Dual-active-site engineering via orbital modulation of Fe single atoms on defective TiO₂ for enhanced photocatalytic NO removal

Nitrogen monoxide (NO), a critical precursor to particulate matter with an aerodynamic diameter ≤ 2.5 μm (PM_2.5) and ozone formation, requires efficient abatement at atmospheric low concentrations (< 1 ppm), yet conventional photocatalysts struggle with deep NO purification due to inefficient trace O₂/NO co-activation. Herein, we engineer Fe single atoms on defective TiO₂ catalyst (Fe_SA/OV-TiO₂, OV refers to oxygen vacancy) with orbital-modulated dual-active sites. Density functional theory calculations and in situ characterizations reveal that surface oxygen vacancies drive robust O₂ activation to generate reactive oxygen species (ROS). Adjacent Fe single atoms enable targeted NO chemisorption via their d-orbital hybridization with antibonding π* orbital of NO. This synergy shifts the NO oxidation pathway from ·O₂⁻-dominated Eley–Rideal (E–R) to dual-activated Langmuir–Hinshelwood (L–H) pathways, where pre-adsorbed NO directly reacts with ROS, forming thermodynamically stable bidentate nitrate. Crucially, Fe_SA/OV-TiO₂ achieves 75% NO conversion efficiency with 98% nitrate selectivity under visible light irradiation, outperforming defective TiO₂-OV by 1.4-fold while maintaining the good activity over five cycles without deactivation. The work establishes orbital-level dual-site engineering as a strategy for developing high-efficiency photocatalysts for air pollution remediation.