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Visible-Light-Driven Conversion of CO₂ to CH₄ with an Organic Sensitizer and an Iron Porphyrin Catalyst

Rao, Heng, Lim, Chern-Hooi, Bonin, Julien, Miyake, Garret M., Robert, Marc
Journal of the American Chemical Society 2018 v.140 no.51 pp. 17830-17834
acidity, carbon dioxide, carbon monoxide, catalysts, dyes, feedstocks, formic acid, iron, irradiation, methane, photochemistry, photosensitizing agents, porphyrins
Using a phenoxazine-based organic photosensitizer and an iron porphyrin molecular catalyst, we demonstrated photochemical reduction of CO₂ to CO and CH₄ with turnover numbers (TONs) of 149 and 29, respectively, under visible-light irradiation (λ > 435 nm) with a tertiary amine as sacrificial electron donor. This work is the first example of a molecular system using an earth-abundant metal catalyst and an organic dye to effect complete 8e–/8H⁺ reduction of CO₂ to CH₄, as opposed to typical 2e–/2H⁺ products of CO or formic acid. The catalytic system continuously produced methane even after prolonged irradiation up to 4 days. Using CO as the feedstock, the same reactive system was able to produce CH₄ with 85% selectivity, 80 TON and a quantum yield of 0.47%. The redox properties of the organic photosensitizer and acidity of the proton source were shown to play a key role in driving the 8e–/8H⁺ processes.