Singlet Oxygen Formation Mechanism for the H2O2-Based Fenton-like Reaction Catalyzed by the Carbon Nitride Homojunction.

The singlet oxygen (1O2) oxidation process activated by metal-free catalysts has recently attracted considerable attention for organic pollutant degradation; however, the 1O2 formation remains controversial. Simultaneously, the catalytic activity of the metal-free catalyst limits the practical application. In this study, carbon nitride (HCCN) containing an intramolecular homojunction, a kind of metal-free catalyst, exhibits excellent activity compared to g-C3N4 (CN) and crystalline carbon nitride (HCN) for tetracycline hydrochloride degradation through the H2O2-based Fenton-like reaction. The rate constant for HCCN increased about 16.1 and 8.9 times than that of CN and HCN, respectively. The activity of HCCN was enhanced, and the dominant reactive oxygen species (ROS) changed from hydroxyl radicals (•OH) to 1O2 with an increase in pH from 4.5 to 11.5. A novel formation pathway of 1O2 was revealed. This result is different from the normal reference, in which •OH is always the primary ROS in the H2O2-based Fenton-like reaction. This study may provide a possible strategy for the investigation on the nonradical oxidation process in the Fenton-like reaction.