SO 2 structure and reactivity on clean and sulfur modified Pd(100)

Abstract

Below 120 K SO 2 was found to adsorb intact on clean and sulfided Pd(100) surfaces. The molecule bonds to the surface via its sulfur atom with preferential occupation of a four-fold hollow binding site. The SO 2 molecular plane apparently lies normal to the surface, but evidence of in-plane tilting and PdO binding was found, in sharp contrast to the out-of-plane tilt observed on Ag(110). This difference is explained in terms of the surface electronic structure of the two metals. Decomposition of SO 2 on Pd(100) occurs above 240 K on the clean surface to yield adsorbed atomic oxygen and SO which in turn dissociates at higher temperatures. For initially high SO 2 coverages an SO 4 species is formed above 300 K by reaction of SO 2 with oxygen formed in SO 2 dissociation. A quarter monolayer of preadsorbed sulfur blocks SO 2 decomposition, but the geometry and stability of adsorbed SO 2 is not strongly perturbed at this sulfur coverage, suggesting that SO 2 adsorbs in the four-fold hollow. At a sulfur coverage of 0.5 ML, adsorbed SO 2 bonds weakly via the sulfur atom, and the molecule is no longer tilted. SO 2 desorption is complete by 170 K at this sulfur coverage.