Highly selective synthesis of surface FeIV=O with nanoscale zero-valent iron and chlorite for efficient oxygen transfer reactions

Modulating Iron Crystals with Lattice Chalcophile-Siderophile Elements for Selective Dechlorinations Over Hydrogen Evolution

Selective dechlorination of organic chlorides over hydrogen evolution reaction (HER) remains a challenge because of their coincidence. Nanoscale zerovalent iron (nFe0) draws a promising picture of in situ groundwater dechlorination, but its indiscriminate reactivity limits the application. Here, nFe0 crystals are designed with electron shuttles and improved hydrophobic nature based on elemental chalcophile-siderophile characteristics, where chalcophile-siderophile S served as a bridge to allow impregnating nFe0 crystals with weakly siderophile and strongly chalcophile Cu. Even impregnations of lattice chalcophile-siderophile elements into the nFe0 crystals are evidenced at both intraparticle and individual-particle levels. The modulated Fe microenvironment and physicochemical properties broke the reactivity-selectivity-longevity-stability trade-off. Compared to nFe0, superhydrophobic Cu─S─nFe0 with lattice expansion promoted dechlorination by 20-fold but inhibited HER by 150-fold, utilizing ≈80-100% electrons from the Fe0 reservoir. This work demonstrates the concept of engineering nFe0 lattice with tunable structure-property relationships, mimicking reductive dehalogenases by selectively interacting with halocarbon functional groups for efficient dehalogenation and sustainable groundwater remediation.

Isotope Techniques in Chemical Wastewater Treatment: Opportunities and Uncertainties

A comprehensive and in-depth analysis of reaction mechanisms is essential for advancing chemical water treatment technologies. However, due to the limitations of conventional experimental and analytical methods, the types of reactive species and their generation pathways are commonly debatable in many aqueous systems. As highly sensitive diagnostic tools, isotope techniques offer deeper insights with minimal interference from reaction conditions. Nevertheless, precise interpretations of isotope results remain a significant challenge. Herein, we first scrutinized the fundamentals of isotope chemistry and highlighted key changes induced by the isotope substitution. Next, we discussed the application of isotope techniques in kinetic isotope effects, presenting a roadmap for interpreting KIE in sophisticated systems. Furthermore, we summarized the applications of isotope techniques in elemental tracing to pinpoint reaction sites and identify dominant reactive species. Lastly, we propose future research directions, highlighting critical considerations for the rational design and interpretation of isotope experiments in environmental chemistry and related fields.

Differential Impacts of Pyrophosphate on Ferrates(VI, V, and IV): Through Its Unique Inhibition to Identify Fe(V) Species

High-valent iron species [Fe(V) and Fe(IV)] exhibit remarkable oxidative activity in environmental chemistry. However, the distinctions between the properties of Fe(V) and Fe(IV) remain poorly understood due to the challenges of distinguishing them. Herein, using pyrophosphate as a model ligand, we comprehensively investigated the influence of oxo-ligands on the reactivity of high-valent iron(VI, V, IV) species. An innovative strategy to selectively generate Fe(IV) using the Fe(VI)-initiated system was proposed, enabling an in-depth investigation of the interaction between Fe(IV) and pyrophosphate. The results reveal that pyrophosphate strongly inhibits Fe(V) oxidation, while it has minimal impact on the reactivity of Fe(VI) and Fe(IV). Based on ligand field theory, pyrophosphate complexation can induce iron 3d orbital resplitting, leading to spin electron rearrangement. Specifically, the hexa-coordinated Fe(V)-oxo complex ligated by pyrophosphate exhibits higher orbital energy, reducing its stability and effective collisions with contaminants, whereas, the potential Jahn-Teller distortion of the Fe(IV)-oxo complex could enhance its stability and preserve its significant reactivity. Given its selective inhibition of Fe(V) oxidation, pyrophosphate can emerge as a promising targeted quenching agent for Fe(V) species. This study provides valuable theoretical insights to guide the identification and characterization of intermediate iron species in iron-based oxidation processes.

