Modulating Electronic Structure Engineering of Atomically Dispersed Cobalt Catalyst in Fenton-like Reaction for Efficient Degradation of Organic Pollutants

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.

Cobalt-modification on UiO-bpydc MOF facilitates ligand-to-metal charge transfer for superior visible-light photocatalytic degradation of refractory fluoroquinolone antibiotics

Pollutant removal through green photocatalysis combined with advanced oxidation processes (AOPs) is critical for efficient wastewater treatment but is limited by poor light harvesting and inefficient oxidant activation. This study addresses these challenges through developing a Co-incorporated UiO-bpydc MOF for enhanced visible-light-driven photocatalysis via peroxymonosulfate (PMS) bridged ligand-to-metal charge transfer (LMCT). The MOF was synthesized through direct cobalt complexation into the UiO-bpydc framework for enabling visible light absorption. The UiO-bpydc(Co) achieved 95.8 % degradation of lomefloxacin (LOM) within 30 min in the presence of PMS, attributing to narrowed bandgap (i.e., 2.82 eV), improved charge transfer via Co centers, and increased pollutant affinity due to electron-rich ligand. Additionally, the generation of long-lifespan singlet oxygen (1O2, 41.8 %) was identified as the key reactive species. Theoretical calculations indicated a reduced HOMO-LUMO gap upon the formation of a -Co-OOSO3 bridge, which promotes carrier separation and improves pollutant-catalyst interactions. The degradation pathways and toxicity evolution of intermediates were clarified, while the exceptional stability, recyclability, and broad pollutant applicability of UiO-bpydc(Co) demonstrate its potential for utilization in oxidative environments This work highlights the potential of transition metal doping to alter the electronic structure of MOFs for target-specific catalytic reactions, offering new opportunities for advanced environmental remediation technologies.

From Single-Atom to Dual-Atom: A Universal Principle for the Rational Design of Heterogeneous Fenton-like Catalysts

Developing efficient heterogeneous Fenton-like catalysts is the key point to accelerating the removal of organic micropollutants in the advanced oxidation process. However, a general principle guiding the reasonable design of highly efficient heterogeneous Fenton-like catalysts has not been constructed up to now. In this work, a total of 16 single-atom and 272 dual-atom transition metal/nitrogen/carbon (TM/N/C) catalysts for H2O2 dissociation were explored systematically based on high-throughput density functional theory and machine learning. It was found that H2O2 dissociation on single-atom TM/N/C exhibited a distinct volcano-type relationship between catalytic activity and •OH adsorption energy. The favorable •OH adsorption energies were in the range of -3.11 ∼ -2.20 eV. Three different descriptors, namely, energetic, electronic, and structural descriptors, were found, which can correlate the intrinsic properties of catalysts and their catalytic activity. Using adsorption energy, stability, and activation energy as the evaluation criteria, two dual-atom CoCu/N/C and CoRu/N/C catalysts were screened out from 272 candidates, which exhibited higher catalytic activity than the best single-atom TM/N/C catalyst due to the synergistic effect. This work could present a conceptually novel understanding of H2O2 dissociation on TM/N/C and inspire the structure-oriented catalyst design from the viewpoint of volcano relationship.

Integrative Lattice and Surface Engineering of Nanoscale Fe0 for Superior Dechlorination of Trichloroethene in Groundwater: Coordination in Reactivity, Selectivity, and Stability

Nanoscale zero-valent iron (nFe0) materials hold great promise in environmental remediation, yet achieving high reactivity, selectivity, and stability in reduction remains a long-standing challenge. Here we address this challenge by employing Ni lattice and FeS surface engineering to fabricate novel nFe0-based nanomaterials (dubbed as FeNix@FeSy), featuring FeNi as the core and FeS as the shell. The FeNi5@FeS10 delivered approximately 242.7- and 81.2-times higher reactivity and selectivity, respectively, over unmodified nFe° for the remediation of trichloroethene (TCE; a notorious environmental pollutant), while maintaining high stability in groundwater remediation. We found that the core composition (i.e., Ni/Fe ratio) of FeNix@FeSy primarily determined reactivity, governed by a tradeoff between the galvanic effect and lattice strain, while shell properties mainly controlled selectivity, despite some interactions between them. Density functional theory (DFT) calculations revealed that the FeS surface served as a favorable adsorption site for TCE, and the low energy barriers (TS2, 0.19 eV) of FeNi5@FeS10 facilitated the cleavage of the first chlorine from TCE. Moreover, the core-shell structure promoted electron transfer from the core to the shell and TCE. This integrative lattice and surface engineering strategy provides a new avenue for designing advanced functional materials for environmental remediation and beyond.

