Morphology and surface reactions of Rh/VOx/SiO2 model catalysts

The interaction between rhodium, the support oxide SiO2, and the promotor o xide VOx has been studied for a Rh/VOx/SiO2/Mo model system. This system co nsists of two monolayers of vanadium oxide on a thin silicon oxide layer, b oth prepared by chemical vapor deposition on molybdenum with a nominal load of one monolayer rhodium. The changes of binding energies of surface speci es and the morphology of the surface after heating in ultra high vacuum (uh v) were analysed by low-energy ion scattering (ISS) and X-ray photoelectron spectroscopy (XPS). No formation of rhodium silicide was found on the VOx promoted system after thermal treatments up to 873 K in ultra high vacuum, in contrast to results reported for the system without vanadia. A reaction mechanism is proposed in which oxygen from VOx adsorbs on the rhodium surfa ce. Consequences of these reactions for the surface morphology are discusse d.