Effect of Sodium on Ni-Promoted MoS₂ Catalyst for Hydrodesulfurization Reaction: Combined Experimental and Simulation Study

The presence of Na in Ni-promoted MoS₂ provides an interesting case study as Na gets inadvertently incorporated during the reverse micelle synthesis of the nanocatalyst. The effect of Na in Ni-promoted MoS₂ during hydrodesulfurization (HDS) of dibenzothiophene (DBT) is investigated here through combined experiment and density functional theory (DFT) studies. Computations suggest that Na replaces otherwise more HDS active Ni sites, which are likely to be present as metal atoms on S edge or on metallic edge sites of MoS₂(100). The presence of Na dopant instead of Ni results in the molecular hydrogen dissociation step becoming more endothermic, leading to the lowering of HDS catalytic activity. The HDS of DBT decreases with an increase of Na concentration in Ni-promoted MoS₂. However, the concentration of Na has a nonmonotonic effect on the selectivity of different mechanistic pathways of HDS. The selectivity toward a cost-effective direct desulfurization (DDS) pathway increases up to an optimal Na concentration, after which the selectivity decreases as observed from experiments and corroborated by DFT studies. However, DDS selectivity always remains higher, which is the critical feature of Na incorporation in Ni-promoted MoS₂.