Evidence of octahedral Co-Mo-S sites in hydrodesulfurization catalysts as determined by resonant inelastic X-ray scattering and X-ray absorption spectroscopy

Abstract

Fundamental understanding of the active sites in CoMo/Al2O3 catalysts is crucial to improve the production of clean transportation fuels by hydrodesulfurization (HDS). In this work, the coordination state and number of active sites (“Co-Mo-S”) in sulfided CoMo/Al2O3 catalysts prepared with and without chelating additives were determined with in situ 1s2p Resonant Inelastic X-ray Scattering (RIXS) and X-ray Absorption Spectroscopy (XAS). Based on evaluation of thiophene HDS activity as function of the Co/Mo ratio, it was determined that only for Co/Mo < 0.1 cobalt is exclusively in interaction with the MoS2 phase, making these compositions ideal for identifying the nature of the active cobalt sites for hydrogenolysis. These cobalt promoter atoms were in centrosymmetric coordination to the MoS2 phase with six sulfur ligands coordinated to each cobalt center. The octahedral coordination of cobalt in Co-Mo-S in alumina-supported HDS catalysts contrasts the tetrahedral cobalt coordination found in Co-promoted MoS2 model systems supported on gold or carbon. We attribute the difference to more disordered MoS2 structures in technical alumina-supported HDS catalysts. The number of Co-Mo-S sites depended strongly on the catalyst preparation method. Chelating additives, such as citric acid and nitrilotriacetic acid, increased the number of Co-Mo-S sites without significantly altering their intrinsic activity. These results demonstrate that state-of-the-art structural characterization tools, such as RIXS, identify the structure of active sites in actual catalysts and that these structures may be different from model systems. Combining such characterization tools with guided catalyst design can provide insight into the structure of amorphous, nanostructured materials aiding the development of improved heterogeneous catalysts.