Single-Atom Mn–N4 Site-Catalyzed Peroxone Reaction for the Efficient Production of Hydroxyl Radicals in an Acidic Solution

A novel electrocatalyst from TOCN/CGG hydrogel-supported Fe-rich sludge and its performance in treating azo dyes-contaminated water

Monolithic electrocatalysts are desired for the electro-Fenton oxidation system. We used a hydrogel consisting of TEMPO-oxidized cellulose nanofibers (TOCN) and cationic guar gum (CGG) to disperse and support Fe-rich sludge and finally obtained a Fe-doped biochar (denoted as C-Sludge@TOCN/CGG) after the freeze-drying and carbonization. This C-Sludge@TOCN/CGG exhibited a porous structure with evenly-distributed Fe due to the inherently three-dimensional porous structure of TOCN/CGG hydrogel and the abundant carbon content. Importantly, Fe and FeO existed in C-Sludge@TOCN/CGG due to the presence of TOCN and CGG during the pyrolysis. The electrochemical properties of C-Sludge@TOCN/CGG demonstrated its good electrocatalytic activity and stability with few side reactions. It had good performance in the electrocatalytic degradation of various azo dyes, attributed to the synergistic integration of TOCN/CGG-derived carbon matrix and carbonized Fe-rich sludge particles. Specifically, two transient radicals (i.e. ·OH and ·O2-) primarily improved the electrocatalytic degradation performance of C-Sludge@TOCN/CGG. This C-Sludge@TOCN/CGG also efficiently degraded a papermill-sourced wastewater containing direct red 23, direct yellow 11, direct black 19 and toner, in which the COD value decreased from 365.12 to 179.13 mg/L within 9 h. This work provides an example of utilizing renewable materials and solid waste to design electrocatalysts to address the wastewater issue.

Low-coordinated Mn-N2 sites in graphene oxide induce peroxydisulfate activation for tetracycline degradation: Process optimization and theoretical calculation

Atom-dispersed low-coordinated transition metal-Nx catalysts exhibit excellent efficiency in activating peroxydisulfate (PDS) for environmental remediation. However, their catalytic performance is limited due to metal-N coordination number and single-atom loading amount. In this study, low-coordinated nitrogen-doped graphene oxide (GO) confined single-atom Mn catalyst (Mn-SA/NGO) was synthesized by molten salt-assisted pyrolysis and coupled to PDS for degradation of tetracycline (TC) in water. Aberration-corrected high-angle annular dark-field scanning transmission electron microscopy (AC-HAADF-STEM) and X-ray absorption fine structure spectroscopy (XAFS) analysis showed the successful doping of single-atom Mn (weight percentage 1.6%) onto GO and the formation of low-coordinated Mn-N2 sites. The optimized parameters obtained by Box-Behnken Design achieved 100% TC removal in both prediction and experimental results. The Mn-SA/NGO + PDS system had strong anti-interference ability for TC removal in the presence of anions. Besides, Mn-SA/NGO possessed good reusability and stability. O2•-, •OH, and 1O2 were the main active species for TC degradation, and the TC mineralization reached 85.1%. Density functional theory (DFT) calculations confirmed that the introduction of single atoms Mn could effectively enhance adsorption and activation of PDS. The findings provide a reference for the synthesis of high-performance single-atom catalysts for effective removal of antibiotics.

Buffer-free ozone-ferrate(VI) systems for enhanced oxidation of electron-deficient contaminants: Synergistic enhancement effects, systematic toxicity assessment, and practical applications

The combination of ozone (O3) and ferrate (Fe(VI)) oxidation technology demonstrates substantial potential for practical applications, though it has been underreported, resulting in gaps in comprehensive activity assessments and thorough exploration of its mechanisms. This study reveals that the previous use of a borate buffer solution obscured certain synergistic reactions between O3 and Fe(VI), causing a reduction of activity by ∼40 % when oxidizing the electron-deficient pollutant atrazine. Consequently, we reassessed the activity and mechanisms using a buffer-salt-free O3/Fe(VI) system. Our findings showed that the hydroxyl radical (·OH) served as the predominant active species, responsible for an impressive 95.9 % of the oxidation activity against electron-deficient pollutants. Additional experiments demonstrated that the rapid production of neglected and really important superoxide radicals (·O2-) could facilitate the decomposition of O3 to generate ·OH and accelerate the reduction of Fe(VI) to Fe(V), reactivating O3 to produce ·OH anew. Intriguingly, as the reaction progressed, the initially depleted Fe(VI) was partially regenerated, stabilizing at over 50 %, highlighting the significant potential of this combined system. Moreover, this combined system could achieve a high mineralization efficiency of 80.4 % in treating actual coking wastewater, complemented by extensive toxicity assessments using Escherichia coli, wheat seeds, and zebrafish embryos, showcasing its robust application potential. This study revisits and amends previous research on the O3/Fe(VI) system, providing new insights into its activity and synergistic mechanisms. Such a combined technology has potential for the treatment of difficult-to-degrade industrial wastewater.

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.

Iron single atoms anchored on ultrathin carbon nitride photocatalyst for visible light-driven water decontamination

Graphitic carbon nitride has gained considerable attention as a visible-light photocatalyst. However, its photocatalytic efficiency is restricted by its limited capacity for absorbing visible light and swift recombination of charge carriers. To overcome this bottleneck, we fabricated an atomic Fe-dispersed ultrathin carbon nitride (Fe-UTCN) photocatalyst via one-step thermal polymerization. Fe-UTCN showed high efficiency in the photodegradation of acetaminophen (APAP), achieving > 90 % elimination within 60-min visible light irradiation. The anchoring of Fe atoms improved the photocatalytic activity of UTCN by narrowing the bandgap from 2.50 eV to 2.33 eV and suppressing radiative recombination. Calculations by density functional theory revealed that the Fe-N4 sites (adsorption energy of - 3.10 eV) were preferred over the UTCN sites (adsorption energy of - 0.18 eV) for the adsorption of oxygen and the subsequent formation of O2•-, the dominant reactive species in the degradation of APAP. Notably, the Fe-UTCN catalyst exhibited good stability after five successive runs and was applicable to complex water matrices. Therefore, Fe-UTCN, a noble-metal-free photocatalyst, is a promising candidate for visible light-driven water decontamination.

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.

Ruthenium-based single atom catalysts: synthesis and application in the electrocatalytic hydrogen evolution reaction

Electrocatalytic water splitting is a promising production method for green hydrogen; however, its practical application is limited by the lack of robust catalysts for the cathode hydrogen evolution reaction (HER). Recently, the use of Ru in electrocatalytic HER has become a research hotspot because Ru has a metal-hydrogen bond strength similar to that of Pt - known for its excellent HER activity - but has a lower cost than Pt. Numerous modification strategies are available to further improve the HER activity of metal Ru such as vulcanisation, phosphating and atomisation. The atomisation strategy has attracted much attention owing to its extremely high Ru atomic utilisation efficiency and tunable electronic structures. However, isolated studies could not effectively address the bottlenecks. Therefore, to promote the effective exploration of Ru-based single-atom catalysts and clarify the research status in this field, studies on related topics (e.g. Ru single-atom catalysts, Ru dual-atom catalysts, composite catalysts containing single-atom Ru and Ru nanoparticles) have been systematically and briefly summarised herein. Finally, the research challenges and prospects of Ru-based single-atom catalysts in the HER field have been discussed, which may provide valuable insights for further research.

