Cavity confinement carbon-based iron monatomic catalytic material as well as preparation method and application thereof

Abstract

The invention discloses a cavity confinement carbon-based iron monatomic catalytic material as well as a preparation method and application thereof. According to the material, a soft and hard template cooperation strategy is adopted, a hollow mesoporous carbon sphere is used as a carrier to anchor atomic-scale dispersion iron sites, and a nanoreactor with a cavity confinement structure is constructed. Compared with a traditional material, the activation efficiency of the material on peroxymonosulfate (PMS) is improved by about 2.9 times, the degradation performance on phenolic pollutants is remarkably enhanced, and the strengthening effect of a confinement structure on mass transfer and reaction kinetics is verified. Through metal-carrier cooperative regulation and control, singlet oxygen (O) is preferentially generated through the reaction, selective oxidation of organic pollutants is achieved through a non-free-radical path, and the anti-interference capacity in complex water is improved. The method is simple in process and relatively low in cost, and the obtained solid catalyst is easy to separate and recover, is suitable for efficient deep treatment of refractory organic pollutants in rural domestic sewage and industrial wastewater, and has a good application prospect.

Description

Technical Field

[0001] This invention relates to the field of advanced oxidation novel water treatment environmental materials technology, specifically to a cavity-confined carbon-based iron single-atom catalytic material and its preparation method and application. Background Technology

[0002] With the large-scale development of livestock farming and the increased use of daily chemical products in rural areas, the concentration of recalcitrant organic pollutants such as phenols in rural domestic sewage has risen significantly. These pollutants tend to accumulate in slow-moving water bodies such as village ditches and ponds, posing a potential threat to regional water environment safety and human health. Therefore, developing technologies suitable for decentralized rural sewage treatment scenarios and capable of achieving stable and in-depth treatment of pollutants is of great practical significance.

[0003] Advanced oxidation technologies are effective methods for treating recalcitrant organic pollutants. Among them, the traditional Fenton process, although widely used, relies on free radicals such as hydroxyl radicals (•OH) for oxidation. In actual water bodies, this process is easily interfered with by complex matrix components such as natural organic matter and inorganic anions, leading to ineffective consumption of oxidants and unstable treatment effects. It is difficult to meet the stable treatment needs of rural sewage in scenarios with large fluctuations in water quality and limited operation and maintenance conditions.

[0004] In recent years, Fenton-like reactions based on persulfate (PMS) activation have attracted attention as an alternative. Especially in heterogeneous catalytic systems (catalyst / PMS), if a non-radical oxidation pathway dominated by singlet oxygen (1O2) can be induced, relatively selective oxidation of electron-rich organic pollutants can be achieved, exhibiting better tolerance to complex aquatic matrices. Therefore, developing catalytic materials capable of efficiently and selectively activating PMS to generate 1O2 is of great significance for improving the efficiency and stability of rural wastewater treatment.

[0005] Carbon-based single-atom catalysts (SACs) have shown great potential in the field of PMS activation due to their near 100% metal atom utilization, highly uniform active sites, and tunable structure. Existing research indicates that by precisely controlling the local microenvironment (including coordinating atoms, coordination number, and electron density) of the single-atom metal center (such as iron or cobalt), specific adsorption and activation behaviors of PMS at the metal sites can be guided, particularly facilitating its cleavage via terminal oxygen coordination, thereby promoting the selective generation of 1O2. For example, Chinese patent CN116832810B discloses an iron single-atom catalyst for activating PMS to generate 1O2 and degrade pollutants, and reports its good degradation performance over a wide pH range and under certain background interference. This technology validates the feasibility of enhancing 1O2 generation capacity by constructing atomically dispersed metal active sites.

[0006] However, existing carbon-based single-atom catalysts face a key bottleneck in practical water treatment applications: limited mass transfer of reactants and insufficient accessibility to internal active sites. Most current carbon-based SACs use microporous carbon materials as supports, whose pore structures are typically narrow, tortuous, and disordered. This makes it difficult for larger PMS molecules and pollutant molecules to diffuse rapidly and sufficiently to the isolated metal active sites embedded deep within the carbon framework. The consequence is: (1) A large number of highly intrinsically active single-atom sites inside the material cannot be effectively utilized, resulting in low material utilization. (2) The overall reaction rate is controlled by diffusion rather than intrinsic catalytic kinetics, which limits the generation efficiency of 1O2 and the degradation rate of pollutants; (3) In actual complex water bodies, slow mass transfer will further amplify the competitive effect of background substances and affect the stability of treatment effect.

