Search

Graphical abstract Highlights • Reduced graphene oxide and N-doped graphene were prepared by microwave irradiation. • Surface engineered graphene exhibited high catalytic ozonation activity. • Microwave reduction created more edging sites and graphitic nitrogen species. • Diverse reactive species, O 2 −, OH and 1O 2 , were possible for 4-nitrophenol degradation. Abstract N-doped graphene has demonstrated exceptional activities in versatile metal-free catalytic processes. In this study, reduced graphene oxide (rGO) and N-doped rGO were synthesized by a facile approach via microwave reduction with a low energy input and short reaction time. The activities of the derived carbocatalysts were evaluated by catalytic ozonation of 4-nitrophenol (4-NP). Compared with thermally annealed rGOs in argon atmosphere, microwave treated rGO demonstrated a better performance in catalytic oxidation, and N-doping would further improve the catalytic activity. It is discovered that microwave irradiation not only gave rise to more edging sites and dangling bonds in rGO, making higher catalytic potentials for ozone decomposition than that from thermal annealing, but also resulted in a higher concentration of N dopants. XPS studies revealed that more graphitic N species were incorporated into the carbon basal plane during the microwave reduction processes. The reactive oxygen species (ROS) in 4-NP oxidation were evaluated and identified by liquid-phase electron spin resonance (ESR) and radical scavenging tests, which indicated the generation of O 2 −, OH and 1O 2 for 4-NP degradation. This study provides a facile protocol for fabricating advanced nanocarbon materials for green oxidation and enables new insights in catalytic ozonation with state-of-the-art carbocatalysis. [ABSTRACT FROM AUTHOR]

Highlights: [•] Higher numbers of basic sites on AC benefited the oxalate ozonation. [•] Pyridine groups on AC possibly decreased the surface oxidation of oxalate. [•] Pyrrole groups on AC may enhance HO generation in bulk solution. [•] Basic oxygen-containing groups and basal planes on AC enhanced surface oxidation. [•] Basicity of AC indirectly contribute to the weakened HO oxidation in bulk solution. [ABSTRACT FROM AUTHOR]

Electro-peroxone process utilizes the in-situ generated H 2 O 2 by electro-reduction of O 2 to react with ozone molecule to produce hydroxyl radical, and achieves high efficiency in the wastewater treatment. However, detailed reaction mechanism in this process is still deficient. In this paper, a three-electrode system was adopted in the electro-peroxone process, and the origin of the synergy was revealed with the aid of the rotating ring disk electrodes. It was found that besides the main reaction between O 3 and H 2 O 2 , O 3 is also electro-reduced to O 3 − and then transformed into OH. Meanwhile, O 3 electro-reduction on the cathode can also possibly generate O 2 and promote H 2 O 2 production. Higher concentration of OH − near the cathode originated from O 2 and O 3 reduction may also react with O 3 to produce H 2 O 2 . These two pathways contributed to the higher amount of H 2 O 2 than the system with O 2 bubbling only, and ultimately contributed to produce more OH. All these reactions are synthetically responsible for the higher degradation rate of sodium oxalate in the electro-peroxone process than those in O 3 -electrolysis and ozonation processes, respectively. Though this system was quite different from normal electro-peroxone process, the conclusions are useful for understanding the practical reactions possibly happened on electrodes in the electro-peoxone process. [ABSTRACT FROM AUTHOR]

• Global softness of the NHCs was employed to correlate with the ozonation efficiency. • Higher global softness of NHCs resulted in a greater degradation efficiency. • Fewer N atoms in heterocyclic ring generated more OH from ozone dissociation. • Reactive sites for both O 3 and radical attack were located by FO theory. • Combined DFT and TOF-MS results were used to identify the hydroxylation products. Nitrogen-containing heterocyclic contaminants (NHCs) have aroused intense environmental issues in waterbody because of their high toxicity and strong recalcitrance against natural degradation. In this study, ozonation degradation on four typical NHCs - benzotriazole (BTA), benzimidazole (BMZ), indazole (IDZ), and indole (IDO) was carried out to investigate the effects of N atoms on degradation efficiency. Global softness of the NHCs was calculated by density functional theory (DFT) and correlated with ozonation efficiency. It was discovered that higher global softness of the NHCs resulted in the greater degradation efficiency. Mineralization results together with the quenching tests results suggested that NHCs with ortho-positioned N atoms in heterocyclic ring are easier to be mineralized, while the presence of para-positioned N atoms increased the recalcitrance for destruction. Apart from hydroxyl radicals, superoxide radicals and singlet oxygens were also identified by electron paramagnetic resonance (EPR) spectra. Condensed Fukui index was employed to probe the potential active sites for both direct ozone oxidation and radical-based attack. Single point energies were further calculated among these active sites to seek the initial-stage hydroxylation intermediates. Combined the calculation results with the identified intermediates from time-of-flight mass spectroscopy (TOF-MS), degradation routes of the NHCs were proposed. This study discovered structure-dependent behavior of NHCs on ozonation efficiency and envisaged a more accurate strategy for establishing the degradation pathway. [ABSTRACT FROM AUTHOR]