Cathode-mediated electrochemical conversion of phenol to benzoquinone in wastewater: High yield rate and low energy consumption

Selective conversion of organic pollutants in wastewater into value-added chemicals is a promising strategy for sustainable water management. Electrochemical processes offer attractive features of precise control over reaction pathway to achieve desired products, however, the traditional anode-mediated processes still face challenges of over-oxidation by the inevitably formed of hydroxyl radical (HO•). Herein, we proposed a new cathode-mediated approach for selective conversion of phenol to p-benzoquinone (p-BQ) through peroxymonosulfate (PMS) activation. A core-shell layered mesoporous spherical iron-based carbon catalyst (denoted Fe/C-MS) was rationally designed to initiate the reactions, where the first shell layer composed of mesoporous carbon provided a confined environment to enrich PMS and phenols, and the electronic configuration of encapsulated Fe species favored the formation of high-valent ion-oxo species (FeIV=O) during PMS activation. Notably, the electrochemical process with Fe/C-MS and PMS (denoted Fe/C-MS-E/PMS) achieved a high yield of p-BQ at 80.2 % and a selectivity of 93.7 % within 5 min, resulting in an ultra-low energy consumption (0.07 KWh/mol phenol). The p-BQ production rate reached an impressive value of 1002.5 %/h, 30-500 times higher than the traditional chemical and anodic oxidation methods. The applicability of this cathode-mediated process was further validated by its successful treatment of real coking wastewater, underscoring the potential as a sustainable strategy for selective conversion of phenol to desired products with high yield and low energy consumption. All the findings available in this study drive us to image that the long-neglected cathode-mediated process, if rationally designed, may serve as an attractive strategy for more sustainable resource recovery during wastewater treatment.

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

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

Green Glyphosate Treatment with Ferrihydrite and CaO2 via Forming Surface Ternary Complex

Glyphosate (PMG) is a globally used broad-spectrum herbicide and receives environmental concerns because of its moderate persistence and potential carcinogenicity. Traditional PMG treatment methods often suffer from the generation of a more toxic and persistent aminomethylphosphonic acid (AMPA) intermediate. Herein, we develop a green method with ferrihydrite (FH) and CaO2 (FH/CaO2) via regulating the coordination of PMG with FH and Ca2+, where the phosphonate group of PMG preferentially binds to FH and its carboxylate side complexes with Ca2+ released by CaO2, forming a FH-PMG-Ca ternary surface complex. This unique ternary complex can redistribute electrons within the PMG molecule for its C-P activation and C-N bond stabilization, favoring the selective C-P bond attack of superoxide radical produced by the Fenton reaction between CaO2-derived H2O2 and FH, thus generating environment-friendly glycine instead of AMPA. The FH/CaO2 process realizes over 99% PMG degradation in industrial wastewater within 1 h, with residual PMG < 0.1 ppm and AMPA < 40 ppb. More importantly, the CaO2 consumption was as low as 3.1 mg of CaO2/mg of PMG, one-fifth those of previously reported CaO2-based counterparts. This study provides an effective and environment-friendly PMG treatment strategy and highlights the importance of surface coordination modes on the degradation pathway of PMG.

Synergistic activation of peroxymonosulfate by highly dispersed iron-based sulfur-nitrogen co-doped porous carbon for bisphenol a removal: mechanistic insights and selective oxidation

Efficient and pervasive solutions are urgently needed to mitigate pollution from emerging contaminants in aquatic environments. Activation of peroxymonosulfate (PMS) is commonly employed to remove refractory organic pollutants from water. Herein, we synthesized sulfur-nitrogen co-doped porous carbon materials loaded with highly dispersed iron species (FeSNC) using template-assisted and ligand site construction methods. The uniform doping of N, S, and Fe in the carbon substrate, along with their synergistic effects, significantly enhanced catalytic activity by creating a high degree of defects in the catalyst (I D/I G = 1.47). This enhancement facilitated efficient removal of BPA, achieving an apparent rate constant of up to 2.83 min-1, which was 30 times higher than that of SNC and 6 times higher than that of FeNC. The FeSNC/PMS system demonstrated robust catalytic stability across the pH 3-9 range, and showed minimal sensitivity to environmental factors like the aqueous matrix, with low iron ion dissolution (<0.01 mg L-1) and certain reusability. Mechanistic investigations employing quenching experiments, EPR tests, probe experiments, and electrochemical tests elucidated that the system catalyzed the degradation of BPA via two non-radical pathways: high-valent iron oxidation and singlet oxygen pathways. Additionally, the system further exhibits selective degradation of electron-rich organics (e.g., 4-chlorophenol, sulfamethoxazole, ofloxacin, etc.).

