Adsorption and thermal reactions of alkanethiols on Pt(111): Dependence on the length of the alkyl chain

The adsorption and thermal decomposition of alkanethiols (R-SH, where R = CH₃, C₂H₅, and C₄H₉) on Pt(111) were studied with temperature-programmed desorption (TPD) and X-ray photoelectron spectroscopy (XPS) with synchrotron radiation. Dissociation of sulfhydry 1 hydrogen (RS-H) of alkanethiol results in the formation of alkanethiolate; the extent of dissociation at an adsorption temperature of 110 K depends on the length of the alkyl chain. At small exposure, all chemisorbed CH₃SH, C₂H₅SH, and C₄H ₉SH decompose to desorb hydrogen below 370 K and yield carbon and sulfur on the surface. Desorption of products containing carbon is observed only at large exposure. In thermal decomposition, alkanethiolate is proposed to undergo a stepwise dehydrogenation: R′-CH₂S →R'′CHS→ R'′ CS, R′ = H, CH₃, and C ₃H₇. Further decomposition of the R'-CS intermediate results in desorption of H₂ at 400-500 K and leaves carbon and sulfur on the surface. On the basis of TPD and XPS data, we conclude that the density of adsorption of alkanethiol decreases with increasing length of the alkyl chain. C₄H₉SH is proposed to adsorb mainly with a configuration in which its alkyl group interacts with the surface; this interaction diminishes the density of adsorption of alkanethiols but facilitates dehydrogenation of the alkyl group.