Interplay Between Ionic Liquids, Kolbe Chemistry, and 2D Photocatalyst Supports in Aqueous CO2 Photoreduction over Pd/TiO2 and Pd/g-C3N4

The photocatalytic reduction of CO2 in aqueous media offers a sustainable route for solar-to-fuel conversion, yet remains challenged by CO2’s thermodynamic stability and kinetic inertness, low solubility, and competitive hydrogen evolution. Here, we investigate the interplay between ionic liquids (ILs), photocatalyst supports, and additive composition in directing product selectivity among CO, CH4, and H2. Using imidazolium acetate as a benchmark, we demonstrate that ILs not only pre-activate CO2 but can also undergo decomposition pathways under illumination, notably Kolbe-type reactions leading to methane formation from acetate rather than from CO2. Comparative studies of Pd-decorated TiO2 and g-C3N4 nanosheets reveal distinct behaviors driven by their interfacial interactions with the imidazolim-based ionic liquid: weak interaction with TiO2 strongly promotes hydrogen evolution, whereas strong coupling with g-C3N4 synergizes with C1C4ImOAc to trigger acetate-derived Kolbe reactivity. The systematic evaluation of alternative salts confirms the determinant role of anion basicity and medium-pH-basic anions facilitate CO2 activation, whereas weakly basic or non-coordinating anions favor water splitting. Overall, these results clarify the dual role of ionic liquids as both CO2 activators and sacrificial agents, and highlight design principles for improving product selectivity and efficiency in aqueous CO2 photoreduction systems.