Photoemission studies of S/Co/Mo(110) and S/Ni/Mo(110) surfaces: Co- and Ni-promoted sulfidation of Mo(110)

The properties of a series of X/Mo(110) and S/X/Mo(110) surfaces (X=Co or Ni) have been examined using photoemission, thermal desorption spectroscopy and ab initio SCF calculations. The bimetallic bonds in the Co/Mo(110) and Ni/Mo(110) systems are complex, involving Co(3d,4s)-->Mo(5s,5p) and Ni(3d,4s)-->Mo(5s,5p) electron transfers and a Mo(4d)-->Mo(5s,5p) rehybridization. These redistributions of charge lead to positive core-level shifts for all the metals. The exposure of Mo(110) to large amounts of S-2 gas produces only a chemisorbed layer of sulfur, without forming molybdenum sulfides. The sulfidation of Mo occurs after exposing Co/Mo(110) and Ni/Mo(110) surfaces to S-2. Co and Ni promote the formation of molybdenum sulfides by transferring charge to Mo (favoring in this way an electrophilic attack of S on Mo), and by changing the structure of the surface (making it easier for the penetration of S into the bulk of the sample). Co and Ni exhibit a unique ability to enhance the Mo<->S interactions. A comparison of the behavior of several admetals on S/Mo(110) surfaces indicates that the ''promotional effect'' of an admetal on the sulfidation of Mo increases in the following order: Ag approximate to Zn<Cu much less than Co approximate to Ni. A good correlation exists between the trends seen for the activity of TMS(y)/MoS2 catalysts (TM = Zn, Cu, Co or Ni) in hydrodesulfurization (HDS) reactions and the trends found for the sulfidation of Mo in S/TM/Mo(110) surfaces. Systems that contain Co and Ni display the largest HDS activity and the strongest Mo<->S interactions.