Asymmetrically Coordinated CoB1N3 Moieties for Selective Generation of High‐Valence Co‐Oxo Species via Coupled Electron–Proton Transfer in Fenton‐like Reactions

High‐valence metal species generated in peroxymonosulfate (PMS)‐based Fenton‐like processes are promising candidates for selective degradation of contaminants in water, the formation of which necessitates the cleavage of OH and OO bonds as well as efficient electron transfer. However, the high dissociation energy of OH bond makes its cleavage quite challenging, largely hampering the selective generation of reactive oxygen species. Herein, an asymmetrical configuration characterized by a single cobalt atom coordinated with boron and nitrogen (CoB1N3) is established to offer a strong local electric field, upon which the cleavage of OH bond is thermodynamically favored via a promoted coupled electron–proton transfer process, which serves an essential step to further allow OO bond cleavage and efficient electron transfer. Accordingly, the selective formation of Co(IV)O in a single‐atom Co/PMS system enables highly efficient removal performance toward various organic pollutants. The proposed strategy also holds true in other heteroatom doping systems to configure asymmetric coordination, thus paving alternative pathways for specific reactive species conversion by rationalized design of catalysts at atomic level toward environmental applications and more.


Introduction
The peroxymonosulfate (PMS) activation process lies at the frontier to degrade various contaminants and alleviate the water pollution crisis, and single-atom catalysts (SACs) have received wide attention in it. [1] Typically, atomically dispersed cobalt (Co) atoms anchored on nitrogen-doped carbon substrates are emerging as model catalysts due to their high reactivity and low cost. [2] Free radicals (SO 4 •− /HO•) with strong oxidizing ability are widely recognized as reactive species in SA-Co/PMS systems, [3] they are however prone to be consumed by the coexisting substrates and unfavorable for selective degradation of pollutants. [4] Therefore, unorthodox highvalence Co-Oxo species (i.e., Co(IV)=O) are in the focus, due to the combined merits of appreciable oxidizing ability, higher steady-state concentration, longer lifetime, and excellent anti-interference properties. [5,6] The conversion from Co(II) to Co(IV)=O with the concomitant production of SO 4 2− and H + generally proceeds via O-H and O-O bonds cleavage in Co(II)-PMS complex, coupled with a two-electron transfer process. [6] However, due to the high bond dissociation energy of O-H, [7] the transformation of PMS tends to produce SO 4 •− /HO• by direct cleavage of O-O bond through strong interaction with the heterogeneous SA-Co catalyst. Despite the substantial progress in the formation of highvalence metal species over a large library of SAC systems (e.g., Co, Fe, and Cu) during PMS activation, O-O bond cleavage and electron transfer are dominating pathways, [4,8] leaving the O-H bond cleavage as an essential step unavailable. In this regard, alternative strategies to facilitate O-H bond cleavage are crucial for the selective formation of Co(IV)=O and highly-efficient pollutant removal.
Coupled electron-proton transfer (CEPT), characterized by a concerted elementary step of proton and electron transfer, refers to a nonclassical redox process in which the proton and electron can transfer in two separate acceptors. [9] In this way, the bond dissociation energy falls in a more variable range, [10] which offers possibility to achieve thermodynamically favored O-H bond cleavage. Also, the proton-transfer process in CEPT can downshift the kinetic energy barrier by optimizing local geometries www.advancedsciencenews.com www.advmat.de of catalysts. [11] Therefore, regulating the coordination configurations of single Co atoms to promote CEPT in the Co(II)-PMS complex would be highly desirable for the formation of highvalence Co-Oxo species in SA-Co catalysts.
