Ultrastable Fe-N4-C6O2 Single Atom Sites for Highly Efficient PMS Activation and Enhanced FeIV=O Reactivity

Ultrastable Fe-N4-C6O2 Single Atom Sites for Highly Efficient PMS Activation and Enhanced FeIV=O Reactivity | Research Square window.SnipcartSettings = { analytics: { enabled: false } }; (function() { var accessVector = localStorage.getItem('access_vector') || ''; window.dataLayer = window.dataLayer || []; if (accessVector) { window.dataLayer.push({ user: { profile: { profileInfo: { snid: accessVector } } } }); } })(); (function(w,d,s,l,i){w[l]=w[l]||[];w[l].push({'gtm.start':new Date().getTime(),event:'gtm.js'});var f=d.getElementsByTagName(s)[0],j=d.createElement(s),dl=l!='dataLayer'?'&l='+l:'';j.async=true;j.src='https://www.googletagmanager.com/gtm.js?id='+i+dl;f.parentNode.insertBefore(j,f);})(window,document,'script','dataLayer','GTM-K279D39R'); Browse Preprints In Review Journals COVID-19 Preprints AJE Video Bytes Research Tools Research Promotion AJE Professional Editing AJE Rubriq About Preprint Platform In Review Editorial Policies Our Team Advisory Board Help Center Sign In Submit a Preprint Cite Share Download PDF Article Ultrastable Fe-N 4 -C 6 O 2 Single Atom Sites for Highly Efficient PMS Activation and Enhanced Fe IV =O Reactivity Lizhi Zhang, Tiantian Chen, Ganbing Zhang, Hongwei Sun, Yetong Hua, and 6 more This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-4654905/v1 This work is licensed under a CC BY 4.0 License Status: Published Journal Publication published 10 Mar, 2025 Read the published version in Nature Communications → Version 1 posted You are reading this latest preprint version Abstract The local environment modulation of iron sites in Fe-N 4 single atom catalysts (SACs) plays a crucial role in the efficient peroxymonosulfate (PMS) activation. Many reported modulation strategies involve the partial replacement of N in the first coordination shell of Fe-N 4 sites with foreign elements to facilitate the PMS activation via disrupting the structural symmetry, suffering from undesired catalytic stability. Herein, we demonstrate that Fe-N 4 -C 6 O 2 sites, which are prepared by substituting C in the second coordination shell of Fe-N 4 sites with O, can activate PMS more efficiently and stably by providing an enhanced localized electric field without destroying their symmetric coordination structure in the first coordination shell, and thus achieve an unprecedented catalytic durability of at least 240 h. The O doping in the second coordination shell strengthened the Fe-N bond by reducing the electron density of Fe center, and weakened the amplitude of Fe-N bond from 0.875 ~ 3.175 Å to 0.925 ~ 2.975 Å during the PMS activation, therefore effectively prevented the demetallation of Fe-N 4 sites. Meanwhile, this O doping also lowered the energy of Fe = O σ* orbitals by weakening the coordination field to promote the electrophilic σ-attack of high-valent iron-oxo (Fe IV =O) towards electron-rich contaminants, thus enhancing the bisphenol A degradation rate from 1.08 × 10 3 M − 1 s − 1 to 4.6 × 10 4 M − 1 s − 1 by a factor of 41.6. This work sheds light on the importance of second coordination shell doping on the ultrastability of Fe-N 4 SACs, and provides a novel strategy to design metal SACs by balancing a trade-off between exceptional activity and long-term stability. Physical sciences/Materials science/Materials for energy and catalysis/Porous materials Earth and environmental sciences/Environmental sciences/Environmental chemistry/Pollution remediation Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Figure 6 Introduction Advanced oxidation processes (AOPs) based on peroxymonosulfate (PMS) have attracted great attention for pollutant control and environmental remediation due to their high efficiency in a wide pH range. 1 Recently, single-atom catalysts (SACs) with metal-N 4 (M-N 4 ) sites, where the metal centers are coordinated with four nitrogen atoms, are widely used for the PMS activation because of their outstanding catalytic activity, excellent selectivity, and exceptionally high metal utilization efficiency. 2 – 5 The electron donating potential of these M-N 4 sites can trigger the PMS activation to produce sulfate radicals (SO 4 •− ), hydroxyl radicals (•OH), and high-valence metal-oxo (M IV =O), etc., thus effectively facilitating the degradation of various organic pollutants. 6 – 14 However, the high bond dissociation energies of O-H and O-O bonds in PMS pose a significant challenge to their cleavage, thereby disfavoring the PMS activation at M-N 4 sites. 15 Therefore, the development of high-performance catalysts to promote the PMS activation is crucial for the effective oxidation of contaminants. Recently, many strategies have been proposed for the efficient PMS activation via increasing the density of metal centers or regulating their local environment surrounding in M-SACs. 16 – 19 Among these strategies, the most common one is the partial replacement of N in the first coordination shell with foreign elements, thereby creating an enhanced localized electric field to facilitate the activation of O-H and O-O bonds in PMS. 20 , 21 Unfortunately, this strategy destroys the symmetric structure of M-N 4 sites, potentially decreasing their long-term stability via the accelerated demetallation. For instance, the activity of M-N 4 - x Y x (Y represents P, O, or S) SACs significantly diminishes after only one or two cycles of PMS activation. 20 , 22 – 24 Therefore, it is still a great challenge to balance the high activity and the long durability of SACs for the PMS activation. Different from the first coordination shell doping, the heteroatom doping in the second coordination shell of M-N 4 sites could improve the catalytic performance of M-N 4 sites by modulating the electronic structure of the metal center through d-p long-range interactions, and simultaneously maintain the M-N 4 coordination structure. 25 – 28 Although Li et al. demonstrated that the electron-withdrawing effect of S doping in the second coordination shell was beneficial for anchoring single Fe atoms to enhance nitrate electroreduction activity and stability of Fe-N 4 sites in comparison with Fe-plate, they did not compare the catalytic stability of Fe-N 4 sites with those with S doping in the first or second coordination shells towards nitrate electroreduction. 25 Compared with nitrate electroreduction, the PMS activation imposes much stricter requirements on the catalytic stability of Fe-N 4 sites owing to its strong oxidative environment. Regarding that the electronegativity of O (3.44) is higher than that of S (2.58), the O doping can reduce the electron density in the metal center more significantly through d-p long-range interaction and establish a stronger electric field gradient around M-N 4 sites, facilitating the adsorption and activation of negatively charged PMS (HSO 5 − ). Meanwhile, the O doping in the second coordination shell might favor the formation of carbon vacancies around the metal center and the asymmetric charge distribution to suppress the demetallation and regulate their coordination environment. Herein, we develop a pre-coordination strategy to engineer heteroatom doping by precisely controlling the substitution of neighboring first coordination shell N atoms and distanced second coordination shell C atoms of Fe center, aiming to clarify the effects of O doping in the first or second coordination shell on the reactivity and stability of Fe-N 4 sites through systematical characterization, degradation experiments, density functional theory (DFT) calculations and molecular dynamics (MD) simulations. This study shill shed light on the design of SACs with high PMS activation performance by dealing with a trade-off between exceptional activity and sustainable stability. Results Synthesis and Characterization. Fe-SACs with oxygen doped in the first (Fe-N 2 O 2 -C 10 ) or the second shell (Fe-N 4 -C 6 O 2 ) were synthesized by a pre-coordination strategy (Fig. 1 a). First, we synthesized two core-shell structured carriers with utilizing silica (SiO 2 ) as the core layer (Supplementary Figs. 1a-1c). One of shell layer was composed of nitrogen-doped (N-C) without any oxygen functional groups (SiO 2 @N-C), while the other consisted of resorcinol-formaldehyde (RF) enriched with oxygen functional groups (SiO 2 @RF). Meanwhile, Fe-N/O co-coordination (I) and Fe-N coordination (II) complexes were prepared using (S, S)-(+)-N, N'-bis(3,5-di-tert-butylsalicylidene)-1, 2-cyclohexanediamin as the N/O precursors and 1,10-phenanthroline as the N precursor, respectively. Subsequently, complexes (I) and (II) were deposited on SiO 2 @N-C and SiO 2 @RF using the rotary evaporation technique, which were then subjected to the processes of high-temperature pyrolysis and SiO 2 template removal to obtain Fe-N 2 O 2 -C 10 and Fe-N 4 -C 6 O 2 , respectively. Fe-SAC without O-doping (Fe-N 4 -C 10 ) was also synthesized with the same procedure except for the deposition of complex (I) on SiO 2 . For comparison, N/O co-doped carbon (N/O-C), N-doped carbon (N-C), and Fe nanoparticles supported on N-doped C (Fe NPs/NC) were also fabricated (see Supporting Information for synthesis details and Table S1 ). All the three Fe-SACs were of ultrathin, uniform, hierarchically porous hollow structure (Fig. 1 b and Supplementary Figs. 1–3), facilitating the full exposure of active sites and the mass transfer during the PMS activation. Their powder X-ray diffraction (PXRD) patterns only contained one broad peak at 25° corresponding to the (002) plane of the carbon carrier, without any discernible peaks arisen from crystalline iron or iron oxides (Fig. 1 c). 29 Meanwhile, the existence of iron particles were ruled out by their high-resolution transmission electron microscope (HR-TEM) images and the selected area electron diffraction (SAED) patterns (Supplementary Fig. 1). Aberration-corrected high-angle annular dark field scanning electron microscopy (AC-HAADF STEM) images of Fe-N 4 -C 6 O 2 clearly indicated that bright spots of sizes, consistent with single Fe atoms (1.002 ± 0.173 Å), were uniformly dispersed in the dark O-N/C background (Figs. 1 d- 1 e), 6 confirming its atomically dispersion of Fe. We then checked the electronic structure and coordination environment of Fe atoms in three Fe-SACs by X-ray absorption fine structure (XAFS), XPS, and 57 Fe Mössbauer spectroscopy. As shown in X-ray absorption near-edge structure (XANES) spectra (Fig. 2 a), the pre-edge peak and absorption edge of Fe-N 2 O 2 -C 10 and Fe-N 4 -C 6 O 2 shifted towards higher energies, accompanied by the intensity increase of the white-line peak, indicating that the O doping altered the coordination environment of Fe centers and reduced their electron densities. Obviously, these changes were more pronounced in the XANES spectrum of Fe-N 2 O 2 -C 10 , indicative of its shorter distance and stronger interaction between O and Fe. The Fourier-transformed k 3 -weighted extended X-ray absorption fine structure (FT-EXAFS) spectrum revealed a broad and unsymmetric peak of Fe-N 2 O 2 -C 10 with the peak maximum located at 1.44 Å, close to the Fe-O backscattering, indicating the presence of Fe-N and Fe-O dual coordination in Fe-N 2 O 2 -C 10 (Fig. 2 b). Differently, the main peak of Fe-N 4 -C 6 O 2 and Fe-N 4 -C 10 were symmetric and left-shifted to ~ 1.4 Å, belonging to the single Fe-N scattering path. However, a new peak in the 1–2 Å range of R -space appeared in the spectrum of Fe-N 4 -C 6 O 2 , indicating that the different coordination environments of Fe-N 4 -C 6 O 2 and Fe-N 4 -C 10 . These results confirmed that O was doped into outer coordination shells of n ≥ 2 in Fe-N 4 -C 6 O 2 . Moreover, those peaks with R > 2 Å (pentagram markers) could not be assigned to Fe-Fe scattering path, because the wavelet transforms (WT) EXAFS contours of three Fe-SACs samples had only one intensity maximum corresponding to the Fe-N/O coordination at k value of 5 Å - 1 in the R range of 1–3 Å (Fig. 2 c). Subsequently, quantitative least-squares fitting of the FT-EXAFS curves revealed that the optimal coordination numbers of Fe in all three Fe-SACs were approximately 4 (Fig. 2 d, Supplementary Fig. 4 and Table 2). The distinction lied in the fact that Fe-N 2 O 2 -C 10 encompassed both Fe-N and Fe-O backscattering paths, thereby confirming the successful construction of a Fe-N/O dual coordination environment, while the other two Fe-SACs solely exhibited Fe-N backscattering paths. As expected, two distinct Fe-N bond lengths of 1.91 Å and 2.09 Å were observed in Fe-N 4 -C 6 O 2 , indicating the presence of N ligand in two different coordination forms, while only an average distance of 1.97 Å was found in Fe-N 4 -C 10 . We further characterized Fe-N 4 -C 6 O 2 and Fe-N 4 -C 10 using 57 Fe Mössbauer spectroscopy to elucidate their coordination environments of Fe. Their deconvoluted 57 Fe Mössbauer spectra exclusively exhibited doublets, without any singlet or sextet associated with α-Fe, Fe x C, or Fe x O (Fig. 2 e). According to the isomer shift (δ iso ) and quadrupole splitting (∆E Q ) values (Supplementary Table 3), 57 Fe Mössbauer spectrum of Fe-N 4 -C 6 O 2 could be well fitted with three doublets (D1-D3), corresponding to medium-spin (MS) Fe II N 2+2 (D1), medium-spin (MS) Fe III N 2+2 (D2), and high-spin (HS) N-Fe II N 2+2 (D3), respectively, 30 – 32 wherein Fe-N 2 + 2 represents the defective Fe-N 4 site in the non-intact graphite layer structure (Supplementary Fig. 5). In contrast, four different doublets (D4-D7) could be identified in Fe-N 4 -C 10 , which were assigned to low-spin (LS) Fe III N 4 /C (D4), LS Fe Ⅱ N 4 /C (D5), MS Fe Ⅱ N 4 /C (D6), and HS Fe Ⅱ N 4 /C (D7), respectively. 