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 CO 2 absorption capacity of 3.26 mol per molar IL under ambient conditions to date. More importantly, it was found that the IL–CO 2 complex displayed visible light absorption because of the delocalization effect in the conjugated anion complex, and it could promote the photoreduction of CO 2 over anatase TiO 2 under visible light irradiation, producing CH 4 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 CO 2 to form the IL–CO 2 complex showing visible light absorption, which served as a photosensitizer to promote the photoreduction of CO 2 over TiO 2 under visible light irradiation. This work opens a new way to enhance visible light-driven photoreduction of CO 2, which may have promising application.