Occurrence of both hydroxyl radical and surface oxidation pathways in N-doped layered nanocarbons for aqueous catalytic ozonation

Abstract

Metal-free catalysts such as N-doped nanocarbons are sustainable alternatives to metal-based catalysts for the degradation of persistent organic pollutants (POPs), but cost-efficient methods are required for their large-scale synthesis. In this study, a facile and scalable strategy was established for synthesizing layered N-doped nanocarbons via the pyrolysis of β-cyclodextrin (β-CD) and melamine in a N₂ atmosphere. Compared with undoped pyrolyzed β-CD, N-doping led to a 30.3-fold enhancement in the pseudo-first-order rate constant for catalytic ozonation of oxalic acid (OA), and complete degradation of 50 mg/L OA was achieved in 45 min. Apart from the specific surface area boosting from 78.9 to 16.2 m²/g after N doping, the OA degradation results and material characterization also suggested that quaternary N was the main active site, which was further validated by density functional theory (DFT) simulations. DFT simulations also suggested that C atoms with high charge densities adjacent to N dopants exhibited considerable potential for the catalytic dissociation of ozone. Electron paramagnetic resonance (EPR), radical quenching, and in situ Raman studies indicated occurrence of surface oxidation pathway apart from the radical-based one. Ozone on the catalyst surface was decomposed into surface-adsorbed atomic oxygen (*O(ad)) and free peroxide (*O₂ (free)). Both *O(ad) and OH, which was further evolved on the surface or in bulk solution, contributed to OA destruction. These insights into the catalytic ozonation mechanism on N-doped nanocarbons will advance their practical application to the catalytic degradation of organic pollutants.