Highly porous α-MnO2 nanorods with enhanced defect accessibility for efficient catalytic ozonation of refractory pollutants

Accelerated tetracycline hydrochloride mineralisation by Fe@CeO2-x/MgO complex metal oxides via ozone-catalysed interfacial reactions: The role of oxygen vacancies and multivalent metal cycling

Heterogeneous catalytic ozonation (HCO) shows promising potential for degrading emerging contaminants, but its mineralisation rate needs further improvement due to insufficient electron transfer. To address this limitation, an efficient HCO system was developed by synthesising a Fe@CeO2-x/MgO multi-metal catalyst via an isomorphous substitution strategy, which was then coupled with ozone (O3) for efficient tetracycline hydrochloride (TCH) mineralisation. The results indicated that TCH mineralisation rate reached 0.0204 min-1 in the O3/0.5Fe@CeO2-x/MgO system, 2.43 fold times higher than that of O3 alone. This enhanced mineralisation performance can be attributed to iron substitution, introduction of numerous Lewis acid sites, and shortening of Ce-O bond length, which facilitated the migratory conversion between oxygen vacancies (OVs) and surface lattice oxygen, as well as improved adsorption and electron transfer between Fe@CeO2-x/MgO and O3. Mechanistic analysis revealed that OVs directly activate oxygen to form singlet oxygen and then reacted with O3 to generate superoxide radicals, that subsequently reacted with O3 to form hydroxyl radical (·OH) and recyclable OVs. In addition, O3 could oxidise Fe(Ⅱ) and Ce(Ⅲ) to obtain ·OH through electron transfer. Similarly, intermediate hydroperoxyl could reduce Fe(Ⅲ) and Ce(Ⅳ) to ensure continued free radical production. As a result of enhanced TCH mineralization performance, both toxicity of intermediates and disinfection by-products (DBPs) formation potential were gradually reduced. Notably, DBPs formation potential exhibited a positive linear correlation with total organic carbon. This study offers an effective HCO system to enhance TCH mineralisation through interfacial O3 catalysis rather than simple degradation, and this provided comprehensive insights to improve O3 utilisation and reduce energy consumption of catalytic ozonation processes.

Progress in MnO2/MnO2-based materials catalytic ozonation process for water and wastewater treatment

Heterogeneous catalytic ozonation (HCO) utilizes catalysts to enhance the adsorption and decomposition of ozone (O3), promote the formation of reactive oxygen species (ROS), and improve the removal of organic compounds, thereby overcoming some disadvantages of ozonation. MnO2/MnO2-based materials are widely used as catalysts for HCO due to their multi-valent Mn species, environment friendliness, abundant resources, and high efficiency. This review aims to provide an overview of the advancements in HCO using MnO2/MnO2-based materials, focusing on their preparation, structural characteristics, catalytic performance, and proposed mechanisms. In particular, the effects of MnO2 synthesis methods on the crystalline structure and morphology of catalysts are discussed. Then, the catalytic performances of various catalysts involving different phases, morphologies, and facets are compared. Subsequently, the enhanced applications of MnO2-based catalysts in HCO for water treatment are described, including metals doping, metal oxides combination, and MnO2-carrier. Furthermore, approaches of ROS identification are clarified, and the mechanisms of strengthening catalytic ozonation efficiency by MnO2/MnO2-based catalysts are summarized, containing redox couple theory, oxygen vacancy theory, complexation theory, and surface hydroxyl theory. Finally, the potential applications and perspectives of MnO2/MnO2-based catalysts are proposed. This review plans to bridge the gap between research and practical applications, providing new insights into the application of HCO technologies in water treatment.

MnO2/porous spontaneously polarized ceramic with self-powered electric field and superior charge transfer to catalyze ozonation for efficient demulsification

