Symmetrical synergism and the role of carbon in transition metal sulfide catalytic materials

Considerable progress has been made in understanding the basis for the activity and the selectivity of molybdenum and tungsten based hydrotreating catalysts [R,R. Chianelli, M. Dagge, M.J. Ledoux, Adv. Catal. 40 (1994) 177]. Despite this progress, a number of fundamental questions persist: How does the catalyst function at the surface in the presence of a reacting molecule? What is the stable phase presenting the active sites to the reacting hydrocarbons? What is the function of Co and Ni promoters, and how do they operate? In this paper, we report on recent progress in understanding these and other questions and emphasize two aspects that have been neglected in attempting to answer these questions. The first aspect is the concept of 'symmetrical synergy'. This concept recognizes that Co9S8 can be promoted by Mo in an analogous manner to the well-recognized promotion of MoS2 by Co. The second aspect is the role of carbon in promoted and unpromoted transition metal sulfide (TMS) catalytic materials. Evidence is presented that the active surface in stabilized, catalytically active TMS catalysts is carbided. Thus, the active TMS catalysts should be viewed as sulfide supported transition metal carbides (SSTMC), On the basis of upon this understanding the active phase in the Co and Ni promoted TMS should be viewed as CoMoC and NiMoC phases supported on their respective sulfide bulk phases. (C) 1999 Elsevier Science B.V. All rights reserved.