Lattice-Confined Single-Atom Catalyst: Preparation, Application and Electron Regulation Mechanism

Lattice-confined single-atom catalyst (LC SAC), featuring exceptional activity, intriguing stability and prominent selectivity, has attracted extensive attention in the fields of various reactions (e.g., hydrogen evolution reaction (HER), oxygen evolution reaction (OER), oxygen reduction reaction (ORR), etc.). To design a "smart" LC SAC for catalytic applications, one must systematically comprehend updated advances in the preparation, the application, and especially the peculiar electron regulation mechanism of LC SAC. In this review, the specific preparation methods of LC SAC based on general coordination strategy are updated, and its applications in HER, OER, ORR, N2 reduction reaction (NRR), advanced oxidation processes (AOPs) and so forth are summarized to display outstanding activity, stability and selectivity. Uniquely, the electron regulation mechanisms are first and deeply discussed and can be primarily categorized as electron transfer bridge with monometallic active sites, novel catalytic centers with polymetallic active sites, and positive influence by surrounding environments. In the end, the existing issues and future development directions are put forward with a view to further optimize the performance of LC SAC. This review is expected to contribute to the in-depth understanding and practical application of highly efficient LC SAC.

Coupling Nano and Atomic Electric Field Confinement for Robust Alkaline Oxygen Evolution

The alkaline oxygen evolution reaction (OER) is a promising avenue for producing clean fuels and storing intermittent energy. However, challenges such as excessive OH- consumption and strong adsorption of oxygen-containing intermediates hinder the development of alkaline OER. In this study, we propose a cooperative strategy by leveraging both nano-scale and atomically local electric fields for alkaline OER, demonstrated through the synthesis of Mn single atom doped CoP nanoneedles (Mn SA-CoP NNs). Finite element method simulations and density functional theory calculations predict that the nano-scale local electric field enriches OH- around the catalyst surface, while the atomically local electric field improves *O desorption. Experimental validation using in situ attenuated total reflection infrared and Raman spectroscopy confirms the effectiveness of the nano-scale and atomically electric fields. Mn SA-CoP NNs exhibit an ultra-low overpotential of 189 mV at 10 mA cm-2 and stable operation over 100 hours at ~100 mA cm-2 during alkaline OER. This innovative strategy provides new insights for enhancing catalyst performance in energy conversion reactions.

Accumulation of Long-Lived Photogenerated Holes at Indium Single-Atom Catalysts via Two Coordinate Nitrogen Vacancy Defect Engineering for Enhanced Photocatalytic Oxidation

Visible-light-driven photocatalytic oxidation by photogenerated holes has immense potential for environmental remediation applications. While the electron-mediated photoreduction reactions are often at the spotlight, active holes possess a remarkable oxidation capacity that can degrade recalcitrant organic pollutants, resulting in nontoxic byproducts. However, the random charge transfer and rapid recombination of electron-hole pairs hinder the accumulation of long-lived holes at the reaction center. Herein, a novel method employing defect-engineered indium (In) single-atom photocatalysts with nitrogen vacancy (Nv) defects, dispersed in carbon nitride foam (In-Nv-CNF), is reported to overcome these challenges and make further advances in photocatalysis. This Nv defect-engineered strategy produces a remarkable extension in the lifetime and an increase in the concentration of photogenerated holes in In-Nv-CNF. Consequently, the optimized In-Nv-CNF demonstrates a remarkable 50-fold increase in photo-oxidative degradation rate compared to pristine CN, effectively breaking down two widely used antibiotics (tetracycline and ciprofloxacin) under visible light. The contaminated water treated by In-Nv-CNF is completely nontoxic based on the growth of Escherichia coli. Structural-performance correlations between defect engineering and long-lived hole accumulation in In-Nv-CNF are established and validated through experimental and theoretical agreement. This work has the potential to elevate the efficiency of overall photocatalytic reactions from a hole-centric standpoint.

Tailored Metal-Organic Frameworks for Water Purification: Perfluorinated Fe-MOFs for Enhanced Heterogeneous Catalytic Ozonation

An industrially viable catalyst for heterogeneous catalytic ozonation (HCO) in water purification requires the characteristics of good dispersion of active species on its surface, efficient electron transfer for ozone decay, and maximum active species utilization. While metal-organic frameworks (MOFs) represent an attractive platform for HCO, the metal nodes in the unmodified MOFs exhibit low catalytic activity. Herein, we present a perfluorinated Fe-MOF catalyst by substituting H atoms on the metalated ligands with F atoms (termed 4F-MIL-88B) to induce structure evolution. The Lewis acidity of 4F-MIL-88B was enhanced via the formation of Fe nodes, tailoring the electron distribution on the catalyst surface. As a result of catalyst modification, the rate constant for degradation of the target compounds examined increased by ∼700% compared with that observed for the unmodified catalyst. Experimental evidence and theoretical calculations showed that the modulated polarity and the enhanced electron transfer between the catalyst and ozone molecules contributed to the adsorption and transformation of O3 to •OH on the catalyst surface. Overall, the results of this study highlight the significance of tailoring the metalated ligands to develop highly efficient and stable MOF catalysts for HCO and provide an in-depth mechanistic understanding of their structure-function evolution, which is expected to facilitate the applications of nanomaterial-based processes in water purification.

Boundary-Rich Carbon-Based Electrocatalysts with Manganese(II)-Coordinated Active Environment for Selective Synthesis of Hydrogen Peroxide

Coordinated manganese (Mn) electrocatalysts owing to their electronic structure flexibility, non-toxic and earth abundant features are promising for electrocatalytic reactions. However, achieving selective hydrogen peroxide (H2 O2 ) production through two electron oxygen reduction (2e-ORR) is a challenge on Mn-centered catalysts. Targeting this goal, we report on the creation of a secondary Mn(II)-coordinated active environment with reactant enrichment effect on boundary-rich porous carbon-based electrocatalysts, which facilitates the selective and rapid synthesis of H2 O2 through 2e-ORR. The catalysts exhibit nearly 100 % Faradaic efficiency and H2 O2 productivity up to 15.1 mol gcat -1  h-1 at 0.1 V versus reversible hydrogen electrode, representing the record high activity for Mn-based electrocatalyst in H2 O2 electrosynthesis. Mechanistic studies reveal that the epoxide and hydroxyl groups surrounding Mn(II) centers improve spin state by modifying electronic properties and charge transfer, thus tailoring the adsorption strength of *OOH intermediate. Multiscale simulations reveal that the high-curvature boundaries facilitate oxygen (O2 ) adsorption and result in local O2 enrichment due to the enhanced interaction between carbon surface and O2 . These merits together ensure the efficient formation of H2 O2 with high local concentration, which can directly boost the tandem reaction of hydrolysis of benzonitrile to benzamide with nearly 100 % conversion rate and exclusive benzamide selectivity.

