Coadsorption of cyclic hydrocarbons and cesium on platinum(111): electronic and ensemble effects

Abstract

The adsorption and dehydrogenation of benzene, cyclohexane, and cyclopentene were studied on cesium-precovered Pt(111) surfaces with thermal desorption mass spectroscopy (TDS), X-ray photoelectron spectroscopy (XPS), and Auger electron spectroscopy (AES). Increases in the molecular TDS peak temperatures for cyclohexane and cyclopentene occurred for increasing cesium coverages in the range {Theta}{sub Cs} = 0 to {approximately}0.10, indicating that small amounts of cesium adatoms electronically stabilize these hydrocarbon molecules on the surface. The reverse occurs for benzene. At higher cesium coverages ({Theta}{sub Cs} {ge} 0.15), the molecular TDS peaks for benzene, cyclohexane, and cyclopentene shifted to lower temperatures. This shift is attributed to steric hindrance of the preferred hydrocarbon adsorption site by cesium adatoms. Coadsorbed cesium also stabilized hydrogen adatoms and accelerated dehydrogenation of certain hydrocarbon fragments, as indicated by peak shifts in the H{sub 2} TDS spectra from adsorbed benzene, cyclohexane, and cyclopentene. In addition, as the precoverage of cesium on the Pt(111) surface was increased, the probability that the adsorbed molecular hydrocarbon undergoes dehydrogenation during TDS decreased, while the probability for molecular desorption increased.