The mechanism of thiophene hydrodesulfurization (HDS) on clean/sulficled beta-Mo2C(0 0 1) slabs was studied based on density functional theory (DFT) calculations with different thiophene configurations. Self-consistent, gradient-corrected, periodic DFF calculations were performed. The eta(5)-configuration of thiophene on the Mo atom of Mo carbide preferred the eta(1)-configuration of the adsorption of the thiophene sulfur atom and induced the subsequent scission of the C-S bonds. The addition of hydrogen atoms to the adsorbed C4 species led to the formation of trans-2-butene and S-Mo bonding on the clean beta-Mo2C(0 0 1) slab, while cis-2-butene was formed as a result of hydrogen addition on the sulfided beta-Mo2C(0 0 1) slab. The difference between the formation of trans-2-butene for the beta-Mo2C(0 0 1) slab and cis-2-butene for the sulfided beta-Mo2C(0 0 1) was discussed. (C) 2008 Elsevier B.V. All rights reserved.