Silicate-Confined Hydrogen on Nanoscale Zerovalent Iron for Efficient Defluorination Reactions

Defluorination reactions are increasingly vital due to the extensive use of organofluorine compounds with robust carbon-fluorine (C-F) bonds; particularly, the efficient defluorination of widespread and persistent per- and polyfluoroalkyl substances under mild conditions is crucial due to their accumulation in the environment and human body. Herein, we demonstrate that surface-modified silicate of pronounced proton affinity can confine active hydrogen (•H) onto nanoscale zerovalent iron (nZVI) by withdrawing electrons from nZVI to react with bound protons, generating confined active hydrogen (•H*) for efficient defluorination under ambient conditions. The exposed silicon cation (Siσ+) of silicate functions as a Lewis acid site to activate the C-F bond by forming Siσ+...F--C and substantially lowers the energy barrier of nucleophilic •H* attack, thereby facilitating selective C-F hydrodefluorination and subsequent fluorine immobilization. In a column flow reactor, silicate-modified nZVI efficiently removes perfluorooctanoic acid (PFOA) of concentrations ranging from 0.24 to 24 μmol/L with 75-92% defluorination efficiencies, 8 times higher than those of nZVI, generating environmentally friendly alkyl carboxylic acids as the primary products. Besides PFOA, this novel nZVI also realizes deep defluorination of other organofluorine compounds, including perfluorooctanesulfonates and fluoroquinolones, demonstrating its superior defluorination potential.

Activation of ClO2 by Nanoscale Zero-Valent Iron for Efficient Soil Polycyclic Aromatic Hydrocarbon Degradation: New Insight into the Relative Contribution of Fe(IV) and Hydroxyl Radicals

Recently, the activation of chlorine dioxide (ClO2) by metal(oxide) for soil remediation has gained notable attention. However, the related activation mechanisms are still not clear. Herein, the variation of iron species and ClO2, the generated reactive oxygen species, and the toxicity of the degradation intermediates were explored and evaluated with nanoscale zero-valent iron (nFe0) being employed to activate ClO2 for soil polycyclic aromatic hydrocarbon (PAH) removal. With an optimized ClO2/nFe0 molar ratio of 15:1 and a soil/water ratio of 3:1, the degradation efficiency of phenanthrene improved 12% in comparison with that of a ClO2-alone system. The presence of nFe0 significantly promoted ClO2 consumption (improved 85.4%) but restrained ClO2- generation (reduced 22.5%). The surface Fe(II) and soluble Fe(II) in the ClO2/nFe0 system was 2.0-fold and 2.8-fold that in the nFe0 system after 2 min. Electron paramagnetic resonance analysis, along with quenching experiments, revealed that Fe(IV), HOCl, and •OH dominated phenanthrene degradation in a ClO2/nFe0 system, with oxidation contributions, respectively, of 34.3%, 52.8% and 12.9%. The degradation intermediates of PAHs in the ClO2/nFe0 system had lower estimated toxicity than those of the ClO2 system. The lettuces grown in ClO2/nFe0-treated soil displayed better results in bioassay indexes than those grown in ClO2-treated soil. This study offers new perspectives for the remediation of organic-pollutant-contaminated soil by using metal-activated ClO2 technology.

Insights into the Roles of Different Iron Species on Zeolites for N2O Selective Catalytic Reduction by CO

Iron zeolites are promising candidates for mitigating nitrous oxide (N2O), a potent greenhouse gas and contributor to stratospheric ozone destruction. However, the atomic-level mechanisms by which different iron species, including isolated sites, clusters, and particles, participate in N2O decomposition in the presence of CO still remain poorly understood, which hinders the application of the reaction in practical technology. Herein, through experiments and density functional theory (DFT) calculations, we identified that isolated iron sites were active for N2O activation to generate adsorbed O* species, which readily reacted with CO following the Eley-Rideal (E-R) mechanism. In contrast, Fe2O3 particles exhibited a different reaction pathway, directly reacting with CO to generate oxygen vacancies (Ov), which could efficiently dissociate N2O following the Mars-van Krevelen (MvK) mechanism. Moreover, the transformation of iron oxide clusters into undercoordinated FeOx species by CO was also revealed through various techniques, such as CO-temperature-programmed reduction (TPR), and ab initio molecular dynamics (AIMD) simulations. Our study provides deeper insights into the roles of different iron species in N2O-SCR by CO, and is anticipated to facilitate the understanding of multicomponent catalysis and the design of efficient iron-containing catalysts for practical applications.

