COBALT PORPHYRIN-CATALYZED REDUCTION OF CO2 - RADIATION-CHEMICAL, PHOTOCHEMICAL, AND ELECTROCHEMICAL STUDIES

Several cobalt porphyrins (CoP) have been reduced by radiation chemica l, photochemical, and electrochemical methods, in aqueous and organic solvents. In aqueous solutions, the (CoP)-P-1 state is stable at high pH but is shorter lived in neutral and acidic solutions. Stable (CoP)- P-1 is also observed in organic solvents and is unreactive toward CO2. One-electron reduction of (CoP)-P-1 leads to formation of a species t hat is observed as a transient intermediate by pulse radiolysis in aqu eous solutions and as a stable product following reduction by Na in te trahydrofuran solutions. The spectrum of this species is not the chara cteristic spectrum of a metalloporphyrin pi-radical anion and is ascri bed to (CoP)-P-0. This species binds and reduces CO2. Catalytic format ion of CO and HCO2- is confirmed by photochemical experiments in aceto nitrile solutions containing triethylamine as a reductive quencher. Ca talytic reduction of CO2 is also confirmed by cyclic voltammetry in ac etonitrile and butyronitrile solutions and is shown to occur at the po tential at which (CoP)-P-1 is reduced to (CoP)-P-0. As compared with C oTPP, fluorinated derivatives are reduced, and catalyze CO2 reduction, at less negative potentials.