Relationship between the hydrodesulfurization of thiophene, dibenzothiophene, and 4,6-dimethyl dibenzothiophene and the local structure of Co in Co–Mo–S sites: Infrared study of adsorbed CO

Infrared spectroscopy of adsorbed CO and HDS activity for thiophene, dibenzothiophene, and 4,6-dimethyl dibenzothiophene were performed using Mo/Al2O3, CoMo/Al2O3, and CoMo–EDTA/Al2O3 catalysts. The results show differences between the FT-IR absorption coefficients of CO adsorbed on Co–Mo–S sites for CoMo/Al2O3, and CoMo–EDTA/Al2O3, despite that the vibration frequency of the CO – Co–Mo–S peak in both cases is observed at ∼2071 cm−1. The absorption coefficient is bigger when the level of promotion increases. We propose that the increased absorption coefficient is associated with the change in the local structure of the surface Co sites from tetrahedral/pseudo-tetrahedral, in partially promoted crystallites, to square planar structure in totally promoted zones of MoS2 crystallites. For simple molecules like thiophene, the HDS activity does not change if Co is in either tetrahedral/pseudo-tetrahedral or square planar structures. In contrast, an open structure like square planar Co favors the accessibility of 4,6-DMDBT to the Co–Mo–S sites, increasing its HDS activity.