The breaking of the C-S bond is a crucial step in hydrodesulfurization, the removal of the sulfur atom from sulfur-containing molecules in crude oil. Thus the hydrogenolysis reaction of CH3SH to CH4 was studied by means of density functional theory on the catalytically active (100) edge of 2H-MoS2, with and without Co and Ni promoter atoms. Thiol adsorption, C-S bond breaking, and the formation and desorption of CH4 were investigated with different sulfur and hydrogen surface coverages. CH3SH first adsorbs molecularly with its S atom in a bridging mode between two surface Mo atoms, followed by S-H bond cleavage with moderate activation energy. The subsequent concerted C-S bond breaking and CH4 formation occurs through a reaction of the adsorbed CH3S group with the H atom of a neighbouring SH group at the molybdenum sulfide surface. Sulfur atoms, hydrogen atoms adsorbed on sulfur atoms, and promoter atoms (Co and Ni) at the catalyst surface weaken the bonding of adsorbed CH3S and lower the energy barrier for CH4 formation. Although the reactions of thiols on the metal sulfide surface are similar to reactions on metal surfaces, the chemistry is different. The reactions occur between intermediate alkyl and hydrogen fragments bonded to sulfur atoms, not to metal atoms. (c) 2005 Elsevier Inc. All rights reserved.