D. Behar et al., COBALT PORPHYRIN-CATALYZED REDUCTION OF CO2 - RADIATION-CHEMICAL, PHOTOCHEMICAL, AND ELECTROCHEMICAL STUDIES, The journal of physical chemistry. A, Molecules, spectroscopy, kinetics, environment, & general theory, 102(17), 1998, pp. 2870-2877
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.