Modeling molybdenum carbide-based hydrodesulfurization (HDS) catalysts using carbon-modified Mo(110) surfaces

Abstract

Transition metal carbides, such as Mo2C, have been proposed as substitutes for group VIII metal catalysts, since they exhibit similar catalytic properties in some applications. Mo2C catalysts have shown potential for commercial use in hydrodesulfurization (HDS) processes and tend to resist sulfur poisoning better than platinum group metals. Although these molybdenum carbide catalysts look encouraging as replacements for MoS2-based catalysts, questions remain regarding the fundamental surface chemistry associated with the HDS of organosulfur molecules on carbided and sulfided molybdenum catalyst surfaces. To further investigate the suitability of Mo2C for HDS applications, the interaction of sulfur-containing molecules with molybdenum surfaces was examined by utilizing carbon-modified Mo(110) single crystals as model catalysts. Specifically, the reactivity of ethanethiol and 1,2-ethanedithiol were studied on the clean Mo(110), defective p(4×4)-C/Mo(110), and p(4×4)-C/Mo(110) surfaces using temperature programmed desorption (TPD), Auger electron spectroscopy (AES), and low energy electron diffraction (LEED). Ethanethiol and 1,2-ethanedithiol TPD experiments demonstrated that the presence of multiple sulfhydryl (SH) groups influences surface chemistry, given the differences observed in product distribution. Ethanethiol experiments performed on clean Mo(110) surfaces yielded ethane and ethylene as reaction products, while 1,2-ethanedithiol TPD experiments produced acetylene, ethylene, vinyl thiol, and ethanethiol. Ethanethiol TPD experiments showed that no significant differences in reactivity, selectivity, or reaction pathways exist between clean Mo(110) and the defective p(4×4) surfaces. 1,2-Ethanedithiol TPD experiments performed on the clean Mo(110) and p(4×4)-C/Mo(110) surfaces produced similar reaction products, although significant changes were observed in selectivity. On the clean surface, the major desorption products were acetylene, ethylene, vinyl thiol, and ethanethiol. However, the reaction of 1,2-ethanedithiol on the p(4×4)-C/Mo(110) surface produced only acetylene and ethylene. Thus, complete desulfurization of 1,2-ethanedithiol occurs on the p(4×4) surface upon decomposition, yielding only hydrocarbon products.