External electric field may polarize the polar O-H bond in PMS and facilitate its cleavage, such an efficient trigger has already been verified for O-H bond dissociation of water molecules in hydrogen evolution reactions. [12] Therefore, heterogeneous SA-Co catalysts with a strong built-in electric gradient surrounding the coordination sites may boost the CEPT from the O-H bond during PMS activation. Note that the traditional symmetric CoN 4 configuration finds its efficacy in the generation of SO 4 •− /HO• but fails to activate O-H bond, as the local electric field is quite weak for symmetry reasons. Construction of an asymmetric coordination microenvironment with high polarity is seemingly plausible for the generation of a stronger external electric field. Ideally, non-metallic heteroatom doping (e.g., sulfur, phosphorous, and boron) is promising to reshape the local environment of the carbon-based substrates to afford strong polarity. [13] Due to the similarity in atomic radius and the large difference in electronegativity between boron (B) and nitrogen (N), B tends to partially replace N atoms in covalent substrate and directly coordinates with Co, while the altered configuration comes with larger electric field contributions. The resulting asymmetric structure is then expected to facilitate proton transfer with minimized energy barriers, and finally to boost a catalytic cycle based on high-valence Co-Oxo species.
In this work, we successfully established a covalent matrix supported by CoB 1 N 3 single-atom configuration as a proofof-concept. The catalytic system shows high removal efficiency toward various organic pollutants via selectively generated highvalence Co-Oxo species (Co(IV)=O). Specifically, sulfamethoxazole (SMX) could be completely removed within 15 seconds in CoB 1 N 3 /PMS system with a pseudo-first-Order kinetic constant (k) reaching high up to 0.30 s −1 , 9.1 times higher than that of the traditional CoN 4 /PMS system. Enabled by in situ Raman, synchrotron-radiation-based Fourier-transform infrared (SR FTIR) spectra and density functional theory (DFT) calculations, we found that during the first step of PMS adsorption on CoB 1 N 3 moiety, a barrier-free intramolecular proton transfer from O-H to the edge O atom in PMS, coupled with strong electron transfer between Co and adsorbed O, is activated under the local electric field. The CEPT process further contributes to rapid conversion to high-valence Co-Oxo species for subsequent catalytic reaction. These findings provide a reference for the design of Fenton-like catalysts with high activity and selectivity. Furthermore, the CEPT mechanism highlights the exciting pathways to explain the formation of other high-valence metal species, such as Mn, Fe, Cu, etc.

Results and Discussion
The B-doped single-atom Co catalyst (termed as Co SA -BNC, NC denotes N-doped carbon) was prepared by using chitosan, boric acid, and cobalt dichloride as precursors, followed by segmental pyrolysis under vacuum and then 20% H 2 /Ar atmosphere. Meanwhile, Co SA -NC was fabricated as a reference sample by calcining the mixture of chitosan and cobalt dichloride under a vacuum (Figure 1a). The Co contents of Co SA -NC and Co SA -BNC, determined by inductively coupled plasma optical emission spectroscopy (ICP-OES), are 1.5 and 0.8 wt.%, respectively (Table S1, Supporting Information). Besides, B doping promotes the formation of carbon defects, as revealed by the higher I D /I G values of Co SA -BNC and BNC relative to Co SA -NC and NC ( Figure S1, Supporting Information). Moreover, B doping endows Co SA -BNC with improved hydrophilicity ( Figure S2, Supporting Information) and larger surface area (Table S1, Supporting Information), which exposes more active sites for catalytic reactions.
X-ray diffraction (XRD) patterns were first recorded to roughly evaluate the dispersion status of Co in the Co SA -BNC and Co SA -NC ( Figure 1b). Note that Co nanoparticles were co-formed after pyrolysis (termed as Co NPs -NC), which were removed (Co SA -NC) after acid leaching treatment, the resultant Co SA -NC is then characterized by amorphous structure (Figure S3a,b, Supporting Information). In contrast, Co SA -BNC exhibits a peak-free diffraction pattern without acid treatment (Figure 1b), and the highly dispersed Co species are also confirmed by the transmission electron microscopy (TEM) images ( Figure S3c, Supporting Information), which suggests that B doping promotes the atomiclevel dispersion of Co. The aberration-corrected high-angle annular dark-field scanning transmission electron microscopy (AC-HAADF-STEM) images (Figure 1c,d; Figure S3d,e, Supporting Information) verify that Co atoms are atomically isolated on the covalent substrate in both Co SA -NC and Co SA -BNC. In addition, C, N, B, and Co atoms are distributed uniformly throughout the Co SA -BNC catalyst, as evidenced by the energy-dispersive Xray spectroscopy (EDS) mapping images (Figure 1e). These results collectively point to the formation of single Co atoms in Co SA -BNC and Co SA -NC.
