Hierarchical TiO2-g-C3N4 photocatalyst with purification effect for NOx oxidation under cyan light

NOx photocatalytic oxidation is a promising approach for practical NOx abatement at sub-ppb levels and under ambient conditions. However, conventional photocatalysts are characterized by their low NOx adsorption capacity and activity under visible light. This results in poor NO (g) conversion and may form more toxic NO2 (g), hampering the utilization of photocatalysis in NOx removal. Herein, we employed a hierarchical macromesoporous structure composed of Z-scheme TiO2-g-C3N4 hybrid embedded in parallel TiO2 channels as a high adsorption capacity and low band gap photocatalyst for NOx oxidation. This unique structure provides aligned macroporous channels and mesoporous walls for efficient NOx adsorption and storage of NOx oxidation product (i.e., NO 3 (s)) while enabling visible light absorption. NOx oxidation was conducted in a laminar-flow reactor under 50% relative humidity and simultaneous exposure to blue and green lights (cyan light). TiO2-gC3N4 showed NO conversion (37%) which is four times higher than that of TiO2 (7%) and two times higher than that of g-C3N4 (17%). In addition, TiO2-g-C3N4 hybrids showed high NOx adsorption capacity and converted NO to nontoxic NO 3 with minimal formation of NO2, evident by its positive DeNOx index (+74 ppm) in contrast to gC3N4 with a negative DeNOx (-4 ppm) and TiO2 with a slightly positive DeNOx index (+15 ppm). The study demonstrates the delicate relationship between H2O adsorption on the hybrid structure and NOx photooxidation reaction and provides detailed mechanistic insights on the photocatalytic response. It is expected that the results of this study will benefit ongoing efforts to design visible-light active photocatalysts.