Genesis of Active Phase and Support Effect in Ultradispersed Mo Sulfide Catalysts
Daria Ryaboshapka, Thomas Len, Pascal Bargiela, M. Aouine, C. Geantet, Valérie Briois, L. Piccolo, P. Afanasiev
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 > Mo/TiO 2 > Mo/Al 2 O 3 ≈ Mo/SiO 2 .