To further explore the electronic structure and coordination environments of single Co atoms in Co SA -BNC and Co SA -NC, soft X-ray absorption spectroscopy (XAS), hard XAS, and X-ray photoelectron spectroscopy (XPS) were performed. X-ray absorption near edge structure (XANES) of Co K-edge spectra reveal an increased electropositivity of Co after B doping, although the valence states of both Co SA -NC and Co SA -BNC are close to +2 after hydrogen treatment (Figure 2a; Figure S4, Supporting Information). Compared to Co SA -NC, the decreased intensity of pre-edge peak and increased intensity of white-line peak in Co SA -BNC suggest the reduced symmetry and rearranged charge polarization, [14] demonstrating the altered coordination environment of Co atoms upon B doping. The extended X-ray absorption fine structure (EXAFS) spectra indicated the absence of a peak assigned to Co-Co bond (2.17 Å) in R space, [15] further verifying the atomic dispersion of Co ( Figure 2b). This is also in line with the wavelet transform (WT)-EXAFS results (Figure 2c), in which the maximum intensity observed in Co SA -BNC and Co SA -NC is clearly separated from the plots of Co foil which is mainly located at 7.5 Å −1 (Co-Co bond). [15] Also, the center of Co SA -BNC shows a slight negative shift compared to that of Co SA -NC (Figure 2c), suggesting that the Co atoms in Co SA -BNC might coordinate with foreign atoms with a smaller atomic number than N or C, which is assigned to B atoms in our case. Next, we identified the coordination structure of Co SA -NC as symmetrical CoN 4 . As shown in the N K-edge spectrum of Co SA -NC ( Figure S5, Supporting Information), Co atoms are coordinated with pyridinic N, [16] and the Co-N coordination number in Co SA -NC is determined to be 4.1 based on the fitting results of EXAFS curve (Figure S6 and Table S2, Supporting Information), demonstrating the successful preparation of traditional CoN 4 as a reference sample. We then tried to probe the interplay between Co and B/N in Co SA -BNC. The N K-edge XANES spectrum in Figure 2d reveals the formation of Co-pyridinic N bond. The L-edge position of Co SA -BNC falls between that of Co SA -NC and CoB (Figure 2e), implying the formation of Co-B besides Co-N bond, as further verified by the B 1s XPS spectrum, in which the peak at 186.8 eV is associated with Co-B interaction ( Figure 2f). [17] Meanwhile, the peaks located at 288.0 and 289.1 eV in the C K-edge spectrum also suggest the formation of C-N and C-B bonds in Co SA -BNC ( Figure S7, Supporting Information). [16b,18] The combined results reveal the coexistence of Co-B-C and Co-N-C. Serving as the anchoring sites for single Co atoms, the pyridinic N and newly incorporated B can explain the promoted single atom isolation after B doping, which is directly formed without acid leaching treatment. Considering that the oxygen(O)-rich chitosan may share coordination sites with Co atoms in the synthesized Co SA -BNC, we recorded Co 2p and O 1s XPS spectra and found no signals assigned to Co-O bonding ( Figure S8, Supporting Information). Quantitatively, we simulated the EXAFS (Figure 2g; Table S2) spectra of Co SA -BNC and found that the coordination numbers of Co-N and Co-B were 3.3 and 1.3, respectively, suggesting the asymmetric CoB 1 N 3 moiety is preferred. We also checked the rationality of CoB 1 N 3 as the main configuration by DFT calculations ( Figure S9, Supporting Information). Specifically, we investigated the possibility to anchor a Co atom onto different B, N codoped carbon substrates (B x N 4−x , x = 1, 2, 3). The estimated binding energies of atomic Co with B 1 N 3 -, B 2 N 2 -, and B 3 N 1 -carbon are −5.5, −4.8, and −4.6 eV, respectively. The highest binding energy clearly indicates the dominant configuration of CoB 1 N 3 among the three reasonable structures. To obtain a comprehensive understanding of the local geometrical structure, we further conducted the XAENS spectra simulation of CoN 3 B 