Asymmetrically Coordinated CoB1 N3 Moieties for Selective Generation of High-Valence Co-Oxo Species via Coupled Electron-Proton Transfer in Fenton-like Reactions

High-performance nitrogen-doped carbon catalyst with Co-Cu-CuxO interfaces via bimetallic ion exchange-carbonization: Synergistic Co/Cu interactions and nonradical activation mechanism for micropollutant removal

Mono-metal active sites, with their restricted electron transfer ability, typically lead to lower redox reaction efficiency, which hampers peroxymonosulfate (PMS) activation and reduces antibiotic degradation effectiveness. In this work, a novel nitrogen-doped carbon catalyst with Co-Cu-CuxO interfaces was synthesized by pyrolyzing a Zn-based elliptical two-dimensional template through a Co2+/Cu2+ bimetallic ion exchange process. The synthesized samples were comprehensively characterized using a range of physicochemical analysis techniques. Furthermore, the catalytic performance was systematically evaluated under varying conditions, including peroxymonosulfate dosage, tetracycline concentration, solution pH, and the influence of co-existing ions and organic matter in water. The results indicated that the optimized 1:1-950 catalyst achieved over 96 % degradation of tetracycline (TC) through PMS activation, with a reaction rate constant (k) of 0.038 min-1, significantly outperforming both the mono-metal ion exchange group and the non-metal ion exchange group. This improvement was attributed to the synergistic effects of Co(II)/Co(III) and Cu(I)/Cu(II) redox reactions at the Co-Cu-CuxO interfaces. Quenching experiments, electron spin resonance (ESR), and electrochemical analyses revealed that non-radical reactive oxygen species (ROS), such as singlet oxygen (1O2) and high-valent metal-oxo species (e.g., Cu(III)-oxo and Co(IV)-oxo), played a key role in the degradation process. The degradation pathways for TC were proposed using high-performance liquid chromatography-mass spectrometry (HPLC-MS), and the environmental safety of the catalytic system was confirmed through physiological testing on mung bean growth. This work presents an efficient approach for PMS activation in TC degradation, using nitrogen-doped carbon catalysts with Co-Cu-CuxO interfaces synthesized via bimetallic ion exchange and carbonization strategy, with promising applications in advanced wastewater treatment.

Mo doping modulates peroxymonosulfate activation of cobalt carbon nanotube-based catalysts for efficient multi-pollutants removal: Oxygen vacancies trigger the evolution of high-valence cobalt-oxo species

Formation of defective catalysts with oxygen-rich vacancies via ion doping represents an advanced strategy for enhancing catalytic activity. Cobalt oxides supported on carbon nanotubes (CNTs) were strategically designed to enhance peroxomonosulfate (PMS) through molybdenum (Mo)-doped oxygen vacancies (Vo) management to achieve the application of high-valent cobalt oxygen (Co(IV)O) dominated degradation mechanism. The first-order rate constant for tetracycline hydrochloride degradation in the CoMo/CNTs/PMS system (0.081 min-1) was twice that of the Co/CNTs system. Additionally, the system exhibited high resistance to interference and excellent pH adaptability. The system demonstrated a high removal efficiency (91.3 %-100 %) for the emerging contaminant (sodium p-perfluorous nonenoxybenzene sulfonate), as well as other organic pollutants such as carbamazepine and imidacloprid. The prepared catalyst membranes exhibited stability and effectively degraded tetracycline hydrochloride over 10 h of continuous-flow experiments, highlighting their potential for practical applications. Theoretical calculations revealed that molybdenum doping reduced the formation energy of oxygen vacancies, while these vacancies shifted the d-band center of cobalt (Co) in CoMo/CNTs upward, bringing it closer to the Fermi energy level. Furthermore, enhanced charge transfer and stronger peroxide bond stretching were observed during PMS adsorption, thus promoting the chemical reaction of PMS adsorbed on CoMo/CNTs. More significantly, the oxygen-rich vacancies in CoMo/CNTs lowered the energy barrier for Co(IV)=O generation. This study provides insights into the mechanism of ionic doping in PMS activation by metal-based catalysts, thereby expanding the application of defective catalysts in environmental remediation.

State-Of-The-Art Structural Regulation Methods and Quantum Chemistry for Carbon-Based Single-Atom Catalysts in Advanced Oxidation Process: Critical Perspectives into Molecular Level

Advanced oxidation processes (AOPs) by carbon-based single-atom catalysts (SACs) are recognized as an attractive scientific frontier for water treatment, with the outstanding benefits of ultra-effective and anti-interference capability. However, most of the research has paid more attention to the performance of SACs, while the in-depth understanding of catalytic regulation by molecular interaction is relatively deficient. This critical review delves into deciphering the catalytic mechanism through a micro-level, which makes it more convenient to interpret apparent catalytic phenomena. It first summarizes basic theories of quantum chemistry, which provide mechanism interpretation and prediction for molecular-oxidation systems. Additionally, corresponding oxidation pathways of common oxidants are underscored. Following the oxidants, state-of-the-art regulation methods are discussed with special attention to involved molecular interactions and pollutants. Particularly, the preliminary insights into the "oxidant-catalyst-pollutants" internal relationships are provided to help construct the SAC-AOP system from a molecular standpoint. Meanwhile, some cutting-edge laboratory devices and pilot-scale engineering are presented to illustrate the ultimate purpose of scientific molecular exploration. Eventually, relative challenges of SACs-AOPs upon the design of catalytic systems and investigation methods are provided. This review aims to promote the large-scale potential of SACs-based AOPs in practical water treatment by emphasizing the pivotal role of micro-insights.

Site-Specific Spin State Modulation in Spinel Oxides for Enhanced Nonradical Oxidation

Spinel oxides hold tremendous potential for driving advanced oxidation processes, yet the underlying mechanism for maximizing their activity remains unclear. In this study, we leverage tetrahedral and octahedral site interactions in MnxCo3-xO4 to modulate the spin states, specifically spin alignment and spin moment, thereby enhancing periodate (PI) activation and catalytic performance in contaminant degradation. Through combined experimental and density functional theory (DFT) analyses, we elucidate the role of spin alignment at synergetic tetrahedral and octahedral sites in facilitating quantum spin exchange interactions (QSEI) with an efficient electronic spin channel for charge transfer. Meanwhile, the engineered high spin configuration in CoMn2O4 raises the d-band center, favoring stable PI* surface complex formation and accelerating the rate-determining desorption of IO3 - with a lower-ICOHP value during the catalytic degradation of ciprofloxacin. As a result, the fine-tuned spin state of CoMn2O4 leads to enhanced overall reaction kinetics, with a 2.5-fold increase compared to MnCo2O4 and up to 22-fold increase compared to the octahedrally-active only catalysts. Such a site-specific modulation has been found applicable to other spinel oxides, enlightening fine-tuned electronic structure for maximizing catalytic performance.

Synchronization Strategy for Activity and Stability in Fenton-Like Single-Atom Catalysis

Single-atom catalysts (SACs) have garnered significant attention in the applications of environmental remediation based on Fenton-like systems. Current research on Fenton-like single-atom catalysis often emphasizes catalytic activity and mechanism regulation, while paying limited attention to the simultaneous enhancement of both activity and stability-a critical factor for the practical and scale-up applications of SACs. This review systematically summarizes recent advances in synchronization strategies for improving the activity and stability of Fenton-like single-atom catalysis, with a focus on the design principles and mechanisms of four key strategies: coordination engineering, confinement effects, carrier substitution, and catalytic module design. To the best of knowledge, this represents the first comprehensive review of Fenton-like single-atom catalysis from the perspective of concurrent optimization of activity and stability. Additionally, the auxiliary role of machine learning and lifecycle assessment (LCA) is evaluated in advancing these synchronization strategies. By investigating the interplay among different support materials, coordination configurations, and reaction environments, as well as enlarged modules, key factors governing the stability/activity of SACs are highlighted, and future directions are proposed for developing next-generation catalysts with high efficiency and long-term durability for practical environmental remediation.

