Axial oxygen coordination drives spin-regulated electron transfer in single-atom Fe catalysts for selective pollutant transformation

Domain: biology
Published: 2026-03-30
Editorial outcome: major_revision

Abstract
Precise modulation of spin states of single-atom catalysts (SACs) offers a promising route to fine-tune peroxide activation behaviors and selectivity toward different oxidation pathways. Here, we report a spin-tunable Fe SAC composed of iron phthalocyanine (FePc) axially coordinated via oxygen bridges (-O-) onto annealed nanodiamond (AND), denoted as FePc-O-AND. The axial oxygen coordination induces a spin transition from high-spin (t 5e 3) to an intermediate-spin (t 4e 2) state. This transition generates an unoccupied Fe 3dz2 orbital that enables oriented electron transfer to peracetic acid (PAA) via hydroxyl oxygen coordination. In situ synchrotron-based Fourier-transform infrared spectroscopy (SR-FTIR) reveals a distinct PAA activation pathway involving inner-sphere complexation and a non-radical electron-transfer mechanism. As a result, the FePc-O-AND/PAA system drives non-radical electron-transfer pathway with a high reaction rate (2.11 min−1), selectively converting phenolic pollutants into high-molecular-weight polyphenolic products (n ≥ 5). Density functional theory (DFT) calculations reveal that axial oxygen coordination in FePc-O-AND enhances PAA adsorption energy (−0.89 eV) and induces a favorable inner-sphere interaction with the hydroxyl oxygen, thereby facilitating effective PAA activation. The FePc-O-AND/PAA system exhibits strong resistance to water matrix interferences and maintains high performance over 130 h of continuous-flow operation. These findings establish axial coordination-mediated spin-state regulation as a powerful strategy for engineering SACs for sustainable water purification and recycling of micropollutants.

Introduction
The growing global demand for affordable and clean water has spurred intensive research into advanced remediation technologies to address escalating pollution and resource scarcity challenges1-3. Among them, peracetic acid-based advanced oxidation processes (PAA-AOPs) have emerged as a promising and environmentally friendly technique, offering strong oxidizing power, low residual toxicity, and compatibility with biological treatments4,5. However, the practical implementation of PAA systems is limited by the absence of catalysts that are simultaneously efficient and durable for PAA activation to generate selective and oxidizing reactive species 6.

Single-atom catalysts (SACs), which feature isolated metal centers anchored on carbon supports, offer a promising avenue to address this challenge7, 8. The catalytic activity of SACs can be precisely tuned by engineering their electronic structure and local coordination environment, enabling exceptional reactivity and selectivity9-12. Among various SAC platforms, iron phthalocyanine (FePc) has attracted particular interest due to its well-defined FeN planar structure, redox versatility, and proven capability in activating peroxides via non-radical oxidation pathways13. However, the intrinsic D 4h symmetry of FePc constrains the electron density within the molecular plane, limiting axial orbital accessibility and suppressing out-of-plane interactions with oxidants in AOP systems, thereby enabling efficient substrate-catalyst coupling.

To address this limitation, axial coordination ligand decoration offers an effective strategy to tune SAC activity, as it directly modulates the electronic structure of SACs without altering the precursor materials or synthetic protocols10, 14-16. The accessible axial coordination sites of FePc molecules can be exploited through ligand engineering strategies to break electronic symmetry via controlled orbital hybridization17, 18. Among various approaches, surface functionalization of conductive carbon substrates with oxygen-containing groups enhances electron donation to FePc, facilitating the formation of FeN -O axial coordination bonds.18, 19 However, a key challenge is to establish stable and controlled axial coordination without altering the molecular backbone of FePc or introducing steric hindrance.

In this work, we address this challenge by anchoring FePc onto oxygen-functionalized annealed nanodiamonds (O-AND) to provide a tailored sp2/sp3-hybridization framework. The annealed nanodiamond (AND) substrate provides graphitic sp2 domains for π–π interactions with FePc, while chemical oxidation introduces abundant surface oxygen groups that serve as axial ligands. This dual-functional interface stabilizes a five-coordinate FeN -O structure, wherein an axial Fe–O bond complements the planar FeN core. A comprehensive set of techniques, including quenching experiments, electron paramagnetic resonance (EPR), electrochemical analysis, in situ synchrotron-based spectroscopy, and density functional theory (DFT) modeling, was employed to unravel the PAA activation mechanism and establish structure-activity relationships. Our findings reveal that axial oxygen coordination regulates the Fe center’s spin state and electron density, thereby facilitating PAA activation and enabling selective and sustained polymerization of phenolic pollutants via a non-radical electron transfer pathway. This study provides molecular-level insights into interfacial coordination engineering for designing efficient and robust SACs for sustainable water treatment.

Results
Synthesis and Characterization of Fe-SACs

A single-atom Fe catalyst (denoted as FePc-O-AND) was synthesized via a multi-step strategy involving thermal annealing, chemical oxidation, and axial coordination, as depicted in Fig. 1a. Initially, fresh nanodiamonds with sp3 carbon framework were calcined at high temperature (1100°C, argon atmosphere) to obtain ANDs with sp2/sp3 hybridization. Subsequently, AND was oxidized with nitric acid (HNO ) to introduce abundant oxygen-containing functional groups, affording oxygen-functionalized nanodiamonds (O-AND). Finally, iron phthalocyanine (FePc) molecules were anchored onto O-AND to fabricate FePc-O-AND catalyst by a mechanochemical grinding method. For comparison, the FePc-AND catalyst was prepared using the same procedure without the oxidation step.

The structural integrity of FePc was largely preserved after immobilization onto nanodiamond substrates, as confirmed by consistent characteristic signals in the X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), Raman, and near-edge X-ray absorption fine structure (NEXAFS) spectra. In the XRD patterns (Supplementary Fig. 1), the typical (100) diffraction peak of FePc (JCPDS No. 14-0926) initially appears around 7.1° and remains clearly identifiable in both FePc-AND and FePc-O-AND, albeit with slight leftward shifts to 7.0° and 6.9°, respectively. These shifts are attributed to partial lattice compression of FePc upon anchoring onto sp2 carbon surfaces, while the presence of surface oxygen functionalities in FePc-O-AND serves to buffer or relieve the interfacial strain. Additionally, XRD patterns of a series of transition metal phthalocyanine (TMPc) catalysts confirm that all TMPc-O-AND samples retain the crystalline features of their respective reference TMPc molecules (Supplementary Fig. 2a), confirming preservation of molecular-structure integrity after immobilization19. Notably, increasing FePc loading intensifies the characteristic peaks of FePc while diminishing those of O-AND (Supplementary Fig. 2b), implying potential molecular stacking at high loading levels, which may hinder interaction with the support.

Moreover, the FTIR characteristic C–N (~1120 and 1080 cm−1) and C–H (720 cm−1) vibrational bands of FeTPc are preserved in both FePc-AND and FePc-O-AND (Supplementary Fig. 3). However, distinct blueshift patterns are observed between the two composites. Specifically, the C–H vibrational band exhibits a more pronounced blueshift in FePc-AND, suggesting stronger π–π interactions between the FePc macrocycle and the sp2-hybridized carbon surface20. In contrast, the C–N band shows a greater blueshift in FePc-O-AND, which is indicative of perturbations in the conjugated phthalocyanine ring, likely arising from synergistic effects of axial Fe–O coordination and π–π interactions. Consistent with the FTIR results, Raman spectra (Supplementary Fig. 4) reveal a notable blueshift in C–N skeletal vibrations, particularly for FePc-O-AND, further supporting the presence of interfacial perturbation21-23.

NEXAFS spectroscopy was employed to verify the structural integrity of the FePc macrocycle upon anchoring. As shown in Supplementary Fig. 5, a notable spectral evolution is observed in the C K-edge. Specifically, the peak at 287.2 eV, assigned to σ* (C–H) transitions and serving as a fingerprint of the intermolecular π-π stacking in bulk FePc24, vanishes in the FePc-O-AND spectrum. This spectral quenching confirms that the FePc aggregates are effectively de-aggregated, resulting in a monolayer dispersion on the O-AND surface. Crucially, apart from this stacking-related feature, the other dominant resonances in both the C and N K-edges remain energetically stable25, exhibiting no significant redshift or blueshift. This spectral consistency suggests that the macrocyclic skeleton is preserved, effectively ruling out Fe–N bond cleavage or destructive structural distortion during the loading process.

Scanning electron microscopy (SEM, Supplementary Figs. 6 and 7a) revealed that both FePc-O-AND and FePc-AND retained the spherical cluster configuration of pristine nanodiamonds. High-resolution transmission electron microscopy (HRTEM, Figs. 1b-d) further confirmed the preservation of the onion-like graphitic structure, with nanoparticles exhibiting an average diameter of 5.0 ± 0.2 nm (Supplementary Fig. 8) and well-defined lattice fringes corresponding to the (002) planes of graphite (d = 0.338 nm, Fig. 1c), indicating the unique sp2/sp3 hybridized carbon framework26. Notably, no metal nanoparticles or clusters were observed, suggesting atomic dispersion of Fe species on the O-AND support15. Furthermore, aberration-corrected high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM, Fig. 1d) provided direct evidence of this atomic dispersion, as the discrete bright spots corresponding to individual Fe atoms were clearly identified in FePc-O-AND27, 28. Energy-dispersive X-ray (EDX, Fig. 1e) mapping images further confirmed the uniform distribution of C, N, O, and Fe in the FePc-O-AND catalyst.

In comparison, FePc-AND exhibited markedly fewer atomic Fe signals (Supplementary Fig. 9), which aligns with the lower Fe loading determined by inductively coupled plasma optical emission spectrometry (ICP-OES) (0.91 wt% for FePc-O-AND vs. 0.42 wt% for FePc-AND, Supplementary Fig. 10). Notably, this disparity arises despite the comparable specific surface areas of NAND and O-AND (Supplementary Fig. 11), highlighting the critical role of surface oxygen groups in O-AND in regulating FePc immobilization. Surface oxygen-containing functional groups were quantified using the standard Boehm titration method.29, 30 Prior to anchoring, the surface phenolic –OH concentration on OA-AND was 801.66 mmol/kg (Supplementary Fig. 12). After FePc anchoring, this value decreased to 613.28 mmol/kg, representing 23.5% reduction. Notably, this decrement (Δ = 188.38 mmol/kg) aligns closely with the molar concentration of Fe introduced (0.91 wt% Fe = 162.95 mmol/kg). This quantitative correlation suggests an approximate 1:1 molar stoichiometry between the anchored Fe atoms and the consumed phenolic –OH sites, providing solid evidence that the formation of axial Fe-O coordination is the primary anchoring mechanism. Moreover, nitrogen adsorption-desorption analysis revealed opposite trends in surface area after FePc anchoring. For FePc-AND, the surface area is significantly decreased to 172 m2 g−1, compared with 226 m2 g−1 for AND (Supplementary Fig. 11), indicating dense molecular stacking driven by strong planar π–π interactions, consistent with observations from XRD and FTIR analyses. In contrast, FePc-O-AND exhibited a slight increase in surface area (264 m2 g−1, compared with 242 m2 g−1 of O-AND), likely due to axial Fe–O coordination that spatially lifts FePc molecules from the support surface, suppressing aggregation and enhancing site accessibility. These findings indicate that axial Fe–O coordination not only facilitates higher FePc loading but also promotes active site accessibility and dispersion.

Synchrotron radiation X-ray absorption spectroscopy (XAS) provided atomic-level insights into the coordination environment of Fe sites in the catalysts. X-ray absorption near edge structure (XANES, Fig. 1f) reveals a characteristic pre-edge peak at ~7117 eV for both FePc-AND and FePc-O-AND, confirming preserved square-planar Fe-N 4 coordination. The white line intensity and absorption edge position fall between those of FePc (Fe2+) and Fe2O3 (Fe3+), suggesting intermediate oxidation states between +2 and +3. Notably, a positive edge shift in FePc-O-AND suggests its slightly higher oxidation state than FePc-AND. Fourier transformed extend X-ray absorption fine structure (FT-EXAFS) analysis provides further structural detail. As shown in Fig. 1g, FePc-O-AND presents a primary peak at ~1.44 Å, which lies between typical Fe−O (~1.42 Å, Fe2O3) and the Fe−N (~1.47 Å, FePc) bond lengths, suggesting the coexistence of Fe–O and Fe–N coordination31, 32. In contrast, FePc-AND and pristine FePc shared a similar peak at ~1.42 Å, indicating dominant Fe–N coordination. Quantitative EXAFS fitting further supports these findings (Fig. 1h and Supplementary Table 1). Regarding the fitting models, Fe−N−C multiple scattering paths were explicitly included for the rigid FePc-AND reference. In contrast, for FePc-O-AND, the increased local disorder induced by axial oxygen coordination dampens these long-range signals, underscoring the need for a fitting model that prioritizes the primary Fe−N and Fe−O shells. Specifically, FePc-AND shows an Fe−N coordination number of 4.14 ± 0.45, consistent with a typical FeN 4 structure. In comparison, FePc-O-AND features a similar Fe–N coordination (4.12 ± 0.26), along with an additional Fe–O coordination (1.27 ± 0.09), confirming the presence of an axial oxygen ligand and the formation of a featured five-coordinate FeN 4–O structure. The slightly elevated total coordination number and the enhanced FT-EXAFS peak intensity in FePc-O-AND further validate this assignment. Moreover, the k3-weighted wavelet transforms (WT) EXAFS analysis (Fig. 1i and Supplementary Fig. 13) shows that FePc-O-AND displays a single intensity maximum centered in R-space, similar to FePc and FePc-AND, but with a slightly broader distribution, due to the overlapping contributions of Fe–N and Fe–O. Notably, no Fe–Fe scattering signals are observed in either FePc-O-AND or FePc-AND, ruling out the presence of Fe clusters and confirming atomic dispersion27. Thus, these findings provide evidence for the formation of a FeN 4–O configuration in FePc-O-AND, where the square-planar FeN 4 unit is stabilized by the axial oxygen ligand.