Single Zn Atoms with Acetate-Anion-Enabled Asymmetric Coordination for Efficient H2 O2 Photosynthesis

Exploring unique single-atom sites capable of efficiently reducing O2 to H2 O2 while being inert to H2 O2 decomposition under light conditions is significant for H2 O2 photosynthesis, but it remains challenging. Herein, we report the facile design and fabrication of polymeric carbon nitride (CN) decorated with single-Zn sites that have tailorable local coordination environments, which is enabled by utilizing different Zn salt anions. Specifically, the O atom from acetate (OAc) anion participates in the coordination of single-Zn sites on CN, forming asymmetric Zn-N3 O moiety on CN (denoted as CN/Zn-OAc), in contrast to the obtained Zn-N4 sites when sulfate (SO4 ) is adopted (CN/Zn-SO4 ). Both experimental and theoretical investigations demonstrate that the Zn-N3 O moiety exhibits higher intrinsic activity for O2 reduction to H2 O2 than the Zn-N4 moiety. This is attributed to the asymmetric N/O coordination, which promotes the adsorption of O2 and the formation of the key intermediate *OOH on Zn sites due to their modulated electronic structure. Moreover, it is inactive for H2 O2 decomposition under both dark and light conditions. As a result, the optimized CN/Zn-OAc catalyst exhibits significantly improved photocatalytic H2 O2 production activity under visible light irradiation.

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.

Single-atom Fe nanozymes with excellent oxidase-like and laccase-like activity for colorimetric detection of ascorbic acid and hydroquinone

Although traditional Fe-based nanozymes have shown great potential, generally only a small proportion of the Fe atoms on the catalyst's surface are used. Herein, we synthesized single-atom Fe on N-doped graphene nanosheets (Fe-CNG) with high atom utilization efficiency and a unique coordination structure. Active oxygen species including superoxide radicals (O2•-) and singlet oxygen (1O2) were efficiently generated from the interaction of the Fe-CNG with dissolved oxygen in acidic conditions. The Fe-CNG nanozymes were found to display enhanced oxidase-like and laccase-like activity, with Vmax of 2.07 × 10-7 M∙S-1 and 4.54 × 10-8 M∙S-1 and Km of 0.324 mM and 0.082 mM, respectively, which is mainly due to Fe active centers coordinating with O and N atoms simultaneously. The oxidase-like performance of the Fe-CNG can be effectively inhibited by ascorbic acid (AA) or hydroquinone (HQ), which can directly obstruct the oxidation of 3,3',5,5'-tetramethylbenzidine (TMB). Therefore, a direct and sensitive colorimetric method for the detection of AA and HQ activity was established, which exhibited good linear detection and limit of detection (LOD) of 0.048 μM and 0.025 μM, respectively. Moreover, a colorimetric method based on the Fe-CNG catalyst was fabricated for detecting the concentration of AA in vitamin C. Therefore, this work offers a new method for preparing a single-atom catalyst (SAC) nanozyme and a promising strategy for detecting AA and HQ.

Mn-single-atom nano-multizyme enabled NIR-II photoacoustically monitored, photothermally enhanced ROS storm for combined cancer therapy

RATIONALE

To realize imaging-guided multi-modality cancer therapy with minimal side effects remains highly challenging.

METHODS

We devised a bioinspired hollow nitrogen-doped carbon sphere anchored with individually dispersed Mn atoms (Mn/N-HCN) via oxidation polymerization with triton micelle as a soft template, followed by carbonization and annealing. Enzyme kinetic analysis and optical properties were performed to evaluate the imaging-guided photothermally synergized nanocatalytic therapy.

RESULTS

Simultaneously mimicking several natural enzymes, namely peroxidase (POD), catalase (CAT), oxidase (OXD), and glutathione peroxidase (GPx), this nano-multizyme is able to produce highly cytotoxic hydroxyl radical (•OH) and singlet oxygen (1O2) without external energy input through parallel and series catalytic reactions and suppress the upregulated antioxidant (glutathione) in tumor. Furthermore, NIR-II absorbing Mn/N-HCN permits photothermal therapy (PTT), enhancement of CAT activity, and photoacoustic (PA) imaging to monitor the accumulation kinetics of the nanozyme and catalytic process in situ. Both in vitro and in vivo experiments demonstrate that near-infrared-II (NIR-II) PA-imaging guided, photothermally enhanced and synergized nanocatalytic therapy is efficient to induce apoptosis of cancerous cells and eradicate tumor tissue.

CONCLUSIONS

This study not only demonstrates a new method for effective cancer diagnosis and therapy but also provides new insights into designing multi-functional nanozymes.

Biochar-Based Single-Atom Catalyst with Fe-N3O-C Configuration for Efficient Degradation of Organic Dyes by Peroxymonosulfate Activation

Iron single-atom catalysts (Fe SACs) hold great promise for peroxymonosulfate (PMS) activation and degradation of organic pollutants in wastewater. However, insights into crucial catalytic sites and activation mechanisms of biochar-based Fe SACs for PMS remain a challenge. Herein, cotton stalk-derived biochar-based Fe SACs (Fe SACs-BC) with an asymmetric Fe-N/O-C configuration were prepared, and their PMS activation and acid orange 7 (AO7) degradation mechanisms were investigated. The results showed that the removal efficiency of the Fe SACs-BC catalyst with Fe-N3O-C configuration for AO7 and other five investigated organic dyes reached 95-99% within 15 min. The EPR spectrums, quenching experiments, electrochemical analysis, masking experiments, XPS, and theoretical calculations indicated that degradations of organic dyes were dominated by singlet oxygen, which was generated by direct PMS conversion at the electron-deficient carbon and iron sites in the Fe-N3O-C configuration. The Fe SACs-BC/PMS exhibited high removal efficiency and strong tolerance in different water matrices with a wide pH range, various coexisting anions and interfering substances, showing great potential and applicability for efficient treatment of actual textile wastewaters.

Self-carbon-thermal-reduction strategy for boosting the Fenton-like activity of single Fe-N4 sites by carbon-defect engineering

Carbon-defect engineering in metal single-atom catalysts by simple and robust strategy, boosting their catalytic activity, and revealing the carbon defect-catalytic activity relationship are meaningful but challenging. Herein, we report a facile self-carbon-thermal-reduction strategy for carbon-defect engineering of single Fe-N4 sites in ZnO-Carbon nano-reactor, as efficient catalyst in Fenton-like reaction for degradation of phenol. The carbon vacancies are easily constructed adjacent to single Fe-N4 sites during synthesis, facilitating the formation of C-O bonding and lowering the energy barrier of rate-determining-step during degradation of phenol. Consequently, the catalyst Fe-NCv-900 with carbon vacancies exhibits a much improved activity than the Fe-NC-900 without abundant carbon vacancies, with 13.5 times improvement in the first-order rate constant of phenol degradation. The Fe-NCv-900 shows high activity (97% removal ratio of phenol in only 5 min), good recyclability and the wide-ranging pH universality (pH range 3-9). This work not only provides a rational strategy for improving the Fenton-like activity of metal single-atom catalysts, but also deepens the fundamental understanding on how periphery carbon environment affects the property and performance of metal-N4 sites.

