Properties of pure and sulfided NiMoO₄ and CoMoO₄ catalysts: TPR, XANES and time-resolved XRD studies

X-ray absorption near-edge spectroscopy (XANES) was used to characterize the structural and electronic properties of a series of cobalt- and nickel-molybdate catalysts (AMoO₄·nH₂O, α-AMoO₄, β-AMoO₄; A=Co or Ni). The results of XANES indicate that the Co and Ni atoms are in octahedral sites in all these compounds, while the coordination of Mo varies from octahedral in the α-phases to tetrahedral in the β-phases and hydrate. Time-resolved x-ray diffraction shows a direct transformation of the hydrates into the β-AMoO₄ compounds (following a kinetics of first order) at temperatures between 200 and 350°C. This is facilitated by the similarities that the AMoO₄·nH₂O and β-AMoO₄ compounds have in their structural and electronic properties. The molybdates react with H₂ at temperatures between 400 and 600°C, forming gaseous water and oxides in which the oxidation state of Co and Ni remains +2 while that of Mo is reduced to +5 or +4. After exposing α-NiMoO₄ and β-NiMoO₄ to H₂S, both metals get sulfided and a NiMoS_x phase is formed. 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 reactions.