Unsupported Nimo Catalysts - Influence of the Sulfiding Temperature and Evolution of the Unsupported Nimos Phase During Reaction

A series of unsupported NiMo catalysts, containing 30% Ni atoms, were prepared by the homogeneous sulfide precipitation method (HSP) and sulfided at different temperatures in the range 673-1073 K. XPS, XRD, microelectrophoresis and BET surface area measurements were used to characterize the samples. The catalytic activity of the various catalysts for the hydrodesulfurization of thiophene and the hydrogenation of cyclohexene in a mixture containing both compounds was measured in mild conditions (573 K, 3 MPa). Both the evolution of the ''NiMoS phase'' during catalytic reaction and the influence of the sulfidation temperature on the properties of the unsupported NiMo catalysts were investigated. The sample sulfided at 673 K gave all the characteristic signals of the ''NiMoS phase''. As the sulfidation temperature increases, larger amounts of nickel segregate from ''NiMoS phase'' to form nickel sulfide. The ''NiMoS phase'' was found unstable under reaction conditions. The maximum catalytic activity for both HDS and HYD reactions was obtained when both ''NiMoS phase'' and bulk Ni3S2 were present in the catalyst. It thus appears as a precursor of a very active mixture of phases rather than of the stable catalyst itself. These results are discussed in relation to some models attempting to explain the synergy in the NiMo catalysts. They are compatible with the remote control mechanism.