Amorphous Engineering Driving d-Orbital High Spin Configuration for Almost 100% 1O2-Mediated Fenton-Like Reactions

The inherent atomic disorder in amorphous materials leads to unsaturated atomic sites or dangling bonds, effectively modulating the material's electronic states and rendering it an ideal platform for the growth of single atoms. Herein, the electronic structure of isolated cobalt atoms anchored on amorphous carbon nitride (Co-ACN) is modulated through a substrate amorphization engineering, enabling the thorough removal of pazufloxacin (PZF) in 1 min with a high reaction rate constant (k1) of 3.504 min-1 by peroxymonosulfate (PMS) activation. Experiments and theoretical calculations reveal that Co-ACN exhibited a higher coordination environment (Co-N3) compared to crystalline Co-CCN (Co-N2). Meanwhile, the t2g energy level enhancement of Co 3d orbital promotes electron transition from t2g to eg, inducing more unpaired electrons and thereby driving the transition from a low-spin state (LS, t2g 6eg 1) to a high-spin state (HS, t2g 5eg 2). The HS Co-ACN optimized the d-band center, boosted the electronic transfer, and weakened the interaction between Co 3d and O 2p orbitals of HSO5 -, thereby enabling nearly 100% selective singlet oxygen (1O2) generation, whereas Co-CCN yielded coexisting reactive oxygen species (ROS). This work opens up a new paradigm for regulating the electronic structure of single-atom catalysts at the atomic scale.

Efficacy and durability of cobalt sulfide nanoparticles and axial sulfur-coordinated cobalt single-atom composite sites in hydrogenative nitroaromatics decontamination

Emerging single-atom materials and metal sulfides hold significant promise as alternatives to precious metal catalysts for nitroaromatics conversion; however, their intrinsic activity and durability remain insufficiently understood. Herein, sulfur and nitrogen co-doped carbon matrices incorporating CoS nanoparticles and single-atom Co with Co-N4-S1 coordination were constructed through a facile pyrolysis approach. Advanced characterization techniques, such as X-ray absorption fine structure (XAFS) and aberration-corrected electron microscopy, unveiled unique structural features underpinning exceptional catalytic efficiency and recyclability. The catalyst achieved a specific catalytic rate of 134 min-1 g-1 L for p-nitrophenol (PNP) hydrogenation, outperforming many noble metal-based catalysts. Experimental and theoretical analyses identified the Co-N4-S1 single-atom moiety as the primary active site, demonstrating remarkable structural stability. Axial sulfur coordination was found to fine-tune the electronic state of the central Co atom, mitigating the overbinding of reaction intermediates and enhancing PNP conversion efficiency. In contrast, CoS nanoparticles exhibited limited recyclability, with agglomeration, cobalt hydroxide formation, and dissolution observed during repeated use. This study presents a highly efficient catalyst for nitroaromatics conversion and provides a foundational framework for understanding the durability and mechanistic roles of cobalt-based active sites.

