Cu−Zn−(Mn)−(Fe)−Al Layered Double Hydroxides and Their Mixed Metal Oxides: Physicochemical and Catalytic Properties in Wet Hydrogen Peroxide Oxidation of Phenol

Antifouling catalytic mixed-matrix membranes based on polyethersulfone and composition-optimized Zn-Cu-Fe-O CWAO catalyst under dark ambient conditions

Besides photocatalysts, novel catalytic wet-air oxidation (CWAO) catalysts capable of operating under mild conditions are a potential candidate to fabricate antifouling filtration membranes. This study optimized the CWAO catalyst consisting of three metal oxide components (ZnO, CuO, and Fe₃O₄) and used it to fabricate composite membranes with PES (polyethersulfone). The catalyst was characterized by methods such as FTIR, BET, XRD, UV-Vis DRS, XPS, ESR. The activity of the catalyst and the composite membranes was tested by the Acid Yellow 42 (AY42) degradation experiments in both cases with and without hydrogen peroxide at room conditions with air aeration. The pure water fluxes of composite membranes were also investigated based on a vacuum filtration system. The major degradation pathways of AY42 by the catalyst were proposed from the DFT (Density Functional Theory) and NBO (Natural Bond Orbital) calculations. The results showed that the optimal catalyst has molar ratios of Zn, Cu, and Fe metal ions of 0.05, 0.588, and 0.362, respectively, with AY42 decomposition efficiency of 88% in 3 h. The main factors affecting the catalytic efficiency of the CWAO catalyst determined from the trapping experiment were e^- and O₂. The results from different materials characterization methods have demonstrated the successful synthesis of the catalyst with a high surface area (103.5 m²/g) and small pore diameters (∼10 nm). The AY42 degradation of composite membranes was stable over five repeated cycles with over 70% efficiency. The pure water fluxes of composite membranes have also been significantly improved and are proportional to catalyst contents.

Heterogeneous Fenton catalysts: A review of recent advances

Heterogeneous Fenton catalysts are emerging as excellent materials for applications related to water purification. In this review, recent trends in the synthesis and application of heterogeneous Fenton catalysts for the abatement of organic pollutants and disinfection of microorganisms are discussed. It is noted that as the complexity of cell wall increases, the resistance level towards various disinfectants increases and it requires either harsh conditions or longer exposure time for the complete disinfection. In case of viruses, enveloped viruses (e.g. SARS-CoV-2) are found to be more susceptible to disinfectants than the non-enveloped viruses. The introduction of plasmonic materials with the Fenton catalysts broadens the visible light absorption efficiency of the hybrid material, and incorporation of semiconductor material improves the rate of regeneration of Fe(II) from Fe(III). A special emphasis is given to the use of Fenton catalysts for antibacterial applications. Composite materials of magnetite and ferrites remain a champion in this area because of their easy separation and reuse, owing to their magnetic properties. Iron minerals supported on clay materials, perovskites, carbon materials, zeolites and metal-organic frameworks (MOFs) dramatically increase the catalytic degradation rate of contaminants by providing high surface area, good mechanical stability, and improved electron transfer. Moreover, insights to the zero-valent iron and its capacity to remove a wide range of organic pollutants, heavy metals and bacterial contamination are also discussed. Real world applications and the role of natural organic matter are summarised. Parameter optimisation (e.g. light source, dosage of catalyst, concentration of H2O2 etc.), sustainable models for the reusability or recyclability of the catalyst and the theoretical understanding and mechanistic aspects of the photo-Fenton process are also explained. Additionally, this review summarises the opportunities and future directions of research in the heterogeneous Fenton catalysis.

Enhanced catalytic phenol hydroxylation by CuZnFeAl layered double hydroxides: synergistic effects of Cu+ and oxygen vacancies

In this work, a series of CuZnFeAl-LDH catalysts for phenol oxidation to dihydroxybenzene have been prepared through a co-precipitation method.

