Synergistic Pd 0 /Pd 2+ Sites Enable Selective Nitric Oxide Reduction to Ammonia via a Hydrogen Radical-Mediated Pathway

The photocatalytic nitric oxide reduction reaction (NORR) to NH3 offers a promising route for ammonia (NH3) synthesis, synergistically addressing environmental issues and mitigating the carbon footprint associated with the energyintensive Haber-Bosch process. However, conventional sluggish multi-proton-coupled electron transfer (PCET) and poor regulation of valence states at active sites hinder the efficient NORR. Herein, we propose a hydrogenation pathway driven by hydrogen radicals (• H) that utilizes ethylene glycol (EG) as a hydrogen donor, effectively bypassing the limitations of the conventional PCET process. By precisely modulating the valence states of Pd cocatalysts, we construct a synergistic active center where Pd 2+ sites facilitate the dissociation of C-H bonds from EG to supply adsorbed H species (*H), which are subsequently activated into highly reactive • H by photogenerated electrons at Pd 0 sites. This Pd 2+ /Pd 0 synergistic effect guarantees a steady flux of • H for deep NORR. Optimizing the Pd 2+ /Pd 0 ratio to unity (Pd1.0/TiO2-50) enabled a superior NH3 yield rate of 31.61±1.61 mmol•gcat −1 •h −1 with high NO conversion (91.54±1.82%) and NH3 selectivity (90.14±1.38%) in a single pass flow. Comprehensive in situ characterizations elucidate the causality between Pd valence states and radical generation, offering a new paradigm for designing radical-mediated molecular transformations.