Synergistically Enhanced Oxygen Evolution Catalysis with Surface Modified Halloysite Nanotube

Synergistically increased oxygen evolution reaction (OER) of manganese oxide (MnO₂) catalyst is introduced with surface-modified halloysite nanotube (Fe₃O₄-HNTs) structure. The flake shaped MnO₂ catalyst is attached on the nanotube template (Fe₃O₄-HNTs) by series of wet chemical and hydrothermal method. The strong interaction between MnO₂ and Fe₃O₄-HNTs maximized active surface area and inter-connectivity for festinate charge transfer reaction for OER. The synergistical effect between Fe₃O₄ layer and MnO₂ catalyst enhance the Mn³⁺/Mn⁴⁺ ratio by partial replacement of Mn ions with Fe. The relatively increased Mn^3+/Mn⁴⁺ ratio on MnO₂@FHNTs induced <italic>σ</italic><italic>^*</italic> orbital (e_g) occupation close to single electron, improving the OER performances. The MnO₂@FHNTs catalyst exhibited the reduced overpotential of 0.42 V (E <italic>vs</italic>. RHE) at 10 mA/cm² and Tafel slope of (99 mV/dec), compared with that of MnO₂ with unmodified HNTs (0.65 V, 219 mV/dec) and pristine MnO₂ (0.53 V, 205 mV/dec). The present study provides simple and innovative method to fabricate nano fiberized OER catalyst for a broad application of energy conversion and storage systems.

Technology Advances in Phenol Removals: Current Progress and Future Perspectives

Phenol acts as a pollutant even at very low concentrations in water. It is classified as one of the main priority pollutants that need to be treated before being discharged into the environment. If phenolic-based compounds are discharged into the environment without any treatments, they pose serious health risks to humans, animals, and aquatic systems. This review emphasizes the development of advanced technologies for phenol removal. Several technologies have been developed to remove phenol to prevent environmental pollution, such as biological treatment, conventional technologies, and advanced technologies. Among these technologies, heterogeneous catalytic ozonation has received great attention as an effective, environmentally friendly, and sustainable process for the degradation of phenolic-based compounds, which can overcome some of the disadvantages of other technologies. Recently, zeolites have been widely used as one of the most promising catalysts in the heterogeneous catalytic ozonation process to degrade phenol and its derivatives because they provide a large specific surface area, high active site density, and excellent shape-selective properties as a catalyst. Rational design of zeolite-based catalysts with various synthesis methods and pre-defined physiochemical properties including framework, ratio of silica to alumina (SiO2/Al2O3), specific surface area, size, and porosity, must be considered to understand the reaction mechanism of phenol removal. Ultimately, recommendations for future research related to the application of catalytic ozonation technology using a zeolite-based catalyst for phenol removal are also described.

Reactive Oxygen Species and Catalytic Active Sites in Heterogeneous Catalytic Ozonation for Water Purification

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 O₃ molecules on the catalyst surface are regarded as the key clues for further elucidating the O₃ 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.

Advanced landfill leachate biochemical effluent treatment using Fe-Mn/AC activates O3/Na2S2O8 process: process optimization, wastewater quality analysis, and activator characterization

A novel catalyst of Fe-Mn/AC was prepared and used as a heterogeneous catalyst to activate O₃/Na₂S₂O₈ for landfill leachate biochemical effluent treatment. The experimental results indicated that the highest COD (84%) and color (98%) removal was obtained at Fe-Mn/AC dosage 1.2 g/L, O₃ concentration 1.2 g/L, Na₂S₂O₈ dosage 6 g/L, initial pH 10, and reaction time 100 min. Three-dimensional and excitation emission matrix (3D-EEM) fluorescence spectrometry, Fourier transform infrared spectroscopy (FTIR), and gas chromatography mass spectrometry (GC/MS) of wastewater samples before and after treatment demonstrated that the leachate biochemical effluent contained a large amount of humic and fulvic acid organic compounds. After treatment with this coupling system, both the pollution level of dissolved organic matter (DOM) and the fluorescence intensity declined. The micro morphology of Fe-Mn/AC was characterized using scanning X-ray diffraction patterns (XRD), electron microscope spectra (SEM), X-ray photoelectron spectroscopy (XPS), and Fourier transform infrared (FTIR) spectroscopy. It can be concluded that the microscopic morphology of the catalyst is porous. The main active components are amorphous MnO₂ and multivalent iron oxides. Furthermore, the Fe-Mn/AC catalyst showed great reusability; the removal efficiency of COD was only reduced from 84% to 79% at the fourth reaction. Moreover, the COD removal efficiency could recover to 81% after catalyst regeneration.

