Homogeneous reduction of CO2 by photogenerated pyridinyl radicals

The impact of fossil fuel combustion emissions on Earth's climate would be alleviated by an ideal process able to capture atmospheric CO2 and convert it into fuels and/or chemical feedstocks via an artificial photosynthetic process driven by sunlight. However, the endoergonic reduction of CO2 into organic matter is still very challenging. Indeed, due to its negative electron affinity in the gas phase, conversion of CO2 into the carbon dioxide radical anion is both very slow and hindered by unfavorable thermodynamics.The chemical stabilization of the carbon dioxide radical anion has recently been achieved with the formation of a carbamyl intermediate from the homogeneous reaction between 1-hydroxypyridinyl radicals formed through th electrochemical reduction of pyridimium with dissolved CO2. However, the mechanism has not yet been fully clarified, quantum mechanical calculations having predicted a pyridinium reduction potential significantly more negative than the experimentally reported value. In our study, CO2 reduction was performed under monochromatic irradiation at 254 nm in dearated homogeneous aqueous solutions containing pyridine and an electron donor (2-Pr-OH). The final reaction products, identified by electrospray ionization mass spectrometry, were shown to contain carboxylate functionalities, Accounting for the homogeneous 2-electrons reduction of CO2 by 2-PrOH initiated by electronically excited pyridine.. The evaluated rate constant and activation energy for 1-pyridinyl addition to CO2 are comparable with thermochemical estimates for this reaction.