Model studies in catalysis with uhv-deposited metal particles and clusters

The motivation for extending well-controlled surface and microstructural studies of model catalytic systems into the regime of small metal particles and clusters is discussed. The possibility of accomplishing this successfully at the present time is related to the availability of proven uhv techniques for the deposition and characterization of thin film deposits; the deposits in this case being either continuous supporting substrates or discrete metallic particles ranging in size to less than 10 Å. A review of the known physical properties of small metal aggregates is given relating their thermodynamic, morphological, crystallographic, and electronic properties to size, support, and gaseous environment. Whenever possible, the chemical properties of supported small particles and clusters are correlated with their observed physical characteristics although it is often extremely difficult to separate intrinsic size and electronic effects from support-induced surface compositional changes. Metal/support interactions (MSI) in vacuum and in different gaseous environments are considered and are related to catalyst preparation and sintering processes. A general experimental concept for studies with evaporated model catalysts is discussed and demonstrated with several instrumental approaches. It consists of effectively combining microstructural techniques of high spatial resolution (TEM) with large-area surface-analytical methods of high detection sensitivity. In conclusion, some key research areas are identified where model studies with thin film systems would seem most promising.