Enhanced activation of peroxymonosulfate using oxygen vacancy-enriched FeCo2O4−x spinel for 2,4-dichlorophenol removal: Singlet oxygen-dominated nonradical process

Degradation of 2, 4-dichlorophenol by peroxymonosulfate catalyzed by ZnO/ZnMn2 O4

In this study, a highly efficient peroxymonosulfate (PMS) activator, ZnO/ZnMn2 O4 , was synthesized using a simple one-step hydrothermal method. The resulting bimetallic oxide catalyst demonstrated a homogenous and high-purity composition, showcasing synergistic catalytic activity in activating PMS for degrading 2, 4-dichlorophenol (2, 4-DCP) in aqueous solution. This catalytic performance surpassed that of individual ZnO, Mn2 O3 , and ZnMn2 O4 metal materials. Under the optimized conditions, the removal efficiency of 2, 4-DCP reached approximately 86% within 60 min, and the catalytic ability remained almost constant even after four cycles of recycling. The developed degradation system proved effective in degrading other azo-dye pollutants. Certain inorganic anions such as HPO4 - , HCO3 - , and NO3 - significantly inhibited the degradation of 2, 4-DCP, while Cl- and SO4 2- did not exhibit such interference. Results from electrochemical experiments indicated that the electron transfer ability of ZnO/ZnMn2 O4 surpassed that of individual metals, and electron transfer occurred between ZnO/ZnMn2 O4 and the oxidant. The primary active radicals responsible for degrading 2, 4-DCP were identified as SO4 •- , OH• and O2 •- , generated through the oxidation and reduction of PMS catalyzed by Zn (II) and Mn (III). Furthermore, X-ray photoelectron spectroscopy (XPS) analysis of the fresh and used catalysts revealed that the exceptional electron transfer ability of ZnO facilitated the valence transfer of Mn (III) and the transfer of electrons to the catalyst's oxygen surface, thus enhancing the catalytic efficiency. The analysis of radicals and intermediates indicates that the two main pathways for degrading 2, 4-DCP involve hydroxylation and radical attack on its aromatic ring. PRACTITIONER POINTS: A bimetallic ZnO/ZnMn2 O4 catalyst was synthesized and characterized. ZnO/ZnMn2 O4 can synergistically activate PMS to degrade 2, 4-DCP compared with single metal oxide. Three primary active radicals, O2 •- , • OH, and SO4 •- , were generated to promote the degradation. ZnO promoted electron transfer among the three species of Mn to facilitate oxidizing pollutants. Hydroxylation and radical attack on the aromatic ring of 2, 4-DCP are the two degradation pathways.

Facilitated Visible-Light-Driven Peroxymonosulfate Activation by a Co-Fe Layered Double Hydroxide Derived p-n Heterostructure for Sulfadiazine Degradation: Affecting Parameters, Kinetics, and Mechanistic Insights

The utilization of multivalence ionic metal species generated through a peroxymonosulfate (PMS)-assisted photocatalytic system is a promising platform for the selective degradation of water contaminants. However, achieving an effective electron transport and enhanced separation efficiency for these metal species is a daunting challenge. Thus, our current study addresses this challenge by using a Co-Fe-based layered-double-hydroxide template to synthesize a Co3O4/FeCo2O4 p-n heterojunction composite via a simple monosynthetic route. The resultant composite is thoroughly validated through advanced characterization techniques that efficiently activate PMS for sulfadiazine (SDZ) degradation under visible light, achieving a remarkable degradation efficiency of up to 90%. This accomplishment is attributed to factors including intimate interfacial contact, excellent light harvesting, mesoporosity, and oxygen vacancies within the composite. The formation of a distinct p-n heterojunction following the S-scheme charge dynamic significantly enhances photogenerated carrier separation and reduces charge recombination. The research delves into comprehensive investigations including degradation studies, active species trapping experiments, parameter exploration, and in-depth liquid chromatography-mass spectrometry for analysis of the degradation byproducts and pathway. Induced oxygen vacancies, strategically placed active surface sites, and mesoporosity in the Co3O4/FeCo2O4 composite synergistically boosted the sluggish PMS activation, leading to enhanced SDZ degradation. This study introduces a new perspective by demonstrating the potential of a single-material, mixed-metal oxide-based p-n heterojunction photocatalytic system following the S-scheme charge-transfer route for SDZ degradation. The findings contribute toward emphasizing the importance of tailored composite materials in tackling persistent contaminants.

