Chemistry of thiophene, pyridine, and cyclohexylamine on Ni/MoSx and Ni/S/Mo(110) surfaces: Role of nickel in hydrodesulfurization and hydrodenitrogenation processes

Synchrotron-based high-resolution photoemission has been used to study the interaction of thiophene, pyridine, and cyclohexylamine (CHA) with pure and Ni-promoted MoSx films and S/Mo(110) surfaces. The MoSx films exhibit Mo 3 d and valence spectra that are very similar to those of MoS2. On the MoSx s ystems, the behavior of thiophene closely resembles that seen on MoS2(0002) . The molecules are weakly chemisorbed, and most of them desorb at temperat ures 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. A similar behavior is observed for adsorbed pyridine. In contrast, CHA displays a ri ch chemistry on these surfaces. Mo centers that have a limited number of S vacancies and do not do chemistry with thiophene, pyridine, or H-2 are able to cleave the C-N bond in a nonaromatic H-rich molecule like CHA. The addi tion of Ni enhances the chemical activity of MoSx. On the NiMoSx systems, t he adsorption energies of thiophene and pyridine are 5-10 kcal/mol larger t han those on pure MoSx. But no dissociation of these 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 fa cilitate the cleavage of aromatic C-S or C-N bonds. Hydrogen seems to play an important role in this aspect. Extensive decomposition of thiophene is o bserved after creating S vacancies in MoSx, and Ni/S/Mo(110) surfaces by re action with atomic hydrogen (2H(gas) + S-surface --> H2Sgas + vacancy(surfa ce)). The role of Ni in NiMoSx, catalysts for hydrodesulfurization and hydr odenitrogenation processes is discussed in light of these results.