CoSn(OH)6 nanocubes: Hydroxyl perovskite catalyst for efficient peroxymonosulfate activation in acetamiprid degradation

This study presents the synthesis of a nano-cubic metal hydroxide with a perovskite structure, CoSn(OH)6, for the efficient activation of peroxymonosulfate (PMS) towards the degradation of acetamiprid (ACE) in water treatment. The CoSn(OH)6/PMS system achieved complete degradation of ACE within only 12 min and exhibited outstanding catalytic stability. Our findings indicate that the non-radical mechanism, featuring singlet oxygen (1O2) and Co(IV)=O, is the primary contributor to the degradation process, while the role of radical species such as sulfate radical (SO4·-) and hydroxyl radicals (·OH) is subordinate. These insights were confirmed through trapping experiments, electron paramagnetic resonance (EPR), in situ Raman spectroscopy and steady-state model. This work offers novel perspectives on the application of cobalt-based hydroxide catalysts in PMS activation for the remediation of emerging contaminants in water.

Enhancing 1O2 Generation Performance by Regulating C and N Coordination for Efficient Fenton-Like Catalytic Process

Singlet oxygen (1O2), a vital reactive species, exhibits excellent organic pollutant degradation selectivity in Fenton-like reactions. Recognizing and controlling the structure-activity relationship of single-atom catalysts (SACs) is essential to achieving the highly efficient and selective generation of 1O2 for various practical applications. Here, three iron single-atom catalysts with different coordination configurations (FeSAC─N4, FeSAC─N3-C1, and FeSAC─N2─C2) are prepared to modulate the selective generation of 1O2 by activating peroxymonosulfate (PMS). Replacing N coordinated to Fe atoms with C increases 1O2 selective generation thus enhancing the Fenton-like reaction activity. Specifically, FeSAC─N2─C2 presents the optimal catalytic activity, high stability, and environmental tolerance. Moreover, the 1O2 selectivity increases as the N coordination number decreases, which is in the order of FeSAC─N4 (73%) < FeSAC─N3─C1 (82%) < FeSAC─N2─C2 (90%). DFT calculations demonstrate that replacing N with C enhances the electrophilicity and electron transfer capacity, optimizes the d-band center, facilitates reactant adsorption, and reduces the energy barrier, thus facilitating 1O2 production and enhancing the Fenton-like reaction activity. This study reveals the underlying catalytic trends and mechanisms of catalyst structure-activity relationships for high selective generation of 1O2 by PMS activation, thus providing guidance for developing catalysts capable of highly selective organic pollutant degradation.

Sulfur Bridge Geometry Boosts Selective FeIV═O Generation for Efficient Fenton-Like Reactions

High-valent iron-oxo species (FeIV═O) is a fascinating enzymatic agent with excellent anti-interference abilities in various oxidation processes. However, selective and high-yield production of FeIV═O remains challenging. Herein, Fe diatomic pairs are rationally fabricated with an assisted S bridge to tune their neighbor distances and increase their loading to 11.8 wt.%. This geometry regulated the d-band center of Fe atoms, favoring their bonding with the terminal and hydroxyl O sites of peroxymonosulfate (PMS) via heterolytic cleavage of O─O, improving the PMS utilization (70%), and selective generation of FeIV═O (>90%) at a high yield (63% of PMS) offers competitive performance against state-of-the-art catalysts. These continuous reactions in a fabricated device and technol-economic assessment further verified the catalyst with impressive long-term activity and scale-up potential for sustainable water treatment. Altogether, this heteroatom-bridge strategy of diatomic pairs constitutes a promising platform for selective and efficient synthesis of high-valent metal-oxo species.

Intrinsic strain of defect sites steering chlorination reaction for water purification

Carbon nanotube (CNT)-based heterogeneous advanced oxidation processes (AOPs) used for water purification have been exploited for several decades. Many strategies for modifying CNTs have been utilized to improve their catalytic performance in remediation processes. However, the strain fields of the intrinsic defect sites on CNT steering AOPs (such as chlorination) have not yet been reported. Here, we explored the strained defect sites for steering the chlorination process for water purification. The strained defect sites with the elongated sp2 hybridized C-C bonds boost electronic reactivity with the chlorine molecules via the initial Yeager-type adsorption. As a result, the reactive species in chlorination can be regulated on demand, such as the ratio of high-selectivity ClO• ranging from 38.8% in conventional defect-based systems to 87.5% in our strain-dominated process, which results in the generation of harmless intermediates and even deep mineralization during 2,4-DCP abatement. This work highlights the role that strain fields have on controlling the extent of chlorination reactions.

Surface-hydroxylated single-atom catalyst with an isolated Co-O-Zn configuration achieves high selectivity in regulating active species

Single-atom catalysts (SACs) are emerging as potent tools for the selective regulation of active species, offering substantial promise for green and sustainable Fenton catalysis. However, current SACs face limitations due to the specificity of their supports, which only allow selective regulation within certain oxidant systems. This constraint makes targeted regulation across different systems challenging. In response, this study designs a SAC, termed CoSAs-ZnO, featuring surface hydroxylation and an isolated asymmetric Co-O-Zn configuration. This SAC can realize a nearly 100% selective generation of sulfate radicals (SO4•-) and singlet oxygen (1O2) in peroxymonosulfate (PMS) and peracetic acid (PAA) systems, respectively. Moreover, the PMS-activated system can efficiently treat electron-deficient-dominated and refractory benzoic acid wastewater, achieving 100.0% removal in multiple consecutive pilot-scale experiments. The PAA-activated system facilitates the rapid conversion of benzyl alcohol to benzaldehyde, with a high selectivity of 89.0%. Detailed DFT calculations reveal that the surface hydroxyl groups on ZnO play a critical role in modulating the adsorption configurations of the oxidants, thus enabling the selective generation of specific active species in each system. This study provides insights into the design of SACs for multifunctional applications and paves the way for their deployment in wastewater treatment and high-value chemical conversion.

Robust Fe-N4-C6O2 single atom sites for efficient PMS activation and enhanced FeIV = O reactivity

The microenvironment regulation of Fe-N4 single atom catalysts (SACs) critically governs peroxymonosulfate (PMS) activation. Although conventional heteroatom substitution in primary coordination enhances activity, it disrupts Fe-N4 symmetry and compromises stability. Herein, we propose oxygen doping in the secondary coordination shell to construct Fe-N4-C6O2 SAC, which amplifies the localized electric field while preserving the pristine coordination symmetry, thus trading off its activity and stability. This approach suppresses Fe-N bond structural deformation (bond amplitude reduced from 0.875-3.175 Å to 0.925-2.975 Å) during PMS activation by lowering Fe center electron density to strengthen Fe-N bond, achieving extended catalytic durability (>240 h). Simultaneously, the weakened coordination field lowers the Fe=O σ* orbital energy, promoting electrophilic σ-attack of high-valent iron-oxo towards bisphenol A, and increasing its degradation rate by 41.6-fold. This work demonstrates secondary coordination engineering as a viable strategy to resolve the activity-stability trade-off in SAC design, offering promising perspectives for developing environmental catalysts.

Acidic and Alkaline pH Controlled Oxygen Reduction Reaction Pathway over Co-N4C Catalyst

Enhanced oxygen reduction reaction (ORR) kinetics and selectivity are crucial to advance energy technologies like fuel cells and metal-air batteries. Single-atom catalysts (SACs) with M-N4/C structure have been recognized to be highly effective for ORR. However, the lack of a comprehensive understanding of the mechanistic differences in the activity under acidic and alkaline environments is limiting the full potential of the energy devices. Here, a porous SAC is synthesized where a cobalt atom is coordinated with doped nitrogen in a graphene framework (pCo-N4C). The resulting pCo-N4C catalyst demonstrates a direct 4e- ORR process and exhibits kinetics comparable to the state-of-the-art (Pt/C) catalyst. Its higher activity in an acidic electrolyte is attributed to the tuned porosity-induced hydrophobicity. However, the pCo-N4C catalyst displays a difference in ORR activity in 0.1 m HClO4 and 0.1 m KOH, with onset potentials of 0.82 V and 0.91 V versus RHE, respectively. This notable activity difference in acidic and alkaline media is due to the protonation of coordinated nitrogen, restricted proton coupled electron transfer (PCET) at the electrode/electrolyte interface. The effect of pH over the catalytic activity is further verified by Ab-initio molecular dynamics (AIMD) simulations using density functional theory (DFT) calculations.

