Pickling defect engineered iron monatomic catalyst, method for preparing same, and use thereof

Abstract

The application belongs to the technical field of iron monatomic catalyst preparation, and particularly relates to a pickling defect engineering modified iron monatomic catalyst and a preparation method and application thereof. The pickling defect engineering modified iron monatomic catalyst contains intrinsic defect sites on the surface, the content of Fe element is 0.60-0.67wt%, the coordination form of Fe atom is Fe-Nn, the coordination number n is 4.2+ / -0.3, the Fe-N bond length is 2.01 angstrom, and the valence state of Fe element is between Fe 2+ and Fe 3+ . Through the synergistic strategy of the optimized ratio design of ZIF-8 precursor and the pickling defect engineering of concentrated sulfuric acid, the controllable construction of defect sites and the significant improvement of catalytic activity are realized, the problems of difficult defect regulation, low active site utilization of the existing Fe-N-C monatomic catalyst are solved, the technical bottleneck of rapid activity attenuation of the existing catalyst in the recycling process and limited engineering application is broken, and the preparation process is simple, controllable, environment-friendly, and has technical feasibility and economic rationality.

Description

Technical Field

[0001] This invention belongs to the field of iron single-atom catalyst preparation technology, and in particular to an acid-washing defect-modified iron single-atom catalyst, its preparation method and application. Background Technology

[0002] In recent years, the risk prevention of emerging pollutants has received widespread global attention. Recalcitrant organic pollutants containing aromatic and heterocyclic structures in water bodies (such as sulfonamide antibiotics, bisphenol A, and dyes) exhibit characteristics such as high biotoxicity, high environmental persistence, and significant bioaccumulation, posing hidden environmental risks and posing extreme challenges to remediation. These pollutants are poorly water-soluble and metabolically stable, making water bodies their primary environmental carriers. Traditional methods such as adsorption, flocculation, and biological treatment are insufficient for deep removal, and long-term accumulation poses a serious potential threat to ecosystems and human health. Developing efficient remediation technologies has become an urgent need in the environmental field.

[0003] Compared to traditional treatment methods, advanced oxidation processes (AOPs) achieve rapid degradation of recalcitrant pollutants by generating highly reactive oxygen species, offering advantages such as high removal efficiency and minimal secondary pollution. They have been widely applied in the removal of recalcitrant micropollutants and advanced wastewater treatment. Among these, persulfate (PDS)-based advanced oxidation processes (PDS-AOPs) have shown unique advantages in the treatment of recalcitrant organic pollutants due to the strong stability of the oxidant, its wide applicable pH range, its long-lasting oxidizing capacity, and its ability to generate various highly reactive species through activation. This has led to significant research and attention in recent years. Efficient PDS activation is the core of this process; the catalyst performance directly determines the PDS decomposition efficiency and pollutant degradation effect. Therefore, developing high-performance, low-cost, and environmentally friendly PDS activation catalysts is crucial for promoting the engineering application of this technology.

[0004] Among numerous PDS activation catalysts, single-atom catalysts (SACs) have become a research hotspot in this field due to their high atom utilization, well-defined active sites, and excellent catalytic selectivity. Among them, Fe-NC type single-atom catalysts exhibit good potential in PDS activation due to the synergistic effect of Fe active sites and NC supports. Zeolite imidazolium ester framework (ZIF-8), with its high specific surface area, abundant nitrogen source, and tendency to form a porous carbon framework after pyrolysis, has become a preferred precursor for preparing this type of catalyst. Compared to homogeneous catalytic materials, ZIF-8-derived Fe-NC heterogeneous catalysts can reduce metal ion leaching, achieve recycling, and further reduce the risk of secondary pollution.

[0005] However, in traditional preparation processes, Zn 2+ with Fe 3+The molar ratio control range is relatively wide, and the ratio varies greatly depending on the method, typically ranging from 10:1 to 500:1. Insufficient optimization of the metal ratio can easily lead to poor dispersion of Fe atoms or unstable coordination structure, making agglomeration highly likely. The core reason is that ZIF-8 is a zinc-based MOF material, and its tetrahedral coordination sites are mainly composed of Zn. 2+ Iron ions occupy and form stable Zn-N bonds with 2-methylimidazole, making it impossible for them to integrate into the topological nodes of ZIF-8. They mostly exist as surface adsorption or confined spaces within pores, making it difficult for them to function stably as a framework component. Furthermore, the coordination stability of 2-methylimidazole for iron is much lower than that for zinc. During the early stages of high-temperature pyrolysis, iron atoms are prone to decoupling and migration. During pyrolysis, Zn sublimates and is removed at approximately 900℃. If iron atoms are not effectively anchored by the N / C matrix, they lose spatial isolation and migrate and aggregate. Simultaneously, excessively high local iron concentrations can exceed the anchoring capacity of the N / C matrix, further exacerbating metal aggregation. Moreover, existing ZIF-8-derived Fe-NC single-atom catalysts still have significant shortcomings in practical applications: the difficulty in controlling defect sites, their number and type, hinders the full realization of catalytic activity; the catalytic stability of iron atoms needs improvement, and performance degradation easily occurs during recycling, making it difficult to meet the continuous treatment requirements of practical wastewater. Additionally, the controllability and economy of the preparation process still need optimization, limiting their engineering application. Therefore, developing a Fe-NC single-atom catalyst with precise defect control, excellent catalytic activity and stability, and simple preparation process is of great significance for promoting the engineering application of PDS-AOPs technology in the treatment of recalcitrant wastewater. Summary of the Invention

[0006] The technical problem to be solved by the present invention is to provide an acid-washing defect-modified iron single-atom catalyst, its preparation method and application, to solve the problems of existing ZIF-8 derived Fe-NC single-atom catalysts in the process of persulfate (PDS) activated degradation of recalcitrant organic pollutants in water, which are difficult to control for defect sites and have insufficient catalytic stability, resulting in low PDS activation efficiency and limited engineering applications.

