Single-atom catalysts (SACs) for CO 2 to CO conversion using Cu, Ni, and Co on graphene flakes support; a DFT study

In this study, to convert the CO2 to more useful compound and in order to design the appropriate catalyst for this conversion, a graphene flakes support was considered for three transition metals of the 3d series (Cu, Co and Ni) as single-atom catalysts. Although different spin multiplicities for cobalt and nickel catalysts are possible, their relative energies and frontier orbital properties suggested the catalyst with the higher spin multiplicity is more suitable for this purpose. Moreover, to develop the mechanism of this conversion, two mechanistic routes were designed for the reaction and the structures of all reactants, products, intermediates and transition states of these routes were optimized and their enthalpies and Gibbs free energies were extracted. The final data showed that by the gas phase calculation, copper and cobalt are more favorable than nickel catalyst; copper is thermodynamically favorable and cobalt is kinetically favorable. The solvent effects were also considered using water and PCM methods, which showed, the solvent facilitate the reaction, especially for cobalt; therefore, in the solvent cobalt (and then, copper) is the most appropriate catalyst by both thermodynamic and kinetic data.