Degradation of benzene derivatives in the CuMgFe-LDO/persulfate system: The role of the interaction between the catalyst and target pollutants

A novel insight on the role of interactions between target pollutants and the catalyst in the copper-containing layered double oxide (LDO)-catalyzed persulfate (PS) system was elucidated in the present study. 4-Chlorophenol (4-CP), as a representative benzene derivative with a hydroxyl group, was completely removed within 5 min, which was much faster than the reaction of monochlorobenzene (MCB) without a hydroxyl group, with the degradation efficiency of 31.7% in 240 min. Through the use of radical quenching and surface inhibition experiments, it could be concluded that the interaction between 4-CP and CuMgFe-LDO, rather than free radicals, played a key role in the decomposition of 4-CP, while only the free radicals participated in the MCB degradation process. According to electron paramagnetic resonance and X-ray photoelectron spectroscopy data, the formation of a Cu(II)-complex between phenolic hydroxyl groups and surface Cu(II) was primarily responsible for the degradation of phenolic compounds, in which PS accepted one electron from the complex and generated sulfate radicals and chelated radical cations. The chelated radical cations transferred one electron to Cu(II) followed by Cu(I) generation and pollutant degradation successively.

Catalysis of hydrogen peroxide with Cu layered double hydrotalcite for the degradation of ethylbenzene

A high catalytic system using Cu layered double hydrotalcite (Cu(II)-Mg(II)-Fe(III)LDHs) and hydrogen peroxide (H₂O₂) was developed for the degradation of ethylbenzene. It was identified that the degradation efficiency of ethylbenzene (0.08 mmol L^-1) and TOC removal were 96.1% and 39.7% respectively in the presence of 0.1 g L^-1 Cu(II)-Mg(II)-Fe(III)LDHs with (Cu²⁺ + Mg²⁺)/Fe³⁺ molar ratio of 5.0 and 0.16 mmol L^-1 H₂O₂ in 6.0 h. Based on ESR and XPS data, hydroxyl radicals (•OH) were the predominant free radical specials generated from the catalytic decomposition of H₂O₂ for the degradation of ethylbenzene. The redox of Cu(II)/Cu(III) on the layered Cu(II)-Mg(II)-Fe(III)LDHs surface active sites accounted for the formation of •OH radicals and the cycle of Cu(II) in the Cu(II)-Mg(II)-Fe(III)LDHs/H₂O₂ system were proposed.

Multipath fabrication of hierarchical CuAl layered double hydroxide/carbon fiber composites for the degradation of ammonia nitrogen

In this work, a series of flower-like CuAl layered double hydroxides (LDHs) and hierarchical CuAl/carbon fiber-LDH (CuAl/CF-LDH) materials were synthesized, and these materials were used as catalysts for the degradation of ammonia nitrogen from simulated wastewater. The morphologies and structures of the materials were characterized using scanning electron microscopy (SEM), Fourier transform infrared (FTIR) spectroscopy, Raman spectroscopy (RS), X-ray diffraction (XRD), and the Brunauer-Emmett-Teller (BET) technique. The effects of the catalyst and H₂O₂ loading dosages, reaction temperature, pH, Cu/Al ratio of the samples, and contact time on the degradation process were investigated by degrading ammonia nitrogen under different conditions, and the possible degradation mechanism was discussed. CuAl/CF-LDH exhibited more effectively catalytically degradation of ammonia nitrogen than others as-prepared samples, and removal efficiency reached 99.7% under the optimized conditions. The reusing capability and stability of the materials were studied. Meanwhile, the versatility of the materials was investigated by testing their performance in the absorption of azo dye, the highest removal efficiency was found to be 99.28%. The prepared materials are promising for use as effective catalysts for the degradation of ammonia nitrogen from wastewater.

Effective degradation of phenol via Fenton reaction over CuNiFe layered double hydroxides

A series of CuNiFe layered double hydroxides (LDHs) with various Cu/Ni molar ratios were synthesized as catalysts for Fenton degradation of phenol. It is found that Cu⁺, Cu²⁺, Ni²⁺, Ni³⁺ and Fe³⁺ are present on LDHs, owing to an electron transfer from Ni²⁺ to Cu²⁺ via metal-oxo-metal bridges. At lower Cu/Ni ratios, the highly dispersed MO₆ octahedra and the electron donation effect of Ni facilitate such electron transfer and thus increase the percentage of Cu⁺. The catalytic activity increases with the decrease in Cu/Ni ratio. The most active Cu_0.5Ni_2.5Fe LDH can mineralize 98.9% phenol at ambient pH and less excessive H₂O₂ dosage ( [Formula: see text] /M_phenol = 37). Even at the H₂O₂ dosage close to the theoretical value, around 90% phenol can be mineralized. The structure-activity correlation indicates Cu⁺ which can readily react with H₂O₂ to produce hydroxyl radicals may dominate the reaction. The regeneration of Cu⁺ could be achieved by the electron transfer between Cu²⁺ and Ni²⁺ in LDHs. Moreover, Fe³⁺ can also act as Fenton-like active sites. The special structure of CuNiFe LDHs could offer surface-enriched and easily regenerated Cu⁺ species, leading to the complete mineralization of phenol and the efficient use of H₂O₂.

