Chain-length-dependent structure–activity relationships in the deep oxidation of multi-component aldehydes over an S-scheme ternary bismuth oxynitride/titanium dioxide/graphite heterojunction photocatalyst

Photocatalytic oxidation (PCO) offers a scalable route for indoor aldehyde abatement, yet practical deployment is hindered by potential surface deactivation from competitive adsorption and accumulation of oxygenated intermediates under realistic multi-component and humidified conditions. Here, a series of ternary heterojunctions, comprising bismuth oxynitride (Bi 5 O 7 NO 3 : BN), titanium dioxide (TiO 2 : T), and graphite (G) (denoted as TGBN-x, “x” = BN/TG wt%), are immobilized on honeycomb cordierite and integrated into a portable air purifier (AP) device. In a 23-L closed-loop chamber, these AP-PCO systems are evaluated for abatement of single- and mixed-aldehydes [formaldehyde (FA), acetaldehyde (AA), propionaldehyde (PA), and valeraldehyde (VA)] under varying operating conditions. Under dry conditions, the optimized TGBN-10 filter records 100% FA (5 ppm) removal within 6 min of 1 W UV-LED illumination, while doubling the catalyst loading from 0.05 to 0.1 g enhances the chamber clean air delivery rate (CADR) from 8.3 to 16.0 L min − 1 . It also exhibits superior charge-carrier dynamics (relative to binary and single counterpart catalysts), delivering an 8.93-fold higher specific CADR (SCADR) at 5 ppm FA (near-ambient level) than at 100 ppm (industrially-relevant level), and a 1.70-fold higher SCADR than the binary TG composite under identical conditions. At 100 ppm FA (100 ppm per aldehyde in the mixture), its SCADR increases 2.64-fold (from 7 to 18.51 L min − 1 g − 1 ) when the inlet gas-phase shifts from a multi-component to a single-component system, underscoring the strong dependence of PCO performance on both inlet concentration and gas-phase composition. In a multi-component mixture (total C 0 = 100 ppm), the peak PCO reaches 91% for FA, compared with 23.2% (PA), 60.7% (VA), and 5.8% (AA), while the overall CO 2 yield across all aldehydes attains 95.9%, confirming a deep mineralization pathway. In situ DRIFTS measurements reveal hydroxyl-mediated FA activation via dioxymethylene and formate intermediates, with carboxylate accumulation increasing from valerate to acetate. It also sustains high PCO activity over 5 cycles and UV-driven photo-regeneration in clean air, with minimal activity loss due to reversible carbonaceous fouling rather than structural degradation. Combined photoelectrochemical, EPR, DRIFTS, and XPS findings further support S-scheme-like charge separation in TGBN-10, with graphite likely serving as a conductive interfacial mediator. These findings demonstrate the potential of TGBN-10 as a functional noble-metal-free photocatalyst for aldehyde VOC abatement under representative industrial/indoor-air conditions.