Comediating Adsorption and Electron Transfer via Dual-Active Site Catalyst Construction for Improving the Treatment of Extraction Wastewater

Solvent extraction is widely applied, while extraction wastewater treatment remains a huge challenge because of the stability of extractants. Heterogeneous Fenton-like catalysis is a promising method, but the short half-life of hydroxyl radicals (⋅OH) generated by hydrogen peroxide (H2O2) activation results in unsatisfactory ⋅OH utilization and organics removal. Herein, an efficient strategy for treating extraction wastewater based on comediating adsorption and electron transfer by fluorine and nitrogen co-doped carbon (FNC) catalyst with dual-active site was developed. Specially, N sites adsorb organics and F sites activate H2O2, shortening the migration distance of ⋅OH. Theoretical calculation and di(2-ethylhexyl) phosphoric acid (D2EHPA) extraction wastewater degradation experiment showed that F site with electron acquisition can transfer electrons provided by electron-rich D2EHPA enriched at N sites to H2O2, facilitating the continuous generation of ⋅OH through lowering the energy barrier for H2O2 activation. As a result, 96.49 % D2EHPA in simulated wastewater and 90.26 % total organic carbon in real extraction wastewater were removed. Moreover, FNC catalyst exhibited excellent reusability and ionic adaptability, and can be extended to the removal of various extractants. The proposed dual-active site catalyst provides an effective strategy for Fenton-like reaction to treat refractory extraction wastewater, promoting sustainable development of solvent extraction industry.

Revealing Crucial Influences of Boron Support on Regulating Geometric and Electronic Structures of 3D Catalyst for Hydrogen Evolution and Oxygen Reduction Reactions

Building insights into the structure-performance relationship of catalysts has been emphasized recently. However, it remains a challenge due to catalysts' various and complex structures, especially the easily overlooked influence of the support material. Here, we reveal the crucial influences of boron introduction on synthesizing 3D carbon nanotube monoliths with embedded multistate Co metals, i.e., single atoms, clusters, and nanoparticles (Co-BNCNTs), by an interesting chemical blowing-assisted calcination method. The boron introduction can contribute to forming captivating boron-nitrogen pairs, shaping a 3D frame, and regulating electronic structure. The 3D Co-BNCNT monoliths present good catalytic performance for both the hydrogen evolution reaction (HER) at all pH values and the oxygen reduction reaction (ORR). The theoretical calculations indicate that the B incorporation in Co-NCNTs can optimize the free energy of adsorbed hydrogen and facilitate the O2 adsorption and the protonation of the O2* species. Furthermore, the Co-BNCNTs-based zinc-air battery provides great battery performance with a high power density and discharge-charge durability.

Non-metallic iodine single-atom catalysts with optimized electronic structures for efficient Fenton-like reactions

In this study, we introduce a highly effective non-metallic iodine single-atom catalyst (SAC), referred to as I-NC, which is strategically confined within a nitrogen-doped carbon (NC) scaffold. This configuration features a distinctive C-I coordination that optimizes the electronic structure of the nitrogen-adjacent carbon sites. As a result, this arrangement enhances electron transfer from peroxymonosulfate (PMS) to the active sites, particularly the electron-deficient carbon. This electron transfer is followed by a deprotonation process that generates the peroxymonosulfate radical (SO5•-). Subsequently, the SO5•- radical undergoes a disproportionation reaction, leading to the production of singlet oxygen (1O2). Furthermore, the energy barrier for the rate-limiting step of SO5•- generation in I-NC is significantly lower at 1.45 eV, compared to 1.65 eV in the NC scaffold. This reduction in energy barrier effectively overcomes kinetic obstacles, thereby facilitating an enhanced generation of 1O2. Consequently, the I-NC catalyst exhibits remarkable catalytic efficiency and unmatched reactivity for PMS activation. This leads to a significantly accelerated degradation of pollutants, evidenced by a relatively high observed kinetic rate constant (kobs ~ 0.436 min-1) compared to other metallic SACs. This study offers valuable insights into the rational design of effective non-metallic SACs, showcasing their promising potential for Fenton-like reactions in water treatment applications.

Integration of single-atom photothermal catalysts with surface-localized high temperature in peroxymonosulfate-based Fenton-like systems for enhanced antibiotics degradation

This study delves into integrating single-atom catalysts with photothermal effect in peroxymonosulfate (PMS)-based Fenton-like systems for enhanced pollutant degradation. A single-atom photothermal catalyst (Co/PMCNs) was designed using mesoporous carbon spheres as both a single-atom support and a photothermal material. Near-infrared (NIR) light was employed due to its superior thermal effect and penetration capacity in water. It was found that Co/PMCNs could generate surface-localized high temperatures for accelerating PMS activation and reducing energy gap of activation reactions, leading to improved degradation performance. Surface-localized high temperatures were demonstrated as key in distinguishing photothermal heating from external heat sources for PMS activation. Moreover, this system performed well across various operating conditions and water matrices, with Co/PMCNs showing promising recyclability. This study highlights the impact of surface-localized high temperatures on heterogeneous catalysis under NIR irradiation, and underscores the potential of integrating single-atom catalysts with photothermal effects into advanced oxidation processes for effective water pollution control.

Nano-island-encapsulated cobalt single-atom catalysts for breaking activity-stability trade-off in Fenton-like reactions

Single-atom catalysts (SACs) have been increasingly acknowledged for their performance in sustainable Fenton-like catalysis. However, SACs face a trade-off between activity and stability in peroxymonosulfate (PMS)-based systems. Herein, we design a nano-island encapsulated single cobalt atom (CoSA/Zn.O-ZnO) catalyst to enhance the activity and stability of PMS activation for contaminant degradation via an "island-sea" synergistic effect. In this configuration, small carrier-based ZnO nanoparticles (the "islands") are utilized to confine and stabilize Co single atoms. The expansive ZnO substrate (the "sea") upholds a neutral microenvironment within the reaction system. The CoSA/Zn.O-ZnO/PMS system exhibits a remarkable selectivity in exclusively generating sulfate radicals (SO4•-), leading to a complete removal of various recalcitrant pollutants within a shorter period. Characterized by minimal leaching of active sites, robust catalytic performance, and low-toxicity decontamination, this system proves highly efficient in multiple treatment cycles and complex water matrices. The design effectively breaks the activity-stability trade-off typically associated with SACs.

Low-peroxide-consumption fenton-like systems: The future of advanced oxidation processes

Conventional heterogeneous Fenton-like systems employing different peroxides have been developed for water/wastewater remediation. However, a large population of peroxides consumed during various Fenton-like systems with low utilization efficiency and associated secondary contamination have become the bottlenecks for their actual applications. Recent strategies for lowering the peroxide consumptions to develop economic Fenton-like systems are primarily devoted to the effective radical generation and subsequent high-efficiency radical utilization through catalysts/systems engineering, leveraging emerging nonradical oxidation pathways with higher selectivity and longer life of the reactive intermediate, as well as reactor designs for promoting the mass transfer and peroxides decomposition to improve the yield of radicals/nonradicals. However, a comparative review summarizing the mechanisms and pathways of these strategies has not yet been published. In this review, we endeavor to showcase the designated systems achieving the reduction of peroxides while ensuring high catalytic activity from the perspective of the above strategic mechanisms. An in-depth understanding of these aspects will help elucidate the key mechanisms for achieving economic peroxide consumption. Finally, the existing problems of these strategies are put forward, and new ideas and research directions for lowering peroxide consumption are proposed to promote the application of various Fenton-like systems in actual wastewater purification.

