Catalytic Ozonation of Ketoprofen with In Situ N-Doped Carbon: A Novel Synergetic Mechanism of Hydroxyl Radical Oxidation and an Intra-Electron-Transfer Nonradical Reaction

Abstract

A novel synergetic mechanism of hydroxyl radical (^•OH) oxidation and an intra-electron-transfer nonradical reaction was found in the catalytic ozonation of ketoprofen (KTP) with the in situ N-doped hollow sphere carbon (NHC). Outperforming the conventional ^•OH-based catalytic ozonation process, O₃/NHC not only realized an enhancement of the pseudo-first-order rate constant of 11 times in comparison with that of O₃ alone, but was also endowed with a high stability over a wide pH (4-9) and temperature (15-35 °C) range for the degradation of KTP. The high graphitization degree (<i>I</i>_D/<i>I</i>_G = 0.78-0.88) and low unsaturated oxygen content (0.10-1.38%) of NHC highlighted the dominant role of N-heteroatoms in the O₃/NHC system. The specific effects of different N species were confirmed by a relationship study (N property vs catalytic activity) and X-ray photoelectron spectroscopy characterization. The graphitic N forming in the bulk of the graphitic structure served as the "electron-mobility" region to promote KTP degradation with the transfer of electrons from the KTP molecule to O₃ via a nonradical reaction process. The pyrrolic and pyridinic N located at defects of the graphitic structure acted as the "radical-generation" region to decompose O₃ into ^•OH for degrading KTP by a radical oxidation process. This finding provided a brand new insight into engineering N-doped carbonaceous catalysts precisely in the catalytic ozonation process for the efficient treatment of organic-contaminated water.