[0007] To solve the above problems, the technical solution adopted in this invention is as follows: an acid-washed defect-engineered iron single-atom catalyst with intrinsic defect sites on its surface, an Fe element content of 0.60–0.67 wt%, and the Fe atom coordination form being Fe-N. n The coordination number n is 4.2 ± 0.3, the Fe-N bond length is 2.01 Å, and the valence state of Fe is between Fe... 2+ with Fe 3+ between.

[0008] The preparation method of the above-mentioned acid-washing defect-modified iron single-atom catalyst includes the following steps: S1. Mix zinc source, iron source and 2-methylimidazole into methanol, the zinc source contains Zn 2+ Fe in the iron source 3+ The molar ratio was 20:(3-4), and after solvothermal reaction, the precursor was obtained by washing and drying. S2. The precursor was ground and then pyrolyzed to obtain the FeNC catalyst; S3. The FeNC catalyst was acid-washed with concentrated sulfuric acid, then washed, filtered and dried to obtain a defective iron single-atom catalyst.

[0009] Furthermore, in step S1, the zinc source is Zn(NO3)2·6H2O, and the iron source is Fe(acac)3.

[0010] Furthermore, in step S1, Zn 2+ with Fe 3+ The molar ratio is 20:(3.5~4).

[0011] Furthermore, Zn 2+ Fe 3+ The molar ratio of 2-methylimidazole to 2-methylimidazole is 16:3:64.

[0012] Further, in step S1, the zinc source, iron source and methanol are mixed to obtain phase A solution; 2-methylimidazole and methanol are mixed to obtain phase B solution, the phase A solution and phase B solution have the same volume; phase A solution is poured into phase B solution at a uniform rate within 10-15s and stirred continuously, then sealed and left to stand at room temperature for 22-24h.

[0013] Furthermore, in step S2, the heating rate of pyrolysis is 4-6℃ / min, the pyrolysis temperature is 800-900℃, the holding time is 2-4h, argon gas is continuously introduced during pyrolysis, and the pyrolysis product is ground again.

[0014] Furthermore, in step S3, the concentration of concentrated sulfuric acid is 1 mol / L, the concentration of FeNC catalyst in the acid solution is 500-700 mg / L, the acid washing temperature is 70-90℃, the stirring rate is 300 rpm, and the acid washing time is 6 h.

[0015] The above-mentioned acid-washing defect-modified iron single-atom catalyst is used as a catalyst in the advanced oxidation treatment process of wastewater containing organic matter.

[0016] Furthermore, the organic matter is a pollutant containing aromatic rings and heterocyclic structures, the catalyst concentration is 80-100 mg / L, and the PDS concentration is 0.2-0.3 mol / L.

[0017] The beneficial effects of this invention are: this invention achieves precise control of Zn 2+ with Fe 3+The molar ratio, combined with the volatilization effect of Zn during the pyrolysis of the ZIF-8 precursor, effectively suppresses the aggregation of Fe atoms, ensuring that Fe atoms are uniformly dispersed in single-atom form and form stable Fe-N4 coordination active sites. The achievement of this single-atom dispersion effect relies on a synergistic mechanism of physical confinement and chemical anchoring: the regular pore structure of the ZIF-8 precursor physically confines Fe atoms during high-temperature pyrolysis, spatially restricting their migration and aggregation; simultaneously, in the Fe-NC structure formed by pyrolysis, Fe atoms and N atoms form stable Fe-N4 coordination through strong metal-support interactions, preventing Fe atom decoupling and migration at the chemical bond level. XAFS and AC-HAADF-STEM characterization verified that the catalyst prepared in this invention does not form Fe-Fe bonds, and the Fe element is in a single-atom dispersed state.

[0018] Based on this, the present invention employs concentrated sulfuric acid rinsing with specific parameters to controllably destroy the graphite structure and N coordination of the catalyst, causing some Fe atoms to lose planar support from C atoms and thus escape, while simultaneously damaging the N-C bonds and forming numerous defect vacancies on the catalyst surface. This acidification treatment does not directly damage the first layer of Fe-N4 coordination environment of the retained Fe single-atom active sites; it only slightly elongates the Fe-N bond length from 1.94 Å to 2.01 Å, achieving the controllable introduction of type 585 topological intrinsic defect sites without destroying the core active sites. These defects are distributed in the vicinity of the Fe-N4 active sites in a structure of two five-membered rings sandwiching an eight-membered ring. 57 Fe Mössbauer spectroscopy verification shows that this structure enables high-spin Fe III The proportion of Fe-N4 species increased from 57.6% to 72.7%, which changed the electron spin distribution of the Fe center and laid the structural and electronic basis for the synergistic effect of defect sites and Fe-N4 active sites.

[0019] The aforementioned defect sites and Fe-N4 active sites form a synergistic effect through long-range interactions: on the one hand, the defect sites regulate the planar electron distribution of carbon, causing the d-band center of the Fe-N4 site to shift upwards closer to the Fermi level, increasing the PDS adsorption energy from -2.19 eV to -2.75 eV, while simultaneously lengthening the SO bonds of PDS to promote its activation, significantly enhancing adsorption and activation capabilities; on the other hand, this synergistic effect lowers the reaction energy barrier, allowing Fe... IV =O formation energy is reduced by 0.39 eV, realizing an oxidation pathway from the traditional 1 O2-dominant 1 O and 2Fe IV =O-co-oxidative transformation. Compared to the traditional process where PDS is easily decomposed to form... 1 The present invention addresses the limitation of oxidation selectivity and efficiency caused by O2. 1 O2 and Fe IV=O synergistic oxidation system significantly improves pollutant degradation efficiency and selectivity.

