Photochemical Reduction of CO₂ Catalyzed by Metal Complexes

P. Neta and J. Grodkowski (Guest Researcher)

Objective: To obtain kinetic and mechanistic information on the elementary reactions involved in the photochemical reduction of CO₂ and to develop strategies for conversion of CO₂ into a fuel or feedstock materials.

Problem: Accumulation of CO₂ in the atmosphere from the burning of fossil fuels leads to global warming. It would be advantageous to reduce the amount of CO₂ by converting it into useful chemicals. Reduction of CO₂ can form various compounds including, CO, HCOOH, CH₂O, CH₃OH, and CH₄.

Approach: Iron and cobalt porphyrins and related compounds are studied as catalysts for photochemical reduction of CO₂. Our approach is to attempt to use them in photochemical systems, to demonstrate formation of CO and/or HCOOH from CO₂, and then to examine the mechanism of catalysis and to determine the relevant rate constants by pulse radiolysis.

Results and Future Plans: We have found that iron and cobalt metalloporphyrins (MP) act as effective catalysts for the photochemical reduction of CO₂ to CO and formic acid in dimethylformamide or acetonitrile solutions containing triethylamine as a reductive quencher. In these solutions, M^IIIP is reduced ultimately to M⁰P, which reacts with CO₂ to form CO. In these photochemical studies, the quantum yields were low. In a recent study we have shown that the yield can be dramatically increased by the use of p-terphenyl (TP) as a photosensitizer. TP is very effectively photoreduced by triethylamine (TEA) to form the radical anion, TP· - , which can reduce Co and Fe porphyrins rapidly to the M⁰P state. The metalloporphyrins were destroyed during the photochemical process and yet production of CO continued. These findings suggest that catalytic reduction of CO₂ to CO may be affected by the ferrous ions formed after decomposition of the porphyrin ligand. We have found that the mechanism involves different intermediates. The TP· - radical anion reduces Fe(II), and the Fe(I) ions produced react with CO₂ to form an adduct. Subsequent reduction of the Fe-CO₂ adduct by TP· - or by Fe(I) leads to formation of CO. After extensive irradiation, photochemical production of CO stops. This is caused by competition between CO and CO₂ for the Fe(I) binding sites. In all of the above experiments, the catalysts are either destroyed by side reactions or deactivated by attachment of CO. We plan to investigate various strategies to overcome these limitations. A promising route may be the incorporation of the catalysts into solid support, which will protect against undesired side reactions and/or permit recovery and reuse of the catalysts.

Publications:

Dhanasekaran, T., Grodkowski, J., Neta, P., Hambright, P., and Fujita, E. "p-Terphenyl Sensitized Photoreduction of CO₂ with Cobalt- and Iron-Porphyrins. Interaction Between CO and Reduced Metalloporphyrins," J. Phys. Chem. A 103, 7742 (1999).

Neta, P., "Radiation Chemical Studies of Porphyrins and Metalloporphyrins," in Radiation Chemistry: Present Status and Future Prospects, C. D. Jonah and B. S. M. Rao, eds., Elsevier, 1999 (in press).

Grodkowski, J. and Neta, P., "Cobalt-Corrin Catalyzed Photoreduction of CO₂," J. Phys. Chem. (in press).