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

Published

July 1, 1999

Author(s)

T Dhanasekaran, J Grodkowski, Pedatsur Neta, P Hambright, E Fujita

Abstract

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 M^IIIP -> M^IIP 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 Co^IP and Fe^IP 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 M^IIP to M^IP 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 Fe^IIP and to Fe^IP, 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 Fe^IIP and Fe^IP should not interfere with the formation of Fe⁰P, the active catalyst, the potential for reduction of Fe^IP to Fe⁰P becomes more negative. However, CO probably binds to the hydrogenated products thereby inhibiting the catalytic process.

Citation

Journal of Physical Chemistry A

Volume

103

Pub Type

Journals

Keywords

catalysis, CO₂ reduction, metalloporphyrins, photochemistry, porphyrins, radiolysis

Citation

Dhanasekaran, T. , Grodkowski, J. , Neta, P. , Hambright, P. and Fujita, E. (1999), p-Terphenyl Sensitized Photoreduction of CO₂ With Cobalt- and Iron-Porphyrins. Interaction Between CO and Reduced Metalloporphyrins, Journal of Physical Chemistry A (Accessed April 25, 2024)