P-terphenyl-sensitized photoreduction of CO{sub 2} with cobalt and iron porphyrins. Interaction between CO and reduced metalloporphyrins
Iron and cobalt porphyrins (FeP and CoP) are utilized as electron-transfer mediators to effect photochemical reduction of CO{sub 2} in homogeneous solutions. The species that activate and reduce CO{sub 2} are the Fe{sup 0}P and Co{sup 0}P 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{sup III}P {yields} M{sup II}P 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{sup {sm{underscore}bullet}{minus}}, which has a sufficiently negative reduction potential to reduce Co{sup I}P and Fe{sup 1}P rapidly to their M{sup 0}P 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{sup II}P to M{sup I}P and for reduction of CO{sub 2} 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 H{sub 2}. The hydrogenated porphyrins also catalyze reduction of CO{sub 2}, 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{sup II}P and to Fe{sup I}P but not to Fe{sup 0}P, 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{sup II}P and Fe{sup 1}P should not interfere with the formation of Fe{sup 0}P, the active catalyst, the potential for reduction of Fe{sup I}P to Fe{sup 0}P becomes more negative. However, CO probably binds to the hydrogenated products thereby inhibiting the catalytic process.
- Research Organization:
- National Inst. of Standards and Technology, Gaithersburg, MD (US)
- Sponsoring Organization:
- USDOE; National Aeronautics and Space Administration
- DOE Contract Number:
- AI02-95ER14565; AC02-76CH00016
- OSTI ID:
- 20000081
- Journal Information:
- Journal of Physical Chemistry A: Molecules, Spectroscopy, Kinetics, Environment, amp General Theory, Vol. 103, Issue 38; Other Information: PBD: 23 Sep 1999; ISSN 1089-5639
- Country of Publication:
- United States
- Language:
- English
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