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

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 (ID/IG = 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.