Spinel ferrite nanoparticles as potential materials in chlorophenol removal from wastewater

Persistent organic pollutants (POPs) including chlorophenols (CPs) are increasing in water effluents, creating serious problems for both aquatic and terrestrial lives. Several research attempts have considered the removal of CPs by functionalised nanomaterials as adsorbents and catalysts. Besides the unique crystal structure, spinel ferrite nanomaterials (SFNs) own interesting optical and magnetic properties that give them the potential to be utilised in the removal of different types of CPs. In this review, we highlighted the recent research work that focused on the application of SFNs in the removal of different CP substances based on the number of chlorine atom attached to the phenolic compound. We have also discussed the structure and properties of SFN along with their numerous characterisation tools. We demonstrated the importance of identifying the structure, surface area, porosity, optical properties, etc. in the efficiency of the SFN during the CP removal process. The reviewed research efforts applied photocatalysis, wet peroxide oxidation (WPO), persulfate activated oxidation and adsorption. The studies presented different paths of enhancing the SFN ability to remove the CPs including doping (ion substitution), oxide composite structure and polymer composite structure. Experimental parameters such as temperature, dosage of CPs and SFN structure have shown to have a major effect in the CP removal efficiency. More attention is needed to investigate the different properties of SFN that can be tailored through different techniques and expected to have major role in the removal mechanism of CPs.

Peroxymonosulfate activation by graphene oxide-supported 3D-MoS2/FeCo2O4 sponge for highly efficient organic pollutants degradation

To address conventional powder catalysts' recovery and aggregation issues that greatly restrain their practical application, a recoverable graphene oxide (GO)-supported 3D-MoS₂/FeCo₂O₄ sponge (SFCMG) was developed through a simple impregnation pyrolysis method. SFCMG can efficiently activate peroxymonosulfate (PMS) to produce reactive species for rapid degradation of rhodamine B (RhB), with 95.0% and 100% of RhB being removed within 2 min and 10 min, respectively. The presence of GO enhances the electron transfer performance of the sponge, and the three-dimensional melamine sponge serves as a substrate to provide a highly dispersed carrier for FeCo₂O₄ and MoS₂/GO hybrid sheets. SFCMG exhibits a synergistic catalytic effect of Fe and Co, and facilitates the redox cycles of Fe(III)/Fe(II) and Co(III)/Co(II) by MoS₂ co-catalysis, which enhances its catalytic activity. Electron paramagnetic resonance results demonstrate that SO₄^•-, ·O₂^- and ¹O₂ are all involved in SFCMG/PMS system, and ¹O₂ played a prominent role in RhB degradation. The system has good resistance to anions (Cl^-, SO₄^2-, and H₂PO₄^-) and humic acid and excellent performance for many typical contaminants degradation. Additionally, it works efficiently over a wide pH range (3-9) and possesses high stability and reusability with the metal leaching far below the safety standards. The present study extends the practical application of metal co-catalysis and offers a promising Fenton-like catalyst for the treatment of organic wastewater.