Heterogeneously catalyzed persulfate with a CuMgFe layered double hydroxide for the degradation of ethylbenzene

CuMgFe layered double hydroxide (CuMgFe-LDH) was successfully synthesized and characterized as an efficient catalyst of persulfate (PS) for the degradation of ethylbenzene. Under the conditions of 0.2gL^-1 CuMgFe-LDH and 4.0mmolL^-1 persulfate at pH 7.6, the degradation efficiency of 0.08mmolL^-1 ethylbenzene was 93.7% with TOC removal efficiency of 65.2% in 24h, and the concentration of Cu leached into the solution was as low as 0.095mgL^-1 after the reaction. The reuse of CuMgFe-LDH showed that the catalyst was highly stable after 5 recycles. Electron Spin Resonance (ESR) test and free radical quenching experiment indicated that SO₄^- and OH radicals were the dominant species accounted for the degradation of ethylbenzene in the CuMgFe-LDH/persulfate system. Catalytic mechanism of the formation of a complex of Cu(II)O₃SOOSO₃^2- and the subsequent redox cycle of Cu(II)/Cu(III) accounted for the generation of radicals was proposed.

A novel three-dimensional hierarchical CuAl layered double hydroxide with excellent catalytic activity for degradation of methyl orange

In this study, heterogeneous Fenton-like degradation of the reactive organic dye methyl orange in water was investigated over CuAl–Layered Double Hydroxide (CuAl–LDH) with high copper ions content as a base catalyst prepared by a hydrothermal synthesis.

Cu/Mg/Al/Zr non-noble metal catalysts for o-phenylphenol synthesis

Cu/Mg/Al/Zr non-noble metal catalysts were prepared via decomposition of hydrotalcite-like precursor and showed a high catalysis activity for OPP synthesis.

Highly biocompatible behaviour and slow degradation of a LDH (layered double hydroxide)-coating on implants in the middle ear of rabbits

Chronic inflammation can irreversibly damage components of the ossicular chain which may lead to sound conduction deafness. The replacement of impaired ossicles with prostheses does not reduce the risk of bacterial infections which may lead to loss of function of the implant and consequently to additional damage of the connected structures such as inner ear, meninges and brain. Therefore, implants that could do both, reconstruct the sound conduction and in addition provide antibacterial protection are of high interest for ear surgery. Layered double hydroxides (LDHs) are promising novel biomaterials that have previously been used as an antibiotic-releasing implant coating to curb bacterial infections in the middle ear. However, animal studies of LDHs are scarce and there exist only few additional data on the biocompatibility and hardly any on the biodegradation of these compounds. In this study, middle ear prostheses were coated with an LDH compound, using suspensions of nanoparticles of an LDH containing Mg and Al as well as carbonate ions. These coatings were characterized and implanted into the middle ear of healthy rabbits for 10 days. Analysis of the explanted prostheses showed only little signs of degradation. A stable health constitution was observed throughout the whole experiment in every animal. The results show that LDH-based implant coatings are biocompatible and dissolve only slowly in the middle ear. They, therefore, appear as promising materials for the construction of controlled drug delivery vehicles.

Morphologies, Preparations and Applications of Layered Double Hydroxide Micro-/Nanostructures

Layered double hydroxides (LDHs), also well-known as hydrotalcite-like layered clays, have been widely investigated in the fields of catalysts and catalyst support, anion exchanger, electrical and optical functional materials, flame retardants and nanoadditives. This feature article focuses on the progress in micro-/nanostructured LDHs in terms of morphology, and also on the preparations, applications, and perspectives of the LDHs with different morphologies.