Sabatier Principle-Driven Single-Atom Coordination Engineering for Enhanced Fenton-Like Catalysis

Single-atom catalysts (SACs) are widely employed in Fenton-like catalysis, yet guidelines for their high-performance design remain elusive. The Sabatier principle provides guidance for the ideal catalyst with the highest activity. Herein, the study meticulously engineered a series of SACs featuring a broad distribution of d-band center through single-atom coordination engineering, facilitating a comprehensive exploration of the Sabatier relationship in Fenton-like catalysis. A volcanic correlation between d-band centers and catalytic activity is identified. Theoretical and experimental results show that moderate d-band center and peroxymonosulfate adsorption energy can lead to the lowest reaction barriers in the rate-determining step for generating singlet oxygen, thus enhancing catalytic efficiency toward the Sabatier optimum. As proof of concept, the Fe-N2O2/C catalyst demonstrates a degradation rate constant of 1.89 min-1, surpassing Fe-N4/C by 3.2 times and Fe-O4/C by 272 times. Moreover, Fe-N2O2/C shows exceptional tolerance to various environmental challenges, providing opportunities for achieving nearly eco-friendly pollutant degradation. The findings reveal how to use the Sabatier principle to guide the design of advanced SACs for efficient pollutant removal.

Support work-function dependent Fenton-like catalytic activity of Co single atoms for selective cobalt(IV)=O generation

In Fenton-like reactions, high-valent cobalt-oxo (CoIV=O) has attracted increasing interests due to high redox potential, long lifetime, and anti-interference properties, but its generation is hindered by the electron repulsion between the electron rich oxo- and cobalt centers. Here, we demonstrate CoIV=O generation from peroxymonosulfate (PMS) activation over cobalt single-atom catalysts (Co-SACs) using in-situ Co K-edge X-ray absorption spectra, and discern that CoIV=O generation is dependent on the support work-function (WF) due to the strong electronic metal-support interaction (EMSI). Supports with a high WF value like anatase-TiO2 facilitate the binding of PMS-terminal oxo-ligand to Co sites by extracting Co-d electrons, thus decreasing the generation barrier for the critical intermediate (Co-OOSO32-). The Co atoms anchored on anatase-TiO2 (Co-TiO2) exhibited enhanced CoIV=O generation and superior activity for sulfamethoxazole (SMX) degradation during PMS activation. The normalized steady-state concentration of CoIV=O in Co-TiO2/PMS system was three orders of magnitude higher than that of free radicals, and 1.3- to 11-fold higher than that generated in other Co-SACs/PMS systems. Co-TiO2/PMS sustained efficient removal of SMX with minimal Co2+ leaching under continuous flow operation, suggesting its attractive water purification potential. Overall, these results underscore the significance of support selection for enhanced generation of high-valent metal-oxo species and efficient PMS activation in supported metal SACs.

Bridge-oxygen bonding modulates Ru single atoms for peroxymonosulfate activation: Importance of high-valent Ru species and 1O2

The application of single-atom catalysts (SACs) to advanced oxidation processes (AOPs) based on peroxymonosulfate (PMS) has attracted considerable attention. However, the catalytic pathways and mechanisms underlying these processes remain unclear. In this study, NiFe-LDH was synthesized and single Ru atoms were stably loaded onto it by forming Ru-O-M (M=Ni or Fe) bonds (Ru@NiFe-LDH). This was demonstrated using high-angle annular dark-field scanning TEM (HAADF-STEM) and X-ray absorption fine structure spectra (XANES). The Ru@NiFe-LDH/PMS system showed a high catalytic reactivity (100 % sulfamethoxazole degradation in only 30 min), high stability (97 % reactivity was maintained after continuous operation for 400 min), and wide pH suitability (working pH range 3-11) for AOPs. The crucial roles of the high-valent species (Ru(V) = O) and 1O2 in this reaction were verified. Density functional theory (DFT) calculations revealed that electron transfer produced a positively charged Ru. This enhances the adsorption of negatively charged PMS anions onto the Ru monoatomic sites, thereby, causing the formation of Ru-PMS* complexes. This study implies that the structure-function relationship between organic compounds and SACs plays a significant role in PMS-based AOPs, and provides a comprehensive mechanism for the role of high-valent species in heterogeneous Fenton-like systems.

Electrochemically Assisted Cobalt/MXene Membrane for Effective Water Treatment: Synchronously Improving Catalytic Performance and Anti-Interference Ability

Catalytic membrane technology for water treatment is often constrained by a trade-off between permeability and catalytic efficiency as well as interference from coexisting anions and organic matter in natural water matrices. Herein, a novel cobalt-loaded MXene (Co/MXene) 2D membrane with good hydrophilicity, electrical conductivity, and PMS activation function is constructed. The negative voltage is exerted on the membrane to significantly enhance its PMS activation efficiency and anti-interference capacity toward effective water treatment. Under -2 V, the optimal Co/MXene catalytic membrane displays 100% rhodamine b (RhB) removal within a residence time of only 1.1 s, whose RhB degradation kinetic constant (k of 6.85 s-1) is 17.6 times higher than that of the Co/MXene catalytic membrane alone and is also greatly superior to other advanced catalysts and catalytic membranes. Meanwhile, the catalytic membrane displays obvious anti-interference ability in the presence of various coexisting substances of the water matrix and performs well in treating the secondary effluent of coking wastewater. The radical-dominated (SO4•- and •OH) mechanism accompanied by the nonradical species (1O2 and Co(VI)═O) is revealed in the system, and the reactive species production is obviously enhanced under negative voltage. Experimental results and theoretical calculations jointly confirm the key role of electrochemical assistance in enhancing membrane performance, which not only facilitates cycling of Co3+/Co2+ for enhanced PMS activation via improving PMS adsorption and promoting charge transfer from Co to PMS but also hinders interference from coexisting substances in water via electrostatic repulsion.

Coordination engineering of heterogeneous high-valent Fe(IV)-oxo for safe removal of pollutants via powerful Fenton-like reactions

Coordination engineering of high-valent Fe(IV)-oxo (FeIV=O) is expected to break the activity-selectivity trade-off of traditional reactive oxygen species, while attempts to regulate the oxidation behaviors of heterogeneous FeIV=O remain unexplored. Here, by coordination engineering of Fe-Nx single-atom catalysts (Fe-Nx SACs), we propose a feasible approach to regulate the oxidation behaviors of heterogeneous FeIV=O. The developed Fe-N2 SACs/peroxymonosulfate (PMS) system delivers boosted performance for FeIV=O generation, and thereby can selectively remove a range of pollutants within tens of seconds. In-situ spectra and theoretical simulations suggest that low-coordination Fe-Nx SACs favor the generation of FeIV=O via PMS activation as providing more electrons to facilitate the desorption of the key *SO4H intermediate. Due to their disparate attacking sites to sulfamethoxazole (SMX) molecules, Fe-N2 SACs mediated FeIV=O (FeIVN2=O) oxidize SMX to small molecules with less toxicity, while FeIVN4=O produces series of more toxic azo compounds through N-N coupling with more complex oxidation pathways.

Electron transfer mediated activation of periodate by contaminants to generate 1O2 by charge-confined single-atom catalyst

The electron transfer process (ETP) is able to avoid the redox cycling of catalysts by capturing electrons from contaminants directly. However, the ETP usually leads to the formation of oligomers and the reduction of oxidants to anions. Herein, the charge-confined Fe single-atom catalyst (Fe/SCN) with Fe-N3S1 configuration was designed to achieve ETP-mediated contaminant activation of the oxidant by limiting the number of electrons gained by the oxidant to generate 1O2. The Fe/SCN-activate periodate (PI) system shows excellent contaminant degradation performance due to the combination of ETP and 1O2. Experiments and DFT calculations show that the Fe/SCN-PI* complex with strong oxidizing ability triggers the ETP, while the charge-confined effect allows the single-electronic activation of PI to generate 1O2. In the Fe/SCN + PI system, the 100% selectivity dechlorination of ETP and the ring-opening of 1O2 avoid the generation of oligomers and realize the transformation of large-molecule contaminants into small-molecule biodegradable products. Furthermore, the Fe/SCN + PI system shows excellent anti-interference ability and application potential. This work pioneers the generation of active species using ETP's electron to activate oxidants, which provides a perspective on the design of single-atom catalysts via the charge-confined effect.

