Heterogeneous catalytic ozonation (HCO) processes have been widely studied for water purification. The reaction mechanisms of these processes are very complicated because of the simultaneous involvement of gas, solid, and liquid phases. Although typical reaction mechanisms have been established for HCO, some of them are only appropriate for specific systems. The divergence and deficiency in mechanisms hinders the development of novel active catalysts. This critical review compares the various existing mechanisms and categorizes the catalytic oxidation of HCO into radical-based oxidation and nonradical oxidation processes with an in-depth discussion. The catalytic active sites and adsorption behaviors of O 3 molecules on the catalyst surface are regarded as the key clues for further elucidating the O 3 activation processes, evolution of reactive oxygen species (ROS) or organic oxidation pathways. Moreover, the detection methods of the ROS produced in both types of oxidations and their roles in the destruction of organics are reviewed with discussion of some specific problems among them, including the scavengers selection, experiment results analysis as well as some questionable conclusions. Finally, alternative strategies for the systematic investigation of the HCO mechanism and the prospects for future studies are envisaged.

Metal-containing Fenton catalysts have been widely investigated. Here, we report for the first time a highly effective stable metal-free Fenton-like catalyst with dual reaction centers consisting of 4-phenoxyphenol-functionalized reduced graphene oxide nanosheets (POP-rGO NSs) prepared through surface complexation and copolymerization. Experimental and theoretical studies verified that dual reaction centers are formed on the C-O-C bridge of POP-rGO NSs. The electron-rich center around O is responsible for the efficient reduction of HO to OH, while the electron-poor center around C captures electrons from the adsorbed pollutants and diverts them to the electron-rich area via the C-O-C bridge. By these processes, pollutants are degraded and mineralized quickly in a wide pH range, and a higher HO utilization efficiency is achieved. Our findings address the problems of the classical Fenton reaction and are useful for the development of efficient Fenton-like catalysts using organic polymers for different fields.

Carbon nitride compounds (CN) complexed with the in-situ-produced Cu(II) on the surface of CuAlO substrate (CN-Cu(II)-CuAlO) is prepared via a surface growth process for the first time and exhibits exceptionally high activity and efficiency for the degradation of the refractory pollutants in water through a Fenton-like process in a wide pH range. The reaction rate for bisphenol A removal is ∼25 times higher than that of the CuAlO. According to the characterization, Cu(II) generation on the surface of CuAlO during the surface growth process results in the marked decrease of the surface oxygen vacancies and the formation of the C-O-Cu bridges between CN and Cu(II)-CuAlO in the catalyst. The electron paramagnetic resonance (EPR) analysis and density functional theory (DFT) calculations demonstrate that the dual reaction centers are produced around the Cu and C sites due to the cation-π interactions through the C-O-Cu bridges in CN-Cu(II)-CuAlO. During the Fenton-like reactions, the electron-rich center around Cu is responsible for the efficient reduction of HO to OH, and the electron-poor center around C captures electrons from HO or pollutants and diverts them to the electron-rich area via the C-O-Cu bridge. Thus, the catalyst exhibits excellent catalytic performance for the refractory pollutant degradation. This study can deepen our understanding on the enhanced Fenton reactivity for water purification through functionalizing with organic solid-phase ligands on the catalyst surface.

The application of the classical Fenton reaction has long been limited by several problems, such as metallic sludge and narrow pH range, which derived from the metal components in the catalyst. Developing a metal-free Fenton catalyst may efficiently address these problems. Here, we firstly perform a density functional theory (DFT) study to explore the possibility of developing the 4-phenoxyphenol moleculedoped reduced graphene oxide nanocomposite (rGO-4-PP Nc) as a metal-free Fenton-like catalyst by tuning the electron distribution. The theoretical calculation results reveal that rGO-4-PP Nc can act as an efficient Fenton-like catalyst for H 2 O 2 activation and pollutant degradation through formation of electron-rich O and electron-deficient C centers on the C-O-C bridge. The actual rGO-4-PP Nc is also prepared via a surface complexation and copolymerization process. The experimental evidence, such as that gained from XRD, FIIR and EPR analysis, confirm the theoretical models and the dual-reactioncenter Fenton-like mechanism. This work provides a basis for theoretical calculation to guide the actual synthesis and prediction of catalytic activity of the Fenton-like catalysts, and also offers a creative perspective to develop new generation metal-free Fenton catalysts by tuning the electron distribution using organic polymers.

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N-doped defective nanocarbon (N-DNC) catalysts have been widely studied due to their exceptional catalytic activity in many applications, but the O 3 activation mechanism in catalytic ozonation of N-DNCs has yet to be established. In this study, we systematically mapped out the detailed reaction pathways of O 3 activation on 10 potential active sites of 8 representative configurations of N-DNCs, including the pyridinic N, pyrrolic N, N on edge, and porphyrinic N, based on the results of density functional theory (DFT) calculations. The DFT results indicate that O 3 decomposes into an adsorbed atomic oxygen species (O ads ) and an 3 O 2 on the active sites. The atomic charge and spin population on the O ads species indicate that it may not only act as an initiator for generating reactive oxygen species (ROS) but also directly attack the organics on the pyrrolic N. On the N site and C site of the N 4 V 2 system (quadri-pyridinic N with two vacancies) and the pyridinic N site at edge, O 3 could be activated into 1 O 2 in addition to 3 O 2 . The N 4 V 2 system was predicted to have the best activity among the N-DNCs studied. Based on the DFT results, machine learning models were utilized to correlate the O 3 activation activity with the local and global properties of the catalyst surfaces. Among the models, XGBoost performed the best, with the condensed dual descriptor being the most important feature.

Dual reaction centers are formed between C–O–Cu bonding bridges of Cu-MP NCs owing to cation–π interactions. H2O2 is reduced to ˙OH on electron-rich Cu centers, while pollutants are degraded on electron-deficient centers.

Hydroxyl radical-dominated oxidation in catalytic ozonation is, in particular, important in water treatment scenarios for removing organic contaminants, but the mechanism about ozone-based radical oxidation processes is still unclear. Here, we prepared a series of transitional metal (Co, Mn, Ni) single-atom catalysts (SACs) anchored on graphitic carbon nitride to accelerate ozone decomposition and produce highly reactive •OH for oxidative destruction of a water pollutant, oxalic acid (OA). We experimentally observed that, depending on the metal type, OA oxidation occurred dominantly either in the bulk phase, which was the case for the Mn catalyst, or via a combination of the bulk phase and surface reaction, which was the case for the Co catalyst. We further performed density functional theory simulations and in situ X-ray absorption spectroscopy to propose that the ozone activation pathway differs depending on the oxygen binding energy of metal, primarily due to differential adsorption of O 3 onto metal sites and differential coordination configuration of a key intermediate species, *OO, which is collectively responsible for the observed differences in oxidation mechanisms and kinetics.