The role of structural carbon in transition metal sulfides hydrotreating catalysts

The necessity of improving the hydrotreating sulfide catalysts' efficiency in deep hydrodesulfurization (HDS) is urgent due to new environmental regulations regarding sulfur content in fuels. Progress has been made in understanding the basis for cobalt-promoted molybdenum sulfide activity. However, recent results [R. R. Chianelli and G. Berhault, Catal. Today 53, 357 (1999)] have evidenced that carbon plays an important role in the stabilized structure of active catalysts. Indeed, the active surface in the stabilized, catalytically active transition metal sulfide (TMS) phase is carbided. Moreover, it is well-known that organosulfide treatment of oxide catalysts to produce the active catalytic form results in a better HDS activity, suggesting an initial modification of the sulfide structure in the presence of a carbon source. This study reports the structural and textural modifications of unsupported MoS2 catalyst via CH3-S-CH3 or dibenzothiophene treatment in order to better characterize the formation of structural carbon on MoS2 layered particles. Both bulk characterization techniques (Synchrotron X-ray diffraction, far-IR) and surface-sensitive methods (electron energy loss spectroscopy (EELS), near-edge X-ray absorption fine structure (NEXAFS)) were performed in order to carefully study the final state of carburized unsupported molybdenum sulfide. Complementary electronic microscopy studies were carried out to clarify textural modifications occurring consecutively with this treatment. Evidence is shown that structural carbon processes through a replacement of sulfur atoms at the reactive edges of MoS2 platelets, resulting in a stabilized MoS2-xCx phase. This carburization surface process does not affect the MoS2 bulk structure. Results also demonstrate that carbon stabilizes texturally sulfide particles, keeping crystallites smaller and less stacked. Thus, the active TMS should be viewed as sulfide-supported transition metal carbides. This work is a preliminary study about the neglected role of carbon on TMS-based catalysts operating in hydrotreating conditions. Further studies will point out the catalytic consequences due to the formation of this stabilized phase. (C) 2001 Academic Press.