Genesis of Active Phase and Support Effect in Ultradispersed Mo Sulfide Catalysts

Molybdenum sulfide-based catalysts are widely used forhydrotreatment,hydrogen evolution, and many other reactions. Recently, we demonstratedthat not only the edges of MoS2 slabs but few-atom ultradispersedMoS( x ) clusters also possess high intrinsicactivity. However, the structure and genesis of such ultradispersedspecies remain unknown. Herein, we present a comparative study ofMoS( x ) catalysts ultradispersed on differentsupports (carbons, SiO2, Al2O3, andTiO(2)). Evolution of the Mo species during sulfidation andhydrodesulfurization (HDS) reaction was studied by means of operandoquick X-ray absorption spectroscopy at the Mo K-edge, assisted bychemometric analysis (multivariate curve resolution with alternatingleast squares). Significant differences of the structure of Mo speciesand their temperature evolution as a function of support were observed.The sulfidation pathway involves the formation of oxysulfide and sulfur-richMoS(3)-like intermediates, which are further transformedinto the final MoS x clusters. As comparedwith MoS2 nanoslabs, the coordination numbers of Mo inthe ultradispersed clusters are decreased, and the interatomic Mo-Sand Mo-Mo distances are shortened. Other characterizations,in particular, STEM-ADF, confirm that few-atom clusters and single-atomspecies are predominant in all the catalysts. The materials show highactivity per Mo atom in the HDS of thiophene, varying in steps withMoS( x ) dispersion, as determined from XAS,in the sequence: Mo/carbons > Mo/TiO2 > Mo/Al2O3 & AP; Mo/SiO2.