[0020] In traditional processes, acid leaching easily leads to the loss of Fe element through leaching, the root cause of which lies in the volatilization and removal of Zn during the pyrolysis of the ZIF-8 precursor. If Fe atoms are not effectively anchored by the N / C matrix, they will lose spatial isolation and undergo decoupling and migration, ultimately forming non-atomic iron species such as iron nanoparticles or iron carbide (Fe3C) clusters. These species are mostly attached to the carbon matrix surface through weak interactions and are easily removed during acid leaching. In addition, when the Zn:Fe molar ratio is improperly controlled or the Fe doping amount is too high, it will exceed the anchoring capacity of the N / C matrix, further promoting metal aggregation and forming more non-single-atom iron species. While removing these species, acid leaching also affects the coordinationally unstable Fe-N... x The damage caused by single atomic sites exacerbates the leaching loss of Fe.

[0021] To address the aforementioned problems, this invention leverages the synergistic stabilizing effect of the Fe-NC framework structure and defect sites to reduce Fe leaching loss through both coordination enhancement and doping optimization. Regarding coordination enhancement, intrinsic defects introduced through defect engineering synergistically interact with the nitrogen-doped carbon matrix, optimizing the coordination environment of Fe atoms and strengthening the Fe-N matrix. x Coordination bonding stabilizes the highly stable Fe-N4 coordination structure, allowing Fe atoms to be more firmly anchored to the carbon support and effectively resisting acidic media corrosion. In terms of doping optimization, precise control of the Zn:Fe molar ratio in the ZIF-8 precursor maximizes the anchoring of Fe atoms to the carbon matrix in single-atom form, preventing the formation of non-atomic iron species such as iron nanoparticles that are easily removed by acid leaching. These synergistic stabilizing effects significantly reduce Fe leaching loss, improve the catalyst's cycle stability and long-term performance, and provide reliable support for its engineering applications.

[0022] Compared with the prior art, the technical solution provided by the present invention has the following beneficial technical effects: 1. This invention achieves controllable construction of defect sites and significant improvement in catalytic activity through a synergistic strategy of optimized ZIF-8 precursor ratio design and concentrated sulfuric acid washing defect engineering, solving the problems of difficult defect control and low active site utilization in existing Fe-NC single-atom catalysts. On the one hand, through Zn 2+ with Fe 3+The optimized molar ratio effectively suppressed the aggregation of Fe atoms, ensuring that Fe is uniformly dispersed in single-atom form and forms stable Fe-N4 coordination active centers. On the other hand, through specific acid washing process parameters, intrinsic defect structures such as nitrogen vacancies and carbon defects can be controllably introduced. These defect sites and Fe-N4 active centers form a synergistic effect, significantly enhancing the adsorption and activation capacity for PDS. Experiments have demonstrated that the prepared FeNC-D catalyst significantly improves the reaction rate constant for PDS degradation of pollutants compared to the unmodified FeNC catalyst, and exhibits superior degradation efficiency for recalcitrant organic pollutants such as SIZ, fully leveraging the atom utilization advantage of single-atom catalysts.

[0023] 2. The catalyst described in this invention exhibits excellent stability, outstanding recyclability and continuous operation capability, overcoming the technical bottleneck of rapid activity decay and limited engineering applications of existing catalysts during recycling. The synergistic effect of the Fe-NC framework structure and defect sites in this invention effectively reduces Fe leaching and enhances the structural stability of the catalyst. Experimental data shows that after five cycles, the FeNC-D catalyst maintains a pollutant removal rate of over 90%; in continuous flow experiments, the removal rate reaches 98% after 120 hours of continuous operation, significantly outperforming the recyclability and long-term operational stability of existing Fe-NC single-atom catalysts. This effectively reduces the catalyst replacement frequency and the reagent consumption cost for wastewater treatment, providing reliable technical support for engineering-scale continuous treatment.

[0024] 3. The preparation process described in this invention is simple, controllable, and environmentally friendly, possessing both technical feasibility and economic rationality. The preparation process involves only three core steps: solvothermal synthesis, high-temperature pyrolysis, and acid washing modification. Key process parameters are clearly defined, requiring no complex equipment or harsh reaction conditions, exhibiting good repeatability and facilitating large-scale production. Furthermore, the catalyst is a heterogeneous system with low Fe leaching, minimizing the risk of secondary pollution, and is recyclable. Combined with the strong stability and wide applicable pH range of PDS oxidant, this further reduces the overall cost of treating recalcitrant wastewater and enhances the technology's potential for engineering applications. Attached Figure Description

[0025] Figure 1 This is an SEM image of the catalyst prepared in Example 1 of the present invention.

[0026] Figure 2 These are AC-HAADF-STEM images of the catalyst prepared in Example 1, wherein (a), (b), and (c) are AC-HAADF-STEM images at a working voltage of 200 kV, and (d), (e), and (f) are AC-HAADF-STEM images at a working voltage of 300 kV.

[0027] Figure 3 These are the FeK-edge XANES spectra of the catalyst prepared in Example 1 and the reference samples (iron foil, FePc, FeO, Fe2O3).

[0028] Figure 4 The images show the FeK-edge Fourier transform (FT) k² weighted χ(k) function spectra of the catalyst prepared in Example 1 and the reference sample.

[0029] Figure 5 The image shows the EXAFS fitting curve of the catalyst prepared in Example 1 in R space (the inset shows the FeNC-D active center model).

[0030] Figure 6 These are the EXAFS wavelet transform spectra of the catalyst prepared in Example 1 and the reference sample.

[0031] Figure 7 The results are from the SIZ degradation experiments of the FeNC-D / PDS system and other reference systems in performance test 2.

[0032] Figure 8 It is the reaction kinetic constant of the FeNC-D / PDS system and other reference systems in performance test 2.

[0033] Figure 9 It is the normalized reaction kinetic constant of the FeNC-D / PDS system and other reference systems in performance test 2.

[0034] Figure 10 The results are from the quenching experiment of the FeNC-D / PDS system in performance test 3, where the quencher concentrations are [TBA]=500 mM, [MeOH]=500 mM, [FFA]=20 mM, and [p-BQ]=0.5 mM.