Atomically Dispersed Cobalt Anchored on Hollow Tubular Carbon Nitride Mediates Direct Electron Transfer and Oxygen-Related Active Species Path for Activation of Permonosulfate

Atomically dispersed catalysts anchored on nitrogen-rich substrates present promising application potential for the persulfate-based advanced oxidation process. Nevertheless, efficient activation efficiency and a clear activated mechanism of persulfate remain challenging in carbon nitride-based single-atom catalysts (SACs). To these, combined with the regulation strategy of metal-ligand section and carrier's architecture, an atomically dispersed Co single-atom catalyst anchored on regular hollow tubular carbon nitride (Co/TCN SAC) herein was devised and utilized to activate permonosulfate. As a result, Co/TCN SACs show excellent catalytic performance for the degradation of common antibiotics. Combined with X-ray absorption fine structure and theory calculation, it is confirmed that superficially anchored CoO3 sites of the Co2N2O2-CoO3 unit are the catalytic active center for peroxymonosulfate (PMS) activation. The electrochemical test and in situ electron paramagnetic resonance results demonstrate radical (SO4•- and •OH) and nonradical (electron transfer process and 1O2) paths contributing to the superior catalytic performance. In addition, the catalyst exhibits high reaction efficiency and structural stability considering water quality parameters. Finally, a continuous and efficient device was operated on a laboratory scale, which exhibited satisfactory efficiency in continuously removing electron-rich antibiotics such as tetracycline. This work reveals the atomic-level modulation of cobalt atomic sites on hollow tubular carbon nitride and their structure-activity relationship with persulfate activation.

Microenvironment Engineering of Heterogeneous Catalysts for Liquid-Phase Environmental Catalysis

Environmental catalysis has emerged as a scientific frontier in mitigating water pollution and advancing circular chemistry and reaction microenvironment significantly influences the catalytic performance and efficiency. This review delves into microenvironment engineering within liquid-phase environmental catalysis, categorizing microenvironments into four scales: atom/molecule-level modulation, nano/microscale-confined structures, interface and surface regulation, and external field effects. Each category is analyzed for its unique characteristics and merits, emphasizing its potential to significantly enhance catalytic efficiency and selectivity. Following this overview, we introduced recent advancements in advanced material and system design to promote liquid-phase environmental catalysis (e.g., water purification, transformation to value-added products, and green synthesis), leveraging state-of-the-art microenvironment engineering technologies. These discussions showcase microenvironment engineering was applied in different reactions to fine-tune catalytic regimes and improve the efficiency from both thermodynamics and kinetics perspectives. Lastly, we discussed the challenges and future directions in microenvironment engineering. This review underscores the potential of microenvironment engineering in intelligent materials and system design to drive the development of more effective and sustainable catalytic solutions to environmental decontamination.

Proton vs Electron: The Dual Role of Redox-Inactive Metal Ions in Permanganate Oxidation Kinetics

Redox-inactive metal-ion-driven modulation of the oxidation behavior of high-valent metal-oxo complex has garnered significant interest in biological and chemical synthesis; however, their role in permanganate (Mn(VII)) oxidation for the removal of organic pollutants has been largely neglected. Here, we uncover the impact of six metal ions (i.e., Ca2+, Mg2+, Ni2+, Zn2+, Al3+, and Sc3+) presenting in water environments on Mn(VII) activity. These ions uniformly boost the electron and oxygen transfer capabilities of Mn(VII) while impeding proton transfer, as evidenced by electrochemical tests, thioanisole probe analysis, and the kinetic isotope effect. The observed effects are intricately linked to the Lewis acidity of the metal ions. Further mechanistic insights reveal that Mn(VII) can interact with metal ions without direct reduction. Such interactions modify the electronic configuration of Mn(VII) and create an acidic microenvironment, thus increasing its electrophilicity and the energy barrier for the abstraction of proton from organic substrates. More importantly, the efficacy of Mn(VII) in removing phenolic pollutants is regulated by these ions through changing the driving force for proton and electron transfer, i.e., facilitated at pH > 4.5 and inhibited at lower pH. The contribution of active Mn intermediates is also discussed to reveal the oxidative mechanism of the metal ion/Mn(VII) system. These findings not only facilitate the rational design of Mn(VII) oxidation conditions in the presence of metal ions for water decontamination but also offer an alternative paradigm for enhancing electrophilic oxidation.

Functional biochar accelerates peroxymonosulfate activation for organic contaminant degradation via the specific B-C-N configuration

Functional biochar designed with heteroatom doping facilitates the activation of peroxymonosulfate (PMS), triggering both radical and non-radical systems and thus augmenting pollutant degradation efficiency. A sequence of functional biochar, derived from hyperaccumulator (Sedum alfredii) residues, was synthesized via sequential doping with boron and nitrogen. The SABC-B@N-2 exhibited outstanding catalytic effectiveness in activating PMS to degrade the model pollutant, acid orange 7 (Kobs = 0.0655 min-1), which was 6.75 times more active than the pristine biochar and achieved notable mineralization efficiency (71.98%) at reduced PMS concentration (0.1 mM). Relative contribution evaluations, using steady-state concentrations combined with electrochemical and in situ Raman analyses, reveal that co-doping with boron and nitrogen alters the reaction pathway, transitioning from PMS activation through multiple reactive oxygen species (ROSs) to a predominantly non-radical process facilitated by electron transfer. Moreover, the previously misunderstood concept that singlet oxygen (1O2) plays a central role in the degradation of AO7 has been clarified. Correlation analysis and density functional theory calculations indicate that the distinct BCN configuration, featuring the BC2O group and pyridinic-N, is fundamental to the active site. This research substantially advances the sustainability of phytoremediation by offering a viable methodology to synthesize highly catalytic functional biochar utilizing hyperaccumulator residues.

Revealing the Synergistic Effect of Cation and Anion Vacancies on Enhanced Fenton-Like Reaction: The Electron Density Modulation of O 2p-Co 3d Bands

Defect engineering is considered as a flexible and effective mean to improve the performance of Fenton-like reactions. Herein, a simple method is employed to synthesize Co3O4 catalysts with Co-O vacancy pairs (VP) for peroxymonosulfate (PMS) activation. Multi-scaled characterization, experimental, and simulation results jointly revealed that the cation vacancies-VCo contributed to enhanced conductivity and anion vacancies-VO provided a new active center for the 1O2 generation. Co3O4-VP can optimize the O 2p and Co 3d bands with the strong assistance of synergistic double vacancies to reduce the reaction energy barrier of the "PMS → Co(IV) = O → 1O2" pathway, ultimately triggering the stable transition of mechanism. Co3O4-VP catalysts with radical-nonradical collaborative mechanism achieve the synchronous improvement of activity and stability, and have good environmental robustness to favor water decontamination applications. This result highlights the possibility of utilizing anion and cation vacancy engineering strategies to rational design Co3O4-based materials widely used in catalytic reactions.

Pore modulation of single atomic Fe sites for ultrafast Fenton-like chemistry with amplified electron migration oxidation

The limited interaction between pollutants, oxidants, and the surface catalytic sites of single atom catalysts (SACs) restricts the water decontamination effectiveness. Confining catalytic sites within porous structures enables the localized enrichment of reactants for optimized reaction kinetics, while the specific regulatory mechanisms remain unclear. Herein, SACs with porous modification significantly improves the utilization of peroxymonosulfate (PMS) and pollutant degradation activity. Confining catalytic sites in porous structure effectively reduces the mass transfer distance between radicals (SO4•- and •OH) and pollutants, thereby improving reaction performance. Pore modulation changes the surface electronic structure, leading to a significant improvement in the electron migration process. The system shows significant potential in effectively oxidizing various common emerging pollutants, and exhibits robust resistance to interference from environmental matrices. Moreover, a quantitative evaluation using life cycle assessment (LCA) indicates that the pFe-SAC/PMS system showcases superior environmental importance and practicality.