1 and CoN 4 configurations ( Figure S10, Supporting Information). The simulated XANES spectrum of CoN 3 B 1 matches well with the experimental one, which further confirms the main component of Co SA -BNC. In comparison, the simulated XANES spectra of CoN 3 B 1 Figure 2. a) Co K-edge XANES spectra, b) FT k 3 -weighted Co K-edge EXAFS spectra, and c) WT-EXAFS plots of Co SA -BNC and the reference samples (Co foil and Co SA -NC). d) N K-edge soft XAS spectrum of the Co SA -BNC. e) Co L-edge soft XAS spectra of Co SA -BNC, Co SA -NC, and CoB. f) B 1s XPS spectrum of Co SA -BNC. g) FT-EXAFS fitting curves of Co SA -BNC at Co K-edge. and CoN 4 exhibit similar tendencies in terms of the difference in white-line peak intensity ( Figure S10c, Supporting Information), keeping consistent with the experimental results. The combined results confirm the formation of symmetric CoN 4 in Co SA -NC and heteroatom-substituted asymmetric CoB 1 N 3 in Co SA -BNC.
SMX was initially selected to evaluate the catalytic performance of Co SA -BNC and Co SA -NC in PMS activation process. The adsorption rates of NC, BNC, Co SA -NC, and Co SA -BNC on SMX are all <5%, their contribution to the degradation results can be neglected (Figures S11 and S12, Supporting Information). Impressively, SMX could be completely removed within 15 s in Co SA -BNC/PMS system with a k value of 0.30 s −1 , which is 9.1 times higher than that of Co SA -NC (Figure 3a). Besides, CoB 1 N 3 exhibits excellent structural integrity and recycling performance in PMS activation (Figures S13 and S14, Supporting Informa-tion), which could remove 92.2% of SMX even in the fourth cycle ( Figure S14, Supporting Information). Electron spin resonance (EPR) tests were conducted to probe the potential reactive species such as •OH, SO 4 •or 1 O 2 ( Figure S15, Supporting Information), as widely reported in PMS activation process. The concentrations of •OH and SO 4 •− in Co SA -BNC are slightly lower than that in Co SA -NC, matching well with the probe experiments using benzoic acid (BA) as a probe molecule ( Figure S16, Supporting Information). On the other hand, there is negligible 1 O 2 generation in the two systems ( Figure S17, Supporting Information). Quenching experiments show that the SMX degradation efficiency only decreases to 81% (k = 0.20 s −1 ) and 89% (k = 0.29 s −1 ) when methanol (MeOH, radical scavenger for SO 4 •− and •OH) or tert-Butanol (TBA, radical scavenger for •OH) is added, respectively. Therefore, these free radicals are not dominant reactive oxygen species in Co SA -BNC/PMS, although they constitute the main contributors in Co SA -NC (Figure 3b; Figure S18, Supporting Information). [2a] This motivated us to explore the possible contribution from high-valence Co-Oxo species, which has been reported in homogenous Co 2+ or heterogenous CoCO 3catalyzed PMS activation processes. [5b,6] Herein, methyl phenyl sulfoxide (PMSO) was used to probe the existence of high-valence Co-Oxo species, as it readily turns to sulfone (PMSO 2 ) via oxygen transfer. [19] As expected, a much higher concentration of PMSO 2 is detected in Co SA -BNC/PMS (Figure 3c; Figure S19, Supporting Information), demonstrating that Co SA -BNC significantly promotes the formation of high-valence Co-Oxo species. The 18 Figure S20, Supporting Information). [20] Accordingly, the degradation efficiency is remarkably reduced when PMSO is added (Figure 3b), highlighting the primary contribution from high-valence Co-Oxo species in pollutant removal. [8f] We also found that the consumed PMS is only 6.2% in the presence of bare PMSO ( Figure S21, Supporting Information), this indicates that PMSO suppresses SMX degradation primarily by quenching the active species rather than consuming PMS. Moreover, cyclic voltammetry (CV) curves and in situ Raman spectra were collected, and the combined results keep consistent with those of PMSO probe experiments and quenching tests. Specifically, a distinct peak at 1.4-1.5 