[0007] This problem is particularly prominent in rural decentralized wastewater treatment scenarios that require rapid response and efficient treatment. Therefore, although existing technologies have recognized the application potential of carbon-based SACs in 1O2-dominated Fenton-like systems, there is still a lack of effective solutions to fundamentally address the problems of limited mass transfer and difficulty in utilizing active sites caused by microstructural defects in traditional carbon supports.

[0008] In summary, current technology urgently needs to overcome the structural limitations of traditional carbon supports and design and develop a novel carbon-based single-atom catalyst that combines highly efficient mass transfer channels with a high-density, accessible active surface. This catalyst should significantly improve the transport efficiency of reactants to active sites while maintaining high intrinsic activity and 1O2 selectivity at the single-atom sites, thereby enhancing the overall performance of Fenton-like reactions to better meet the treatment needs of rural domestic sewage, which is characterized by large fluctuations in water quality, limited operational conditions, and high treatment efficiency requirements. Summary of the Invention

[0009] Based on the above-mentioned technical problems, the purpose of this invention is to provide a cavity-confined carbon-based iron single-atom material, its preparation method, and its application. This material improves the accessibility of reactants to iron single-atom active sites by constructing a directionally connected, open, accessible, and uniformly scaled nanoscale confinement space, shortens the mass transfer and diffusion path of reactants, and enhances the local enrichment effect of reactants near the active sites. This promotes the synergistic enhancement of electron transfer and interfacial reaction kinetics during the oxidant activation process, achieving efficient generation of 1O2 in Fenton-like reactions and rapid degradation of organic pollutants. Consequently, it is better suited for the stable and efficient treatment needs under complex water quality conditions such as rural domestic sewage.

[0010] This invention protects a method for preparing a cavity-confined carbon-based iron single-atom catalytic material, specifically comprising the following steps: Step 1, Preparation of silica template: Ammonia water is added to a mixed solvent of ethanol and deionized water and ultrasonically dispersed. Under vigorous stirring, an ethanol solution of tetraethoxysilane is added dropwise. After continuing to stir and react, the mixture is centrifuged, washed, and dried to obtain a SiO2 hard template. The stirring speed is 600-1200 rpm and the stirring reaction time is 12-30 h. Further, in step 1, the amount of ammonia added is 5-20 mL; the volume ratio of ethanol to deionized water is 0.3-1.5; and the tetraethoxysilane ethanol solution is a mixed solution with a volume ratio of tetraethoxysilane:ethanol = 5-20:80-100.

[0011] Step 2, coating of iron-containing polydopamine precursor on template surface and construction of mesoporous structure: The SiO2 hard template prepared in Step 1 is ultrasonically dispersed in a mixed solvent of ethanol and deionized water. Block copolymer Pluronic F127 and dopamine hydrochloride are added and ultrasonically dissolved until fully dissolved. Iron source is added under stirring at 200-800 rpm. After uniform mixing, pore-forming agent is gradually added and stirring is continued for 1-6 h. Then, ammonia is added dropwise to polymerize dopamine, forming an iron-containing polydopamine coating layer on the surface of SiO2 hard template, obtaining SiO2@PDA-Fe composite. The composite is washed and dried to obtain SiO2@PDA-Fe powder. Further, in step 2, the amount of SiO2 hard template used is 0.2–2.0 g; the total volume of ethanol and deionized water is 60–150 mL; the amount of Pluronic F127 added is 0.2–2.0 g; the amount of dopamine hydrochloride added is 0.3–3.0 g; the iron source is ferrous salt or ferric salt, and the amount of iron source added is 1–20 mg; the stirring speed is 200–800 rpm; the porogen is 1,3,5-trimethylbenzene; the volume of the porogen added is 1.0–10.0 mL, and stirring continues for 1–6 h after addition; the volume of ammonia water added dropwise is 1–10 mL, and stirring continues for 4–12 h after dropwise addition; the composite is washed with deionized water and ethanol, and then dried at 40–80 °C for 6–24 h to obtain SiO2@PDA-Fe powder.