Innovative asymmetric CoSA-N-Ti3C2Tx catalysis: unleashing superoxide radicals for rapid self-coupling removal of phenolic pollutant

The polymerization pathway of contaminants rivals the traditional mineralization pathway in water purification technologies. However, designing suitable oxidative environments to steer contaminants toward polymerization remains challenging. This study introduces a nitrogen-oxygen double coordination strategy to create an asymmetrical microenvironment for Co atoms on Ti3C2Tx MXenes, resulting in a novel Co-N2O3 microcellular structure that efficiently activates peroxymonosulfate. This unique activation capability led to the complete removal of various phenolic pollutants within 3 min, outperforming the representative Co single-atom catalysts reported in the past three years. Identifying and recognizing reactive oxygen species highlight the crucial role of ⋅O2 -. The efficient pollutant removal occurs through a ⋅O2 --mediated radical pathway, functioning as a self-coupling reaction rather than deep oxidation. Theoretical calculations demonstrate that the electron-rich pollutants transfer more electrons to the catalyst surface, inducing the reduction of dissolved oxygen to ⋅O2 - in the Co-N2O3 microregion. In a practical continuous flow-through application, the system achieved 100 % acetaminophen removal efficiency in 6.5 h, with a hydraulic retention time of just 0.98 s. This study provides new insights into the previously underappreciated role of ⋅O2 - in pollutant purification, offering a simple strategy for advancing aggregation removal technology in the field of wastewater treatment.

Angstrom Confinement-Triggered Adaptive Spin State Transition of CoMn Dual Single Atoms for Efficient Singlet Oxygen Generation

To achieve high selectivity in the transformation from peroxymonosulfate to singlet oxygen, adaptive tuning of atomic spin state as the peroxymonosulfate structure varied is crucial. The angstrom confinement can effectively tune spin state, but developing an adaptive angstrom-confined atomic system is challenging. Angstrom-confined cobalt (Co) manganese (Mn) dual single atoms within flexible 2D carbon nitride interlayer are constructed to drive adaptive tuning of spin state by changing atomic coordination under angstrom confinement. The in situ characterizations and density functional theory calculations showed that medium-spin Co in Co─N4 absorbed electrons after the adsorption of peroxymonosulfate on CoMn dual single-atom sites and then cleaved O─H of peroxymonosulfate to facilitate *SO5 generation, while the introduction of *SO5 increased interlayer distance and then cleaved Co─N and Mn─N, resulting in the spin state transition from medium to high. Subsequently, the high-spin Co and Mn in Co─N2 and Mn─N2 desorbed the *O2 from *SO5, restoring the initial medium spin state. The adaptive spin state transition enhanced 38.6-fold singlet oxygen yield compared to the unconfined control. The proposed angstrom-confined diatomic strategy is applicable to serial diatomic catalysts, providing an efficient and universal design scheme for singlet oxygen-mediated selective wastewater treatment technology at the atomic level.

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.

Atomically Dispersed Fe-Mo Catalysts Mediate Fenton-Like Reaction to Efficiently Degrade Chlorophenol Pollutants Through Synergistic Oxidation and Dechlorination Reactions

Chlorophenols are difficult to degrade and mineralize by traditional advanced oxidation processes due to the strong electronegativity of chlorine. Here, a dual-site atomically dispersed catalyst (FeMoNC) is reported, which Fe/Mo supported on mesoporous nitrogen-doped carbon is prepared through high-temperature migration. The FeMoNC exhibits a high dechlorination rate of 93.3% within 1 min. Theoretical calculation suggested that the doping of high-valence Mo6+ as the electron reservoir, promoted electronic delocalization at Fe sites, thereby enhancing the adsorption and dissociation of peroxymonosulfate (PMS), subsequent generation of Fe (IV) = O and singlet oxygen (1O2) species. An interesting finding is that Mo sites can adsorb chlorine sites in 4-chlorophenol (4-CP) and induce C─Cl bond fracture. Thus, the FeMoNC/PMS system has high catalytic performance due to the synergistic effects of Mo-induced dechlorination and non-radical species (Fe(IV) = O and 1O2) as the degradation pathways, the degradation efficiency of 99.1% of 4-CP within 5 min without significant performance decline after 168 h ≈15,120-bed volumes. These findings can advance mechanistic understanding of PMS activation at the molecular level and guide the rational design of efficient eco-friendly single-atom catalysts (SACs) catalysts with bimetallic atomic sites.

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.

