XPS characterization of carbon nanotube supported CoMo hydrodesulfurization catalysts

In this paper, the effect of catalytic support and sulfiding method on the chemical state of supported Co-MO catalysts is studied by XPS. After sulfidation with in-situ method, the majority of molybdenum in CNT supported CoMo catalyst is transferred to a species with a formal chemical state Mo(IV) in MoS₂ phase, and the rest to Mo(V) which consists of Mo coordinated both to O and S, such as MoO₂S²₂-_- and MoO₃S²-⁻. In case of CoMo/γ-Al₂O₃ catalyst sulfided with in-situ method, a fraction of molybdenum is transferred to formal state Moo in the form of MoS₂, but there is still a mount of unreduced Mo(VI) phase which is difficult to be sulfided. In CoMo/CNT catalytic system sulfided with ex-situ method, Mo(IV) in the form of MoS₂ is detected along with a portion of unreduced Mo(VI) phase, suggesting that not all the Mo phases are reduced and sulfided by ex-situ method. As for CoMo/γ-Al₂O₃, a portion of molybdenum is sulfided to intermediate reduced state Mo(V) which consists of Mo coordinated both to O and S. such as MoO₂S^2–₂ and MoO₃S^2–, in addition, there is still a fraction of unreduced Mo(VI) phase. XPS analyses results suggest that CNT support facilitates the reduction and sulfidation of active species to a large extent, and that alumina support strongly interacts with active species, hereby producing a fraction of phase which resists complete sulfiding. Catalytic measurements of catalysts in the HDS of dibenzothiophene (DBT) show that CoMo/CNT catalysts are of higher HDS activity and selectivity than CoMo/γ-A₂O₃ catalyst, which is in good relation with the sulfiding behavior of the corresponding catalyst.