To probe the local chemical bonding environment in the catalysts, synchrotron-based NEXAFS measurements were conducted at Fe L-edge, as well as the N and O K-edge. The Fe L-edge spectra consist of two spin-orbit split regions: the L 3 edge (706−712 eV) corresponding to 2p → 3d transitions and the L 2 edge (718−726 eV) corresponding to 2p → 3d transitions33. These features are highly sensitive to both the oxidation and spin states of the Fe centers. In the L 3 region, FePc-O-AND exhibits two distinct peaks at ~708.2 and ~709.5 eV, attributed to Fe2+ and Fe3+ species, respectively, indicating a mixed-valence state (Supplementary Fig. 14a). Semi-quantitative peak deconvolution of the L-edge spectra reveals a Fe2+/Fe3+ ratio that closely aligns with that obtained from X-ray photoelectron spectroscopy (XPS) analysis. Specifically, the Fe2+/Fe3+ ratio in FePc-O-AND is 1.30 (Supplementary Fig. 15c), which is higher than that in FePc-AND (Supplementary Fig. 15d), suggesting that axial Fe–O coordination stabilizes the lower-valent Fe2+ state by facilitating electron delocalization through the oxygen ligand. At the O K-edge (Supplementary Fig. 14b), FePc-O-AND exhibits a distinct feature at 531.8 eV, attributed to transitions from the O 1s level to unoccupied states of O 2p orbitals hybridized with Fe 3d states34, 35. Although this energy region may also induce contributions from π* transitions associated with C–O–C groups24, the pronounced enhancement observed in FePc-O-AND suggests the dominant role of Fe–O coordination. Additionally, the broader feature centered around 540 eV is assigned to σ*(C–O) transitions from oxygen-containing functional groups. In parallel, the O 1s XPS spectrum (Supplementary Fig. 16a) also shows a distinct Fe–O peak centered at ~530.4 eV and enhanced C–O–C signals, further validating the formation of Fe–O bonding. In the N K-edge spectrum, FePc-O-AND displays enhanced absorption associated with graphitic and pyrrolic N species (Supplementary Fig. 14c), while signals from pyridinic N and Fe–N coordination are diminished compared to FePc-AND36. These observations suggest that the introduction of oxygen disrupts the original Fe–N coordination and promotes a more delocalized nitrogen configuration within the phthalocyanine ring. Consistently, the N 1s spectra indicate that FePc-O-AND presents higher proportions of graphitic and pyrrolic N, whereas FePc-AND is dominated by pyridinic N and Fe–N coordination (Supplementary Fig. 16b). Moreover, a quantitative analysis based on XPS results was performed to further validate the stoichiometry of the axial coordination. Due to the limited sensitivity of the Fe 2p signal, the Fe concentration was indirectly deduced from the N 1s concentration. Given the preserved FePc stoichiometry (Fe : N = 1: 8), the surface Fe concentration was calculated to be 0.30 at% based on the measured total N 1s signal (2.39 at%, Supplementary Fig. 16b). This value shows excellent agreement with the concentration of the specific axial Fe−O species (0.29 at%) derived from O 1s deconvolution (Supplementary Fig. 16a), yielding a Fe:O ratio of approximately 1:1 (0.30 at% Fe vs. 0.29 at% Fe−O). This quantitative consistency further corroborates the mono-axial coordination model proposed by EXAFS.

In summary, these findings confirm that FePc was successfully immobilized onto nanodiamond substrates, resulting in distinct, well-defined interfacial environments. The introduction of surface oxygen functionalities enables axial Fe–O coordination, which not only enhances Fe anchoring efficiency and atomic dispersion but also modulates the Fe valence state and coordination geometry, forming a stable FeN 4–O configuration. This five-coordinate structure facilitates electron delocalization, stabilizes Fe2+ species, and improves molecular dispersion, which underpins the superior catalytic performance of FePc-O-AND.

Performance evaluation and reactive species generation. The PAA activation performance of Fe-SACs was evaluated using 4-chlorophenol (4-CP) as a model pollutant. Observed degradation rate constants (k obs) were derived from pseudo-first-order kinetic fitting. Among the tested catalysts, FePc-O-AND/PAA exhibited exceptional performance (k obs = 2.11 min−1) (Supplementary Fig. 17). In contrast, negligible 4-CP removal was observed in control experiments by catalyst adsorption (Supplementary Fig. 18) or catalyst-free oxidant decomposition, indicating that pollutant removal primarily originated from catalytic oxidation. Moreover, control experiments were conducted to investigate the influence of residual precursor compounds, including 0.4 mM H 2O 2 and 1.5 mM CH 3COOH, that are typically co-present in commercial PAA solutions. Using either H 2O 2 or CH 3COOH alone as the oxidant resulted in no observable 4-CP removal (Supplementary Fig. 18). This confirms that these co-existing H 2O 2 and CH 3COOH make negligible contributions to the overall catalytic oxidation process. Compared to FePc (k obs = 0.21 min−1) and FePc-AND (k obs = 0.77 min−1, Supplementary Fig. 17), FePc-O-AND exhibited a 10-fold and nearly 3-fold enhancement, respectively, highlighting the pivotal role of axial Fe–O coordination in boosting catalytic efficiency in PAA activation. Moreover, FePc-O-AND outperformed a range of state-of-the-art heterogeneous materials, including metal, carbon, metal-carbon composites, and other SACs based on a normalized kinetic rate constant (k-value, Fig. 2a and Supplementary Table 2).

Moreover, the effects of FePc precursor loading and central metal identity were systematically investigated to further optimize the catalytic performance. As shown in Supplementary Fig. 19a, increasing the FePc precursor loading from 0 to 0.01 g led to a progressive rise in activity, peaking at 0.01 g. Further increases resulted in declined performances, likely due to FePc aggregation and reduced dispersion. A similar volcano-like trend was observed in the turnover frequency (TOF, defined as the 4-CP removal rate per Fe atom), with FePc-O-AND at 0.01 g exhibiting the highest TOF (0.62 min−1, Supplementary Fig. 20). In addition, replacing Fe with other transition metals (e.g., Co, Cu, Mn) resulted in markedly reduced activity for PAA activation (Supplementary Fig. 19b), confirming the superior intrinsic activity of Fe centers with axial coordination.

Given the markedly enhanced catalytic activity of FePc-O-AND, the underlying mechanism was further investigated, with particular emphasis on identifying the reactive species involved in PAA activation. As shown in Fig. 2b and Supplementary Figs. 21 and 22, Electron paramagnetic resonance (EPR) spectroscopy revealed no characteristic signals using 5,5-dimethyl-1-pyrroline-N-oxide (DMPO) (in both H 2O and MeOH) or 2,2,6,6-tetramethyl-4-piperidinol (TEMP), indicating the absence of free radicals including organic radicals (CH 3COOO•, CH 3COO•, and •CH 3), hydroxyl radicals (•OH), superoxide radicals (O 2•−), and singlet oxygen (1O 2). Consistently, the introduction of their corresponding scavengers, including MeOH (for free organic radicals and •OH), CHCl 3 (for O 2•−), TEMP (for 1O 2), as well as the molecular probe PMSO (for high-valent Fe species), resulted in negligible inhibition of 4-CP oxidation (Fig. 2c and Supplementary Fig. 23). To further rule out the involvement of 1O 2, a solvent isotope experiment was conducted based on its longer lifetime in D 2O (22 to 70 μs) compared to H 2O (2.9 to 4.6 μs) 37. No enhancement in k obs was observed in D 2O (Supplementary Fig. 24), further confirming the absence of 1O 2 in the FePc-O-AND/PAA system. A previous study38 reported that FeN 4 sites can activate PAA to produce high-valent Fe species via electron transfer and O–O bond cleavage, as evidenced by selective methyl phenyl sulfoxide (PMSO) oxidation to the featured product of phenyl sulfone (PMSO 2) via oxygen atom transfer reaction of FeIV=O 6. However, the addition of PMSO at a range of concentrations (0.05, 0.1, 0.5, and 1.0 mM) did not suppress 4-CP removal, and the yield of PMSO 2 remained below 40% in all cases (Supplementary Fig. 25), revealing that high-valent iron species were not generated. Collectively, these results indicate that PAA activation in both FePc-O-AND and FePc-AND proceeds primarily via other non-radical oxidation mechanisms, rather than through radical, singlet oxygen, or high-valent metal-oxo pathways.

To explore the electron transfer process (ETP), in situ Raman spectroscopy was performed to probe the formation of reactive intermediates during PAA activation. As shown in Fig. 2f, PAA alone exhibited a characteristic O–O stretching peak at 876 cm−1 39, 40. Upon the introduction of FePc-O-AND, a distinct new peak emerged at 889 cm−1, which is assigned to the O–O stretching vibration of the adsorbed peroxy species, corroborating the formation of the metastable active catalyst-PAA complex (Cat-PAA*)41-43. The electron transfer behavior of the Cat-PAA* complex was then evaluated using open circuit potential (OCP) analysis. Upon PAA addition, the OCP of the FePc-O-AND-GCE rapidly increased and stabilized at approximately 0.91 V after 4600 s, whereas FePc-AND reached a lower equilibrium value of 0.81 V (Fig. 2d), indicating that axial Fe–O coordination facilitates the formation of a more oxidizing catalyst surface. Subsequent addition of 4-CP led to a sharp decline in OCP in both systems, suggesting the extraction of electrons from the pollutant to Cat-PAA* via ETP. In parallel, a significant current drop following PAA addition in time-current analysis (Supplementary Fig. 26), implying the occurrence of electron transfer from the catalyst to PAA. To investigate if ETP proceeds via direct pollutant-oxidant contact or catalyst-mediated long-range electron transfer, a dual-chamber galvanic cell separated by a proton exchange membrane (PEM) was constructed (the inset of Fig. 2e). When FePc-O-AND was employed as the mediator, a current of 35 μA was generated (Fig. 2e), along with 48% 4-CP removal (Supplementary Fig. 27). In contrast, FePc-AND produced only 20 μA and 13% degradation, while bare electrodes yielded negligible activity. These results strongly support that the catalyst-mediated ETP regime is the primary driving force for pollutant degradation in both FePc-O-AND/PAA and FePc-AND/PAA systems.

The selective oxidation behavior of the Cat-PAA* complex was further examined across a series of organic pollutants with different molecular structures and thus distinct redox properties. As presented in Supplementary Fig. 28, the FePc-O-AND/PAA system rapidly removed electron-rich compounds such as bisphenol A (BPA), phenol (PE), and 4-chlorophenol (4-CP); while electron-deficient substrates, like benzoic acid, exhibited negligible removal (< 2% removal), demonstrating the featured selectivity of ETP for FePc-O-AND/PAA. To quantitatively evaluate this trend, lnk obs was plotted against the half-wave potential (φ 1/2) of each compound. A strong linear correlation was observed for the FePc-O-AND/PAA system (R2 = 0.939, slope = −10.01, Fig. 2g), demonstrating that the oxidation efficiency is affected by the electron-donating ability of the substrate under the ETP regime. In comparison, the FePc-AND/PAA system showed a less steep slope (R2 = 0.892, slope = −6.18, Fig. 2h), suggesting a weaker kinetic dependence on the substrate’s single-electron oxidation potential. In addition to substrate selectivity, oxidant utilization efficiency was also evaluated. As shown in Supplementary Fig. 29, the FePc-O-AND/PAA system exhibits a markedly lower PAA consumption rate (0.20 min−1) compared to the FePc-AND/PAA system (0.34 min−1). To quantitatively assess oxidant efficiency, the PAA utilization efficiency was defined as the molar ratio of pollutant removed to PAA consumed (Δ[4-CP]/Δ[PAA]). At 2 min, this value reached 0.26 for the FePc-O-AND system, substantially higher than that of FePc-AND (0.10), demonstrating a significantly improved oxidant economy in the FePc-O-AND/PAA system. This enhancement stems from the formation of surface-confined, metastable reactive complexes (catalyst-PAA*) rather than free radicals. These complexes increase the redox potential of the FePc-O-AND, enabling efficient electron abstraction from electron-rich pollutants and thereby promoting near-complete PAA conversion into acetic acid.

To elucidate the oxidation mechanisms of 4-CP on different catalyst surfaces, surface-adsorbed oligomeric products were extracted using two complementary protocols. A methanol/toluene mixture was used for ultra-high performance liquid chromatography-quadrupole time-of-flight mass spectrometry (UPLC-QTOF-MS) detection, whereas tetrahydrofuran (THF) was selected to extract polymeric products, which were subsequently isolated via rotary evaporation prior to matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF-MS) analysis. The MALDI-TOF-MS spectrum revealed that the polymeric products possess a repeating unit with a mass of m/z ≈ 126.6 Da (Supplementary Fig. 30), in excellent agreement with the theoretical mass of the chlorophenoxyl moiety (ClPhO). This observation provides direct evidence for an oxidative coupling-driven polymerization pathway.

The generation of surface-bound chlorophenoxyl radicals (ClPhO•) was further corroborated using 2,2,6,6-tetramethyl piperidine-1-yl) oxyl (TEMPO) and ferulic acid (FA) as specific diagnostic probes. A pronounced and gradual attenuation of the TEMPO signal was observed, attributable to the coupling between TEMPO and the generated surface-bound ClPhO• species, yielding the diamagnetic TEMPO-OPhCl adduct (Supplementary Fig. 31)44. In parallel, quenching experiments employing FA, a selective scavenger for ClPhO•, showed an apparent concentration-dependent suppression of 4-CP degradation (Supplementary Fig. 32)44-46. These results provide compelling evidence that surface-bound ClPhO• species act as the pivotal intermediates driving the formation of polymeric products. The surface-confined nature of ClPhO• intermediates is further corroborated by the spatial distribution of the degradation products. The polymeric products were found exclusively associated with the catalyst surface and necessitated exhaustive solid-phase extraction for recovery, whereas the filtrate remained product-free. Moreover, such interfacial confinement maintains a high local concentration of ClPhO• at the catalyst surface, which kinetically favors ClPhO• self-coupling and subsequent polymerization. This mechanism effectively bypasses the bulk-phase dilution that would otherwise promote non-selective oxidation in the aqueous solution.

Notably, MALDI-TOF-MS spectra highlighted a distinct variation in product distribution (Supplementary Fig. 30): the polymeric species formed in the FePc-O-AND/PAA system extended to higher mass ranges (m/z ~ 900) than those in the FePc-AND/PAA system (m/z < 700). Subsequent UPLC-QTOF-MS analysis provided molecular insights into these polymeric structures . As shown in Supplementary Figs. 33-40, 4-CP underwent hydroxylation to generate species such as P143 without experiencing ring-opening reactions, oxidized by both FePc-O-AND-PAA* and FePc-AND-PAA*. In both systems, low-molecular-weight oligomers were observed on the catalyst surface, including dimers (e.g., P253), trimers (e.g., P379), and tetramers (e.g., P505). Thus, the surface-bound phenoxyl radicals formed through one-electron oxidation of 4-CP via ETP can undergo further coupling to form larger molecules. Strikingly, high-order polymers (n ≥ 5), such as pentamers to octamers (e.g., P631, P773, P899, Supplementary Fig. 33), were exclusively identified in the FePc-O-AND/PAA system. This suggests that axial Fe−O coordination enabled faster 4-CP oxidation or sustained electron abstraction from larger oligomer substrates, thereby accelerating the generation and accumulation of surface-bound phenoxyl radicals. These highly reactive intermediates undergo rapid intermolecular radical coupling, leading to further polymerization into larger-molecule polymeric products.45 The divergence in product distribution underscores the key role of Fe electronic structure in governing the oxidation-coupling behavior. Specifically, the FePc-O-AND/PAA* complex, with its higher oxidation potential and stronger inner-sphere electron transfer capability, promotes more effective oxidative radical propagation. In contrast, FePc-AND, lacking axial coordination, exhibits limited polymerization capacity, with maximal products consisting of four aromatic units (Supplementary Fig. 38).

