Oxygen vacancy mediated surface charge redistribution of Cu-substituted LaFeO3 for degradation of bisphenol A by efficient decomposition of H2O2

Recent progress in defect-engineered metal oxides for photocatalytic environmental remediation

Rapid industrial advancement over the last few decades has led to an alarming increase in pollution levels in the ecosystem. Among the primary pollutants, harmful organic dyes and pharmaceutical drugs are directly released by industries into the water bodies which serves as a major cause of environmental deterioration. This warns of a severe need to find some sustainable strategies to overcome these increasing levels of water pollution and eliminate the pollutants before being exposed to the environment. Photocatalysis is a well-established strategy in the field of pollutant degradation and various metal oxides have been proven to exhibit excellent physicochemical properties which makes them a potential candidate for environmental remediation. Further, with the aim of rapid industrialization of photocatalytic pollutant degradation technology, constant efforts have been made to increase the photocatalytic activity of various metal oxides. One such strategy is the introduction of defects into the lattice of the parent catalyst through doping or vacancy which plays a major role in enhancing the catalytic activity and achieving excellent degradation rates. This review provides a comprehensive analysis of defects and their role in altering the photocatalytic activity of the material. Various defect-rich metal oxides like binary oxides, perovskite oxides, and spinel oxides have been summarized for their application in pollutant degradation. Finally, a summary of existing research, followed by the existing challenges along with the potential countermeasures has been provided to pave a path for the future studies and industrialization of this promising field.

Effect of Interfacial Action on the Generation and Transformation of Reactive Oxygen Species in Tripolyphosphate-Enhanced Heterogeneous Fe3O4/O2 Systems

It has been reported that tripolyphosphate (TPP) can enhance the oxygenation of natural Fe(II)-containing minerals to produce reactive oxygen species (ROS). However, the molecular structure of the TPP-Fe(II) mineral surface complex and the role of this complex in the generation and transformation of ROS have not been fully characterized. In the present study, a heterogeneous magnetite (Fe3O4)/O2/TPP system was developed for the degradation of p-nitrophenol (PNP). The results showed that the addition of TPP significantly accelerated the removal of PNP in the Fe3O4/O2 system and extended the range of effective pH to neutral. Experiments combined with density functional theory calculations revealed that the activation of O2 mainly occurs on the surface of Fe3O4 induced by a structural Fe(II)-TPP complex, where the generated O2•- (intermediate active species) can be rapidly converted into H2O2, and then the •OH generated by the Fenton reaction is released into the solution. This increases the concentration of •OH produced and the efficiency of •OH produced relative to Fe(II) consumed, compared with the homogeneous system. Furthermore, the binding of TPP to the surface of Fe3O4 led to stretching and even cleavage of the Fe-O bonds. Consequently, more Fe(II)/(III) atoms are exposed to the solvation environment and are available for the binding of active O2 and O2•-. This study demonstrates how common iron minerals and O2 in the natural environment can be combined to yield a green remediation technology.

Preparation of Novel C/N-Doped LaFeO3 Type Perovskite for Efficient Photocatalytic Degradation of Sodium Humate

LaFeO3 chalcocite precursor was prepared by solid-phase milling method, and LaFeO3-type chalcocite composite catalyst, referred to as LFCN catalyst, was synthesized by in situ doping of carbon and nitrogen (urea, melamine, dicyandiamide, and carbon powder), The catalytic performance of the catalysts was investigated by the different mass ratios of LaFeO3 chalcocite precursor and carbon and nitrogen (1:1, 1:2, and 2:1) and the degradation mechanism. Various characterization analyses, such as X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and Brunauer-Emmett-Teller (BET), showed that the doped composite LFCN catalysts exhibited a hemispherical network structure with a larger specific surface area than that of the pure phase LaFeO3 material. In addition, the LaFeO3 material adjusted the electronic structure of the original LaFeO3 chalcogenide material to a certain extent after in situ doping with organic C and N elements, which enhanced its lattice oxygen oxidation ability. In the study of the catalytic degradation of sodium humate solution under natural light conditions, the catalytic performance was significantly improved compared to that of the pure phase LaFeO3, and 10 mg of the catalyst degraded 30 mg/L of sodium humate solution in 50 min, with a degradation rate increasing from 40 to 98%. The degradation rate increased from 40 to 98% after 4 applications, indicating that the LFCN catalyst has good stability and significant catalytic degradation performance.

