Spectroelectrochemical Study of CO2 Reduction on TiO2 Electrodes in Acetonitrile

Abstract

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.