Hydrogen Radical-Mediated Nitric Oxide Reduction to Ammonia Over Synergistic Pd⁰/Pd²⁺ Dual Sites

The photocatalytic nitric oxide reduction reaction (NORR) to NH₃ offers a promising route for ammonia (NH₃) synthesis, addressing environmental issues and mitigating the carbon footprint associated with the energy-intensive Haber-Bosch process. However, conventional sluggish multi-proton-coupled electron transfer (PCET) and poor regulation of valence states at active sites hinder the efficiency of NORR. Herein, we propose a hydrogenation pathway driven by hydrogen radicals (^•H) utilizing ethylene glycol (EG) as a hydrogen donor, which effectively bypasses the limitations of the conventional PCET process. By precisely modulating the valence states of Pd cocatalysts, we construct synergistic Pd⁰/Pd²⁺ dual active sites, where Pd²⁺ 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 Pd⁰ sites. This synergistic effect of Pd⁰/Pd²⁺ guarantees a steady flux of ^•H for deep NORR. Optimizing the Pd⁰/Pd²⁺ ratio to unity (Pd_1.0/TiO₂-50) enabled a superior NH₃ yield rate of 31.61 ± 1.61 mmol∙g_cat ^-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 and understanding the radical-mediated chemistry.