In Situ Synthesis of Defect-Engineered MOFs as a Photoregenerable Catalytic Adsorbent: Understanding the Effect of LML, Adsorption Behavior, and Photoreaction Process

Defect-engineering is an exciting strategy for the modification of metal-organic frameworks (MOFs), which can go beyond the limit of conventional MOFs, tailor material properties, and incorporate multiple functionalities. Herein, based on the large mixed-linker approach, we successfully integrated tetrakis(4-carboxyphenyl)porphyrin (TCPP) into stable UiO-66 via an in situ one-pot synthetic method and used the obtained material for the removal of diclofenac (DF). TCPP@UiO-66 maintained the structure, excellent stability, and porosity of UiO-66. The defect density significantly affected the phase purity, crystallite morphology, and properties of TCPP@UiO-66s. Owing to the delicate balance between defects, stability, and porosity, TCPP@UiO-66(25%) was the optimal material in our system. The pseudo-second-order kinetic model and the Sips isothermal model described the adsorption of DF onto defect-engineered MOFs, and the adsorption capacity was 590 mg/g. Electrostatic interaction, Lewis acid-base interaction, π-π interaction, hydrogen bonding, and anion-π interaction were possible adsorption mechanisms. Moreover, under simulated sunlight irradiation, TCPP@UiO-66(25%) was catalytically active for the degradation of DF with a removal efficiency of 99%. It displayed good recyclability during three reaction cycles. The result of electron spin resonance revealed the generation of ¹O₂, implying the occurrence of type II photosensitization reaction. Meanwhile, the first-order rate constants of DF photodegradation after the addition of scavengers confirmed that h⁺ is also a key reactive species. Both the energy transfer from TCPP to triplet oxygen and the electron transfer from TCPP to Zr clusters contributed to the degradation of DF. The degradation byproducts of DF were monitored by three-dimensional excitation-emission matrix (3D EEM). Therefore, TCPP@UiO-66(25%) was an attractive photoregenerable catalytic adsorbent for the effective removal of DF. Combining the advantages of the parent framework and the functional linker, our strategy expands the functionality of the stable MOFs for potential applications in environmental remediation.

Insights into Heteroatom-Doped Graphene for Catalytic Ozonation: Active Centers, Reactive Oxygen Species Evolution, and Catalytic Mechanism

To guide the design of novel graphene-based catalysts in catalytic ozonation for micropollutant degradation, the mechanism of catalytic ozonation with heteroatom-doped graphene was clarified. Reduced graphene oxide doped with nitrogen, phosphorus, boron, and sulfur atoms (N-, P-, B-, and S-rGO) were synthesized, and their catalytic ozonation performances were evaluated in the degradation of refractory organics and bromate elimination simultaneously. Doping with heteroatoms, except sulfur, significantly improved the catalytic ozonation activity of graphene. Introducing sulfur atoms destroyed the stability of graphene during ozonation, with the observed partial performance improvement caused by surface adsorption. Degradation pathways for selected refractory organics were proposed based on the intermediates identified using high-resolution Orbitrap mass spectroscopy and gas chromatographic-mass spectroscopy. Three and six new unopened intermediates were identified in benzotriazole and p-chlorobenzoic acid degradation, respectively. Roles of chemical functional groups, doped atoms, free electron, and delocalized π electron of heteroatom-doped graphene in catalytic ozonation were identified, and contributions of these active centers to the formation of reactive oxygen species (ROS), including hydroxyl radicals, superoxide radicals, singlet oxygen, and H₂O₂, were evaluated. A mechanism for catalytic ozonation by heteroatom-doped graphene was proposed for the first time.

Preparation of BiF3/BiOBr heterojunctions from microwave-assisted method and photocatalytic performances

Novel BiF₃/BiOBr heterojunction photocatalysts were prepared from a fast and stable microwave-assisted method, and characterized by X-ray diffractometry, X-ray photoelectron spectroscopy, scanning electron microscopy, ultraviolet-visible spectroscopy and fluorescence spectroscopy. The photocatalytic activity of BiF₃/BiOBr heterojunctions under light irradiation was significantly higher than pure BiOBr or BiF₃, and was maximized at the Br:F molar ratio of 1:1, as the targeted 20 mg/L Rhodamine B (RhB) solution was completely degraded within 40 min. This was mainly because the unique BiF₃/BiOBr heterojunction formed during photocatalytic degradation accelerated the photoelectron and hole separation, effectively enhanced the quantum efficiency, and thereby strengthened the photocatalytic activity.

Photocatalytic performance of Ag2S/ZnO/ZnS nanocomposites with high visible light response prepared via microwave-assisted hydrothermal two-step method

A series of different ratios of Ag₂S/ZnO/ZnS nanocomposites with visible light response were prepared by a microwave-assisted hydrothermal two-step method, whose composition, crystalline structure, morphology and surface physicochemical properties were well-characterized via X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), UV-vis diffuse reflectance spectroscopy (UV-vis/DRS), photoluminescence spectrum (PL), scanning electron microscopy (SEM), high-resolution transmission electron microscopy (HR-TEM) and N₂ adsorption-desorption measurements. Results showed that as-composites mainly consisted of ZnS crystal phase, whose grain size increased obviously compared with non Ag₂S samples. At the same time, due to the introduction of narrow band gap Ag₂S, the synthesized composite can effectively increase the visible optical absorption of ZnO/ZnS composites. Among them, 1% Ag₂S/ZnO/ZnS showed a mixed structure of nano-line and nano-particle, of which BET value increased significantly, and the morphology was more excellent. Photocatalytic activities of a series of Ag₂S/ZnO/ZnS composites under different light sources were studied using methyl orange as a model molecule, and 1% Ag₂S/ZnO/ZnS was taken as the best one. Meanwhile, 1% Ag₂S/ZnO/ZnS also showed a good degradation effect on other dyes with different structures, and its degradation efficiency did not change significantly after three cycles, showing certain stability. In addition, composites with Ag₂S loading of 1% possessed the highest hydrogen production ability of photolysis water, indicating that the introduction of Ag₂S had significantly enhanced the catalytic performance.