A Transmetalation Synthetic Strategy to Engineer Atomically Dispersed MnN2 O2 Electrocatalytic Centers Driving High-Performance LiS Battery

Sluggish sulfur redox reaction (SROR) kinetics accompanying lithium polysulfides (LiPSs) shuttle effect becomes a stumbling block for commercial application of LiS battery. High-efficient single atom catalysts (SACs) are desired to improve the SROR conversion capability; however, the sparse active sites as well as partial sites encapsulated in bulk-phase are fatal to the catalytic performance. Herein, high loading (5.02 wt.%) atomically dispersed manganese sites (MnSA) on hollow nitrogen-doped carbonaceous support (HNC) are realized for the MnSA@HNC SAC by a facile transmetalation synthetic strategy. The thin-walled hollow structure (≈12 nm) anchoring the unique trans-MnN2 O2 sites of MnSA@HNC provides a shuttle buffer zone and catalytic conversion site for LiPSs. Both electrochemical measurement and theoretical calculation indicate that the MnSA@HNC with abundant trans-MnN2 O2 sites have extremely high bidirectional SROR catalytic activity. The assembled LiS battery based on the MnSA@HNC modified separator can deliver a large specific capacity of 1422 mAh g-1 at 0.1 C and stable cycling over 1400 cycles with an ultralow decay rate of 0.033% per cycle at 1 C. More impressively, a flexible pouch cell on account of the MnSA@HNC modified separator may release a high initial specific capacity of 1192 mAh g-1 at 0.1 C and uninterruptedly work after the bending-unbending processes.

DNA-modulated single-atom nanozymes with enhanced enzyme-like activity for ultrasensitive detection of dopamine

Despite the current progress in optimizing and tailoring the performance of nanozymes through structural and synthetic adaptation, there is still a lack of dynamic modulation approaches to alter their catalytic activity. Here, we demonstrate that DNA can act as an auxiliary regulator via a straightforward incubation method with Fe-N-C single-atom nanozymes (SAzymes), causing a leap in the enzyme-like activity of Fe-N-C from moderate to a higher level. The DNA-assisted enhancement is attributed to the increased substrate affinity of Fe-N-C nanozymes through electrostatic attraction between the substrate and DNA. Based on the prepared DNA/Fe-N-C system, colorimetric sensors for dopamine (DA) detection were constructed. Surprisingly, the incorporation of DNA not only enabled the detection of DA in a low concentration range, but also greatly improved the sensitivity with a 436-fold decrease in detection limit. The quantitative determination of DA was achieved in two-segment linear ranges of 0.01-4 μM and 5-100 μM with an ultralow detection limit of 9.56 nM. The DNA/Fe-N-C system shows superior performance compared to the original Fe-N-C system, making it an ideal choice for nanozyme-based biosensors. This simple design approach has paved the way for enhancing nanozyme activity and is expected to serve as a general strategy for optimizing biosensor performance.

Universal Water Disinfection by the Introduction of Fe-N3 Traps between g-C3N4 Layers under Visible Light

Efficient inactivation of bacteria in the sewage via a photocatalytic process represents a promising strategy for the efficient chemical utilization of solar energy. Herein, uniformly dispersed Fe atoms were embedded between layers of g-C3N4 photocatalysts (CNFx), which were facilely prepared by thermal treatment. The optimized photocatalyst (CNF100) first showed excellent photoactivity for killing a variety of bacteria (93.0% for E. coli, 93.9% for Salmonella, and 96.2% for S. aureus) under visible light irradiation. The superior activity can be attributed to the formation of shallow electron traps (Fe-N3) that can capture excitons of excited states, which promote the charge transfer and energy transfer process of activated adsorbed molecular oxygen, respectively, forming reactive oxygen species, improving separation efficiency of photoexcited electrons and holes, and the Fe-N3 traps can also be used as photosensitive sites to broaden the absorption range of visible light. This strategy of constructing shallow electronic traps lays a theoretical foundation for the design of new environmentally friendly and efficient water disinfectants.

Carbon-based single-atom catalysts in advanced oxidation reactions for water remediation: From materials to reaction pathways

Single-atom catalysts (SACs) have been widely recognized as state-of-the-art catalysts in environment remediation because of their exceptional performance, 100% metal atomic utilization, almost no secondary pollution, and robust structures. Most recently, the activation of persulfate with carbon-based SACs in advanced oxidation processes (AOPs) raises tremendous interest in the degradation of emerging contaminants in wastewater, owning to its efficient and versatile reactive oxidant species (ROS) generation. However, the comprehensive and critical review unraveling the underlying relationship between structures of carbon-based SACs and the corresponding generated ROS is still rare. Herein, we systematically summarize the fundamental understandings and intrinsic mechanisms between single metal atom active sites and produced ROS during AOPs. The types of emerging contaminants are firstly elaborated, presenting the prior pollutants that need to be degraded. Then, the preparation and characterization methods of carbon-based SACs are overviewed. The underlying material structure-ROS type relationship in persulfate-based AOPs is discussed in depth to expound the catalytic mechanisms. Finally, we briefly conclude the current development of carbon-based SACs in AOPs and propose the prospects for rational design and synthesis of carbon-based SACs with on-demand catalytic performances in AOPs in future research.

Heterogeneous Fe-Co dual-atom catalyst outdistances the homogeneous counterpart for peroxymonosulfate-assisted water decontamination: New surface collision oxidation path and diatomic synergy

Heterogeneous catalysts lag far behind their homogeneous counterparts in activating peroxymonosulfate (PMS) for water decontamination due to the low site intrinsic activity and sluggish mass transfer. The single-atom catalyst can bridge the gap between heterogeneous and homogeneous catalysts, but the difficulty to break scaling relations originating from the site monotony restricts further efficiency upgradation. Herein through modulating the crystallinity of NH₂-UIO-66, a porous carbon support with ultrahigh surface area (1721.71 m² g^-1) is obtained to anchor the dual-atom FeCoN₆ site, which exhibits superior turnover frequency over single-atom FeN₄ and CoN₄ sites (13.07 versus 9.97, 9.07 min^-1). The as-synthesized composite thus outperforms the homogeneous catalytic system (Fe³⁺+Co²⁺) for sulfamethoxazole (SMZ) degradation, and the catalyst-dose-normalized kinetic rate constant (99.26 L min^-1 g^-1) exceeds reported values by 1∼2 orders of magnitude. Moreover, only 20 mg of the catalyst can run a fluidized-bed reactor to realize continuous zero discharge of SMZ in multiple actual waters for up to 8.33 h. Unlike all reported reaction routes, the catalysis on the diatomic site follows a new surface collision oxidation path, i.e. the dispersed catalyst adsorbs PMS to generate surface-activated PMS with high potential, which collides with surrounding SMZ and directly seizes electron from it to induce pollutant oxidation. Theoretical calculation indicates that the enhanced activity of FeCoN₆ site stems from the diatomic synergy, leading to stronger PMS adsorption, larger near-Fermi-level density of states and optimal global Gibbs free energy evolution. Overall, this work provides an effective strategy of constructing heterogeneous dual-atom catalyst/PMS process to achieve faster pollution control than homogeneous system, and sheds light on the interatomic synergetic mechanism for PMS activation.

