A study of methanation kinetics on a clean and a titania-covered platinum foil

Abstract

We have investigated the detailed methanation kinetics for a clean and a titania-covered Pt foil. Pressure dependences for H 2 and CO were measured between 0.01 and 100 Torr, and Auger electron spectra (AES) were taken of the surfaces following reaction. For the foil not covered by titania, AES indicated that there is no carbon on the surface following reaction, even for H 2 CO pressure ratios as low as 0.03. Kinetic measurements indicated that the reaction order for CO changed from first order at low pressures to zero order at higher pressures, while the reaction was always first order for H 2 except for low CO pressures. The methanation kinetics could be modeled by a Langmuir-Hinshelwood rate expression in which CO and hydrogen are competitively adsorbed. The adsorption equilibrium constants in the rate expression for both CO and H 2 were identical to those obtained from adsorption and desorption measurements in ultrahigh vacuum experiments, indicating that the dominant adsorbed species during reaction are likely the same as those species measured in adsorption. The results for the titania-covered foil showed that methanation on this surface is very different. AES indicated that significant quantities of a reactive carbon were present on the surface at low H 2 CO pressure ratios. The reaction was again zero order in CO at high CO pressures; however, the reaction became half order rather than first order at low CO pressures. The reasons for the enhanced methanation activity of titania-supported catalysts are discussed in terms of these results.