Adsorption of thiophene on surfaces of clean and Ni-promoted molybdenum sulfide

Synchrotron-based high-resolution photoemission has been used to study the interaction of thiophene with pure and Ni promoted MoSx films. The films exhibit Mo 3d and valence spectra that are very similar to those of MoS2. On the MoSx systems, the chemistry of thiophene closely resembles that seen on MoS2(0002). The molecules are weakly chemisorbed, and most of them desorb at temperatures around 200 K. A small fraction of the adsorbed thiophene is bonded to Mo sites that have S vacancies and desorbs between 250 and 300 K. The addition of Ni enhances the chemical activity of MoSx. On the NiMoSx systems, the adsorption energy of thiophene is at least 5-10 kcal mol(-1) larger than on pure MoSx. No dissociation of the molecules is observed on the NiMoSx surfaces. The Ni<->S interactions reduce the reactivity of nickel, and the presence of this metal alone is not enough to promote or facilitate the cleavage of C-S bonds in hydrodesulfurization (HDS) reactions. Hydrogen seems to play an important role in this aspect. The behavior of NiMoSx catalysts in HDS processes is discussed in the light of these results. (C) 1999 Elsevier Science B.V. All rights reserved.