Vanadium Oxide Overlayers on Rhodium: Influence of the Reduction Temperature on the Composition and on the Promoting Effect in CO Hydrogenation

Abstract

Vanadia adlayers on a Rh surface promote the hydrogenation of CO. This promotion was investigated with particular emphasis on the effect of the oxidation state of vanadium. Inverse model catalysts were prepared by UHV deposition of submonolayer amounts of metallic vanadium on a Rh foil in an oxygen atmosphere. After oxidation at 673 K the catalyst was reduced in hydrogen under specified conditions and the surface composition was determined by Auger electron spectroscopy. The kinetics of the reaction were then measured at 573 K and atmospheric pressure. The reaction rate is always increased with respect to the bare Rh surface but depends strongly on the reduction temperature. After reduction up to 673 K the observed rate increase can be correlated to the existence of two VOx adlayer phases of different oxygen content. Enhanced CO dissociation at the perimeter sites of the VOx islands leads to enhanced initial hydrogenation rates but also to enhanced deactivation of the catalyst. Raising the reduction temperature above 773 K results in the formation of metallic V, which is partially dissolved in the bulk, resulting in a V/Rh subsurface alloy with a Rh-terminated surface. This surface is particularly active for CO hydrogenation and less susceptible to deactivation by carbon deposition.