Regulating nitrogen transformation via oxygen-vacancy-mediated metal-support interaction in Ru/TiO2 single-atom catalysts for catalytic subcritical water oxidation of nitrobenzene

Catalytic subcritical water oxidation (CSWO) is a promising technology for treating refractory organic pollutants, yet its application to nitrogen-containing wastewater is limited by inefficient nitrogen conversion and the accumulation of nitrogenous intermediates. Herein, predominantly Ru/TiO 2 single-atom catalysts with tunable oxygen vacancy concentrations were developed for nitrobenzene degradation. The optimized 800-Ru/TiO 2 catalyst achieved mineralization and total nitrogen (TN) removal rates of 87.8% and 85.2%, respectively. A pronounced decoupling between organic matter mineralization and nitrogen conversion was observed, with nitrogen transformation identified as the rate-limiting step. Reaction condition analysis reveals that nitrogen transformation is governed by a competition between selective N 2 formation and over-oxidation to NO 3 − , and the coupling of NH 4 + and NO 2 − is identified as the key pathway for N 2 generation, whereas excessive oxidation promotes nitrate accumulation. The introduction of atomically dispersed Ru significantly enhances nitrogen conversion and shifts the reaction pathway toward selective N 2 formation. Structural characterization and electronic analyses demonstrate that oxygen-vacancy-enriched TiO 2 stabilizes Ru single atoms and induces strong metal-support interaction (SMSI), which is associated with enhances oxidant activation and promotes nitrogen redox coupling. The catalyst exhibits good stability over 92% performance retention after five cycles. This work provides insights into the relationship between catalyst structure, electronic properties, and nitrogen transformation, and offers a strategy for achieving efficient and selective nitrogen removal in CSWO systems.