Chemical Properties of Zn/S/Mo(110) and Co/S/Mo(110) Surfaces: Reaction with Hydrogen and Formation of Hydrogen Sulfide

The chemical and electronic properties of a series of Zn/S/Mo(110) and Co/S/Mo(110) systems have been investigated using photoemission, thermal desorption mass spectroscopy, and hydrogen (H 2, D 2, or D) chemisorption. Sulfur multilayers supported on Mo(110) are very reactive toward admetals like Zn and Co. The behavior of the Zn/S/Mo(110) and Co/S/Mo(110) systems indicates that Zn and Co promote Mo ↔ S interactions, inducing the formation of molybdenum sulfide films. The ZnMoS and CoMoS films were unreactive toward H 2 or D 2 under ultrahigh vacuum conditions. As gas-phase hydrogen atoms (D) impinged on the surfaces, gaseous hydrogen sulfide was formed. Thus, the slow step in the D 2,gas + S solid → D 2 S gas reaction is the dissociation of molecular hydrogen. A good correlation exists between trends seen in the hydrodesulfurization (HDS) activity of ZnMoS and CoMoS catalysts and trends found for the sulfidation of Mo and hydrogenation of S in ZnMoS and CoMoS films. The systems that contain Co show the larger HDS activity, the stronger metal ↔ metal interactions with a subsequent increase in the reactivity of Mo toward S-containing molecules, and the bigger tendency to create unsaturated Mo sites through the hydrogenation of Mo-bonded S atoms.