V assigned to Co(III)/Co(IV) oxidation process is detected in the CV curve of Co SA -BNC/PMS system (Figure 3d), while the peak at ≈1.1 V in Co SA -NC/PMS signifies the Co(II)/Co(III) oxidation process. [21] The higher electron-donating and catalytic ability of Co SA -BNC enables O 2 or H + in solution (stemming from the dissociation of BS or PMS) to accept the electrons from the catalyst. The peak centered ≈ −0.6 V is then likely to relate with oxygen reduction or hydrogen evolution reaction, [22] which is however absent in Co SA -NC with much lower activity. The valence transformation from Co(III) to Co(IV) is also tracked by the in situ Raman spectra of Co SA -BNC/PMS. In addition to the generation and conversion of Co(III) located at 436 cm −1 , a signal assigned to Co(IV) at 578 cm −1 gradually increases (Figure 3e). [23] It is worth mentioning that the high catalytic performance at seconds scale ( Figure 3a) does not contradict the in situ Raman signals occurring at minutes scale (Figure 3e, Experimental Section) because the latter is inherently limited by Raman testing conditions which only allows the minimum time interval of 2 min for signal collection to assure reasonable control. Relative to that of Co SA -NC which generates radicals via a one-electron transfer process, Co SA -BNC is found to transfer twice as many electrons as Co SA -NC through the rotating disk electrode tests ( Figure S22, Supporting Information), suggesting that Co SA -BNC prefers to activate PMS via the two-electron transfer pathway. These findings directly support the formation of high-valence Co-Oxo species (Co(IV)=O) in Co SA -BNC/PMS system. In summary, the combination of PMSO probe experiments, quenching tests, CV curves, and in situ Raman confirm that CoB 1 N 3 configuration significantly promotes the generation of high-valence Co(IV)=O species in PMS activation process, which in turn substantially advances pollutant removal performance. The proposed strategy to break the symmetry of metal coordination environment also finds its universality in phosphorous-doped single-atom Co catalyst (termed as Co SA -PNC) to selectively generate high-valence Co-Oxo species (Experimental Procedures in Figure S23, Tables S3 and S4, Supporting Information). As a result of the degradation process oxidized by Co(IV)=O in Co SA -BNC/PMS system, TOC removal efficiency of SMX reaches 58.9% with an extremely low catalyst dosage (0.01 g L −1 ), which climbs to 87.7% and 93.1% when the dosage increases to 0.05 and 0.1 g L −1 , respectively, suggesting that Co SA -BNC could achieve high-level water purification with proper catalyst dosage (Figures S24 and S25, Supporting Information). Due to the inherent strong oxidizing ability of high-valence Co species, oxygen transfer, aniline oxidation, N-S bond cleavage, polymerization, and a series of ring-cleavage processes can be achieved in the SMX oxidation process (Figures S26-S28, Supporting Information). [4,6] In our system, we also detected the oxidation product of olefins. For example, two carbons can be detached from TP116 (C 4 H 8 N 2 O 2 ) by olefin oxidation to form TP106 (C 2 H 6 N 2 O 3 ). Two and four carbons can be detached from TP502 (C 20 H 18 N 6 O 6 S 2 ) via ring-cleavage and olefin oxidation to form TP526 (C 18 H 18 N 6 O 9 S 2 ) and TP518 (C 16 H 18 N 6 O 10 S 2 ), respectively. The final mineralized products are assumed to be CO 2 and water, suggesting the potential application of high-valence Co species in the mineralization of organic pollutants. Depending on the differences in electron-losing ability of organic pollutants, Co SA -BNC can also achieve selective removal toward various pollutants (Figure 3f; Figures S29 and S30, Tables S5-S7 (Sup-porting Information), Experimental Section), including methylene blue (MB), sulfamethoxazole (TTC), bisphenol A (BPA), ciprofloxacin (CIP) and rhodamine B (RhB), benzoic acid (BA) and nitrobenzene (NB), suggesting its favorable practicability.
