Air‐Level Oxygen Enables 100% Selectivity in Urea Synthesis via Photocatalytic C─N Coupling of CO and Ammonia

The clean-energy-driven synthesis of urea from carbon- and nitrogen-containing small molecules has garnered significant interest but remained great challenges to achieve with high selectivity. Herein, we present a photocatalytic pathway for the selective urea synthesis through the oxidative coupling between CO and NH₃. The key factor in this process is the atmospheric O₂ level, which plays a crucial role in controlling both the urea production rate and its selectivity. Using oxygen-deficient TiO₂ under an air-level (20%) O₂ atmosphere, we achieved a urea generation rate of 54.31 mg g_cat ^-1 h^-1 with 100% selectivity. This rate is 38.52 times higher than under oxygen-free conditions, while further increasing the O₂ level significantly reduces selectivity. Mechanistic studies reveal that the process begins with the oxidation of NH₃ to •NH₂ through oxidative radicals generated on TiO₂, especially the oxygen-derived O₂ ^•-. This •NH₂ radicals then couple with CO to form urea. The concentration of radicals is controlled by the O₂ level, with the optimal concentration under air-level O₂ enabling efficient NH₃ oxidation to •NH₂ while preventing over-oxidation.