ABSTRACT
In spite of their wide and longstanding use, an explanation of the active sites in these catalysts needs a complete formulation. Fifteen years ago Massoth reviewed the field of Mo and W based hydrotreating catalysts (2). At the time of the Massoth article, it was not clear what role the alumina support played in the activity and selectivity of these catalysts originates in the sulfide phase. Since the availability of probes such as EXAFS (Extended X-ray Fine Structure), it has become clear that the alumina's primary function is to disperse and stabilize the M0S2 and WS2 hydrotreating catalysts (3). The role of the Co and Ni promoter phase has been extensively studied with much progress being made in elucidating its role. In past years, three models have been proposed to explain the action of the promoter in the Mo and W sulfides, in the first model (the intercalation model) (4) it is proposed that the Co or Ni atoms are intercalated into octahedral sites at the edges between M0S2 or WS2 slabs. The second model or synergistic model proposes that the promoter phase (Nİ3S2 or CoqSg ) exist as a separated phase. Finally, the Topsoe s model or C0M0S, NiMoS phase where the promoter atoms as in the intercalation model are located on in octahedral sites but in the same plane as the Mo atoms.
