The different mechanisms of CO2 methanation on Ni(111) surfaces have been investigated by density functional theory with and without the formation of CO as an intermediate. The most stable adsorption configurations of all reaction species (O, OH, CO, CH, CH2, CH3, CH4, HCOO, C(OH)(2), CH2O, etc.) in three paths of CO2 methanation adsorbed on the Ni(111) surface are described. The energy barriers and reaction energies for the overall processes involved in the various paths are presented. The rate-determining steps for the three mechanisms are HCOO -> CO + OH for path 1, CO -> C + O for path 2 and CO2 +2H -> C(OH)(2) for path 3 with maximum energy barriers of 3O6.8 kJ/mol, 237.4 kJ/mol and 292.3 kJ/mol, respectively. Path 2 is therefore the optimum of the three mechanisms. The path starts with CO2 dissociation into CO and O, CO decomposition into C and O species and C species hydrogenation to form CH4: CO2 -> CO + O -> C + O +4H -> CH2 + 2H -> CH3 +H -> CH4. (C) 2015 Elsevier B.V. All rights reserved.