Influence of support and promotor on the catalytic activity of Rh/VOx/SiO2model catalysts

A comparative study of the catalytic activities of a polycrystalline Rh foi l with Rh/SiO2 and Rh/VOx/SiO2 model catalysts was carried out using the CO and CO2 hydrogenation to methane as a test reaction. The model systems con sisted 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 rhodi um (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/VOx/SiO2). The reactions of CO/H-2 and CO2/H-2 and over the mode l catalysts and the Rh foil were performed at atmospheric pressure and elev ated temperatures up to 773 K in a flow-reactor system. In both reactions t he VOx-promoted reactions the catalyst is more active at equal temperatures than the unpromoted and the activity for methane in CO2/H-2 reactions for both catalysts is much higher than in CO/H-2 reactions. activity of the rho dium clusters on SiO2 with a size of 1 to 2 nm was found to be on average 4 times higher for CO/H-2 and 27 times higher for CO2/H-2 as compared to the Rh foil. The reasons for the difference in activity are discussed with res pect to the Rh cluster size, the total CO coverage, the CO dissociation and the VOx oxidation state during CO and CO2 adsorption and reaction.