Fast Electron Transfer and•OH Formation: Key Features for High Activity in Visible-Light-Driven Ozonation with C₃N₄Catalysts

Photocatalytic ozonation of wastewater pollutants by sunlight is a highly attractive technology close to real application. Understanding this process on the atomic scale and under realistic working conditions is challenging but vital for the rational design of catalysts and photocatalytic decontamination systems. Here we study two highly active C 3 N 4 photocatalysts (bulk C 3 N 4 and a nanosheet-structured C 3 N 4 ) under simultaneous visible-light irradiation and O 3 bubbling in water by in situ EPR spectroscopy coupled with an online spin-trapping technique. The photoexcitation of electrons to the conduction band (CB-e – ), their further trapping by dissolved O 2 and O 3, and the evolution of reactive oxygen species (ROS) have been semiquantitatively visualized. A dual role of O 3 in boosting the CB-e – to • OH conversion is confirmed: (i) an inlet 2.1 mol % O 3 /O 2 gas mixture can trap about 2–3 times more CB-e – upon aqueous C 3 N 4 suspension than pure O 2 and further produce • OH by a robust • O 3 – -mediated one-electron-reduction pathway (O 3 → • O 3 – → HO 3 • → • OH); (ii) O 3 can readily take CB-e – back from • O 2 – to form • O 3 –, thus blocking the inefficient H 2 O 2 -mediated three-electron-reduction route (O 2 → • O 2 – → HO 2 • → H 2 O 2 → • OH) but further strengthening the • O 3 – -mediated pathway. In the presence of 2.1 mol % O 3 /O 2, the • OH yield increases by 17 and 5 times, and consequently, the mineralization rate constant of oxalic acid increases by 84 and 41 times over bulk C 3 N 4 and NS C 3 N 4, respectively. This work presents an attractive opportunity to boost the yield of ROS species ( • OH) for water purification by visible-light-driven photocatalysis and provides a powerful tool to monitor complex photocatalytic reactions under practical conditions.