EPR spin trapping evidence of radical intermediates in the photo-reduction of bicarbonate/CO2 in TiO2 aqueous suspensions

Abstract

Using the EPR spin trapping technique, we prove that simultaneous reactions take place in illuminated suspensions of TiO₂ in aqueous carbonate solutions (pH ≈ 7). The adsorbed HCO₃⁻ is reduced to formate as directly made evident by the detection of formate radicals (˙CO₂⁻). In addition, the amount of OH˙ radicals from the photo-oxidation of water shows a linear dependence on the concentration of bicarbonate, indicating that electron scavenging by HCO₃⁻ increases the lifetime of holes. In a weakly alkaline medium, photo-oxidation of HCO₃⁻/CO₃²⁻ to ˙CO₃⁻ interferes with the oxidation of water. A comparative analysis of different TiO₂ samples shows that formation of ˙CO₂⁻ is influenced by factors related to the nature of the surface, once expected surface area effects are accounted for. Modification of the TiO₂ surface with noble metal nanoparticles does not have unequivocal benefits: the overall activity improves with Pd and Rh but not with Ru, which favours HCO₃⁻ photo-oxidation even at pH = 7. In general, identification of radical intermediates of oxidation and reduction reactions can provide useful mechanistic information that may be used in the development of photocatalytic systems for the reduction of CO₂ also stored in the form of carbonates.