13 , 30 , 33 The relatively higher intensity ratio of D-band to G-band (I D /I G = 1.07) in Fe-N 4 -C 6 O 2 than that of Fe-N 4 -C 10 (0.94) evidenced that the O doping created more defects (Supplementary Fig. 6). Meanwhile, the presence of Fe-N and N-O bonds were also confirmed by HR-XPS spectra of Fe 2p, N 1s, and O 1s in Fe-N 4 -C 6 O 2 (Supplementary Figs. 7–8). Therefore, we concluded that two O atoms were doped in the second coordination shell of Fe-N 4 -C 6 O 2 . Considering the presence of doped O atoms in the second coordination shell of Fe-N 4 -C 6 O 2 , we constructed three possible Fe-N 4 -C 6 O 2 models using DFT (Fig. 2 g). Specifically, the two O atoms could be situated at either ipsilateral "a" and "b" positions (configuration I), or ipsilateral "b" and "c" positions (configuration II), or opposite "b" and "d" positions (configuration III) within the second coordination shell, respectively. However, the optimization of configuration I led to the connection of C atoms at positions "c" and "d", which contradicts the Fe-N 2 + 2 structure and consequently resulted in its exclusion. The stability of configuration II was significantly lower than that of configuration III, despite its conformity to the requirements for the Fe-N 2 + 2 structure. Therefore, configuration III was selected as the Fe-N 4 -C 6 O 2 model in this study. The formation of configuration III could be attributed to the replacement of C 1 atoms in the second coordination shell of Fe-N 4 by highly electronegative O atoms, resulting in a deviation of Fe-N bond length and a detachment of O atoms from the C 2 atom due to the eight-electron rule. 27 During calcination, the collision of C 2 atoms with N 2 molecules in irregular thermal motion would lead to their release from the catalyst surface to form carbon vacancies (CVs), accompanying with a configuration transformation from Fe-N 4 to Fe-N 2 + 2 (Supplementary Fig. 9). Subsequently, the significantly enhanced Lorenz curve signal of Fe-N 4 -C 6 O 2 indicated that the O doping and the introduction of CVs in the second coordination shell effectively facilitated the formation of unpaired electrons in Fe-N 4 -C 6 O 2 (Fig. 2 h), which contributed to the improvement of electron transfer efficiency in the catalytic reaction. 34 – 36 Furthermore, partial density of state (PDOS) analysis demonstrated that the O doping in the second coordination shell could effectively modulate the electron distribution within the Fe 3d orbitals through d-p long-range interaction, shifting the d-band center of Fe in Fe-N 4 -C 6 O 2 (-1.842 eV vs -2.137 eV for Fe-N 4 -C 10 ) closer to the Fermi level (E f ) (Fig. 2 i), which might enhance the intrinsic reactivity of Fe-SAC towards the PMS activation. PMS Activation and Pollutant Degradation. We utilized DFT calculations to investigate the influence of O doping on the PMS activation and the reactive oxygen species (ROS) formation, and employed an asterisk (*) to denote the surface adsorbed species and O 1 /O 2 /O 3 to represent the three different types of oxygen atoms in PMS (Supplementary Figs. 10–11). As expected, the higher positive charge of Fe center and its surrounding strong local electric field in Fe-N 2 O 2 -C 10 promoted the PMS adsorption and triggered the rapid cleavage of peroxide (O 2 -O 3 ) bond (Fig. 3 a), while the low electron density of Fe center facilitated its robust interaction with O 1 /O 2 /O 3 , delocalizing Fe atom from the carrier plane and elongating the Fe-N and Fe-O bonds to 2.161 ~ 2.826 Å to potentially result in demetallation. In contrast, the reduction of positive charge on Fe centers and their surrounding local electric field in Fe-N 4 -C 6 O 2 and Fe-N 4 -C 10 failed to satisfy the requirements for direct breakage of O 2 -O 3 bond induced by their PMS adsorption, and thus they may form complexes (I), (II), and (III) with PMS via O 1 , O 2 , and O 3 , respectively (Fig. 3 a and Supplementary Fig. 10). However, the activation of the O 2 -O 3 bond only occurred in complex (III). Therefore, complex (III) was adopted for the following calculations. Subsequently, the formation of Fe IV =O and radicals in the three Fe-SACs/PMS systems was explored theoretically. As shown in Figs. 3 b-c, it was also thermodynamically feasible for the generation of Fe IV =O in the Fe-N 2 O 2 -C 10 /PMS system via the traditional pathway of *(I) → *PMS (Ⅱ) → *OH (Ⅲ) → *OH*OH (Ⅳ) → transition state (TS, Ⅴ) → *O*H 2 O (Ⅵ) → *O (Ⅶ). 38 Differently, the appropriate distance between O 1 and H of *PMS(Fe-N 4 -C 6 O 2 ) and *PMS(Fe-N 4 -C 10 ) facilitated the formation of intramolecular hydrogen bonds, and the interaction between Fe and O 3 weakened the O 2 -O 3 and O 3 -H bonds, thus favoring their cleavage to produce Fe IV =O through the non-classical coupled electron-proton transfer (CEPT) pathway (Figs. 3 d- 3 e and Supplementary Fig. S11). 21 In this pathway, protons were easily transferred from O 3 to O 1 , and the two remaining electrons in the O 3 2p orbital would be coupled with the two electrons of Fe to form the Fe = O bond. Notably, the distantly doped O and CVs promoted the PMS adsorption by lowering the adsorption energy from 0.14 eV for Fe-N 4 -C 10 to -0.05 eV for Fe-N 4 -C 6 O 2 , and affected the *PMS structure by providing a moderately enhanced local electric field, including elongated O 2 -O 3 (1.494 Å for Fe-N 4 -C 6 O 2 vs 1.480 Å for Fe-N 4 -C 10 ) and O 3 -H (1.026 Å for Fe-N 4 -C 6 O 2 vs 1.017 Å for Fe-N 4 -C 10 ) bonds, as well as a reduced distance between O 1 and H (1.650 Å for Fe-N 4 -C 6 O 2 vs 1.698 Å for Fe-N 4 -C 10 ). These changes effectively reduced the energy barrier for the formation of Fe IV =O via the CETP pathway in the Fe-N 4 -C 6 O 2 /PMS system. Finally, the thermodynamic feasibility of radical formation in three Fe-SACs/PMS systems through the single electron transfer pathway was calculated, obeying the trend of Fe-N 2 O 2 -C 10 /PMS with Gibbs free energy (ΔG) of -1.21 eV > Fe-N 4 -C 10 /PMS with ΔG of -0.70 eV > Fe-N 4 -C 6 O 2 /PMS with ΔG of -0.41 eV (Supplementary Fig. 12). However, the significantly lower ΔG associated with the Fe IV =O formation (-2.94 eV for Fe-N 2 O 2 -C 10 , -2.31 eV for Fe-N 4 -C 6 O 2 , -2.34 eV for Fe-N 4 -C 10 ) suggested that the Fe IV =O formation was much easier than that of radicals in three Fe-SACs/PMS systems. We then evaluated the PMS activation performance of Fe-SACs by the BPA removal (Supplementary Fig. 13), and found that the O doping significantly enhanced the activity of Fe-SACs and N/O-C, while Fe-N 4 -C 6 O 2 exhibited the best catalytic activity among the three Fe-SACs samples (Fig. 4 a). BPA could be completely removed in 30 s with a pseudo-first-order rate constant ( k ) as high as 13.299 min − 1 at 0.1 mg mL − 1 of Fe-N 4 -C 6 O 2 and 400 µM of PMS (Supplementary Fig. 14). Impressively, the BPA removal performance of Fe-N 4 -C 6 O 2 even surpassed those of homogeneous Fe 2+ , commercial Fe 3 O 4 and Fe NPs/NC, nZVI/kaolinite, and other reported M-SACs (Fig. 4 b and Supplementary Fig. 15). More importantly, Fe-N 4 -C 6 O 2 displayed a robust pH compatibility and an exceptional resistance to the interference of inorganic ions and common water matrices (Supplementary Fig. 16), and the Fe-N 4 -C 6 O 2 /PMS system could also efficiently remove other pollutants (Supplementary Figs. 17–19 and Table 4). Interestingly, electron-rich phenolic contaminants, such as 2,4-dichlorophenol (2,4-DCP), p-chlorophenol (4-CP), and phenol (PE), could be removed with significantly higher rates compared to contaminants with electron-withdrawing groups, such as p-nitrophenol (PNP), fipronil (FP), p-nitrobenzyl alcohol (PNBA), and p-nitrobenzaldehyde (PNBD) in the Fe-N 4 -C 6 O 2 /PMS system, which is a preliminary indication of the possible predominance of selective non-radical oxidations in this system 38 . Meanwhile, the indispensable role of atomically dispersed Fe sites for the effective PMS activation was confirmed by the much lower activity of N/O-C and N-C. Therefore, the specific activity of individual Fe site was further investigated, and the BPA turnover frequency (TOF) of Fe sites was in the order of Fe-N 2 O 2 -C 10 > Fe-N 4 -C 6 O 2 > Fe-N 4 -C 10 , which suggested that the O doping in the first coordination shell could enhance the activity of Fe sites more effectively than the O doping in the second coordination shell (Supplementary Fig. 20). Impressively, the overall apparent activity of Fe-N 4 -C 6 O 2 was surprisingly higher than that of Fe-N 2 O 2 -C 10 (Fig. 4 a). Regarding that the Fe content of Fe-N 2 O 2 -C 10 was much lower than that of Fe-N 4 -C 6 O 2 (Supplementary Table 5), we thus increased the Fe content of Fe-N 2 O 2 -C 10 by increasing the proportion of Fe precursors, which instead resulted in its significant metal agglomeration and catalytic reactivity decrease (Supplementary Fig. 21), suggesting that the O doping in the first coordination shell disfavored the stability of high-loading Fe single atoms. Subsequently, continuous catalytic BPA degradation experiments were carried out to examine the stability of Fe-SACs during the reaction (Fig. 4 c), which revealed the most robust stability of Fe-N 4 -C 6 O 2 among the three Fe-SACs, even though its BPA adsorption capacity was not as good as that of Fe-N 2 O 2 -C 10 (Fig. 4 d). After 120 h of continuous treatment, the iron leaching ratio of Fe-N 4 -C 6 O 2 was merely 8.0%, much lower than those of Fe-N 2 O 2 -C 10 (67.4%) and Fe-N 4 -C 10 (30.5%) (Fig. 4 e). Strikingly, the reactivity of Fe-N4-C 6 O 2 did not decline after 240 h of reaction and its structure kept almost unchanged even after 500 h of operation (Supplementary Figs. 22–25). Considering the complexity of pH in different waters, we also monitored the iron leaching of three Fe-SACs under extreme acidic and basic conditions (Fig. 4 f), and found that the Fe leaching rate of Fe-N 4 -C 6 O 2 was even less than that of Fe-N 4 -C 10 at acidic pH and almost the same as that of Fe-N 4 -C 10 in alkaline solution. Additionally, the demetallation rate of Fe-N 2 O 2 -C 10 was much higher than that of Fe-N 4 -C 10 , with a factor of 4.5 or 99 under acidic or alkaline conditions. Therefore, the O doping in the first coordination shell significantly enhanced the activity of single-atom metal sites at the expense of their stability, but a well-balanced relationship between activity and stability of single-atom metal sites could be achieved by rationally designing the Fe-SAC coordination configuration with the O doping in the second coordination shell. Regarding that the demetallation of Fe-SACs initiates from the elongation and breakage of Fe-N/O bonds, MD simulations were conducted to explore the structure-dependent stability of Fe-SACs in terms of Fe-N/O bond length fluctuations. A relatively shorter Fe-N/O bond length and a narrower range of fluctuations indicate the stable Fe-N/O bonds of the catalyst with less demetallation tendency during reactions. As shown in Fig. 4 g, the radial distribution function (RDF) of Fe-N/O, denoted as g Fe−N/O (r), was obtained by calculating and counting the frequency of the occurrence of N/O atoms at a distance r from Fe atom. At room temperature, the g Fe−N (r) of Fe-N 4 -C 10 SAC contained four distinct peaks in the range of 0.875 ~ 3.175 Å. However, the fluctuations of N (1.775 ~ 3.525 Å) and O (1.775 ~ 3.175 Å) in Fe-N 2 O 2 -C 10 SAC were farther away from the Fe centre, which indicated that the destruction of its symmetric coordination structure weakened the Fe-N and Fe-O bonds, thereby increasing the demetallation tendency. Fascinatingly, the thermal motion amplitude of Fe-N bonds in Fe-N 4 -C 6 O 2 decreased to 0.925 ~ 2.975 Å, revealing its much stronger Fe-N bond to resist external perturbations. These results further illustrated that the O doping in the second coordination shell could reinforce the interaction between Fe single atoms and coordinating N atoms by reducing the electron density of Fe, effectively inhibiting the leaching of Fe atoms during the PMS activation. Mechanism Investigation. We subsequently investigated the reactive species generated in the three Fe-SACs/PMS systems. The contribution of superoxide radicals (O 2 •− ) to the BPA degradation was first excluded through the superoxide dismutase (SOD) quenching experiments and electron paramagnetic resonance (EPR) measurements (Fig. 5 a and Supplementary Fig. 26a). The presence of methanol (MeOH) only slightly inhibited the BPA degradation in the three Fe-SACs/PMS systems, indicative of weak contribution of SO 4 •− and •OH to the BPA degradation (Fig. 5 a and Supplementary Table 6). When using 5,5-dimethyl-1-pyridone-N-oxide (DMPO) as a trapping agent, only a signal of DMPO oxides (DMPOX) was observed in the three Fe-SAC/PMS systems, which might be originated from oxidation of DMPO by Fe IV =O or other non-radical species, without the appearance of •OH and SO 4 •− signals (Supplementary Figs. 26b-d). The above results revealed that non-radical oxidation pathways such as electron transfer process (ETP), Fe IV =O or singlet oxygen ( 1 O 2 ) oxidation mainly took place in these systems, while the inhibitory effect of MeOH may not be solely attributed to the radicals quenching, but to its competitive adsorption on Fe-SACs with BPA and/or its competitive reaction with other ROS, such as Fe IV =O 39 . The relatively weak inhibition effects of methyl phenyl sulfoxide (PMSO) on Fe-N 4 -C 10 /PMS (35.1%) and Fe-N 2 O 2 -C 10 /PMS (36.8%) systems suggested the dominant role of ETP or 1 O 2 oxidation process other than the Fe IV =O oxidation in these two systems. Surprisingly, PMSO was found to suppress 84.1% of BPA degradation in the Fe-N 4 -C 6 O 2 /PMS system, demonstrating that the doping of O in the second coordination shell could shift the BPA degradation pathway from the ETP or 1 O 2 oxidation process to the Fe IV =O counterpart. Given that the reactions of PMSO with •OH, SO 4 •− and Fe IV =O would form different products (Eqs. S15-S17), where PMSO can be oxidized to methyl phenyl sulfone (PMSO 2 ) through the oxygen atom transfer (OAT) pathway by Fe IV =O, we utilized the PMSO oxidation to further check the generation of radicals and Fe IV =O, 40 and observed that the amount of PMSO 2 generated in the Fe-N 2 O 2 -C 10 /PMS system (5.5 µM) was only 23% of that in Fe-N 4 -C 10 /PMS (24.4 µM) and 16% of that in Fe-N 4 -C 6 O 2 /PMS (34.4 µM) (Supplementary Fig. 27). Meanwhile, the conversion of PMSO 2 in the Fe-N 4 -C 10 /PMS and Fe-N 4 -C 6 O 2 /PMS systems was close to 100%, confirming that Fe IV =O was predominantly generated in these two systems rather than radicals. The efficient generation of Fe IV =O in the Fe-N 4 -C 6 O 2 /PMS and Fe-N 4 -C 10 /PMS systems was further confirmed by the EPR measurements (Fig. 5 b). Subsequently, we investigated the ETP and the 1 O 2 oxidation processes in the three Fe-SACs/PMS systems and observed EPR signals of 2,2,6,6-tetramethylpiperidine-N-oxyl (TEMPO) in these three systems. However, the replacement of H 2 O with D 2 O did not enhance the TEMPO signal (Supplementary Fig. 28), and the degradation rates of BPA and 2,4-DCP did not show a positive correlation with pH after excluding radicals and Fe IV =O interference (Supplementary Fig. 29). These results ruled out the generation of 1 O 2 . 