Selective Arene Photonitration via Iron-Complex β-Homolysis

Nitroaromatics, as an important member and source of nitrogen-containing aromatics, is bringing enormous economic benefits in fields of pharmaceuticals, dyes, pesticides, functional materials, fertilizers, and explosives. Nonetheless, the notoriously polluting nitration industry, which suffers from excessive discharge of fumes and waste acids, poor functional group tolerance, and tremendous purification difficulty, renders mild, efficient, and environmentally friendly nitration a formidable challenge. Herein, we develop a visible-light-driven biocompatible arene C-H nitration strategy with good efficiency and regioselectivity, marvelous substrate applicability and functional group tolerance, and wide application in scale-up synthesis, total synthesis, and late-stage functionalization. A nitryl radical delivered through unusual β-homolysis of a photoexcited ferric-nitrate complex is proposed to be the key nitrification reagent in this system.

Enhanced the removal of norfloxacin by oxalated zero-valent iron with rich surface Fe(II) sites activating the chlorite

Oxalic acid-modified ball-milled zero-valent iron (OA-ZVIbm) was employed to activate sodium chlorite (ClO2-) for the removal of norfloxacin (NOR). The complete removal of 20 mg/L NOR was achieved within 60 min by the OA-ZVIbm/ClO2- process. Compared with the ZVIbm/ClO2- process which was the ball-milled zero-valent iron (ZVIbm) activate sodium chlorite, the reaction activity of the OA-ZVIbm/ClO2- process was increased by 102.6 times. Through scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), electrochemical testing, and density functional theory (DFT) calculations, which has been confirmed that the introduction of oxalic acid can significantly increase the surface Fe(II) content of OA-ZVIbm, and accelerate the electron transfer rate of iron nuclei, thereby improve the efficiency of ClO2- activation for the removal of NOR. The role of various active species in NOR removal, which were •O2-, 1O2, Fe(IV), ClO2, and •OH, was elucidated through free radical quenching experiments, electron paramagnetic resonance (EPR) spectroscopy, and quantitative detection of active species. These active species all participated in the reaction, while •O2- played a dominant role in the reaction because it could transform into other active species, such as (•OH, 1O2). Inorganic anions and natural organic matter have no significant effect on the removal of NOR in the OA-ZVIbm/ClO2- process. The protonation of oxalic acid ensured its good pH applicability range (pH = 2-11), thus exhibiting excellent performance in NOR removal in real water bodies. This further demonstrates that OA-ZVIbm prepared by oxalic acid ball milling modification is an efficient ClO2- activator, offering promising prospects for antibiotic removal technology.

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

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

FePO4/WB as an efficient heterogeneous Fenton-like catalyst for rapid removal of neonicotinoid insecticides: ROS quantification, mechanistic insights and degradation pathways

Iron-based catalysts for peroxymonosulfate (PMS) activation hold considerable potential in water treatment. However, the slow conversion of Fe(III) to Fe(II) restricts its large-scale application. Herein, an iron phosphate tungsten boride composite (FePO4/WB) was synthesized by a simple hydrothermal method to facilitate the Fe(III)/Fe(II) redox cycle and realize the efficient degradation of neonicotinoid insecticides (NEOs). Based on electron paramagnetic resonance (EPR) characterization, scavenging experiments, chemical probe approaches, and quantitative tests, both radicals (HO• and SO4⋅-) and non-radicals (1O2 and Fe(IV)) were produced in the FePO4/WB-PMS system, with relative contributions of 3.02 %, 3.58 %, 6.24 %, and 87.16 % to the degradation of imidacloprid (IMI), respectively. Mechanistic studies revealed that tungsten boride (WB) promoted the reduction of FePO4, and the generated Fe(II) dominantly activated PMS through a two-electron transfer to form Fe(IV), while a minority of Fe(II) engaged in a one-electron transfer with PMS to produce SO4⋅-, HO•, and 1O2. In addition, four degradation pathways of NEOs were proposed by analyzing the byproducts using UPLC-Q-TOF-MS/MS. Besides, seed germination experiments revealed the biotoxicity of NEOs was significantly reduced after degradation via the FePO4/WB-PMS system. Meanwhile, the recycling experiments and continuous flow reactor experiments showed that FePO4/WB exhibited high stability. Overall, this study provided a new perspective on water remediation by Fenton-like reaction. ENVIRONMENTAL IMPLICATION: Neonicotinoids (NEOs) are a type of insecticide used widely around the world. They've been found in many aquatic environments, raising concerns about their possible negative effects on the environment and health. Iron-based catalysts for peroxymonosulfate (PMS) activation hold great promise for water purification. However, the slow conversion of Fe(III) to Fe(II) restricts its large-scale application. Herein, iron phosphate tungsten boride composite (FePO4/WB) was synthesized by a simple hydrothermal method to facilitate the Fe(III)/Fe(II) redox cycle and realize the efficient degradation of NEOs. The excellent stability and reusability provided a great prospect for water remediation.