Applications of vacancy defect engineering in persulfate activation: Performance and internal mechanism

The vacancy defects in heterogeneous catalysts have received extensive attention for persulfate (PS) activation. Vacancy defects can tune the electronic structure of metal oxides and generate unsaturated coordination sites. Meanwhile, the adsorption energy of reactants on catalyst surface is optimized. Thereby, the reaction energy barrier between catalysts and PS decreases, which could promote catalytic activation and accelerate pollutants degradation. Nowadays, oxygen vacancy (OV), nitrogen vacancy (NV), sulfur vacancy (SV), selenium vacancy (SeV) and titanium vacancy (TiV) have been widely studied with great potential for water remediation. So far, no review was reported regarding the vacancy activated persulfate systems. This paper summarized the types, preparation, mechanism and applications of vacancy in PS systems systematically. In addition, we put forward possible development of vacancy engineering in PS activation systems. It is expected that this review will contribute to the controllable synthesis and applications of vacancies in catalysts for PS activation and contaminants removal.

Recyclable Fe/S co-doped nanocarbon derived from metal-organic framework as a peroxymonosulfate activator for efficient removal of 2,4-dichlorophenol

In this study, a recyclable Fe/S co-doped nanocarbon (Fe/S-NC) was successfully prepared by the pyrolysis of ZIF-8 confined with Fe(II) and added S. Characterization showed that a highly graphitized carbon-based material co-doped with sulfur and iron was successfully prepared. This Fe/S-NC can efficiently activate PMS to remove organic pollutants in water. The effect of different synthesis conditions on the degradation efficiency of 2,4-DCP was studied by orthogonal experiments. The optimized Fe/S-NC/PMS system exhibited excellent catalytic performance and could degrade more than 99.7% of 2,4-DCP within 30 min. Even after 5 cycles, the degradation efficiency could still be maintained above 96.3%, which proved that the catalytic system had good cycle performance. In addition, the effect of pH on catalytic performance showed that the degradation rate of 2,4-DCP exceeds 96.7% in the pH range of groundwater (pH = 5-9). We had confirmed that the free radicals that caused 2,4-DCP degradation were SO₄^·-, ·OH, O₂^·-, and ¹O₂, which played important roles in degrading organic pollutants. These research results show that the Fe/S-NC/PMS system can be used as an efficient, stable, and environmentally friendly system to treat organic pollutants in groundwater.

Selective Degradation of Electron-Rich Organic Pollutants Induced by CuO@Biochar: The Key Role of Outer-Sphere Interaction and Singlet Oxygen

Efficient degradation of organic pollutants by oxidative radicals is challenging in the complex soil environment because of the invalid consumption of radicals by nontarget background substances and the generation of secondary halogenated organic pollutants. Nonradical-based oxidation is a promising pollutant removal method due to its high selectivity and environmental adaptability. Herein, a biochar-supported sheetlike CuO (e-CuO@BC) was developed, which exhibited efficient activation of peroxydisulfate (PDS) via nonradical pathways. The activation mechanisms were identified as (i) formation of surface-bonding active complexes via an outer-sphere interaction between e-CuO@BC and PDS and (ii) the continuous generation of ¹O₂ by the cycling of the Cu(I)/Cu(II) redox couple. In addition, the activation of PDS primarily occurred at the crystal facet (001) of e-CuO occupied by Cu atoms and was well facilitated by the Cu-O-C bond, which induced electron-rich centers around CuO. Two oxidative species from PDS activation, including surface-bonding active complexes and ¹O₂, showed a highly selective degradation toward electron-rich pollutants. Moreover, a highly efficient mineralization of organic pollutants and an effective inhibition on the generation of toxic byproducts (i.e., halogenated organics) was indicated by the intermediate and final degradation products. This study provides a comprehensive understanding of the heterogeneous activation process of PS by the e-CuO@BC catalyst for electron-rich organic pollutant removal.

