(692a) Structure-Activity Relations for the Reactions of Alkyl-Sulfur Compounds with Model Metal-Oxide Surfaces

The reactions of alkyl thiols(R-SH), sulfides(R-S-R'), and disulfides(R2S2) on metal oxide surfaces are of interest in applications ranging from desulfurization of petroleum feedstocks to the production of useful chemicals such as dimethylsulfoxide. In order to understand the fundamental surface chemistry involved in these systems we have been studying the reactions of thiols, sulfides and disulfides on several model metal-oxide catalysts. In particular, TPD and XPS were used to study the interaction of thiols and other sulfur containing probe molecules with ZnO(0001)/ (000-1) / ( 10-10) , CeO2(111), and V2O5/CeO2(111). The reactions of thiols and sulfides on ZnO(10-10) and(0001) surfaces, and CeO(111) were found to be highly structure-sensitive. For example, (CH3)2S is the main product from the reaction of CH3SH on ZnO(0001) and only a minor product on ZnO(10-10). Saturation coverage was also found to be structure-sensitive as the coverage of thiols on ZnO(0001) was found to be much greater on ZnO(10-10) while the saturation coverage of disulfides was similar for both surfaces. The reaction active sites on the ZnO surfaces were found to include coordinationally unsaturated adjacent oxygen-zinc surface ions that are found at the edges of steps covering the surfaces of ZnO. Alkyl thiols and disulfides were also found to react on CeO2(111) where the active sites were found to be clustered surface oxygen vacancies. V2O5/CeO2(111) is not initially an active catalyst for thiols and disulfides yet becomes active upon sulfidation by exposure to disulfides and subsequent heating. Monolayer and multilayer sulfided V2O5/CeO2(111) are active for the reactions of alkyldisulfides, while sulfided multilayer V2O5/CeO2(111) is active for alkyldisulfides and is inactive for alkylthiols.