Ultradispersed MoSx catalysts: evolution of active species and effect of the support
Résumé
In the field of hydrodesulfurization (HDS) catalysis, lamellar MoS2 is the most studied system. Active sites are located at the edges/corners of MoS2 slabs. Recently along with slabs, clusters of several atoms and single atoms (SAs) of Mo were observed by STEM in sulfide catalysts ( , ) and proven to have high HDS activity ( ). However, the structure and evolution of such species during catalyst activation and under reaction conditions are still unknown.
In this work, we have prepared sulfide Mo catalysts containing mostly MoSx clusters and SAs and studied their activity in HDS and the effect of the support on the formation of these species. Five catalysts were synthesized: MoSx supported on activated carbon (AC), graphene, silica (KIT-6), TiO2 and Al2O3 with 1 wt.% Mo. Conventional characterizations (TEM, UV-vis DRS) and STEM-ADF confirmed the absence of MoS2 slabs. For all samples, the specific HDS activity per Mo atom was higher than that of a 10%MoS2/Al2O3 reference (Fig. 1a). The structure and evolution of the Mo species during sulfidation and HDS reaction were studied by means of operando QXAS at the ROCK beamline of SOLEIL synchrotron.
As determined from MCR-ALS analysis of QXAS spectra, the evolution during sulfidation of the initial oxidic Mo oligomers proceeds via an oxysulfidic state, then MoS3-like species, and finally MoSx species are formed. On all the supports, the coordination number (CN) of Mo is equal to or lower than 1 (Fig. 1b), and much lower than for 10%MoS2/Al2O3 (CN = 2.3). The lowest Mo CNs were obtained on TiO2, AC and graphene. The mechanisms of ultradispersion stabilization seem different: carbons provide a large enough surface area to distribute the Mo precursor, while TiO2 binds it strongly, allowing to obtain highly dispersed MoSx clusters and SAs.
In conclusion, for the first time, we demonstrated that MoSx clusters supported on carbons and oxides are active in HDS in the absence of MoS2 slabs. The support greatly influences the nature of the species: silica and alumina favor the formation of larger and less active clusters, while more active sulfidic oligomers are obtained on carbons and titania