Tuning the intrinsic catalytic sites of magnetite to concurrently enhance the reduction of H2O2 and O2: Mechanism analysis and application potential evaluation

Heterogeneous Fenton-like process based on H₂O₂ activation has been widely tested for water purification, but its application still faces some challenges such as the use of high doses of chemicals (including catalysts and H₂O₂). Herein, a facile co-precipitation method was utilized for small-scale production (∼50 g) of oxygen vacancies (OVs)-containing Fe₃O₄ (V_o-Fe₃O₄) for H₂O₂ activation. Experimental and theoretical results collaboratively verified that H₂O₂ adsorbed on the Fe site of Fe₃O₄ tended to lose electrons and generate O₂^•-. While the localized electron from OVs of V_o-Fe₃O₄ could assist in donating electrons to H₂O₂ adsorbed on OVs sites, this allowed more H₂O₂ to be activated to ^•OH, which was 3.5 folds higher than Fe₃O₄/H₂O₂ system. Moreover, the OVs sites promoted dissolved oxygen activation and decreased the quenching of O₂^•- by Fe(III), thus promoting the generation of ¹O₂. Consequently, the fabricated V_o-Fe₃O₄ achieved much higher oxytetracycline (OTC) degradation rate (91.6%) than Fe₃O₄ (35.4%) at a low catalyst (50 mg/L) and H₂O₂ dosage (2 mmol/L). Importantly, further integration of V_o-Fe₃O₄ into fixed-bed Fenton-like reactor could effectively eliminate OTC (>80%) and chemical oxygen demand (COD) (21.3%∼50%) within the running period. This study provides promising strategies for enhancing the H₂O₂ utilization of Fe mineral.

Tetracycline hydrochloride degradation by activation of peroxymonosulfate with lanthanum copper Ruddlesden-Popper perovskite oxide: Performance and mechanism

ABO₃-type perovskite oxides have been regarded as a kind of potential catalyst for peroxymonosulfate (PMS) activation. But some limitations such as specific pH conditions and coexisting ion interference restrict its practical application. Herein, a lanthanum copper Ruddlesden-Popper perovskite oxide (La₂CuO₄) was successfully synthesized through the sol-gel process and applied in the activation of PMS. And for the first time the La₂CuO₄/PMS system was used for tetracycline hydrochloride (TC-HCl) degradation. Results showed that La₂CuO₄ was a potential PMS activation catalyst in the removal of antibiotics. At optimized condition (0.2 g/L catalysts, 1 mM PMS, pH₀ 6.9), 96.05% of TC-HCl was removed in 30 min. In experiments of debugging control conditions, over a wide pH range of 3-11, more than 90% of TC-HCl can be removed. In the natural water treatment process, TC-HCl removal rates of about 84.2% and 70.3% were obtained in tap water and River water, respectively. According to the reusability and stability tests and the results of FTIR and XPS analysis, La₂CuO₄ had high structural and chemical stability. Electron paramagnetic resonance (EPR) suggested that the active species including ·OH, SO₄^-· and ¹O₂ were detected in degradation reaction. Finally, reasonable reaction mechanisms and possible degradation pathways of TC-HCl were proposed. These results indicate that La₂CuO₄ can act as a potential catalyst for PMS activation to degrade TC-HCl in water.

Co3O4/CoFe2O4 Hollow Nanocube Multifunctional Nanozyme with Oxygen Vacancies for Deep-Learning-Assisted Smartphone Biosensing and Organic Pollutant Degradation

Although the application of nanozymes has been widely studied, it is still a huge challenge to develop highly active and multifunctional nanozyme catalysts with a wider application prospect. Co₃O₄/CoFe₂O₄ hollow nanocubes (HNCs) with oxygen vacancies were proposed in this study, which had a porous oxide heterostructure with CoFe₂O₄ as the core and Co₃O₄ as the shell. The Co₃O₄/CoFe₂O₄ HNCs had three enzyme activities: peroxidase-like, oxidase-like, and catalase-like. Combining XPS depth profiling with density functional theory (DFT), the catalytic mechanism of peroxidase-like activity was explored in depth, which was mainly originated from ·OH produced by the synergistic effect between the outer oxygen and inner oxygen and electron transfer between Co and Fe. A colorimetry/smartphone dual sensing platform was designed based on the peroxidase-like activity. Especially, a multifunctional intelligent sensing platform based on deep learning-YOLO v3 algorithm-assisted smartphone was constructed to realize real-time and rapid in situ detection of l-cysteine, norfloxacin, and zearalenone. Surprisingly, the detection limit of norfloxacin was low at 0.015 μM, which was better than that of the newly published detection method in the field of nanozymes. Meanwhile, the detection mechanism of l-cysteine and norfloxacin was successfully investigated by in situ FTIR. In fact, it also showed outstanding applications in detecting l-cysteine in the food environment and norfloxacin in drugs. Furthermore, Co₃O₄/CoFe₂O₄ HNCs also could degrade 99.24% of rhodamine B, along with good reusability even after 10-cycle runs. Therefore, this work provided an in-depth understanding of the synergistic effect between the outer and inner oxygen in the reaction mechanism and an efficient method for establishing a deep-learning-assisted intelligent detection platform. In addition, this research also offered a good guideline for the further development and construction of nanozyme catalysts with multienzyme activities and multifunctional applications.