To study the intrinsic formation mechanism of high-valence Co-Oxo species, DFT calculations were carried out to explore the possible intermediates in the dynamic catalytic process. We initially proposed that single Co atoms could catalyze the adsorbed PMS, which then directly split into SO 4 2− and H + to generate Co(IV)=O, [19] but it turned out to be thermodynamically unfavorable ( Figure S31, Supporting Information), implying that other intermediates might be involved for the formation of Co(IV)=O. To directly investigate the surface chemical evolution of singleatom Co sites during PMS activation process, in situ Raman spectra were collected in both CoB 1 N 3 and CoN 4 systems. As depicted in Figure 4a, the peaks centered at 881, 978 and 1060 cm −1 in CoB 1 N 3 /PMS system can be assigned to the adsorbed PMS (HSO 5 − ), SO 4 2and HSO 4 − , respectively. [24] A sharp signal of PMS appeared at 2 min and then rapidly decreases, accompanied by continuous accumulation of SO 4 2with prolonged reaction time, implying the rapid PMS activation process (Figures S32-S34, Supporting Information). Besides, a peak at 835 cm −1 stemmed from the adsorbed peroxo species is clearly observed during the activation process, which indicates the strong Co-PMS interactions. [4] Notably, a sharp signal of HSO 4 − rapidly appears at 4 minutes, which further intensifies and then weakens, suggesting that HSO 4 − might serve as a key intermediate in the process of Co(IV)=O formation. Comparatively, the signals of HSO 5 − and SO 4 2only experienced a moderate increase in CoN 4 /PMS system (Figure 4b), reflecting the weaker adsorption and activation of PMS. The signal of HSO 4 − remains invisible, signifying that HSO 4 − contributes little to the formation of free radicals. These results are further supported by the synchrotron radiation-based Fourier-transform infrared (SR FTIR) spectra (Figure 4c), the distinct peak assigned to HSO 4 − (centered at 1150 cm −1 ) in CoB 1 N 3 /PMS is not detectable in CoN 4 /PMS. [25] Given that B doping alters the symmetry of conventional CoN 4 configuration, non-uniform distribution of electrons and enhanced local electric field in asymmetric CoB 1 N 3 are expected to trigger the CEPT process, which can facilitate the hetero-cleavage of polar O-H bond. The dissociated proton is prone to be captured by the edge O atoms (depicted as an intramolecular protontransfer process), leaving two electrons retained in the O 2p orbital. Consequently, they are coupled with the other two electrons from single-atom Co donator to form Co=O double bond, further contributing to O-O bond cleavage. As a result, Co(II) is successfully transformed into Co(IV)=O accompanied by generation of HSO 4 − , which gradually transforms into H + and SO 4 2-(path 1 in Figure 4d). We notice that the redox potential of Co(IV)/Co(II) is recognized to be higher than HSO 5 − /SO 4 2− (>1.88 V vs 1.82 V), [26] however, the overall potential of PMS for oxidizing Co(II) to Co(IV) depends on the integrated contribution from E(HSO 5 − /=O) and E(HSO 5 − /SO 4 2− ) ( Figure S35a, Supporting  Information). Therefore, the extra potential E(HSO 5 − /=O) of PMS provides possibility to drive Co(II)/Co(IV) transformation. Although the transformation from Co(II) to Co(IV) is dominated by two-electron transfer, we are not committed to excluding the existence of intermediate states, such as Co(III) ( Figure S35b, Supporting Information). Temperature-dependent magnetization tests reveal that the single Co atoms in Co SA -BNC possess a high spin state ( Figure S35c, Supporting Information). Upon simulating the rearrangement of d-electrons with a square planar crystal field, [27] we found that the two most easily lost electrons are located in different energy levels ( Figure S35d, Supporting Information), making them capable of losing in a suc-cessive way. Due to O-H bond cleavage can take place when the two-electron transfer occurs between Co-O in CoB 1 N 3 /PMS, free radicals are not likely to be produced even when Co(III) intermediates exist. We also