Single-Atom Catalysts in Environmental Engineering: Progress, Outlook and Challenges

Recently, single-atom catalysts (SACs) have attracted wide attention in the field of environmental engineering. Compared with their nanoparticle counterparts, SACs possess high atomic efficiency, unique catalytic activity, and selectivity. This review summarizes recent studies on the environmental remediation applications of SACs in (1) gaseous: volatile organic compounds (VOCs) treatment, NO_x reduction, CO₂ reduction, and CO oxidation; (2) aqueous: Fenton-like advanced oxidation processes (AOPs), hydrodehalogenation, and nitrate/nitrite reduction. We present the treatment activities and reaction mechanisms of various SACs and propose challenges and future opportunities. We believe that this review will provide constructive inspiration and direction for future SAC research in environmental engineering.

Efficacy and mechanism of peroxymonosulfate activation by single-atom transition metal catalysts for the oxidation of organic pollutants: Experimental validation and theoretical calculation

Single-atom catalysts can activate peroxymonosulfate (PMS) to enhance its oxidation of organic pollutants in water treatment. We synthesized a series of carbon-supported single-atom transition metal catalysts (MnN@C, FeN@C, CoN@C, NiN@C, and CuN@C) with similar compositions and structures. Their catalytic activity toward PMS activation and oxidation mechanisms were investigated using acid orange 7 (AO7) as a model pollutant. The degradation rate (min^-1·mol^-1·g·m^-2) of AO7 followed order: FeN@C/PMS (7.576 × 10³) > MnN@C/PMS (5.104 × 10³) > CoN@C/PMS (1.919 × 10³) ≫ NiN@C/PMS (0.058 × 10³) > CuN@C/PMS (0.035 × 10³). Electron transfer mediated by surface-activated PMS was found to be the main regime of AO7 oxidation in the catalytic systems. Density functional theory calculations indicated that the degradation of AO7 was promoted by the intense adsorption of PMS and the electron transfer between AO7 and the surface-activated PMS on the catalyst. The cleavage of the naphthalene ring and the azo group was the primary degradation pathway. The toxicity of the products was significantly reduced. This research provides valuable findings for preparing highly efficient single-atom transition metal catalysts for PMS-based degradation of toxic and refractory organic pollutants from water.

Z-scheme WOx/Cu-g-C3N4 heterojunction nanoarchitectonics with promoted charge separation and transfer towards efficient full solar-spectrum photocatalysis

Construction of Z-scheme heterojunctions has been considered one superb method in promoting solar-assisted charge carrier separation of carbon-based materials to achieve efficient utilization of solar energy in hydrogen production and CO₂ reduction. One interesting concept in nanofabrication that has become trend recent years is nanoarchitectonics. A heterostructure photocatalyst constructed based on the idea of nanoarchitectonics using the combination of g-C₃N₄, metal and an additional semiconducting nanocomposite is investigated in this paper. Z-scheme tungsten oxide incorporated copper modified graphitic carbon nitride (WO_x/Cu-g-C₃N₄) heterostructures are fabricated via immobilization of WO_x on Cu nanoparticles modified superior thin g-C₃N₄ nanosheets. Mechano-chemical pre-reaction and a two-step high-temperature thermal polymerization process are the keys in attaining homogeneous distribution of Cu nanoparticles in g-C₃N₄ nanosheets. The horizontal growth of homogeneously distributed WO_x nanobelts on Cu modified g-C₃N₄ (Cu-g-C₃N₄) base via solvothermal synthesis is achieved. The photocatalytic performances of the heterostructures are evaluated through water splitting and CO₂ photoreduction measurements in full solar spectrum irradiation condition. The presence of Cu nanoparticles in the composite system improves charge transport between g-C₃N₄ and WO_x and thus enhances the photocatalytic performances (H₂ generation and CO₂ photoreduction) of the composite material, while the presence of WO_x nanocomposites enhances light absorption of the composite material in the near infrared range. The synthesized heterostructure with optimized WO_x to Cu-g-C₃N₄ ratio and in case of no co-catalyst addition exhibits enhanced photocatalytic H₂ evolution (4560 μmolg^-1h^-1) as well as excellent CO₂ reduction rate (5.89 μmolg^-1h^-1 for CO generation).

Engineering Single-Atom Nanozymes for Catalytic Biomedical Applications

Nanomaterials with enzyme-mimicking properties, coined as nanozymes, are a promising alternative to natural enzymes owing to their remarkable advantages, such as high stability, easy preparation, and favorable catalytic performance. Recently, with the rapid development of nanotechnology and characterization techniques, single atom nanozymes (SAzymes) with atomically dispersed active sites, well-defined electronic and geometric structures, tunable coordination environment, and maximum metal atom utilization are developed and exploited. With superior catalytic performance and selectivity, SAzymes have made impressive progress in biomedical applications and are expected to bridge the gap between artificial nanozymes and natural enzymes. Herein, the recent advances in SAzyme preparation methods, catalytic mechanisms, and biomedical applications are systematically summarized. Their biomedical applications in cancer therapy, oxidative stress cytoprotection, antibacterial therapy, and biosensing are discussed in depth. Furthermore, to appreciate these advances, the main challenges, and prospects for the future development of SAzymes are also outlined and highlighted in this review.

Targeted Decomplexation of Metal Complexes for Efficient Metal Recovery by Ozone/Percarbonate

Traditional methods cannot efficiently recover Cu from Cu(II)-EDTA wastewater and encounter the formation of secondary contaminants. In this study, an ozone/percarbonate (O₃/SPC) process was proposed to efficiently decomplex Cu(II)-EDTA and simultaneously recover Cu. The results demonstrate that the O₃/SPC process achieves 100% recovery of Cu with the corresponding k_obs value of 0.103 min^-1 compared with the typical ^•OH-based O₃/H₂O₂ process (81.2%, 0.042 min^-1). The carbonate radical anion (CO₃^•-) is generated from the O₃/SPC process and carries out the targeted attack of amino groups of Cu(II)-EDTA for decarboxylation and deamination processes, resulting in successive cleavage of Cu-O and Cu-N bonds. In comparison, the ^•OH-based O₃/H₂O₂ process is predominantly responsible for the breakage of Cu-O bonds via decarboxylation and formic acid removal. Moreover, the released Cu(II) can be transformed into stable copper precipitates by employing an endogenous precipitant (CO₃^2-), accompanied by toxic-free byproducts in the O₃/SPC process. More importantly, the O₃/SPC process exhibits excellent metal recovery in the treatment of real copper electroplating wastewater and other metal-EDTA complexes. This study provides a promising technology and opens a new avenue for the efficient decomplexation of metal-organic complexes with simultaneous recovery of valuable metal resources.

