The dissociation and hydrogenation of CO2 on Cu(100) surfaces that are modified by introducing Co nanoclusters with different size into the top layer have been investigated using density functional theory method. Our results show that on all surfaces the Co atoms are the sites for the adsorption of CO2, and in the early stage of introducing Co dopant, the chemisorption behavior of CO2 is sensitive to the amount of Co atom. According to the predicted pathways for the dissociation of CO2 to CO, it is interesting that the energy barrier decreases first and then increases as more Co atoms are dispersed on the surface, forming a "V" shape. The minimum energy barrier of CO2 decomposition is predicted on the Cu(100) surface that contains four Co atoms aggregated together on the top layer, namely Co-4/Cu(100) bimetallic surface. The most favorable reaction pathway for the hydrogenation of CO to methanol on such surface is further determined, which follows the sequence of CO*-> HCO*-> H2CO*-> H3CO*-> (HCOH)-C-3*, and the rate-limiting step is the hydrogenation of H3CO species with an activation barrier of 106.4 kJ/mol. It is noted that with respect to the pure Cu(100), since more stronger Co-O adsorption bonds are formed on the Co-modified surface, the stability of formaldehyde intermediate is significantly enhanced. Correspondingly, the introducing of Co-4 cluster tends to improve the productivity and selectivity towards methanol synthesis on Cu(100) surface. (C) 2017 Elsevier B.V. All rights reserved.