(69c) Hierarchical Inorganic Assemblies for the Photocatalytic Reduction of CO2 by H2O

The multi-electron processes of CO₂ reduction and H₂O oxidation, which constitute the essential reactions of sunlight to fuel conversion require efficient coupling of light absorber and catalyst. Using all-inorganic heterobinuclear units such as ZrOCo(II) anchored on high surface area silica, we have developed photodeposition methods for the directional assembly of nanometer sized catalysts for H₂O oxidation or CO₂ reduction with proper coupling to the binuclear light absorber. Copper oxide nanoclusters were assembled adjacent to the Zr acceptor sites of the heterobinuclear unit, and light driven reduction of CO₂ upon excitation of the ZrOCo unit demonstrated. Illumination of the ZrOCo chromophore coupled to an Ir oxide nanocatalyst for H₂O oxidation  allow us to reduce CO₂ by taking the electrons from water, thereby closing the photocatalytic cycle. Time resolved FT-IR spectroscopy provides detailed mechanistic insight into the multi-electron reactions on the metal oxide catalyst surface under reaction conditions. To achieve CO₂ photoreduction by H₂O under product separation, Co₃O₄-SiO₂ core-shell nanotubes were developed in which the inner surface acts as efficient water oxidation catalyst while the nanoscale silica layer with embedded molecular wires serves as a proton conducting, gas impermeable separation membrane with tight control of electron transport. Using nanofabrication based on sacrificial Si nanowire array methods, macroscale arrays of Co oxide-silica nanotubes are being developed for accomplishing the complete photocatalytic cycle in a macroscale assembly.