METHYL THIOLATE ON NI(111) - MULTIPLE ADSORPTION SITES AND MECHANISTIC IMPLICATIONS

High resolution XPS, HREELS, and TPD data indicate two distinct bindin g modes for methyl thiolate with clearly distinct reactivites on the N i(111) surface. At low temperatures methyl thiolate is adsorbed primar ily at low coordination, probably bridging, sites with the C-S bond ti lted toward the surface and undergoes hydrogenolysis near 275 K. Betwe en 150 and 250 K, a fraction of the bridge bonded thiolates are conver ted to thiolates in hollow sites with their C-S bonds oriented nearly perpendicular to the surface. The C-S bond in the hollow bonded thiola te is more stable and does not undergo hydrogenolysis until around 300 K. Overall, about 80% of the chemisorbed thiolates undergo hydrogenol ysis (an overall disproportionation reaction) to form methane in the 2 50-350 K temperature range. The remaining 20% of the thiolates are com pletely dehydrogenated to form adsorbed atomic carbon and sulfur. Coad sorbed hydrogen favors formation of the thiolate in the bridge site. I n the presence of coadsorbed deuterium, a substantial fraction of the methane formed is multiply deuterated (-d(2) and higher); however, no kinetic isotope effect is observed for the methane formation reaction. These results suggest C-S bond breaking as the rate determining step for methane formation.