This review deals with the structure of the active phase in sulfidic Co Mo and Ni Mo hydrotreating catalysts. Various models describing the catalyst and its interaction with the reaction environment are discussed in the light of the evolution of the active phase during the catalyst life cycle. Special attention is paid to the contribution of Mossbauer Emission Spectroscopy (MES), Extended X-ray Absorption Fine Structure (EXAFS), High Resolution Transmission Electron Microscopy (HR-TEM) and Molecular Modeling to the unraveling of the active phase structure. It is concluded that MoS 2 sintering and Co 9 S 8 (Ni 3 S 2 , NiS) segregation during the catalyst life cycle force the shift of the actual reaction mechanism from that involving a single site or an ensemble of sites to that of a close cooperation between segregated components. The adsorption of reactants can take place in many diferent ways, through the heteroatom or through the ring, on the Mo sites as well as on the Co (Ni) sites. The exact state of each active site depends on the reaction environment. If the catalyst is S deficient, the classical adsorption mechanism involving the S vacancy is dominant. With a fully sulfided catalyst surface the adsorption takes place through S S bonds. The catalyst is a dynamic evolving system adapting itself to its ever changing reaction environment. Each catalyst component fulfills multiple functions in determining the catalyst structure as well as its interaction with the reactant molecules.

On the flexibility of the active phase in hydrotreating catalysts

The dispersion and homogeneity of two series of sulfidic Al2O3 supported Co-Mo and Ni-Mo catalysts were investigated by transmission electron microscopy (TEM). Dispersion data were obtained by a novel semi-quantitative method for the evaluation of TEM micrographs. A detailed discussion of this method is given. It appears that the hydrodesulfurization and hydrodenitrogenation activities of the catalyst samples are proportional to the MoS2 dispersion. High-activity commercial catalysts are found to have very high MoS2 dispersions.

MoS2 structures in high-activity hydrotreating catalysts: I. Semi-quantitative method for evaluation of transmission electron microscopy results. Correlations between hydrodesulfurization and hydrodenitrogenation activities and MoS2 dispersion

Sulfidation and Thiophene Hydrodesulfurization Activity of Nickel Tungsten Sulfide Model Catalysts, Prepared without and with Chelating Agents

The Effect of Support Interaction on the Sulfidability of Al2O3- and TiO2-Supported CoW and NiW Hydrodesulfurization Catalysts

Crystallite Size Determination of Highly Dispersed Unsupported MoS2Catalysts

Influence of sulphidation conditions on the properties of NiW/Al2O3 hydrotreating catalysts

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Influence of sulphidation conditions on the properties of NiW/Al2O3 hydrotreating catalysts