Properties of pure and sulfided NiMoO_4 and CoMoO_4 catalysts: tpr, xanes and time-resolved xrd studies

摘要

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 (AMoO_4.NH_2O, alpha AMoO_4, 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 alpha -phases to tetrahedral in the beta -phases and hydrate. Time-resolved x-ray diffraction shows a direct transformation of the hydrates into the beta -AMoO_4 compounds (following a kinetics of first order) at temperatures between 200 and 350 deg C. This is facilitated by the similarities that the AMoO_4.nH_2O and beta -AMoO_4 compounds have in their structural and electronic properties. The molybdates react with H_2 at temperatures between 400 and 600 deg C, formin 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 alpha -NiMoO_4 and beta -NiMoO_4 to H_2S, both metals get sulfided and a NiMoS_x phase is formed. For the beta phase of NiMoO_4 the sulfidation of Mo is more extensive than for the alpha phase, making the former a better precursor for catalysts of hydrodesulfurization reactions.