Revealing the Overlooked Catalytic Ability of γ-Al2O3: Efficient Activation of Peroxymonosulfate for Enhanced Water Treatment

Activated alumina (γ-Al2O3) is one of the few nanomaterials manufactured at a ton-scale and successfully implemented in large-scale water treatment. Yet its role in advanced oxidation processes (AOPs) has primarily been limited to functioning as an inert carrier due to its inherently nonredox nature. This study, for the first time, presents the highly efficient capability of γ-Al2O3 to activate peroxymonosulfate (PMS) for selectively eliminating electron-rich organic pollutants in the presence of Cl-. Through experimental and theoretical analysis, we revealed that γ-Al2O3, characterized by uniquely strong Lewis acid sites, enabled robust inner-sphere complexation between PMS and Al(III) sites, triggering the oxidation of Cl- to free chlorine through a distinctive, low-energy-barrier Eley-Rideal pathway. Such a unique pathway resulted in a 42.7-fold increase in free chlorine generation, culminating in a remarkable 145.9-fold enhancement in the degradation of carbamazepine (CBZ) compared with the case without γ-Al2O3. Furthermore, this catalyst exhibited high oxidant utilization efficiency, stable performance in real-world environmental matrices, and sustained long-term activation for over 1206 bed volumes (BV) with a CBZ removal rate exceeding 90% in fixed-bed experiments. These favorable features render γ-Al2O3 an extremely promising nanomaterial for sustainable water treatment initiatives.

Portable smartphone-assisted amperometric immunosensor based on CoCe-layered double hydroxide for rapidly immunosensing erythromycin

Herein, we have manufactured a newly designed bifunctional impedimetric and amperometric immunosensor for rapidly detecting erythromycin (ERY) in complicated environments and food stuffs. For this, bimetallic cobalt/cerium-layered double hydroxide nanosheets (CoCe-LDH NSs), which was derived from Co-based zeolite imidazole framework via the structure conversion, was simultaneously utilized as the bioplatform for anchoring the ERY-targeted antibody and for modifying the gold and screen printed electrode. Basic characterizations revealed that CoCe-LDH NSs was composed of mixed metal valences, enrich redox, and abundant oxygen vacancies, facilitating the adhesion on the electrode, the antibody adsorption, and the electron transfers. The manufactured impedimetric and amperometric immunosensor based on CoCe-LDH has showed the comparable sensing performance, having a wide linear detection range from 1.0 fg mL-1 to 1.0 ng mL-1 with the ultralow detection limit toward ERY. Also, the portable, visualized, and efficient analysis of ERY was then attained at the smartphone-assisted CoCe-LDH-based SPE.

N-coordinated iron sites dispersed in porous carbon frameworks to activate peroxymonosulfate for efficient sulfisoxazole degradation and real hospital wastewater decontamination

Herein, an N-coordinated Fe site dispersed in porous carbon frameworks (Fe-NC) fabricated from zeolitic imidazolate frameworks encapsulated with iron acetylacetonate (Fe(acac)3 @ZIFs) was employed to activate peroxymonosulfate (PMS) for the attenuation of sulfisoxazole (SIZ) and treating real hospital wastewater. The constructed Fe-NC/PMS system exhibited good catalytic stability for SIZ degradation, maintaining excellent degradation performance over multiple cycles with virtually no leaching. The quenching experiments, electron paramagnetic resonance (EPR) capture analyses, and semi-quantitative measurements showed that singlet oxygen (1O2) and high-valent metal-oxo species were mainly responsible for SIZ degradation by Fe-NC/PMS. Significantly, ultra-performance liquid chromatography coupled to quadrupole time-of-flight mass spectrometry (UHPLC-Q-TOF-MS/MS) was used to trace 134 pharmaceutical contaminants in real hospital wastewater. Effective degradation was achieved for 87 % of the pharmaceutical contaminants by the Fe-NC/PMS process. Seventy-four pharmaceutical contaminants were eliminated. Taken together, this work successfully established the Fe-NC/PMS technology using the developed iron-based materials and explored its application to real hospital wastewater treatment, providing an eco-friendly and effective strategy for treating wastewater.