[0035] Figure 11 The TEMP capture in the FeNC-D / PDS system and other reference systems in performance test four. 1 EPR spectrum of O2.

[0036] Figure 12 These are the EPR spectra of various systems using DMPO as a capture agent in performance test four.

[0037] Figure 13 This is the result of the cyclic test of SIZ degradation by the FeNC-D / PDS system in performance test five.

[0038] Figure 14 This is the result of the cyclic test of SIZ degradation by the FeNC / PDS system in performance test five.

[0039] Figure 15 This is a graph showing the removal efficiency of SIZ by the FeNC-D / PDS system in the continuous flow experiment of the performance test. Detailed Implementation

[0040] The present invention will be further described below with reference to the accompanying drawings and embodiments.

[0041] The acid-washing defect-modified iron single-atom catalyst of this invention is a Fe-NC type catalyst with uniformly dispersed Fe single atoms, maintaining a regular hexahedral macroscopic structure, and each particle has a diameter of approximately 80 nm. The coordination mode of the Fe atoms is Fe-N. n Its average coordination number n is 4.2 ± 0.3, meaning it is predominantly Fe-N4, with a small number of coordination groups existing as Fe-N3 and Fe-N5 due to defects. The Fe-N bond length is 2.01 Å, and the valence state of Fe is between Fe... 2+ with Fe 3+ The catalyst surface contains intrinsic defect sites such as nitrogen vacancies and carbon defects, and the Fe element content is 0.60–0.67 wt%.

[0042] The preparation method of the acid-washing defect-modified iron single-atom catalyst of the present invention includes the following steps: S1. Mix zinc source, iron source and 2-methylimidazole into methanol, the zinc source contains Zn 2+ Fe in the iron source 3+ The molar ratio of the two components is 20:(3-4). After a solvothermal reaction, the precursor is obtained by washing and drying.

[0043] The zinc source is Zn(NO3)2·6H2O, the iron source is Fe(acac)3, and the Zn... 2+ with Fe 3+ The preferred molar ratio for Zn is 20:(3.5–4). 2+ Fe 3+ The optimal molar ratio of 2-methylimidazole to 2-methylimidazole is 16:3:64.

[0044] Methanol is used as the solvent for the solvothermal reaction. First, the zinc source and iron source are mixed with methanol to obtain phase A solution. Then, 2-methylimidazole is mixed with methanol to obtain phase B solution. The volumes of phase A solution and phase B solution are the same. Phase A solution is poured into phase B solution at a uniform rate within 10-15 seconds, with a stirring rate of 400 rpm during the pouring process. After the pouring is completed, stirring is continued at a stirring rate of 600 rpm for 60 minutes. Then, the mixture is sealed and allowed to stand at room temperature for 22-24 hours.

[0045] During washing, the product is centrifuged and washed three times with methanol. Drying is performed under vacuum at 60°C. After drying, the precursor must be thoroughly pulverized and ground.

[0046] The precursor obtained in this step is Fe-dorped@ZIF-8 (iron-doped dimethylimidazolium zinc salt).

[0047] S2. The precursor is ground and then pyrolyzed to obtain the FeNC catalyst.

[0048] The pyrolysis heating rate is 4-6℃ / min, the pyrolysis temperature is 800-900℃, the holding time is 2-4h, argon gas is continuously introduced as a protective atmosphere during pyrolysis, and the pyrolysis product is ground again.

[0049] S3. The FeNC catalyst was acid-washed with concentrated sulfuric acid, then washed, filtered and dried to obtain a defective iron single-atom catalyst (FeNC-D).

[0050] The concentration of concentrated sulfuric acid was 1 mol / L, the concentration of FeNC catalyst in the acid solution was 500-700 mg / L, the acid washing temperature was 70-90℃, the stirring rate was 300 rpm, and the acid washing time was 6 h.

[0051] After acid washing, the solid was washed with pure water until neutral, filtered, and dried in a vacuum drying oven at 60℃ for 20-28 hours. After drying, it was ground to obtain the FeNC-D catalyst.

[0052] The core purpose of acid etching defect engineering is to introduce intrinsic defect sites on the surface of FeNC catalyst through the etching effect of concentrated sulfuric acid, while removing some unstable Fe species, and finally obtain FeNC-D catalyst that has both single-atom dispersion characteristics and defect structure.

[0053] The catalyst prepared by this invention can be used as a catalyst in advanced oxidation treatment processes for wastewater containing organic matter. The organic matter includes pollutants with aromatic or heterocyclic structures such as sulfamethoxazole (SIZ), sulfamethoxazole (SMX), bisphenol A (BPA), and rhodamine B (RhB). The catalyst dosage is 80–100 mg / L, and the PDS dosage is 0.2–0.3 mol / L. The reaction system temperature is controlled at 25 ± 1℃, and the stirring rate is 300 rpm. No additional pH adjustment is required, and the reaction can proceed efficiently at room temperature. The wastewater treatment time can be determined according to the type and concentration of organic pollutants, generally controlled at 10–120 min, until the pollutant removal rate reaches equilibrium. During the reaction, the reaction solution is extracted through a 0.22 μm polytetrafluoroethylene (PTFE) filter membrane. After terminating the reaction with Na2S2O3 solution, the residual pollutant concentration is measured.

[0054] This invention demonstrates through experiments that the FeNC-D catalyst has excellent recycling performance, with a pollutant removal rate still above 90% after 5 cycles; in continuous flow experiments, the pollutant removal rate can reach 98% after 120 hours of continuous operation, significantly reducing the catalyst consumption cost for wastewater treatment. Example

[0055] 4.76 g (16 mM) Zn(NO3)2·6H2O and 1.12 g (3 mM) Fe(acac)3 were added to 60 mL of analytical grade methanol and stirred until completely dissolved to obtain phase A solution. Separately, 5.26 g (64 mM) 2-methylimidazole (2-MeIM) was added to 60 mL of analytical grade methanol and stirred until completely dissolved to obtain phase B solution. Phase A solution was slowly poured into phase B solution at a rate of 5 mL / s while stirring continuously at 400 rpm. The stirring rate was then adjusted to 600 rpm and stirred vigorously for 60 min. The reaction vessel was sealed with a sealing film and placed in a cool, dark place at room temperature for 24 h. After the solution separated into clear liquid and precipitate, the upper clear liquid was slowly poured off, retaining the lower turbid precipitate. In this step, Zn... 2+ with Fe 3+ The molar ratio is 16:3. During the stirring process, it is necessary to ensure that the two phase solutions are fully mixed. During the settling stage, avoid shaking the container to prevent the precipitate from dispersing.