29 , 41 Consequently, we conclude that the ETP process governed the BPA degradation in the three Fe-SACs/PMS systems, where Fe-SACs mediate the electron transfer from BPA to PMS, 42 , which was further confirmed through open circuit potential measurements (Supplementary Fig. 30). Furthermore, Fe-N 2 O 2 -C 10 and Fe-N 4 -C 6 O 2 possessed the stronger potential fluctuations than Fe-N 4 -C 10 after the addition of PMS and BPA, suggesting that the O doping in either the first or second coordination shell could promote the ETP. This is because the doped O could reduce the energy of the lowest unoccupied molecular orbital (LUMO) in the *PMS complexes by modulating the molecular orbital energy levels and electron distributions of the Fe-SACs, which narrowed the energy gap with the highest occupied molecular orbital (HOMO) of BPA, thus effectively facilitating the transfer of electrons from BPA to PMS (Supplementary Fig. 31). We then quantified the steady-state concentrations of different ROS through measuring the competition kinetics between ROS and probe compounds to deeply investigate the effect of O doping in the second coordination shell on the catalytic process and the Fe IV =O reactivity (Figs. 5 c- 5 e, Supplementary Fig. 32, Tables 7–8). As illustrated in Fig. 5 c, the steady-state concentrations of Fe IV =O produced in the Fe-N 4 -C 10 /PMS and Fe-N 4 -C 6 O 2 /PMS system were 1.09 × 10 − 6 and 1.71 × 10 − 6 mol L − 1 , respectively, which were 7–8 orders of magnitude higher than that of •OH and SO 4 •− , strongly validating that Fe-N 4 -C 10 /PMS and Fe-N 4 -C 6 O 2 /PMS were more likely to produce Fe IV =O species rather than radicals, consistent with theoretical predictions. However, the second-order reaction rate constant of BPA and Fe IV =O species produced in the Fe-N 4 -C 10 /PMS system was only 1.08 × 10 3 M − 1 s − 1 , much lower than that of the radicals (1.37 × 10 9 -1.7 × 10 10 M − 1 s − 1 ), 43–45 rendering the contribution of Fe IV =O to the BPA oxidation to be only 8.4%, much lower than those of ETP (60.0%) and radical oxidation (31.6%) (Figs. 5 d- 5 e and Supplementary Table 7). These facts suggested that the Fe IV =O species produced in the Fe-N 4 -C 10 /PMS system possessed low reactivity towards BPA. Surprisingly, the second-order reaction rate constant of BPA and Fe IV =O species produced in the Fe-N 4 -C 6 O 2 /PMS system increased to 4.6 × 10 4 M − 1 s − 1 , 41.6-fold that of Fe-N 4 -C 10 /PMS (Figs. 5 c- 5 e), suggesting that the O doping in the second coordination shell could significantly increase the reactivity of Fe IV =O towards BPA. Consequently, the contribution of Fe IV =O to the BPA removal sharply increased to 82.74% in the Fe-N 4 -C 6 O 2 /PMS system. Theoretically, either π*(d xz/yz -p x/y ) or σ*( \({d}_{{Z}^{2}}\) -p z ) antibonding orbitals at the Fe = O fragment in Fe IV =O could receive foreign electrons to disrupts the Fe = O bond, triggering the terminal O transfer and the substrate oxidation (Supplementary Fig. 33a). The Mössbauer spectroscopy indicated the O doping and the CVs generation could significantly increase the spin state of Fe-N 4 -C 6 O 2 (Supplementary Fig. 33b). According to the ligand field theory, the metal center typically possesses a high spin state when it is coordinated with a weak-field ligand and vice versa. Therefore, the enhanced spin state indicated that the O doping and the CVs generation could effectively weaken the strength of coordination field, resulting in a decrease in the energy of the Fe 3d orbitals to further reduce the orbital energy of Fe = O fragment in Fe IV =O, and improving the oxidative reactivity of Fe IV =O. This provides a new possibility for improving the activity of Fe IV =O. Subsequently, we employed DFT calculations to clarify the increased intrinsic reactivity of Fe IV =O produced in the Fe-N 4 -C 6 O 2 /PMS system. As shown in Fig. 5 f, the Fe IV =O produced in the Fe-N 4 -C 10 /PMS system possessed the π- and σ-attack pathways for the pollutants oxidation. Its Fe/O (d xz/yz -p x /p y ) π* (β) orbitals (-2.023 eV) and ( \({d}_{{z}^{2}}\) - \({p}_{z}\) ) σ* (α) orbitals (-1.986 eV) could accept electrons from pollutant substrates, thereby reducing the bond order of Fe = O and favoring the terminal O transfer. In the π-attack pathway, the pollutants needed to overlap orbitals from a direction perpendicular to the Fe = O bond, which resulted in a spatial collision between the pollutants and the equatorial ligands of Fe IV =O produced in the Fe-N 4 -C 10 /PMS system. In the σ-attack pathway, its ( \({d}_{{z}^{2}}\) - \({p}_{z}\) ) σ* (α) orbital had a high energy (-1.986 eV), requiring a considerable energy barrier to be overcome to initiate σ-attack. Overall, the two reaction pathways of Fe IV =O produced in the Fe-N 4 -C 10 /PMS system exhibited moderate reactivity towards pollutant. For the Fe IV =O produced in the Fe-N 4 -C 6 O 2 /PMS system, the orbital components became more complicate and the orbital energy levels were significantly lowered due to the doping of oxygen atoms and the decrease of ligand field intensity. Among them, the energies of its ( \({d}_{{z}^{2}}\) - \({p}_{z}\) , \({d}_{{x}^{2}-{y}^{2}-}\text{L}\text{e}\text{q}\) ) σ* (β) and ( \({d}_{{z}^{2}}\) - \({p}_{z}\) , Leq) σ* (α) orbitals were respectively reduced to -4.527 eV and − 2.636 eV, significantly lower than that of its (d yz -p y ) π* (β) orbital (-2.620 eV). As a result, the electrons of the pollutant could fill the low-lying σ* orbitals of Fe IV =O produced in the Fe-N 4 -C 6 O 2 /PMS system along the Z-axis without steric hindrance, which facilitated the formation of the pollutant-O-Fe collinear transition state, thereby accelerating the reaction between the pollutant with Fe IV =O. Finally, we determined the main intermediates in the degradation process by chromatography-mass spectrometry (LC-MS) and gas chromatography-mass spectrometry (GC-MS) techniques (Supplementary Figs. 34–37, Tables 9–10), and proposed two potential BPA degradation pathways in the Fe-N 4 -C 6 O 2 /PMS system (Supplementary Fig. 38, details can be found in the Supporting Information). In these two pathways, BPA was oxidized by Fe IV =O through the OAT reaction, electrophilic addition, and H-abstraction/O-rebound mechanism (Fig. 6 ). The core of these three oxidation mechanisms was the filling of pollutant electrons into the low-lying σ* orbitals of Fe/O to form pollutant-O-Fe intermediates, thus realizing the oxidation of pollutant and the reduction of Fe IV =O. Notably, the decrease of total organic carbon (TOC) in BPA solution reached as high as 90% in 2 min when the concentration of Fe-N 4 -C 6 O 2 SAC was 0.1 g L − 1 , and it was close to 80% in 2 min even when the catalyst concentration was reduced by half (Supplementary Fig. 39), which indicated that the Fe-N 4 -C 6 O 2 SAC exhibited a good performance in AOPs, which was able to cause the organic compounds to be degraded into harmless ones completely and reduce the impact on the environment. Conclusion In summary, we have reported the first synthesis of Fe-N 4 -C 6 O 2 SACs by substituting C in the second coordination shell of Fe-N 4 sites with O and demonstrated these novel Fe-N 4 -C 6 O 2 sites could activate PMS more efficiently and stably by providing an enhanced localized electric field without destroying their symmetric coordination structure in the first coordination shell, thus achieving exceptional activity and unprecedented catalytic durability. The O doping in the second coordination shell could enhance the strength of the Fe-N bond by reducing the electron density of Fe center and weaken the amplitude of Fe-N bond during the PMS activation, therefore effectively preventing the demetallation of Fe-N 4 sites. More importantly, this O doping also lowered the energy of Fe = O σ* orbitals by weakening the coordination field to promote the electrophilic σ-attack of Fe IV =O towards BPA, thus greatly enhancing its degradation rate by a factor of 41.6. This work sheds light on the importance of second coordination shell doping on the ultrastability of Fe-N 4 SACs, and provides a novel strategy to design metal SACs with a trade-off between exceptional activity and long-term stability. Methods Data Availability. All study data are included in the article and/or Supplementary Information. For detailed information regarding materials and characterization, please refer to Supplementary Text 1. The synthesis of carriers and catalysts are included in Supplementary Text 2. Details of the degradation experiments and methods of analysis can be found in Supplementary Texts 3 and 4, respectively. The formulae for k , TOF, k -value and TOC are listed in Supplementary Text 5. Continuous flow experiments and preparation of electrodes are detailed in Supplementary Texts 6 and 7, respectively. See Supplementary Text 8 for quantification of ROS. Theoretical. The theoretical calculations were performed using Gaussian 16 47 software and Multiwfn 3.8 program 48 on the basis of DFT. The dynamic simulations elucidating the material's dynamic behaviors were conducted through the application of Born−Oppenheimer molecular dynamics (BOMD) simulations, as implemented within the openly accessible CP2K/Quickstep package (SupplementaryText. 9). Declarations Acknowledgments This work was financially supported by the National Natural Science Foundation of China (U22A20402, 22076059, 22376076, 22076061, 21936003), international Joint Research Center for Intelligent Biosensing Technology and Health, Shenzhen Science and Technology Program (JCYJ20220818095601002), and the Fundamental Research Funds for the Central Universities (CCNU22JC014). Author Contributions Statement H. X. and L. Z. Z. supervised the project and provided financial support. T. T. C. conceived and designed the experiments. G. B. Z. performed DFT calculations and its analysis; Y. T. H., S. Y., D. D. Z., H. X. D., Y. L. X., and S. H. H., provided experimental assistance; T. T. C., H. X., H. W. S., G. B. Z. and L. Z. Z. wrote the paper. Competing Interest Statement The authors declare no competing interests. References Yao, Y. et al. 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Multiwfn: a multifunctional wavefunction analyzer. J. Comput. Chem. 33 , 580-592 (2012). Additional Declarations There is NO Competing Interest. Supplementary Files Supplementaryinformation.docx Cite Share Download PDF Status: Published Journal Publication published 10 Mar, 2025 Read the published version in Nature Communications → Version 1 posted You are reading this latest preprint version Research Square lets you share your work early, gain feedback from the community, and start making changes to your manuscript prior to peer review in a journal. As a division of Research Square Company, we’re committed to making research communication faster, fairer, and more useful. We do this by developing innovative software and high quality services for the global research community. Our growing team is made up of researchers and industry professionals working together to solve the most critical problems facing scientific publishing. Also discoverable on Platform About Our Team In Review Editorial Policies Advisory Board Help Center Resources Author Services Accessibility API Access RSS feed Manage Cookie Preferences © Research Square 2026 | ISSN 2693-5015 (online) Privacy Policy Terms of Service Do Not Sell My Personal Information {"props":{"pageProps":{"initialData":{"identity":"rs-4654905","acceptedTermsAndConditions":true,"allowDirectSubmit":false,"archivedVersions":[],"articleType":"Article","associatedPublications":[],"authors":[{"id":325525326,"identity":"9c999985-c18b-4ef0-8144-556d70ecf9fd","order_by":0,"name":"Lizhi Zhang","email":"data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAZAAAAAyAQMAAABI0h/eAAAABlBMVEX///8AAABVwtN+AAAACXBIWXMAAA7EAAAOxAGVKw4bAAAA+0lEQVRIiWNgGAWjYDACCRBRIMFgwMx8AMJhSCBGiwFIC1sCg0QC8VpAiMcAqpqAFvnZzc8efjGwsDdn5/n8wfLHYQZ+9hwDhp87cGthnHPM3FjGQILZspl3m4REwmEGyZ43Boy9Z3BrYZZIMJOWMJBgMzjMu40BpMXgRo4BM2Mbbi1sEunfQFp4DA7zPP4A0mJPSAuPRI6Z5AcDCQmgFgawwwwkCGiRkMgpkwYqMzA4zGYmIZGWziNx5lnBwV48WuRnpG+T/FFRZ29w/vDjzxI21nL87ckbH/zEowUcBDwwBjCSwOwD+DUAA/oHjPGBkNJRMApGwSgYkQAAXclEVyH/GWsAAAAASUVORK5CYII=","orcid":"https://orcid.org/0000-0002-6842-9167","institution":"Shanghai Jiao Tong University","correspondingAuthor":true,"prefix":"","firstName":"Lizhi","middleName":"","lastName":"Zhang","suffix":""},{"id":325525327,"identity":"f554dae4-a368-4b42-bfea-a04002adf346","order_by":1,"name":"Tiantian Chen","email":"","orcid":"","institution":"Central China Normal University","correspondingAuthor":false,"prefix":"","firstName":"Tiantian","middleName":"","lastName":"Chen","suffix":""},{"id":325525328,"identity":"7c869eb4-3c1a-48f9-ae27-84c30442fcdb","order_by":2,"name":"Ganbing Zhang","email":"","orcid":"","institution":"Hubei University","correspondingAuthor":false,"prefix":"","firstName":"Ganbing","middleName":"","lastName":"Zhang","suffix":""},{"id":325525329,"identity":"0b986b8b-74d6-49cb-bbc7-6329236c11f1","order_by":3,"name":"Hongwei Sun","email":"","orcid":"https://orcid.org/0000-0001-8758-9337","institution":"Central China Normal University","correspondingAuthor":false,"prefix":"","firstName":"Hongwei","middleName":"","lastName":"Sun","suffix":""},{"id":325525330,"identity":"730f111b-8ebe-4a01-bab6-e34ddc137745","order_by":4,"name":"Yetong Hua","email":"","orcid":"","institution":"Central China Normal University","correspondingAuthor":false,"prefix":"","firstName":"Yetong","middleName":"","lastName":"Hua","suffix":""},{"id":325525331,"identity":"56f8e95a-8c04-4c9a-8435-780d31fcc5cf","order_by":5,"name":"Shu Yang","email":"","orcid":"","institution":"Central China Normal University","correspondingAuthor":false,"prefix":"","firstName":"Shu","middleName":"","lastName":"Yang","suffix":""},{"id":325525332,"identity":"bf3ecfb1-18fd-4bb3-ae73-9ad20e93cb44","order_by":6,"name":"Dandan Zhou","email":"","orcid":"","institution":"Central China Normal University","correspondingAuthor":false,"prefix":"","firstName":"Dandan","middleName":"","lastName":"Zhou","suffix":""},{"id":325525333,"identity":"a1b93eb2-5fc4-473d-9f7a-d064c77fa86f","order_by":7,"name":"Haoxin Di","email":"","orcid":"","institution":"Central China Normal University","correspondingAuthor":false,"prefix":"","firstName":"Haoxin","middleName":"","lastName":"Di","suffix":""},{"id":325525334,"identity":"f88ff616-9759-4691-8276-1f69def3b944","order_by":8,"name":"Yiling Xiong","email":"","orcid":"","institution":"Central China Normal University","correspondingAuthor":false,"prefix":"","firstName":"Yiling","middleName":"","lastName":"Xiong","suffix":""},{"id":325525335,"identity":"db3feac0-5fac-4b60-8021-6f68714703ed","order_by":9,"name":"Shenghuai Hou","email":"","orcid":"","institution":"Central China Normal University","correspondingAuthor":false,"prefix":"","firstName":"Shenghuai","middleName":"","lastName":"Hou","suffix":""},{"id":325525336,"identity":"dfd7da49-0349-46c7-be40-5b5733ef35ce","order_by":10,"name":"Hui Xu","email":"","orcid":"https://orcid.org/0000-0003-1999-9806","institution":"","correspondingAuthor":false,"prefix":"","firstName":"Hui","middleName":"","lastName":"Xu","suffix":""}],"badges":[],"createdAt":"2024-06-28 12:55:09","currentVersionCode":1,"declarations":"","doi":"10.21203/rs.3.rs-4654905/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-4654905/v1","draftVersion":[],"editorialEvents":[{"content":"https://doi.org/10.1038/s41467-025-57643-7","type":"published","date":"2025-03-10T04:00:00+00:00"}],"editorialNote":"","failedWorkflow":false,"files":[{"id":60123199,"identity":"9946fcf3-6b4e-459d-97de-19b3116bd49b","added_by":"auto","created_at":"2024-07-12 05:20:07","extension":"jpeg","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":4030041,"visible":true,"origin":"","legend":"\u003cp\u003e(a) Schematic diagram of the Fe-N\u003csub\u003e4\u003c/sub\u003e-C\u003csub\u003e6\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e synthesis. (b) TEM images of Fe-N\u003csub\u003e4\u003c/sub\u003e-C\u003csub\u003e6\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e. (c) PXRD pattern of different catalysts. (d) HADDF-STEM images of Fe-N\u003csub\u003e4\u003c/sub\u003e-C\u003csub\u003e6\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e. (e) The particle size of Fe in Fe-N\u003csub\u003e4\u003c/sub\u003e-C\u003csub\u003e6\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e.