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Properties of pure and sulfided NiMoO<inf>4</inf> and CoMoO<inf>4</inf> catalysts: TPR, XANES and time-resolved XRD studies

Abstract:

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 NiMoSx 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.