Probing the Active Sites of MoS2 Based Hydrotreating Catalysts Using Modulation Excitation Spectroscopy

The reactive surface sites of MoS2 hydrotreating catalysts (unpromoted as well as Co- and Ni-promoted) supported on MgAl2O4 spinel were investigated with respect to the substitution of sulfur by oxygen using in situ XAS coupled with modulation excitation spectroscopy (MES). Specifically, MES experiments were carried out by periodically cycling between a H2O and H2S containing hydrogen gas mixture at 400 °C. Due to the low fraction of S–O exchange, conventional XANES and EXAFS data hardly showed any changes when these catalysts were exposed to increasing ratios of H2O to H2S in an H2 atmosphere. XANES and EXAFS data extracted at the Mo K-edge by MES analysis showed that for approximately 1% of the Mo atoms, sulfur atoms are replaced by oxygen atoms when exposed to H2O, causing partial oxidation of these active sites. The reaction is reversible and Mo returns to its initial sulfide phase when H2O is removed and H2S is supplied in the feed. In the case of Co- and Ni-promoted catalysts, the magnitude of S–O e...