How g-C₃N₄ Works and Is Different from TiO₂ as an Environmental Photocatalyst: Mechanistic View

Graphitic carbon nitride (CN) as a popular visible light photocatalyst needs to be better understood for environmental applications. The behaviors of CN as an environmental photocatalyst were systematically studied in comparison with a well-known TiO₂ photocatalyst. The two photocatalysts exhibit different photocatalytic oxidation (PCO) behaviors and dependences on the experimental conditions (e.g., pH, Pt loading, and the kind of organic substrate and scavenger). The PCO of organic substrates was significantly enhanced by loading Pt on TiO₂ under UV light (λ > 320 nm), whereas Pt-CN exhibited a lower PCO activity than bare CN under visible light (λ > 420 nm). While the presence of Pt enhances the charge separation in both TiO₂/UV and CN/visible light systems (confirmed by transient IR absorption spectroscopic analysis), the opposite effects of Pt are ascribed to the different mechanisms of ^•OH generation in the two photocatalytic systems. The negative effect of Pt on CN is ascribed to the fact that Pt catalytically decomposes in situ-generated H₂O₂ (a main precursor of OH radical), which hinders ^•OH production. The production of OH radicals on CN is favored only at acidic pH but ¹O₂ generation is dominant in alkaline pH. The pH-dependent behaviors of reactive oxygen species generation on CN were confirmed by electron paramagnetic resonance spin trap measurements.