Interaction of SO 2 with single crystal metal surfaces

Abstract

Studying the interaction of SO 2 with metal surfaces under UHV conditions, a question of central interest is whether the molecule dissociates (leaving back the catalyst poison sulphur on the surface) or not. A spontaneous or a thermally activated dissociation of SO 2 occurs on Fe, Rh, W, Ni, Pd and Pt. On Cu and Ag a strong chemisorption, but only a partial dissociation induced by defects or coadsorbed alkalis, and on Au no chemisorption at all were observed. In this paper a comparison of our results obtained for the chemisorption and multilayer adsorption of SO 2 on Cu(111), Ag(111), Ag(100) and Ag(110) in the temperature range between 80 K and 900 K is given. By combining highly resolved TPD-measurements, isothermal and temperature-programmed ΔΦ-experiments after different stages of exposure and molecular beam backscattering measurements (MBBS) —assisted by LEED, AES and isotope mixing experiments — a destinction between ordinary desorption and desorption after a reorientation process during the heating procedure could be made. Whereas on clean Ag surfaces adsorption and desorption of SO 2 are observed only below 300 K, on Cs-precovered Ag desorption of SO 2 takes place even above 600 K. Finally, results concerning the different stages of SO 2 multilayer adsorption (bi-, tri-, multilayers) are presented showing a characteristic dependence of the layer growth on the adsorption temperature, the impinging SO 2 flux density and on the surface structure.