The active phases in heterogeneous HDS catalysts

Commercial heterogeneous HDS catalysts for refinery use consist, almost without exception, of nickel- and/or cobalt-promoted molybdenum oxide located on a high surface area (approx. 300 m g ) alumina or silica-alumina support. Cobalt and nickel promoters increase the catalytic activity, particularly towards thiophenes whether Co or Ni is used as a promoter depends on the specific function for which the catalyst should be optimal. The catalyst material is shaped into porous pellets, a few millimeters in size, and these pellets are loaded into the reactor, forming a catalyst bed of 30-200 m volume. During start-up of a freshly loaded reactor, the catalyst bed, which is in the oxidic form, is sulfided, typically by treatment with an oil feed which has been spiked with a reactive sulfur compound that readily generates H2S in situ. The oxidic precursor phases (non-stoichiometric CoMo or NiMo surface oxides) are thereby converted into sulfidic phases termed Co-Mo-S and Ni-Mo-S. The conversion from the oxidic phase to the sulfidic is accompanied by a reduction in Mo oxidation state from +6 to +4. 

The use of Ni- or Co-promoted molybdenum-based catalysts in refinery opera- 

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