Ozone (O3) demulsification shows great potential in emulsion wastewater treatment due to its strong oxidative properties. However, the low mass transfer efficiency and oxidation selectivity of O3 cannot be ignored. Herein, a MnO2/porous spontaneously polarized ceramic (MnO2/PSPC) composite with strong interfacial interactions and self-powered electric field was prepared for heterogeneous catalytic ozonation (HCO) to achieve efficient demulsification. Excellent remanent polarization (0.00858 μC/cm2) together with systematic electrochemical characterizations of MnO2/PSPC demonstrated its significant charge transfer capability, which is essential for the subsequent reduction of Mn4+ in the HCO demulsification process. O3- MnO2/PSPC exhibited excellent demulsification performance with 99 % demulsification rate of cetyltrimethylammonium bromide-stabilized emulsion within 30 min, outperforming O3 (130 min), O3-MnO2 (60 min), and O3-PSPC (90 min). O3-MnO2/PSPC showed effective demulsification of non-/anionic surfactant stabilized emulsions and excellent stability after 5 cycles. Density functional theory calculations together with characterizations illustrate that potential difference-induced rapid electron transfer and water flow-induced self-powered electric field were the fundamental motivation for the fast Mn3+/Mn4+ cycle and O3 adsorption/decomposition to generate reactive oxygen species (ROS). Notably, the oxidation of surfactants by ROS led to the coalescence of the oil droplets. This study provides an efficient, sustainable, and energy-efficient method to improve the O3 demulsification performance.

Comparative study on CeO2 catalysts with different morphologies and exposed facets for catalytic ozonation: performance, key factor and mechanism insight

Morphology and facet effects of metal oxides in heterogeneous catalytic ozonation (HCO) are attracting increasing interests. In this paper, the different HCO performances for degradation and mineralization of phenol of seven ceria (CeO2) catalysts, including four with different morphologies (nanorod, nanocube, nanooctahedron and nanopolyhedron) and three with the same nanorod morphology but different exposed facets, are comparatively studied. CeO2 nanorods with (110) and (100) facets exposed show the best performance, much better than that of single ozonation, while CeO2 nanocubes and nanooctahedra show performances close to single ozonation. The underlying reason for their different HCO performances is revealed using various experimental and density functional theory (DFT) calculation results and the possible catalytic reaction mechanism is proposed. The oxygen vacancy (OV) is found to be pivotal for the HCO performance of the different CeO2 catalysts regardless of their morphology or exposed facet. A linear correlation is discerned between the rate of catalytic decomposition of dissolved ozone (O3) and the density of Frenkel-type OV. DFT calculations and in-situ spectroscopic studies ascertain that the existence of OV can boost O3 activation on both the hydroxyl (OH) and Ce sites of CeO2. Conversely, various facets without OV exhibit similar O3 adsorption energies. The OH group plays an important role in activating O3 to produce hydroxyl radical (∙OH) for improved mineralization. This work may offer valuable insights for designing Facet- and OV-regulated catalysts in HCO for the abatement of refractory organic pollutants.

Catalytic ozonation of reverse osmosis concentrate from coking wastewater reuse by surface oxidation over Mn-Ce/γ-Al2O3: Effluent organic matter transformation and its catalytic mechanism

Degradation of organics in high-salinity wastewater is beneficial to meeting the requirement of zero liquid discharge for coking wastewater treatment. Creating efficient and stable performance catalysts for high-salinity wastewater treatment is vital in catalytic ozonation process. Compared with ozonation alone, Mn and Ce co-doped γ-Al2O3 could remarkably enhance activities of catalytic ozonation for chemical oxygen demand (COD) removal (38.9%) of brine derived from a two-stage reverse osmosis treatment. Experimental and theoretical calculation results indicate that introducing Mn could increase the active points of catalyst surface, and introducing Ce could optimize d-band electronic structures and promote the electron transport capacity, enhancing HO• bound to the catalyst surface ([HO•]ads) generation. [HO•]ads plays key roles for degrading the intermediates and transfer them into low molecular weight organics, and further decrease COD, molecular weights and number of organics in reverse osmosis concentrate. Under the same reaction conditions, the presence of Mn/γ-Al2O3 catalyst can reduce ΔO3/ΔCOD by at least 37.6% compared to ozonation alone. Furthermore, Mn-Ce/γ-Al2O3 catalytic ozonation can reduce the ΔO3/ΔCOD from 2.6 of Mn/γ-Al2O3 catalytic ozonation to 0.9 in the case of achieving similar COD removal. Catalytic ozonation has the potential to treat reverse osmosis concentrate derived from bio-treated coking wastewater reclamation.

