Using Transient XAS to Detect Minute Levels of Reversible S-O Exchange at the Active Sites of MoS2-Based Hydrotreating Catalysts: Effect of Metal Loading, Promotion, Temperature, and Oxygenate Reactant

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Modulation excitation spectroscopy (MES) coupled with time-resolved X-ray absorption spectroscopy (XAS) was applied to unpromoted and Co-promoted Mo-based hydrotreating catalysts (i.e., Mo and CoMo catalysts) for investigating the minute as well as instantaneous differences in active site composition under the influence of H2S and H2O. Transient experiments during alternating sulfur- and water-rich conditions were performed at temperatures between 400 and 500 °C using catalysts with varying Co/Mo metal loadings to study the correlative effect of these factors on the stability of the active sites. For both types of catalysts, the features of the demodulated Mo K-edge spectrum matching the difference spectra of the MoS2 and MoO3 references indicate a reversible oxidation–sulfidation process during H2O/H2S cycling. In the case of CoMo catalysts with increasing metal loadings, transient XAS at the Mo K- as well as Co K-edge revealed minute levels of S-O exchange at both sulfided Mo and Co sites, although 26 to 47% of the Co was bound as stable CoAl2O4, which does not form a bulk sulfide. Thus, even in the presence of multiple Co phases (oxide and sulfide) and a low number of active sites relative to the total number of Mo atoms, transient XAS can successfully detect the changes occurring over both the active Co and Mo sites. Furthermore, the comparison of the amplitude of the demodulated spectra showed that Mo atoms are more prone to S-O exchange with increasing temperature, while Co promotion stabilized the molybdenum sulfide. MES-coupled XAS experiments with 1-propanol performed to evaluate the degree of S-O exchange in the presence of another oxygenate reactant revealed the higher affinity of water toward these sulfided Mo sites as compared to 1-propanol.