The trapping of methane on ordered structures of CO on Pt(111)

Abstract

The dynamics of methane trapping on CO-covered Pt(111) in low coverage, c(√3×5)rect, and c(4×2) structures was investigated using supersonic molecular beam techniques at a surface temperature of 50 K; at this temperature methane was stably adsorbed on the clean (A. F. Carlsson and R. J. Madix, to be published) surface, but not in multilayers (A. F. Carlsson and R. J. Madix, to be published), and thus trapped amidst adsorbed CO molecules. Molecular trapping was enhanced to greater degrees with increasing CO coverage, and the methane uptake decreased with increasing CO coverage, as would be expected. The trapping probability further increased as methane covered the Pt(111)–CO surface; the modified Kisliuk model [J. Chem. Phys. 92, 1397 (1990); J. Phys. Chem. 95, 2461 (1991)] describes the coverage-dependent trapping probability. Methane adsorption may occur directly on the surface, or via two entrance channels into an extrinsic precursor, where the trapping probability is higher. The angular dependence of methane trapping on the CO-covered surface suggests an increasing corrugation in the gas–surface potential as the CO coverage increases; the corrugation may also contribute to the enhancement of the overall trapping probability by facilitating interconversion of perpendicular to parallel momentum during the gas–surface collision.