Photoelectron spectroscopic studies of deposited metal clusters and model catalyst systems

Abstract: Metal clusters are small metal aggregates between atomic and metal systems. Their physical and chemical properties have been observed to change with size. Their surface fraction is large which makes them of particular interest for catalysis. Most presently used catalyst systems consist of small metal particles on porous oxide supports. By replacing these particles with selected clusters it may be possible to produce better catalysts. To do this, fundamental knowledge of clusters is important and this can be gained by applying surface science techniques, such as photoelectron and X-ray absorption spectroscopy.Practical catalytic systems are difficult to study with surface science techniques because of their rather complex nature. Therefore model systems have been developed to enable detailed studies of clusters. The results presented apply to one such model system, metal particles on a thin alumina film grown through oxidation of a NiAl(110) surface. The structure of this film and the interaction between it and deposited particles is discussed.The electronic structure of deposited metal clusters change with size, approaching a bulk-like character as the size increases. This has been observed both for the valence band of Pd particles and the core levels of Pd and Rh particles. It is also found that the Rh particles deposited on the thin alumina film undergo a transition from electronically-isolated to metallic anchored character in the size range of 1000-3000 atoms.The chemical reactivity of clusters is known to change with particle size. This has been observed for CO interaction with both Pd and Rh clusters. Of special interest is the observation that the heat-induced CO dissociation ability of Rh particles is strongly size-dependent, exhibiting a maximum for particles with around 1000 atoms. In this case the important factor for the catalyst is not to exhibit as much surface as possible, but to exhibitthe right type of surface. This is an example of how knowledge about the size-dependent properties of clusters can be important for the application of catalyst systems. The development of a size-selected cluster source and results from it is presented.