Chalcogen Elements in Regulating the Local Electron Density of Cu2X for an Efficient Heterogeneous Fenton-like Process

In this work, a novel strategy for Fenton activity improvement of Cu₂X was reported, in which the local electron density of Cu sites was regulated via manipulation of simple chalcogen elements (O, S, and Se). Among them, Cu₂Se catalysts show excellent catalytic activity to activate H₂O₂ for the complete removal of ofloxacin (10 mg/L) at an initial pH of 6.5 within 120 min. Radical scavenger experiments and electron spin resonance spectroscopy confirm that ^•OH radicals are the primary oxygen reactive species to drive ofloxacin degradation. In addition, density functional theory calculations further proved that electrons would migrate from X and accumulate on Cu active sites in the order Se > S > O. Compared with Cu₂O and Cu₂S, the highly concentrated electron density of Cu atoms in Cu₂Se not only decreased the activation energy of the Fenton-like reaction but also boosted the Cu²⁺/Cu⁺ cycle with the generation of more ^•OH radicals (18-66 μm) and the maintenance of high stability of catalysts, leading to excellent catalytic activity and application potential. We believe this work will lay the foundation for designing excellent Fenton catalysts for practical applications since developing a heterogeneous Fenton system with the highest oxidation efficiency has always been the long-term goal in this field.

Cobalt mediated perovskite as efficient Fenton-like catalysts for the tetracycline removal over a neutral condition: The importance of superoxide radical

Cobalt mediated perovskite oxides (Ca-Fe-Co-x) were prepared for heterogeneous Fenton-like, which exhibited excellent tetracycline (TC) degradation efficiency and wider pH suitability (3-11). Experimental results showed that Ca-Fe-Co-1.0 sample displayed the highest degradation rate could reach 80.5% under neutral conditions, and maintain at around 80% after four cycles. The analysis of degradation mechanism showed that the redox of Fe²⁺/Fe³⁺ and Co²⁺/Co³⁺ significant enhanced the activation of H₂O₂ to superoxide radical (∙O₂^-). Meanwhile, the hydroxyl radical (∙OH) was also detected by ESR analysis. In addition, the possible degradation pathway and mechanism of TC were deduced via UPLC-QTOF/MS analysis and density functional theory (DFT) calculations. The toxicity of TC and its intermediates were also evaluated by the ECOSAR software. The Ca-Fe-Co-1.0/nanocellulose aerogel (NCA) displayed highly removal efficiency of TC wastewater in the long-term operation conduction. This study provided a feasible method to design and synthesis heterogeneous Fenton-like catalysts for antibiotic degradation.

Copper-based Ruddlesden-Popper perovskite oxides activated hydrogen peroxide for coal pyrolysis wastewater (CPW) degradation: Performance and mechanism

Coal pyrolysis wastewater (CPW) contained all kinds of toxic and harmful components, which would seriously threaten the natural environment and human health. However, the traditional advanced oxidation processes frequently failed to remove phenolic substances. An A₂BO₄-type perovskite (La₂CuO₄) was successfully synthesized through sol-gel process and first applied in the treatment of CPW. More than 90% of 3, 5-dimethylphenol (DMP) was removed within 200 min at neutral conditions. Moreover, La₂CuO₄ also displayed excellent catalytic activity and stability in the actual CPW treatment process. Results demonstrated that DMP was removed through the attack of ∙OH, ∙O₂^- and ¹O₂ in La₂CuO₄/H₂O₂ system. The La₂CuO₄ were more favorable for H₂O₂ activation and have a lower adsorption energy than LaFeO₃. XPS of fresh and spent La₂CuO₄ illustrated that the decomposition of hydrogen peroxide (H₂O₂) was mainly due to the redox cycle between surface copper and oxygen species. Moreover, the possible degradation pathway of DMP was deduced by identifying degradation products and analyzing density functional theory (DFT) calculations. This research provided a novel strategy for the development of perovskite-based catalytic materials on the treatment of practical CPW.

