Vanadium Oxide Deposited on an Rh Foil: CO and CO2 Hydrogenation Reactivity

Abstract

The effects of submonolayer deposits of vanadia on the rates of CO and CO2 hydrogenation of an Rh foil have been examined. Auger electron spectroscopy (AES), ion scattering spectroscopy (ISS), and temperature-programmed desorption (TPD) have been used to determine the oxide coverage, and X-ray photoelectron spectroscopy (XPS) has been used to determine the oxidation state for VOx deposits on a rhodium foil. After oxidation (3 × 10−6 O2 at 350°C), the vanadium valence state is 3+, which corresponds to an oxide stoichiometry of V2O3. CO chemisorption reduces 34% of the V3+ to V2+ at low oxide coverages. The absolute amount of V2+ in the oxide overlayer after CO titration is found to maximize at a coverage of 0.5 ML. The rates of CO and CO2 hydrogenation increase upon addition of VOx to an Rh foil. For CO hydrogenation, the rate maximizes at two times that of the rate on the clean surface at an oxide coverage of 0.4 ML. For CO2 hydrogenation, the rate maximizes at six times that of the clean surface rate at an oxide coverage of 0.6 ML. The oxide promoter also alters the activation energies, partial pressure dependences, and selectivities for the hydrogenation reactions. A comparison of the kinetic and spectroscopic data reveals a strong correlation between the degree of reducibility of the oxide overlayer and the amount of rate enhancement for CO and CO2 hydrogenation.