EUROPEAN CONGRESS AND EXHIBITION ON ADVANCED MATERIALS AND PROCESSES - EUROMAT 2021

In CO2 photoreduction, the structural characteristics of the photocatalyst influences the selectivity of the reaction and the concentration of products. High specific surface area (SSA) and porosity allow for more reactants to bind on the surface of the photocatalyst. This adsorption potentially facilitates the one-electron transfer to CO2 a crucial step in CO2 photoreduction. Carbon-containing impurities are often present on the surface of metal oxides. Those impurities under light can form apparent products, not originating from CO2 photoreduction, thus overestimating the photocatalyst’s performance.
In this work, experiments were performed under high-purity conditions where all potential impurity sources were considered. Extensive blank measurements under pure He were performed to remove impurities (cleaning step) and to determine the true origin of the detected products. Three samples were synthesized and tested: a mesoporous TiO2 (m-TiO2), and two ZnO/m-TiO2 mixtures (3 or 10% ZnO) using incipient wetness impregnation. Both m-TiO2 and 3%_ZnO/m-TiO2 produced similar amounts of CH4 (≈95 ppm), four times more than 10%_ZnO/m-TiO2 (200 W Hg/Xe lamp, 1.5% CO2 in He). The addition of a co-catalyst at high loadings can lead to the formation of aggregates which may increase charge recombination events. Under He, H2 was formed for all samples. The addition of CO2 suppressed H2 by 40 and 47% for the 3 and 10%_ZnO/m-TiO2 respectively, while an 80% increase was observed for m-TiO2 because of the lower SSA of the 3% and 10% ZnO/m-TiO2 samples compared to the pure m-TiO2, and the potential blocking of H2O binding sites from CO2. The addition of H2 to the CO2/He reaction mixture lowered CH4 production in m-TiO2. Both ZnO/m-TiO2 samples benefited from H2, increasing their CH4 yields up to five times (for 10%_ZnO/m-TiO2).

Acknowledgments
The authors acknowledge BMBF (Bundesministerium für Bildung und Forschung) for funding through projects PROPHECY (CO2Plus, 033RC003) and PRODIGY (CO2WIN, 033RC024A).