Spectroelectrochemical Study of CO₂ Reduction on TiO₂ Electrodes in Acetonitrile

One of the main current goals of humanity is the CO₂ conversion into high energy compounds for facilitating both a diminution of the CO₂ atmospheric levels and the development of energy storage strategies. In many studies, TiO₂ has been successfully used as a photocatalyst for CO₂ reduction, but there is still a lack of understanding of its catalytic behavior. In this context, CO₂ reduction has been studied on nanoporous TiO₂ electrodes in acetonitrile media by means of (spectro) electrochemical methods (ATR-IR and UV-vis). Importantly, the onset of the cathodic Faradaic processes related with CO₂ reduction on TiO₂ electrodes is located at -0.81 V versus SHE, which is less negative than that observed for metal electrodes under similar conditions. UV-vis spectroelectrochemical results indicate that the electrocatalytic behavior of TiO₂ is related to the generation of oxygen vacancies and Ti³⁺ sites at its surface and promoted by electrolytes with nonintercalating cations in agreement with recent results on WO₃ electrodes. ATR-IR spectroelectrochemical measurements allow for monitoring of the TiO₂/solution interfacial state as reduction proceeds. Specifically, IR bands for carbon monoxide and carbonyl groups related with carbonate and oxalate are observed. Additionally, a chromatographic analysis shows CO and oxalate as main products. With controlled water addition (0.5 M), methanol and CO were found to be the main products. Based on these results, a mechanism for CO₂ reduction on TiO₂ electrodes is presented in which the regeneration of the TiO₂ surface by oxide electrodissolution/deposition is a critical step.