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 β-phase of NiMoO₄, the sulfidation of Mo is more extensive than for the α-phase, making the former a better precursor for catalysts of hydrodesulfurization (HDS) reactions. The Ni L_II-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 ∼2.48 Å) and a NiMoS_x 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 (MoS_x and NiS_y). The latter is associated with the formation of S-O bonds (SO₄^2-). The formation of sulfates is also supported by the O K-edge NEXAFS spectra. The partially sulfided NiMoO₄ catalysts (both α- and β-isomorphs) have a much lower thermal stability in a reducing environment than pure NiMoO₄ and MoS₂. The sulfided molybdates react with H₂ in TPR producing H₂O and H₂S at temperatures above 400 K.