Ferrous Ions as Catalysts for Photochemical Reduction of CO₂ in Homogeneous Solutions

Ferrous ions are found to catalyze the photochemical reduction of CO 2 to CO in acetonitrile (ACN) solutions containing triethylamine (TEA) or triethanolamine (TEOA) as reductive quenchers and p -terphenyl (TP) as a photosensitizer. TP is photoreduced to form the radical anion, TP •-, which reduces Fe(II) ions. The rate constant for the reduction of Fe(II) by TP •- was determined by pulse radiolysis in ACN/TEOA solutions and found to be (1.2 ± 0.4) × 10 8 L mol -1 s -1 . The Fe(I) ions produced are expected to react with CO 2 to form an adduct that is equivalent to the adduct known to be formed upon reaction of CO 2 •- with Fe(II) complexes. Subsequent reduction of the Fe−CO 2 adduct leads to formation of CO. Fe(I) can also undergo protonation and form H 2 . After extensive irradiation, photochemical production of CO stops, probably due to competition between CO and CO 2 for the Fe(I) binding sites. Addition of CO to the solution enhances the photochemical production of H 2 .