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

Abstract

Molybdenum sulfide-based catalysts are widely used for hydrotreatment, hydrogen evolution, and many other reactions. Recently, we demonstrated that not only the edges of MoS 2 slabs but few-atom ultradispersed MoS x clusters also possess high intrinsic activity. However, the structure and genesis of such ultradispersed species remain unknown. Herein, we present a comparative study of MoS x catalysts ultradispersed on different supports (carbons, SiO 2 , Al 2 O 3 , and TiO 2 ). Evolution of the Mo species during sulfidation and hydrodesulfurization (HDS) reaction was studied by means of operando quick X-ray absorption spectroscopy at the Mo K-edge, assisted by chemometric analysis (multivariate curve resolution with alternating least squares). Significant differences of the structure of Mo species and their temperature evolution as a function of support were observed. The sulfidation pathway involves the formation of oxysulfide and sulfur-rich MoS 3 -like intermediates, which are further transformed into the final MoS x clusters. As compared with MoS 2 nanoslabs, the coordination numbers of Mo in the ultradispersed clusters are decreased, and the interatomic Mo−S and Mo−Mo distances are shortened. Other characterizations, in particular, STEM-ADF, confirm that few-atom clusters and single-atom species are predominant in all the catalysts. The materials show high activity per Mo atom in the HDS of thiophene, varying in steps with MoS x dispersion, as determined from XAS, in the sequence: Mo/carbons