Chemical instability of graphene oxide following exposure to highly reactive radicals in advanced oxidation processes

Two-dimensional layered carbon-based catalytic ozonation for water purification: Rational design of catalysts and an in-depth understanding of the interfacial reaction mechanism

This review renewed insight into the existing complex and contradictory mechanisms of catalytic ozonation by two-dimensional layered carbon-based materials (2D-LCMs) for degradation toxic refractory organics in aqueous solution. Migration and capture of active electrons are central to catalytic ozonation reactions, which was not studied or reviewed more clearly. Based on this perspective, the catalytic ozonation potential of 2D-LCMs synthesized by numerous methods is firstly contrasted to guide the design of subsequent carbon based-catalysts, and not limited to 2D-LCMs. Matching ROS to active sites is a key step in understanding the catalytic mechanism. The structure-activity relationships between reported numerous active sites and ROS evolution is then constructed. Result showed that OH could be produced by -OH, -C=O, -COOH groups, defective sites, immobilized metal atoms, doped heteroatoms and photo-induced electrons; and O₂^- could be produced by -OH groups and sp²-bonded carbon. The normalized model further be used to visually compare the contribution degree of various regulatory methods to performance improvement. More importantly, this review calls for 2D-LCMs-based catalytic ozonation to be studied without circumventing the issue of structural stability, which would lead to many proposals of catalysts and its involved catalytic reaction mechanism being meaningless.

Piezoelectric polarization modulated novel Bi2WO6/g-C3N4/ZnO Z-scheme heterojunctions with g-C3N4 intermediate layer for efficient piezo-photocatalytic decomposition of harmful organic pollutants

It is of great significance to understand the role of carrier in piezocatalysis of composites by studying the separation mode of carriers under dynamic polarization field. Herein, the separation and migration pathways of carriers under piezoelectric field are investigated by synthesizing heterojunctions with Bi₂WO₆ (BWO) nanosheets grown vertically on g-C₃N₄ (CN) coated ZnO nanorods and directly on ZnO. Compared with the photocatalysis, the piezocatalytic efficiency of Rhodamine B (RhB) by BWO/ZnO is significantly increased to 0.121 min^-1, which indicated the polarization field promotes band tilt and Z-scheme formation. After introducing the CN interlayer, the piezocatalytic efficiency of BWO/CN/ZnO is further improved (0.217 min^-1), which can be attributed to the unique core-shell structure with Z-scheme heterojunctions. This unique structure provides more active sites and excited carrier concentration, the intermediate layer CN also reduces the direct contact and recombination of electrons and holes controlled by polarization potential at the interface between BWO and ZnO. This work deeply analyzes the influence of carrier concentration, separation efficiency and transport process on piezocatalysis, which provides a reference for the design of efficient catalysts.

Advanced activation of persulfate by polymeric g-C3N4 based photocatalysts for environmental remediation: A review

Photocatalytic materials for photocatalysis is recently proposed as a promising strategy to address environmental remediation. Metal-free graphitic carbon nitride (g-C₃N₄), is an emerging photocatalyst in sulfate radical based advanced oxidation processes. The solar-driven electronic excitations in g-C₃N₄ are capable of peroxo (O‒O) bond dissociation in peroxymonosulfate/peroxydisulfate (PMS/PDS) and oxidants to generate reactive free radicals, namely SO₄^•- and OH^• in addition to O₂^•- radical. The synergistic mechanism of g-C₃N₄ mediated PMS/PDS photocatalytic activation, could ensure the generation of OH^• radicals to overcome the low reductive potential of g-C₃N₄ and fastens the degradation reaction rate. This article reviews recent work on heterojunction formation (type-II heterojunction and direct Z-scheme) to achieve the bandgap for extended visible light absorption and improved charge carrier separation for efficient photocatalytic efficiency. Focus is placed on the fundamental mechanistic routes followed for PMS/PDS photocatalytic activation over g-C₃N₄-based photocatalysts. A particular emphasis is given to the factors influencing the PMS/PDS photocatalytic activation mechanism and the contribution of SO₄^•- and OH^• radicals that are not thoroughly investigated and require further studies. Concluding perspectives on the challenges and opportunities to design highly efficient persulfate-activated g-C₃N₄ based photocatalysts toward environmental remediation are also intensively highlighted.

