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 gcat -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.