An STM investigation of sulfur and alkoxide adsorption on Ni(100)

The effect of sulfur on alkoxide formation and decomposition on the Ni(100) surface has been investigated with STM and LEED. At low coverage sulfur adsorbs into a p(2 x 2) structure, in agreement with LEED measurements and previous STM results. With increasing sulfur coverage, the p(2 x 2) structure saturates the surface and scattered domains of c(2 x 2) appear. Further increases in sulfur coverage affect increases in c(2 x 2) domain sizes; the state of the sulfur-covered surface up to 0.43 ML is characterized by p(2 x 2) and c(2 x 2) domains. STM measurements of the evolution of the sulfur-covered surface with D(2)S(g) adsorption are suggestive of sulfur nucleation and growth at multiple sites on the surface. Alkoxide formation on these surfaces was studied following exposure to ROH (R = CH(3), CH(3)CH(2), CH(3)CH(2)CH(2), and C(6)H(5)). The alkoxy surface intermediates adsorbed in p(2 x 2)-S vacancies and, in the case of phenoxy, between hollow sites. Agreement between the methoxy coverage determined by XPS and the fraction of the surface covered with p(2 x 2)-S, as determined by STM, suggests that the p(2 x 2) vacancies are the sites of methoxy adsorption, and hence the active sites for selective poisoning.