Facile synthesis of Mn, Ce co-doped g-C3N4 composite for peroxymonosulfate activation towards organic contaminant degradation

Peroxymonosulfate (PMS)-based advanced oxidation processes for wastewater treatment have received extensive attention in the past years. Here, a novel Mn, Ce co-modified g-C₃N₄ (MnCe-CN) composite was successfully synthesized by one-step pyrolysis for activating PMS. The physical and chemical characterization of MnCe-CN/PMS was conducted, indicating that Mn and Ce were evenly distributed on g-C₃N₄ and existed in the form of Mn-N structure and CeO₂, respectively. The MnCe-CN/PMS system could effectively degrade pollutants such as acetaminophen (ACT), methylparaben (MeP), p-nitrophenol (PNP), and 2,4-dichlorophenol (2,4-DCP). Among them, 2,4-DCP could be rapidly degraded, reaching 100% within 30 min. The masking experiments and electrochemical testing results revealed that 2,4-DCP was degraded via superoxide radicals (O₂˙-), singlet oxygen (¹O₂), and electron transfer path. The cyclic experiments and real water treatment experiments testified that the oxidative system had excellent stability and applicability. This study provides a facile synthetic method to fabricate bimetallic co-modified g-C₃N₄ for the enhancement of PMS activation.

Interfacial engineering coupling with tailored oxygen vacancies in Co2Mn2O4 spinel hollow nanofiber for catalytic phenol removal

Herein, one-dimensional Co₂Mn₂O₄ (CMO) hollow nanofibers with a general spinel structure were constructed by electrospinning and tunning thermal-driven procedures. The resultant catalyst was endowed with appreciable active interfacial engineering effect, which revealed improved peroxymonosulfate (PMS) activation efficiency in catalytic phenol degradation with nearly 12.9 folds increment in reaction rate constant compared to the hydrothermally synthesized counterpart. Besides, tailored oxygen-vacancy sites including chemical environment and contents in the bimetallic spinel were rationally validated compared to the monometal spinel counterparts. The improved catalytic phenol degradation by reactive-oxidative-species (ROS) from PMS was well correlated with the more active Co(II) and Mn(II) species, reactive active oxygen-vacancy and the interfacial engineering effect in the CMO catalyst. These correlations were comprehensively demonstrated by various characterization techniques, catalytic results, and Density-Functional-Theoretical (DFT) calculations of the adsorption and activation of PMS. Besides, the results revealed that the specific content of cobalt species in the structural unit of the Co₂Mn₂O₄ spinel resulting from the optimized thermal treatment could further improve the catalytic activity by the intermetallic synergy along with the beneficial electron transfer cycles. This work provides a practical understanding of the improvement of interfacial systems in catalysis efficiency and environmental remediation.

Mechanism study of CoS2/Fe(III)/peroxymonosulfate catalysis system: The vital role of sulfur vacancies

Peroxymonosulfate (PMS) activation methods have attractive advantages in advanced oxidation process (AOPs) due to their powerful ability of directly or indirectly generating various reactive oxygen species (ROS). Herein, trace amount of Fe(III) ions were added into the commercial-CoS₂/PMS system to improve the CoS₂/PMS decomposition for organics removal. The organics removal efficiency could reach >90% towards methylene blue (MB), diclofenac sodium (DCF), sulfamethoxazole (SMX) and bisphenol A (BPA) in the CoS₂/Fe(III)/PMS system, with the kinetic apparent rate constant k_obs of 0.141, 0.206, 0.247 and 0.091 min^-1, respectively. The synergistic effect between Fe(III) ions and sulfur-vacancies on CoS₂ for PMS degradation were revealed for the first time in cobalt sulfides/PMS system. Quenching experiments and ESR analysis proved that ¹O₂ was the major ROS and was produced mainly by the hydrolysis of SO₅^•-. Besides, the high degradation efficiency was obtained by the contribution of SO₄^•- and ^•OH. Electron spin-resonance spectroscopy (ESR), cyclic voltammetry (CV) and Raman spectrum data revealed that the addition of Fe(III) ions could optimize the intensity of sulfur vacancies on the CoS₂ surface, which hindered the PMS reduction ability of Co(II), but accelerated the PMS oxidation to form ¹O₂. The degradation path of MB was analyzed by liquid chromatograph-mass spectrometer (LC-MS). The mechanism studies speculated that the sulfur vacancies of CoS₂ provided the binding sites for Fe(III) ions with Co(II), which facilitated the PMS activation by Co(III).