[0056] The collected precipitate was washed three times by centrifugation with analytical grade methanol until the washing liquid became clear. The solid material obtained by centrifugation was then dried in a vacuum drying oven at 60°C to obtain the catalyst precursor Fe-dorped@ZIF-8 (iron-doped dimethylimidazolium zinc salt).

[0057] The dried Fe-dorped@ZIF-8 precursor was thoroughly pulverized and ground, then placed in a quartz boat in a tube furnace. Argon gas was introduced into the tube furnace as a protective atmosphere, and the temperature was increased from room temperature to 900°C at a rate of 5°C / min. The furnace was then calcined at 900°C for 3 hours. After the tube furnace cooled naturally to room temperature, the calcined product was ground thoroughly again to obtain the FeNC iron single-atom catalyst.

[0058] Meanwhile, the control catalyst NC was prepared: except that Fe(acac)3 was not dissolved in the A phase solution, the rest of the preparation steps were completely consistent with the synthesis process of the FeNC catalyst above.

[0059] Next, the FeNC iron single-atom catalyst was added to concentrated sulfuric acid with a concentration of 1 mol / L and stirred until homogeneous, forming a turbid solution with a catalyst concentration of 600 mg / L. In this step, it is essential to ensure sufficient contact between the FeNC catalyst and the concentrated sulfuric acid to prevent catalyst agglomeration and uneven acid washing.

[0060] The above turbid liquid was placed in a constant temperature water bath, the water bath temperature was set to 80℃, and the magnetic stirrer was turned on and stirred continuously at a stirring rate of 300 rpm for 6 hours for acid washing.

[0061] After acid washing, the reaction product was repeatedly washed with pure water until the filtrate was neutral. Then, the solid and liquid were separated by a vacuum filtration device, and the solid material was collected. The solid material was placed in a vacuum drying oven at 60°C and dried for 24 hours. After drying, it was taken out and ground to obtain the defective iron single-atom catalyst FeNC-D.

[0062] Performance Test 1 The FeNC-D catalyst prepared in Example 1 was characterized in terms of structure, elemental composition and Fe atom coordination environment: the FeNC-D catalyst was systematically characterized by scanning electron microscopy (SEM), aberration-corrected scanning transmission electron microscopy (AC-HAADF-STEM), X-ray absorption fine structure spectroscopy (XAFS), and inductively coupled plasma optical emission spectrometry (ICP-OES).

[0063] The results are as follows Figures 1 to 6 As shown, Figure 1 This is a SEM image of the FeNC-D catalyst; Figure 2 (a), (b), and (c) are AC-HAADF-STEM images of the FeNC-D catalyst at a working voltage of 200 kV, and (d), (e), and (f) are AC-HAADF-STEM images of the FeNC-D catalyst at a working voltage of 300 kV. Figure 3 These are FeK-edge XANES spectra of the FeNC-D catalyst and reference samples (iron foil, FePc, FeO, Fe2O3); Figure 4 The images show the FeK-edge Fourier transform (FT) k² weighted χ(k) function spectra of the FeNC-D catalyst and the reference sample. Figure 5 The image shows the EXAFS fitting curve of the FeNC-D catalyst in R space (the inset shows the FeNC-D active site model). Figure 6 These are the EXAFS wavelet transform spectra of the FeNC-D catalyst and the reference sample.

[0064] Depend on Figure 1 SEM results show that after acid washing with concentrated sulfuric acid at 80℃, the FeNC-D catalyst still maintains the regular hexahedral macroscopic structure derived from ZIF-8, with a single particle diameter of about 80nm, which is basically consistent with the particle size of the un-acid-washed FeNC catalyst, indicating that the acid washing defect engineering did not destroy the overall morphology and structure of the catalyst.

[0065] Depend on Figure 2The AC-HAADF-STEM results show that at a working voltage of 200 kV, there are a considerable number of defect vacancies in the visual signal of C atoms in the FeNC-D catalyst. Even with the influence of catalyst particle thickness, the defect structure can still be clearly observed. At a working voltage of 300 kV, the graphitized C atom signal is shielded to the maximum extent, and the signal spot of Fe atoms has sufficient contrast. Dispersed single atom sites, adjacent paired single atom sites, and obvious defect sites can be observed. No Fe particle agglomeration was found, proving that FeNC-D is a single atom catalyst containing defect structures.

[0066] Depend on Figures 3 to 6 XAFS test results show that the FeK-edge XANES lines of the FeNC-D catalyst are located between Fe2O3 and FePc, indicating that the valence state of Fe atoms is between Fe2O3 and FePc. 2+ with Fe 3+ Between; its Fourier transform (FT) k² weighted EXAFS spectrum shows a single Fe-N coordination characteristic signal peak at ~1.45 Å, without the Fe-Fe bond signal near ~2.2 Å, which, compared with iron foil, proves that Fe exists in single-atom form; the EXAFSR space fitting curve and parameters show that the Fe atoms in the FeNC-D catalyst are mainly in Fe-N4 coordination form, with a coordination number of 4.2±0.3 and a corresponding Fe-N bond length of 2.01 Å. Compared with the un-washed FeNC catalyst, the N coordination number is basically the same, indicating that the defect treatment did not directly damage the first coordination environment of the Fe single-atom active site, but the Fe-N bond length was lengthened from 1.94 Å to 2.01 Å, reflecting that the NC bonds outside the first coordination environment were indirectly damaged; Figure 6 Wavelet transform contour plot at 3.5 Å -1 The maximum intensity was observed nearby, with only Fe-N bond signals detected and no Fe-Fe bond signals detected, further confirming the single-atom dispersion state of Fe atoms.

