Metal-free catalysts of graphitic carbon nitride–covalent organic frameworks for efficient pollutant destruction in water

Abstract

Novel metal-free catalysts via integration of covalent organic framework (COF) and graphitic carbon nitride (g-C3N4@COF) with a high graphitization degree and nitrogen content were fabricated and exhibited an outstanding activity in a wide pH range for peroxymonosulfate (PMS)-driven oxidation of refractory organic pollutants in water. Scanning electron microscopy images showed many aggregated COFs crystals anchored on the irregular g-C3N4 surface to form 3D structures. The precursors (urea, melamine, and dicyandiamide) of g-C3N4 determined the porous structures and properties of the g-C3N4@COF materials. The hybrids possessed superior reactivity in Orange II removal (100%) compared to pristine g-C3N4 (10%) and COF (5%), benefiting from high-temperature pyrolysis to generate crystal carbon and modulate nitrogen doping. Besides, removal efficiency of target pollutants depended on the oxidant dosages (0.33–1.30 mM), initial concentrations of organics (10–40 mg/L), temperatures (5–45 °C), pHs (1.72–10.3), and anions (Cl−, SO42-, NO3-, HCO3-, CO32-, and HCOO-). Quenching experiments and electron paramagnetic resonance demonstrated that non-radical singlet oxygen (1O2) was the dominant species for the oxidation of organic pollutants via electron transfer in the g-C3N4@COF/PMS system. It was inferred that the good balance between graphitization degree and nitrogen content benefited to enhancing the catalytic performance for the refractory pollutant degradation. The present investigation provides a new avenue for the design and construction of metal-free hybrid composites for environmental remediation.