ESR of SO2− and SO − anion-radicals adsorbed on supported vanadium silica gel catalysts

Abstract

ESR spectroscopic measurements have been carried out on unsupported CoMo, NiMo, NiW sulfide catalysts with two objectives: the first is to determine the role of Group VIII metal promoter atom in hydrotreating catalysts, while the second is to identify differences, if any, between Mo and W based catalysts. This study shows two ranges of promoter concentrations which deserve more attention, viz., low promoter concentration range (r = Group VIII metal atom/{Group VIII + Group VI-B} metal atoms < 0.05), and high promoter concentration (0.1 < r < 0.5). The presence of trivalent Group VI-B metal species has been detected in both cases. In the low concentration range, addition of Group VIII metal atom has been found to suppress ESR signals due to defects present in the host dichalcogenide (MoS2 or WS2). In the region corresponding to high promoter content, strong ferromagnetic behavior is observed. These results are explained invoking current theories proposed for HDS catalysis. A comparison of ESR spectra of Mo and W catalysts shows that although paramagnetic properties of Mo-based catalysts are not very different from those exhibited by W-based catalysts and their responses to addition of Group VIII metal are quite similar, a stronger electronic interaction is observed in WS2-based catalysts.