Using density functional theory (DFT) together with periodic slab models, the adsorption and dehydrogenation mechanisms of methane on clean and oxygen-covered Pd (1 0 0) surfaces have been studied systematically. Different adsorption geometries were investigated for CH4 and related intermediates (CH3, CH2, CH, C, H, O and OH). It was found that CH4 and CH3 prefer to adsorb on the top site, CH2 and OH are favorable on the bridge site, while CH, C, 0 and H species adsorb preferentially on the hollow site. In addition, this work identified the stable co-adsorption configurations for the relevant co-adsorption groups. It was concluded that the effect of co-adsorbed oxygen atom tends to weaken the adsorbate-substrate interaction on the Pd (1 0 0) surface. Finally, transition states, energy barriers and reaction energies were determined to confirm the mechanism of dehydrogenation of CH4 on clean and oxygen-covered Pd (1 0 0) surfaces. The existence of oxygen atom increases the energy barriers obviously and inhibits the dissociation of CH x (x=1, 2 and 4) except for CH3 group. (C) 2014 Elsevier B.V. All rights reserved.