Morphology Control Studies of MnTiO3 Nanostructures with Exposed {0001} Facets as a High-Performance Catalyst for Water Purification

Novel single-crystal hexagonal MnTiO₃ nanosheets with exposed {0001} facets have been synthesized via a simple one-pot hydrothermal method using NaOH as a mineralizer and tetraethylammonium hydroxide (TEAH) as a morphology controller. The intermediate morphologies of MnTiO₃ nanostructures such as nanoparticles, nanowires, nanorods, and nanodiscs are trapped kinetically by adjusting the synthesis conditions. This approach enables us to elucidate the growth mechanisms of MnTiO₃ nanosheets based on the tetraethylammonium cation adsorption abilities on different MnTiO₃ crystal facets combined with density functional theory calculations. Dissolution and recrystallization processes are involved during the MnTiO₃ crystallization. The surface-controlled MnTiO₃ has been found to be effective as a catalyst for ozonation in the degradation of 4-chlorophenol (4-CP). Within typical experimental conditions (catalyst dosage = 0.3 g L^-1, [4-CP]₀ = 50 mg L^-1, [O₃] = 20 mg L^-1, gas flow = 0.1 L min^-1, pH 6.8, and T = 293 K), the total organic carbon (TOC) removal efficiency of 4-CP in catalytic ozonation with well-structured MnTiO₃ (MnTiO₃-180-10 sample) was 76.3% after 60 min, compared with only 22.1 and 38.5% TOC removal in the absence of catalyst and with uncontrolled MnTiO₃ (MnTiO₃-no TEAH sample), respectively. Benefiting from the high exposure percentage of {0001} facet, mixed-valences of manganese, surface hydroxyl groups, and the enrichment Lewis acid sites provided by Mn and Ti, the morphology-controlled MnTiO₃ nanosheets can be applied as heterogeneous catalytic ozonation catalysts which exhibit excellent pollutant degradation. We anticipate that MnTiO₃ can be a promising candidate material for the application in remediation of organic pollutants in water.

Industrial wastewater advanced treatment via catalytic ozonation with an Fe-based catalyst

An Fe-based catalyst was used as a heterogeneous catalyst for the ozonation of industrial wastewater, and key operational parameters (pH and catalyst dosage) were studied. The results indicated that the Fe-based catalyst significantly improved the mineralization of organic pollutants in wastewater. TOC (total organic carbon) removal was high, at 78.7%, with a catalyst concentration of 200 g/L, but only 31.6% with ozonation alone. The Fe-based catalyst significantly promoted ozone decomposition by 70% in aqueous solution. Hydroxyl radicals (·OH) were confirmed to be existed directly via EPR (electron paramagnetic resonance) experiments, and ·OH were verified to account for about 34.4% of TOC removal with NaHCO₃ as a radical scavenger. Through characterization by SEM-EDS (field emission scanning electron microscope with energy-dispersive spectrometer), XRD (X-ray powder diffraction) and XPS (X-ray photoelectron spectroscopy), it was deduced that FeOOH on the surface of the catalyst was the dominant contributor to the catalytic efficiency. The catalyst was certified as having good stability and excellent reusability based on 50 successive operations and could be used as a filler simultaneously. Thereby, it is a promising catalyst for practical industrial wastewater advanced treatment.

A novel catalytic ceramic membrane fabricated with CuMn2O4 particles for emerging UV absorbers degradation from aqueous and membrane fouling elimination

A novel catalytic ceramic membrane (CM) for improving ozonation and filtration performance was fabricated by surface coating CuMn₂O₄ particles on a tubular CM. The degradation of ultraviolet (UV) absorbers, reduction of toxicity, elimination of membrane fouling and catalytic mechanism were investigated. The characterization results suggested the particles were well-fixed on membrane surface. The modified membrane showed improved benzophenone-3 removal performance (from 28% to 34%), detoxification (EC₅₀ as 12.77%) and the stability of catalytic activity. In the degradation performance of model UV absorbers, the developed membrane significantly decreased the UV254 and DOC values in effluent. Compared with a virgin CM, this CM ozonation increased water flux as 29.9% by in-situ degrade effluent organic matters. The CuMn₂O₄ modified membrane enhanced the ozone self-decompose to generate O₂^- and initiated the chain reaction of ozone decomposition, and subsequently reacted with molecule ozone to produce OH. Additionally, CM was able to promote the interaction between ozone and catalyst/organic chemicals to form H₂O₂ that promoted the formation of OH. This catalytic ceramic membrane combining with ozonation showed potential applications in emerging pollutant degradation and membrane fouling elimination, and acted as a novel ternary technology for wastewater treatment and water reuse.