Chemistry of Thiophene, Pyridine, and Cyclohexylamine on Ni/MoS_xand Ni/S/Mo(110) Surfaces: Role of Nickel in Hydrodesulfurization and Hydrodenitrogenation Processes

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Abstract

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 3d and valence spectra that are very similar to those of MoS2. On the MoSx systems, the behavior 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. A similar behavior is observed for adsorbed pyridine. In contrast, CHA displays a rich chemistry on these surfaces. Mo centers that have a limited number of S vacancies and do not do chemistry with thiophene, pyridine, or H2 are able to cleave the C−N bond in a nonaromatic H-rich molecule like CHA. The addition of Ni enhances the chemical activity of MoSx. On the NiMoSx systems, the adsorption energies of thiophene and pyridine are 5−10 kcal/mol larger than 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 facilitate 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 observed after creating S vacancies in MoSx and Ni/S/Mo(110) surfaces by reaction with atomic hydrogen (2Hgas + Ssurface → H2Sgas + vacancysurface). The role of Ni in NiMoSx catalysts for hydrodesulfurization and hydrodenitrogenation processes is discussed in light of these results.

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