Degradation of diclofenac sodium by peroxymonosulfate activated with a sulfur-doped chitosan ferrocarbon material: Synergistic interaction of free radical and nonfree radical pathways

The synthesis of efficient and stable peroxymonosulfate (PMS) catalysts by doping naturally degradable and functional group-rich chitosan (CS) with nonmetallic atoms remains challenging. In this study, an environmentally friendly electron-rich S-doped CS ferrocarbon material (Fe-S-CN) was synthesized via the sol-gel method, and the resulting material exhibited excellent catalytic activity (up to 98.6 % diclofenac sodium (DCF) removal in 5 min), wide pH applicability, environmental tolerance and renewability. Moreover, Fe-S-CN synergistically activated PMS via both the radical pathway (superoxide radical (O2•-)) and the nonradical pathway (single-linear oxygen (1O2) and electron transfer processes (ETP)) to efficiently mineralize DCF. O2•- originates from the self-decomposition of PMS, whereas 1O2 is due to the oxidation of PMS and further conversion of O2•-. In addition, Fe species, graphitic N and thiophene S are the major active sites in Fe-S-CN. The susceptibility sites of DCF and its possible degradation pathways in the Fe-S-CN/PMS system were inferred in conjunction with density functional theory (DFT) calculation. The present study creates a promising scenario for the synergistic effect of easily overlooked heteroatom doping in chitosan iron-carbon materials in the removal of difficult-to-biodegrade organic matter from water bodies.

Picolinic acid-mediated Mn(II) activated periodate for ultrafast and selective degradation of emerging contaminants: Key role of high-valent Mn-oxo species

The utilization of periodate (PI, IO4-) in metal-based advanced oxidation processes (AOPs) for the elimination of emerging contaminants (ECs) have garnered significant attention. However, the commonly used homogeneous metal catalyst Mn(II) performs inadequately in activating PI. Herein, we exploited a novel AOP technology by employing the complex of Mn(II) with the biodegradable picolinic acid (PICA) to activate PI for the degradation of electron-rich pollutants. The performance of the Mn(II)-PICA complex surpassed that of ligand-free Mn(II) and other Mn(II) complexes with common aminopolycarboxylate ligands. Through scavenger, sulfoxide-probe transformation, and 18O isotope-labeling experiments, we confirmed that the dominant reactive oxidant generated in the Mn(II)-PICA/PI system was high-valent manganese-oxo species (Mn(V)=O). Due to its reliance on Mn(V)=O, the Mn(II)-PICA/PI process exhibited remarkable selectivity and strong anti-interference during EC oxidation in complex water matrices. Nine structurally diverse pollutants were selected for evaluation, and their lnkobs values in the Mn(II)-PICA/PI system correlated well with their electrophilic/nucleophilic indexes, EHOMO, and vertical IP (R2 = 0.79-0.94). Additionally, IO4- was converted into non-toxic iodate (IO3-) without producing undesired iodine species such as HOI, I2, and I3-. This study provides a novel protocol for metal-based AOPs using PI in combination with chelating agents and high-valent metal-oxo species formation during water purification.

Synergistic Coordination in Cu Single-Atom Catalysts Enhances High-Valent Copper-Oxo Species for Efficient PMS Activation

In the domain of heterogeneous catalytic activation of peroxymonosulfate (PMS), high-valent metal-oxo (HVMO) species are widely recognized as potent oxidants for the abatement of organic pollutants. However, the generation selectivity and efficiency of HVMO are often constrained by stringent requirements for catalyst adsorption sites and electron transfer efficiency. In this study, a single-atom catalyst, CuSA/CNP&S, is synthesized featuring multiple types (planar/axial) of heteroatom coordination via an H-bond-assisted self-assembly strategy. It is confirmed that CuN3 active centers with axial S coordination are uniformly distributed in a carbon matrix modified by planar P atoms. CuSA/CNP&S activated PMS to selectively generate Cu(III)═OH species as the primary reactive oxygen species (ROS). The pseudo-first-order kinetic rate for bisphenol A degradation reached 1.51 min-1, a 17.57-fold increase compared to the unmodified CuSA/CN catalyst. Additionally, the CuSA/CNP&S catalyst demonstrates high efficiency and durability in removing contaminants from various aqueous matrices. Theoretical calculations and experimental results indicate that the intrinsic electric field generated by distal planar P atoms enhances electron transfer efficiency within the carbon matrix. Meanwhile, axial S coordination elevates the d-band center and tunes the eg * band broadening of Cu, thereby enhancing the adsorption selectivity for the terminal oxygen of PMS. This multitype coordination synergistically mitigates the issues of low selectivity and yield of HVMO species.

