Photo-accelerated Co3+/Co2+ transformation on cobalt and phosphorus co-doped g-C3N4 for Fenton-like reaction

Abstract

Peroxydisulfate (PDS) can be activated by graphitic carbon nitride (g-C₃N₄) under irradiation, and the generated oxygen reactive species (ROSs) can degrade organic pollutants effectively. However, the photocatalytic activity of pristine g-C₃N₄ is limited due to its narrow light absorption range and rapid electron–hole recombination. Herein, cobalt and phosphorus co-doped g-C₃N₄ (Co, P-CN) was synthesized by a facile one-pot pyrolysis method. Co and P co-doping enhances the light absorption and charge separation, thus greatly promoting the photocatalytic activation of PDS. Additionally, the presence of doped Co²⁺ can chemically activate PDS, with Co²⁺ transforming into Co³⁺ simultaneously. Recycling tests reveal that Co²⁺ in Co, P-CN can maintain a high level attributing to the fast Co³⁺/Co²⁺ transformation rate under light irradiation. The density functional theory (DFT) calculations show that the Co–N_x species formed by chemical coordination between Co and g-C₃N₄ in Co, P-CN could effectively activate PDS. The synergistic effect of photo-catalytic and chemical-catalytic driven PDS activation and a faster Co³⁺/Co²⁺ redox cycle result in the enhanced bisphenol A (BPA) degradation with a reaction rate constant (k) of 0.375 min⁻¹, which is 7 times larger than that of pristine g-C₃N₄. This work provides an effective method for the in situ regeneration of active low-valent transition metal ions, and reveals the mechanism of synergistic activation of PDS by photo-catalysis and chemical-catalysis for pollutant degradation.