[0012] The optimized iron source is (NH4)2Fe(SO4)2·6H2O.

[0013] Step 3, carbonization and template removal: The SiO2@PDA-Fe powder obtained in Step 2 is subjected to programmed heating heat treatment under an inert atmosphere to carbonize it and obtain a carbon / SiO2 composite intermediate; then the carbon / SiO2 composite intermediate is heated in sodium hydroxide solution to etch and remove the SiO2 template, washed until neutral and dried to obtain the cavity-confined carbon-based iron single-atom catalyst Fe-NC.

[0014] Further, in step 3, the heating rate of the heat treatment is 0.5–5 °C / min; the heat treatment includes a first stage of heating to 250–450 °C and holding for 1–6 h, and a second stage of heating to 650–950 °C and holding for 0.5–5 h; the concentration of the sodium hydroxide solution used for etching to remove the SiO2 hard template is 1–8 mol / L; the etching temperature is 50–100 °C, and the etching time is 4–24 h.

[0015] The present invention also protects the cavity-confined carbon-based iron single-atom catalytic material obtained by the above method, wherein the catalytic material Fe-NC has a cavity-confined structure, the structure comprising: a hollow spherical shell composed of nitrogen-doped carbon; and atomically dispersed iron-nitrogen active sites Fe-Nx uniformly anchored on the inner and outer surfaces of the shell; wherein the shell has a mesoporous structure that extends through the inside and outside, and the pore size of the mesopores ranges from 2 to 50 nm.

[0016] This invention also protects the application of the above-mentioned cavity-confined carbon-based iron single-atom catalytic material, which is used as a catalyst in the degradation of organic pollutants by activated permonosulfate (PMS), and the activated permonosulfate degradation of organic pollutants is used to treat water bodies containing phenolic organic matter.

[0017] Furthermore, the application includes adding the catalytic material and persulfate to rural domestic sewage containing phenolic organic pollutants to catalytically degrade the phenolic organic pollutants.

[0018] This invention also protects a method for degrading phenolic organic pollutants in water, comprising: adding the above-mentioned catalytic material and persulfate to rural domestic sewage containing phenolic organic pollutants, and carrying out a catalytic reaction, the specific method being: First, prepare 100 mL of a 10 mg / L MeP solution and place it in a 150 mL beaker. Add predetermined concentrations of quenching / scavenging agents to the MeP solution and mix well. The quenching / scavenging agents include tert-butanol (TBA), methanol (MeOH), p-benzoquinone (p-BQ), L-histidine (L-his), and dimethyl sulfoxide (DMSO), which are used to scavenge hydroxyl radicals (•OH), sulfate radicals (SO4•⁻), superoxide anion radicals (O2•⁻), singlet oxygen (1O2), and high-valence ferrooxide species [Fe(IV)=O], respectively. Then, add 0.005 g of Fe-NC material to the system and ultrasonically disperse for 10 s. Next, add 0.2 mL of 100 mmol / L PMS solution. Place the beaker on a magnetic stirrer and stir the reaction at 600 rpm. Samples are taken at 0, 1, 2, 4, 6, 8, and 10 min of reaction time, with 1.0 g of sample taken each time. mL; after the sample solution was filtered through a 0.22 μm filter membrane, 0.5 mL of methanol was added to terminate the reaction.

[0019] Compared with existing technologies, the present invention has the following beneficial effects: 1. This invention adopts a sequential assembly strategy of soft template and hard template, using hollow mesoporous carbon spheres as the framework and anchoring atomically dispersed iron active sites to construct a nano-confined reactor Fe-NC catalytic material with cavity confinement characteristics, thereby achieving high-density and stable loading of available single atomic sites.

