Post-synthetic Metalation on the Ionic TiO 2 Surface to Enhance Metal-CO 2 Interaction During Photochemical CO 2 Reduction.

During the photochemical CO 2 reduction reaction, CO 2 adsorption on the catalyst's surface is a crucial step where the binding mode of the [metal-CO 2 ] adduct directs the product selectivity and efficiency. Herein, an ionic TiO 2 nanostructure stabilized by polyoxometalates (POM), ([POM] x @TiO 2 ), 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]@TiO 2 ). The microscopic and spectroscopic studies affirm the copper exchange without altering the TiO 2 core and weak coordination of copper (II) ions to the POMs' surface. Band structure analysis suggests the photo-harvesting efficiency of the TiO 2 core with the conduction band edge higher than the reduction potential of Cu II/I and multi-electron CO 2 reduction potentials. Photochemical CO 2 reduction with Cu x [POM]@TiO 2 results in 30 μmol g cat. -1 CO (79 %) and 8 μmol g cat -1 of CH 4 (21 %). Quasi-in-situ Raman study provides evidence in support of CO 2 adsorption on the Cu x [POM]@TiO 2 surface. 13 C and D 2 O labeling studies affirm the {Cu-[CO 2 ] - } adduct formation. Despite the photo-harvesting ability of Na x [POM]@TiO 2 itself, the poor CO 2 adsorption ability of sodium ions highlights the crucial role of copper ion CO 2 photo-reduction. Characterization of the {M-[η 2 -CO 2 ] - } species via surface tuning validates the CO 2 activation and photochemical reduction pathway proposed earlier.