The ecotoxicity of 4-CP and its degradation products toward fish, daphnia, and green algae was initially predicted using the ECONSAR program. The simulation indicated that the polymeric intermediates could exhibit higher theoretical toxicity than the parent compound (Supplementary Figs. 41, 42 and Supplementary Tables 3,4). However, under practical reaction conditions, these aggregated products primarily accumulate on the surface of the heterogeneous catalyst, effectively preventing their leaching and mitigating secondary water pollution. To validate this, toxicity assays were conducted using the post-reaction effluent, which represents a realistic matrix containing both the residual PAA and the soluble organic degradation products. The results demonstrate that the treated effluent does not exert inhibitory effects on E. coli (Supplementary Fig. 43 and Supplementary Table 5). Collectively, these findings confirm that the potentially toxic polymers remain immobilized and that the residual PAA concentration is reduced below the inhibitory threshold for bacterial growth, either due to rapid consumption or conversion into biodegradable organic carbon.

Molecular Interaction Between Catalyst and PAA. To elucidate the molecular interaction between Fe-SACs and reaction components, a series of site-blocking and coordination-interference experiments were conducted. Because KSCN was found to directly react with PAA (Supplementary Fig. 44), 1,10-phenanthroline was instead selected as a selective chelating agent to probe the accessibility and involvement of Fe active sites47. As displayed in Supplementary Fig. 45, a clear dose-dependent inhibition of 4-CP removal was observed upon introducing phenanthroline. This result confirms that the Fe species are chemically accessible and act as the indispensable active centers for PAA activation. In contrast, 1,10-phenanthroline had no observable effect on 4-CP adsorption (Supplementary Fig. 46a), implying that PAA and 4-CP bind to different sites on the catalyst. Further evidence was provided by a pre-mixing experiment: when 4-CP was pre-adsorbed before PAA addition, the degradation performance exhibited only a slight decrease in both the FePc-O-AND and FePc-AND systems (Supplementary Fig. 46b), indicating that PAA preferentially coordinates to the Fe site, regardless of the presence of 4-CP. These findings suggest that axial Fe–O coordination enhances both the stability and selectivity of PAA binding at the Fe center.

Having established that Fe centers are critical for PAA activation, we further investigated the coordination mode between PAA and Fe-SACs to distinguish between inner-sphere and outer-sphere electron transfer pathways. The nature of the interaction between the catalyst and the reactants was further identified by regulating the ionic strength under different concentrations of NaClO 4. An elevation in ionic strength markedly influences outer-sphere interactions, such as electrostatic bonding between the reactants and catalyst surface, whereas it exerts a negligible influence on inner-sphere complexation, which involves covalent bonding or partially covalent interactions. As shown in Fig. 2i, the FePc-O-AND/PAA system exhibited negligible changes in degradation efficiency across varying NaClO 4 concentrations, indicating that electron transfer proceeds via a robust inner-sphere coordination pathway. In contrast, the FePc-AND/PAA system displayed a marked decrease in reactivity with increasing NaClO 4 concentrations, suggesting an outer-sphere ETP because of the weaker electrostatic interactions. The observed ionic-strength sensitivity underscores the susceptibility of FePc-AND to perturbations in the electrical double layer by water background factors, whereas FePc-O-AND maintains stable catalytic behavior under high-salinity conditions (Supplementary Fig. 47). These results further validate that axial Fe–O coordination facilitates a more stable inner-sphere binding between catalyst and PAA, strengthening electronic coupling and enabling high selectivity and superior immunity to co-existing inorganic ions in complex water matrices.

In the FePc-O-AND system featuring axial oxygen coordination, the electronic properties of substituent groups on the phthalocyanine ring exert a significant influence on catalytic performance. As shown in Supplementary Fig. 48a, the electron-withdrawing sulfonic group (FePc-SO 3H-O-AND) yielded the highest 4-CP degradation rate constant (k obs = 2.89 min−1), whereas the electron-donating amidogen group (FePc-NH 2-O-AND) exhibited markedly lower activity (k obs = 0.64 min−1). This contrast is attributed to the fact that the electron-withdrawing group in phthalocyanine decreased the electron density at the Fe center, thereby enhancing its electrophilicity and enabling it to interact with PAA to form the PAA* complex. Conversely, the electron-donating group increases the electron density around Fe, diminishing its oxidative reactivity, which might be ascribed to a weaker radical-mediated process. Notably, in the absence of axial coordination (Supplementary Fig. 48b), the performance disparity among substituents becomes negligible, implying that the electron transfer primarily proceeds via an outer-sphere pathway with limited sensitivity to electronic modulation. These results underscore that axial oxygen coordination not only facilitate inner-sphere electron transfer between Fe and PAA but also provides a molecular platform for precise electronic tuning through in-plane ligand functionalization, offering a versatile strategy for precisely engineering high-performance single-atom catalysts.

Theoretical Calculation and Catalytic Mechanism. To unravel the insights into the regulation of axial oxygen to the catalytic behaviors of single-atom Fe centers, a combination of theoretical modeling and spectroscopic characterizations was conducted. Based on EXAFS fitting and XPS analysis, structural models of FePc, FePc-O-AND (FeN 4-O) and FePc-AND (FeN 4) were constructed and geometrically optimized (Supplementary Fig. 49). The formation enthalpy (ΔH) of FePc-O-AND (–2.60 eV) is significantly lower than that of FePc-AND (–1.93 eV), indicating that axial oxygen coordination leads to a thermodynamically more stable configuration.19 Furthermore, the dissociation energy (E diss) of Fe-O bond was calculated to be 1.04 eV (Supplementary Table 6), which suggests a moderately strong and stable Fe-O interaction. This energy magnitude indicates a robust coordination interaction, confirming the thermal and structural stability of the axial oxygen ligand during the catalytic process. In FePc-AND, electrons are primarily transferred from the sp²-hybridized carbon support to the Fe center via the conjugated phthalocyanine macrocycle, resulting in a symmetric charge accumulation around the FeN 4 site (1.80 e−, Fig. 3a). In contrast, FePc-O-AND exhibits a distinct dual-channel electron transfer process. While 1.26 e− is still delivered from the carbon support through the phthalocyanine ring, an additional 0.55 e− is migrated from the Fe center to the axially coordinated oxygen ligand due to its stronger electronegativity (Fig. 3b). This electron delocalization from Fe to O induces more spatially polarized charge distribution, breaking the in-plain symmetry of the FeN 4 center and reinforcing the covalent Fe–O interaction, which in turn enhances PAA adsorption through improved electronic and geometric compatibility. Bader charge analysis further confirms that the Fe center in FePc-O-AND has a lower electron density (6.69 e−), compared to FePc-AND (6.88 e−) and FePc (6.83 e−, Supplementary Table 7), reflecting electron withdrawal by the electronegative axial oxygen. This redistribution is accompanied by a slight elongation of the Fe–N bond in FePc-O-AND (~1.96 Å vs. ~1.94 Å in FePc), whereas FePc-AND retains the same bond length as pristine FePc (Supplementary Table 8). Moreover, projected density of states (PDOS) further reveals strong orbital hybridization between Fe 3d and O 2p orbitals near the Fermi level (−1.0 to −0.4 eV, Fig. 3c), confirming covalent Fe–O bonding. Axial O coordination also perturbs the 3d orbital occupation and spin state of the Fe center48-50. As shown in Fig. 3d, FePc-AND exhibits a t 2g 5e g 3 configuration, in which all 3d orbitals are occupied, including the d z 2, limiting the Fe site’s ability to engage in axial coordination or out-of-plane electron transfer51, 52. In contrast, FePc-O-AND adopts a t 2g 4e g 2 configuration with an unoccupied d z 2 orbital, providing an axial electron-accepting channel for the hydroxyl oxygen of PAA to enhance axial coordination, charge transfer, and catalytic activation to form the non-radical complex.

To validate spin state variations, EPR spectroscopy, 57Fe Mössbauer spectroscopy, and zero-field cooling (ZFC) magnetic susceptibility (χ m) measurements were conducted53. EPR spectra (g = 2.002) revealed signal intensities in the order of FePc-AND > FePc-O-AND > FePc (Supplementary Fig. 50), consistent with the number of unpaired electrons and spin multiplicity. ZFC results further supported this trend54-56. 57Fe Mössbauer spectroscopy elucidated the spin state transitions based on isomer shift (IS) and quadrupole splitting (QS) parameters. As shown in Fig. 3e and Supplementary Table 9, the pristine FePc displays a typical low-spin (LS) Fe(II) doublet (IS = 0.16 mm·s−1, QS = 2.41 mm·s−1) 57, 58. Upon anchoring on AND (FePc-AND), the strong support-induced π-π interaction shifts the center to a high-spin (HS) Fe(III) state (IS = 0.17 mm·s−1, QS = 0.93 mm·s−1)59. Notably, the FePc-O-AND system exhibits a more complex spectrum deconvoluted into three distinct components corresponding to medium-spin (MS, 21.06%), high-spin (58.33%), and low-spin (20.61%) species. This distribution arises because the synergistic interplay between the axial Fe-O coordination and the basal π-π interactions, which together stabilize a substantial fraction of MS Fe centers. The MS configuration is electronically labile, enabling a dynamic redistribution among the MS, HS, and LS states. This mechanism provides a compelling explanation for the observed mixed Fe3+/Fe2+ oxidation states and highlights the crucial role of spin-state dynamics in facilitating efficient electron transfer at the catalytic center. The effective magnetic moments (µ eff) of FePc, FePc-AND and FePc-O-AND were determined to be 0.01, 1.50, and 2.15 µB, respectively (Fig. 3f), corresponding to approximately 0, 1 and 2 unpaired d electrons. These values suggest a low-spin (LS) configuration for FePc, an intermediate-spin (IS) state for FePc-O-AND, and a higher spin state for FePc-AND. Fe L-edge XANES analysis confirmed this order, showing an increase in L 3/L 2 intensity ratio from FePc < FePc-O-AND < FePc-AND (Supplementary Fig. 51). Volcano-type correlations were observed between the lnk obs for 4-CP oxidation and L 3/L 2 intensity ratios (Fig. 3g) as well as e g/t 2g orbital occupancy ratios (Supplementary Figs. 52-54). FePc-O-AND, with respective values of 3.20 and 0.626 for these descriptors, demonstrates the highest catalytic efficiency.

To explore adsorption mechanisms, DFT calculations were performed to examine PAA coordination. PAA contains three distinct oxygen atoms: peroxy oxygen (O1), hydroxyl oxygen (O2), and carbonyl oxygen (O3), enabling different coordination modes: Type I (O1), Type II (O2), and Type III (O3) (Fig. 3h and Supplementary Fig. 55). FePc-O-AND favored hydroxyl oxygen coordination (Type II) with an adsorption energy of −0.89 eV, while FePc-AND preferred carbonyl oxygen binding (Type III, −0.53 eV). This difference confirms that the axial oxygen coordination not only altered the binding mode of PAA, but also strengthened catalyst-substrate interaction. In situ synchrotron-based Fourier-transform infrared spectroscopy (SR-FTIR) further confirmed activation pathway differences. Reference IR spectra of PAA, CH 3COOH, and H 2O 2 were first recorded to identify characteristic vibrational bands (Supplementary Fig. 56). Upon mixing PAA with FePc-O-AND, intensities at 1713, 1640, and 1371 cm−1 increased (Fig. 3i and Supplementary Fig. 57), accompanied by redshifts of the corresponding vibrational bands. These peaks are assigned to O–H and O–O vibrational modes of the activated PAA* species (Fig. 3j). In contrast, FePc-AND induced changes at 1758 and 1223 cm−1, related to C=O−O and C−O modes (Fig. 3k and Supplementary Fig. 58), suggesting a fundamentally different activation mechanism from that of FePc-O-AND. PDOS analysis further confirms strong orbital overlap between Fe 3d and O 2p orbitals in FePc-O-AND (Supplementary Figs. 59 and 60)50, 60. Geometrically, the interaction shows significant enhancement: the Fe–O bond length (PAA* to Fe) shortened from 2.45 Å (FePc-AND) to 2.30 Å (FePc-O-AND), while the O–O bond stretched slightly from 1.45 Å to 1.46 Å (Supplementary Fig. 61 and Supplementary Table 10). These changes reflect a stronger binding affinity and more effective activation of the peroxide bond. This surface-bound species acts as the critical intermediate driving the oxidation process via a direct, non-radical electron transfer mechanism, distinct from pathways involving conventional reactive species such as free radicals, singlet oxygen, or high-valent metal species. In summary, the axially coordinated O atom in FePc-O-AND dramatically alters the ligand field environment around the Fe center. This modulation optimizes the electronic structure to promote PAA activation, forming a surface-confined PAA* complex without decomposition via a non-radical inner-sphere pathway, ultimately enhancing catalytic performance.

Catalytic Stability and Environmental Applications. To assess the long-term stability and environmental adaptability of FePc-O-AND, a comprehensive evaluation was conducted involving batch cycling, complex matrix resistance, and continuous-flow operation. As shown in Fig. 4a, the FePc-O-AND/PAA system maintained > 90% 4-CP oxidation efficiency over four consecutive cycles, whereas FePc-AND experienced a sharp decline to below 50%, indicating rapid catalyst deactivation. This result highlights the crucial role of axial Fe–O coordination in enhancing catalyst durability under oxidative conditions, as it enables electron transfer from the AND support to retain the surface redox property of molecular single Fe catalysts. In terms of matrix tolerance, FePc-O-AND displayed exceptional resistance to various water constituents, including inorganic anions (e.g. Cl−, NaHCO 3), natural organic matter (HA), and variable pH environments (Fig. 4b and Supplementary Figs. 62 and 63). Even in real water matrices such as seawater, river water, lake water, secondary effluent, and tap water, FePc-O-AND retained high catalytic activity, underscoring its robustness and practical applicability. For long-term operational testing, FePc-O-AND was immobilized on cotton fiber and integrated into a continuous-flow reactor (Fig. 4c). The catalyst maintained 100% 4-CP removal for 130 h (Fig. 4d), treating over 10 L of simulated wastewater. In contrast, FePc-AND showed rapidly declined activity within 5 h. Importantly, Fe leaching (0.142 μg L−1) from FePc-O-AND remained well below the limit of the Chinese drinking water standard (300 μg L−1, GB 5749−2022), confirming the strong chemical stability of the single-atom Fe centers (Supplementary Fig. 64).