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.

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.

Overcoming metals redox rate limitations in spinel oxide-driven Fenton-like reactions via synergistic heteroatom doping and carbon anchoring for efficient micropollutant removal

The transition metals redox rate limitations of spinel oxides during Fenton-like reactions hinder its efficient and sustainable treatment of actual wastewater. Herein, we propose to optimize the electronic structure of Co-Mn spinel oxide (CM) via sulfur doping and carbon matrix anchoring synergistically, enhancing the radicals-nonradicals Fenton-like processes for efficient water decontamination. Activating peroxymonosulfate (PMS) with optimised spinel oxide (CMSAC) achieved near-complete removal of ofloxacin (10 mg/L) within 6 min, showing 8.4 times higher efficiency than CM group. Significantly higher yields of SO4·- and high-valent metal species in CMSAC/PMS system provided exceptional resistance to co-existing anions, enabling efficient removal of various emerging contaminants in high salinity leachate. Specifically, sulfur coordination and carbon anchoring-induced oxygen vacancy synergistically improved the electronic structure and electron transfer efficiency of CMSAC, thus forming highly reactive Co sites and significantly reducing the energy barrier for Co(IV)=O generation. The reductive sulfur species facilitated the conversion of Co(III) to Co(II), thereby maintaining the stability of the catalytic activity of CMSAC. This work developed a synergistic optimization strategy to overcome the metals redox rate limitations of spinel oxides in Fenton-like reactions, providing deep mechanistic insights for designing Fenton-like catalysts suitable for practical applications.

Advanced Characterization Techniques and Theoretical Calculation for Single Atom Catalysts in Fenton-like Chemistry

Single-atom catalysts (SACs) have attracted extensive attention due to their unique catalytic properties and wide range of applications. Advanced characterization techniques, such as energy-dispersive X-ray spectroscopy, X-ray photoelectron spectroscopy, transmission electron microscopy, scanning electron microscopy, and X-ray absorption fine-structure spectroscopy, have been used to investigate the elemental compositions, structural morphologies, and chemical bonding states of SACs in detail, aiming at unraveling the catalytic mechanism. Meanwhile, theoretical calculations, such as quantum chemical calculations and kinetic simulations, were used to predict the catalytic reaction pathways, active sites, and reaction kinetic behaviors of SACs, providing theoretical guidance for the design and optimization of SACs. This review overviews advanced characterization techniques and theoretical calculations for SACs in Fenton-like chemistry. Moreover, this work highlights the importance of advanced characterization techniques and theoretical calculations in the study of SACs and provides perspectives on the potential applications of SACs in the field of environmental remediation and the challenges of practical engineering.

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.

Tuning electronic structure of metal-free dual-site catalyst enables exclusive singlet oxygen production and in-situ utilization