Single-Atom MnN5 Catalytic Sites Enable Efficient Peroxymonosulfate Activation by Forming Highly Reactive Mn(IV)-Oxo Species

Four-nitrogen-coordinated transitional metal (MN₄) configurations in single-atom catalysts (SACs) are broadly recognized as the most efficient active sites in peroxymonosulfate (PMS)-based advanced oxidation processes. However, SACs with a coordination number higher than four are rarely explored, which represents a fundamental missed opportunity for coordination chemistry to boost PMS activation and degradation of recalcitrant organic pollutants. We experimentally and theoretically demonstrate here that five-nitrogen-coordinated Mn (MnN₅) sites more effectively activate PMS than MnN₄ sites, by facilitating the cleavage of the O-O bond into high-valent Mn(IV)-oxo species with nearly 100% selectivity. The high activity of MnN₅ was discerned to be due to the formation of higher-spin-state N₅Mn(IV)═O species, which enable efficient two-electron transfer from organics to Mn sites through a lower-energy-barrier pathway. Overall, this work demonstrates the importance of high coordination numbers in SACs for efficient PMS activation and informs the design of next-generation environmental catalysts.

Single-Atom Fe-N4 Sites for Catalytic Ozonation to Selectively Induce a Nonradical Pathway toward Wastewater Purification

Nonradical oxidation has been determined to be a promising pathway for the degradation of organic pollutants in heterogeneous catalytic ozonation (HCO). However, the bottlenecks are the rational design of catalysts to selectively induce nonradicals and the interpretation of detailed nonradical generation mechanisms. Herein, we propose a new HCO process based on single-atom iron catalysts, in which Fe-N₄ sites anchored on the carbon skeleton exhibited outstanding catalytic ozonation activity and stability for the degradation of oxalic acid (OA) and p-hydroxybenzoic acid (pHBA) as well as the advanced treatment of a landfill leachate secondary effluent. Unlike traditional radical oxidation, nonradical pathways based on surface-adsorbed atomic oxygen (*O_ad) and singlet oxygen (¹O₂) were identified. A substrate-dependent behavior was also observed. OA was adsorbed on the catalyst surface and mainly degraded by *O_ad, while pHBA was mostly removed by O₃ and ¹O₂ in the bulk solution. Density functional theory calculations and molecular dynamics simulations revealed that one terminal oxygen atom of ozone preferred bonding with the central iron atom of Fe-N₄, subsequently inducing the cleavage of the O-O bond near the catalyst surface to produce *O_ad and ¹O₂. These findings highlight the structural design of an ozone catalyst and an atomic-level understanding of the nonradical HCO process.

Enhanced redox cycle of Fe3+/Fe2+ on Fe@NC by boron: Fast electron transfer and long-term stability for Fenton-like reaction

In this work, Fe@NC/B material is successfully synthesized and in-situ supported on the surface of amorphous boron (B) using a simple pyrolysis method. The interface between Fe species and B is improved by introducing N-doped carbon (NC) layers as intermediate, fast electron transfer from B to Fe@NC can therefore be achieved, thus could promote the fast redox cycle of Fe³⁺/Fe²⁺. The obtained material can therefore activate peroxymonosulfate (PMS) effectively to degrade Bisphenol A (BPA), a fast degradation rate and a very long lifetime in a continous tubular reactor are realized. Moreover, experiments and DFT calculation indicate that Fe²⁺ containing species are the dominated active sites, while the exposed B atoms and structure defect of B can also activate PMS directly to produce SO₄^•- and ¹O₂ species for BPA degradation. In addition, boric acid is the oxidation product of B, which can be dissolved into the aqueous solution and expose fresh B species again for PMS activation. The combination of B with Fe@NC provide novel materials for long term PMS activation, thus could promote the real application of persulfates on an industrial scale.

Complexation and degradation of tetracycline by activation of molecular oxygen with biochar-supported nano-zero-valent copper composite

Nano-zero-valent copper (nZVC) is a superior molecular oxygen (O₂) activator for the abatement of organic pollutants due to its high electron utilization rate. However, the activation efficiency of O₂ is compromised by the agglomeration tendency of nZVC particles and the concomitant reduction of the available active sites. To address this problem, the biochar (BC) with porous structure and abundant surface functional groups is utilized to disperse and stabilize nZVC for O₂ activation (simplified as the nZVC/BC/O₂ system) for efficient removal of tetracycline (TC). The nZVC/BC composite possesses a high specific area with well-distributed nZVC particles on the BC surface, which guarantees the superior dispersion and high reactivity in the activation of O₂. The efficacy of the nZVC/BC/O₂ system for TC abatement is evaluated and the underlying mechanism is elucidated. The results show that nZVC/BC/O₂ system can achieve excellent removal of TC with the efficiencies of more than 85% in the pH range of 4.0-9.0, which originated from the combined action of complexation and degradation. The degradation is dominated by reactive oxygen species (ROS) including •OH, •O₂^- and ¹O₂ generated by Cu⁰/Cu⁺ activated O₂ while the generation of Cu²⁺ via oxygen oxidation on the surface of nZVC/BC can remove TC by complexation adsorption. This study highlights the complexation and degradation in the removal of TC and can be expected to exhibit application prospects in the water and wastewater treatment.

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

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

Mn-N-C catalysts derived from metal triazole framework with hierarchical porosity for efficient oxygen reduction

Manganese and nitrogen co-doped porous carbon (Mn-N-C) are proposed as one of the most up-and-coming non-precious metal electrocatalysts to substitute Pt-based in the oxygen reduction reaction (ORR). Herein, we chose metal triazole frameworks as carbon substrate with hierarchical porosity for trapping and anchoring Mn-containing gaseous species by a mild one-step pyrolysis method. The optimized Mn-N-C electrocatalyst with a large metal content of 1.71 wt% and a volume ratio of 0.86 mesopores pore delivers a superior ORR activity with a half-wave potential (E_1/2) of 0.92 V in 0.1 M KOH and 0.78 V in 0.1 M HClO₄. Moreover, the modified Mn-N-C catalyst showed superior potential cyclic stability. TheE_1/2remained unchanged in 0.1 M KOH and only lost 6 mV in 0.1 M HClO₄after 5000 cycles. When applied as the cathode catalyst in Zn-air battery, it exhibited a maximum peak power density of 176 mW cm^-2, demonstrating great potential as a usable ORR catalyst in practical devices.