\u003c/p\u003e","description":"","filename":"floatimage1.jpeg","url":"https://assets-eu.researchsquare.com/files/rs-4654905/v1/296ffdb886c79a73bb677ca8.jpeg"},{"id":60123198,"identity":"cf3ed86e-b4d9-4b12-bc4a-35a146b5e6ca","added_by":"auto","created_at":"2024-07-12 05:20:07","extension":"jpeg","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":906947,"visible":true,"origin":"","legend":"\u003cp\u003e(a) XANES, (b) FT-EXAFS spectra (\u003cem\u003ek\u003c/em\u003e\u003csup\u003e3\u003c/sup\u003e-weighted), and (c) WT-EXAFS spectra of different Fe foil and Fe-SACs. (d) corresponding EXAFS \u003cem\u003eR\u003c/em\u003e-space fitting curve of Fe-N\u003csub\u003e4\u003c/sub\u003e-C\u003csub\u003e6\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e. \u003csup\u003e57\u003c/sup\u003eFe Mössbauer spectroscopy of (e) Fe-N\u003csub\u003e4\u003c/sub\u003e-C\u003csub\u003e6\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e and (f) Fe-N\u003csub\u003e4\u003c/sub\u003e-C\u003csub\u003e10\u003c/sub\u003e. (g) Optimized geometries of three types possible Fe-N\u003csub\u003e4\u003c/sub\u003e-C\u003csub\u003e6\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e active site structure. (h) EPR spectra and (i) PDOS of Fe-SACs.\u003csup\u003e37\u003c/sup\u003e\u003c/p\u003e","description":"","filename":"floatimage2.jpeg","url":"https://assets-eu.researchsquare.com/files/rs-4654905/v1/af6bdcd1cc723faead308d57.jpeg"},{"id":60123200,"identity":"3b618ee8-8e63-4c7b-9158-46c853705d7b","added_by":"auto","created_at":"2024-07-12 05:20:07","extension":"jpeg","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":907727,"visible":true,"origin":"","legend":"\u003cp\u003e(a) The adsorption configuration of PMS on Fe-N\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e-C\u003csub\u003e10\u003c/sub\u003e, Fe-N\u003csub\u003e4\u003c/sub\u003e-C\u003csub\u003e6\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e and Fe-N\u003csub\u003e4\u003c/sub\u003e-C\u003csub\u003e10\u003c/sub\u003e. (b) The free energy for the Fe\u003csup\u003eIV\u003c/sup\u003e=O generation in the Fe-N\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e-C\u003csub\u003e10\u003c/sub\u003e/PMS system. (c) The structures of reaction intermediates for the Fe\u003csup\u003eIV\u003c/sup\u003e=O generation. (d) The free energies for the Fe\u003csup\u003eIV\u003c/sup\u003e=O generation in the Fe-N\u003csub\u003e4\u003c/sub\u003e-C\u003csub\u003e6\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e/PMS and Fe-N\u003csub\u003e4\u003c/sub\u003e-C\u003csub\u003e10\u003c/sub\u003e/PMS systems. (e) The schematic diagram of the CEPT process.\u003c/p\u003e","description":"","filename":"floatimage3.jpeg","url":"https://assets-eu.researchsquare.com/files/rs-4654905/v1/e460c8cf7e612e7961870922.jpeg"},{"id":60123201,"identity":"7191fe5c-0044-4a72-9dba-c59ead62ad73","added_by":"auto","created_at":"2024-07-12 05:20:07","extension":"jpeg","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":911990,"visible":true,"origin":"","legend":"\u003cp\u003e(a) The BPA removal curves. (b) Comparison of the \u003cem\u003ek\u003c/em\u003e-value of BPA removal by PMS activated with different catalysts. Experimental conditions (a-b): [Catalysts] = 0.1 g L\u003csup\u003e-1\u003c/sup\u003e, [BPA] = 50 μM, [PMS]\u003csub\u003e0\u003c/sub\u003e = 400 μM, T = 25 ℃, pH = 7. (c) Stability testing of Fe-SACs. average flow rate: 45 L/m\u003csup\u003e2\u003c/sup\u003e/h, catalyst dosage: 20 mg. (d) BPA adsorption of Fe-SACs. Demetallation rates (e) in continuous catalysis test (f) and acid and alkali resistance test of Fe-SACs, t=36h. (g) Fe-N and Fe-O radical distribution function profiles of Fe-SACs at 25°C. (h) Snapshots of Fe-SACs obtained from MD simulations at 25°C.\u003c/p\u003e","description":"","filename":"floatimage4.jpeg","url":"https://assets-eu.researchsquare.com/files/rs-4654905/v1/5c3b7139f48909e5cfca402b.jpeg"},{"id":60123202,"identity":"5eac97f1-b51c-4dc8-94bf-ab148d8ac91e","added_by":"auto","created_at":"2024-07-12 05:20:07","extension":"jpeg","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":696849,"visible":true,"origin":"","legend":"\u003cp\u003e(a) Effect of scavengers on BPA degradation in the Fe-SACs/PMS system. Experimental conditions: [BPA] = 50 μM, [catalysts] = 0.05 g L\u003csup\u003e-1\u003c/sup\u003e, [PMS]\u003csub\u003e0\u003c/sub\u003e = 400 μM, T=25 ℃, pH = 7, [SOD] = 100 U mL\u003csup\u003e-1\u003c/sup\u003e if need, [MeOH/PMS] = 1000 if need, [PMSO/PMS] = 100 if need. (b) EPR spectra of Fe-N\u003csub\u003e4\u003c/sub\u003e-C\u003csub\u003e6\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e and Fe-N\u003csub\u003e4\u003c/sub\u003e-C\u003csub\u003e10\u003c/sub\u003e under different conditions. (c) Steady-state concentrations, (d) observed reaction rate constants, and (e) oxidation contributions of different ROS to BPA degradation in the Fe-SACs/PMS system. (f) The molecular orbital composition and energy level of Fe-SACs.\u003csup\u003e46\u003c/sup\u003e The above illustration displays only the orbitals in the vicinity of the LUMO orbital. The N-Fe-N axis in Fe\u003csup\u003eIV\u003c/sup\u003e=O(Fe-N\u003csub\u003e4\u003c/sub\u003e-C\u003csub\u003e6\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e) where N is directly connected to O is defined as the x-axis.\u003c/p\u003e","description":"","filename":"floatimage5.jpeg","url":"https://assets-eu.researchsquare.com/files/rs-4654905/v1/1aec577739a21aad1ac9308c.jpeg"},{"id":60123990,"identity":"0de9a5e9-b609-421f-97d6-17a8d101466c","added_by":"auto","created_at":"2024-07-12 05:28:07","extension":"jpeg","order_by":6,"title":"Figure 6","display":"","copyAsset":false,"role":"figure","size":113272,"visible":true,"origin":"","legend":"\u003cp\u003eReaction mechanisms diagram of Fe\u003csup\u003eIV\u003c/sup\u003e=O with pollutants.\u003c/p\u003e","description":"","filename":"floatimage6.jpeg","url":"https://assets-eu.researchsquare.com/files/rs-4654905/v1/61d59e73da55c78db8dde65a.jpeg"},{"id":78227061,"identity":"2cf23426-e7cf-4f13-ae66-758a6720afea","added_by":"auto","created_at":"2025-03-11 07:07:21","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":8554861,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-4654905/v1/070d75df-ddc8-49d4-a6ca-a55612cb0af0.pdf"},{"id":60123204,"identity":"cb674b17-a08c-4c66-9a7b-ac2ef1bb6f51","added_by":"auto","created_at":"2024-07-12 05:20:07","extension":"docx","order_by":1,"title":"","display":"","copyAsset":false,"role":"supplement","size":34816154,"visible":true,"origin":"","legend":"","description":"","filename":"Supplementaryinformation.docx","url":"https://assets-eu.researchsquare.com/files/rs-4654905/v1/bda21f74029de222b62cc3d9.docx"}],"financialInterests":"There is \u003cb\u003eNO\u003c/b\u003e Competing Interest.","formattedTitle":"\u003cp\u003eUltrastable Fe-N\u003csub\u003e4\u003c/sub\u003e-C\u003csub\u003e6\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e Single Atom Sites for Highly Efficient PMS Activation and Enhanced Fe\u003csup\u003eIV\u003c/sup\u003e=O Reactivity\u003c/p\u003e","fulltext":[{"header":"Introduction","content":"\u003cp\u003eAdvanced oxidation processes (AOPs) based on peroxymonosulfate (PMS) have attracted great attention for pollutant control and environmental remediation due to their high efficiency in a wide pH range.\u003csup\u003e\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e\u003c/sup\u003e Recently, single-atom catalysts (SACs) with metal-N\u003csub\u003e4\u003c/sub\u003e (M-N\u003csub\u003e4\u003c/sub\u003e) sites, where the metal centers are coordinated with four nitrogen atoms, are widely used for the PMS activation because of their outstanding catalytic activity, excellent selectivity, and exceptionally high metal utilization efficiency.\u003csup\u003e\u003cspan additionalcitationids=\"CR3 CR4\" citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e5\u003c/span\u003e\u003c/sup\u003e The electron donating potential of these M-N\u003csub\u003e4\u003c/sub\u003e sites can trigger the PMS activation to produce sulfate radicals (SO\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e\u0026bull;\u0026minus;\u003c/sup\u003e), hydroxyl radicals (\u0026bull;OH), and high-valence metal-oxo (M\u003csup\u003eIV\u003c/sup\u003e=O), etc., thus effectively facilitating the degradation of various organic pollutants.\u003csup\u003e\u003cspan additionalcitationids=\"CR7 CR8 CR9 CR10 CR11 CR12 CR13\" citationid=\"CR6\" class=\"CitationRef\"\u003e6\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e14\u003c/span\u003e\u003c/sup\u003e However, the high bond dissociation energies of O-H and O-O bonds in PMS pose a significant challenge to their cleavage, thereby disfavoring the PMS activation at M-N\u003csub\u003e4\u003c/sub\u003e sites.\u003csup\u003e\u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e15\u003c/span\u003e\u003c/sup\u003e Therefore, the development of high-performance catalysts to promote the PMS activation is crucial for the effective oxidation of contaminants.\u003c/p\u003e \u003cp\u003eRecently, many strategies have been proposed for the efficient PMS activation via increasing the density of metal centers or regulating their local environment surrounding in M-SACs.\u003csup\u003e\u003cspan additionalcitationids=\"CR17 CR18\" citationid=\"CR16\" class=\"CitationRef\"\u003e16\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e19\u003c/span\u003e\u003c/sup\u003e Among these strategies, the most common one is the partial replacement of N in the first coordination shell with foreign elements, thereby creating an enhanced localized electric field to facilitate the activation of O-H and O-O bonds in PMS.\u003csup\u003e\u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e20\u003c/span\u003e,\u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e21\u003c/span\u003e\u003c/sup\u003e Unfortunately, this strategy destroys the symmetric structure of M-N\u003csub\u003e4\u003c/sub\u003e sites, potentially decreasing their long-term stability via the accelerated demetallation. For instance, the activity of M-N\u003csub\u003e4\u003c/sub\u003e-\u003csub\u003ex\u003c/sub\u003eY\u003csub\u003ex\u003c/sub\u003e (Y represents P, O, or S) SACs significantly diminishes after only one or two cycles of PMS activation.\u003csup\u003e\u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e20\u003c/span\u003e,\u003cspan additionalcitationids=\"CR23\" citationid=\"CR22\" class=\"CitationRef\"\u003e22\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR24\" class=\"CitationRef\"\u003e24\u003c/span\u003e\u003c/sup\u003e Therefore, it is still a great challenge to balance the high activity and the long durability of SACs for the PMS activation.\u003c/p\u003e \u003cp\u003eDifferent from the first coordination shell doping, the heteroatom doping in the second coordination shell of M-N\u003csub\u003e4\u003c/sub\u003e sites could improve the catalytic performance of M-N\u003csub\u003e4\u003c/sub\u003e sites by modulating the electronic structure of the metal center through d-p long-range interactions, and simultaneously maintain the M-N\u003csub\u003e4\u003c/sub\u003e coordination structure.\u003csup\u003e\u003cspan additionalcitationids=\"CR26 CR27\" citationid=\"CR25\" class=\"CitationRef\"\u003e25\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR28\" class=\"CitationRef\"\u003e28\u003c/span\u003e\u003c/sup\u003e Although Li et al. demonstrated that the electron-withdrawing effect of S doping in the second coordination shell was beneficial for anchoring single Fe atoms to enhance nitrate electroreduction activity and stability of Fe-N\u003csub\u003e4\u003c/sub\u003e sites in comparison with Fe-plate, they did not compare the catalytic stability of Fe-N\u003csub\u003e4\u003c/sub\u003e sites with those with S doping in the first or second coordination shells towards nitrate electroreduction.\u003csup\u003e\u003cspan citationid=\"CR25\" class=\"CitationRef\"\u003e25\u003c/span\u003e\u003c/sup\u003e Compared with nitrate electroreduction, the PMS activation imposes much stricter requirements on the catalytic stability of Fe-N\u003csub\u003e4\u003c/sub\u003e sites owing to its strong oxidative environment.\u003c/p\u003e \u003cp\u003eRegarding that the electronegativity of O (3.44) is higher than that of S (2.58), the O doping can reduce the electron density in the metal center more significantly through d-p long-range interaction and establish a stronger electric field gradient around M-N\u003csub\u003e4\u003c/sub\u003e sites, facilitating the adsorption and activation of negatively charged PMS (HSO\u003csub\u003e5\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e). Meanwhile, the O doping in the second coordination shell might favor the formation of carbon vacancies around the metal center and the asymmetric charge distribution to suppress the demetallation and regulate their coordination environment. Herein, we develop a pre-coordination strategy to engineer heteroatom doping by precisely controlling the substitution of neighboring first coordination shell N atoms and distanced second coordination shell C atoms of Fe center, aiming to clarify the effects of O doping in the first or second coordination shell on the reactivity and stability of Fe-N\u003csub\u003e4\u003c/sub\u003e sites through systematical characterization, degradation experiments, density functional theory (DFT) calculations and molecular dynamics (MD) simulations. This study shill shed light on the design of SACs with high PMS activation performance by dealing with a trade-off between exceptional activity and sustainable stability.\u003c/p\u003e"},{"header":"Results","content":"\u003cp\u003e \u003cb\u003eSynthesis and Characterization.