Turn Waste into Treasure: Gas–Solid Photoreduction of NO in Flow for Separation-Free Ammonia Synthesis

Photo/electrocatalytic ammonia (NH₃) synthesis via nitrous oxide (NO<i>x</i>) reduction is challenged by low NO solubility in electrolytes, inefficient electron and proton supply, and the high cost of separating NH₃ from dilute NH₄⁺ product solutions. In this study, we present a continuous, separation-free gas-solid photoreduction strategy for direct NH₃ gas production from flue gas, enabled by dynamic band structure regulation of the photocatalyst. Comprehensive in situ characterization and theoretical analysis demonstrate that this approach utilizes photoexcited oxygen vacancies (PV_O) in TiO₂ to generate high-energy electrons in deep conduction bands, thereby enhancing the reduction capacity, accelerating electron transfer, and lowering the activation energy for both H₂O dissociation and NO hydrogenation. The real-time tracking of NO radicals in a gas-solid system using operando EPR provides crucial mechanistic insights into high-energy electron behavior and the NO-to-NH₃ conversion pathway, where these high-energy electrons not only promote efficient NO-to-NH₃ conversion but also prevent toxic N₂O byproduct formation, ensuring selective and efficient reduction. The optimized TiO₂-PV_O catalyst achieves an NH₃ production rate of 79.86 ± 3.48 μmol g^-1 h^-1 with nearly 100% selectivity, a 16-fold improvement over pristine TiO₂. This study offers valuable insights into solar-driven gas-solid NH₃ synthesis from flue NO<i>x</i>.