Selective Oxygen Activation to Reactive Oxygen Species on a Carbon Layer-Encapsulated CuxO Electrocatalyst for Water Purification

In situ synthesis of reactive oxygen species (ROS) on demand via oxygen activation (OA) is significant in biological, chemical, and environmental fields. Thus, the design of OA catalysts with adequate reactivity, durability, and selectivity is critical but challenging. Here, we report a Cu_xO@C core@shell photoelectrode prepared by encapsulating Cu/Cu₂O/CuO into a carbon layer through anodic electropolymerization (electrophoresis-coupled self-assembly of carbon quantum dots). Theoretical prediction and experiments indicate that the carbon layer can effectively facilitate optical trapping and charge transfer, thus promoting photoelectric conversion and anti-photocorrosion performance of Cu_xO@C. The inner Cu_xO core acts as an electron reservoir and continuously injects electrons into the outer carbon layer shell, and the carbon atoms adjacent to oxygen-enriched functional groups (C-O-C and -COOH) in the electron-rich carbon layer work as the reactive sites to adsorb O₂ and donate electrons to the antibonding orbital [lowest unoccupied molecular orbital (π*)] of dioxygen. Optimized adsorption and hydrogenation of the critical intermediates (*O₂, *OOH, and *H₂O₂) and thermodynamically tunable O-O bond cleavage enable O₂ being selectively reduced to the superoxide anion and hydroxyl radical via the mixed multielectron processes consisting of one- and three-electron pathways. Sulfamethoxazole, an emerging refractory organic contaminant widely present in the environment, can be effectively degraded (∼100% removal) in such an electrochemical platform, benefiting from the abundant ROS generated in situ. Our findings demonstrate an innovative strategy to develop highly efficient and selective OA catalysts for practical water purification.

Single atom Mn anchored on N-doped porous carbon derived from spirulina for catalyzed peroxymonosulfate to degradation of emerging organic pollutants

Highly efficient single atom catalysts are critical to substantially promote for peroxymonosulfate (PMS) activation to organic pollutant degradation, but it remains a challenge at present. Herein, single atom Mn anchored on N-doped porous carbon (SA-Mn-NSC) was synthesized by ball milling of Mn-doped carbon nitride and spirulina biochar to dominantly activate PMS. The precursor of carbon nitride and spirulina possessed a strong coordinating capability for Mn(II), facilitating the formation of highly dispersed nitrogen-coordinated Mn sites (Mn-N₄). The SA-Mn-NSC catalyst exhibited high activity and stability in the heterogeneous activation of PMS to degrade a wide range of pollutants within 10 min, showing an outstanding degradation rate constant of 0.31 min^-1 in enrofloxacin (ENR) degradation. The high surface density of Mn-N₄ sites and abundant interconnected meso-macro pores were highly favorable for activating PMS to produce ¹O₂ and high-valent manganese (Mn(IV)) for pollutant degradation. This work offers a new pathway of using a low-cost and easily accessible single-atom catalysts (SACs) and could inspire more catalytic oxidation strategies.

MnOx@N-doped carbon nanosheets derived from Mn-MOFs and g-C3N4 for peroxymonosulfate activation: Electron-rich Mn center induced by N doping

The fabrication of metal-carbon hybrids with heteroatom doping from manganese-metal organic frameworks (MOFs) has rarely been reported for peroxymonosulfate (PMS) activation. In this work, novel MnO_x@N-doped carbon (MnO_x@NC) nanosheets were prepared using 2D manganese-1,4 benzenedicarboxylic acid-based MOFs (Mn-MOFs) and different proportions of graphitic carbon nitride (g-C₃N₄, additional N source and carbon source) to activate PMS for sulfamethoxazole (SMX) removal. The polarization difference induced by Mn-N coordination during the carbonization process made C an electron-poor center and Mn an electron-rich center, thus providing more Mn(II) for PMS activation. Benefiting from the highest Mn(II) content, the most uniform and exposed MnO_x active sites, abundant N active species and rich defective sites, MnO_x@NC-20 showed excellent degradation (72.9% within 5 min) and mineralization performance (47.40% within 60 min) for SMX. Nonradical and radical processes worked together in MnO_x@NC-20/PMS/SMX system, where singlet oxygen (¹O₂) dominated the degradation of SMX. N-doped carbon not only exhibited dragging and protection effects on MnO_x, but also provided adsorption sites for PMS and pollutants, thus reducing their migration distance. Moreover, the electrons of organic substrates could be captured by the electron-poor carbon layer and then transported to the electron-rich Mn center, thus improving the utilization efficiency of PMS and the redox of Mn. This study provides a facile optimization method to prepare MOFs-derived carbon catalysts with improved stability and catalytic performance.

Atomically dispersed Mn boosting photoelectrochemical SARS-CoV-2 spike protein immunosensing on carbon nitride

The sudden outbreak of coronavirus disease (COVID-19) triggered by SARS-CoV-2 infection has created a terrifying situation around the world. The spike protein of SARS-CoV-2 can act as an early biomarker for COVID-19. Therefore, controlling the spread of COVID-19 requires a low-cost, fast-response, and sensitive monitoring technique of spike protein. Herein, a photoelectrochemical (PEC) immunosensor for the detection of spike protein was constructed using the nanobody and an Mn (Ⅱ) modified graphitic carbon nitride (Mn/g-C₃N₄). The introduction of atomically dispersed Mn (Ⅱ) can accelerate the effective transfer and separation of photogenerated electron-hole pairs, which significantly boosts PEC performance of g-C₃N₄, thereby improving the detection sensitivity. As a recognition site, nanobody can achieve high-affinity binding to the spike protein, leading to a high sensitivity. The linear detection range of the proposed PEC immunosensor was 75 fg mL^-1 to 150 pg mL^-1, and the limit of detection was calculated to be 1.22 fg mL^-1. This stable and feasible PEC immunosensor would be a promising diagnostic tool for sensitively detecting spike protein of SARS-CoV-2.

Catalysts containing Fe and Mn from dewatered sludge showing enhanced electrocatalytic degradation of triclosan

The present work demonstrates a simple one-step pyrolysis method for the synthesis of a catalytic sludge-based carbon (SBC) biochar containing Fe and Mn from dehydrated sludge with added KMnO₄ and Fe(II). The electrocatalytic degradation of triclosan (TCS) in water was evaluated using an Fe/Mn-SBC cathode to promote a heterogeneous Fenton-like reaction. The catalyst generated at 500 °C exhibited an abundant porous structure and a relatively high surface area, and produced an electrode with better conductivity and electron diffusion. The presence of metal oxides changed the surface structure defects of this biochar and enhanced its catalytic performance while increasing the electrochemically active surface area by 72.68 mF/cm² compared with plain SBC. TCS was degraded (91.3%) within 180 min by oxygen species generated in situ on an Fe/Mn-SBC cathode because the activation energy for oxygen reduction was lowered by 4.62 kJ/mol. The degradation of TCS followed pseudo first-order kinetics and was controlled by TCS diffusion and interfacial chemical reactions between adsorbed TCS and the electrode. Possible TCS degradation pathways were devised based on the main intermediates, and ¹O₂ was found to be more important than •OH radicals. Through toxicity test and prediction, the toxicity of degradation was gradually reduced. This study demonstrates a simple and ecofriendly method for the electrocatalytic degradation of organic pollutants.

