Is CN Chemically Stable toward Reactive Oxygen Species in Sunlight-Driven Water Treatment?

Reactive oxygen species (ROS) are key oxidants for the degradation of organic pollutants in sunlight-driven photocatalytic water treatment, but their interaction with the photocatalyst is easily ignored and, hence, is comparatively poorly understood. Here we show that graphitic carbon nitride (CN, a famous visible-light-responsive photocatalyst) is chemically stable toward ozone and superoxide radical; in contrast, hydroxyl radical (OH) can tear the heptazine unit directly from CN to form cyameluric acid and further release nitrates into the aqueous environment. The ratios of released nitrogen from nanosheet-structured CN and bulk CN that finally exists in the form of NO reach 9.5 and 6.8 mol % in initially ultrapure water, respectively, after 10 h treatment by solar photocatalytic ozonation, which can rapidly generate abundant OH to attack CN. On a positive note, in the presence of organic pollutants which compete against CN for OH, the CN decomposition has been completely or partially blocked; therefore, the stability of CN under practical working conditions has been obviously preserved. This work supplements the missing knowledge of the chemical instability of CN toward OH and calls for attention to the potential deactivation and secondary pollution of catalysts in OH-involved water treatment processes.