EPR spin trapping evidence of radical intermediates in the photo-reduction of bicarbonate/CO2 in TiO2 aqueous suspensionsElectronic supplementary information (ESI) available. See DOI: 10.1039/c4pp00467a

• Main Authors: Molinari, Alessandra, Samiolo, Luca, Amadelli, Rossano
• Format: Article
• Language: English
• ISSN: 1474-905X; 1474-9092
• DOI: 10.1039/c4pp00467a

Using the EPR spin trapping technique, we prove that simultaneous reactions take place in illuminated suspensions of TiO 2 in aqueous carbonate solutions (pH 7). The adsorbed HCO 3 − is reduced to formate as directly made evident by the detection of formate radicals (&z.rad;CO 2 − ). In addition, the amount of OH&z.rad; radicals from the photo-oxidation of water shows a linear dependence on the concentration of bicarbonate, indicating that electron scavenging by HCO 3 − increases the lifetime of holes. In a weakly alkaline medium, photo-oxidation of HCO 3 − /CO 3 2− to &z.rad;CO 3 − interferes with the oxidation of water. A comparative analysis of different TiO 2 samples shows that formation of &z.rad;CO 2 − is influenced by factors related to the nature of the surface, once expected surface area effects are accounted for. Modification of the TiO 2 surface with noble metal nanoparticles does not have unequivocal benefits: the overall activity improves with Pd and Rh but not with Ru, which favours HCO 3 − 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 CO 2 also stored in the form of carbonates. Storage of CO 2 as carbonates is a strategy for its reductive activation. Carbonates are reduced at pH 5.5-7.5 with photoexcited TiO 2 . EPR is convenient for detecting reaction intermediates.