Catalytic Activation of Hydrogen Peroxide Using Highly Porous Hydrothermally Modified Manganese Catalysts for Removal of Azithromycin Antibiotic from Aqueous Solution

Hydrogen peroxide catalytic activation holds great promise in the treatment of persistent pollutants. In this study, the novel Mn-Acacair/Al, Mn-Acacarg/Al and Mn-BTCarg/Al catalysts, supported on Al2O3, were applied for rapid hydrogen peroxide activation and azithromycin antibiotic removal. The catalysts were prepared by the calcination-hydrothermal method under air or argon atmosphere. The characterization confirmed that the modification of manganese with acetylacetonate and benzene-1,3,5-tricarboxylic acid (H3BTC) O-donor ligands highly improves the catalyst porosity, amorphousity, and abundance of coordinately unsaturated sites, which facilitate the generation of reactive oxygen species. The hydrogen peroxide activation and azithromycin removal reached 98.4% and 99.3% after 40 min using the Mn-BTCarg/Al catalyst with incredible stability and reusability. Only a 5.2% decrease in activity and less than 2% manganese releasing in solutions were detected after five regeneration cycles under the optimum operating conditions. The removal intermediates were identified by LC-MS/MS analysis, and the pathways were proposed. The hydroxylation and decarboxylation reactions play a key role in the degradation reaction.

Ca and Cu doped LaFeO3 to promote coupling of photon carriers and redox cycling for facile photo-Fenton degradation of bisphenol A

Enhancements in the light response and hydrogen peroxide utilization are critical to the catalytic performance of heterogeneous Fenton-like perovskites. Here, in this research, oxygen vacancy-enriched La_0.9Ca_0.1Cu_0.5Fe_0.5O_3-δ was prepared by a co-precipitation method with Cu substitution and Ca doping and demonstrated excellent performance for the degradation of bisphenol A. Both total organic carbon (TOC) removal and hydrogen peroxide utilization were close to 90% within 120 min at pH 3-7, where the TOC removal and hydrogen peroxide utilization were 2.5 times and 5.5 times of LaFeO₃ in the absence of Ca and Cu doping. It demonstrated excellent stability to light irradiation and oxidation with respect to cycling and metal ion leaching. This revealed that oxygen vacancies were enriched in the catalyst with the substitution of Ca and Cu and contributed to the recombination of photogenerated electrons, thereby increasing the reduction efficiency of copper ions and accelerating the redox cycling of iron ions.

Self-Accelerating Interfacial Catalytic Elimination of Gaseous Sulfur-Containing Volatile Organic Compounds as Microbubbles in a Facet-Engineered Three-Dimensional BiOCl Sponge Fenton-Like Process

The elimination of gaseous sulfur-containing volatile organic compounds (S-VOCs) by a microbubble-assisted Fenton-like process is an innovative strategy. Herein, we established a microbubble-assisted Fenton-like process to eliminate malodorous microbubble CH₃SH as representative gaseous S-VOCs, in which BiOCl nanosheets loaded on a three-dimensional sponge were exposed to (001) or (010) facets and induced Fenton-like interface reactions. Intriguingly, the microbubble-assisted Fenton-like process significantly removed 99.9% of CH₃SH, higher than that of the macrobubble-assisted Fenton-like process (39.0%). The self-accelerating interfacial catalytic mechanism was in-depth identified by in situ ATR-FTIR, PTR-TOF-MS, EPR, and DFT computational study. The extraordinary elimination performance of microbubble-assisted Fenton-like process lies in the enhancing dissolution/mass transfer of gaseous CH₃SH in the gas/liquid phase and the tight contact between CH₃SH-microbubbles and 3D-BiOCl sponge due to the low rising velocity (0.13 mm s^-1) and negative charge (-45.53 mV) of CH₃SH-microbubbles, as well as the effective generation of ¹O₂ by activating the enriched dissolved oxygen in CH₃SH-microbubble via effective electron-polarized sites on 3D-BiOCl sponge. Furthermore, CH₃SH-microbubbles transferred electrons to H₂O₂ through electron-rich oxygen vacancy centers of the 3D-BiOCl sponge to generate more •OH, thus achieving excellent elimination performance. Overall, this study demonstrates the enhanced self-accelerating interfacial catalytic elimination by S-VOC microbubble and provides the underlying mechanisms.

