Characterization of pure and sulfided NiMoO4 catalysts using synchrotron-based X-ray absorption spectroscopy (XAS) and temperature-programmed reduction (TPR)

This study aims at characterizing the properties of pure and sulfided NiMoO4 catalysts using synchrotron-based near-edge X-ray absorption fine structure (NEXAFS) and temperature-programmed reduction (TPR). Mo L-II-edge and M-III-edge NEXAFS spectra indicate that on reaction with H2S, the Mo component of NiMoO4 gets partially reduced with the formation of MoS2 type species. For the beta-phase of NiMoO4, the sulfidation of Mo is more extensive than for the ct-phase, making the former a better precursor for catalysts of hydrodesulfurization (HDS) reactions. The Ni Lu-edge features are relatively insensitive to the changes accompanying the partial sulfidation of NiMoO4. The sulfidation of the Ni component is confirmed by analysis of the Ni K-edge extended X-ray absorption fine structure (EXAFS) spectra which show the formation of Ni-S bonds (bond length similar to 2.48 Angstrom) and a NiMoSx phase. The S K-edge NEXAFS spectra show the presence of at least two types of sulfur species, one associated with a formal oxidation state of 2- and another associated with a formal oxidation state of 6+. We attribute the former to the presence of metal-sulfur bonds (MoSx and NiSy). The latter is associated with the formation of S-O bonds (SO42-). The formation of sulfates is also supported by the O K-edge NEXAFS spectra. The partially sulfided NiMoO4 catalysts (both alpha- and beta-isomorphs) have a much lower thermal stability in a reducing environment than pure NiMoO4 and MoS2. The sulfided molybdates react with H-2 in TPR producing H2O and H2S at temperatures above 400 K.