\u003c/b\u003e Fe-SACs with oxygen doped in the first (Fe-N\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e-C\u003csub\u003e10\u003c/sub\u003e) or the second shell (Fe-N\u003csub\u003e4\u003c/sub\u003e-C\u003csub\u003e6\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e) were synthesized by a pre-coordination strategy (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003ea). First, we synthesized two core-shell structured carriers with utilizing silica (SiO\u003csub\u003e2\u003c/sub\u003e) as the core layer (Supplementary Figs.\u0026nbsp;1a-1c). One of shell layer was composed of nitrogen-doped (N-C) without any oxygen functional groups (SiO\u003csub\u003e2\u003c/sub\u003e@N-C), while the other consisted of resorcinol-formaldehyde (RF) enriched with oxygen functional groups (SiO\u003csub\u003e2\u003c/sub\u003e@RF). Meanwhile, Fe-N/O co-coordination (I) and Fe-N coordination (II) complexes were prepared using (S, S)-(+)-N, N'-bis(3,5-di-tert-butylsalicylidene)-1, 2-cyclohexanediamin as the N/O precursors and 1,10-phenanthroline as the N precursor, respectively. Subsequently, complexes (I) and (II) were deposited on SiO\u003csub\u003e2\u003c/sub\u003e@N-C and SiO\u003csub\u003e2\u003c/sub\u003e@RF using the rotary evaporation technique, which were then subjected to the processes of high-temperature pyrolysis and SiO\u003csub\u003e2\u003c/sub\u003e template removal to obtain Fe-N\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e-C\u003csub\u003e10\u003c/sub\u003e and Fe-N\u003csub\u003e4\u003c/sub\u003e-C\u003csub\u003e6\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e, respectively. Fe-SAC without O-doping (Fe-N\u003csub\u003e4\u003c/sub\u003e-C\u003csub\u003e10\u003c/sub\u003e) was also synthesized with the same procedure except for the deposition of complex (I) on SiO\u003csub\u003e2\u003c/sub\u003e. For comparison, N/O co-doped carbon (N/O-C), N-doped carbon (N-C), and Fe nanoparticles supported on N-doped C (Fe NPs/NC) were also fabricated (see Supporting Information for synthesis details and Table \u003cspan refid=\"MOESM1\" class=\"InternalRef\"\u003eS1\u003c/span\u003e).\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eAll the three Fe-SACs were of ultrathin, uniform, hierarchically porous hollow structure (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003eb and Supplementary Figs.\u0026nbsp;1\u0026ndash;3), facilitating the full exposure of active sites and the mass transfer during the PMS activation. Their powder X-ray diffraction (PXRD) patterns only contained one broad peak at 25\u0026deg; corresponding to the (002) plane of the carbon carrier, without any discernible peaks arisen from crystalline iron or iron oxides (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003ec).\u003csup\u003e\u003cspan citationid=\"CR29\" class=\"CitationRef\"\u003e29\u003c/span\u003e\u003c/sup\u003e Meanwhile, the existence of iron particles were ruled out by their high-resolution transmission electron microscope (HR-TEM) images and the selected area electron diffraction (SAED) patterns (Supplementary Fig.\u0026nbsp;1). Aberration-corrected high-angle annular dark field scanning electron microscopy (AC-HAADF STEM) images of Fe-N\u003csub\u003e4\u003c/sub\u003e-C\u003csub\u003e6\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e clearly indicated that bright spots of sizes, consistent with single Fe atoms (1.002\u0026thinsp;\u0026plusmn;\u0026thinsp;0.173 \u0026Aring;), were uniformly dispersed in the dark O-N/C background (Figs.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003ed-\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003ee),\u003csup\u003e\u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e6\u003c/span\u003e\u003c/sup\u003e confirming its atomically dispersion of Fe.\u003c/p\u003e \u003cp\u003eWe then checked the electronic structure and coordination environment of Fe atoms in three Fe-SACs by X-ray absorption fine structure (XAFS), XPS, and \u003csup\u003e57\u003c/sup\u003eFe M\u0026ouml;ssbauer spectroscopy. As shown in X-ray absorption near-edge structure (XANES) spectra (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003ea), the pre-edge peak and absorption edge of Fe-N\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e-C\u003csub\u003e10\u003c/sub\u003e and Fe-N\u003csub\u003e4\u003c/sub\u003e-C\u003csub\u003e6\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e shifted towards higher energies, accompanied by the intensity increase of the white-line peak, indicating that the O doping altered the coordination environment of Fe centers and reduced their electron densities. Obviously, these changes were more pronounced in the XANES spectrum of Fe-N\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e-C\u003csub\u003e10\u003c/sub\u003e, indicative of its shorter distance and stronger interaction between O and Fe. The Fourier-transformed \u003cem\u003ek\u003c/em\u003e\u003csup\u003e\u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e3\u003c/span\u003e\u003c/sup\u003e-weighted extended X-ray absorption fine structure (FT-EXAFS) spectrum revealed a broad and unsymmetric peak of Fe-N\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e-C\u003csub\u003e10\u003c/sub\u003e with the peak maximum located at 1.44 \u0026Aring;, close to the Fe-O backscattering, indicating the presence of Fe-N and Fe-O dual coordination in Fe-N\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e-C\u003csub\u003e10\u003c/sub\u003e (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003eb). Differently, the main peak of Fe-N\u003csub\u003e4\u003c/sub\u003e-C\u003csub\u003e6\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e and Fe-N\u003csub\u003e4\u003c/sub\u003e-C\u003csub\u003e10\u003c/sub\u003e were symmetric and left-shifted to ~\u0026thinsp;1.4 \u0026Aring;, belonging to the single Fe-N scattering path. However, a new peak in the 1\u0026ndash;2 \u0026Aring; range of \u003cem\u003eR\u003c/em\u003e-space appeared in the spectrum of Fe-N\u003csub\u003e4\u003c/sub\u003e-C\u003csub\u003e6\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e, indicating that the different coordination environments of Fe-N\u003csub\u003e4\u003c/sub\u003e-C\u003csub\u003e6\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e and Fe-N\u003csub\u003e4\u003c/sub\u003e-C\u003csub\u003e10\u003c/sub\u003e. These results confirmed that O was doped into outer coordination shells of n\u0026thinsp;\u0026ge;\u0026thinsp;2 in Fe-N\u003csub\u003e4\u003c/sub\u003e-C\u003csub\u003e6\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e. Moreover, those peaks with R\u0026thinsp;\u0026gt;\u0026thinsp;2 \u0026Aring; (pentagram markers) could not be assigned to Fe-Fe scattering path, because the wavelet transforms (WT) EXAFS contours of three Fe-SACs samples had only one intensity maximum corresponding to the Fe-N/O coordination at \u003cem\u003ek\u003c/em\u003e value of 5 \u0026Aring;\u003csup\u003e-\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e\u003c/sup\u003e in the \u003cem\u003eR\u003c/em\u003e range of 1\u0026ndash;3 \u0026Aring; (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003ec). Subsequently, quantitative least-squares fitting of the FT-EXAFS curves revealed that the optimal coordination numbers of Fe in all three Fe-SACs were approximately 4 (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003ed, Supplementary Fig.\u0026nbsp;4 and Table\u0026nbsp;2). The distinction lied in the fact that Fe-N\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e-C\u003csub\u003e10\u003c/sub\u003e encompassed both Fe-N and Fe-O backscattering paths, thereby confirming the successful construction of a Fe-N/O dual coordination environment, while the other two Fe-SACs solely exhibited Fe-N backscattering paths. As expected, two distinct Fe-N bond lengths of 1.91 \u0026Aring; and 2.09 \u0026Aring; were observed in Fe-N\u003csub\u003e4\u003c/sub\u003e-C\u003csub\u003e6\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e, indicating the presence of N ligand in two different coordination forms, while only an average distance of 1.97 \u0026Aring; was found in Fe-N\u003csub\u003e4\u003c/sub\u003e-C\u003csub\u003e10\u003c/sub\u003e.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eWe further characterized Fe-N\u003csub\u003e4\u003c/sub\u003e-C\u003csub\u003e6\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e and Fe-N\u003csub\u003e4\u003c/sub\u003e-C\u003csub\u003e10\u003c/sub\u003e using \u003csup\u003e57\u003c/sup\u003eFe M\u0026ouml;ssbauer spectroscopy to elucidate their coordination environments of Fe. Their deconvoluted \u003csup\u003e57\u003c/sup\u003eFe M\u0026ouml;ssbauer spectra exclusively exhibited doublets, without any singlet or sextet associated with α-Fe, Fe\u003csub\u003ex\u003c/sub\u003eC, or Fe\u003csub\u003ex\u003c/sub\u003eO (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003ee). According to the isomer shift (δ\u003csub\u003eiso\u003c/sub\u003e) and quadrupole splitting (∆E\u003csub\u003eQ\u003c/sub\u003e) values (Supplementary Table\u0026nbsp;3), \u003csup\u003e57\u003c/sup\u003eFe M\u0026ouml;ssbauer spectrum of Fe-N\u003csub\u003e4\u003c/sub\u003e-C\u003csub\u003e6\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e could be well fitted with three doublets (D1-D3), corresponding to medium-spin (MS) Fe\u003csup\u003eII\u003c/sup\u003eN\u003csub\u003e2+2\u003c/sub\u003e (D1), medium-spin (MS) Fe\u003csup\u003eIII\u003c/sup\u003eN\u003csub\u003e2+2\u003c/sub\u003e (D2), and high-spin (HS) N-Fe\u003csup\u003eII\u003c/sup\u003eN\u003csub\u003e2+2\u003c/sub\u003e (D3), respectively,\u003csup\u003e\u003cspan additionalcitationids=\"CR31\" citationid=\"CR30\" class=\"CitationRef\"\u003e30\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR32\" class=\"CitationRef\"\u003e32\u003c/span\u003e\u003c/sup\u003e wherein Fe-N\u003csub\u003e2\u0026thinsp;+\u0026thinsp;2\u003c/sub\u003e represents the defective Fe-N\u003csub\u003e4\u003c/sub\u003e site in the non-intact graphite layer structure (Supplementary Fig.\u0026nbsp;5). In contrast, four different doublets (D4-D7) could be identified in Fe-N\u003csub\u003e4\u003c/sub\u003e-C\u003csub\u003e10\u003c/sub\u003e, which were assigned to low-spin (LS) Fe\u003csup\u003eIII\u003c/sup\u003eN\u003csub\u003e4\u003c/sub\u003e/C (D4), LS Fe\u003csup\u003eⅡ\u003c/sup\u003eN\u003csub\u003e4\u003c/sub\u003e/C (D5), MS Fe\u003csup\u003eⅡ\u003c/sup\u003eN\u003csub\u003e4\u003c/sub\u003e/C (D6), and HS Fe\u003csup\u003eⅡ\u003c/sup\u003eN\u003csub\u003e4\u003c/sub\u003e/C (D7), respectively.\u003csup\u003e\u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e,\u003cspan citationid=\"CR30\" class=\"CitationRef\"\u003e30\u003c/span\u003e,\u003cspan citationid=\"CR33\" class=\"CitationRef\"\u003e33\u003c/span\u003e\u003c/sup\u003e The relatively higher intensity ratio of D-band to G-band (I\u003csub\u003eD\u003c/sub\u003e/I\u003csub\u003eG\u003c/sub\u003e = 1.07) in Fe-N\u003csub\u003e4\u003c/sub\u003e-C\u003csub\u003e6\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e than that of Fe-N\u003csub\u003e4\u003c/sub\u003e-C\u003csub\u003e10\u003c/sub\u003e (0.94) evidenced that the O doping created more defects (Supplementary Fig.\u0026nbsp;6). Meanwhile, the presence of Fe-N and N-O bonds were also confirmed by HR-XPS spectra of Fe 2p, N 1s, and O 1s in Fe-N\u003csub\u003e4\u003c/sub\u003e-C\u003csub\u003e6\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e (Supplementary Figs.\u0026nbsp;7\u0026ndash;8). Therefore, we concluded that two O atoms were doped in the second coordination shell of Fe-N\u003csub\u003e4\u003c/sub\u003e-C\u003csub\u003e6\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e.\u003c/p\u003e \u003cp\u003eConsidering the presence of doped O atoms in the second coordination shell of Fe-N\u003csub\u003e4\u003c/sub\u003e-C\u003csub\u003e6\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e, we constructed three possible Fe-N\u003csub\u003e4\u003c/sub\u003e-C\u003csub\u003e6\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e models using DFT (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003eg). Specifically, the two O atoms could be situated at either ipsilateral \"a\" and \"b\" positions (configuration I), or ipsilateral \"b\" and \"c\" positions (configuration II), or opposite \"b\" and \"d\" positions (configuration III) within the second coordination shell, respectively. However, the optimization of configuration I led to the connection of C atoms at positions \"c\" and \"d\", which contradicts the Fe-N\u003csub\u003e2\u0026thinsp;+\u0026thinsp;2\u003c/sub\u003e structure and consequently resulted in its exclusion. The stability of configuration II was significantly lower than that of configuration III, despite its conformity to the requirements for the Fe-N\u003csub\u003e2\u0026thinsp;+\u0026thinsp;2\u003c/sub\u003e structure. Therefore, configuration III was selected as the Fe-N\u003csub\u003e4\u003c/sub\u003e-C\u003csub\u003e6\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e model in this study. The formation of configuration III could be attributed to the replacement of C\u003csub\u003e1\u003c/sub\u003e atoms in the second coordination shell of Fe-N\u003csub\u003e4\u003c/sub\u003e by highly electronegative O atoms, resulting in a deviation of Fe-N bond length and a detachment of O atoms from the C\u003csub\u003e2\u003c/sub\u003e atom due to the eight-electron rule.\u003csup\u003e\u003cspan citationid=\"CR27\" class=\"CitationRef\"\u003e27\u003c/span\u003e\u003c/sup\u003e During calcination, the collision of C\u003csub\u003e2\u003c/sub\u003e atoms with N\u003csub\u003e2\u003c/sub\u003e molecules in irregular thermal motion would lead to their release from the catalyst surface to form carbon vacancies (CVs), accompanying with a configuration transformation from Fe-N\u003csub\u003e4\u003c/sub\u003e to Fe-N\u003csub\u003e2\u0026thinsp;+\u0026thinsp;2\u003c/sub\u003e (Supplementary Fig.\u0026nbsp;9). Subsequently, the significantly enhanced Lorenz curve signal of Fe-N\u003csub\u003e4\u003c/sub\u003e-C\u003csub\u003e6\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e indicated that the O doping and the introduction of CVs in the second coordination shell effectively facilitated the formation of unpaired electrons in Fe-N\u003csub\u003e4\u003c/sub\u003e-C\u003csub\u003e6\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003eh), which contributed to the improvement of electron transfer efficiency in the catalytic reaction.\u003csup\u003e\u003cspan additionalcitationids=\"CR35\" citationid=\"CR34\" class=\"CitationRef\"\u003e34\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR36\" class=\"CitationRef\"\u003e36\u003c/span\u003e\u003c/sup\u003e Furthermore, partial density of state (PDOS) analysis demonstrated that the O doping in the second coordination shell could effectively modulate the electron distribution within the Fe 3d orbitals through d-p long-range interaction, shifting the d-band center of Fe in Fe-N\u003csub\u003e4\u003c/sub\u003e-C\u003csub\u003e6\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e (-1.842 eV vs -2.137 eV for Fe-N\u003csub\u003e4\u003c/sub\u003e-C\u003csub\u003e10\u003c/sub\u003e) closer to the Fermi level (E\u003csub\u003ef\u003c/sub\u003e) (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003ei), which might enhance the intrinsic reactivity of Fe-SAC towards the PMS activation.