Synergistic strategy of solute environment and phase control of Pb-based anodes to solve the activity-stability trade-off

The contradiction between the activity and stability of metal anodes exists extensively, especially in acid electrooxidation under industrial-level current density. Although the anode modification enhanced the initial activity of anodes, its long-term activity is limited by anode slime accumulation. Herein, a synergistic strategy, coupling the solute environment with the phase control of anodes, is proposed to achieve the trade-off between activity and stability of Pb-based anodes in concentrated sulfuric acid electrolysis. Non-exogenous Mn2+ motivated a series of positive behaviours of reactive-oxygen-species capture, anode reconstruction and corrosion-dependent activity alleviation. The synergistic effects, which are crystal phase-dependent, mainly benefit from the continuous self-healing ability of the specific crystal phase of MnO2 on the anodes by the coexisted Mn2+. Compared with Mn2+/α-MnO2, Mn2+/γ-MnO2 exhibited outperformed activity and stability in boosting oxygen evolution reaction (OER) and reducing hazardous pollutants, which resulted from the energy difference in the rate-determining step of OER and in the selectivity priority of Mn2+/MnO2 oxidation pathway. Interestingly, the pre-coated γ-MnO2 on the anode also presents excellent inheritance, guaranteeing the unchanged crystal phase of MnO2 and the high performance in ultra-low hazardous slime generation in subsequent Mn2+ oxidation. The sustainability of Mn2+/γ-MnO2 was proved in the operating hydrometallurgy conditions on Pb-based anodes. This strategy offers a promising approach for this common issue in electrooxidation-related areas.

Preparation of Heterogeneous Fenton Catalysts Cu-Doped MnO2 for Enhanced Degradation of Dyes in Wastewater

Herein, a series of heterogeneous Fenton catalysts, Cu doped MnO2 (CDM), with different Cu/Mn molar ratios were prepared via a hydrothermal reaction. Meanwhile, detailed characterizations were used to study the structures of CDM, and it is amazing that the morphology of CDM changed from nanowires to nanoflowers with an increasing amount of Cu doped. Apart from this, both the specific surface area and oxygen vacancy increased obviously with the increasing Cu/Mn molar ratio. Then, the degradation of different dyes was utilized to evaluate the catalytic activity of different CDM with H2O2 used as the oxidizing agent, and the 50%-CDM with the highest content of Cu doped displayed the best catalytic activity. Herein, the degradation efficiency (D%) of Congo red (CR) solution with low concentration (60 mg/L) reached 100% in 3 min, while the D% of CR solution with a high concentration (300 mg/L) reached 99.4% after 5 min with a higher dosage of H2O2. Additionally, the 50%-CDM also displayed excellent reusability, for which the D% values were still higher than 90% after the 14th cycles. Based on the structure characteristics and mechanism analysis, the excellent catalytic capacity of 50%-CDM was due to the combined influence of large specific surface area and abundant oxygen vacancy. Thus, a promising heterogeneous Fenton catalyst was developed in this study, which proved the treatment efficiency of actual dye wastewater.

Switching reactive oxygen species reactions derived from Mn-Pt anchored zeolite for selective catalytic ozonation

Rationally switching reactive oxygen species (ROS) reactions in advanced oxidation processes (AOPs) is urgently needed to improve the adaptability and efficiency for the engineering application. Herein we synthesized bimetallic Mn-Pt catalysts based on zeolite to realize the switching of ROS reactions in catalytic ozonation for sustainable degradation of organic pollutants from water. The ROS reactions switched from singlet oxygen (1O2, 71.01%) to radical-dominated (93.79%) pathway by simply introducing defects and changing Pt/Mn ratios. The oxygen vacancy induced by anchoring Mn-Pt species from zeolite external surface (MnPt/H-Beta) to internal framework (MnPt@Si-Beta) exposes more electron-rich Pt2+/Pt4+ redox sites, accelerating the decomposition of O3 to generate •OH via electron transfer and switching ROS reactions. The Mn site acted as a bridge plays a critical role in conducting electrons from organic pollutants to Pt sites, which solidly solves the electron loss of catalysts, facilitating the efficient degradation of pollutants. A 34.7-fold increase in phenol degradation compared with the non-catalytic ozonation and an excellent catalytic stability are achieved by MnPt@Si-Beta/O3. The 1O2-dominated ROS reaction originated from MnPt/H-Beta/O3 exhibits superior performances in anti-interference for Cl-, HCO3-, NO3-, and SO4-. This work establishes a novel strategy for switching ROS reactions to expand the targeted applications of O3 based AOPs for environmental remediation.