To unveil the structural origin of this stability, post-reaction characterizations were conducted. XRD patterns showed no significant change in crystallinity after cycling (Supplementary Fig. 65), and HAADF-STEM images confirmed the retention of atomically dispersed Fe species without aggregation (Supplementary Figs. 66 and 67), indicating structural robustness during repeated PAA activation. At the atomic level, in situ Fe K-edge XAS analysis revealed slight enhancement of the main XANES absorption peak after reaction (Supplementary Fig. 68) without a shift in the edge position, indicating preserved Fe oxidation state. Corresponding EXAFS spectra showed a reversible shift of the Fe–N/O peak from 1.43 Å to 1.46 Å upon PAA addition, which returned to 1.43 Å after the introduction of 4-CP (Supplementary Fig. 69). This reversible shift reflects dynamic coordination between Fe and PAA, and suggests that the Fe–O axial bond enables a reversible inner-sphere PAA activation pathway, and the surface-bound PAA* will be consumed for pollutant oxidation to restore the catalytic site, preserving the catalyst integrity. Moreover, sp3-carbon-rich pristine nanodiamonds (PND) were used to prepare FePc-O-PND to investigate the synergy between axial oxygen coordination and the support-dependent electronic interaction. FT-EXAFS analysis confirmed that intrinsic surface oxygen on PND induces the formation of axial Fe–O bonds (Supplementary Fig. 70), similar to FePc-O-AND. Interestingly, FePc-O-PND exhibited appreciable initial activity to FePc-O-AND (k obs = 0.87 min−1), confirming that axial oxygen coordination is the decisive factor for triggering the catalytic reaction. However, its cycling stability was significantly inferior (Supplementary Fig. 71). This sharp contrast is attributed to the lack of a conjugated sp2 carbon network on PND. In the optimal FePc-O-AND system, the graphitized AND support provides a strong π-π electronic coupling with the FePc planar ring. This interaction acts as a structural anchor, stabilizing the active center against leaching or demetalation. Thus, the superior performance of FePc-O-AND arises from the synergy of axial oxygen coordination and FePc-AND π-π interactions, which promote both intrinsic activity and stability during PAA activation.

Methods
Chemicals and Reagents. Reagents used in this study are provided in the Supporting Information.

Catalyst Preparation. The synthesis of FePc-O-AND is similar to that of Yang et al.63 First, 1 g of commercial nanodiamond powder was placed in a porcelain boat and heated in a tube furnace under a continuous nitrogen flow. The temperature was raised to 1100 °C at a rate of 5 °C min⁻¹ and maintained for 4 h. After cooling to room temperature, the resulting nanodiamond material was designated as AND. Next, 1 g of AND powder was placed in a 250 mL round-bottom flask with 100 mL of concentrated nitric acid (69 wt%). The suspension was heated to 100 °C and stirred at 800 rpm under reflux for 4 h using a water-cooled condenser. After oxidation, the mixture was cooled to room temperature, and the solid was collected by centrifugation. The product was then washed repeatedly with ultrapure water until the supernatant reached neutral pH, and dried in an oven at 50 °C overnight. The oxidized material was referred to as O-AND. Subsequently, 0.01 g of FePc was dissolved in 2 mL of ethyl acetate, stirred, and sonicated for 5 minutes for dispersion. Then, 0.1 g of O-AND was placed in a mortar, mixed with the FePc solution, and ground in a fume hood for 20 minutes, with ethyl acetate replenished as needed. The ground powder was dried in a vacuum oven at 60 °C for 12 h. Finally, the dried powder was placed in a porcelain boat, heated in a tube furnace under continuous argon flow at a rate of 5 °C min−1 to 300 °C, and maintained for 1 h. After cooling to room temperature, the FePc-O-AND material was obtained.

The synthesis of the FePc-AND catalyst follows a similar procedure to that of FePc-O-AND, except that AND does not undergo nitric acid oxidation. FePc-O-PND was prepared similarly to FePc-O-AND, except that the PND support was not annealed or oxidized. By varying the metal precursor, single-atom catalysts with different metal centers and axial O coordination can be obtained. Specifically, replacing 0.01 g of FePc with copper phthalocyanine (CuPc), cobalt phthalocyanine (CoPc), or manganese phthalocyanine (MnPc) to yield CuPc-O-AND, CoPc-O-AND, and MnPc-O-AND catalysts, respectively. By altering the substituents on the FePc precursor, single-atom iron catalysts (Fe-SACs) with different axial O-coordination can be synthesized. For instance, replacing 0.01 g of FePc with tetraamino iron phthalocyanine, tetracarboxyl iron phthalocyanine, or tetrasulfonic iron phthalocyanine produces FePc-NH₂-O-AND, FePc-COOH-O-AND, and FePc-SO 3H-O-AND catalysts, respectively.

Experimental Procedure. Degradation experiments were performed in a 100 mL beaker under room temperature conditions with continuous magnetic stirring. The solution containing 0.25 mM PAA and 0.02 mM of the target pollutant was initially adjusted to the desired pH with NaOH or H 2SO 4. Then, the reaction was initiated by adding the desired amount of catalyst powder. Samples were taken from the reaction mixture at predetermined time intervals, filtered through a 0.22 μm membrane filter, and quenched with Na 2S 2O 3 solution for subsequent analysis. For the stability assessment, the catalysts used were collected after each run, washed several times with ultrapure water, and dried at 60 °C for reuse. All experiments were conducted in triplicate, and the mean values along with their standard deviations were presented. Equation parameters were estimated by fitting the model equations via linear regression with 95% confidence intervals. Real water samples were collected from the following sources: lake water from Donghu Lake (Wuhan, China), river water from the Yangtze River (Wuhan section, China), seawater from Glenelg Beach (Adelaide, Australia), and secondary sedimentation tank effluent from a wastewater treatment plant in Chongqing, China. The detailed water quality parameters are provided in Supplementary Table 13.

Characterization. XRD was conducted on a Rigaku MiniFlex 600 X-ray diffractometer. HRTEM images were acquired with a Philips CM 200 microscope. XANES spectra for the O, N K-edge and Fe L-edge were obtained using the Soft X-ray Spectroscopy beamline at the Australian Synchrotron in Melbourne. XPS spectra were obtained employing an SPECS (SPECS GmbH, Germany) electron spectrometer. The Brunauer–Emmett–Teller (BET) surface area and pore distribution were measured by nitrogen adsorption–desorption on a V-Sorb 2800P analyzer. HAADF images and energy dispersive spectrometer (EDS) elemental mapping analysis were obtained using the STEM mode on an aberration-corrected FEI Titan Themis 80-200, operated at 200 kV. The actual Fe loadings and Fe leaching concentration during the reaction were determined by ICP-OES measurements. The in-situ SR-FTIR signal was monitored using the FT-IR Spectroscopy beamline at the Australian Synchrotron in Melbourne. The detailed SR-FTIR experiment is provided in the Supporting Information. The polymerization and coupling products of 4-chlorophenol (4-CP) were analyzed using ultrahigh performance liquid chromatography with quadrupole time-of-flight initial mass spectrometry (UPLC-QTOF-MS, Waters Xevo G2-XS QTOF, Waters) and matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF-MS, UltrafleXtreme, Bruker). The toxicity of the 4-CP and the polymeric products to three aquatic organisms, fish, daphnia, and green algae, was predicted by the ECOSAR Program.

Analytical Methods. The concentration of emerging pollutants was measured by high-performance liquid chromatography (HPLC, Shimadzu LC-40D XR, Japan) equipped with a C−18 column and a UV detector. The detailed composition of the mobile phase for each pollutant are summarized in Supplementary Table 11. The residual PAA concentration during the reaction was quantified by the iodometric titration method using a UV–vis spectrophotometer. In detail, a sample of 0.2 ml was collected, mixed with 4.8 ml KI of 0.2 M, and measured immediately at  max = 352 nm without adding any quenchers. EPR spectra were recorded on Bruker EMXplus-6/1, Germany, with DMPO and TEMP as the spin-trapper reagents for radicals and 1O 2, respectively. For the investigation of O 2•−), ultrapure water was replaced by methanol for the test. In situ Raman spectra were measured using Raman spectroscopy (Horiba LabRAM HR Evolution) equipped with a green laser at 532 nm to identify the phase composition of catalysts and the reactive complex formed during PAA activation. Detailed characterization methods and electrochemical measurements are shown in the Supporting Information.

Theoretical Calculations. All DFT calculations were carried out using the Vienna Ab initio Simulation Package (VASP). The details are shown in the Supporting Information. The optimized atomic coordinates for all studied models are available in Supplementary Data 1.

