Simulating alveoli-inspired air pockets in a ZnO/NiMoO4/C3N4 catalyst filter for toluene entrapment and photodecomposition

Highlights: • A ZnO nanorod-supported NiMoO₄/C₃N₄ layer constitutes an alveoli catalyst. • Air pockets in the alveoli catalyst physically entrap VOC molecules. • A concentration gradient and ZnO selectivity control diffusion into air pockets. • Separated adsorptive catalytic sites maintain high VOC degradation kinetics. • Controlled VOC mass transfer to the catalyst results in high quantum yields. Here, we propose an alveoli–inspired catalyst to address the susceptibility of photocatalytic air oxidation systems to fluctuations in volatile organic contaminant (VOC) loads. An alveoli structure was fabricated by covering ZnO nanorods grown on a stainless–steel mesh (SSM) with a porous NiMoO₄/C₃N₄ layer. The alveoli catalyst regulates VOC mass transfer from the air to the catalyst surface using air pockets that capture VOC molecules by diffusion driven by a concentration gradient. Air pockets act as localized reservoirs of molecules that prevent scarcity and congestion at the catalyst surface at low and high VOC loads, respectively. The presence of air pockets in the catalyst assembly and its potential to capture VOC was confirmed by a distinct bimodal adsorption configuration. A ZnO/NiMoO₄/C₃N₄@SSM (ZNC@SSM) catalyst with air pockets achieved a high degree of toluene adsorption (6.1 μmol·m^–2). Toluene selectivity of ZnO controlled the delivery of molecules to active catalyst sites, resulting in 95% toluene conversion in 90 min. Synergetic toluene adsorption in air pockets and degradation on catalytic sites helped achieve a quantum yield of 4.14 × 10^–05 molecules/photon. A figure of merit reflecting fundamental system parameters was compared with previous photocatalytic systems to evaluate the practicality of ZNC@SSM.