[0020] 2. The constructed cavity confinement structure provides an effective platform for revealing and utilizing the confinement effect. Compared with the unconfined comparative material, the activation efficiency of the Fe-NC nanoconfinement reactor for PMS is increased by about 2.9 times, and the degradation performance of phenolic pollutants can be significantly improved simultaneously, thus verifying the enhancing effect of cavity confinement on mass transfer and interfacial reaction kinetics.

[0021] 3. By regulating the reaction pathway of active sites through the synergistic effect of metal-carrier, the system preferentially generates singlet oxygen 1O2 and achieves oxidative degradation in a non-radical manner, thereby improving the selective removal capacity of organic pollutants and enhancing the resistance to interference under complex water matrix conditions.

[0022] 4. The preparation process is simple and controllable, the raw materials are widely available and the cost is low. The resulting solid catalytic material is easy to add, separate and recycle. It is suitable for the efficient and in-depth treatment of recalcitrant organic pollutants in rural domestic sewage and industrial wastewater and has good engineering application prospects. Attached Figure Description

[0023] Figure 1 SEM image of the SiO2 hard template prepared in this invention; Figure 2These are XRD images of Fe-NC, Fe-NC-Si, and NC in embodiments of the present invention; Figure 3 SEM images of Fe-NC prepared according to the present invention; Figure 4 HRTEM and EDS surface scan images of Fe-NC prepared according to the present invention; Figure 5 The graph shows a comparison of the performance of Fe-NC-type Fenton reaction in degrading MeP prepared in this invention. Figure 6 The effect of Fe-NC prepared in this invention on MeP degradation in a PMS system under different quenching agents. Detailed Implementation

[0024] The technical solution of the present invention will be clearly and completely described below with reference to the embodiments. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative effort are within the scope of protection of the present invention.

[0025] Example 1 A method for preparing a cavity-confined carbon-based iron single-atom catalytic material specifically includes the following steps: 1. Preparation of silica template 10 mL of ammonia solution was added to a mixed solvent of 40 mL ethanol and 50 mL deionized water and ultrasonically dispersed. While stirring vigorously at 1000 rpm, a mixed solution of 9 mL tetraethoxysilane and 91 mL ethanol was added dropwise. The reaction was then continued with stirring at 800 rpm for 22 hours at room temperature. The resulting SiO2 hard template was obtained by centrifugation, washing, and drying. See attached SEM image for details. Figure 1 The results showed that the SiO2 template exhibited a spherical structure, as expected.

[0026] 2. Coating of iron-containing polydopamine precursor on template surface and construction of mesoporous structure Take 0.8 g of the SiO2 hard template prepared in step 1 and ultrasonically disperse it in a mixed solvent of 30 mL ethanol and 60 mL deionized water. Add 1.0 g of block copolymer Pluronic F127 and 1.5 g of dopamine hydrochloride and ultrasonicate until fully dissolved. Add 4 mg of (NH4)2Fe(SO4)2·6H2O as an iron source under stirring at 500 rpm. After mixing evenly, gradually add 4.0 mL of 1,3,5-trimethylbenzene and continue stirring for 3 h. Then, add 3.75 mL of ammonia water dropwise to the above suspension to polymerize dopamine. Continue stirring for 8 h after the addition. An iron-containing polydopamine coating layer is formed on the surface of the SiO2 hard template to obtain the SiO2@PDA-Fe composite. Wash the composite with deionized water and ethanol and dry it at 60 °C for 12 h to obtain SiO2@PDA-Fe powder.

[0027] 3. Carbonization and template removal The SiO2@PDA-Fe powder obtained in step 2 was subjected to programmed heating under an inert atmosphere, with the temperature increased to 350 ℃ at a rate of 1 ℃ / min and held for 3 h; then the temperature was increased to 800 ℃ at a rate of 1 ℃ / min and held for 2 h to carbonize it and obtain a carbon / SiO2 composite intermediate; subsequently, the carbon / SiO2 composite intermediate was placed in a sodium hydroxide solution with a concentration of 4 mol / L and heated at 80 ℃ for 12 h to etch and remove the SiO2 template, washed until neutral and dried to obtain the cavity-confined carbon-based iron single-atom catalyst Fe-NC.

