Adsorption and Thermal Decomposition of Alkanethiols on Cu(110)

Abstract

Adsorption and thermal decomposition of alkanethiols (RSH, R = CH 3, C 2 H 5, and C 4 H 9 ) 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, CH 3 SH and C 2 H 5 SH dissociate to form surface thiolates and hydrogen, whereas C 4 H 9 SH adsorbs molecularly on the surface at 100 K; adsorbed C 4 H 9 SH 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. CH 3 generated from CH 3 S reacts with surface hydrogen to evolve CH 4, but at a large coverage can also undergo coupling to form C 2 H 6 . The thermal reaction of surface C 2 H 5 formed from C 2 H 5 S produces C 2 H 6 through hydrogenation and C 2 H 4 through β-hydride elimination, the latter with desorption of H 2; the ratio of products C 2 H 4 and C 2 H 6 varies with the adsorption site of surface C 2 H 5 S. The C 4 H 9 group of surface C 4 H 9 S undergoes exclusively elimination of β-hydride to form C 4 H 8 . To a small extent the alkyl moiety also undergoes dehydrogenation, resulting in deposition of carbon on the surface. Three CH 4 desorption states are proposed to correspond to CH 3 S intermediates with distinct adsorption sites after decomposition of CH 3 SH, whereas two C 2 H 5 S sites and one C 4 H 9 S site are proposed for cases of C 2 H 5 SH and C 4 H 9 SH, respectively. The distribution of desorption products depends on the nature of the adsorption site of a particular alkanethiolate.