i.e., hydration state of the oxide precursor, composition of the sulfiding mixture H2/H2S or N2/H2S, temperature, and time duration. The important role of these experimental conditions on the detection of transient species during sulfidation has been pointed out. Different intermediates such as oxysulfides, sulfido compounds, and MoS3 appear to be the dominant ones, whose existence depends on the previously mentioned parameters. The crystallite size of the MoS2, a function of the Mo loading and determined by HREM, influences the position and width of the ^385-cnr1 line.
Bulk nickel molybdenum sulfides have been studied by X-ray photoelectron spectroscopy (XPS) and compared to a sulfided / -1203 catalyst. Several types of Ni species have been identified on both catalytic systems. A particular Ni species has been observed and assigned to Ni located at the edges of the MoS2 slabs in a decoration position forming the so-called NiMoS phase. In bulk nickel molybdenum sulfides, ion scattering spectroscopy (ISS) and XPS results show that, for an atomic ratio Ni/(Ni + Mo) between 0.2 and 0.8, a particular surface composition exists, resulting from the existence of a phase of composition Ni/(Ni + Mo) between 0.2 and 0.3 and identified as the "NiMoS" phase. A comparison with the Ni-W system shows that the Ni species in decoration position is present in both the Moand W-based catalysts and can be assumed to be responsible for the synergy effect observed in these catalysts. On alumina, existence of this NiMoS phase has been found to be dependent on the sulfidation temperature of the oxide precursor.
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