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

Abstract:

This study aims at characterizing the properties of pure and sulfided NiMoO<inf>4</inf> catalysts using synchrotron-based near-edge X-ray absorption fine structure (NEXAFS) and temperature-programmed reduction (TPR). Mo L<inf>II</inf>-edge and M<inf>III</inf>-edge NEXAFS spectra indicate that on reaction with H<inf>2</inf>S, the Mo component of NiMoO<inf>4</inf> gets partially reduced with the formation of MoS<inf>2</inf> type species. For the β-phase of NiMoO<inf>4</inf>, 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<inf>II</inf>-edge features are relatively insensitive to the changes accompanying the partial sulfidation of NiMoO<inf>4</inf>. 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<inf>x</inf> 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<inf>x</inf> and NiS<inf>y</inf>). The latter is associated with the formation of S-O bonds (SO<inf>4</inf><sup>2-</sup>). The formation of sulfates is also supported by the O K-edge NEXAFS spectra. The partially sulfided NiMoO<inf>4</inf> catalysts (both α- and β-isomorphs) have a much lower thermal stability in a reducing environment than pure NiMoO<inf>4</inf> and MoS<inf>2</inf>. The sulfided molybdates react with H<inf>2</inf> in TPR producing H<inf>2</inf>O and H<inf>2</inf>S at temperatures above 400 K.