Long-range interactions driving neighboring Fe-N4 sites in Fenton-like reactions for sustainable water decontamination

Actualizing efficient and sustainable environmental catalysis is essential in global water pollution control. The single-atom Fenton-like process, as a promising technique, suffers from reducing potential environmental impacts of single-atom catalysts (SACs) synthesis and modulating functionalized species beyond the first coordination shell. Herein, we devised a high-performance SAC possessing impressive Fenton-like reactivity and extended stability by constructing abundant intrinsic topological defects within carbon planes anchored with Fe-N4 sites. Coupling atomic Fe-N4 moieties and adjacent intrinsic defects provides potent synergistic interaction. Density functional theory calculations reveal that the intrinsic defects optimize the d-band electronic structure of neighboring Fe centers through long-range interactions, consequently boosting the intrinsic activity of Fe-N4 sites. Life cycle assessment and long-term steady operation at the device level indicate promising industrial-scale treatment capability for actual wastewater. This work emphasizes the feasibility of synergistic defect engineering for refining single-atom Fenton-like chemistry and inspires rational materials design toward sustainable environmental remediation.

Materials Containing Single-, Di-, Tri-, and Multi-Metal Atoms Bonded to C, N, S, P, B, and O Species as Advanced Catalysts for Energy, Sensor, and Biomedical Applications

Modifying the coordination or local environments of single-, di-, tri-, and multi-metal atom (SMA/DMA/TMA/MMA)-based materials is one of the best strategies for increasing the catalytic activities, selectivity, and long-term durability of these materials. Advanced sheet materials supported by metal atom-based materials have become a critical topic in the fields of renewable energy conversion systems, storage devices, sensors, and biomedicine owing to the maximum atom utilization efficiency, precisely located metal centers, specific electron configurations, unique reactivity, and precise chemical tunability. Several sheet materials offer excellent support for metal atom-based materials and are attractive for applications in energy, sensors, and medical research, such as in oxygen reduction, oxygen production, hydrogen generation, fuel production, selective chemical detection, and enzymatic reactions. The strong metal-metal and metal-carbon with metal-heteroatom (i.e., N, S, P, B, and O) bonds stabilize and optimize the electronic structures of the metal atoms due to strong interfacial interactions, yielding excellent catalytic activities. These materials provide excellent models for understanding the fundamental problems with multistep chemical reactions. This review summarizes the substrate structure-activity relationship of metal atom-based materials with different active sites based on experimental and theoretical data. Additionally, the new synthesis procedures, physicochemical characterizations, and energy and biomedical applications are discussed. Finally, the remaining challenges in developing efficient SMA/DMA/TMA/MMA-based materials are presented.

Inverted loading strategy regulates the Mn-OV-Ce sites for efficient fenton-like catalysis

Regulating interfacial active sites to improve peroxymonosulfate (PMS) activation efficiency is a hot topic in the heterogeneous catalysis field. In this study, we develop an inverted loading strategy to engineer asymmetric Mn-OV-Ce sites for PMS activation. Mn3O4@CeO2 prepared by loading CeO2 nanoparticles onto Mn3O4 nanorods exhibits the highest catalytic activity and stability, which is due to the formation of more oxygen vacancies (OV) at the Mn-OV-Ce sites, and the surface CeO2 layer effectively inhibits corrosion by preventing the loss of manganese ion active species into the solution. In situ characterizations and density functional theory (DFT) studies have revealed effective bimetallic redox cycles at asymmetric Mn-OV-Ce active sites, which promote surface charge transfer, enhance the adsorption reaction activity of active species toward pollutants, and favor PMS activation to generate (•OH, SO4•-, O2•- and 1O2) active species. This study provides a brand-new perspective for engineering the interfacial behavior of PMS activation.

Atomic-Level Engineered Cobalt Catalysts for Fenton-Like Reactions: Synergy of Single Atom Metal Sites and Nonmetal-Bonded Functionalities

Single atom catalysts (SACs) are atomic-level-engineered materials with high intrinsic activity. Catalytic centers of SACs are typically the transition metal (TM)-nonmetal coordination sites, while the functions of coexisting non-TM-bonded functionalities are usually overlooked in catalysis. Herein, the scalable preparation of carbon-supported cobalt-anchored SACs (CoCN) with controlled Co─N sites and free functional N species is reported. The role of metal- and nonmetal-bonded functionalities in the SACs for peroxymonosulfate (PMS)-driven Fenton-like reactions is first systematically studied, revealing their contribution to performance improvement and pathway steering. Experiments and computations demonstrate that the Co─N3C coordination plays a vital role in the formation of a surface-confined PMS* complex to trigger the electron transfer pathway and promote kinetics because of the optimized electronic state of Co centers, while the nonmetal-coordinated graphitic N sites act as preferable pollutant adsorption sites and additional PMS activation sites to accelerate electron transfer. Synergistically, CoCN exhibits ultrahigh activity in PMS activation for p-hydroxybenzoic acid oxidation, achieving complete degradation within 10 min with an ultrahigh turnover frequency of 0.38 min-1, surpassing most reported materials. These findings offer new insights into the versatile functions of N species in SACs and inspire rational design of high-performance catalysts in complicated heterogeneous systems.

Correlating active sites and oxidative species in single-atom catalyzed Fenton-like reactions

Single-atom catalysts (SACs) have gained widespread popularity in heterogeneous catalysis-based advanced oxidation processes (AOPs), owing to their optimal metal atom utilization efficiency and excellent recyclability by triggering reactive oxidative species (ROS) for target pollutant oxidation in water. Systematic summaries regarding the correlation between the active sites, catalytic activity, and reactive species of SACs have rarely been reported. This review provides an overview of the catalytic performance of carbon- and metal oxide-supported SACs in Fenton-like reactions, as well as the different oxidation pathways induced by the metal and non-metal active sites, including radical-based pathways (e.g., ·OH and SO4˙-) and nonradical-based pathways (e.g. 1O2, high-valent metal-oxo species, and direct electron transfer). Thereafter, we discuss the effects of metal types, coordination environments, and spin states on the overall catalytic performance and the generated ROS in Fenton-like reactions. Additionally, we provide a perspective on the future challenges and prospects for SACs in water purification.

Bioinspired axial S-coordinated single-atom cobalt catalyst to efficient activate peroxymonosulfate for selective high-valent Co-Oxo species generation

The efficient activation and selective high-valent metal-oxo (HVMO) species generation remain challenging for peroxymonosulfate (PMS)-based advanced oxidation processes (PMS-AOPs) in water purification. The underlying mechanism of the activation pathway is ambiguous, leading to a massive dilemma in the control and regulation of HVMO species generation. Herein, bioinspired by the bio-oxidase structure of cytochrome P450, the axial coordination strategy was adopted to tailor a single-atom cobalt catalyst (CoN4S-CB) with an axial S coordination. CoN4S-CB high-selectively generated high-valent Co-Oxo species (Co(IV)=O) via PMS activation. Co(IV)=O demonstrated an ingenious oxygen atom transfer (OAT) reaction to achieve the efficient degradation of sulfamethoxazole (SMX), and this allowed robust operation in various complex environments. The axial S coordination modulated the 3d orbital electron distribution of the Co atom. Density functional theory (DFT) calculation revealed that the axial S coordination decreased the energy barrier for PMS desorption and lowered the free energy change (ΔG) for Co(IV)=O generation. CoN4S-PMS* had a narrow d-band close to the Fermi level, which enhanced charge transfer to accelerate the cleavage of O-O and O-H bonds in PMS. This work provides a broader perspective on the activator design with natural enzyme structure-like active sites to efficient activate PMS for selective HVMO species generation.

