Oxidation of S at Fe metal and oxide surfaces and interfaces

Temperature-programmed desorption (TPD) and Auger electron spectroscopy (AES) were used to study the interactions of H2S and S adsorbed on oxidized Fe and metallic Fe surfaces, in the presence of H2O or O-2 vapor. Our results reveal contrasting chemical behavior between adsorbed S on a clean Fe(poly) metal surface and adsorbed S on an Fe oxide surface. SO2 formation occurs at the Fe oxide surface due to a reaction between adsorbed S and H2O or O-2 vapor. On the Fe(poly) metal surface, no such SO2 formation was observed. The chemical behavior of S located at the metal-oxide interface was also investigated. Our results demonstrate the deleterious effect of S when present at the metal-oxide interface. S located at the metal-oxide interface reacts with the oxide to form SO2. This SO2 formation results in destruction of metal-oxide bonds at the interface and causes oxide destabilization with consequent detachment of the oxide from the metal In contrast to the interfacial S, S adsorbed on the oxide surface does not form SO2 under UHV conditions. SO2 formation on the oxide surface occurs only in the presence of H2O or O-2 vapor. In addition, some degradation of an oxide was observed due to S deposited on top of the oxide surface. Our results indicate that the initial adsorption site of the S atom (metal versus oxide) exerts a critical influence on subsequent oxidation chemistry. (C) 1997 Elsevier Science B.V.