Abstract: The reaction pathway of C–H bond breaking of methane on the clean Pd(111) and O-modified Pd(111) surfaces was investigated by the first-principles density functional theory generalized gradient approximation calculations with the slab model. Geometries of reactants, transition states, and products were calculated. Adsorption energy of possible species and activation energy barriers of the reaction were also obtained. The calculated results show that methane favors such a configuration that one hydrogen points towards the surface in the top site. Methyl is adsorbed in the top site, and hydroxyl, oxygen, and hydrogen are all adsorbed in the fcc site. On the clean Pd(111) surface, the activation energy of 0.97 eV is smaller than that of 1.42 eV in the case of oxygen-modified (oxygen atom acts as a “spectator”) Pd(111) surface, which indicates that the presence of oxygen atom inhibited the C–H bond cleavage. Compared with the case that only the surface oxygen atom exists (oxygen atom participates in the reaction), the activation energy decreased from 1.43 to 0.72 eV when the subsurface oxygen atom exists. This suggests that the subsurface oxygen atom promotes the activation of methane molecule. On the oxygen-modified Pd(111), the activation energy of the reactions forming methyl and hydrogen, and methyl and hydroxyl is 1.42 and 1.43 eV, respectively, which indicates that the reaction possibility is equivalent.

Key words: methane, decomposition, palladium, oxygen atom modification, density functional theory, generalized gradient approximation, slab model