DFT study of the sulfidation pretreatment of molybdenum carbides in the hydrodechlorination reaction of chlorobenzene

The effect of the pre-adsorption of sulfur on the hexagonal-Mo₂C-(001) surface in the hydrodechlorination (HDC) reaction of chlorobenzene was studied using density functional theory (DFT) calculations. The results related with the coordination modes and the adsorption energies of the aromatic molecule suggest that the main effect of the sulfur incorporation into the surface is to lead towards a weaker chlorobenzene adsorption that will benefit the continuity of the catalytic cycle and therefore to avoid the carbonization and chlorination of the catalyst. The study of the HDC mechanism was also performed and two different reaction paths were considered. The calculated energy barriers indicate that both mechanisms may occur at the normal reaction temperature (350°C). These latter approaches involve new roles of the superficial sulfur as atomic or radical hydrogen scavenger for the S-Hformation and as a precursor of the σ chlorobenzene coordination, necessary for effective hydrogenation in the proposed mechanisms.