Catalytic reduction of proton, oxygen and carbon dioxide with cobalt macrocyclic complexes

The conversion of solar energy into chemical energy by the reduction of small molecules provides a promising solution for the effective energy storage and transport. In this manuscript, we have highlighted our recent researches on the catalysis of cobalt-macrocycle complexes for the reduction of O₂, proton and CO₂. We have successfully clarified the reaction mechanisms of catalytic O₂ reduction with cobalt phthalocyanine (CoII(Pc)) and cobalt chlorin (CoII(Ch)) based on detailed kinetic study under homogeneous conditions. The presence of proton-accepting moieties on these macrocyclic ligands enhances the electron-accepting ability, leading to the efficient catalytic two-electron reduction of O₂ to produce hydrogen peroxide (H₂O₂) with high stability and less overpotential in acidic solutions. When CoII(Ch) is adsorbed on multi-walled carbon nanotubes (MWCNTs) and employed as an electrocatalyst, CO₂ was successfully reduced to form CO with a Faradaic efficiency of 89% at an applied potential of -1.1 V vs. NHE in an aqueous solution. Finally, photocatalytic H₂ evolution was attained from ascorbic acid with CoII(Ch) as a catalyst and [Ru(bpy)₃]2+ (bpy = 2,2'-bipyridine) as a photocatalyst via a one-photon two-electron process.