Revealing Cu⁰-Cu⁺ Synergistic Catalysis for Rational Design of Cu/TiO₂ Composites toward Nitrate Reduction into...

The size effect of supported metal particles profoundly influences catalytic performance, but how to determine the optimal size to enhance the catalytic activity still faces significant challenges. This work systematically investigates the complete reaction mechanism as well as region-dependent catalytic behaviors of nitrate reduction into ammonia on representative Cu/TiO₂ composites, using extensive ab initio molecular dynamics (AIMD) simulations and density functional theory calculations. Mechanistically, we demonstrated the direct N-O bond cleavage of NO₃* via a tilted tridentate configuration whereas a prehydrogenation assisted dissociation pathway in the forms of HO-NO* and HN-OH* for NO₂* and NO* intermediates on supported copper particles. Geometrically, interface Cu⁺ sites show excellent deoxygenation activity with reduced reaction barriers for NO₃* + * → NO₂* + O*, while surface Cu⁰ boosts the hydrogenation of subsequent intermediates. Together with the surmountable diffusion of NO_x species as well as electronic/geometric correlation analysis, we identified a novel mechanism of Cu⁰-Cu⁺ synergistic catalysis that realizes the superior activity of supported Cu for nitrate reduction in experiments. Further, by employing a hemispherical wetting model and coordinating Cu⁰/Cu⁺ ratios, our results suggest that supported Cu nanoparticles with a diameter of 1-2 nm are the best catalysts for photocatalytic nitrate reduction. This study provides a general approach for designing the optimal size of supported metal particle catalysts.