Electron Delocalization Realizes Speedy Fenton‐Like Catalysis over a High‐Loading and Low‐Valence Zinc Single‐Atom Catalyst

Abstract A zinc (Zn)‐based single‐atom catalyst (SAC) is recently reported as an active Fenton‐like catalyst; however, the low Zn loading greatly restricts its catalytic activity. Herein, a molecule‐confined pyrolysis method is demonstrated to evidently increase the Zn loading to 11.54 wt.% for a Zn SAC (ZnSA‐N‐C) containing a mixture of Zn−N4 and Zn−N3 coordination structures. The latter unsaturated Zn−N3 sites promote electron delocalization to lower the average valence state of Zn in the mix‐coordinated Zn−Nx moiety conducive to interaction of ZnSA‐N‐C with peroxydisulfate (PDS). A speedy Fenton‐like catalysis is thus realized by the high‐loading and low‐valence ZnSA‐N‐C for PDS activation with a specific activity up to 0.11 min L−1 m−2, outstripping most Fenton‐like SACs. Experimental results reveal that the formation of ZnSA‐N‐C−PDS* complex owing to the strong affinity of ZnSA‐N‐C to PDS empowers intense direct electron transfer from the electron‐rich pollutant toward this complex, dominating the rapid bisphenol A (BPA) elimination. The electron transfer pathway benefits the desirable environmental robustness of the ZnSA‐N‐C/PDS system for actual water decontamination. This work represents a new class of efficient and durable Fenton‐like SACs for potential practical environmental applications.


Text S2. Catalytic Tests and Analyses
The catalytic performance was assessed by adding ZnSA-N-C (0.1 g L −1 ) into 40 mL of bisphenol A (BPA) solution (0.1 mM) under magnetic stirring in a constant temperaturecontrolled water bath at 30 °C.After pre-interaction for 30 min to ensure the uniform suspension, PDS (2 mM) was injected into the suspension to initiate the Fenton-like reaction.At predetermined time intervals, 1 mL aliquots of reaction solution were withdrawn by syringe and immediately filtered through a Millipore filter (0.22 μm) for analysis.The batch experiments were carried out in at least two duplicates.The initial solution pH (before PDS addition) was adjusted by dilute sodium hydroxide and S3 sulfuric acid solutions.The long-term durability of ZnSA-N-C was evaluated via a homemade continuous-flow column reactor.
The concentration of BPA was measured by a 1260 Infinity HPLC (Agilent, USA) with a UV detector and a Poroshell 120 EC-C18 column (4.6  100 mm, 2.7 μm) at a detection wavelength of 225 nm.A mixture of methanol/water (70:30, v/v) was used as a mobile phase, and the flowing rate was set as 1.0 mL min −1 .The total organic carbon (TOC) concentration was determined by a TOC-L analyzer (Shimadzu, Japan).The amount of zinc ions leaching from the catalyst after catalytic reaction was measured by inductively coupled plasma optical emission spectrometry (ICP-OES) (Avio 200, PerkinElmer).The PDS concentration was analyzed with spectrophotometric method on a Hach DR 6000 UV-vis spectrometer.Measurement of residual sulfate anion (SO4 2− ) was conducted by a Thermo ICS-600 ion chromatography (IC).Electron paramagnetic resonance (EPR) measurements for in situ detection of hydroxyl ( • OH)/sulfate (SO4 •-) and superoxide (O2 •-) radicals were undertaken on a Bruker A300 spectrometer with 5,5-dimethyl-1-pyrroline N-oxide (DMPO) as the spin-trapping agent in the aqueous and methanol media, respectively.Singlet oxygen ( 1 O2) was detected by EPR spectrometry with the spin-trapping agent of 2,2,6,6-tetramethyl-4-piperidinol (TMP) in aqueous solution.The generation of O2 •-was also investigated via a nitroblue tetrazolium (NBT) method.Electrochemical measurements including cyclic voltammetry (CV), open-circuit potential (OCP), and chronoamperometry (CP) were conducted in a standard three-electrode cell system on a CHI 700E electrochemical workstation with a sodium sulfate solution (0.5 M) as an electrolyte.The as-prepared S4 samples, Pt foil and Ag/AgCl electrode were utilized as working electrode, counter electrode and reference electrode, respectively.The CV analysis was conducted at a scan rate of 50 mV s −1 .For volatile product identification, 50 mL of the sample was extracted with 1.5 mL of dichloromethane, and the extract was analyzed on a gas chromatography-mass spectrometer (GC-MS, Shimadzu GC/MS-QP2020 NX) in EI mode using a full scan range of 40-400 m/z S8 (DB-1701 column, 30 m  0.25 mm  0.25 μm; injector 280 °C, oven 50 °C held for 5 min, and then ramped to 250 °C at 5 °C min −1 , and auxiliary 270 °C).The in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) analysis of BPA degradation over ZnSA-N-C via PDS activation was performed on a PerkinElmer Frontier FTIR spectrometer.