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "This manuscript presents a compelling study on axial oxygen coordination engineering in single-atom Fe catalysts for PAA activation. The work demonstrates strong experimental rigor through the integration of advanced characterization techniques with theoretical modeling. However, several aspects require revisions.\n1. The manuscript's scientific novelty lies in the spin-state regulation strategy through axial oxygen coordination, which represents a meaningful advancement in single-atom catalyst design. The demonstrated selectivity toward polyphenolic product formation offers potential for waste-to-value applications. However, the environmental implications of this selectivity (i.e., fate of polyphenolic products in natural systems) need more thorough discussion.\n2. Peracetic Acid (PAA) was used as the reagent of reactive species generation. This manuscript should provide quantitative data on PAA utilization efficiency and residual concentrations during the degradation process. Time-resolved PAA consumption profiles alongside pollutant degradation kinetics, PAA utilization efficiency calculations (mol PAA consumed per mol pollutant removed), Detailed analysis of PAA transformation products beyond the brief mention of acetic acid and oxygen\n3. PAA residuals in treated water should be assessed for their potential impact on downstream biological treatment processes, such as discussion of the trade-off between incomplete PAA utilization and the formation of polyphenolic products. The environmental implications of PAA residuals are insufficiently addressed. Given that PAA is used in drinking water treatment, organic residual as the assimilable organic carbon for microorganism proliferation should be discussed.\n4. The quantitative analysis of Fe:O ratio in the axial coordination structure requires more rigorous characterization. EXAFS fitting shows Fe-O coordination number of 1.27 ± 0.09. The manuscript should provide direct evidence for the proposed 1:1 Fe:O stoichiometry using quantitative XPS deconvolution or other complementary techniques\n5. Axial oxygen plays vital role during this oxidation process, and it requires more detailed analysis. The Fe-O bond length (~1.96 Å from EXAFS) suggests a relatively weak coordination. DFT calculations should explore the bond dissociation energy and thermal stability. The manuscript claims ‘all O on nanodiamond are occupied by Fe atom, but provides no quantitative evidence. Surface titration or IR spectroscopy of surface OH groups before and after FePc anchoring should be provided\n6. Line 163-164 states all Fe-SAC exist as FeN4-O, but the evidence is incomplete. The EXAFS fitting suggests mixed coordination environments, not exclusively FeN4-O. XPS shows mixed Fe2+/Fe3+ oxidation states, indicating heterogeneity in the Fe coordination environment. Mössbauer spectroscopy would provide definitive evidence for the coordination environment and spin state distribution\n7. Line 278 proposes phenoxyl radical formation, but EPR shows no radical signals. This contradiction needs to be addressed. If phenoxyl radicals are truly intermediates, they should be detectable by EPR under appropriate conditions (low temperature, rapid freeze-quench). Alternative explanations for the polymerization mechanism should be considered (e.g., direct electron transfer without free radical intermediates)\n8. The manuscript should distinguish between surface-bound radical intermediates and free radicals in solution\n9. The proposed electron transfer pathway lacks mechanistic rigor. The Cat-PAA* complex formation (889 cm-1 Raman peak) needs more detailed vibrational assignment. DFT calculations should provide the complete reaction coordinate for PAA activation, including transition states and activation barriers\n10. The manuscript inadequately addresses H2O2 co-existence in commercial PAA. Commercial PAA solutions typically contain 1-15% H2O2. The manuscript should clarify the H2O2 content in their PAA reagent. Control experiments with H2O2 alone using both catalysts should be provided to delineate PAA-specific vs H2O2-mediated pathways\n11. DFT computational details require expansion. The choice of functional (likely PBE or PBE+U based on VASP usage) should be validated against experimental binding energies. Dispersion corrections are crucial for π-π stacking interactions but are not mentioned\n12. Figure 2f and 2h titles appear to have labeling inconsistencies. Figure 2i title needs correction.", "reviewer_2": "This work reports a promising strategy of axial oxygen coordination to tune the spin state of single-atom Fe catalysts for efficient peracetic acid (PAA) activation, with potential implications for water purification. The core hypotheses (axial O-induced spin transition, inner-sphere non-radical mechanism) are supported by a combination of experiments and DFT calculations. However, several critical gaps in experimental detail, insufficient mechanistic evidence, and unclear presentation of key data limit the rigor of the work. Below are specific comments for revision:\n(1)The mechanochemical grinding step for FePc immobilization is inadequately described. No details on grinding conditions (e.g., mortar/pestle material, grinding force control, ethyl acetate replenishment volume/frequency) or post-grinding drying were provided in raw manuscript. The oxidation of annealed nanodiamonds (AND) with HNO₃ is described as “refluxed for 4 h” but lacks critical parameters such as acid concentration, reflux stirring speed, and post-oxidation purification.\n(2)EXAFS fitting (Fe K-edge) omits critical details about multiple scattering and fitting models.Author fails to clarify whether multiple scattering (e.g., Fe–N–C) was included in the fitting model.\n(3)PAA concentration quantification (iodometric titration) lacks interference control. Iodometric titration is susceptible to oxidation by other reactive species (e.g., O₂•⁻, if present) or catalyst surface oxidants.\n(4)Current evidence for spin transition (EPR, magnetic susceptibility, NEXAFS L3/L2 ratio) is indirect. EPR only reports g = 2.002 signal intensity, which is sensitive to unpaired electrons but not uniquely diagnostic of spin state. Magnetic susceptibility (μeff = 1.50 μB for FePc-AND, 2.15 μB for FePc-O-AND) is consistent with high- and intermediate-spin, therefore Mössbauer spectroscopy—the gold standard for Fe spin/oxidation state analysis—is required.\n(5)The manuscript uses PDOS to show Fe 3d-O 2p hybridization but does not compare DFT-predicted orbital energies with experimental data (e.g., XPS valence band spectra, which directly probe valence orbital occupancy).\n(6)Quench experiments to rule out high valence Fe species are incomplete. The manuscript uses PMSO (0.1 mM?) to detect FeIV=O but does not report the concentration dependence (e.g., 0.05–0.5 mM) to ensure PMSO is in excess. In addition, time-dependent PMSO₂ yield is needed to confirm no delayed formation.\n(7)The manuscript refers to FePc-AND as “high-spin” (t2g5eg3) and FePc-O-AND as “intermediate-spin” (t2g4eg2), but in one instance it mentions “low-spin (LS) configuration for FePc”—this is confusing, as FePc’s spin state is not clearly defined earlier.\n(8)Manuscript’s core conclusion relies on the claim that “axial oxygen in FePc-O-AND originates from O-AND, but it fails to explicitly rule out the possibility of oxygen being introduced from the FePc precursor itself or during FePc immobilization (e.g., FePc degradation/oxidation under grinding or annealing steps). This ambiguity undermines the causal link between “O-AND-derived axial O” and the observed spin transition/catalytic enhancement. I suggest author to synthesize a control catalyst: “FePc adsorbed on oxygen-free supports (e.g., pristine nanodiamonds, denoted as FePc-PND)”, and characterize its Fe coordination environment via EXAFS (to check for Fe–O bonds) and catalytic activity (to compare with FePc-O-AND/FePc-AND). If FePc-PND has no Fe–O bonds and low activity, it confirms O-AND as the axial O source.\n（9）The manuscript claims that “the structural integrity of FePc was largely preserved after immobilization onto nanodiamond substrates” (LINE 94), but this conclusion is supported only by indirect evidence of (e.g., XRD characteristic peaks of FePc, FTIR C–N/C–H bands), while critical evidence for “no local structural alteration” of FePc is missing. This insufficiency raises doubts about whether FePc itself undergoes unreported changes (e.g., phthalocyanine ring distortion, Fe–N bond micro-modification, or in-situ introduction of oxygen-containing groups) during immobilization—changes that could independently modulate the Fe center’s electronic state, thereby confounding the causal link between “axial O from O-AND” and the observed spin transition/catalytic activity.", "reviewer_3": "This manuscript, entitled “Axial Oxygen Coordination Drives Spin-Regulated Electron Transfer in Single-Atom Fe Catalysts for Selective Pollutant Transformation,” presents a solid and interesting study. However, several issues regarding experimental design, writing clarity, and result interpretation should be addressed. Detailed comments are provided below:\n1. In Lines 114–115, the provided SEM images do not clearly show the spherical cluster morphology described. The authors should include more dispersed and distinct SEM images to better illustrate the morphology of FePc-O-AND and FePc-AND.\n2. In Line 117, size distribution diagrams of the nanoparticles should be provided.\n3. The nomenclature used for FePc-O-AND materials in the text (e.g., Line 145) is inconsistent with that in the figures. The authors should ensure uniformity throughout.\n4. In Lines 158–160, the claim that “X-ray absorption near edge structure (XANES, Fig. 1f) ……confirming preserved square-planar Fe-N4 coordination” is not sufficiently supported by the XANES results. Additional evidence or clarification is needed.\n5. Detailed electrochemical experimental procedures should be included in the Supporting Information. Conditions for degradation experiments should be clearly stated in the figure captions.\n6. In Lines 285–293, more comprehensive evidence is required. For example, through matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF-MS). If 4-CP is removed via coupling into larger molecules, the authors should attempt to detect polymeric products using solvents such as ethanol or toluene to dissolve oxidation products adsorbed on the materials. Additionally, a preliminary toxicity assessment of the oxidation products is recommended.\n7. In Lines 288–293: Since KSCN is known to rapidly consume PMS in heterogeneous/PMS systems, potentially leading to misinterpretation of quenching effects, the authors should clarify whether KSCN directly reacts with PAA, resulting in significant PAA depletion and thus the observed inhibition of 4-CP removal.\n8. In Lines 304–307: According to earlier descriptions, FePc-O-AND/PAA achieves complete 4-CP removal within 2 minutes. Why does the control group (0 mM NaClO4) require 5 minutes? Furthermore, the statement that “FePc-O-AND/PAA system exhibited negligible changes in degradation kinetics” is not accurate. First, Fig. 2i does not provide kinetic data; second, if kinetic constants were calculated, a significant inhibitory effect would likely be evident.\n9. In Line 401, data on the effect of HA on 4-CP degradation are missing.\n10. In Lines 402–403, experiments evaluating the impact of real water matrices (e.g., river water or treated wastewater) on degradation performance should be added. Water quality parameters of these actual water samples should also be provided.\n11. In Lines 457–458, data on PAA consumption are missing.\n12. Several errors and inconsistencies throughout the manuscript require careful revision. Examples include:\n(1) Line 86: “AND were oxidized”;\n(2) Line 163: “these findings provide evidence” (not “evidences”);\n(3) Line 165: The full term for “NEXAFS” should be provided upon first use;\n(4) Lines 209–211: The text description contradicts Supplementary Fig. 17a;\n(5) Lines 311–312: The claim that “the degradation performance remained unchanged in both the FePc-O-AND and FePc-AND systems” is not fully consistent with the slight decrease shown in the results. The description should accurately reflect the experimental outcomes;\n(6) Line 391: Remove the erroneous word “acts” after “performance.”;\n(7) Verb tenses should be checked and made consistent throughout the manuscript.\n13. Several issues were identified in the references:\n(1) Lines 63–65: Authoritative references should be added where appropriate. This issue occurs in multiple instances;\n(2) Line 178: The citation of Reference 30 does not align with the discussion context “which is assigned to hybridized Fe 3d–O 2p orbitals30, 31”. A more suitable reference should be selected;\n(3) Lines 594–595 and 636–637: Page numbers are missing for References 24 and 40."}, "author_response": "We sincerely thank the Reviewers for their constructive and encouraging comments, which have been invaluable in improving the clarity and scientific quality of our review. We have carefully addressed all suggestions and revised the manuscript accordingly, with the changes highlighted in blue in the revised version. A detailed, point-by-point response to each comment is provided below. We hope that the revisions satisfactorily address the concerns raised and meet the high standards of the Editor and Reviewers for publication in Nature Communications.\nResponse to Reviewers’ Comments\nReviewer #1 (Comments for the Author):\nGeneral comments: This manuscript presents a compelling study on axial oxygen coordination engineering in single-atom Fe catalysts for PAA activation. The work demonstrates strong experimental rigor through the integration of advanced characterization techniques with theoretical modeling. However, several aspects require revisions.\nResponse: We appreciate the Reviewer’s positive comments and valuable suggestions, and we accept them all. The manuscript has been carefully revised in accordance with the Reviewer’s comments.\nComment 1. The manuscript’s scientific novelty lies in the spin-state regulation strategy through axial oxygen coordination, which represents a meaningful advancement in single-atom catalyst design. The demonstrated selectivity toward polyphenolic product formation offers potential for waste-to-value applications. However, the environmental implications of this selectivity (i.e., fate of polyphenolic products in natural systems) need more thorough discussion.\nResponse: We sincerely thank the Reviewer’s insightful comment. In the revised manuscript, we have supplemented more discussions on the environmental implications of the polyphenolic products.\n“Notably, FePc-O-AND enables efficient phenol-to-polyphenol transformation through extended oxidative polymerization, attributable to its higher redox potential and faster electron-transfer capability. The polymerization mechanism offers a distinct environmental advantage by facilitating a sustainable “waste-to-resource” strategy. Specifically, the polymerization process converts dissolved phenolic pollutants into insoluble solid-state polymers that preferentially adhere to heterogeneous surfaces, thereby achieving efficient sequestration and rapid water purification61. Unlike mineralization, this phase transformation enables the straightforward extraction and recovery of organic carbon, which can be subsequently upcycled into high-value functional materials for applications such as chemicals and catalyst precursors62.” (Please see Page 26)\nReference\n61. Duan, P.-J.; Liu, J.-Y.; Chen, L.; Li, M.-X.; Pan, J.-W.; Zhang, Z.-Q.; Bai, C.-W.; Chen, X.-J.; Yu, H.-Q.; Chen, F., Polymeric products deactivate carbon-based catalysts in catalytic oxidation reactions. Nat. Water 2025, 3 (2), 178-190.\n62. Ren, W.; Zhang, Q.; Chen, J.; Xiao, X.; Duan, X.; Luo, X.; Wang, S., Catalytic resource recovery for transformation of the wastewater industry. Nat. Water2025, 3 (11), 1228-1242.\nComment 2-1. Peracetic Acid (PAA) was used as the reagent of reactive species generation. This manuscript should provide quantitative data on PAA utilization efficiency and residual concentrations during the degradation process. Time-resolved PAA consumption profiles alongside pollutant degradation kinetics, PAA utilization efficiency calculations (mol PAA consumed per mol pollutant removed), detailed analysis of PAA transformation products beyond the brief mention of acetic acid and oxygen.\nResponse: We appreciate the Reviewer’s insightful suggestion. In the revised manuscript, we have provided a comprehensive analysis of PAA transformation, including PAA consumption kinetics, utilization efficiency calculations, and transformation products.\n“In addition to substrate selectivity, oxidant utilization efficiency was also evaluated. As shown in Supplementary Fig. 29, the FePc-O-AND/PAA system exhibits a markedly lower PAA consumption rate (0.20 min−1) compared to the FePc-AND/PAA system (0.34 min−1). To quantitatively assess oxidant efficiency, the PAA utilization efficiency was defined as the molar ratio of pollutant removed to PAA consumed (Δ[4-CP]/Δ[PAA]). At 2 min, this value reached 0.26 for the FePc-O-AND system, substantially higher than that of FePc-AND (0.10), demonstrating a significantly improved oxidant economy in the FePc-O-AND/PAA system. This enhancement stems from the formation of surface-confined, metastable reactive complexes (catalyst-PAA*) rather than free radicals. These complexes increase the redox potential of the FePc-O-AND, enabling efficient electron abstraction from electron-rich pollutants and thereby promoting near-complete PAA conversion into acetic acid.” (Please see Page 15)\nComment 3. PAA residuals in treated water should be assessed for their potential impact on downstream biological treatment processes, such as discussion of the trade-off between incomplete PAA utilization and the formation of polyphenolic products. The environmental implications of PAA residuals are insufficiently addressed. Given that PAA is used in drinking water treatment, organic residual as the assimilable organic carbon for microorganism proliferation should be discussed.\nResponse: We appreciate the Reviewer’s thoughtful comment. The toxicity of the post-reaction effluent was evaluated through E. coli cultivation experiments. The results and detailed discussions have been added to the revised manuscript and Supplementary Information.