DFT insights into the direct desulfurization pathways of DBT and 4,6-DMDBT catalyzed by Co-promoted and Ni-promoted MoS2 corner sites

The direct desulfurization (DDS) mechanisms of DBT and 4,6-DMDBT over the different Co-promoted and Ni-promoted MoS2 corner sites were systematically investigated using DFT calculations. Innovatively, the adsorption processes of both sulfur-containing compounds were evaluated by dispersion corrected methods (DFT-D) firstly. After that, the effect of corner vacancy on the direct desulfurization mechanism was discussed in detail. And the results showed that single corner vacancy provided sufficient support for the DDS reaction of DBT, while double corner vacancies revealed necessary for the adsorption and subsequent CS bond scission of 4,6-DMDBT. However, based on the DFT calculations, the formation energy of the double vacancies was relatively high, which was considered to be present at a very low possibility under the hydrodesulfurization (HDS) condition, thus the DDS activity of 4,6-DMDBT was relatively low. Compared to CoMoS, it could also be found that the corner vacancy of NiMoS showed more superiorities, not only in the contributions to the CS bond cleavages of DBT and 4,6-DMDBT but also in the generation and regeneration processes of active sites. Consequently, the barriers of CS bond cleavages over NiMoS corner sites were as low as 1.16 and 1.22 eV for DBT and 4,6-DMDBT, respectively. Furthermore, the formation energy of the single corner vacancy over NiMoS was 0.78 eV. The relative DFT researches provided atomic-scale insights into the catalytic cycle and the role of corner vacancy in the HDS reaction on Co/Ni-promoted catalysts, which were of significance for the design of highly active Mo-based HDS catalysts.