Post-synthetic Metalation on the Ionic TiO₂ Surface to Enhance Metal-CO₂ Interaction During Photochemical CO₂ Reduction

During the photochemical CO₂ reduction reaction, CO₂ adsorption on the catalyst's surface is a crucial step where the binding mode of the [metal-CO₂] adduct directs the product selectivity and efficiency. Herein, an ionic TiO₂ nanostructure stabilized by polyoxometalates (POM), ([POM]_x@TiO₂), is prepared and the sodium counter ions present on the surface to balance the POMs’ charge are replaced with copper(II) ions, (Cu_x[POM]@TiO₂). The microscopic and spectroscopic studies affirm the copper exchange without altering the TiO₂ core and weak coordination of copper (II) ions to the POMs’ surface. Band structure analysis suggests the photo-harvesting efficiency of the TiO₂ core with the conduction band edge higher than the reduction potential of Cu^II/I and multi-electron CO₂ reduction potentials. Photochemical CO₂ reduction with Cu_x[POM]@TiO₂ results in 30 μmol g_cat.⁻¹ CO (79 %) and 8 μmol g_cat⁻¹ of CH₄ (21 %). Quasi-in-situ Raman study provides evidence in support of CO₂ adsorption on the Cu_x[POM]@TiO₂ surface. ¹³C and D₂O labeling studies affirm the {Cu-[CO₂]⁻} adduct formation. Despite the photo-harvesting ability of Na_x[POM]@TiO₂ itself, the poor CO₂ adsorption ability of sodium ions highlights the crucial role of copper ion CO₂ photo-reduction. Characterization of the {M-[η₂-CO₂]⁻} species via surface tuning validates the CO₂ activation and photochemical reduction pathway proposed earlier.