\u003c/p\u003e \u003cp\u003e \u003cb\u003ePMS Activation and Pollutant Degradation.\u003c/b\u003e We utilized DFT calculations to investigate the influence of O doping on the PMS activation and the reactive oxygen species (ROS) formation, and employed an asterisk (*) to denote the surface adsorbed species and O\u003csub\u003e1\u003c/sub\u003e/O\u003csub\u003e2\u003c/sub\u003e/O\u003csub\u003e3\u003c/sub\u003e to represent the three different types of oxygen atoms in PMS (Supplementary Figs.\u0026nbsp;10\u0026ndash;11). As expected, the higher positive charge of Fe center and its surrounding strong local electric field in Fe-N\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e-C\u003csub\u003e10\u003c/sub\u003e promoted the PMS adsorption and triggered the rapid cleavage of peroxide (O\u003csub\u003e2\u003c/sub\u003e-O\u003csub\u003e3\u003c/sub\u003e) bond (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003ea), while the low electron density of Fe center facilitated its robust interaction with O\u003csub\u003e1\u003c/sub\u003e/O\u003csub\u003e2\u003c/sub\u003e/O\u003csub\u003e3\u003c/sub\u003e, delocalizing Fe atom from the carrier plane and elongating the Fe-N and Fe-O bonds to 2.161\u0026thinsp;~\u0026thinsp;2.826 \u0026Aring; to potentially result in demetallation. In contrast, the reduction of positive charge on Fe centers and their surrounding local electric field in Fe-N\u003csub\u003e4\u003c/sub\u003e-C\u003csub\u003e6\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e and Fe-N\u003csub\u003e4\u003c/sub\u003e-C\u003csub\u003e10\u003c/sub\u003e failed to satisfy the requirements for direct breakage of O\u003csub\u003e2\u003c/sub\u003e-O\u003csub\u003e3\u003c/sub\u003e bond induced by their PMS adsorption, and thus they may form complexes (I), (II), and (III) with PMS via O\u003csub\u003e1\u003c/sub\u003e, O\u003csub\u003e2\u003c/sub\u003e, and O\u003csub\u003e3\u003c/sub\u003e, respectively (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003ea and Supplementary Fig.\u0026nbsp;10). However, the activation of the O\u003csub\u003e2\u003c/sub\u003e-O\u003csub\u003e3\u003c/sub\u003e bond only occurred in complex (III). Therefore, complex (III) was adopted for the following calculations.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eSubsequently, the formation of Fe\u003csup\u003eIV\u003c/sup\u003e=O and radicals in the three Fe-SACs/PMS systems was explored theoretically. As shown in Figs.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003eb-c, it was also thermodynamically feasible for the generation of Fe\u003csup\u003eIV\u003c/sup\u003e=O in the Fe-N\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e-C\u003csub\u003e10\u003c/sub\u003e/PMS system via the traditional pathway of *(I) \u0026rarr; *PMS (Ⅱ) \u0026rarr; *OH (Ⅲ) \u0026rarr; *OH*OH (Ⅳ) \u0026rarr; transition state (TS, Ⅴ) \u0026rarr; *O*H\u003csub\u003e2\u003c/sub\u003eO (Ⅵ) \u0026rarr; *O (Ⅶ).\u003csup\u003e\u003cspan citationid=\"CR38\" class=\"CitationRef\"\u003e38\u003c/span\u003e\u003c/sup\u003e Differently, the appropriate distance between O\u003csub\u003e1\u003c/sub\u003e and H of *PMS(Fe-N\u003csub\u003e4\u003c/sub\u003e-C\u003csub\u003e6\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e) and *PMS(Fe-N\u003csub\u003e4\u003c/sub\u003e-C\u003csub\u003e10\u003c/sub\u003e) facilitated the formation of intramolecular hydrogen bonds, and the interaction between Fe and O\u003csub\u003e3\u003c/sub\u003e weakened the O\u003csub\u003e2\u003c/sub\u003e-O\u003csub\u003e3\u003c/sub\u003e and O\u003csub\u003e3\u003c/sub\u003e-H bonds, thus favoring their cleavage to produce Fe\u003csup\u003eIV\u003c/sup\u003e=O through the non-classical coupled electron-proton transfer (CEPT) pathway (Figs.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003ed-\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003ee and Supplementary Fig. S11).\u003csup\u003e\u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e21\u003c/span\u003e\u003c/sup\u003e In this pathway, protons were easily transferred from O\u003csub\u003e3\u003c/sub\u003e to O\u003csub\u003e1\u003c/sub\u003e, and the two remaining electrons in the O\u003csub\u003e3\u003c/sub\u003e 2p orbital would be coupled with the two electrons of Fe to form the Fe\u0026thinsp;=\u0026thinsp;O bond. Notably, the distantly doped O and CVs promoted the PMS adsorption by lowering the adsorption energy from 0.14 eV for Fe-N\u003csub\u003e4\u003c/sub\u003e-C\u003csub\u003e10\u003c/sub\u003e to -0.05 eV for Fe-N\u003csub\u003e4\u003c/sub\u003e-C\u003csub\u003e6\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e, and affected the *PMS structure by providing a moderately enhanced local electric field, including elongated O\u003csub\u003e2\u003c/sub\u003e-O\u003csub\u003e3\u003c/sub\u003e (1.494 \u0026Aring; for Fe-N\u003csub\u003e4\u003c/sub\u003e-C\u003csub\u003e6\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e vs 1.480 \u0026Aring; for Fe-N\u003csub\u003e4\u003c/sub\u003e-C\u003csub\u003e10\u003c/sub\u003e) and O\u003csub\u003e3\u003c/sub\u003e-H (1.026 \u0026Aring; for Fe-N\u003csub\u003e4\u003c/sub\u003e-C\u003csub\u003e6\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e vs 1.017 \u0026Aring; for Fe-N\u003csub\u003e4\u003c/sub\u003e-C\u003csub\u003e10\u003c/sub\u003e) bonds, as well as a reduced distance between O\u003csub\u003e1\u003c/sub\u003e and H (1.650 \u0026Aring; for Fe-N\u003csub\u003e4\u003c/sub\u003e-C\u003csub\u003e6\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e vs 1.698 \u0026Aring; for Fe-N\u003csub\u003e4\u003c/sub\u003e-C\u003csub\u003e10\u003c/sub\u003e). These changes effectively reduced the energy barrier for the formation of Fe\u003csup\u003eIV\u003c/sup\u003e=O via the CETP pathway in the Fe-N\u003csub\u003e4\u003c/sub\u003e-C\u003csub\u003e6\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e/PMS system. Finally, the thermodynamic feasibility of radical formation in three Fe-SACs/PMS systems through the single electron transfer pathway was calculated, obeying the trend of Fe-N\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e-C\u003csub\u003e10\u003c/sub\u003e/PMS with Gibbs free energy (ΔG) of -1.21 eV\u0026thinsp;\u0026gt;\u0026thinsp;Fe-N\u003csub\u003e4\u003c/sub\u003e-C\u003csub\u003e10\u003c/sub\u003e/PMS with ΔG of -0.70 eV\u0026thinsp;\u0026gt;\u0026thinsp;Fe-N\u003csub\u003e4\u003c/sub\u003e-C\u003csub\u003e6\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e/PMS with ΔG of -0.41 eV (Supplementary Fig.\u0026nbsp;12). However, the significantly lower ΔG associated with the Fe\u003csup\u003eIV\u003c/sup\u003e=O formation (-2.94 eV for Fe-N\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e-C\u003csub\u003e10\u003c/sub\u003e, -2.31 eV for Fe-N\u003csub\u003e4\u003c/sub\u003e-C\u003csub\u003e6\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e, -2.34 eV for Fe-N\u003csub\u003e4\u003c/sub\u003e-C\u003csub\u003e10\u003c/sub\u003e) suggested that the Fe\u003csup\u003eIV\u003c/sup\u003e=O formation was much easier than that of radicals in three Fe-SACs/PMS systems.\u003c/p\u003e \u003cp\u003eWe then evaluated the PMS activation performance of Fe-SACs by the BPA removal (Supplementary Fig.\u0026nbsp;13), and found that the O doping significantly enhanced the activity of Fe-SACs and N/O-C, while Fe-N\u003csub\u003e4\u003c/sub\u003e-C\u003csub\u003e6\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e exhibited the best catalytic activity among the three Fe-SACs samples (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003ea). BPA could be completely removed in 30 s with a pseudo-first-order rate constant (\u003cem\u003ek\u003c/em\u003e) as high as 13.299 min\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e at 0.1 mg mL\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e of Fe-N\u003csub\u003e4\u003c/sub\u003e-C\u003csub\u003e6\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e and 400 \u0026micro;M of PMS (Supplementary Fig.\u0026nbsp;14). Impressively, the BPA removal performance of Fe-N\u003csub\u003e4\u003c/sub\u003e-C\u003csub\u003e6\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e even surpassed those of homogeneous Fe\u003csup\u003e2+\u003c/sup\u003e, commercial Fe\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e and Fe NPs/NC, nZVI/kaolinite, and other reported M-SACs (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003eb and Supplementary Fig.\u0026nbsp;15). More importantly, Fe-N\u003csub\u003e4\u003c/sub\u003e-C\u003csub\u003e6\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e displayed a robust pH compatibility and an exceptional resistance to the interference of inorganic ions and common water matrices (Supplementary Fig.\u0026nbsp;16), and the Fe-N\u003csub\u003e4\u003c/sub\u003e-C\u003csub\u003e6\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e/PMS system could also efficiently remove other pollutants (Supplementary Figs.\u0026nbsp;17\u0026ndash;19 and Table\u0026nbsp;4). Interestingly, electron-rich phenolic contaminants, such as 2,4-dichlorophenol (2,4-DCP), p-chlorophenol (4-CP), and phenol (PE), could be removed with significantly higher rates compared to contaminants with electron-withdrawing groups, such as p-nitrophenol (PNP), fipronil (FP), p-nitrobenzyl alcohol (PNBA), and p-nitrobenzaldehyde (PNBD) in the Fe-N\u003csub\u003e4\u003c/sub\u003e-C\u003csub\u003e6\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e/PMS system, which is a preliminary indication of the possible predominance of selective non-radical oxidations in this system\u003csup\u003e\u003cspan citationid=\"CR38\" class=\"CitationRef\"\u003e38\u003c/span\u003e\u003c/sup\u003e. Meanwhile, the indispensable role of atomically dispersed Fe sites for the effective PMS activation was confirmed by the much lower activity of N/O-C and N-C. Therefore, the specific activity of individual Fe site was further investigated, and the BPA turnover frequency (TOF) of Fe sites was in the order of Fe-N\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e-C\u003csub\u003e10\u003c/sub\u003e\u0026thinsp;\u0026gt;\u0026thinsp;Fe-N\u003csub\u003e4\u003c/sub\u003e-C\u003csub\u003e6\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e\u0026thinsp;\u0026gt;\u0026thinsp;Fe-N\u003csub\u003e4\u003c/sub\u003e-C\u003csub\u003e10\u003c/sub\u003e, which suggested that the O doping in the first coordination shell could enhance the activity of Fe sites more effectively than the O doping in the second coordination shell (Supplementary Fig.\u0026nbsp;20). Impressively, the overall apparent activity of Fe-N\u003csub\u003e4\u003c/sub\u003e-C\u003csub\u003e6\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e was surprisingly higher than that of Fe-N\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e-C\u003csub\u003e10\u003c/sub\u003e (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003ea). Regarding that the Fe content of Fe-N\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e-C\u003csub\u003e10\u003c/sub\u003e was much lower than that of Fe-N\u003csub\u003e4\u003c/sub\u003e-C\u003csub\u003e6\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e (Supplementary Table\u0026nbsp;5), we thus increased the Fe content of Fe-N\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e-C\u003csub\u003e10\u003c/sub\u003e by increasing the proportion of Fe precursors, which instead resulted in its significant metal agglomeration and catalytic reactivity decrease (Supplementary Fig.\u0026nbsp;21), suggesting that the O doping in the first coordination shell disfavored the stability of high-loading Fe single atoms. Subsequently, continuous catalytic BPA degradation experiments were carried out to examine the stability of Fe-SACs during the reaction (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003ec), which revealed the most robust stability of Fe-N\u003csub\u003e4\u003c/sub\u003e-C\u003csub\u003e6\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e among the three Fe-SACs, even though its BPA adsorption capacity was not as good as that of Fe-N\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e-C\u003csub\u003e10\u003c/sub\u003e (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003ed). After 120 h of continuous treatment, the iron leaching ratio of Fe-N\u003csub\u003e4\u003c/sub\u003e-C\u003csub\u003e6\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e was merely 8.0%, much lower than those of Fe-N\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e-C\u003csub\u003e10\u003c/sub\u003e (67.4%) and Fe-N\u003csub\u003e4\u003c/sub\u003e-C\u003csub\u003e10\u003c/sub\u003e (30.5%) (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003ee). Strikingly, the reactivity of Fe-N4-C\u003csub\u003e6\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e did not decline after 240 h of reaction and its structure kept almost unchanged even after 500 h of operation (Supplementary Figs.\u0026nbsp;22\u0026ndash;25). Considering the complexity of pH in different waters, we also monitored the iron leaching of three Fe-SACs under extreme acidic and basic conditions (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003ef), and found that the Fe leaching rate of Fe-N\u003csub\u003e4\u003c/sub\u003e-C\u003csub\u003e6\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e was even less than that of Fe-N\u003csub\u003e4\u003c/sub\u003e-C\u003csub\u003e10\u003c/sub\u003e at acidic pH and almost the same as that of Fe-N\u003csub\u003e4\u003c/sub\u003e-C\u003csub\u003e10\u003c/sub\u003e in alkaline solution. Additionally, the demetallation rate of Fe-N\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e-C\u003csub\u003e10\u003c/sub\u003e was much higher than that of Fe-N\u003csub\u003e4\u003c/sub\u003e-C\u003csub\u003e10\u003c/sub\u003e, with a factor of 4.5 or 99 under acidic or alkaline conditions. Therefore, the O doping in the first coordination shell significantly enhanced the activity of single-atom metal sites at the expense of their stability, but a well-balanced relationship between activity and stability of single-atom metal sites could be achieved by rationally designing the Fe-SAC coordination configuration with the O doping in the second coordination shell.