Text S3. Characterizations
The morphology of and elemental distribution of samples were observed by a FEI Quanta FEG 250 scanning electron microscopy (SEM) and a FEI Tecnai G2 F20 transmission electron microscopy (TEM) equipped with an energy dispersive X-ray (EDX) spectroscopy.The atomic dispersion of zinc atoms on the surface of catalyst was analyzed on an aberration-corrected high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM, FEI Themis Z).The phase structures of samples were investigated using a Bruker D8 Advance diffractometer with Cu Kα radiation (λ = 1.54178Å).The N2 adsorption/desorption isotherm measurements were performed by a Micrometrics ASAP 2460 apparatus at 77K.Raman and in situ Raman spectra were recorded on a LabRAM HR Evolution (HORIBA, France) with a S5 out on a Thermo ESCALAB 250 Xi instrument with monochromatic Al Kα radiation.
The zinc content of sample was measured by inductively coupled plasma optical emission spectrometry (ICP-OES, Avio 200, PerkinElmer).Fourier transform infrared (FTIR) spectra were acquired with a Nicolet iS10 spectrometer with samples dispersed in KBr at a resolution of 4 cm −1 .X-ray absorption fine structure (XAFS) spectroscopy analysis was conducted in a transmission mode at beamline 14W1B of the Shanghai Synchrotron Radiation Facility (SSRF).Data processing of the X-ray absorption near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) spectra were carried out using the IFEFFIT package.The solid-state EPR spectra of catalysts were collected from a Bruker A300 spectrometer.

Text S4. Galvanic Oxidation System (GOS) Experiment
For the preparation of ZnSA-N-C-coated graphite electrode, 5 mg of catalyst powder was added into a mixture of Nafion solution (50 μL) and absolute ethanol (2.95 mL) under sonication for 30 min.Then, the suspension (3 mL) was pipetted onto both sides of graphite sheets and the process was repeated once to ensure an even coating of the catalyst.A commercial salt bridge was used to connect the two half cells and retain electrical neutrality during the measurement.A tinned wire was employed to connect the two electrodes for electron transfer.

Text S5. Theoretical Computations
All calculations were performed using the DMol 3 code base on the spin-unrestricted density function theory (DFT). [1]The electronic exchange and correlation effects were descried by the generalized gradient approximation (GGA) with the Perdew-Burke-S6 Ernzerhof (PBE) function. [2]The Grimme's methods were added in order to describe the van der Waals interactions.The All Electron Relativistic was employed to include all electrons explicitly and introduce some relativistic effects into the core.Furthermore, the double numerical plus polarization (DNP) basis set was selected.A smearing of 0.005 Ha of the orbital occupation was applied to speed up electronic convergence.The real-space global orbital cutoff radius was selected as 4.6Å for the sake of high-quality results.The convergence tolerance of energy, maximum force and displacement were 1  10 -5 Ha, 0.002 Ha Å -1 and 0.005 Å, respectively.The formation energy (Ef) was defined as follows: [3] where EZnNxCy is the total energy of Zn−Nx moiety; μ is the chemical potential of the corresponding species.The reference states are chosen to be perfect graphene for C, nitrogen molecule for N, and isolated zinc atom for Zn.S3.Structure parameters of ZnSA-N-C extracted from the EXAFS fitting of Zn K-edge using the Zn-N3 core structure according to DFT calculations (see Figure S8a for its atomic structure).Co-N2 (0.2) 880.4 BPA ( 11) PMS (0.60) 100% (5 min) 0.695 0.004 [24]   Co-N-CNTs (0.1) 178.5 SMX ( 10) PMS (0.30) 100% (16 min) 0.157 0.008 [25]   Mn-ISAs@CN (0.To ascertain the importance of phenanthroline for sequestering the Zn species, the Znsupported carbon nitride (denoted as Zn Nps-C3N4) was prepared by the same process as Zn-C3N4 but without the addition of phenanthroline.X-ray diffraction (XRD) patterns (Figure S1a) show that both Zn Nps-C3N4 and Zn-C3N4 samples maintain the basic heterocyclic skeleton of C3N4, where the peaks at approximately 13.6° and 26.8° correspond to the (100) and (002) planes regarding the in-plane repeating units of continuous heptazine framework and the stacking of conjugated aromatic structure, respectively. [29]Compared with pristine C3N4, the (100) peak in Zn Nps-C3N4 shifts negatively, while the (002) peak presents an obvious positive shift (by ~0.9°) with an evidently increased intensity.Besides, a new weak peak emerges at approximately 56.9°

