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

Photo/electrocatalytic ammonia (NH3) synthesis via nitrous oxide (NOx) 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 (PVO) 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-PVO 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 NOx.