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The promoting action of nickel on sulfided Mo catalysts has been examined on two series of thiophene-sulfided NiMo/Al2O3 catalysts with constant Mo content (7 and 16% MoO3, respectively) and variable Ni concentration. The catalysts are characterized by their activity for thiophene HDS, by the fraction of Ni or Mo which can be extracted in an acidic or basic medium, and by their sulfur content. Comparison with “one-component” Ni/Al2O3 and Mo/Al2O3 catalysts shows that the dissolution of molybdenum is unaffected by the presence of nickel; by contrast, molybdenum lowers the non-extractable fraction of nickel, and thus partially hampers the migration of the promoter ions into the alumina matrix. The changes in the catalytic activity are discussed according to the amount of “free” (extractable) nickel NiF: three different roles of nickel are found and related to the nature of the surface phases. In the main promotion range of the first series (0.1 < NiF/Mo < 0.6), the results are consistent with increasing formation of a “NiMoS” phase which consists of small Mo entities associating almost two promoter atoms to three molybde num atoms. For the high-Mo series, interaction between the Mo units leads to a different NiMoS association in larger patches, which limits at a lower NiF/Mo ratio the favorable effect of added nickel. Higher Ni content results for both series in bulk Ni3S2, unsuitable for further development in activity. At low NiF/Mo ratio, the catalyst structure is not elucidated. |