[0067] Inductively coupled plasma optical emission spectrometry (ICP-OES) analysis revealed that the Fe content in the un-acid-washed FeNC catalyst was 1.14 wt%, while the Fe content in the FeNC-D catalyst after acid washing defect engineering treatment was 0.64 wt%, representing a Fe loss rate of approximately 43%. This phenomenon is due to the defects in the graphite structure and N coordination of the catalyst caused by acid washing, resulting in the loss of some Fe atoms due to the loss of support from the C atom plane. This is a situation that is difficult to avoid in defect engineering.

[0068] Performance Test 2 The difference in catalytic performance between the FeNC-D catalyst and the FeNC catalyst prepared in Example 1 for activating PDS to degrade sulfamethoxazole (SIZ) was examined to verify the effect of acid washing defect engineering on improving catalyst activity. SIZ was prepared into a 20 μmol / L solution using deionized water to simulate wastewater. Three sets of experiments were set up as follows: Group 1 (FeNC-D / PDS system): PDS and FeNC-D catalyst were added to the simulated wastewater, with the PDS concentration controlled at 0.25 mol / L and the FeNC-D catalyst dosage at 100 mg / L; Group 2 (FeNC / PDS system): PDS and FeNC catalyst were added to the simulated wastewater, with the PDS concentration controlled at 0.25 mol / L and the FeNC catalyst dosage at 100 mg / L. Group 3 (Adsorption control group): FeNC-D or FeNC catalyst (both added at a dosage of 100 mg / L) was added to the simulated wastewater, without the addition of PDS.

[0069] All experiments were conducted at 25±1℃ and 300rpm with stirring, using a 250mL beaker as the reaction vessel.

[0070] During the simulated wastewater treatment process, 1 mL of the reaction solution was extracted through a 0.22 μm PTFE filter membrane using a syringe at regular intervals. This solution was then injected into a liquid chromatography vial containing 20 μL of 20 mM Na2S2O3 solution to terminate the reaction. Subsequently, the SIZ concentration was determined using high performance liquid chromatography (HPLC).

[0071] The initial concentration of SIZ was denoted as C0. The change in C / C0 over treatment time was calculated, and the reaction rate constant was also calculated. To eliminate the influence of Fe loss on activity, the reaction rate constant was normalized (calculated per 1 mg Fe loading). The results are as follows: Figures 7 to 9 As shown.

[0072] from Figures 7 to 9 It can be seen that there is no significant difference in the adsorption effect of FeNC-D and FeNC on SIZ in the adsorption control group, and the adsorption amount is extremely low, indicating that the adsorption effect of the catalyst does not make a substantial contribution to the removal of SIZ. The performance of the FeNC-D / PDS system in degrading SIZ is significantly better than that of the FeNC / PDS system, and the corresponding reaction rate constant is higher. After normalization treatment, the catalytic activity of FeNC-D is still significantly higher than that of FeNC, which confirms that even when acid washing defect engineering leads to the loss of some Fe elements (Fe content decreases from 1.14wt% to 0.64wt%), it can still effectively improve the intrinsic activity of the catalyst, fully demonstrating the synergistic effect of defect sites and Fe-N4 active sites.

[0073] Performance Test 3 Quenching experiments were used to identify the core reactive oxygen species (ROS) in the degradation of SIZ by the FeNC-D / PDS system, and to clarify the contribution of different reactive species to the degradation reaction. The experimental conditions were consistent with those in performance test 2: a 250 mL beaker was used as the reaction vessel, 100 mL of 20 μmol / L SIZ solution was used as simulated wastewater, the FeNC-D catalyst dosage was 100 mg / L, the PDS concentration was 0.25 mol / L, the reaction temperature was 25 ± 1 °C, and the stirring speed was 300 rpm.

[0074] (2) Five groups of experiments were set up, and the specific groups are as follows: Group 1 (blank control group): No quenching agent was added; Group 2 (TBA group): Tert-butanol (TBA) was added before the reaction started, and the concentration of TBA in the system was controlled at 500 mM (specific quenching). • OH); Group 3 (MeOH group): Methanol (MeOH) was added before the reaction started, and the concentration of MeOH in the system was controlled at 500mM (quenching). • OH and organic free radicals (RO) • ); Group 4 (FFA group): Furan-methanol (FFA) was added before the reaction started, and the FFA concentration in the system was controlled at 20 mM (specific quenching). 1 O2); Group 5 (p-BQ group): p-benzoquinone (p-BQ) was added before the reaction started, and the concentration of p-BQ in the system was controlled to be 0.5 mM (quenching O2). •– ).

[0075] (3) The experimental procedure was consistent with that of performance test 2: During the reaction, 1 mL of the reaction solution was extracted through a 0.22 μm PTFE filter membrane at regular intervals, and 20 μL of 20 mM Na2S2O3 solution was added to terminate the reaction. The SIZ concentration was determined by HPLC, and the SIZ degradation rate at different reaction times was calculated. The results are as follows: Figure 10 As shown.

[0076] Figure 10 These are the quenching test results of the FeNC-D / PDS system in performance test two, where the quencher concentrations were [TBA]=500 mM, [MeOH]=500 mM, [FFA]=20 mM, and [p-BQ]=0.5 mM. Figure 10 It can be seen that the blank control group (without quencher) had the highest SIZ degradation efficiency; after adding different quenchers, the degradation reaction was inhibited to varying degrees, with the FFA group showing the most significant inhibition, resulting in a substantial decrease in the SIZ degradation rate, indicating that... 1O2 contributed the most to the degradation reaction; while the inhibitory effects of the TBA group, MeOH group, and p-BQ group were weaker, indicating that... • OH, RO • and O2 •– It is not the dominant active species in the system. In summary, it can be determined that the core reactive oxygen species for SIZ degradation in the FeNC-D / PDS system is... 1 O2.

