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 sul fided at different temperatures in the range 673-1073 K. XPS, XRD, mic roelectrophoresis and BET surface area measurements were used to chara cterize the samples. The catalytic activity of the various catalysts f or the hydrodesulfurization of thiophene and the hydrogenation of cycl ohexene in a mixture containing both compounds was measured in mild co nditions (573 K, 3 MPa). Both the evolution of the ''NiMoS phase'' dur ing catalytic reaction and the influence of the sulfidation temperatur e on the properties of the unsupported NiMo catalysts were investigate d.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 sulfid e. The ''NiMoS phase'' was found unstable under reaction conditions. T he maximum catalytic activity for both HDS and HYD reactions was obtai ned when both ''NiMoS phase'' and bulk Ni3S2 were present in the catal yst. It thus appears as a precursor of a very active mixture of phases rather than of the stable catalyst itself. These results are discusse d in relation to some models attempting to explain the synergy in the NiMo catalysts. They are compatible with the remote control mechanism.