Critical review of perovskite-based materials in advanced oxidation system for wastewater treatment: Design, applications and mechanisms

Perovskite has been widely concerned in the field of modern environmental catalysis due to its low price, high stability, excellent catalytic activity, diverse structure and strong conversion adaptability. In recent years, people have been working on the coupling of perovskite catalysts and advanced oxidation processes (AOPs) on the removal of organic pollutants from wastewater. In this review, we classified perovskites of different designs and summarized the application and basic reaction mechanisms of each perovskite in different AOPs. This review helps scientists selecting and designing more effective perovskite catalysts for AOPs by summarizing the applications and reaction mechanisms of perovskite in AOPs. At the end of the review, the challenges and future directions of perovskite in removing organic pollutants from wastewater are discussed.

Phenolic compounds degradation: Insight into the role and evidence of oxygen vacancy defects engineering on nanomaterials

Oxygen vacancy as a typical point defect has incited substantial interest in photocatalysis due to its profound impact on optical absorption response and facile isolation of photocarriers. The presence of oxygen vacancy can introduce the midgap defect states, which promote extended absorption in the visible region. The redistribution of electron density at the surface can stimulate the adsorption and activation kinetics of adsorbates, manifesting optimal photocatalytic performance. Despite such alluring outcomes, the ambiguity in understanding the precise location, appropriate concentration, and oxygen vacancy role is still a long-standing task. The present review article comprehensively outlines the identification of oxygen vacancy defects at bulk or on the surface and its ultimate effect on the photocatalytic degradation of phenolic compounds. Particular emphasis has been drawn to summarize the critical influence of oxygen vacancy on different factors such as crystal structure, bandgap energy, electronic structure, and charge carrier mobility by integrating experimental results and theoretical calculations. We have also explored the reaction pathways and the intermediate chemistry of phenol photodegradation by analyzing the molecular activation (O₂, H₂O, and sulphate activation) through oxygen vacancy defects. Finally, the review concludes with the various challenges and future perspectives, aiming to provide a firm base for further progressions towards photocatalysis.

Cancer therapeutic strategies based on metal ions

As a necessary substance to maintain the body's normal life activities, metal ions are ubiquitous in organisms and play a major role in various complex physiological and biochemical processes, such as material transportation, energy conversion, information transmission, metabolic regulation, etc. Their abnormal distribution/accumulation in cells can interfere with these processes, causing irreversible physical damage to cells or activating biochemical reactions to induce cell death. Therefore, metal ions can be exploited against a wide spectrum of cancers with high efficiency and without drug resistance, which can effectively inhibit the growth of cancer cells by triggering biocatalysis, breaking the osmotic balance, affecting metabolism, interfering with signal transduction, damaging DNA, etc. This perspective systematically summarizes the latest research progress of metal ion-based anti-tumor therapy, and emphasizes the challenges and development directions of this type of therapeutic strategy, hoping to provide a general implication for future research.

Enhanced Fenton-like efficiency by the synergistic effect of oxygen vacancies and organics adsorption on FexOy-d-g-C3N4 with Fe‒N complexation

d-g-C₃N₄-Fe composites was prepared via a self-assembly and calcination process. According to measurements and density functional theory (DFT) computations, the complexation of iron and pyridinic N of g-C₃N₄ (Fe‒N) occurred with Fe(III)-π interaction, causing more oxygen vacancies (OVs) with more electrons in iron oxides. In the catalyst air-saturated suspension, the adsorbed pollutants complexed surface Fe(III) through their hydroxyl group donated electrons to around OVs, reducing the surface Fe(III) to Fe(II) and were destructed by Fe(III)-π interaction of the complexation. The addition of H₂O₂ mainly acted as acceptor being reduced ^•OH at the OV centers, causing higher degradation rate of pollutants due to both ^•OH and the surface reaction. However, for the adsorbed hydrophobic pollutants onto the sites of peripheral structure in g-C₃N₄, H₂O₂ was mainly decomposed into O₂ by the synergistic effect of iron species and OVs. Therefore, the catalyst exhibited high Fenton-like efficiency for the degradation of hydroxyl-containing pollutants and hydrophobic pollutants mixing with the former. Our results demonstrate that the Fe(III)-π interaction could carry out the oxidation of pollutants on the catalyst surface, decreasing the consumption of H₂O₂, and the role of OVs depends on pollutant adsorption patterns.

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.