\n“To validate this, toxicity assays were conducted using the post-reaction effluent, which represents a realistic matrix containing both the residual PAA and the soluble organic degradation products. The results demonstrate that the treated effluent does not exert inhibitory effects on E. coli (Supplementary Fig. 43 and Supplementary Table 5). Collectively, these findings confirm that the potentially toxic polymers remain immobilized and that the residual PAA concentration is reduced below the inhibitory threshold for bacterial growth, either due to rapid consumption or conversion into biodegradable organic carbon.” (Please see Page 17)\n“Supplementary Text 5. Evaluation of toxicity by culture experiments with E. coli.\nThe E. coli strain ATCC 25922 was used for the toxicity evaluation. The bacteria were first activated by inoculating a single colony into 3 mL of sterile Luria-Bertani (LB) broth and incubating at 37 °C with shaking at 200 rpm for 15 h. After activation, the bacterial suspension was diluted with sterile phosphate-buffered saline (PBS) to a concentration of 106 CFU/mL. For each test group, 0.5 mL of the diluted bacterial suspension was added to 0.5 mL of the sample solution in 1.5 mL sterile centrifuge tubes. The control group contained 0.5 mL bacterial suspension mixed with 0.5 mL PBS. All groups were incubated statically at 37 °C for 24 h. After incubation, each bacterial solution was serially diluted 10-fold with sterile PBS, and 100 µL of each dilution was spread onto LB agar plates. The plates were incubated at 37 °C for 18 h, and the number of bacterial colonies was counted. Only plates with 30–300 colonies were selected for quantification according to GB4789.2-2016. The bacterial concentration (CFU/mL) was calculated as: CFU/mL = colony number × dilution factor × 10 (for the 0.1 mL volume plated). All procedures were carried out under sterile conditions using a biosafety cabinet. All consumables were sterilized either by autoclaving at 121 °C for 15–20 min or by ultraviolet irradiation (254 nm) for 25 min prior to use.”\nComment 4. The quantitative analysis of Fe:O ratio in the axial coordination structure requires more rigorous characterization. EXAFS fitting shows Fe−O coordination number of 1.27 ± 0.09. The manuscript should provide direct evidence for the proposed 1:1 Fe:O stoichiometry using quantitative XPS deconvolution or other complementary techniques.\nResponse: We appreciate the Reviewer’s thoughtful comment. We employed an indirect quantitative XPS analysis to verify the stoichiometry. By deducing the surface Fe content from the N 1s signal and comparing it with the deconvoluted Fe−O species, we obtained a ratio remarkably close to 1:1 (0.30 at% Fe vs. 0.29 at% Fe−O). The detailed calculations and discussion have been incorporated into the revised manuscript.\n“Moreover, a quantitative analysis based on XPS results was performed to further validate the stoichiometry of the axial coordination. Due to the limited sensitivity of the Fe 2p signal, the Fe concentration was indirectly deduced from the N 1s concentration. Given the preserved FePc stoichiometry (Fe : N = 1: 8), the surface Fe concentration was calculated to be 0.30 at% based on the measured total N 1s signal (2.39 at%, Supplementary Fig. 16b). This value shows excellent agreement with the concentration of the specific axial Fe−O species (0.29 at%) derived from O 1s deconvolution (Supplementary Fig. 16a), yielding a Fe:O ratio of approximately 1:1 (0.30 at% Fe vs. 0.29 at% Fe−O). This quantitative consistency further corroborates the mono-axial coordination model proposed by EXAFS.” (Please see Page 10)\nComment 5-1. Axial oxygen plays vital role during this oxidation process, and it requires more detailed analysis. The Fe-O bond length (~1.96 Å from EXAFS) suggests a relatively weak coordination. DFT calculations should explore the bond and thermal stability.\nResponse: We appreciate the Reviewer’s insightful comment. The DFT calculations were performed to determine the Fe-O bond dissociation energy and the comparative formation enthalpies. The relevant results and discussion have been added to the revised manuscript and Supporting Information.\n“The formation enthalpy (ΔH) of FePc-O-AND (–2.60 eV) is significantly lower than that of FePc-AND (–1.93 eV), indicating that axial oxygen coordination leads to a thermodynamically more stable configuration.19 Furthermore, the dissociation energy (Ediss) of Fe-O bond was calculated to be 1.04 eV (Supplementary Table 6), which suggests a moderately strong and stable Fe-O interaction. This energy magnitude indicates a robust coordination interaction, confirming the thermal and structural stability of the axial oxygen ligand during the catalytic process.” (Please see Pages 19-20)\nComment 5-2. The manuscript claims ‘all O on nanodiamond are occupied by Fe atom, but provides no quantitative evidence. Surface titration or IR spectroscopy of surface OH groups before and after FePc anchoring should be provided.\nResponse: We appreciate the Reviewer’s excellent suggestion, and we accept it. In response, we quantified the surface oxygen functionalities before and after FePc anchoring using the standard Boehm titration method, as advised. The detailed calculations and results have been added to the revised manuscript and Supplementary Information.\n“In comparison, FePc-AND exhibited markedly fewer atomic Fe signals (Supplementary Figs. 7 and 8), which aligns with the lower Fe loading determined by ICP-OES (0.91 wt% for FePc-O-AND vs. 0.42 wt% for FePc-AND, Supplementary Fig. 9). Notably, this disparity arises despite the comparable specific surface areas of AND and O-AND (Supplementary Fig. 10), highlighting the critical role of surface oxygen groups in O-AND in regulating FePc immobilization. Surface oxygen-containing functional groups were quantified using the standard Boehm titration method.29, 30 Prior to anchoring, the surface phenolic –OH concentration on O-AND was 801.66 mmol/kg (Supplementary Fig. 12). After FePc anchoring,this value decreased to 613.28 mmol/kg, representing 23.5% reduction. Notably, this decrement (Δ = 188.38 mmol/kg) aligns closely with the molar concentration of Fe introduced (0.91 wt% Fe = 162.95 mmol/kg). This quantitative correlation suggests an approximate 1:1 molar stoichiometry between the anchored Fe atoms and the consumed phenolic –OH sites, providing solid evidence that the formation of axial Fe-O coordination is the primary anchoring mechanism.” (Please see Page 7)\nComment 6. Line 163-164 states all Fe-SAC exist as FeN4-O, but the evidence is incomplete. The EXAFS fitting suggests mixed coordination environments, not exclusively FeN4-O. XPS shows mixed Fe2+/Fe3+ oxidation states, indicating heterogeneity in the Fe coordination environment. Mössbauer spectroscopy would provide definitive evidence for the coordination environment and spin state distribution.\nResponse: We appreciate and accept the Reviewer’s excellent suggestion. The 57Fe Mössbauer spectroscopy analyses of FePc, FePc-AND, and FePc-O-AND were carefully analyzed to definitively determine the coordination environment and spin state distribution. The results and discussion have been added to the revised manuscript.\n“57Fe Mössbauer spectroscopy elucidated the spin state transitions based on isomer shift (IS) and quadrupole splitting (QS) parameters. As shown in Fig. 3e and Supplementary Table 9, the pristine FePc displays a typical low-spin (LS) Fe(II) doublet (IS = 0.16 mm·s−1, QS = 2.41 mm·s−1) 57, 58. Upon anchoring on AND (FePc-AND), the strong support-induced π-π interaction shifts the center to a high-spin (HS) Fe(III) state (IS = 0.17 mm·s−1, QS = 0.93 mm·s−1)59. Notably, the FePc-O-AND system exhibits a more complex spectrum deconvoluted into three distinct components corresponding to medium-spin (MS, 21.06%), high-spin (58.33%), and low-spin (20.61%) species. This distribution arises because the synergistic interplay between the axial Fe-O coordination and the basal π-π interactions, which together stabilize a substantial fraction of MS Fe centers. The MS configuration is electronically labile, enabling a dynamic redistribution among the MS, HS, and LS states. This mechanism provides a compelling explanation for the observed mixed Fe3+/Fe2+ oxidation states and highlights the crucial role of spin-state dynamics in facilitating efficient electron transfer at the catalytic center.” (Please see Page 21)\nComment 7. Line 278 proposes phenoxyl radical formation, but EPR shows no radical signals. This contradiction needs to be addressed. If phenoxyl radicals are truly intermediates, they should be detectable by EPR under appropriate conditions (low temperature, rapid freeze-quench). Alternative explanations for the polymerization mechanism should be considered (e.g., direct electron transfer without free radical intermediates).\nResponse: We appreciate the Reviewer’s critical assessment. We would like to clarify that the absence of radical signals in the initial EPR spectra was due to the absence of the pollutant (4-CP) in those specific control experiments. Surface-bound phenoxyl radicals (PhO•) are derived from the oxidation of the pollutant substrate; thus, they cannot be generated without 4-CP. To definitely address this and confirm the formation of PhO• intermediates, we performed additional EPR experiments using TEMPO as a spin probe in the FePc-O-AND/PAA/4-CP system. Moreover, we employed ferulic acid (FA), a specific scavenger for phenoxyl radicals, to verify their mechanistic role. The results unequivocally support the involvement of phenoxyl radicals in the polymerization process. Detailed data and discussions have been added to the revised manuscript and Supporting Information.\n“The generation of surface-bound chlorophenoxyl radicals (ClPhO•) was further corroborated using 2,2,6,6-tetramethyl piperidine-1-yl) oxyl (TEMPO) and ferulic acid (FA) as specificdiagnostic probes. A pronounced and gradual attenuation of the TEMPO signal was observed, attributable to the coupling between TEMPO and the generated surface-bound ClPhO• species, yielding the diamagnetic TEMPO-OPhCl adduct (Supplementary Fig. 31)44. In parallel, quenching experiments employing FA, a selective scavenger for ClPhO•, showed an apparent concentration-dependent suppression of 4-CP degradation (Supplementary Fig. 32)44-46. These results provide compelling evidence that surface-bound ClPhO• species act as the pivotal intermediates driving the formation of polymeric products. The surface-confined nature of ClPhO• intermediates is further corroborated by the spatial distribution of the degradation products. The polymeric products were found exclusively associated with the catalyst surface and necessitated exhaustive solid-phase extraction for recovery, whereas the filtrate remained product-free. Moreover, such interfacial confinement maintains a high local concentration of ClPhO• at the catalyst surface, which kinetically favors ClPhO• self-coupling and the subsequent polymerization pathway. This mechanism effectively bypasses the bulk-phase dilution that would otherwise promote non-selective oxidation in the aqueous solution.” (Please see Pages 15-16)\nComment 8. The manuscript should distinguish between surface-bound radical intermediates and free radicals in solution.\nResponse: We thank the Reviewer for this important suggestion to distinguish free and surface-bound radical species. In the revised manuscript, we have updated the terminology from “phenoxyl radicals” to “surface-bound phenoxyl radicals”, and clarified the distinction via selective quenching tests as follows.\n“In parallel, quenching experiments employing FA, a selective scavenger for ClPhO•, showed an apparent concentration-dependent suppression of 4-CP degradation (Supplementary Fig. 32)44-46. These results provide compelling evidence that surface-bound ClPhO• species act as the pivotal intermediates driving the formation of polymeric products. The surface-confined nature of ClPhO• intermediates is further corroborated by the spatial distribution of the degradation products. The polymeric products were found exclusively associated with the catalyst surface and necessitated exhaustive solid-phase extraction for recovery, whereas the filtrate remained product-free. Moreover, such interfacial confinement maintains a high local concentration of ClPhO• at the catalyst surface, which kinetically favors ClPhO• self-coupling and subsequent polymerization. This mechanism effectively bypasses the bulk-phase dilution that would otherwise promote non-selective oxidation in the aqueous solution.” (Please see Page 16)\nComment 9-1. The proposed electron transfer pathway lacks mechanistic rigor. The Cat-PAA* complex formation (889 cm−1 Raman peak) needs more detailed vibrational assignment.\nResponse: We appreciate the Reviewer’s careful review and kind suggestion. In the revised manuscript, we have provided a detailed analysis of the Raman feature at 889 cm−1, assigned to the O–O stretching vibration of the adsorbed PAA molecule.\n“Upon the introduction of FePc-O-AND, a distinct new peak emerged at 889 cm−1, which is assigned to the O–O stretching vibration of the adsorbed peroxy species, corroborating the formation of the metastable active catalyst-PAA complex (Cat-PAA*)41-43.” (Please see Page 13)\nComment 9-2. DFT calculations should provide the complete reaction coordinate for PAA activation, including transition states and activation barriers.\nResponse: We appreciate the Reviewer’s valuable suggestion. The activation of PAA proceeds via the formation of a metastable surface-bound complex (Cat-PAA*) rather than a concerted bond-cleavage pathway typical of radical, singlet oxygen, or high-valent metal species generation. Consequently, a full reaction coordinate with transition state (TS) and activation barriers was not constructed. Instead, we focused on the adsorption configurations, binding energies, orbital hybridization, and charge redistribution, which provide critical insights into the electron-transfer initiation and reaction.\n“PDOS analysis further confirms strong orbital overlap between Fe 3d and O 2p orbitals in FePc-O-AND (Supplementary Figs. 59 and 60)50, 60. Geometrically, the interaction shows significant enhancement: the Fe–O bond length (PAA* to Fe) shortened from 2.45 Å (FePc-AND) to 2.30 Å (FePc-O-AND), while the O–O bond stretched slightly from 1.45 Å to 1.46 Å (Supplementary Fig. 61 and Supplementary Table 10). These changes reflect a stronger binding affinity and more effective activation of the peroxide bond. This surface-bound species acts as the critical intermediate driving the oxidation process via a direct, non-radical electron transfer mechanism, distinct from pathways involving conventional reactive species such as free radicals, singlet oxygen, or high-valent metal species. In summary, the axially coordinated O atom in FePc-O-AND dramatically alters the ligand field environment around the Fe center. This modulation optimizes the electronic structure to promote PAA activation, forming a surface-confined PAA* complex without decomposition via non-radical inner-sphere pathway, ultimately enhancing catalytic performance.” (Please see Page 23)\nComment 10. The manuscript inadequately addresses H2O2 co-existence in commercial PAA. Commercial PAA solutions typically contain 1-15% H2O2. The manuscript should clarify the H2O2 content in their PAA reagent. Control experiments with H2O2 alone using both catalysts should be provided to delineate PAA-specific vs H2O2-mediated pathways.\nResponse: We appreciate the Reviewer’s very considerate suggestion and we accept it. We quantified the residual H2O2 content in the commercial PAA stock solution and assessed the potential contributions of H2O2 and acetic acid. The corresponding results and discussion have been added to the revised manuscript as follows.\n“Moreover, control experiments were conducted to investigate the influence of residual precursor compounds, including 0.4 mM H2O2 and 1.5 mM CH3COOH, that are typically co-present in commercial PAA solutions. Using either H2O2 or CH3COOH alone as the oxidant resulted in no observable 4-CP removal (Supplementary Fig. 18). This confirms that these co-existing H2O2 and CH3COOH make negligible contributions to the overall catalytic oxidation process.” (Please see Page 11)\nComment 11. DFT computational details require expansion. The choice of functional (likely PBE or PBE+U based on VASP usage) should be validated against experimental binding energies. Dispersion corrections are crucial for π-π stacking interactions but are not mentioned.\nResponse: We appreciate the Reviewer’s kind comment regarding providing the computational methodology. The computational details were added in Supplementary Text 2.\n“Supplementary Text 2. Computational details.\nDFT calculations were performed by using the Vienna ab initio simulation package (VASP)S1, S2. The electron-ion potential was described by the projected augmented wave method (PAW)S3, and a plane-wave energy cut-off of 450 eV was set in this study. The Perdew-Burk-Ernzerhof (PBE) within the generalized gradient approximation (GGA) was used to describe the exchange-correlation functionalS4. The convergence criterion of the force was -0.03 eV/Å and the convergence threshold for the total energy was 10-5 eV. Since GGA cannot correctly describe strongly correlated systems containing partially filled d subshells, the GGA+U method is applied to describe partially filled d orbitals by considering Coulomb and exchange corrections. We used a correlation energy (U) of 4 eV and an exchange energy (J) of 1 eV for TM d orbitals. The Brillouin-zone integration was approximated using a 5×5×1 grid, with the Monkhorst-Pack k-point mesh. A vacuum space of 15 Å in the Z-direction was chosen to avoid artificial interaction. The effect of van der Waals (vdW) interactions was included for weak interaction cases using the semiempirical correction scheme of Becke-Jonson damping, DFT-D3S5.”\nComment 12. Figure 2f and 2h titles appear to have labeling inconsistencies. Figure 2i title needs correction.\nResponse: We appreciate the Reviewer’s careful review and gentle reminder. The figure captions in Fig. 2f and 2h have been revised for consistency as follows, and the title of Fig. 2i has been corrected.