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eRegarding that the demetallation of Fe-SACs initiates from the elongation and breakage of Fe-N/O bonds, MD simulations were conducted to explore the structure-dependent stability of Fe-SACs in terms of Fe-N/O bond length fluctuations. A relatively shorter Fe-N/O bond length and a narrower range of fluctuations indicate the stable Fe-N/O bonds of the catalyst with less demetallation tendency during reactions. As shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003eg, the radial distribution function (RDF) of Fe-N/O, denoted as g\u003csub\u003eFe\u0026minus;N/O\u003c/sub\u003e(r), was obtained by calculating and counting the frequency of the occurrence of N/O atoms at a distance r from Fe atom. At room temperature, the g\u003csub\u003eFe\u0026minus;N\u003c/sub\u003e(r) of Fe-N\u003csub\u003e4\u003c/sub\u003e-C\u003csub\u003e10\u003c/sub\u003e SAC contained four distinct peaks in the range of 0.875\u0026thinsp;~\u0026thinsp;3.175 \u0026Aring;. However, the fluctuations of N (1.775\u0026thinsp;~\u0026thinsp;3.525 \u0026Aring;) and O (1.775\u0026thinsp;~\u0026thinsp;3.175 \u0026Aring;) in Fe-N\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e-C\u003csub\u003e10\u003c/sub\u003e SAC were farther away from the Fe centre, which indicated that the destruction of its symmetric coordination structure weakened the Fe-N and Fe-O bonds, thereby increasing the demetallation tendency. Fascinatingly, the thermal motion amplitude of Fe-N bonds in Fe-N\u003csub\u003e4\u003c/sub\u003e-C\u003csub\u003e6\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e decreased to 0.925\u0026thinsp;~\u0026thinsp;2.975 \u0026Aring;, revealing its much stronger Fe-N bond to resist external perturbations. These results further illustrated that the O doping in the second coordination shell could reinforce the interaction between Fe single atoms and coordinating N atoms by reducing the electron density of Fe, effectively inhibiting the leaching of Fe atoms during the PMS activation.\u003c/p\u003e \u003cp\u003e \u003cb\u003eMechanism Investigation.\u003c/b\u003e We subsequently investigated the reactive species generated in the three Fe-SACs/PMS systems. The contribution of superoxide radicals (O\u003csub\u003e2\u003c/sub\u003e\u003csup\u003e\u0026bull;\u0026minus;\u003c/sup\u003e) to the BPA degradation was first excluded through the superoxide dismutase (SOD) quenching experiments and electron paramagnetic resonance (EPR) measurements (Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003ea and Supplementary Fig.\u0026nbsp;26a). The presence of methanol (MeOH) only slightly inhibited the BPA degradation in the three Fe-SACs/PMS systems, indicative of weak contribution of SO\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e\u0026bull;\u0026minus;\u003c/sup\u003e and \u0026bull;OH to the BPA degradation (Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003ea and Supplementary Table\u0026nbsp;6). When using 5,5-dimethyl-1-pyridone-N-oxide (DMPO) as a trapping agent, only a signal of DMPO oxides (DMPOX) was observed in the three Fe-SAC/PMS systems, which might be originated from oxidation of DMPO by Fe\u003csup\u003eIV\u003c/sup\u003e=O or other non-radical species, without the appearance of \u0026bull;OH and SO\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e\u0026bull;\u0026minus;\u003c/sup\u003e signals (Supplementary Figs.\u0026nbsp;26b-d). The above results revealed that non-radical oxidation pathways such as electron transfer process (ETP), Fe\u003csup\u003eIV\u003c/sup\u003e=O or singlet oxygen (\u003csup\u003e\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e\u003c/sup\u003eO\u003csub\u003e2\u003c/sub\u003e) oxidation mainly took place in these systems, while the inhibitory effect of MeOH may not be solely attributed to the radicals quenching, but to its competitive adsorption on Fe-SACs with BPA and/or its competitive reaction with other ROS, such as Fe\u003csup\u003eIV\u003c/sup\u003e=O\u003csup\u003e39\u003c/sup\u003e. The relatively weak inhibition effects of methyl phenyl sulfoxide (PMSO) on Fe-N\u003csub\u003e4\u003c/sub\u003e-C\u003csub\u003e10\u003c/sub\u003e/PMS (35.1%) and Fe-N\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e-C\u003csub\u003e10\u003c/sub\u003e/PMS (36.8%) systems suggested the dominant role of ETP or \u003csup\u003e\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e\u003c/sup\u003eO\u003csub\u003e2\u003c/sub\u003e oxidation process other than the Fe\u003csup\u003eIV\u003c/sup\u003e=O oxidation in these two systems. Surprisingly, PMSO was found to suppress 84.1% of BPA degradation in the Fe-N\u003csub\u003e4\u003c/sub\u003e-C\u003csub\u003e6\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e/PMS system, demonstrating that the doping of O in the second coordination shell could shift the BPA degradation pathway from the ETP or \u003csup\u003e\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e\u003c/sup\u003eO\u003csub\u003e2\u003c/sub\u003e oxidation process to the Fe\u003csup\u003eIV\u003c/sup\u003e=O counterpart. Given that the reactions of PMSO with \u0026bull;OH, SO\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e\u0026bull;\u0026minus;\u003c/sup\u003e and Fe\u003csup\u003eIV\u003c/sup\u003e=O would form different products (Eqs. S15-S17), where PMSO can be oxidized to methyl phenyl sulfone (PMSO\u003csub\u003e2\u003c/sub\u003e) through the oxygen atom transfer (OAT) pathway by Fe\u003csup\u003eIV\u003c/sup\u003e=O, we utilized the PMSO oxidation to further check the generation of radicals and Fe\u003csup\u003eIV\u003c/sup\u003e=O,\u003csup\u003e40\u003c/sup\u003e and observed that the amount of PMSO\u003csub\u003e2\u003c/sub\u003e generated in the Fe-N\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e-C\u003csub\u003e10\u003c/sub\u003e/PMS system (5.5 \u0026micro;M) was only 23% of that in Fe-N\u003csub\u003e4\u003c/sub\u003e-C\u003csub\u003e10\u003c/sub\u003e/PMS (24.4 \u0026micro;M) and 16% of that in Fe-N\u003csub\u003e4\u003c/sub\u003e-C\u003csub\u003e6\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e/PMS (34.4 \u0026micro;M) (Supplementary Fig.\u0026nbsp;27). Meanwhile, the conversion of PMSO\u003csub\u003e2\u003c/sub\u003e in the Fe-N\u003csub\u003e4\u003c/sub\u003e-C\u003csub\u003e10\u003c/sub\u003e/PMS and Fe-N\u003csub\u003e4\u003c/sub\u003e-C\u003csub\u003e6\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e/PMS systems was close to 100%, confirming that Fe\u003csup\u003eIV\u003c/sup\u003e=O was predominantly generated in these two systems rather than radicals. The efficient generation of Fe\u003csup\u003eIV\u003c/sup\u003e=O in the Fe-N\u003csub\u003e4\u003c/sub\u003e-C\u003csub\u003e6\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e/PMS and Fe-N\u003csub\u003e4\u003c/sub\u003e-C\u003csub\u003e10\u003c/sub\u003e/PMS systems was further confirmed by the EPR measurements (Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003eb).\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eSubsequently, we investigated the ETP and the \u003csup\u003e\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e\u003c/sup\u003eO\u003csub\u003e2\u003c/sub\u003e oxidation processes in the three Fe-SACs/PMS systems and observed EPR signals of 2,2,6,6-tetramethylpiperidine-N-oxyl (TEMPO) in these three systems. However, the replacement of H\u003csub\u003e2\u003c/sub\u003eO with D\u003csub\u003e2\u003c/sub\u003eO did not enhance the TEMPO signal (Supplementary Fig.\u0026nbsp;28), and the degradation rates of BPA and 2,4-DCP did not show a positive correlation with pH after excluding radicals and Fe\u003csup\u003eIV\u003c/sup\u003e=O interference (Supplementary Fig.\u0026nbsp;29). These results ruled out the generation of \u003csup\u003e\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e\u003c/sup\u003eO\u003csub\u003e2\u003c/sub\u003e.\u003csup\u003e\u003cspan citationid=\"CR29\" class=\"CitationRef\"\u003e29\u003c/span\u003e,\u003cspan citationid=\"CR41\" class=\"CitationRef\"\u003e41\u003c/span\u003e\u003c/sup\u003e Consequently, we conclude that the ETP process governed the BPA degradation in the three Fe-SACs/PMS systems, where Fe-SACs mediate the electron transfer from BPA to PMS,\u003csup\u003e\u003cspan citationid=\"CR42\" class=\"CitationRef\"\u003e42\u003c/span\u003e\u003c/sup\u003e, which was further confirmed through open circuit potential measurements (Supplementary Fig.\u0026nbsp;30). Furthermore, Fe-N\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e-C\u003csub\u003e10\u003c/sub\u003e and Fe-N\u003csub\u003e4\u003c/sub\u003e-C\u003csub\u003e6\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e possessed the stronger potential fluctuations than Fe-N\u003csub\u003e4\u003c/sub\u003e-C\u003csub\u003e10\u003c/sub\u003e after the addition of PMS and BPA, suggesting that the O doping in either the first or second coordination shell could promote the ETP. This is because the doped O could reduce the energy of the lowest unoccupied molecular orbital (LUMO) in the *PMS complexes by modulating the molecular orbital energy levels and electron distributions of the Fe-SACs, which narrowed the energy gap with the highest occupied molecular orbital (HOMO) of BPA, thus effectively facilitating the transfer of electrons from BPA to PMS (Supplementary Fig.\u0026nbsp;31).\u003c/p\u003e \u003cp\u003eWe then quantified the steady-state concentrations of different ROS through measuring the competition kinetics between ROS and probe compounds to deeply investigate the effect of O doping in the second coordination shell on the catalytic process and the Fe\u003csup\u003eIV\u003c/sup\u003e=O reactivity (Figs.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003ec-\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003ee, Supplementary Fig.\u0026nbsp;32, Tables\u0026nbsp;7\u0026ndash;8). As illustrated in Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003ec, the steady-state concentrations of Fe\u003csup\u003eIV\u003c/sup\u003e=O produced in the Fe-N\u003csub\u003e4\u003c/sub\u003e-C\u003csub\u003e10\u003c/sub\u003e/PMS and Fe-N\u003csub\u003e4\u003c/sub\u003e-C\u003csub\u003e6\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e/PMS system were 1.09 \u0026times; 10\u003csup\u003e\u0026minus;\u0026thinsp;6\u003c/sup\u003e and 1.71 \u0026times; 10\u003csup\u003e\u0026minus;\u0026thinsp;6\u003c/sup\u003e mol L\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e, respectively, which were 7\u0026ndash;8 orders of magnitude higher than that of \u0026bull;OH and SO\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e\u0026bull;\u0026minus;\u003c/sup\u003e, strongly validating that Fe-N\u003csub\u003e4\u003c/sub\u003e-C\u003csub\u003e10\u003c/sub\u003e/PMS and Fe-N\u003csub\u003e4\u003c/sub\u003e-C\u003csub\u003e6\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e/PMS were more likely to produce Fe\u003csup\u003eIV\u003c/sup\u003e=O species rather than radicals, consistent with theoretical predictions. However, the second-order reaction rate constant of BPA and Fe\u003csup\u003eIV\u003c/sup\u003e=O species produced in the Fe-N\u003csub\u003e4\u003c/sub\u003e-C\u003csub\u003e10\u003c/sub\u003e/PMS system was only 1.08 \u0026times; 10\u003csup\u003e3\u003c/sup\u003e M\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e s\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e, much lower than that of the radicals (1.37 \u0026times; 10\u003csup\u003e9\u003c/sup\u003e-1.7 \u0026times; 10\u003csup\u003e10\u003c/sup\u003e M\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e s\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e),\u003csup\u003e43\u0026ndash;45\u003c/sup\u003e rendering the contribution of Fe\u003csup\u003eIV\u003c/sup\u003e=O to the BPA oxidation to be only 8.4%, much lower than those of ETP (60.0%) and radical oxidation (31.6%) (Figs.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003ed-\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003ee and Supplementary Table\u0026nbsp;7). These facts suggested that the Fe\u003csup\u003eIV\u003c/sup\u003e=O species produced in the Fe-N\u003csub\u003e4\u003c/sub\u003e-C\u003csub\u003e10\u003c/sub\u003e/PMS system possessed low reactivity towards BPA. Surprisingly, the second-order reaction rate constant of BPA and Fe\u003csup\u003eIV\u003c/sup\u003e=O species produced in the Fe-N\u003csub\u003e4\u003c/sub\u003e-C\u003csub\u003e6\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e/PMS system increased to 4.6 \u0026times; 10\u003csup\u003e4\u003c/sup\u003e M\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e s\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e, 41.6-fold that of Fe-N\u003csub\u003e4\u003c/sub\u003e-C\u003csub\u003e10\u003c/sub\u003e/PMS (Figs.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003ec-\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003ee), suggesting that the O doping in the second coordination shell could significantly increase the reactivity of Fe\u003csup\u003eIV\u003c/sup\u003e=O towards BPA. Consequently, the contribution of Fe\u003csup\u003eIV\u003c/sup\u003e=O to the BPA removal sharply increased to 82.74% in the Fe-N\u003csub\u003e4\u003c/sub\u003e-C\u003csub\u003e6\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e/PMS system.\u003c/p\u003e \u003cp\u003eTheoretically, either π*(d\u003csub\u003exz/yz\u003c/sub\u003e-p\u003csub\u003ex/y\u003c/sub\u003e) or σ*(\u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\({d}_{{Z}^{2}}\\)\u003c/span\u003e\u003c/span\u003e-p\u003csub\u003ez\u003c/sub\u003e) antibonding orbitals at the Fe\u0026thinsp;=\u0026thinsp;O fragment in Fe\u003csup\u003eIV\u003c/sup\u003e=O could receive foreign electrons to disrupts the Fe\u0026thinsp;=\u0026thinsp;O bond, triggering the terminal O transfer and the substrate oxidation (Supplementary Fig.\u0026nbsp;33a). The M\u0026ouml;ssbauer spectroscopy indicated the O doping and the CVs generation could significantly increase the spin state of Fe-N\u003csub\u003e4\u003c/sub\u003e-C\u003csub\u003e6\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e (Supplementary Fig.