Synergistic oxygen vacancy-rich CuO/visible light activation of peroxymonosulfate for degradation of rhodamine B: fast catalyst synthesis and degradation mechanism

This work outlines the synthesis of copper oxide nanoparticles (CuO-SC) loaded with a number of oxygen vacancies by a fast sodium citrate assisted precipitation method with no need of calcination. X-ray diffraction, scanning electron microscopy, UV-Vis diffuse reflectance spectroscopy, time-resolved fluorescence lifetime and electrochemical impedance spectra were used to characterize the as-synthesized nanocomposites. The results indicated that the CuO-SC nanoparticles had regular fusiform shape with high surface area, wide light harvesting window, fast charge transport and high carrier concentration. As a result, the catalytic activity of the CuO-SC/peroxymonosulfate (PMS)/visible light (Vis) system for the degradation of rhodamine B (RhB) was much higher than that of as-prepared CuO nano powder in the absence of sodium citrate. Almost 98.0% of the initial RhB dyes was decomposed in 20 min with 0.12 g L⁻¹ PMS and 0.3 g L⁻¹ catalyst. Meantime, it exhibited high catalytic stability with little deactivation after four runs and a wide application range of pH. Moreover, RhB can be readily degraded with backgrounds of Cl⁻, NO₃⁻, SO₄²⁻, HCO₃⁻ and low concentration of humic acid in a CuO-SC/PMS/Vis system. Combined with the results of electron spin resonance paramagnetic spectroscopy, X-ray photoelectron spectroscopy and radical quenching experiments, holes, superoxide radicals and a small amount of sulfate radicals, hydroxyl radicals and singlet oxygen were involved in the CuO-SC/PMS/Vis system. Furthermore, a possible degradation mechanism based on the synergistic effect of radical reaction and non-radical reaction was proposed based on the above results.

Oxygen vacancy-rich CuO nanoparticles synthesized via a sodium citrate strategy exhibit extremely excellent visible light-assisted catalytic behavior.

Noble-metal-free Fe3O4/Co3S4 nanosheets with oxygen vacancies as an efficient electrocatalyst for highly sensitive electrochemical detection of As(III)

The electrochemical method for highly sensitive determination of arsenic(III) in real water samples with noble-metal-free nanomaterials is still a difficult but significant task. Here, an electrochemical sensor driven by noble-metal-free layered porous Fe₃O₄/Co₃S₄ nanosheets was successfully employed for As(III) analysis, which was prepared via a facile two-step method involves a hydrothermal treatment and a subsequent sulfurization process. As expected, the electrochemical detection of As(III) in 0.1 M HAc-NaAc (pH 6.0) by square wave anodic stripping voltammetry (SWASV) with a considerable sensitivity of 4.359 μA/μg·L^-1 was obtained, which is better than the commonly used noble metals modified electrodes. Experimental and characterization results elucidate the enhancement of As(III) electrochemical performance could be attributed to its nano-porous structure, the presence of oxygen vacancies and strong synergetic coupling effects between Fe₃O₄ and Co₃S₄ species. Besides, the Fe₃O₄/Co₃S₄ modified screen printed carbon electrode (Fe₃O₄/Co₃S₄-SPCE) shows remarkable stability and repeatability, valuable anti-interference ability and could be used for detection in real water samples. Consequently, the results confirm that as-prepared porous Fe₃O₄/Co₃S₄ nanosheets is identified as a promising modifier to detect As(III) in real sample analysis.

Regulation of growth temperature on structure, magnetism of epitaxial FeCo2O4 films

Epitaxial FeCo2O4 films were grown by pulsed laser deposition on Al2O3 (0001) substrates at different growth temperatures in this work.