Deactivation and regeneration of carbon nanotubes and nitrogen-doped carbon nanotubes in catalytic peroxymonosulfate activation for phenol degradation: variation of surface functionalities

The reuse, deactivation and regeneration of carbon nanotubes (CNT) and N-doped carbon nanotubes (NCNT) were studied in catalytic peroxymonosulfate (PMS) activation for phenol degradation. The results showed that for catalytic PMS activation, marked deactivation was observed on both CNT and NCNT, resulting in marked variation of the surface functionalities of the catalysts. Catalytic PMS activation led to markedly increased oxygen-containing functionalities and decreased points of zero charge (PZCs) of CNT and NCNT. The catalytic activity of CNT was strongly dependent on the initial PMS concentration but was independent of the initial phenol concentration. Furthermore, the dependency of the CNT activity on the initial PMS concentration closely followed the Langmuir-Hinshelwood model, indicating that the catalytic activation of adsorbed PMS was the rate controlling step. For the used CNT and NCNT, chemical reduction by NaBH4 or thermal treatment regeneration under inert atmosphere could effectively remove surface O-containing functionalities and enhance PZCs, restoring their catalytic activities; meanwhile, the N-containing functionalities of NCNT decreased with regeneration treatment, resulting in a negative impact on catalyst regeneration. The present findings indicate that surface functionalities are closely correlated with catalyst deactivation and regeneration, playing crucial roles in the catalytic activation of PMS.

Nanoparticles of magnetite anchored onto few-layer graphene: A highly efficient Fenton-like nanocomposite catalyst

Developing a catalyst with high efficiency and recyclability is an important issue for the heterogeneous Fenton-like systems. In this study, magnetic Fe₃O₄ and reduced graphene oxide (RGO) nanocomposites were prepared by a facile alkaline-thermal precipitation method and employed as a highly effective heterogeneous Fenton-like catalyst for methyl orange (MO) degradation. Characterization of these nanocomposites by XRD, FTIR, Raman, FESEM and TEM revealed that nanoparticles (NPs) of Fe₃O₄ were tightly anchored on the few-layer RGO sheets. The anchoring of Fe₃O₄ NPs and the reduction of GO were achieved in one pot without adding any other reducing agents. Based on the measurements of GO surface Zeta potentials, a possible anchoring mechanism of Fe₃O₄ NPs onto RGO sheets was given. The Fe₃O₄/RGO nanocomposites exhibited much higher Fenton-like catalytic efficiency for MO degradation than pure Fe₃O₄ NPs. This degradation process followed the first-order kinetics model, where k₁ and T complied with the Arrhenius equation with E_a of 12.79 kJ/mol and A of 8.20 s^-1. Magnetic measurements revealed that Fe₃O₄/RGO nanocomposites were ferromagnetic as indicated by the presence of magnetic hysteresis loops. The Fe₃O₄/RGO nanocomposites showed good stability and recyclability. Hydroxyl radicals, OH were determined as the dominant oxidative species in Fe₃O₄/RGO-H₂O₂ system and the Fenton-like mechanism for MO degradation in water was proposed and discussed.

Is C3N4 Chemically Stable toward Reactive Oxygen Species in Sunlight-Driven Water Treatment?

Reactive oxygen species (ROS) are key oxidants for the degradation of organic pollutants in sunlight-driven photocatalytic water treatment, but their interaction with the photocatalyst is easily ignored and, hence, is comparatively poorly understood. Here we show that graphitic carbon nitride (C₃N₄, a famous visible-light-responsive photocatalyst) is chemically stable toward ozone and superoxide radical; in contrast, hydroxyl radical (^•OH) can tear the heptazine unit directly from C₃N₄ to form cyameluric acid and further release nitrates into the aqueous environment. The ratios of released nitrogen from nanosheet-structured C₃N₄ and bulk C₃N₄ that finally exists in the form of NO₃^- reach 9.5 and 6.8 mol % in initially ultrapure water, respectively, after 10 h treatment by solar photocatalytic ozonation, which can rapidly generate abundant ^•OH to attack C₃N₄. On a positive note, in the presence of organic pollutants which compete against C₃N₄ for ^•OH, the C₃N₄ decomposition has been completely or partially blocked; therefore, the stability of C₃N₄ under practical working conditions has been obviously preserved. This work supplements the missing knowledge of the chemical instability of C₃N₄ toward ^•OH and calls for attention to the potential deactivation and secondary pollution of catalysts in ^•OH-involved water treatment processes.