\u0026nbsp;33b). According to the ligand field theory, the metal center typically possesses a high spin state when it is coordinated with a weak-field ligand and vice versa. Therefore, the enhanced spin state indicated that the O doping and the CVs generation could effectively weaken the strength of coordination field, resulting in a decrease in the energy of the Fe 3d orbitals to further reduce the orbital energy of Fe\u0026thinsp;=\u0026thinsp;O fragment in Fe\u003csup\u003eIV\u003c/sup\u003e=O, and improving the oxidative reactivity of Fe\u003csup\u003eIV\u003c/sup\u003e=O. This provides a new possibility for improving the activity of Fe\u003csup\u003eIV\u003c/sup\u003e=O. Subsequently, we employed DFT calculations to clarify the increased intrinsic reactivity of Fe\u003csup\u003eIV\u003c/sup\u003e=O produced in the Fe-N\u003csub\u003e4\u003c/sub\u003e-C\u003csub\u003e6\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e/PMS system. As shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003ef, the Fe\u003csup\u003eIV\u003c/sup\u003e=O produced in the Fe-N\u003csub\u003e4\u003c/sub\u003e-C\u003csub\u003e10\u003c/sub\u003e/PMS system possessed the π- and σ-attack pathways for the pollutants oxidation. Its Fe/O (d\u003csub\u003exz/yz\u003c/sub\u003e-p\u003csub\u003ex\u003c/sub\u003e/p\u003csub\u003ey\u003c/sub\u003e) π* (β) orbitals (-2.023 eV) and (\u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\({d}_{{z}^{2}}\\)\u003c/span\u003e\u003c/span\u003e-\u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\({p}_{z}\\)\u003c/span\u003e\u003c/span\u003e) σ* (α) orbitals (-1.986 eV) could accept electrons from pollutant substrates, thereby reducing the bond order of Fe\u0026thinsp;=\u0026thinsp;O and favoring the terminal O transfer. In the π-attack pathway, the pollutants needed to overlap orbitals from a direction perpendicular to the Fe\u0026thinsp;=\u0026thinsp;O bond, which resulted in a spatial collision between the pollutants and the equatorial ligands of Fe\u003csup\u003eIV\u003c/sup\u003e=O produced in the Fe-N\u003csub\u003e4\u003c/sub\u003e-C\u003csub\u003e10\u003c/sub\u003e/PMS system. In the σ-attack pathway, its (\u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\({d}_{{z}^{2}}\\)\u003c/span\u003e\u003c/span\u003e-\u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\({p}_{z}\\)\u003c/span\u003e\u003c/span\u003e) σ* (α) orbital had a high energy (-1.986 eV), requiring a considerable energy barrier to be overcome to initiate σ-attack. Overall, the two reaction pathways of Fe\u003csup\u003eIV\u003c/sup\u003e=O produced in the Fe-N\u003csub\u003e4\u003c/sub\u003e-C\u003csub\u003e10\u003c/sub\u003e/PMS system exhibited moderate reactivity towards pollutant. For the Fe\u003csup\u003eIV\u003c/sup\u003e=O produced in the Fe-N\u003csub\u003e4\u003c/sub\u003e-C\u003csub\u003e6\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e/PMS system, the orbital components became more complicate and the orbital energy levels were significantly lowered due to the doping of oxygen atoms and the decrease of ligand field intensity. Among them, the energies of its (\u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\({d}_{{z}^{2}}\\)\u003c/span\u003e\u003c/span\u003e-\u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\({p}_{z}\\)\u003c/span\u003e\u003c/span\u003e, \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\({d}_{{x}^{2}-{y}^{2}-}\\text{L}\\text{e}\\text{q}\\)\u003c/span\u003e\u003c/span\u003e) σ* (β) and (\u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\({d}_{{z}^{2}}\\)\u003c/span\u003e\u003c/span\u003e-\u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\({p}_{z}\\)\u003c/span\u003e\u003c/span\u003e, Leq) σ* (α) orbitals were respectively reduced to -4.527 eV and \u0026minus;\u0026thinsp;2.636 eV, significantly lower than that of its (d\u003csub\u003eyz\u003c/sub\u003e-p\u003csub\u003ey\u003c/sub\u003e) π* (β) orbital (-2.620 eV). As a result, the electrons of the pollutant could fill the low-lying σ* orbitals of Fe\u003csup\u003eIV\u003c/sup\u003e=O produced in the Fe-N\u003csub\u003e4\u003c/sub\u003e-C\u003csub\u003e6\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e/PMS system along the Z-axis without steric hindrance, which facilitated the formation of the pollutant-O-Fe collinear transition state, thereby accelerating the reaction between the pollutant with Fe\u003csup\u003eIV\u003c/sup\u003e=O.\u003c/p\u003e \u003cp\u003eFinally, we determined the main intermediates in the degradation process by chromatography-mass spectrometry (LC-MS) and gas chromatography-mass spectrometry (GC-MS) techniques (Supplementary Figs.\u0026nbsp;34\u0026ndash;37, Tables\u0026nbsp;9\u0026ndash;10), and proposed two potential BPA degradation pathways in the Fe-N\u003csub\u003e4\u003c/sub\u003e-C\u003csub\u003e6\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e/PMS system (Supplementary Fig.\u0026nbsp;38, details can be found in the Supporting Information). In these two pathways, BPA was oxidized by Fe\u003csup\u003eIV\u003c/sup\u003e=O through the OAT reaction, electrophilic addition, and H-abstraction/O-rebound mechanism (Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003e). The core of these three oxidation mechanisms was the filling of pollutant electrons into the low-lying σ* orbitals of Fe/O to form pollutant-O-Fe intermediates, thus realizing the oxidation of pollutant and the reduction of Fe\u003csup\u003eIV\u003c/sup\u003e=O. Notably, the decrease of total organic carbon (TOC) in BPA solution reached as high as 90% in 2 min when the concentration of Fe-N\u003csub\u003e4\u003c/sub\u003e-C\u003csub\u003e6\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e SAC was 0.1 g L\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e, and it was close to 80% in 2 min even when the catalyst concentration was reduced by half (Supplementary Fig.\u0026nbsp;39), which indicated that the Fe-N\u003csub\u003e4\u003c/sub\u003e-C\u003csub\u003e6\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e SAC exhibited a good performance in AOPs, which was able to cause the organic compounds to be degraded into harmless ones completely and reduce the impact on the environment.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e"},{"header":"Conclusion","content":"\u003cp\u003eIn summary, we have reported the first synthesis of Fe-N\u003csub\u003e4\u003c/sub\u003e-C\u003csub\u003e6\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e SACs by substituting C in the second coordination shell of Fe-N\u003csub\u003e4\u003c/sub\u003e sites with O and demonstrated these novel Fe-N\u003csub\u003e4\u003c/sub\u003e-C\u003csub\u003e6\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e sites could activate PMS more efficiently and stably by providing an enhanced localized electric field without destroying their symmetric coordination structure in the first coordination shell, thus achieving exceptional activity and unprecedented catalytic durability. The O doping in the second coordination shell could enhance the strength of the Fe-N bond by reducing the electron density of Fe center and weaken the amplitude of Fe-N bond during the PMS activation, therefore effectively preventing the demetallation of Fe-N\u003csub\u003e4\u003c/sub\u003e sites. More importantly, this O doping also lowered the energy of Fe\u0026thinsp;=\u0026thinsp;O σ* orbitals by weakening the coordination field to promote the electrophilic σ-attack of Fe\u003csup\u003eIV\u003c/sup\u003e=O towards BPA, thus greatly enhancing its degradation rate by a factor of 41.6. This work sheds light on the importance of second coordination shell doping on the ultrastability of Fe-N\u003csub\u003e4\u003c/sub\u003e SACs, and provides a novel strategy to design metal SACs with a trade-off between exceptional activity and long-term stability.\u003c/p\u003e"},{"header":"Methods","content":"\u003cp\u003e\u003cstrong\u003eData Availability.\u003c/strong\u003e All study data are included in the article and/or Supplementary Information.\u0026nbsp;For detailed information regarding materials and characterization, please refer to\u0026nbsp;Supplementary\u0026nbsp;Text 1. The synthesis of carriers and catalysts are included in Supplementary Text 2. Details of the degradation experiments and methods of analysis can be found in Supplementary Texts 3 and 4, respectively. The formulae for \u003cem\u003ek\u003c/em\u003e, TOF, \u003cem\u003ek\u003c/em\u003e-value and TOC are listed in Supplementary Text 5. Continuous flow experiments and preparation of electrodes are detailed in Supplementary Texts 6 and 7, respectively. See Supplementary Text 8 for quantification of ROS.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eTheoretical.\u003c/strong\u003e The theoretical calculations were performed using Gaussian 16\u0026nbsp;\u003csup\u003e47\u003c/sup\u003e software and Multiwfn 3.8 program\u0026nbsp;\u003csup\u003e48\u003c/sup\u003e on the basis of DFT. The dynamic simulations elucidating the material's dynamic behaviors were conducted through the application of Born−Oppenheimer molecular dynamics (BOMD) simulations, as implemented within the openly accessible CP2K/Quickstep package (SupplementaryText. 9).\u003c/p\u003e"},{"header":"Declarations","content":"\u003cp\u003eAcknowledgments\u003c/p\u003e\n\u003cp\u003eThis work was financially supported by the National Natural Science Foundation of China (U22A20402, 22076059, 22376076, 22076061,\u0026nbsp;21936003), international Joint Research Center for Intelligent Biosensing Technology and Health,\u0026nbsp;Shenzhen Science and Technology Program\u0026nbsp;(JCYJ20220818095601002), and the Fundamental Research Funds for the Central Universities (CCNU22JC014).\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eAuthor Contributions Statement\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eH. X. and L. Z. Z.\u0026nbsp;supervised the project and provided financial support. T. T. C. conceived and designed the experiments. G. B. Z. performed DFT calculations and its analysis; Y. T. H., S. Y., D. D. Z., H. X. D., Y. L. X., and S. H. H., provided experimental assistance; T. T. C., H. X., H. W. S., G. B. Z. and L. Z. Z. wrote the paper.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eCompeting Interest Statement\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe authors declare no competing interests.\u0026nbsp;\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\n\u003cli\u003eYao, Y.\u003cem\u003e et al.\u003c/em\u003e Rational regulation of Co-N-C coordination for high-efficiency generation of \u003csup\u003e1\u003c/sup\u003eO\u003csub\u003e2\u003c/sub\u003e toward nearly 100% selective degradation of organic pollutants. \u003cem\u003eEnviron. Sci. 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Chem.\u003c/em\u003e \u003cstrong\u003e33\u003c/strong\u003e, 580-592 (2012). \u003c/li\u003e\n\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":true,"hideJournal":false,"highlight":"","institution":"","isAcceptedByJournal":true,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"nature-portfolio","isNatureJournal":true,"hasQc":false,"allowDirectSubmit":false,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"","title":"Nature Portfolio","twitterHandle":"","acdcEnabled":false,"dfaEnabled":false,"editorialSystem":"ejp","reportingPortfolio":"","inReviewEnabled":true,"inReviewRevisionsEnabled":false},"keywords":"","lastPublishedDoi":"10.21203/rs.3.rs-4654905/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-4654905/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eThe local environment modulation of iron sites in Fe-N\u003csub\u003e4\u003c/sub\u003e single atom catalysts (SACs) plays a crucial role in the efficient peroxymonosulfate (PMS) activation. Many reported modulation strategies involve the partial replacement of N in the first coordination shell of Fe-N\u003csub\u003e4\u003c/sub\u003e sites with foreign elements to facilitate the PMS activation via disrupting the structural symmetry, suffering from undesired catalytic stability. Herein, we demonstrate that Fe-N\u003csub\u003e4\u003c/sub\u003e-C\u003csub\u003e6\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e sites, which are prepared by substituting C in the second coordination shell of Fe-N\u003csub\u003e4\u003c/sub\u003e sites with O, can activate PMS more efficiently and stably by providing an enhanced localized electric field without destroying their symmetric coordination structure in the first coordination shell, and thus achieve an unprecedented catalytic durability of at least 240 h. The O doping in the second coordination shell strengthened the Fe-N bond by reducing the electron density of Fe center, and weakened the amplitude of Fe-N bond from 0.875\u0026thinsp;~\u0026thinsp;3.175 \u0026Aring; to 0.925\u0026thinsp;~\u0026thinsp;2.975 \u0026Aring; during the PMS activation, therefore effectively prevented the demetallation of Fe-N\u003csub\u003e4\u003c/sub\u003e sites. Meanwhile, this O doping also lowered the energy of Fe\u0026thinsp;=\u0026thinsp;O σ* orbitals by weakening the coordination field to promote the electrophilic σ-attack of high-valent iron-oxo (Fe\u003csup\u003eIV\u003c/sup\u003e=O) towards electron-rich contaminants, thus enhancing the bisphenol A degradation rate from 1.08 \u0026times; 10\u003csup\u003e3\u003c/sup\u003e M\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e s\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e to 4.6 \u0026times; 10\u003csup\u003e4\u003c/sup\u003e M\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e s\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e by a factor of 41.6. This work sheds light on the importance of second coordination shell doping on the ultrastability of Fe-N\u003csub\u003e4\u003c/sub\u003e SACs, and provides a novel strategy to design metal SACs by balancing a trade-off between exceptional activity and long-term stability.\u003c/p\u003e","manuscriptTitle":"Ultrastable Fe-N4-C6O2 Single Atom Sites for Highly Efficient PMS Activation and Enhanced FeIV=O Reactivity","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2024-07-12 05:19:56","doi":"10.21203/rs.3.rs-4654905/v1","editorialEvents":[],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"nature-communications","isNatureJournal":true,"hasQc":false,"allowDirectSubmit":false,"externalIdentity":"NCOMMS","sideBox":"Learn more about [Nature Communications](http://www.nature.com/ncomms/)","snPcode":"","submissionUrl":"https://mts-ncomms.nature.com/","title":"Nature Communications","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"ejp","reportingPortfolio":"Nature Communications","inReviewEnabled":true,"inReviewRevisionsEnabled":false}}],"origin":"","ownerIdentity":"e5b03f29-b662-4ec1-8b21-83849d4a9ab4","owner":[],"postedDate":"July 12th, 2024","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"published-in-journal","subjectAreas":[{"id":34488831,"name":"Physical sciences/Materials science/Materials for energy and catalysis/Porous materials"},{"id":34488832,"name":"Earth and environmental sciences/Environmental sciences/Environmental chemistry/Pollution remediation"}],"tags":[],"updatedAt":"2025-03-11T07:07:13+00:00","versionOfRecord":{"articleIdentity":"rs-4654905","link":"https://doi.org/10.1038/s41467-025-57643-7","journal":{"identity":"nature-communications","isVorOnly":false,"title":"Nature Communications"},"publishedOn":"2025-03-10 04:00:00","publishedOnDateReadable":"March 10th, 2025"},"versionCreatedAt":"2024-07-12 05:19:56","video":"","vorDoi":"10.1038/s41467-025-57643-7","vorDoiUrl":"https://doi.org/10.1038/s41467-025-57643-7","workflowStages":[]},"version":"v1","identity":"rs-4654905","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-4654905","identity":"rs-4654905","version":["v1"]},"buildId":"8U1c8b4HqxoKbykW_rLl7","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}