied, in situ, by a large number of surface science techniques. In order to get relevant information from those studies it is necessary to control the nucleation and growth in order to get uniform collections of metal particles. The preparation conditions and the characterisation methods will be reviewed. Particles with well-defined shapes are obtained by epitaxial growth at high temperature on clean ordered surfaces. The electronic properties of the small metal particles depend not only on their size but also on their shape. The chemisorption properties are strongly related to the surface structure of the particles. The interplay between the surface structure, the local electronic properties and the adsorption energy will be discussed for CO chemisorption. The presence of the support plays an important role in the control of the particle morphology. Furthermore, it can increase the adsorption rate. The intrinsic heterogeneity of the supported model catalysts has to be taken into account to understand in detail the catalytic reactions. The reaction rate cannot be considered as an average on the different crystalline facets present on the particle. Finally, we will discuss the possibility to study in situ and at the atomic level simple chemical reactions on supported catalysts.