Investigating Metal Oxide Nanoparticles as Photocatalysts for Carbon Dioxide Reduction Reaction

Metal oxide nanoparticles were investigated for their application as photocatalysts for the gas-phase photocatalytic conversion of carbon dioxide to value-added chemicals and fuels. A multi-photoreactor catalyst screening system was developed to screen a broad set of nanoparticles for photocatalytic activity. Carbon-13 labelled carbon dioxide (13CO2) isotope tracing was carried out to discriminate whether the products arose from CO2 conversion or from reactions involving residual carbon contamination. A set of metal oxide photocatalysts were investigated at temperatures ranging from 80 °C to 190 °C in atmospheres containing both H 2 and CO2. A nanoparticle heterostructure composed of Cu sputtered into a mesoporous TiO2 nanoparticle film was synthesized. These films showed evidence of increased hydrocarbon production under illumination, but no 13C labelled product was measured.;Hydroxylated indium sesquioxide (In2O3-xOH y) nanoparticles were synthesized and investigated for the photocatalytic conversion of CO2. It was found that In2O3-xOH y nanoparticles are capable of photocatalytically and thermally driving the reverse water gas shift (RWGS) reaction at temperatures above 130 °C. Both 13CO and trace 13CH4 were measured. A strong relationship between the CO2 adsorption capacity and the relative surface concentrations of surface defects, such as hydroxides and oxygen vacancies, and photocatalytic activity was established. It was found that there is strong dependence on illumination intensity and wavelength, suggesting that the improved activity under illumination is from light.;A combined computational and experimental study investigating the (111) surface and kinetics of the RWGS reaction over In2O3-xOH y was carried out. It was found that a surface site containing a surface hydroxide adjacent to a surface oxygen vacancy was capable of the heterolytic splitting of H2 and the subsequent reduction of CO2 into CO. This proposed mechanism is analogous to a surface frustrated Lewis pair (FLP) and this discovery represents the first time such a system is shown to be present on the surface of a photocatalyst. Kinetic measurements revealed a 1st order dependence on CO2 in the dark and light, whereas a zero-order dependence was observed for H2. These observations are consistent with the proposed mechanism from theory, which is similar to the Eley-Rideal mechanism.