Oxygen Doping Cooperated with Co-N-Fe Dual-Catalytic Sites: Synergistic Mechanism for Catalytic Water Purification within Nanoconfined Membrane

Atom-site catalysts, especially for graphitic carbon nitride-based catalysts, represents one of the most promising candidates in catalysis membrane for water decontamination. However, unravelling the intricate relationships between synthesis-structure-properties remains a great challenge. This study addresses the impacts of coordination environment and structure units of metal central sites based on Mantel test, correlation analysis, and evolution of metal central sites. An optimized unconventional oxygen doping cooperated with Co-N-Fe dual-sites (OCN Co/Fe) exhibits synergistic mechanism for efficient peroxymonosulfate activation, which benefits from a significant increase in charge density at the active sites and the regulation in the natural population of orbitals, leading to selective generation of SO4 •-. Building upon these findings, the OCN-Co/Fe/PVDF composite membrane demonstrates a 33 min-1 ciprofloxacin (CIP) rejection efficiency and maintains over 96% CIP removal efficiency (over 24 h) with an average permeance of 130.95 L m-2 h-1. This work offers a fundamental guide for elucidating the definitive origin of catalytic performance in advance oxidation process to facilitate the rational design of separation catalysis membrane with improved performance and enhanced stability.

Nanoconfinement steers nonradical pathway transition in single atom fenton-like catalysis for improving oxidant utilization

The introduction of single-atom catalysts (SACs) into Fenton-like oxidation promises ultrafast water pollutant elimination, but the limited access to pollutants and oxidant by surface catalytic sites and the intensive oxidant consumption still severely restrict the decontamination performance. While nanoconfinement of SACs allows drastically enhanced decontamination reaction kinetics, the detailed regulatory mechanisms remain elusive. Here, we unveil that, apart from local enrichment of reactants, the catalytic pathway shift is also an important cause for the reactivity enhancement of nanoconfined SACs. The surface electronic structure of cobalt site is altered by confining it within the nanopores of mesostructured silica particles, which triggers a fundamental transition from singlet oxygen to electron transfer pathway for 4-chlorophenol oxidation. The changed pathway and accelerated interfacial mass transfer render the nanoconfined system up to 34.7-fold higher pollutant degradation rate and drastically raised peroxymonosulfate utilization efficiency (from 61.8% to 96.6%) relative to the unconfined control. It also demonstrates superior reactivity for the degradation of other electron-rich phenolic compounds, good environment robustness, and high stability for treating real lake water. Our findings deepen the knowledge of nanoconfined catalysis and may inspire innovations in low-carbon water purification technologies and other heterogeneous catalytic applications.

From waste corn straw to graphitic porous carbon: A trade-off between specific surface area and graphitization degree for efficient peroxydisulfate activation

Electron transfer pathways have been verified as overriding regimes when peroxydisulfate (PDS) was activated by porous carbon. The incorporation of graphitic structure into carbon matrix was favorable to the rapid electron transfer, but excessive graphitization would deteriorate the specific surface area (SSA), weakening the catalytic performance. The reasonable trade-off between SSA and graphitization degree was necessary and challenging for the preparation of efficient carbon based PS-activators. Herein, a series of graphitic porous carbon with discrepant SSA and graphitic structure were fabricated. The incorporation of graphitization tracks into ultra-thin edges on porous carbon film was verified by multifarious structural characterization. After trade-off, the optimum catalyst exhibited superior catalytic performance with degradation rate constant (kobs) exceeding that of ungraphitized precursor by up to 16.0 times. Mechanistic investigations substantiated that the sufficient SSA of catalyst provided favorable conditions for its affinity towards PDS and sulfadiazine (SDZ), resulting in the formation of PDS* complexes and SDZ adsorption, while the appropriate graphitization degree ensured the reinforced electron transfer rate, which collectively accelerated SDZ oxidation through electron-transfer pathway. The multivariate linear regression model linking kobs to SSA and graphitization degree was established providing basis to construct efficient catalysts for PDS activation.

Interfacial OOH* mediated Fe(II) regeneration on the single atom Co-N-C catalyst for efficient Fenton-like processes

Fe(II) regeneration is decisive for highly efficient H2O2-based Fenton-like processes, but the role of cobalt-containing reactive sites in promoting Fe(II) regeneration was overlooked. Herein, a single atom Co-N-C catalyst was employed in Fe(II)/H2O2 system to promote the degradation of diverse organic contaminants. The EPR and quenching experiments indicated Co-N-C significantly enhanced the generation of superoxide species, and accelerated hydroxyl radical generation for pollutant degradation. The electrochemical and surface composition analyses demonstrated the enhanced H2O2 activation and Fe(III)/Fe(II) recycling on the catalyst. Furthermore, in-situ Raman characterization with shell-isolated gold nanoparticles was employed to visualize the interfacial reactive intermediates and their time-resolved interaction. The accumulation of interfacial CoOOH* was confirmed when Co-N-C activated H2O2 alone, but it rapidly transformed into FeOOH* upon Fe(II) addition. Besides, the temporal variation of OOH* intermediates and the relative intensity of Co(III)-O and Co(IV)=O peaks depicted the dynamic interaction of reactive intermediates along the H2O2 consumption. With this basis, we proposed a mechanism of interfacial OOH* mediated Fe(II) regeneration, which overcame the kinetical limitation of Fe(II)/H2O2 system. Therefore, this study provided a primary effort to elucidate the overlooked role of interfacial CoOOH* in the Fenton-like processes, which may inspire the design of more efficient catalysts.

Advances in Atomically Dispersed Metal and Nitrogen Co-Doped Carbon Catalysts for Advanced Oxidation Technologies and Water Remediation: From Microenvironment Modulation to Non-Radical Mechanisms

Atomically dispersed metal and nitrogen co-doped carbon catalysts (M-N-C) have been attracting tremendous attentions thanks to their unique MNx active sites and fantastic catalytic activities in advanced oxidation technologies (AOTs) for water remediation. However, precisely tailoring the microenvironment of active sites at atomic level is still an intricate challenge so far, and understanding of the non-radical mechanisms in persulfate activation exists many uncertainties. In this review, latest developments on the microenvironment modulation strategies of atomically dispersed M-N-C catalysts including regulation of central metal atoms, regulation of coordination numbers, regulation of coordination heteroatoms, and synergy between single-atom catalysts (SACs) with metal species are systematically highlighted and discussed. Afterwards, progress and underlying limitations about the typical non-radical pathways from production of singlet oxygen, electron transfer mechanism to generation of high-valent metal species are well demonstrated to inspire intrinsic insights about the mechanisms of M-N-C/persulfate systems. Lastly, perspectives for the remaining challenges and opportunities about the further development of carbon-based SACs in environment remediation are also pointed out. It is believed that this review will be much valuable for the further design of active sites in M-N-C/persulfate catalytic systems and promote the wide application of SACs in various fields.

Discriminating the Active Ru Species Towards the Selective Generation of Singlet Oxygen from Peroxymonosulfate: Nanoparticles Surpass Single-Atom Catalysts

Singlet oxygen (1O2) is an exceptional reactive oxygen species in advanced oxidation processes for environmental remediation. Despite single-atom catalysts (SACs) representing the promising candidate for the selective generation of 1O2 from peroxymonosulfate (PMS), the necessity to meticulously regulate the coordination environment of metal centers poses a significant challenge in the precisely-controlled synthetic method. Another dilemma to SACs is their high surface free energy, which results in an inherent tendency for the surface migration and aggregation of metal atoms. We here for the first time reported that Ru nanoparticles (NPs) synthesized by the facile pyrolysis method behave as robust Fenton-like catalysts, outperforming Ru SACs, towards efficient activation of PMS to produce 1O2 with nearly 100 % selectivity, remarkably improving the degradation efficiency for target pollutants. Density functional theory calculations have unveiled that the boosted PMS activation can be attributed to two aspects: (i) enhanced adsorption of PMS molecules onto Ru NPs, and (ii) decreased energy barriers by offering adjacent sites for promoted dimerization of *O intermediates into adsorbed 1O2. This study deepens the current understanding of PMS chemistry, and sheds light on the design and optimization of Fenton-like catalysts.