S-Doping Regulated Iron Spin States in Fe-N-C Single-Atom Material for Enhanced Peroxidase-Mimicking Activity at Neutral pH

Designing biomimetic nanomaterials with peroxidase (POD)-like activity at neutral pH remains a significant challenge. An S-doping strategy is developed to afford an iron single-atom nanomaterial (Fe1@CN-S) with high POD-like activity under neutral conditions. To the best of knowledge, there is the first example on the achievement of excellent POD-like activity under neutral conditions by regulating the active site structure. S-doping not only promotes the dissociation of the N─H bond in 3,3″,5,5″-tetramethylbenzidine (TMB), but also facilitates the desorption of OH* by the transformation of iron species' spin states from middle-spin (MS FeII) to low-spin (LS FeII). Meanwhile, LS FeII sites typically have more unfilled d orbitals, thereby exhibiting stronger interactions with H2O2 than MS FeII, which can enhance POD-like activity. Finally, a one-pot visual detection of glucose at pH 7 is performed, demonstrating the best selectivity and sensitivity than previous reports.

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.

Cu1-Fe Dual Sites for Superior Neutral Ammonia Electrosynthesis from Nitrate

The electrochemical nitrate reduction reaction (NO3RR) is able to convert nitrate (NO3 -) into reusable ammonia (NH3), offering a green treatment and resource utilization strategy of nitrate wastewater and ammonia synthesis. The conversion of NO3 - to NH3 undergoes water dissociation to generate active hydrogen atoms and nitrogen-containing intermediates hydrogenation tandemly. The two relay processes compete for the same active sites, especially under pH-neutral condition, resulting in the suboptimal efficiency and selectivity in the electrosynthesis of NH3 from NO3 -. Herein, we constructed a Cu1-Fe dual-site catalyst by anchoring Cu single atoms on amorphous iron oxide shell of nanoscale zero-valent iron (nZVI) for the electrochemical NO3RR, achieving an impressive NO3 - removal efficiency of 94.8 % and NH3 selectivity of 99.2 % under neutral pH and nitrate concentration of 50 mg L-1 NO3 --N conditions, greatly surpassing the performance of nZVI counterpart. This superior performance can be attributed to the synergistic effect of enhanced NO3 - adsorption on Fe sites and strengthened water activation on single-atom Cu sites, decreasing the energy barrier for the rate-determining step of *NO-to-*NOH. This work develops a novel strategy of fabricating dual-site catalysts to enhance the electrosynthesis of NH3 from NO3 -, and presents an environmentally sustainable approach for neutral nitrate wastewater treatment.

Natural Attenuation of 2,4-Dichlorophenol in Fe-Rich Soil during Redox Oscillations: Anoxic-Oxic Coupling Mechanism

