Metal-free activation of peroxymonosulfate by boron and nitrogen co-doped graphene nanotubes for catalytic oxidation of 4-hydroxybenzoic acid

Abstract

Recently metal-free catalysts become very promising in environmental catalysis own to the nature of being free of metals for avoiding metal leaching-related secondary contamination. Herein, a series of boron and nitrogen co-doped graphene nanotubes were first synthesised by thermal treatment of urea, boric acid, and polyethene glycol (PEG, 2000). The materials fabricated under varied thermal conditions, e.g., different pyrolysis temperature and retention time, were characterised through advanced physiochemical techniques. The as-prepared materials showed outstanding catalytic activity for degradation of 4-hydroxybenzoic acid (HBA) via peroxymonosulfate (PMS) activation, whereas the catalyst pyrolysed at 1100 ​°C for 6 ​h (BNG-1100-6h) was found to be the best candidate for environmental remediation, thanks to its engineered surface, exposed active sites, and well-tuned functional groups. Based on the optimal carbocatalyst, reaction conditions such as catalyst loading, PMS dosage, solution pH, and reaction temperature were thoroughly investigated to make it a cost-effective catalytic system. A thermal regenerative path was adopted to enhance the catalyst stability and reusability. Quenching tests and electron paramagnetic resonance (EPR) spectroscopic analysis further revealed the dominant role of singlet oxygen (¹O₂), a non-radical reactive species, in the degradation of HBA. The current research work will not only provide a facile strategy for development of a carbocatalytic system but also open a new perspective for degradation of emerging contaminants such as HBA via a non-radical route.