noted that path 1 is more favorable when the Co SA -BNC/PMS system is situated in an alkaline solution, where the dissociated hydrogen can combine with OH − to form H 2 O and accelerate the cleavage of O-H bond, further promoting the formation of Co(IV)=O by PMS activation and contributing to higher k value ( Figures S36-S38, Supporting Information). On the other hand, PMS adsorbed on the single-atom Co sites in symmetric CoN 4 without a strong local electric field directly accepts one electron from single Co atoms to trigger O-O bond cleavage, which results in the generation of free radicals, without forming HSO 4 − (path 2 in Figure 4d). [28] The HSO 4 − detected by in situ Raman and SR-FTIR spectra indicates the intramolecular proton-transfer process in CoB 1 N 3 /PMS, and the coupled strong electron transfer is also revealed by open-circuit potential (OCP) measurements and projected density of states (PDOS) calculations (Figure 4e). The OCP of both CoN 4 and CoB 1 N 3 systems increases when PMS is added, elucidating that single Co atoms donate electrons to PMS. The much higher kinetics and larger potential gap in CoB 1 N 3 /PMS arise from the greatly enhanced electron transfer, which is further confirmed by adding SMX and a more evident decrease of OCP ( Figure S39, Supporting Information). Besides, PDOS calculations show that the single Co atoms in CoB 1 N 3 possess a d-band center of −1.11 eV, which is much higher than that in CoN 4 (−2.17 eV), indicating the enhanced adsorption ability and better affinity to PMS. [29] In addition, the electrons of the Co 3d orbital in CoB 1 N 3 are strongly overlapped with the adsorbed O 2p orbital in PMS, implying the intensified interaction and easier electron transfer between Co SA -BNC and PMS, matching well with OCP tests. Collectively, the promoted CEPT process for the formation of Co(IV)=O in Co SA -BNC/PMS is confirmed.
Low-temperature EPR spectra were recorded to evaluate the electronic state near central Co atoms. The signal intensity of CoB 1 N 3 is dramatically strengthened relative to that of CoN 4 , implying a higher concentration of free electrons (Figure 4f; Figure S40, Supporting Information). These unpaired electrons of single Co atoms in CoB 1 N 3 are more likely to be attracted by the asymmetrically coordinated electron-withdrawing atoms with strong electronegativity, causing non-uniform distribution of electrons to create a local electric field. Electron localization function analysis was further conducted to simulate the electron distribution around central Co sites (Figure 4g). Obviously, the electrons of four pyridinic N atoms are uniformly distributed on a highly symmetrical CoN 4 configuration. However, loss of electrons appears near Co-B bond when one pyridinic N is replaced by one B atom to form asymmetric CoB 1 N 3 , due to the larger electronegativity of N than B (3.04 vs 2.04). As such, a local polar electric field is built along the B→Co→N direction, which is also a reflection of the tailored spin state of Co atoms after B doping ( Figure S35a, Supporting Information). This field is interpreted as an external force to facilitate proton transfer from O-H bond to the edge O atom. Energy barrier calculations also highlight that the proton transfer could proceed in a barrier-free manner under the local electric field of CoB 1 N 3 ( Figure S41, Supporting Information). DFT calculations of the corresponding energy profiles about the intermediate structures in CoB 1 N 3 and CoN 4 are depicted in Figure 4h, Figures S42 and S43 (Supporting Information). For CoN 4 , free radicals are more likely to generate through path 2. However, the adsorption model with an intramolecular proton transfer to firstly break O-H bond is preferred in CoB 1 N 3 , followed by a more thermodynamically favored O-O cleavage to form Co(IV)=O. The maximum total energy released during the reaction on CoB 1 N 3 sites is 4.96 eV, significantly larger than that of CoN 4 (1.88 eV), again signifying the much higher conversion potential to form high-valence Co(IV)=O in CoB 1 N 3 .

