THE INTERACTION OF NI AND FE WITH SULFUR AND MOLYBDENUM-SULFIDE SURFACES - A TDS, XPS AND HYDROGEN-CHEMISORPTION STUDY

Sulfur multilayers, containing S-n species (n = 2, 4, and 8), are very reactive toward admetals like nickel and iron. Ni and Fe atoms suppor ted on sulfur films at 200-300 K exhibit core-level binding energies a nd band structures very similar to those of nickel and iron sulfides. In contrast, Ni atoms supported on molybdenum-sulfide surfaces remain in a metallic state. NiMoS and FeMoS films can be generated by heating Ni/S-film/Mo(110) and Fe/S-film/Mo(110) systems to high temperature. The behavior of the Ni/S-film/Mo(110) and Fe/S-film/Mo(110) systems in dicates that Ni and Fe promote Mo<->S interactions and the subsequent formation of molybdenum sulfides. On TM/MoSx and TM/S/Mo(110) surfaces (TM = Ni or Fe), the slow step in the D-2,D-gas + S-solid-->D2Sgas re action is the dissociation of molecular hydrogen. Ni/MoSx and Fe/MoSx surfaces interact strongly with atomic hydrogen (D), sorbing this elem ent and forming gaseous hydrogen sulfide. The sorption of D produces u niform changes in the electronic properties of the MoSx substrate, wit h positive binding energy shifts (0.3-0.4 eV) in the core levels of mo lybdenum and sulfur. Most of the sorbed hydrogen evolves into gas phas e as D-2 at temperatures between 350 and 500 K. Trends seen in the hyd rodesulfurization activity of NiMoS and FeMoS catalysts are analyzed f ollowing our results for the sulfidation of Mo and the hydrogenation o f S in NiMoS and FeMoS films.