[0077] Performance Test 4 Electron paramagnetic resonance (EPR) spectroscopy, combined with a specific trapping agent, was used to further confirm the types of reactive oxygen species in the FeNC-D / PDS system and verify the core conclusions drawn from the quenching experiment. Experimental conditions: The basic parameters of the reaction system were kept consistent with those of performance tests II and III, namely, FeNC-D catalyst dosage of 100 mg / L, PDS concentration of 0.25 mol / L, and reaction temperature of 25℃; reference systems were set up, including PDS system alone, FeNC / PDS system, and FeNC-D system alone.

[0078] Testing equipment and trapping agent: Electron paramagnetic resonance (EPR) spectroscopy was used for detection; 2,2,6,6-tetramethylpiperidine (TEMP) was selected as the trapping agent. 1 5,5-Dimethyl-1-pyrrolidone-N-oxide (DMPO) is a specific O2 scavenger as a hydroxyl radical ( • OH) and superoxide radicals (O2) •– ) capture agent.

[0079] Experimental steps: Group 1 (TEMP capture) 1 O2): Add TEMP scavenger to 2 mL of deionized water to achieve a concentration of 100 mmol / L, then add PDS and FeNC-D catalyst to start the reaction. The reference system was prepared in the same proportion. During the reaction, samples were aspirated with a capillary tube at regular intervals, and the signal changes were detected by EPR.

[0080] Group 2 (DMPO capture) • OH / O2 •– ): Add DMPO scavenging agent to 2 mL of deionized water to achieve a concentration of 100 mmol / L. The addition ratio of other reagents, reaction and detection steps are the same as those in group 1.

[0081] Test results are as follows Figure 11 and Figure 12 As shown: Figure 11 Capture for TEMP 1 The EPR spectrum of O2 shows a clear 1:1:1 triplet symmetric characteristic peak in the FeNC-D / PDS system.1 The characteristic signal of the reaction between O2 and TEMP to generate tetramethylpiperidine oxide (TEMPO), with stable peak intensity, confirms the presence of [a specific substance / component] in the system. 1 O2; This characteristic peak was not detected in the standalone PDS system or the standalone FeNC-D system. The triplet intensity of the FeNC / PDS system was weaker than that of the FeNC-D / PDS system, further indicating that the defect structure of FeNC-D can promote the activation and generation of PDS. 1 O2. Figure 12 Capture for DMPO • OH / O •– No EPR spectrum was detected in the FeNC-D / PDS system and all reference systems. • The 1:2:2:1 quartet characteristic peak corresponding to OH did not show O2. •– The corresponding complex multiplets, with only a weak background signal, indicate that there are almost no reactive substances in the system. • OH and O2 •– This excludes the dominant role of the free radical degradation pathway.

[0082] The EPR test results are completely consistent with the quenching experiment conclusions of performance test three, jointly confirming that the core reactive oxygen species in the FeNC-D / PDS system for degrading SIZ is... 1 O2 clarified the non-radical pathway of catalytic degradation, laying the foundation for subsequent probe experiments to verify it.

[0083] Performance Test 5 The cycling stability of the defective iron single-atom catalyst (FeNC-D) prepared in Example 1 was examined, and compared with the FeNC catalyst, to verify the effect of acid washing defect engineering on improving the reusability of the catalyst. The experimental conditions were consistent with those in performance test 2: a 250 mL beaker was used as the reaction vessel, 100 mL of 20 μmol / L SSIZ solution was used as simulated wastewater, the dosage of FeNC-D and FeNC catalysts was 100 mg / L, the PDS concentration was 0.25 mol / L, the reaction temperature was 25 ± 1 °C, and the stirring speed was 300 rpm.

[0084] Two sets of cyclical experiments were set up, namely: Group 1 (FeNC-D / PDS system): The degradation reaction was started under the above conditions. After the reaction was completed, the reaction solution was centrifuged at 8000 rpm for 10 min and the precipitated FeNC-D catalyst was collected. The catalyst was washed twice with methanol to remove the surface adsorbed contaminants and reaction residues. Then it was dried in a vacuum drying oven at 60℃ for 6 h. After drying, it was ground and used for the next cycle experiment. Group 2 (FeNC / PDS system): FeNC catalyst was treated using the same recovery, washing, and drying process, and cyclic degradation experiments were conducted.

[0085] Both sets of experiments were repeated 5 times, and the reaction parameters in each cycle were exactly the same as those in the first reaction.

[0086] The test procedure was the same as in Example 4: During each reaction cycle, samples were periodically taken and filtered through a 0.22 μm PTFE membrane. The reaction was terminated by adding 20 μL of 20 mM Na₂S₂O₃ solution. The SIZ concentration was determined by HPLC, and the SIZ removal rate for each cycle was calculated. The results are as follows: Figure 13 and Figure 14 As shown.

[0087] Depend on Figure 13 and Figure 14 It was found that the catalytic performance of the FeNC / PDS system rapidly declined with increasing cycle number, and the SIZ removal rate decreased significantly after several cycles. In contrast, the FeNC-D / PDS system exhibited excellent cycle stability, maintaining a SIZ removal rate of over 90% after five cycles without significant activity degradation. This result indicates that after acid leaching defect engineering modification, the FeNC-D catalyst can rapidly degrade intermediate products accumulated during the SIZ reaction, exhibiting more stable oxidation capacity. Furthermore, the synergistic effect of the Fe-NC framework structure and defect sites reduces Fe leaching loss, significantly improving the catalyst's structural stability and reusability, thus expanding its practical application scope.

[0088] Performance Test Six The performance of the defective iron single-atom catalyst (FeNC-D) prepared in Example 1 was investigated in a continuous flow scenario to verify its potential for engineering continuous treatment of wastewater containing sulfamethoxazole (SIZ).