Conclusion
Our experimental results and theoretical analyses clearly demonstrated that the asymmetrical CoB 1 N 3 configuration dramatically enhances the activation of PMS to selectively generate highvalence Co-Oxo species, which allows ultrafast removal efficiency toward various organic pollutants. The doping of B atoms induces a strong local electric field around the atomic Co, thus promoting the CEPT process during the first step of PMS adsorption, facilitating peroxy O-O bond cleavage in the conversion pathway to Co(IV)=O. This work highlights that the selective generation of high-valence metal species in Fenton-like reactions can be achieved through rational design of catalysts at atomic level. Moreover, the promoted CEPT enabled by local electric field in asymmetric configurations of the large SAC family, also provides exciting opportunities for the development of catalysts to cleave O-H bonds in organometallic and/or inorganometallic catalysis.

Experimental Section
Synthesis of Co SA -BNC and Co SA -NC: Chitosan (1 g), acetic acid (0.25 mL), CoCl 2 ·6H 2 O (60 mg), and H 3 BO 3 (180 mg) were mixed in 50 mL deionized water by continuous stirring to obtain a homogeneous gel solution. Then, the mixture was spread in a petri dish (diameter: 10 cm) and dried at 60°C under a vacuum for 24 h. The purple solid was heated to 550°C with a ramping rate of 10°C min −1 and kept for 2 h under vacuum. Then, the product was ground into a fine powder and washed three times with 20 mL water and ethanol, respectively, followed by drying at 60°C for 8 h. The powder was further mixed with 180 mg H 3 BO 3 and ground thoroughly, followed by annealing under 400°C for 2 h in H 2 (20% H 2 /Ar) atmosphere. Finally, the powder was washed and dried with the same procedures as described above. The obtained dark powder was denoted as Co SA -BNC. Similarly, 1 g chitosan, 0.25 mL acetic acid, 50 mL H 2 O, and 60 mg CoCl 2 ·6H 2 O were mixed and dried, followed by pyrolysis at 550°C for 2 h under vacuum. The product was denoted as Co NPs -NC. Then, the product was leached in 0.25 m H 2 SO 4 solution for 10 min. After washing with water and ethanol for three times and drying at 60°C for 8 h, the obtained powder was denoted as Co SA -NC.
Soft XAFS and hard XAFS Measurements: Soft XAS spectra were recorded at Beamlines MCD-A and MCD-B (Soochow Beamline for Energy Materials) in the National Synchrotron Radiation Laboratory (NSRL) in Hefei, China. Finally, the spectra were normalized to yield an edge jump to one. Hard XAFS spectra of Co K-edge were measured at the BL14W1 beamline stations in Shanghai Synchrotron Radiation Facility (SSRF) (Shanghai, China). Before the analysis at the beamline, samples were pressed into thin sheets (diameter: 1 cm) and sealed using Kapton tape film. The XAFS spectra were recorded at room temperature using a 4-channel Silicon Drift Detector (SDD) Bruker 5040. Using Si (311) double-crystal monochromator, the data collection was carried out in transmission mode at ambient conditions. The fitting details were shown in Supporting Information.
Measurements of Catalytic Performance: To evaluate the catalytic performance, catalysts were added into 50 mL of pollutant solution (45 μm), followed by the addition of 0.8 mm PMS. The pollutant solution was further added with 50 mm phosphate-buffered saline (PBS, 17.2 mm KH 2 PO 4 mixed with 32.7 mm Na 2 HPO 4 ·12H 2 O) to maintain a stable pH value of 7. The sample was directly mixed with an equal volume of methanol and filtered with a filter membrane (Teflon, 0.22 μm) for further testing. To obtain reliable results in the catalytic reactions using Co SA -BNC which could rapidly degrade pollutants, ten independent parallel experiments were