Single-atom site catalysts for environmental remediation: Recent advances

Single-atom site catalysts (SACs) can maximize the utilization of active metal species and provide an attractive way to regulate the activity and selectivity of catalytic reactions. The adjustable coordination configuration and atomic structure of SACs enable them to be an ideal candidate for revealing reaction mechanisms in various catalytic processes. The minimum use of metals and relatively tight anchoring of the metal atoms significantly reduce leaching and environmental risks. Additionally, the unique physicochemical properties of single atom sites endow SACs with superior activity in various catalytic processes for environmental remediation (ER). Generally, SACs are burgeoning and promising materials in the application of ER. However, a systematic and critical review on the mechanism and broad application of SACs-based ER is lacking. Herein, we review emerging studies applying SACs for different ERs, such as eliminating organic pollutants in water, removing volatile organic compounds, purifying automobile exhaust, and others (hydrodefluorination and disinfection). We have summarized the synthesis, characterization, reaction mechanism and structural-function relationship of SACs in ER. In addition, the perspectives and challenges of SACs for ER are also analyzed. We expect that this review can provide constructive inspiration for discoveries and applications of SACs in environmental catalysis in the future.

Self-cycled photo-Fenton-like system based on an artificial leaf with a solar-to-H2O2 conversion efficiency of 1.46

Millions of families around the world remain vulnerable to water scarcity and have no access to drinking water. Advanced oxidation processes (AOPs) are an effective way towards water purification with qualified reactive oxygen species (ROSs) while are impeded by the high-cost and tedious process in either input of consumable reagent, production of ROSs, and the pre-treatment of supporting electrolyte. Herein, we couple solar light-assisted H₂O₂ production from water and photo-Fenton-like reactions into a self-cyclable system by using an artificial leaf, achieving an unassisted H₂O₂ production rate of 0.77 μmol/(min·cm²) under 1 Sun AM 1.5 illumination. Furthermore, a large (70 cm²) artificial leaf was used for an unassisted solar-driven bicarbonate-activated hydrogen peroxide (BAP) system with recycled catalysts for real-time wastewater purification with requirements for only water, oxygen and sunlight. This demonstration highlights the feasibility and scalability of photoelectrochemical technology for decentralized environmental governance applications from laboratory benchtops to industry.

Continuous generation of reactive oxygen species is desirable in the advanced oxidation process. Here, the authors report a self-cycled photoFenton-like with a scalable artificial leaf for production of H₂O₂ from water with solar-to-H₂O₂ efficiency of 1.46%.

Boosting the photocatalytic activation of molecular oxygen and photodegradation of tetracycline: The role of interfacial synergistic effect of cocatalyst and dopants

Utilizing reactive oxygen species (ROS), which are generated by the activation of molecular oxygen (O₂) in oxidation reaction, is a promising method for pollutant degradation. However, it is limited by the commonly low efficiency of O₂ activation and carrier separation. Herein, as a model system, Ag cocatalyst and Cl doping modified g-C₃N₄ (Ag/Cl-CN) was constructed to improve the ability of O₂ activation. Results showed that Ag/Cl-CN could effectively convert more O₂ into ROS than pristine g-C₃N₄ (CN), and individually decorated CN (Ag-CN and Cl-CN). A series of experiments and DFT calculations revealed that the deposition of Ag could promote charge separation resulting in more charges accumulated around O₂ and the introduction of Cl led to a stronger adsorption capacity for O₂. Therefore, due to the synergistic effect of Ag cocatalyst and Cl dopant, Ag/Cl-CN generated higher concentrations of O₂^- and displayed much better activity for photocatalytic degradation of tetracycline (TC) than CN, Ag-CN and Cl-CN.

In-situ electronic structure redistribution tuning of single-atom Mn/g-C3N4 catalyst to trap atomic-scale lead(II) for highly stable and accurate electroanalysis

Constructing catalysts with simple structures, uniform effective sites, and excellent performance is crucial for understanding the reaction mechanism of target pollutants. Herein, the single-atom catalyst of Mn-intercalated graphitic carbon nitride (Mn/g-C₃N₄) was prepared. It was found that the intercalated Mn atoms acted as strong electron donors to effectively tune the electronic structure distribution of the in-situ N atoms, providing a large number of negative potential atomic-scale sites for catalytic reactions. In the detection, the in-situ N atom established an electron bridge for the transient electrostatic trapping of free Pb(II), which induced Pb-N-Mn coordination bonding. Even in g-C₃N₄-loaded Mn nanoparticles, the N atom was again confirmed to be the interaction site for coupling with Pb. And the Mn^II-N₄-C/Mn^IV-N₄-C cycle actively participated in the electrocatalysis of Pb(II) was confirmed. Moreover, Mn/g-C₃N₄ achieved highly stable and accurate detection for Pb(II) with a sensitivity of 2714.47 µA·µM^-1·cm^-2. And excellent reproducibility and specific detection of real water samples made the electrode practical. This study contributes to understanding the changes in the electronic structure of chemically inert substrates after single-atom intercalation and the interaction between contaminants and the microstructure of sensitive materials, providing a guiding strategy for designing highly active electrocatalytic interfaces for accurate electroanalysis.

Degradation of Organic Contaminants by Reactive Iron/Manganese Species: Progress and Challenges

Many iron(II, III, VI)- and manganese(II, IV, VII)-based oxidation processes can generate reactive iron/manganese species (RFeS/RMnS, i.e., Fe(IV)/Fe(V) and Mn(III)/Mn(V)/Mn(VI)), which have mild and selective reactivity toward a wide range of organic contaminants, and thus have drawn significant attention. The reaction mechanisms of these processes are rather complicated due to the simultaneous involvement of multiple radical and/or nonradical species. As a result, the ambiguity in the occurrence of RFeS/RMnS and divergence in the degradation mechanisms of trace organic contaminants in the presence of RFeS/RMnS exist in literature. In order to improve the critical understanding of the RFeS/RMnS-mediated oxidation processes, the detection methods of RFeS/RMnS and their roles in the destruction of trace organic contaminants are reviewed with special attention to some specific problems related to the scavenger and probe selection and experimental results analysis potentially resulting in some questionable conclusions. Moreover, the influence of background constituents, such as organic matter and halides, on oxidation efficiency of RFeS/RMnS-mediated oxidation processes and formation of byproducts are discussed through their comparison with those in free radicals-dominated oxidation processes. Finally, the prospects of the RFeS/RMnS-mediated oxidation processes and the challenges for future applications are presented.