Calcium Fluoride/Manganese Dioxide Nanocomposite with Dual Enzyme-like Activities for Uric Acid Sensing: A Comparative Study of Enzyme and Nonenzyme Methods

The debate over enzyme methods versus nonenzyme methods in the field of nanosensing has lasted for decades despite hundreds of published studies on this topic. In this study, we first present a comparative analysis of these methods using a reaction based on the CaF2/MnO2 nanocomposite (CM Nc) with dual-enzyme activity, presenting oxidase- and peroxidase-like activities. Uric acid (UA) is a byproduct of purine metabolism in the body, and abnormal levels can cause many diseases; hence, tracking the amount of UA in human serum is crucial. The enzyme method was established using uricase and CM Nc: UA produced H2O2 when catalyzed by uricase; H2O2 was then catalyzed into reactive oxygen species (ROS) by the peroxidase activity of the CM Nc; this ROS oxidized 3,3',5,5'-tetramethylbenzidine (TMB), which was oxidized into blue oxidized TMB (oxTMB). The nonenzyme method was built on the scavenging effect of UA on the ROS, which prevented the catalytic capability of CM Nc toward TMB and induced blue oxTMB fading. The results of further tests revealed the good selectivity of the enzyme method compared to the fast response of the nonenzyme method. Additionally, both methods were effective in determining the UA concentration in human serum. The two separate methods can also independently verify each other, increasing the accuracy of the detection results in accordance with the relatively independent detection principles. This research provided theoretical backing for the practical design of multienzyme nanozyme catalysts, which can facilitate the precise detection of UA in biochemical products.

Degradation of Sodium Acetate by Catalytic Ozonation Coupled with a Mn-Functionalized Fly Ash: Reaction Parameters and Mechanism

Supported ozone catalysts usually take alumina, activated carbon, mesoporous molecular sieve, graphene, etc. as the carrier for loading metal oxide via the impregnation method, sol-gel method and precipitation method. In this work, a Mn-modified fly ash catalyst was synthesized to reduce the consumption and high unit price of traditional catalyst carriers like alumina. As a solid waste discharged from coal-fired power plants fueled by coal, fly ash also has porous spherical fine particles with constant surface area and activity, abd is expected to be applied as the main component in the synthesis of ozone catalyst. After the pretreatment process and modification with MnOx, the obtained Mn-modified fly ash exhibited stronger specific surface area and porosity combined with considerable ozone catalytic performance. We used sodium acetate as the contaminant probe, which is difficult to directly decompose with ozone as the end product of ozone oxidation, to evaluate the performance of this Mn-modified fly. It was found that ozone molecules can be transformed to generate ·OH, ·O2- and 1O2 for the further oxidation of sodium acetate. The oxygen vacancy produced via Mn modification plays a crucial role in the adsorption and excitation of ozone. This work demonstrates that fly ash, as an industrial waste, can be synthesized as a potential industrial catalyst with stable physical and chemical properties, a simple preparation method and low costs.

Enhanced Catalytic Ozonation by Mn-Ce Oxide-Loaded Al2O3 Catalyst for Ciprofloxacin Degradation

Catalytic ozonation is an effective and promising advanced oxidation technology for organic pollutant removal. Herein, CexMn_1-xO₂ metal oxides loaded on Al₂O₃ catalysts (Mn-Ce/Al₂O₃) were synthesized for catalytic ozonation of the wastewater containing ciprofloxacin. The morphology, crystal structure, and specific surface area of the prepared catalyst were characterized. The characteristics of the Mn-Ce/Al₂O₃ catalyst revealed that the loaded MnO₂ could interfere with the formed CeO₂ crystals and then produced complex CexMn_1-xO₂ oxides. Compared with an ozone-alone system (47.4%), the ciprofloxacin degradation efficiency in the Mn-Ce/Al₂O₃ catalytic ozonation system elevated to 85.1% within 60 min. The ciprofloxacin degradation kinetic rate over the Mn-Ce/Al₂O₃ catalyst is 3.0 times that of the ozone-alone system. The synergetic corporation of redox pairs between Mn(III)/Mn(IV) and Ce(III)/Ce(IV) in the Mn-Ce/Al₂O₃ catalyst could accelerate ozone decomposition to generate active oxygen species and further significantly improve the mineralization efficiency of ciprofloxacin. The work demonstrates the great potential of developing dual-site ozone catalysts for advanced treatment of wastewater.