\n“Figure 2. (f) In situ Raman spectra of FePc-O-AND/PAA and FePc-AND/PAA systems. The correlation of lnk of pollutant removal to φ in the (g) FePc-O-AND/PAA and (h) FePc-AND systems. (i) Effect of NaClO4 on 4-CP removal in the FePc-O-AND/PAA and FePc-AND/PAA systems. Conditions: [Catalyst] = 0.1 g/L, [PAA] = 0.25 mM, [4-CP] = 0.02 mM, and pH 7.”\nReviewer #2(Comments for the Author):\nGeneral comments: This work reports a promising strategy of axial oxygen coordination to tune the spin state of single-atom Fe catalysts for efficient peracetic acid (PAA) activation, with potential implications for water purification. The core hypotheses (axial O-induced spin transition, inner-sphere non-radical mechanism) are supported by a combination of experiments and DFT calculations. However, several critical gaps in experimental detail, insufficient mechanistic evidence, and unclear presentation of key data limit the rigor of the work. Below are specific comments for revision:\nResponse: Many thanks for the Reviewer’s insightful comments and valuable suggestions, which are all accepted.\nComment 1. The mechanochemical grinding step for FePc immobilization is inadequately described. No details on grinding conditions (e.g., mortar/pestle material, grinding force control, ethyl acetate replenishment volume/frequency) or post-grinding drying were provided in raw manuscript. The oxidation of annealed nanodiamonds (AND) with HNO3 is described as “refluxed for 4 h” but lacks critical parameters such as acid concentration, reflux stirring speed, and post-oxidation purification.\nResponse: We appreciate the Reviewer’s careful review and gentle reminder. We have now added detailed information on the grinding procedure and critical parameters for the acid-refluxed treatment in the revised manuscript.\n“Next, 1 g of AND powder was placed in a 250 mL round-bottom flask with 100 mL of concentrated nitric acid (69 wt%). The suspension was heated to 100 °C and stirred at 800 rpm under reflux for 4 h using a water-cooled condenser. After oxidation, the mixture was cooled to room temperature, and the solid was collected by centrifugation. The product was then washed repeatedly with ultrapure water until the supernatant reached neutral pH, and dried in an oven at 50 °C overnight. The oxidized material was referred to as O-AND.” (Please see Page 27)\nComment 2. EXAFS fitting (Fe K-edge) omits critical details about multiple scattering and fitting models. Author fails to clarify whether multiple scattering (e.g., Fe–N–C) was included in the fitting model.\nResponse: We appreciate the Reviewer’s insightful comment. As suggested, we have carefully considered multiple scattering paths in our EXAFS analysis.Regarding the fitting models, the Fe−N−C multiple scattering paths for FePc-AND were explicitly included. Due to the high rigidity of the FePc macrocycle and the low Debye-Waller factor (σ2 = 0.010±0.002), the MS configuration contributes a discernible signal, which was necessary to achieve the optimal fit. In contrast, for FePc-O-AND, the axial oxygen coordination induces a slight elongation of the Fe−N bond (from 1.95 Å to 2.04 Å) and an increase in static disorder (σ2 = 0.015±0.009). This increased local disorder effectively dampens the long-range geometric correlations required to produce detectable multiple-scattering contributions. Consequently, including complex multiple-scattering paths for this sample resulted in over-parameterization without statistically improving fit quality. Therefore, a model prioritizing the primary Fe−N and Fe−O shells was adopted, which ensures a highly reliable structural assignment.\n“Quantitative EXAFS fitting further supports these findings (Fig.1h and Supplementary Table 1). Regarding the fitting models, Fe−N−C multiple scattering paths were explicitly included for the rigid FePc-AND reference. In contrast, for FePc-O-AND, the increased local disorder induced by axial oxygen coordination dampens these long-range signals, underscoring the need for a fitting model that prioritizes the primary Fe−N and Fe−O shells. Specifically, FePc-AND shows an Fe−N coordination number of 4.14 ± 0.45, consistent with a typical FeN4 structure. In comparison, FePc-O-AND features an additional Fe–O coordination (1.27 ± 0.09), confirming the presence of an axial oxygen ligand and the formation of a featured five-coordinate FeN4–O structure. The slightly elevated total coordination number and the enhanced FT-EXAFS peak intensity in FePc-O-AND further validate this assignment.” (Please see Page 8)\nComment 3. PAA concentration quantification (iodometric titration) lacks interference control. Iodometric titration is susceptible to oxidation by other reactive species (e.g., O2•–, if present) or catalyst surface oxidants.\nResponse: We appreciate the Reviewer’s insightful and considerate comment regarding potential interference with PAA concentration quantification. During the tests, all samples were first filtered to exclude catalyst surface-confined ROS. Furthermore, since the reaction proceeds via a surface-confined mechanism, bulk superoxide radicals (O2•−) and other reactive oxygen species are absent, eliminating the risk of solution-phase interference. To rigorously validate the iodometric titration (KI method), we employed the N,N-diethyl-p-phenylenediamine (DPD) method for cross-validation. As shown in Fig. R1, the quantification results from the KI method aligned perfectly with those from the DPD method, showing a relative error of 3.37% across all data points. This strong correlation confirms the reliability of the iodometric titration for this study.\nComment 4. Current evidence for spin transition (EPR, magnetic susceptibility, NEXAFS L3/L2 ratio) is indirect. EPR only reports g = 2.002 signal intensity, which is sensitive to unpaired electrons but not uniquely diagnostic of spin state. Magnetic susceptibility (μeff = 1.50 μB for FePc-AND, 2.15 μB for FePc-O-AND) is consistent with high- and intermediate-spin, therefore Mössbauer spectroscopy—the gold standard for Fe spin/oxidation state analysis—is required.\nResponse: We appreciate the Reviewer’s valuable suggestion. In response, the 57Fe Mössbauer spectra of FePc, FePc-AND, and FePc-O-AND were conducted and carefully analyzed to elucidate the oxidation and spin states of the Fe centers. The results and discussion have been added to the revised manuscript.\n“57Fe Mössbauer spectroscopy elucidated the spin state transitions based on isomer shift (IS) and quadrupole splitting (QS) parameters. As shown in Fig. 3e and Supplementary Table 9, the pristine FePc displays a typical low-spin (LS) Fe(II) doublet (IS = 0.16 mm·s−1, QS = 2.41 mm·s−1) 57, 58. Upon anchoring on AND (FePc-AND), the strong support-induced π-π interaction shifts the center to a high-spin (HS) Fe(III) state (IS = 0.17 mm·s−1, QS = 0.93 mm·s−1)59. Notably, the FePc-O-AND system exhibits a more complex spectrum deconvoluted into three distinct components corresponding to medium-spin (MS, 21.06%), high-spin (58.33%), and low-spin (20.61%) species. This distribution arises because the synergistic interplay between the axial Fe-O coordination and the basal π-π interactions, which together stabilize a substantial fraction of MS Fe centers. The MS configuration is electronically labile, enabling a dynamic redistribution among the MS, HS, and LS states. This mechanism provides a compelling explanation for the observed mixed Fe3+/Fe2+ oxidation states and highlights the crucial role of spin-state dynamics in facilitating efficient electron transfer at the catalytic center.” (Please see Page 21)\nComment 5. The manuscript uses PDOS to show Fe 3d-O 2p hybridization but does not compare DFT-predicted orbital energies with experimental data (e.g., XPS valence band spectra, which directly probe valence orbital occupancy).\nResponse: We appreciate the reviewer for this insightful comment. While XPS valence band spectra can indeed provide insight into the occupied density of states, they lack orbital specificity and cannot directly resolve contributions from specific hybridized orbitals such as Fe 3d–O 2p. Instead, to experimentally probe the Fe–O electronic interaction, we employed O K-edge X-ray absorption spectroscopy (XAS), which is sensitive to transitions from the O 1s core level to unoccupied O 2p-derived states that are hybridized with Fe 3d orbitals. Therefore, although we did not perform a direct comparison with XPS valence band spectra, the experimental XAS results provided clear evidence of Fe–O orbital mixing, which is consistent with the hybridization features predicted by DFT. The corresponding results and discussion have been added to the revised manuscript.\n“At the O K-edge (Supplementary Fig. 14b), FePc-O-AND exhibits a distinct feature at 531.8 eV, attributed to transitions from the O 1s level to unoccupied states of O 2p orbitals hybridized with Fe 3d states34, 35. Although this energy region may also induce contributions from π* transitions associated with C–O–C groups24, the pronounced enhancement observed in FePc-O-AND suggests the dominant role of Fe–O coordination. Additionally, the broader feature centered around 540 eV is assigned to σ*(C–O) transitions from oxygen-containing functional groups.” (Please see Page 10)\nComment 6. Quench experiments to rule out high valence Fe species are incomplete. The manuscript uses PMSO (0.1 mM?) to detect FeIV=O but does not report the concentration dependence (e.g., 0.05–0.5 mM) to ensure PMSO is in excess. In addition, time-dependent PMSO2 yield is needed to confirm no delayed formation.\nResponse: We appreciate the Reviewer’s great suggestion. As advised, additional PMSO quenching experiments over a range of concentrations (0.05, 0.1, 0.5, and 1.0 mM) were performed in both the FePc-AND/PAA and FePc-O-AND/PAA systems to demonstrate the concentration dependence. The associated time-dependent PMSO2 yield was also provided in the revised manuscript.\n“However, the addition of PMSOat a range of concentrations (0.05, 0.1, 0.5, and 1.0 mM) did not suppress 4-CP removal, and the yield of PMSO2 remained below 40% in all cases (Supplementary Fig. 25), revealing that high-valent iron species were not generated.” (Please see Page 13)\nComment 7. The manuscript refers to FePc-AND as “high-spin” (t2g5eg3) and FePc-O-AND as “intermediate-spin” (t2g4eg2), but in one instance it mentions “low-spin (LS) configuration for FePc”—this is confusing, as FePc’s spin state is not clearly defined earlier.\nResponse: We appreciate the Reviewer’s gentle reminder. As advised, the spin state of Fe in FePc was determined by 57Fe Mössbauer spectroscopy (see responses to Comment 4).\nComment 8. Manuscript’s core conclusion relies on the claim that “axial oxygen in FePc-O-AND originates from O-AND, but it fails to explicitly rule out the possibility of oxygen being introduced from the FePc precursor itself or during FePc immobilization (e.g., FePc degradation/oxidation under grinding or annealing steps). This ambiguity undermines the causal link between “O-AND-derived axial O” and the observed spin transition/catalytic enhancement. I suggest author to synthesize a control catalyst: “FePc adsorbed on oxygen-free supports (e.g., pristine nanodiamonds, denoted as FePc-PND)”, and characterize its Fe coordination environment via EXAFS (to check for Fe–O bonds) and catalytic activity (to compare with FePc-O-AND/FePc-AND). If FePc-PND has no Fe–O bonds and low activity, it confirms O-AND as the axial O source.\nResponse: Thanks for the Reviewer’s very thoughtful suggestion. Following this advice, we used pristine nanodiamonds (PND) as the support to synthesize the control catalyst. Notably, characterization revealed pristine PNDs naturally possess abundant surface sp3–C–O groups. Consequently, the resulting sp3-carbon-supported catalyst sample also possesses Fe–O coordination, denoted as FePc-O-PND. Thus, FePc-O-PND was selected as a model catalyst to decouple the effects of axial oxygen coordination on sp3 (FePc-O-PND) and hybrid (sp2/sp3) carbon support. The relevant discussions have been added to the revised manuscripts.\n“Moreover, sp3-carbon-rich pristine nanodiamonds (PND) were used to prepare FePc-O-PND to investigate the synergy between axial oxygen coordination and the support-dependent electronic interaction. FT-EXAFS analysis confirmed that intrinsic surface oxygen on PND induces the formation of axial Fe–O bonds (Supplementary Fig. 70), similar to FePc-O-AND. Interestingly, FePc-O-PND exhibited appreciable initial activity to FePc-O-AND (kobs = 0.87 min−1), confirming that axial oxygen coordination is the decisive factor for triggering the catalytic reaction. However, its cycling stability was significantly inferior (Supplementary Fig. 71). This sharp contrast is attributed to the lack of a conjugated sp2 carbon network on PND. In the optimal FePc-O-AND system, the graphitized AND support provides a strong π-π electronic coupling with the FePc planar ring. This interaction acts as a structural anchor, stabilizing the active center against leaching or demetalation. Thus, the superior performance of FePc-O-AND arises from the synergy of axial oxygen coordination and FePc-AND π-π interactions, which promote both intrinsic activity and stability during PAA activation.” (Please see Page 25)\nComment 9. The manuscript claims that “the structural integrity of FePc was largely preserved after immobilization onto nanodiamond substrates” (LINE 94), but this conclusion is supported only by indirect evidence of (e.g., XRD characteristic peaks of FePc, FTIR C–N/C–H bands), while critical evidence for “no local structural alteration” of FePc is missing. This insufficiency raises doubts about whether FePc itself undergoes unreported changes (e.g., phthalocyanine ring distortion, Fe–N bond micro-modification, or in-situ introduction of oxygen-containing groups) during immobilization—changes that could independently modulate the Fe center’s electronic state, thereby confounding the causal link between “axial O from O-AND” and the observed spin transition/catalytic activity.)\nResponse: We appreciate the Reviewer’s thoughtful concern regarding the structural change of FePc during the synthesis. In response, we performed NEXAFS at the C and N K-edges to demonstrate the structural robustness and integrity. The relevant discussions were added in the revised manuscripts.\n“The structural integrity of FePc was largely preserved after immobilization onto nanodiamond substrates, as confirmed by consistent characteristic signals in the X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), Raman, and near-edge X-ray absorption fine structure (NEXAFS) spectra.” (Please see Page 5)\n“NEXAFS spectroscopy was employed to verify the structural integrity of the FePc macrocycle upon anchoring. As shown in Supplementary Fig. 5, a notable spectral evolution is observed in the C K-edge. Specifically, the peak at 287.2 eV, assigned to σ* (C–H) transitions and serving as a fingerprint of the intermolecular π-π stacking in bulk FePc24, vanishes in the FePc-O-AND spectrum. This spectral quenching confirms that the FePc aggregates are effectively de-aggregated, resulting in a monolayer dispersion on the O-AND surface. Crucially, apart from this stacking-related feature, the other dominant resonances in both the C and N K-edges remain energetically stable25, exhibiting no significant redshift or blueshift. This spectral consistency suggests that the macrocyclic skeleton is preserved, effectively ruling out Fe–N bond cleavage or destructive structural distortion during the loading process.” (Please see Page 6)\nReviewer #3(Comments for the Author):\nGeneral comments: This manuscript, entitled “Axial Oxygen Coordination Drives Spin-Regulated Electron Transfer in Single-Atom Fe Catalysts for Selective Pollutant Transformation,” presents a solid and interesting study. However, several issues regarding experimental design, writing clarity, and result interpretation should be addressed. Detailed comments are provided below:\nResponse: Many thanks for the Reviewer’s insightful comments and considerate suggestions, all of which are fully accepted.\nComment 1. In Lines 114–115, the provided SEM images do not clearly show the spherical cluster morphology described. The authors should include more dispersed and distinct SEM images to better illustrate the morphology of FePc-O-AND and FePc-AND.\nResponse: We appreciate the Reviewer’s considerate suggestion. In response, we have performed high-resolution SEM imaging on gold-coated samples to improve the clarity. The updated SEM images now distinctly reveal the dispersed nanospheres and aggregated particle morphology of both FePc-O-AND and FePc-AND.\nComment 2. In Line 117, size distribution diagrams of the nanoparticles should be provided.\nResponse: We appreciate the Reviewer’s great suggestion. To quantitatively evaluate the particle size, we performed a statistical analysis based on the TEM images. The resulting particle size distribution histograms have been provided to the revised manuscript.\n“High-resolution transmission electron microscopy (HRTEM, Figs. 1b-d) further confirmed the preservation of the onion-like graphitic structure, with nanoparticles exhibiting an average diameter of 5.0 ± 0.2 nm (Supplementary Fig. 8) and well-defined lattice fringes corresponding to the (002) planes of graphite (d = 0.338 nm, Fig. 1c), indicating the unique sp2/sp3 hybridized carbon framework26.” (Please see Page 6)\nComment 3. The nomenclature used for FePc-O-AND materials in the text (e.g., Line 145) is inconsistent with that in the figures. The authors should ensure uniformity throughout.\nResponse: We appreciate the Reviewer’s careful review and gentle reminder. We have now carefully revised the entire manuscript, including the figures and captions, to ensure consistent nomenclature for the FePc-based materials.\nComment 4. In Lines 158–160, the claim that “X-ray absorption near edge structure (XANES, Fig. 1f) ……confirming preserved square-planar Fe-N4 coordination” is not sufficiently supported by the XANES results. Additional evidence or clarification is needed.\nResponse: We appreciate the Reviewer’s thoughtful suggestion. To confirm the preserved square-planar Fe-N4 coordination, we performed NEXAFS analyses for both C and N K-edges. The relevant discussions were added to the revised manuscripts.