[0089] Experimental setup: A 30mm×100mm sealed reaction vessel was used as the main reactor. 200mg FeNC-D catalyst was loaded onto polyurethane foam and fixed inside the reactor. The reactor inlet and outlet were connected to hoses respectively, and the fluid flow rate was controlled by a peristaltic pump to form a continuous flow treatment system.

[0090] Experimental conditions: Prepare 20 μmol / L SIZ simulated wastewater with deionized water and prepare 0.5 mmol / L PDS solution separately. Mix the two solutions evenly in the specified ratio and use the mixture as the influent. Continuously introduce the mixed influent into the reactor at a flow rate of 1 mL / min using a peristaltic pump. After fully contacting and reacting with the FeNC-D catalyst, the solution is discharged into the effluent container from the outlet. No additional temperature or pH adjustment is required during the entire experimental process; maintaining room temperature is sufficient.

[0091] Test Procedure: During the operation of the continuous flow system, samples were taken at the outlet at regular intervals. The samples were filtered through a 0.22 μm PTFE membrane, and the reaction was terminated by adding 20 μL of 20 mM Na₂S₂O₃ solution. The residual concentration of SIZ in the effluent was determined by HPLC. The initial SIZ concentration in the influent was recorded as C₀. The SIZ removal rate at different operating times was calculated. Continuous monitoring was conducted for 120 hours. The results are as follows: Figure 15 As shown.

[0092] Depend on Figure 15 The FeNC-D catalyst exhibits excellent stability in the continuous flow treatment system: in the initial stage of continuous operation, the SIZ removal rate is close to 100%; with the extension of operating time, the removal rate does not show significant decline, and after 120 hours of continuous operation, the SIZ removal rate still remains at approximately 98%. This result demonstrates that the FeNC-D catalyst can still efficiently activate PDS to degrade pollutants in practical applications with dynamic continuous influent. Its structural stability and catalytic activity are not affected by fluid scouring or continuous reaction, fully reflecting the engineering application value of this catalyst and providing reliable technical support for large-scale wastewater treatment.

[0093] The above description is merely a preferred embodiment of the present invention and is not intended to limit the invention. Various modifications and variations can be made to the present invention by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. An acid-washing defect-modified iron single-atom catalyst, characterized in that, The surface contains intrinsic defect sites, and the Fe element content is 0.60–0.67 wt%, with the Fe atoms in the Fe-N coordination form. n The coordination number n is 4.2 ± 0.3, the Fe-N bond length is 2.01 Å, and the valence state of Fe is between Fe... 2+ with Fe 3+ between; Its preparation method includes the following steps: S1. A zinc source, an iron source, and 2-methylimidazole are mixed in methanol. The iron source is Fe(acac)3. After a solvothermal reaction, the mixture is washed and dried to obtain the precursor. S2. The precursor was ground and then pyrolyzed to obtain the FeNC catalyst; S3. The FeNC catalyst was acid-washed with concentrated sulfuric acid at a concentration of 1 mol / L and an acid-washing temperature of 70–90 °C. Then, it was washed, filtered, and dried to obtain a single-atom iron catalyst containing defects.

2. The preparation method of the acid-washing defect-modified iron single-atom catalyst according to claim 1, characterized in that, Includes the following steps: S1. Mix zinc source, iron source, and 2-methylimidazole into methanol. The iron source is Fe(acac)3, and the zinc source contains Zn. 2+ Fe in the iron source 3+ The molar ratio was 20:(3-4), and after solvothermal reaction, the precursor was obtained by washing and drying. S2. The precursor was ground and then pyrolyzed to obtain the FeNC catalyst; S3. The FeNC catalyst was acid-washed with concentrated sulfuric acid at a concentration of 1 mol / L and an acid-washing temperature of 70–90 °C. Then, the catalyst was washed, filtered, and dried to obtain a single-atom iron catalyst containing defects.

3. The preparation method of the acid-washing defect-modified iron single-atom catalyst as described in claim 2, characterized in that: In step S1, the zinc source is Zn(NO3)2. 6H2O.

4. The preparation method of the acid-washing defect-modified iron single-atom catalyst as described in claim 2, characterized in that: In step S1, Zn 2+ with Fe 3+ The molar ratio is 20:(3.5~4).

5. The preparation method of the acid-washing defect-modified iron single-atom catalyst as described in claim 4, characterized in that: Zn 2 + Fe 3+ The molar ratio of 2-methylimidazole to 2-methylimidazole is 16:3:

64.

6. The preparation method of the acid-washing defect-modified iron single-atom catalyst as described in claim 2, characterized in that: In step S1, zinc source, iron source and methanol are mixed to obtain phase A solution; 2-methylimidazole and methanol are mixed to obtain phase B solution, phase A solution and phase B solution have the same volume; phase A solution is poured into phase B solution at a uniform rate within 10-15s and stirred continuously, then sealed and left to stand at room temperature for 22-24h.

7. The preparation method of the acid-washing defect-modified iron single-atom catalyst as described in claim 2, characterized in that: In step S2, the heating rate of pyrolysis is 4-6℃ / min, the pyrolysis temperature is 800-900℃, the holding time is 2-4h, argon gas is continuously introduced during pyrolysis, and the pyrolysis product is ground again.

8. The preparation method of the acid-washing defect-modified iron single-atom catalyst as described in claim 2, characterized in that: In step S3, the concentration of FeNC catalyst in the acid solution is 500-700 mg / L, the stirring rate is 300 rpm, and the acid washing time is 6 h.

9. The application of the pickling defect-modified iron single-atom catalyst as described in claim 1 as a catalyst in an advanced oxidation treatment process for wastewater containing organic matter.

10. The application as described in claim 9, characterized in that, The organic matter consists of pollutants containing aromatic rings and heterocyclic structures. The concentration of the catalyst is 80–100 mg / L, and the concentration of PDS is 0.2–0.3 mol / L.

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