Developing eco-friendly catalysts for effective water purification with minimal oxidant use is imperative. Herein, we present a metal-free and nitrogen/fluorine dual-site catalyst, enhancing the selectivity and utilization of singlet oxygen (1O2) for water decontamination. Advanced theoretical simulations reveal that synergistic fluorine-nitrogen interactions modulate electron distribution and polarization, creating asymmetric surface electron configurations and electron-deficient nitrogen vacancies. These properties trigger the selective generation of 1O2 from peroxymonosulfate (PMS) and improve the utilization of neighboring reactive oxygen species, facilitated by contaminant enrichment at the fluorine-carbon Lewis-acid adsorption sites. Utilizing these insights, we synthesize the catalyst through montmorillonite (MMT)-assisted pyrolysis (NFC/M). This method leverages the role of MMT as an in-situ layer-stacked template, enabling controlled decomposition of carbon, nitrogen, and fluorine precursors and resulting in a catalyst with enhanced structural adaptability, reactive site accessibility, and mass-transfer capacity. The NFC/M demonstrates an impressive 290.5-fold increase in phenol degradation efficiency than the single-site analogs, outperforming most of metal-based catalysts. This work not only underscores the potential of precise electronic and structural manipulations in catalyst design but also advances the development of efficient and sustainable solutions for water purification.

The "4 + 1" strategy fabrication of iron single-atom catalysts with selective high-valent iron-oxo species generation

Single-atom catalysts (SACs) with atomic dispersion active sites have exhibited huge potentials in peroxymonosulfate (PMS)-based Fenton-like chemistry in water purification. However, four-N coordination metal (MN4) moieties often suffer from such problems as low selectivity and narrow workable pH. How to construct SACs in a controllable strategy with optimized electronic structures is of great challenge. Herein, an innovative strategy (i.e., the "4 + 1" fabrication) was devised to precisely modulate the first-shell coordinated microenvironment of FeN4 SAC using an additional N (SA-FeN5). This leads to almost 100% selective formation of high-valent iron-oxo [Fe(IV)═O] (steady-state concentration: 2.00 × 10-8 M) in the SA-FeN5/PMS system. In-depth theoretical calculations unveil that FeN5 configuration optimizes the electron distribution of monatomic Fe sites, which thus fosters PMS adsorption and reduces the energy barrier for Fe(IV)═O generation. SA-FeN5 was then attached to polyvinylidene difluoride membrane for a continuous flow device, showing long-term abatement of the microcontaminant. This work furnishes a general strategy for effective PMS activation and selective high-valent metal-oxo species generation by high N-coordination number regulation in SACs, which would provide guidance in the rational design of superior environmental catalysts for water purification.

Synergetic Manipulation Mechanism of Single-Atom M-N4 and M-OH (M = Mn, Fe, Co, Ni) Sites for Ozone Activation: Theoretical Prediction and Experimental Verification

Carbon-based single-atom catalysts (SACs) have been gradually introduced in heterogeneous catalytic ozonation (HCO), but the interface mechanism of O3 activation on the catalyst surface is still ambiguous, especially the effect of a surface hydroxyl group (M-OH) at metal sites. Herein, we combined theoretical calculations with experimental verifications to comprehensively investigate the O3 activation mechanisms on a series of conventional SAC structures with N-doped nanocarbon substrates (MN4-NCs, where M = Mn, Fe, Co, Ni). The synergetic manipulation effect of the metal atom and M-OH on O3 activation pathways was paid particular attention. O3 tends to directly interact with the metal atom on MnN4-NC, FeN4-NC, and NiN4-NC catalysts, among which MnN4-NC has the best catalytic activity for its relatively lower activation energy barrier of O3 (0.62 eV) and more active surface-adsorbed oxygen species (Oads). On the CoN4-NC catalyst, direct interaction of O3 with the metal site is energetically infeasible, but O3 can be activated to generate Oads or HO2 species from direct or indirect participation of M-OH sites. The experimental results showed that 90.7 and 82.3% of total organic carbon (TOC) was removed within 40 min during catalytic ozonation of p-hydroxybenzoic acid with MnN4-NC and CoN4-NC catalysts, respectively. Phosphate quenching, catalyst characterization, and EPR measurement further supported the theoretical prediction. This contribution provides fundamental insights into the O3 activation mechanism on SACs, and the methods and ideals could be helpful for future studies of environmental catalysis.

