Ag "promoted" sulfidation of metal and oxide surfaces: A photoemission study of the interaction of sulfur with Ag/Rh(111) and Ag/ZnO

The interaction of S-2 with Ag/Rh(111) and Ag/ZnO surfaces has been investigated using high-resolution photoemission spectroscopy. In general, silver "promotes" the reaction of S, with both clean Rh(111) and polycrystalline ZnO. On dosing Ag/Rh(111) surfaces with S, at 300 K, the S 2p spectra show the presence of multiple sulfur species (S, S-n polymers). Annealing to 500 K decomposes the sulfur species that have significant S-S bonding and, in addition, results in the formation of metal-sulfur bonds (Rh-S and Ag-S). The formation of Rh-S bonds is indicated by a big drop in the density of states at the Fermi edge, while the formation of Ag-S bonds is evidenced by a slight positive shift (0.4 eV) and narrowing of the Ag 4d band. On Ag/Rh(111) surfaces, the rare of adsorption of S-2 is substantially larger than on Rh(111). A comparison of these results with those previously reported for the S-2/Ag/Mo(110) and S-2/Ag/Pt(111) systems indicates that Ag adatoms have the "ability" to enhance the rate of sulfidation of transition metal surfaces. On zinc oxide the Ag overlayers show a band structure that is significantly different from that of pure metallic Ag. Nevertheless, the reactivity of the Ag/ZnO surfaces towards sulfur adsorption is larger than that of pure ZnO. Ag clusters supported on ZnO react with S-2 to form AgSx species which are stable up to temperatures above 500 K. The supported Ag clusters provide a large number of electronic states that are efficient at donating electrons to the S-2(2 pi(g)) orbitals, inducing in this way the breaking of the S-S bonds and the formation of AgSx. When Ag is vapor deposited on S/ZnO surfaces, the sulfur migrates from the oxide to on-top of the Ag overlayer. This migration is favored by differences in the strength of the Ag-->S and ZnO-->S electron-donor interactions. Trends in the poisoning of metal/oxide catalysts by sulfur are discussed in terms of a simple mode based on perturbation theory and the Huckel or tight-binding method. (C) 1998 Elsevier Science B.V. All rights reserved.