The formation of MoSxCy compounds has been observed on/in the surface of molybdenum-sulfide catalysts during the hydrodesulfurization (HDS) process, and it is a major factor for determining the activity of molybdenum-carbide catalysts. Density functional theory (DFT) was employed to investigate the adsorption of sulfur and sulfidation of transition metal carbides from groups 4-6 in the periodic table, including extended surfaces [MC(0 0 1) (M = Ti, V, Mo, Ta)], nanocrystals [M14C 13 (M = Ti, V, Mo)] and metcar [M8C12 (M = Ti, V, Mo)] nanoparticles. It was found that with increasing carbon/metal ratio, the reactivity of the metal carbides towards sulfur decreased in the sequence: Mo2C > M14C13, M8C12 > MC(0 0 1). In terms of sulfidation, M8C12 and MC(0 0 1) display a stronger resistance than M14C13. The presence of corner or edge sites in the M14C13 nanocrystal favors the formation of MoSxCy compounds. Following Sabatier's principle, our results suggest that flat MC(0 0 1) surfaces are too inert to catalyze HDS reactions, while M14C13 is too active to resist the sulfidation that leads to degradation of the carbides. For reactions involving sulfur and sulfur-containing molecules, nanoparticles adopting the special geometry of metcars should display a better catalytic activity than the corresponding bulk materials and carbide nanoparticles that have a cubic-based structure like nanocrystals. Indeed, DFT calculations indicate that Ti 8C12 and Mo8C12 are good catalysts for the HDS of thiophene.
- Density functional theory
- Metal carbide
- Sulfur adsorption
ASJC Scopus subject areas
- Process Chemistry and Technology
- Physical and Theoretical Chemistry