Fundamental studies of the reactions of alkyl thiols and sulfides on metal-oxide model catalysts

The reactions of alkyl thiols(R-SH), sulfides(R-S-R\u27), and disulfides(R2S2) are of interest in applications ranging from desulfurization of petroleum feedstocks to the production of useful chemicals such as dimethylsulfoxide. A surface science approach to was utilized in an effort insight into the reactions of thiols, sulfides and disulfides on metal-oxide catalysts. This investigation employed a combination of surface sensitive techniques such as TPD and XPS to study the interaction of thiols and sulfide probe molecules with the model catalyst surfaces ZnO(0001)/(000 1)/(1010), CeO2(111), and sulfided V2O5/CeO2(111). The reactions of thiols and sulfides on ZnO(1010) and (0001) surfaces, and CeO(111) were found to be highly structure-sensitive. Thiols and disulfides were found to adsorb dissociatively on surface defects of ZnO (1010) and (0001) surfaces, CeO(111), and sulfided 0-5ML V2O5/CeO2(111). The nature and structure of the defects was found to effect reactivity, as did the structure and available surface bonds. For ZnO the defects were adsorption and reactions occur are edges of steps covering the surfaces. For CeO2(111) the defects are oxygen vacancies. Sulfides adsorbed on vanadia, sulfiding and activating monolayer vanadia to thiols and sulfides as well as activating multilayer vanadia for sulfides as well. The adsorption of thiols and sulfides results in thiolates that can react via a number of pathways, including alkyl shifts between thiolates to make disulfide, and from thiolate to surface to make alkoxide. The alkoxides resulting from transfer to the surface undergo hydrogenation, dehydrogenation, oxidation, and elimination paralleling reactions observed for oxygenates on these surfaces. A low temperature formaldehyde product unique to dimethyldisulfide reaction on ZnO arising from possibly a sulfur-based intermediate was also observed