p-Terphenyl-Sensitized Photoreduction of CO2 with Cobalt and Iron Porphyrins. Interaction between CO and Reduced Metalloporphyrins

• Published in: The journal of physical chemistry. A, Molecules, spectroscopy, kinetics, environment, & general theory Vol. 103; no. 38; pp. 7742 - 7748
• Main Authors: Dhanasekaran, T, Grodkowski, J, Neta, P, Hambright, P, Fujita, Etsuko
• Format: Journal Article
• Language: English
• Published: American Chemical Society 23.09.1999
• DOI: 10.1021/jp991423u
• ISSN: 1089-5639

Iron and cobalt porphyrins (FeP and CoP) are utilized as electron-transfer mediators to effect photochemical reduction of CO2 in homogeneous solutions. The species that activate and reduce CO2 are the Fe0P and Co0P formed by reduction of the starting materials. Reduction of the metalloporphyrins (MP) is achieved by photolysis in dimethylformamide or acetonitrile solutions containing triethylamine (TEA) as a reductive quencher. The photoreduction is efficient for the MIIIP → MIIP stage and probably occurs by an intramolecular electron transfer from an axially bound TEA. However, TEA does not bind to the reduced metal complexes, and the quantum efficiency is much lower for the subsequent reduction steps. Considerably higher quantum yields are obtained by adding p-terphenyl (TP) as a sensitizer. TP is very effectively photoreduced by TEA to form the radical anion, TP•-, which has a sufficiently negative reduction potential to reduce CoIP and FeIP rapidly to their M0P state. The rate constants for these reactions, determined by pulse radiolysis, are found to be nearly diffusion-controlled. The quantum yield for the reduction of MIIP to MIP and for reduction of CO2 to CO are increased by more than an order of magnitude in the presence of TP. Side reactions involve hydrogenation of the porphyrin ring and production of H2. The hydrogenated porphyrins also catalyze reduction of CO2, but the photochemical production of CO eventually stops. This limit on catalytic activity is due to destruction of the porphyrin macrocycle and accumulation of CO. CO can bind strongly to FeIIP and to FeIP but not to Fe0P, as demonstrated by electrochemical measurements and by optical spectra of the species produced by sodium reduction in tetrahydrofuran in the presence and absence of CO. Although binding of CO to FeIIP and FeIP should not interfere with the formation of Fe0P, the active catalyst, the potential for reduction of FeIP to Fe0P becomes more negative. However, CO probably binds to the hydrogenated products thereby inhibiting the catalytic process.