Correlation of Active Sites to Generated Reactive Species and Degradation Routes of Organics in Peroxymonosulfate Activation by Co-Loaded Carbon

Peroxymonosulfate (PMS)-based advanced oxidation processes (PMS-AOPs) as an efficient strategy for organic degradation are highly dependent on catalyst design and structured active sites. However, the identification of the active sites and their relationship with reaction mechanisms for organic degradation are not fully understood for a composite catalyst due to the complex structure. Herein, we developed a family of Co encapsulated in N-doped carbons (Co-PCN) with tailored types and contents of active sites via manipulated pyrolysis for PMS activation and ciprofloxacin (CIP) degradation, focusing on the correlation of active sites to generated reactive species and degradation routes of organics. The structure-function relationships between the different active sites in Co-PCN catalysts and reactive oxygen species (ROS), as well as bond breaking position of CIP, were revealed through regression analysis and density functional theory calculation. Co-N_x, O-C═O, C═O, graphitic N, and defects in Co-PCN stimulate the generation of ¹O₂ for oxidizing the C-C bond in the piperazine ring of CIP into C═O. The substitution of F by OH and hydroxylation of the piperazine ring might be induced by SO₄^•- and ^•OH, whose formation was affected by C-O, Co(0), Co-N_x, graphitic N, and defects. The findings provided new insights into reaction mechanisms in PMS-AOP systems and rational design of catalysts for ROS-oriented degradation of pollutants.

Alkylpolyglycoside modified MnFe2O4 with abundant oxygen vacancies boosting singlet oxygen dominated peroxymonosulfate activation for organic pollutants degradation

A novel alkylpolyglycoside (APG)-modified MnFe₂O₄ nanocomposite (APG@MnFe₂O₄) enriched with oxygen vacancies (VOs) was developed via co-precipitation and characterized as a peroxymonosulfate (PMS) activator to degrade 2,4-dichlorophenol (2,4-DCP) as the model contaminant. The APG effectively promoted the in situ formation of VOs on MnFe₂O₄ and subsequently enhanced the production of singlet oxygen (¹O₂). Furthermore, the APG@MnFe₂O₄ initialized an even more efficient non-radical pathway and dominated the degradation of 2,4-DCP. The constructed APG@MnFe₂O₄ exhibited a much higher reaction rate constant (0.0522) by ~12.73 times of that of the bare MnFe₂O₄ (0.0041). The degradation efficiency of 2,4-DCP in the APG@MnFe₂O₄/PMS system approached 93% within 90 min, a rate significantly higher than that in the MnFe₂O₄/PMS system (32%) given the same condition. The reasonable catalytic mechanism can be attributed to the Fe/Mn/VOs species. The APG@MnFe₂O₄ also exhibits universally high removal activity for various pollutants and excellent cyclic stability. Thus, the APG@MnFe₂O₄ is a promising PMS activator, and its utilization offers a useful strategy for developing VOs-enriched MnFe₂O₄ catalysts as a means of eliminating organic pollutants from wastewater.

Advances in Electrochemical and Catalytic Performance of Nanostructured FeCo2 O4 and Its Composites

It is well established that the excessive and uncontrolled use of fossil fuels and organic chemicals have put a risk to the earth's environment and the life that sustains within it. Carbon-free, sustainable, alternative energy technologies have therefore become the prime focus of current research. Smart inorganic materials have emerged as the potential solution to suffice energy needs and remediate the organic pollutants discharged to the environment. One such promising, versatile material is FeCo₂ O₄ which has gained immense research interest in the present decade due to its high efficiency and performance in energy and environmental applications. Innovative material design strategies involving the interplay of nanostructured morphology, chemical composition, redox surface states, and defect engineering have significantly enhanced both electrochemical and catalytic properties of FeCo₂ O₄ . Therefore, this review article aims to provide the first-ever comprehensive account of the latest research and developments in design-synthesis strategies, characterization techniques, and applications of nanostructured FeCo₂ O₄ and its composites in various electrochemical as well as catalytic applications. A detailed account of the nanostructured FeCo₂ O₄ and its composites in various energy storage and conversion devices such as supercapacitors (SCs), batteries, and fuel cells has been presented. Furthermore, a special section has been devoted to highlight the role of FeCo₂ O₄ in enhancing the sluggish reaction kinetics of oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) in water splitting application. This review also highlights the role of nanostructured FeCo₂ O₄ in photocatalytic waste water treatment, gas sensing, and dual-phase membrane technologies wherein FeCo₂ O₄ has demonstrated promising performance.