S7
in Zn Nps-C3N4 assignable to the metallic ZnO. [30]These alternations illuminate that the Zn aggregation occurs in Zn Nps-C3N4 during the calcination process without the S12 addition of phenanthroline, which results in the distortion in the in-plane aromatic stacking to decrease the interlamellar distance of C3N4 and facilitates the crystallization of C3N4. [31]By contrast, the (100) peak almost vanishes in Zn-C3N4 upon adding phenanthroline as a starting material, and meanwhile, the (002) peak shifts slightly toward the higher angle.More importantly, no identifiable metallic Zn phase can be detected in Zn-C3N4.This result confirms the crucial role of phenanthroline for securing and secluding the Zn atoms in the course of calcination, conducive to the confinement of Zn species in the electron-rich cavities of C3N4 containing macrocyclic units with pyridinic N. In this regard, the X-ray photoelectron spectroscopy (XPS) analyses of C3N4, Zn Nps-C3N4, and Zn-C3N4 were performed.By comparing the XPS N 1s spectrum of C3N4 with that of Zn Nps-C3N4, and Zn-C3N4 (Figure S1b), the C−N=C peak shifts toward the lower binding energy slightly (by 0.05 eV) and significantly (by 0.22 eV) for Zn Nps-C3N4 and Zn-C3N4, respectively.In addition, a novel sub-peak appears at approximately 400.32 eV in Zn-C3N4 referring to the Zn−N bond, while this peak is absent in Zn Nps-C3N4.The apparent negative shift of the C−N=C peak and the appearance of Zn−N peak in Zn-C3N4 manifest the binding of Zn and pyridinic N within the cavities of the heptazine units, [32] where the atomic structure of Zn-C3N4 can be observed in Figure S1c.Moreover, the Zn content of Zn-C3N4 was determined to be 3.24 at% by XPS, which is significantly higher that of Zn Nps-C3N4 (0.19 at%), as evidenced by the much stronger Zn 2p peak intensity for Zn-C3N4 relative to that for Zn Nps-C3N4 (Figure S1d).Unfortunately, both Zn Nps-C3N4 and Zn-C3N4 exhibit limited Fenton-like activity in PDS activation (Figure S2).This situation may be

S13 ascribed to the
insufficient electronic structure modulation and the inadequate dosage of C3N4-based materials or PDS, which are not the focus of the present work.It therefore necessitates the conversion of Zn-C3N4 into a nitrogen-doped carbon-based Zn single-atom catalyst.Upon a higher temperature pyrolysis at 800 °C under N2 atmosphere, the Zn-C3N4 can be converted to a high-loading Zn SAC.The above results highlight the advantage of the surface molecule-confined calcination method for the preparation of the Zn SAC, namely ZnSA-N-C, with a high Zn loading up to 11.54 wt%.

Table S1 .
Elemental composition of the as-prepared sample by XPS and ICP-OES.

Table S2 .
Structure parameters of samples extracted from the EXAFS fitting of Zn Kedge using ZnPc as the Zn−N4 core structure.

Table S4 .
Specific activity comparison of ZnSA-N-C with recently reported transitionmetal (Fe, Co, Mn, Cu)-based SACs for the Fenton-like reaction.