Characterization and catalysis studies of small metal particles on planar model oxide supports

Planar model oxide-supported metal catalysts have been prepared by evaporating metals of catalytic interest onto various oxide thin films supported on refractory metal single crystals, followed by annealing to promote the formation of small metal clusters. The employment of these relatively simple supported catalysts prepared in ultrahigh vacuum (UHV) represent a promising method of simulating heterogeneous catalytic processes over high surface area supported catalysts in an experimentally tractable manner. While providing many of the same practical advantages associated with single crystals relative to high surface area supported catalysts, the experiments using planar model catalysts address important issues such as intrinsic particle size effects and metal support interactions that are beyond the scope of single crystal studies. Because these samples are suitable for study using modern UHV based surface analysis techniques, including scanning tunneling and atomic force microscopy, they can be characterized on the atomic level to a degree unlikely to be achieved for the analogous powder catalysts. Kinetics studies for several reactions performed in parallel over single crystals, planar model supported catalysts and conventional high surface area supported powder catalysts allow the catalytic behavior exhibited for a particular reaction to be correlated with specific structural features. The experiments described here establish the viability of these relatively simple systems as representative model catalysts and demonstrate their utility in elucidating heterogeneous catalytic systems, particularly in the context of combined studies involving single crystals and powder catalysts.