Influence of support and promotor on the catalytic activity of Rh/VOx/SiO2 model catalysts

Abstract

A comparative study of the catalytic activities of a polycrystalline Rh foil with Rh/SiO₂ and Rh/VO_x/SiO₂ model catalysts was carried out using the CO and CO₂ hydrogenation to methane as a test reaction. The model systems consisted of a thin silicon oxide layer, prepared by chemical vapor deposition on a Si(100) substrate, and a nominal load of about one monolayer of rhodium (Rh/SiO₂). The influence of the promotor material vanadium oxide on the catalytic activity was studied for a system with two monolayers of vanadium oxide (Rh/VO_x/SiO₂). The reactions of CO/H₂ and CO₂/H₂ over the model catalysts and the Rh foil were performed at atmospheric pressure and elevated temperatures up to 773 K in a flow-reactor system. In both reactions the VO_x-promoted catalyst is more active at equal temperatures than the unpromoted and the activity for methane in CO₂/H₂ reactions for both catalysts is much higher than in CO/H₂ reactions. The activity of the rhodium clusters on SiO₂ with a size of 1 to 2 nm was found to be on average 4 times higher for CO/H₂ and 27 times higher for CO₂/H₂ as compared to the Rh foil. The reasons for the difference in activity are discussed with respect to the Rh cluster size, the total CO coverage, the CO dissociation and the VO_x oxidation state during CO and CO₂ adsorption and reaction.