A novel MnOOH coated nylon membrane for efficient removal of 2,4-dichlorophenol through peroxymonosulfate activation

2,4-Dichlorophenol (2,4-DCP) is a highly toxic water contaminant. In this study, we demonstrate a novel catalytic filtration membrane by coating MnOOH nanoparticles on nylon membrane (MnOOH@nylon) for improved removal of 2,4-DCP through a synergetic "trap-and-zap" process. In this hybrid membrane, the underlying nylon membrane provides high adsorption affinity for 2,4-DCP. While the immobilized MnOOH nanoparticles on the membrane surface provide catalytic property for peroxymonosulfate activation to produce reactive oxygen species (ROS), which migrate with the fluid to the underlying nylon membrane pore channels and react with the adsorbed 2,4-DCP with a much higher rate (0.9575 mg L^-1 min^-1) than that in the suspended MnOOH particle system (0.1493 mg L^-1 min^-1). The forced flow in the small voids of the MnOOH nanoparticle coating layer (< 200 nm) and channels of nylon membrane (~220 nm) is critical to improve the 2,4-DCP adsorption, ROS production, and 2,4-DCP degradation. The hybrid MnOOH@nylon membrane also improves the stability of the MnOOH nanoparticles and the resistibility to competitive anions, due to much higher concentration ratio of the adsorbed 2,4-DCP and produced ROS versus background competitive ions in the membrane phase. This study provides a generally applicable approach to achieve high removal of target contaminants in catalytic membrane processes.

Uniform Mesoporous Amorphous Cobalt-Inherent Silicon Oxide as a Highly Active Heterogeneous Catalyst in the Activation of Peroxymonosulfate for Rapid Oxidation of 2,4-Dichlorophenol: The Important Role of Inherent Cobalt in the Catalytic Mechanism

Amorphous cobalt-inherent silicon oxide (Co-SiOx) was synthesized for the first time and employed as a highly active catalyst in the activation of peroxymonosulfate (PMS) for the rapid oxidation of 2,4-dichlorophenol (2,4-DCP). The characterization results revealed that the 0.15Co-SiOx possessed a high specific surface area of 607.95 m²/g with a uniform mesoporous structure (24.33 nm). The X-ray diffraction patterns indicate that the substituted cobalt atoms enlarge the unit cell parameter of the original SiO₂, and the selected area electron diffraction pattern confirmed the amorphous nature of Co-SiOx. More bulk oxygen vacancies (O_v) existing in the Co-SiOx were identified to be one of the primary contributors to the significantly enhanced catalytic activation of PMS. The cobalt substitution both creates and stabilizes the surficial O_v and forms the adequately active Co(II)-O_v pairs which engine the electron transfer process during the catalytic activities. The active Co(II)-O_v pairs weaken the average electronegativity of Co/Si and Co/O sites, resulting in the prevalent changes in final state energy, which is the main driving cause of the binding energy shifts in the X-ray photoelectron spectroscopy (XPS) spectra of Si and O among all samples. The increase of the relative proportion of Co(III) in the spent Co-SiOx probably causes the binding energy shifts of the Co XPS spectrum compared to that of the Co-SiOx. The amorphous Co-SiOx outperforms stable and quick 2,4-DCP degradation, achieving a much higher kinetic rate of 0.7139 min^-1 at pH = 7.02 than others via sulfate radical advanced oxidation processes (AOPs), photo-Fenton AOPs, H₂O₂ reagent AOPs, and other AOP approaches. The efficient degradation performance makes the amorphous Co-SiOx as a promising catalyst in removing 2,4-DCP or organic-rich pollutants.