A comprehensive review on metal based active sites and their interaction with O3 during heterogeneous catalytic ozonation process: Types, regulation and authentication

Heterogeneous catalytic ozonation (HCO) was a promising water purification technology. Designing novel metal-based catalysts and exploring their structural-activity relationship continued to be a hot topic in HCO. Herein, we reviewed the recent development of metal-based catalysts (including monometallic and polymetallic catalysts) in HCO. Regulation of metal based active sites (surface hydroxyl groups, Lewis acid sites, metal redox cycle and surface defect) and their key roles in activating O₃ were explored. Advantage and disadvantage of conventional characterization techniques on monitoring metal active sites were claimed. In situ electrochemical characterization and DFT simulation were recommended as supplement to reveal the metal active species. Though the ambiguous interfacial behaviors of O₃ at these active sites, the existence of interfacial electron migration was beyond doubt. The reported metal-based catalysts mainly served as electron donator for O₃, which resulted in the accumulation of oxidized metal and reduced their activity. Design of polymetallic catalysts could accelerate the interfacial electron migration, but they still faced with the dilemma of sluggish Me^(n+m)+/Meⁿ⁺ redox cycle. Alternative strategies like coupling active metal species with mesoporous silicon materials, regulating surface hydrophobic/hydrophilic properties, polaring surface electron distribution, coupling HCO process with photocatalysis and H₂O₂ were proposed for future research.

Application of Heterogeneous Catalytic Ozonation in Wastewater Treatment: An Overview

Catalytic ozonation is a non-selective mineralization technology of organic matter in water by using active free radicals generated by ozone degradation. Catalytic ozonation technology can be divided into homogeneous catalytic reactions using metal ions as catalysts and heterogeneous catalytic reactions using solid catalysts. Homogeneous catalytic ozonation technology has many problems, such as low mineralization rate, secondary pollution caused by the introduction of metal ions and low utilization efficiency of oxidants, which limit its practical application. Compared with homogeneous catalytic ozonation technology, heterogeneous catalytic ozonation technology has the advantages of easy recovery, lower cost of water treatment, higher activity and improved mineralization rate of organic matter. This overview classifies and describes catalysts for heterogeneous catalytic ozonation technology, including the different types of metal oxides, metal-free catalysts, and substrates used to immobilize catalysts. In addition, the heterogeneous catalytic ozonation process involved in the multiphase complex reaction process is discussed. The effects of different parameters on the performance of heterogeneous catalytic ozonation are also discussed.

Catalytic Ozonation of Norfloxacin Using Co-Mn/CeO2 as a Multi-Component Composite Catalyst

In this study, a Co-Mn/CeO2 composite was prepared through a facile sol-gel method and used as an efficient catalyst for the ozonation of norfloxacin (NOR). The Co-Mn/CeO2 composite was characterized via XRD, SEM, BET and XPS analysis. The catalytic ozonation of NOR by Co-Mn/CeO2 under different conditions was systematically investigated, including the effect of the initial solution’s pH, Co-Mn/CeO2 composite dose, O3 dose and NOR concentration on degradation kinetics. Only about 3.33% of total organic carbon (TOC) and 72.17% of NOR could be removed within 150 min by single ozonation under the conditions of 60 mg/L of NOR and 200 mL/min of O3 at pH= 7 and room temperature, whereas in the presence of 0.60 g/L of the Co-Mn/CeO2 composite under the same conditions, 87.24% NOR removal was obtained through the catalytic ozonation process. The results showed that catalytic ozonation with the Co-Mn/CeO2 composite could effectively enhance the degradation and mineralization of NOR compared to a single ozonation system alone. The catalytic performance of CeO2 was significantly improved by the modification with Mn and Co. Co-Mn/CeO2 represents a promising way to prepare efficient catalysts for the catalytic ozonation of organic polluted water. The removal efficiency of NOR in five cycles indicates that Co-Mn/CeO2 is stable and recyclable for catalytic ozonation in water treatment.