The interactions of cyclopentane with clean and bismuth-covered Pt(111)

Abstract

The adsorption and reactions of cyclopentane on clean and on Bi-covered Pt(111) surfaces have been studied with thermal desorption mass spectroscopy (TDS), Auger electron spectroscopy (AES), and X-ray photoelectron spectroscopy (XPS). Partial dehydrogenation of adsorbed cyclopentane to c-C 5H 8(a) competes with molecular desorption at 218 K during TDS. At full monolayer coverage, molecular desorption is preferred over dehydrogenation by a factor of ∼ 6. Quantitative XPS indicates that a monolayer of c- C 5H 10 corresponds to a coverage of ∼ 0.22 molecules per Pt surface atom. The intermediate, c-C 5H 8(a), was verified by bismuth-postdosing TDS. It deydrogenates further by 420 K to produce c-C 5H 5(a), and ultimately to graphitic carbon by 820 K. Coadsorbed bismuth adatoms suppress dehydrogenation of adsorbed cyclopentane, and increase molecular desorption. At low Bi coverages, Bi exerts little electronic influence as evidenced by the invariance of the TDS peak temperatures for desorption and for dehydrogenation of cyclopentane. Thus, the mechanism for the poisoning of cyclopentane dehydrogenation by Bi is dominated by an ensemble effect, whereby the Pt sites necessary for hydrogen abstraction are sterically blocked by bismuth. Kinetic modelling of the results suggests that an ensemble of at least five free Pt atoms are required for the dehydrogenation of cyclopentane, in addition to the ∼ 4.5 Pt atoms required for chemisorption. The influence of various forms of surface carbon upon the kinetics suggests that dehydrogenation is not dominated by defect sites.