Adsorption and thermal decomposition of alkanethiols on Cu(110)

Adsorption and thermal decomposition of alkanethiols (RSH, R = CH₃, C₂H₅, and C₄H₉) on a Cu(110) surface have been studied by means of temperature-programmed desorption (TPD) and X-ray photoemission spectroscopy (XPS) with synchrotron radiation. At a small coverage, CH₃SH and C₂H₅SH dissociate to form surface thiolates and hydrogen, whereas C₄H₉SH adsorbs molecularly on the surface at 100 K; adsorbed C₄H₉SH begins to deprotonate at ∼ 170 K. All of these alkanethiolates can decompose to evolve hydrocarbon via scission of the C-S bond, resulting in deposition of sulfur on the surface. CH₃ generated from CH₃S reacts with surface hydrogen to evolve CH₄, but at a large coverage can also undergo coupling to form C₂H₆. The thermal reaction of surface C₂H₅ formed from C₂H₅S produces C₂H₆ through hydrogenation and C₂H₄ through β-hydride elimination, the latter with desorption of H₂; the ratio of products C₂H₄ and C₂H₆ varies with the adsorption site of surface C₂H₅S. The C₄H₉ group of surface C₄H₉S undergoes exclusively elimination of β-hydride to form C₄H₈. To a small extent the alkyl moiety also undergoes dehydrogenation, resulting in deposition of carbon on the surface. Three CH₄ desorption states are proposed to correspond to CH₃S intermediates with distinct adsorption sites after decomposition of CH₃SH, whereas two C₂H₅S sites and one C₄H₉S site are proposed for cases of C₂H₅SH and C₄H₉SH, respectively. The distribution of desorption products depends on the nature of the adsorption site of a particular alkanethiolate.