Theoretical study of electrochemical reduction of CO₂ to CO using a nickel-N₄-Schiff base complex

The electrochemical reduction (ECR) of CO₂ to CO by nickel-N₄-Schiff base complexes as catalysts was investigated using density functional theory (DFT). Three nickel complexes, 1-Ni, 2-Ni, and [2-Ni]^Me were considered. Two CO₂ reduction pathways, i.e., external and internal proton transfer, were proposed and their reaction energy profiles were computed. The external proton transfer pathway which includes three steps has no transition state. The reaction energies for all steps are exothermic and the reaction catalyzed by 1-Ni has the lowest overall reaction energy (-5.72 eV) followed by those by 2-Ni (-5.56 eV) and [2-Ni]^Me (-5.54 eV). The internal proton transfer pathway is composed of four steps. The internal proton transfer step (carboxylic formation) includes a transition state. The CO₂ reduction by [2-Ni]^Me could not proceed via this mechanism, since [2-Ni]^Me does not have an NH group in the ligand and 1-Ni has a lower activation energy (0.83 eV), which is in agreement with the experiment. The charge of the pre-adsorption nickel complex seems to be related to the activity of the catalysts. The catalyst with a less positive nickel charge is more active.