Separation and reutilization of heavy metal ions in wastewater assisted by p-BN adsorbent

Extracting heavy metal ions from wastewater has significant implications for both environmental remediation and resource preservation. However, the conventional adsorbents still suffer from incomplete ion removal and low utilization efficiency of the recovered metals. Herein, we present an extraction and reutilization method assisted by porous boron nitride (p-BN) containing high-density N atoms for metal recovery with simultaneous catalyst formation. The p-BN exhibits stable and efficient metal adsorption performance, particularly for ultra-trace-level water purification. The distribution coefficients towards Pb2+, Cd2+, Co2+ and Fe3+ can exceed 106 mL g-1 and the residual concentrations that reduced from 1 mg L-1 to 0.8-1.3 μg L-1 are much lower than the acceptable limits in drinking water standards of World Health Organization. Meanwhile, the used p-BN after Co ion adsorption can be directly adopted as a high-efficiency catalyst for activating peroxymonosulfate (PMS) in organic pollutant degradation without additional post-treatment, avoiding the secondary metal pollution and the problems of neglected manpower and energy consumption. Moreover, a flow-through multistage utilization system assisted by p-BN/polyvinylidene fluoride (PVDF) membrane is constructed for achieving both metal ion separation and reutilization in the removal of organic pollutants, providing a new avenue for sustainable wastewater remediation.

Ni introduction induced non-radical degradation of bisphenol A in spinel ferrite/H2O2 systems

Herein, we achieved reactive oxygen species manipulation using transition metal spinel ferrites (NixCo1-xFe2O4, x = 0, 0.5, 1) as Fenton-like agents. Specifically, NiFe2O4 mainly produced 1O2 and high-valence metals, while CoFe2O4 mainly produced ˙OH, from H2O2 activation. With bisphenol A as a model pollutant, the NiFe2O4/H2O2 system exhibited good resistance to ion interference.

Utilizing the oxygen-atom trapping effect of Co3O4 with oxygen vacancies to promote chlorite activation for water decontamination

Heterogeneous high-valent cobalt-oxo [≡Co(IV)=O] is a widely focused reactive species in oxidant activation; however, the relationship between the catalyst interfacial defects and ≡Co(IV)=O formation remains poorly understood. Herein, photoexcited oxygen vacancies (OVs) were introduced into Co3O4 (OV-Co3O4) by a UV-induced modification method to facilitate chlorite (ClO2-) activation. Density functional theory calculations indicate that OVs result in low-coordinated Co atom, which can directionally anchor chlorite under the oxygen-atom trapping effect. Chlorite first undergoes homolytic O-Cl cleavage and transfers the dissociated O atom to the low-coordinated Co atom to form reactive ≡Co(IV)=O with a higher spin state. The reactive ≡Co(IV)=O rapidly extracts one electron from ClO2- to form chlorine dioxide (ClO2), accompanied by the Co atom returning a lower spin state. As a result of the oxygen-atom trapping effect, the OV-Co3O4/chlorite system achieved a 3.5 times higher efficiency of sulfamethoxazole degradation (~0.1331 min-1) than the pristine Co3O4/chlorite system. Besides, the refiled OVs can be easily restored by re-exposure to UV light, indicating the sustainability of the oxygen atom trap. The OV-Co3O4 was further fabricated on a polyacrylonitrile membrane for back-end water purification, achieving continuous flow degradation of pollutants with low cobalt leakage. This work presents an enhancement strategy for constructing OV as an oxygen-atom trapping site in heterogeneous advanced oxidation processes and provides insight into modulating the formation of ≡Co(IV)=O via defect engineering.

Precise coordination of high-loading Fe single atoms with sulfur boosts selective generation of nonradicals

Nonradicals are effective in selectively degrading electron-rich organic contaminants, which unfortunately suffer from unsatisfactory yield and uncontrollable composition due to the competitive generation of radicals. Herein, we precisely construct a local microenvironment of the carbon nitride-supported high-loading (~9 wt.%) Fe single-atom catalyst (Fe SAC) with sulfur via a facile supermolecular self-assembly strategy. Short-distance S coordination boosts the peroxymonosulfate (PMS) activation and selectively generates high-valent iron-oxo species (FeIV=O) along with singlet oxygen (1O2), significantly increasing the 1O2 yield, PMS utilization, and p-chlorophenol reactivity by 6.0, 3.0, and 8.4 times, respectively. The composition of nonradicals is controllable by simply changing the S content. In contrast, long-distance S coordination generates both radicals and nonradicals, and could not promote reactivity. Experimental and theoretical analyses suggest that the short-distance S upshifts the d-band center of the Fe atom, i.e., being close to the Fermi level, which changes the binding mode between the Fe atom and O site of PMS to selectively generate 1O2 and FeIV=O with a high yield. The short-distance S-coordinated Fe SAC exhibits excellent application potential in various water matrices. These findings can guide the rational design of robust SACs toward a selective and controllable generation of nonradicals with high yield and PMS utilization.

Electron Delocalization Realizes Speedy Fenton-Like Catalysis over a High-Loading and Low-Valence Zinc Single-Atom Catalyst

A zinc (Zn)-based single-atom catalyst (SAC) is recently reported as an active Fenton-like catalyst; however, the low Zn loading greatly restricts its catalytic activity. Herein, a molecule-confined pyrolysis method is demonstrated to evidently increase the Zn loading to 11.54 wt.% for a Zn SAC (ZnSA -N-C) containing a mixture of Zn-N4 and Zn-N3 coordination structures. The latter unsaturated Zn-N3 sites promote electron delocalization to lower the average valence state of Zn in the mix-coordinated Zn-Nx moiety conducive to interaction of ZnSA -N-C with peroxydisulfate (PDS). A speedy Fenton-like catalysis is thus realized by the high-loading and low-valence ZnSA -N-C for PDS activation with a specific activity up to 0.11 min L-1 m-2 , outstripping most Fenton-like SACs. Experimental results reveal that the formation of ZnSA -N-C-PDS* complex owing to the strong affinity of ZnSA -N-C to PDS empowers intense direct electron transfer from the electron-rich pollutant toward this complex, dominating the rapid bisphenol A (BPA) elimination. The electron transfer pathway benefits the desirable environmental robustness of the ZnSA -N-C/PDS system for actual water decontamination. This work represents a new class of efficient and durable Fenton-like SACs for potential practical environmental applications.

Polar electric field-modulated peroxymonosulfate selective activation for removal of organic contaminants via non-radical electron transfer process

Nonradical electron transfer process (ETP) in peroxomonosulfate (PMS) based advanced oxidation processes (AOPs) is regarded promising for selective degradation of organic contaminants in water, however, the subjective modulation strategy and the definitive mechanistic elucidation of ETP are still lacking. Herein, we proposed a heretofore unreported yet efficient ETP indution approach by construction of polar electrical field on biochar via nonmetallic elements co-doping. Physicochemical characterizations and density functional theory (DFT) calculations verified the electronegativity difference among boron, nitrogen, and sulfur elements bestowed robust local electric fields on biochar surface (BC-BNS), which effectively enhanced the adsorption complexation and charge transfer between biochar and PMS. Compared to the other single-doped or co-doped biochar, BC-BNS exhibited superior catalytic performance of PMS activation for degradation of atrazine (ATZ) (kobs=0.036 min-1), as well as various kinds of electron-rich organics. The remarkable catalytic degradation capacity was further verified in various aqueous matrices and background factors, representing the excellent selectivity. Analysis of contribution from reactive oxygen species and electrochemical testing together substantiated the role of polar electric fields in facilitating the modulation from singlet oxygen (1O2) to ETP as a prevailing mechanism. DFT calculations and apparent interactions revealed the dissociation of S-O bond was thermodynamically favored within this potent localized electric field, which further induced the cleavage of OO bond and ultimately promoted the dual electron transfer between ATZ and PMS. The superiority of BC-BNS/PMS system was further validated with the low ecotoxicity caused by enhanced dechlorination, the low energy consumption, and the long-term effectiveness. The novel modulation principle and atomic-level mechanism exploration gave suggestions for advancing ETP-dominated AOP to remove recalcitrant contaminants during water treatment and restoration.