Efficient Homolytic Cleavage of H2O2 on Hydroxyl-Enriched Spinel CuFe2O4 with Dual Lewis Acid Sites

Traditional H2O2 cleavage mediated by macroscopic electron transfer (MET) not only has low utilization of H2O2, but also sacrifices the stability of catalysts. We present a non-redox hydroxyl-enriched spinel (CuFe2O4) catalyst with dual Lewis acid sites to realize the homolytic cleavage of H2O2. The results of systematic experiments, in situ characterizations, and theoretical calculations confirm that tetrahedral Cu sites with optimal Lewis acidity and strong electron delocalization can synergistically elongate the O-O bonds (1.47 Å → 1.87 Å) in collaboration with adjacent bridging hydroxyl (another Lewis acid site). As a result, the free energy of H2O2 homolytic cleavage is decreased (1.28 eV → 0.98 eV). H2O2 can be efficiently split into ⋅OH induced by hydroxyl-enriched CuFe2O4 without MET, which greatly improves the catalyst stability and the H2O2 utilization (65.2 %, nearly 2 times than traditional catalysts). The system assembled with hydroxyl-enriched CuFe2O4 and H2O2 affords exceptional performance for organic pollutant elimination. The scale-up experiment using a continuous flow reactor realizes long-term stability (up to 600 mL), confirming the tremendous potential of hydroxyl-enriched CuFe2O4 for practical applications.

Active Center Size-Dependent Fenton-Like Chemistry for Sustainable Water Decontamination

Accurately controlling catalytic activity and mechanism as well as identifying structure-activity-selectivity correlations in Fenton-like chemistry is essential for designing high-performance catalysts for sustainable water decontamination. Herein, active center size-dependent catalysts with single cobalt atoms (CoSA), atomic clusters (CoAC), and nanoparticles (CoNP) were fabricated to realize the changeover of catalytic activity and mechanism in peroxymonosulfate (PMS)-based Fenton-like chemistry. Catalytic activity and durability vary with the change in metal active center sizes. Besides, reducing the metal size from nanoparticles to single atoms significantly modulates contributions of radical and nonradical mechanisms, thus achieving selective/nonselective degradation. Density functional theory calculations reveal evolutions in catalytic mechanisms of size-dependent catalytic systems over different Gibbs free energies for reactive oxygen species generation. Single-atom site contact with PMS is preferred to induce nonradical mechanisms, while PMS dissociates and generates radicals on clusters and nanoparticles. Differences originating from reaction mechanisms endow developed systems with size-dependent selectivity and mineralization for treating actual hospital wastewater in column reactors. This work brings an in-depth understanding of metal size effects in Fenton-like chemistry and guides the design of intelligent catalysts to fulfill the demand of specific scenes for water purification.

Coupled Surface-Confinement Effect and Pore Engineering in a Single-Fe-Atom Catalyst for Ultrafast Fenton-like Reaction with High-Valent Iron-Oxo Complex Oxidation

The nanoconfinement effect in Fenton-like reactions shows great potential in environmental remediation, but the construction of confinement structure and the corresponding mechanism are rarely elucidated systematically. Herein, we proposed a novel peroxymonosulfate (PMS) activation system employing the single Fe atom supported on mesoporous N-doped carbon (FeSA-MNC, specific surface area = 1520.9 m2/g), which could accelerate the catalytic oxidation process via the surface-confinement effect. The degradation activity of the confined system was remarkably increased by 34.6 times compared to its analogue unconfined system. The generation of almost 100% high-valent iron-oxo species was identified via 18O isotope-labeled experiments, quenching tests, and probe methods. The density functional theory illustrated that the surface-confinement effect narrows the gap between the d-band center and Fermi level of the single Fe atom, which strengthens the charge transfer rate at the reaction interface and reduces the free energy barrier for PMS activation. The surface-confinement system exhibited excellent pollutant degradation efficiency, robust resistance to coexisting matter, and adaptation of a wide pH range (3.0-11.0) and various temperature environments (5-40 °C). Finally, the FeSA-MNC/PMS system could achieve 100% sulfamethoxazole removal without significant performance decline after 10,000-bed volumes. This work provides novel and significant insights into the surface-confinement effect in Fenton-like chemistry and guides the design of superior oxidation systems for environmental remediation.