Natural attenuation of organic contaminants can occur under anoxic or oxic conditions. However, the effect of the coupling anoxic-oxic process, which often happens in subsurface soil, on contaminant transformation remains poorly understood. Here, we investigated 2,4-dichlorophenol (2,4-DCP) transformation in Fe-rich soil under anoxic-oxic alternation. The anoxic and oxic periods in the alternating system showed faster 2,4-DCP transformation than the corresponding control single anoxic and oxic systems; therefore, a higher transformation rate (63.4%) was obtained in the alternating system relative to control systems (27.9-42.4%). Compared to stable pH in the alternating system, the control systems presented clear OH- accumulation, caused by more Fe(II) regeneration in the control anoxic system and longer oxygenation in the control oxic system. Since 2,4-DCP was transformed by ion exchangeable Fe(II) in soil via direct reduction in the anoxic process and induced ·OH oxidation in the oxic process, OH- accumulation was unbeneficial because it competed for proton with direct reduction and inhibited •OH generation via complexing with Fe(II). However, the alternating system exhibited OH--buffering capacity via anoxic-oxic coupling processes because the subsequent oxic periods intercepted Fe(II) regeneration in anoxic periods, while shorter exposure to O2 in oxic periods avoided excessive OH- generation. These findings highlight the significant role of anoxic-oxic alternation in contaminant attenuation persistently.

Critical Role of Mineral Fe(IV) Formation in Low Hydroxyl Radical Yields during Fe(II)-Bearing Clay Mineral Oxygenation

In subsurface environments, Fe(II)-bearing clay minerals can serve as crucial electron sources for O2 activation, leading to the sequential production of O2•-, H2O2, and •OH. However, the observed •OH yields are notably low, and the underlying mechanism remains unclear. In this study, we investigated the production of oxidants from oxygenation of reduced Fe-rich nontronite NAu-2 and Fe-poor montmorillonite SWy-3. Our results indicated that the •OH yields are dependent on mineral Fe(II) species, with edge-surface Fe(II) exhibiting significantly lower •OH yields compared to those of interior Fe(II). Evidence from in situ Raman and Mössbauer spectra and chemical probe experiments substantiated the formation of structural Fe(IV). Modeling results elucidate that the pathways of Fe(IV) and •OH formation respectively consume 85.9-97.0 and 14.1-3.0% of electrons for H2O2 decomposition during oxygenation, with the Fe(II)edge/Fe(II)total ratio varying from 10 to 90%. Consequently, these findings provide novel insights into the low •OH yields of different Fe(II)-bearing clay minerals. Since Fe(IV) can selectively degrade contaminants (e.g., phenol), the generation of mineral Fe(IV) and •OH should be taken into consideration carefully when assessing the natural attenuation of contaminants in redox-fluctuating environments.

Linking atomic to mesoscopic scales in multilevel structural tailoring of single-atom catalysts for peroxide activation

A key challenge in designing single-atom catalysts (SACs) with multiple and synergistic functions is to optimize their structure across different scales, as each scale determines specific material properties. We advance the concept of a comprehensive optimization of SACs across different levels of scale, from atomic, microscopic to mesoscopic scales, based on interfacial kinetics control on the coupled metal-dissolution/polymer-growth process in SAC synthesis. This approach enables us to manipulate the multilevel interior morphologies of SACs, such as highly porous, hollow, and double-shelled structures, as well as the exterior morphologies inherited from the metal oxide precursors. The atomic environment around the metal centers can be flexibly adjusted during the dynamic metal-oxide consumption and metal-polymer formation. We show the versatility of this approach using mono- or bi-metallic oxides to access SACs with rich microporosity, tunable mesoscopic structures and atomic coordinating compositions of oxygen and nitrogen in the first coordination-shell. The structures at each level collectively optimize the electronic and geometric structure of the exposed single-atom sites and lower the surface *O formation barriers for efficient and selective peroxidase-type reaction. The unique spatial geometric configuration of the edge-hosted active centers further improves substrate accessibility and substrate-to-catalyst hydrogen overflow due to tunable structural heterogeneity at mesoscopic scales. This strategy opens up new possibilities for engineering more multilevel structures and offers a unique and comprehensive perspective on the design principles of SACs.

Iron-Catalyzed C-H Oxygenation Using Perchlorate Enabled by Secondary Sphere Hydrogen Bonds

Perchlorate (ClO4-) is a groundwater pollutant that is challenging to remediate. We report a strategy to use Fe(II) tris(2-pyridylmethyl)amine (TPA) complexes featuring appended aniline hydrogen bonds (H-bonds) to promote ClO4- reduction. These complexes facilitate oxygen atom transfer from ClO4- to PPh3 and C-H oxygenation reactions of organic substrates. Catalytic reactions using 15 mol % afforded excellent yields for oxygenation of anthracene and cyclic alkyl aromatics, and this methodology tolerates aryl halides as well as heterocycles containing either O, S, or N.