Surface reactions on inverse model catalysts: CO adsorption and CO hydrogenation on vanadia- and ceria-modified surfaces of rhodium and palladium

Reducible transition metal oxides are well-known promoters of the hydrogena tion of CO on noble metal surfaces. In this study the promotional effect of vanadia and ceria adlayers on Rh and Pd surfaces was investigated with emp hasis on the effect of the oxidation state on CO adsorption and catalytic a ctivity. "Inverse supported catalysts" were prepared by UHV deposition of V and Ce on the noble metal surface (Rh(111), Pd(111) or Rh foil). After oxi dation and specified reduction, the reaction kinetics on polycrystalline Rh was measured at atmospheric pressure, and the molecular and dissociative c hemisorption of CO on Rh(111) and Pd(111) and the methanation kinetics on R h(111) were investigated by molecular beam techniques. On Rh(111), the prob ability of CO dissociation and the reaction rate are enhanced by submonolay er VOx deposits. Local pressures between 10(-2) and 1 mbar are sufficient t o drive the methanation at 573 K with measurable amounts of products, accom panied by significant restructuring of the catalyst surface. Although the r eaction on Rh is generally promoted by small quantities of vanadia and ceri a, the reaction rates depend strongly on the extent and temperature of hydr ogen reduction. The observed increase of the reaction rate by reduction up to 673 K can be correlated to concomitant changes of the structure and comp osition of the VOx deposits. If the reduction temperature is raised above 6 73 K, metallic V is partially dissolved in the bulk, and the resulting V/Rh subsurface alloy exhibits a particularly high activity. Contrary to vanadi a, ceria islands on Rh promote the initial reaction only after a low-temper ature reduction, but the activity decreases after reduction above 573 K.