Visible Light-Driven Photoreduction of CO2 to CH4 over TiO2 Using a Multiple-Site Ionic Liquid as an Absorbent and Photosensitizer

A novel four-site ionic liquid (IL) was designed via the neutralization of citrazinic acid with tetrabutylphosphonium hydroxide, which had the highest CO2 absorption capacity of 3.26 mol per molar IL under ambient conditions to date. More importantly, it was found that the IL–CO2 complex displayed visible light absorption because of the delocalization effect in the conjugated anion complex, and it could promote the photoreduction of CO2 over anatase TiO2 under visible light irradiation, producing CH4 as the sole carbonaceous product with a rate of 3.52 μmol g–1 h–1 and a selectivity of >96%. The mechanism investigation indicated that the IL absorbed CO2 to form the IL–CO2 complex showing visible light absorption, which served as a photosensitizer to promote the photoreduction of CO2 over TiO2 under visible light irradiation. This work opens a new way to enhance visible light-driven photoreduction of CO2, which may have promising application.