Boosting Nitrogen Selectivity in Photocatalytic Nitrate Reduction via Radical-Mediated Pathway over Cu-Ag Alloy...

Photocatalytic nitrate (NO₃^-) reduction in aqueous systems is an attractive strategy for water remediation; however, directing the reaction toward benign nitrogen gas (N₂) rather than ammonium (NH₄⁺) remains a major challenge. Herein, subnanometer Cu-Ag alloy nanoclusters (ANCs) were anchored on TiO₂ by stepwise photodeposition to achieve efficient and highly selective denitrification. Compared with their monometallic counterparts, the optimized Cu_0.2Ag_0.8/TiO₂ catalyst delivered 94.3% NO₃^- removal and 92.7% N₂ selectivity within 2 h. Combined spectroscopic characterization and density functional theory (DFT) calculations reveal a pronounced alloy synergy in which Ag-rich sites stabilize NO₃^- adsorption, whereas Cu-rich sites promote interfacial electron transfer and activation of nitrogenous intermediates. Mechanistic investigations, including in situ EPR and ¹⁵N isotope labeling, further demonstrate a radical-mediated pathway in which formate oxidation generates ·CO₂^- radicals. These radicals accelerate the selective conversion of nitrite intermediates toward N₂, thereby suppressing over-reduction to ammonia. This study highlights alloy nanocluster engineering coupled with radical chemistry as an effective strategy for controlling product selectivity in aqueous photocatalytic nitrate reduction.