\n“The structural integrity of FePc was largely preserved after immobilization onto nanodiamond substrates, as confirmed by consistent characteristic signals in the X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), Raman, and near-edge X-ray absorption fine structure (NEXAFS) spectra.” (Please see Page 5)\n“NEXAFS spectroscopy was employed to verify the structural integrity of the FePc macrocycle upon anchoring. As shown in Supplementary Fig. 5, a notable spectral evolution is observed in the C K-edge. Specifically, the peak at 287.2 eV, assigned to σ* (C–H) transitions and serving as a fingerprint of the intermolecular π-π stacking in bulk FePc24, vanishes in the FePc-O-AND spectrum. This spectral quenching confirms that the FePc aggregates are effectively de-aggregated, resulting in a monolayer dispersion on the O-AND surface. Crucially, apart from this stacking-related feature, the other dominant resonances in both the C and N K-edges remain energetically stable25, exhibiting no significant redshift or blueshift. This spectral consistency suggests that the macrocyclic skeleton is preserved, effectively ruling out Fe–N bond cleavage or destructive structural distortion during the loading process.” (Please see Page 6)\nComment 5-1. Detailed electrochemical experimental procedures should be included in the Supporting Information.\nResponse: We appreciate the Reviewer’s careful suggestion. The details of electrochemical experimental procedures have been added to the Supporting Information. Furthermore, the specific conditions for the degradation experiments (including pollutant concentration, catalyst loading, and pH) have been supplemented in the figure captions.\n“Supplementary Text 7. Electrochemical measurements.\nThe open-circuit potential was measured on an electrochemical workstation (CHI 760E, Shanghai Chenhua). A glassy carbon electrode (GCE, 3 mm in diameter) loaded with catalyst powder was first prepared. 5 mg of catalyst was dispersed into a solution composed of 5 wt% Nafion perfluorinated resin, isopropanol, and water at the ratio of 1:20:80 (v/v/v), and then the resulting solution was sonicated for 20 min to form a homogenous ink. Subsequently, 20 μL suspension was pipetted onto the surface of GCE and vacuum dried at 60 ℃ for 30 min.”\n“Supplementary Text 8. Galvanic oxidation process (GOP).\nTo probe the direct redox interaction between PAA and 4-CP occurring at the catalyst surface, we performed galvanic cell experiments using a dual-chamber reactor. The catalyst was immobilized on a graphite sheet electrode (2 cm × 2 cm) as follows. 40 mg of catalyst powder was dispersed in 10 mL of a mixture containing 5 wt% Nafion solution, isopropanol, and water at the ratio of 1:20:80 (v/v/v), then the resulting solution was sonicated for 20 min to form a homogenous ink. A total of 1 mL of the catalyst ink was uniformly coated onto one side of the graphite sheet and vacuum-dried at 60 oC for 30 min. This coating procedure was repeated once on the same side and twice on the opposite side to ensure uniform catalyst deposition. In the galvanic cell setup, one chamber was filled with a solution of 20 M 4-CP and 50 mM Na2SO4 as electrolyte, while the other contained only 50 mM Na2SO4. The reaction was initiated by injecting 0.25 mM PAA into the PAA-containing chamber. The current generated during the reaction was recorded using an ammeter connected to both electrodes via copper wires. Samples were periodically collected from the 4-CP chamber for subsequent analysis.”\nComment 5-2. Conditions for degradation experiments should be clearly stated in the figure captions.\nResponse: We appreciate the Reviewer’s careful review and gentle reminder. We have added the experimental conditions in the figure captions, such as Supplementary Figs. 21, 22 and 28.\nComment 6-1. In Lines 285–293, more comprehensive evidence is required. For example, through matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF-MS). If 4-CP is removed via coupling into larger molecules, the authors should attempt to detect polymeric products using solvents such as ethanol or toluene to dissolve oxidation products adsorbed on the materials. Additionally, a preliminary toxicity assessment of the oxidation products is recommended.\nResponse: We appreciate the Reviewer’s constructive comment. We conducted MALDI-TOF-MS analysis to characterize the polymeric products formed during 4-CP removal. To ensure the complete extraction of the oxidation products adsorbed on the catalyst surface, we employed tetrahydrofuran (THF) as the extraction solvent. The selection of THF over ethanol or toluene is due to its superior solubility for high-molecular-weight phenolic oligomers. The results have been added to the revised manuscript as follows.\n“To elucidate the oxidation mechanisms of 4-CP on different catalyst surfaces, surface-adsorbed oligomeric products were extracted using two complementary protocols. A methanol/toluene mixture was used for ultra-high performance liquid chromatography-quadrupole time-of-flight mass spectrometry (UPLC-QTOF-MS) detection, whereas tetrahydrofuran (THF) was selected to extract polymeric products, which were subsequently isolated via rotary evaporation prior to matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF-MS) analysis. The MALDI-TOF-MS spectrum revealed that the polymeric products possess a repeating unit with a mass of m/z ≈ 126.6 Da (Supplementary Fig. 30), in excellent agreement with the theoretical mass of the chlorophenoxyl moiety (ClPhO). This observation provides direct evidence for an oxidative coupling-driven polymerization pathway.” (Please see Page 15)\nComment 6-2. Additionally, a preliminary toxicity assessment of the oxidation products is recommended.\nResponse: We appreciate the Reviewer’s thoughtful suggestion. As advised, we evaluated the toxicity of the degradation intermediates, including the ECOSAR prediction model and E. coli toxicity tests. The results have been added to the revised manuscript as follows.\n“The ecotoxicity of 4-CP and its degradation products toward fish, daphnia, and green algae was initially predicted using the ECOSAR program. The simulation indicated that the polymeric intermediates could exhibit higher theoretical toxicity than the parent compound (Supplementary Figs. 41, 42 and Supplementary Tables 3,4). However, under practical reaction conditions, these aggregated products primarily accumulate on the surface of the heterogeneous catalyst, effectively preventing their leaching and mitigating secondary water pollution. To validate this, toxicity assays were conducted using the post-reaction effluent, which represents a realistic matrix containing both the residual PAA and the soluble organic degradation products. The results demonstrate that the treated effluent does not exert inhibitory effects on E. coli (Supplementary Fig. 43 and Supplementary Table 5). Collectively, these findings confirm that the potentially toxic polymers remain immobilized and that the residual PAA concentration is reduced below the inhibitory threshold for bacterial growth, either due to rapid consumption or conversion into biodegradable organic carbon.” (Please see Page 17)\nComment 7. In Lines 288–293: Since KSCN is known to rapidly consume PMS in heterogeneous/PMS systems, potentially leading to misinterpretation of quenching effects, the authors should clarify whether KSCN directly reacts with PAA, resulting in significant PAA depletion and thus the observed inhibition of 4-CP removal.\nResponse: We appreciate the Reviewer’s considerate comment. The Reviewer is correct that KSCN slowly consumes PAA, as evidenced by our experiment. Therefore, we substituted KSCN with 1,10-phenanthroline (phen), a specific iron chelator, to rigorously test the contribution of metal sites 23. The results confirm that blocking Fe sites greatly inhibits the oxidation reaction, proving their essential role in catalyzing PAA activation for 4-CP removal.\n“Because KSCN was found to directly react with PAA (Supplementary Fig. 44), 1,10-phenanthroline was instead selected as a selective chelating agent to probe the accessibility and involvement of Fe active sites47. As displayed in Supplementary Fig. 45, a clear dose-dependent inhibition of 4-CP removal was observed upon introducing phenanthroline. This result confirms that the Fe species are chemically accessible and act as the indispensable active centers for PAA activation.” (Please see Page 17)\nComment 8-1. In Lines 304–307: According to earlier descriptions, FePc-O-AND/PAA achieves complete 4-CP removal within 2 minutes. Why does the control group (0 mM NaClO4) require 5 minutes?\nResponse: We appreciate the Reviewer’s critical observation. After careful review, we found that the slower degradation (5 min) observed in the initial control group (0 mM NaClO4) in Fig. 2i was likely due to partial catalyst deactivation because of long-term sample storage in air. To address this issue and ensure data reliability, we have synthesized and used the fresh FePc-O-AND catalyst and repeated the experiment under identical conditions. The updated results show complete, reproducible 4-CP removal within 2 min in the control group, in agreement with our earlier description and data.\nComment 8-2. Furthermore, the statement that “FePc-O-AND/PAA system exhibited negligible changes in degradation kinetics” is not accurate. First, Fig. 2i does not provide kinetic data; second, if kinetic constants were calculated, a significant inhibitory effect would likely be evident.\nResponse: We appreciate the Reviewer’s careful observation and gentle reminder. We agree with the Reviewer these mentioned expressions are not accurate. In revision, the statement of “degradation kinetics” has been carefully rephrased to “degradation efficiency” in the revised manuscript.\n“As shown in Fig. 2i, the FePc-O-AND/PAA system exhibited negligible changes in degradation efficiency across varying NaClO4 concentrations……” (Please see Page 18)\nComment 9. In Line 401, data on the effect of HA on 4-CP degradation are missing.\nResponse: We appreciate the Reviewer’s careful suggestion. We have included the specific data on the effect of humic Acid (HA) in the revised manuscript.\nComment 10-1. In Lines 402–403, experiments evaluating the impact of real water matrices (e.g., river water or treated wastewater) on degradation performance should be added.\nResponse: We appreciate the Reviewer’s thoughtful suggestion. We evaluated the degradation performance of 4-CP in three real water matrices: Yangtze River water, East Lake water, and secondary effluent from a municipal wastewater treatment plant. The relevant data have been added to the revised manuscript.\n“Even in real water matrices such as seawater, river water, lake water, secondary effluent, and tap water, FePc-O-AND retained high catalytic activity, underscoring its robustness and practical applicability.” (Please see Page 24)\nComment 10-2. Water quality parameters of these actual water samples should also be provided.\nResponse: We appreciate the Reviewer’s great suggestion. The detailed water quality parameters (e.g., pH, TOC, NH3-N) for these samples have been provided in Supplementary Table 13.\n“Real water samples were collected from the following sources: lake water from Donghu Lake (Wuhan, China), river water from the Yangtze River (Wuhan section, China), seawater from Glenelg Beach (Adelaide, Australia), and secondary sedimentation tank effluent from a wastewater treatment plant in Chongqing, China. The detailed water quality parameters are provided in Supplementary Table 13.” (Please see Page 28)\nComment 11. In Lines 457–458, data on PAA consumption are missing.\nResponse: We appreciate the Reviewer’s careful review and reminder. We have supplemented the manuscript with data on PAA consumption kinetics and the corresponding utilization efficiency.\n“In addition to substrate selectivity, oxidant utilization efficiency was also evaluated. As shown in Supplementary Fig. 29, the FePc-O-AND/PAA system exhibits a markedly lower PAA consumption rate (0.20 min−1) compared to the FePc-AND/PAA system (0.34 min−1). To quantitatively assess oxidant efficiency, the PAA utilization efficiency was defined as the molar ratio of pollutant removed to PAA consumed (Δ[4-CP]/Δ[PAA]). At 2 min, this value reached 0.26 for the FePc-O-AND system, substantially higher than that of FePc-AND (0.10), demonstrating a significantly improved oxidant economy in the FePc-O-AND/PAA system. This enhancement stems from the formation of surface-confined, metastable reactive complexes (catalyst-PAA*) rather than free radicals. These complexes increase the redox potential of the FePc-O-AND, enabling efficient electron abstraction from electron-rich pollutants and thereby promoting near-complete PAA conversion into acetic acid.” (Please see Page 15)\nComment 12-1. Several errors and inconsistencies throughout the manuscript require careful revision. Examples include: Line 86: “AND were oxidized”;\nResponse:We appreciate the Reviewer’s careful proofreading. The phrase “AND were oxidized” has been corrected to “AND was oxidized”in the revised manuscript.\nComment 12-2. Line 163: “these findings provide evidence” (not “evidences”);\nResponse: We appreciate the Reviewer’s careful correction. The phrase “evidences” has been corrected to “evidence”in the revised manuscript.\nComment 12-3. Line 165: The full term for “NEXAFS” should be provided upon first use;\nResponse: We appreciate the Reviewer’s correction. The full term “near-edge X-ray absorption fine structure (NEXAFS)”has been spelled out at the first mention in the revised manuscript.\nComment 12-4. Lines 209–211: The text description contradicts Supplementary Fig. 17a;\nResponse:We appreciate the Reviewer’s gentle reminder. The figure caption has been corrected in the revised Supporting Information to ensure the consistency with the text description.\nComment 12-5. Lines 311–312: The claim that “the degradation performance remained unchanged in both the FePc-O-AND and FePc-AND systems” is not fully consistent with the slight decrease shown in the results. The description should accurately reflect the experimental outcomes;\nResponse: We appreciate the Reviewer’s rigorous observation. To accurately reflect the experimental outcomes, we have amended the description in the revised manuscript.\n“Further evidence was provided by a pre-mixing experiment: when 4-CP was pre-adsorbed before PAA addition, the degradation performance exhibited only a slight decrease in both the FePc-O-AND and FePc-AND systems (Supplementary Fig. 46b), indicating that PAA preferentially coordinates to the Fe site, regardless of the presence of 4-CP.\" (Please see Page 18)\nComment 12-6. Line 391: Remove the erroneous word “acts” after “performance.”;\nResponse: We appreciate the Reviewer’s correction. The erroneous word “acts” has been removed from the revised manuscript.\nComment 12-7. Verb tenses should be checked and made consistent throughout the manuscript.\nResponse:We appreciate the Reviewer’s gentle reminder. We have carefully proofread the full text to ensure consistent verb usage. Specifically, the manuscript has been revised to strictly follow the convention of using the past tense for experiments and results, and the present tense for general statements and conclusions.\nComment 13-1. Several issues were identified in the references: Lines 63–65: Authoritative references should be added where appropriate. This issue occurs in multiple instances;\nResponse:We appreciate the Reviewer’s suggestion. We have updated the citations in Lines 63–65 to better support our claims.\n“Among various approaches, surface functionalization of conductive carbon substrates with oxygen-containing groups enhances electron donation to FePc, facilitating the formation of FeN4-O axial coordination bonds.18, 19” (Please see Page 4)\nComment 13-2. Line 178: The citation of Reference 30 does not align with the discussion context “which is assigned to hybridized Fe 3d–O 2p orbitals30, 31”. A more suitable reference should be selected;\nResponse:We appreciate the Reviewer’s close scrutiny. We have carefully re-examined Reference 30 (Chen et al. 2022). We respectfully submit that this work explicitly supports the assignment of the pre-edge feature. Specifically, the cited paper states: “It corresponds to the transition from the O 1s core state to the unoccupied state of O 2p hybridized with Fe 3d states.” This description aligns precisely with our discussion in the manuscript. However, to ensure maximum clarity and avoid potential ambiguity, we have refined the corresponding sentence in the revised manuscript while retaining this relevant citation. Moreover, we have included an additional reference (Ye et al. 2025) that further supports this orbital hybridization mechanism.\n“......which is attributed to transitions from the O 1s level to unoccupied states of O 2p orbitals hybridized with Fe 3d states34, 35”\nComment 13-3. Lines 594–595 and 636–637: Page numbers are missing for References 24 and 40.\nResponse: We appreciate the Reviewer’s careful correction. The missing pagination details for references 24 and 40 have been added in the revised manuscript."}, {"round": 2, "reviewer_comments": {"reviewer_1": "Authors have well responded to all comments.", "reviewer_2": "authors have addressed my concerns and i agree it to be accepted.", "reviewer_3": "The authors have thoroughly addressed the reviewers’ comments and concerns by conducting carefully designed supplementary experiments and providing comprehensive responses. The logical coherence and scientific content of the manuscript have been substantially strengthened. Accordingly, I recommend acceptance of the manuscript in its current form."}, "author_response": ""}]

Decision letter
Title: Axial Oxygen Coordination Drives Spin-Regulated Electron Transfer in Single-Atom Fe Catalysts for Selective Pollutant Transformation
Manuscript Number: NCOMMS-25-62823
We sincerely thank the Reviewers for their constructive and encouraging comments, which have been invaluable in improving the clarity and scientific quality of our review. We have carefully addressed all suggestions and revised the manuscript accordingly, with the changes highlighted in blue in the revised version. A detailed, point-by-point response to each comment is provided below. We hope that the revisions satisfactorily address the concerns raised and meet the high standards of the Editor and Reviewers for publication in Nature Communications.