[0028] In addition, using the same procedure, Fe-NC-Si and NC were prepared respectively without etching the silica template and without adding an iron source, serving as comparative examples for the application in Example 2. The XRD patterns of the prepared series of materials are attached. Figure 2 SEM images of Fe-NC materials are attached. Figure 3 The HRTEM image and corresponding EDS energy spectrum scan are attached. Figure 4 .

[0029] Example 2 The cavity-confined carbon-based iron single-atom catalytic material Fe-NC, compared with the comparative materials (Fe-NC-Si and NC), exhibits the following specific operating method in its catalytic degradation activity for MeP: 100 mL of a 10 μmol / L MeP solution was prepared and placed in a 150 mL beaker. 0.001–0.01 g of Fe-NC material was added and sonicated for 10 s to ensure uniform dispersion. 0.1–1.0 mL of a 100 mmol / L PMS solution was added, and the reaction was started immediately. Three experimental groups were set up: Fe-NC, Fe-NC-Si, and NC. Fe-NC-Si and NC served as control groups, respectively, with no SiO2 template etching and no iron source added. The beaker was placed on a magnetic stirrer at 600 rpm for the reaction. Samples were taken at 0, 1, 2, 4, 6, 8, and 10 min, 1.0 mL each time, filtered through a 0.22 μm filter, and the reaction was stopped with 0.5 mL of methanol. The samples were quantitatively analyzed by high-performance liquid chromatography (HPLC) equipped with a diode array detector and a C18 HPLC column. The mobile phase was 70% methanol and 30% pure water, and the detection wavelength was 256 nm. nm, results are attached. Figure 5 As shown.

[0030] The results showed that Fe-NC prepared using SiO2 as a template had the strongest catalytic degradation ability for MeP, indicating that the confinement effect can significantly enhance the catalytic degradation ability of Fe-NC.

[0031] Example 3 This invention prepares cavity-confined carbon-based iron single-atom catalytic materials and activates the active species recognition (quenching experiment) of the PMS system, including the following steps: First, prepare 100 mL of a 10 mg / L MeP solution and place it in a 150 mL beaker. Add predetermined concentrations of quenching / scavenging agents to the MeP solution and mix well. The quenching / scavenging agents include tert-butanol (TBA), methanol (MeOH), p-benzoquinone (p-BQ), L-histidine (L-his), and dimethyl sulfoxide (DMSO), which are used to scavenge hydroxyl radicals (•OH), sulfate radicals (SO4•⁻), superoxide anion radicals (O2•⁻), singlet oxygen (1O2), and high-valence ferrooxide species [Fe(IV)=O], respectively. Then, add 0.005 g of Fe-NC material to the system and ultrasonically disperse for 10 s; then add 0.2 mL of 100 mmol / L PMS solution, and place the beaker on a magnetic stirrer to stir the reaction at 600 rpm. Samples were taken at 0, 1, 2, 4, 6, 8, and 10 min of reaction, with 1.0 mL taken each time. The sample solution was filtered through a 0.22 μm filter membrane, and the reaction was terminated by adding 0.5 mL of methanol. The concentration of MeP was quantitatively detected using the same HPLC conditions as in Example 2. The experimental results are shown in the appendix. Figure 6 .

[0032] The results showed that the catalytic degradation of MeP was significantly inhibited after the addition of L-his, indicating that 1O2 is the main active species contributing to the activation of PMS in the cavity-confined carbon-based iron single-atom material reaction system.

[0033] Although embodiments of the invention have been shown and described, it will be understood by those skilled in the art that various changes, modifications, substitutions and alterations can be made to these embodiments without departing from the principles and spirit of the invention, the scope of which is defined by the appended claims and their equivalents.

Claims

1. A method for preparing a cavity-confined carbon-based iron single-atom catalytic material, characterized in that, Specifically, the steps include the following: Step 1, Preparation of silica template: Ammonia water is added to a mixed solvent of ethanol and deionized water and ultrasonically dispersed. Under vigorous stirring, an ethanol solution of tetraethoxysilane is added dropwise. After continuing to stir and react, the mixture is centrifuged, washed, and dried to obtain a SiO2 hard template. The stirring speed is 600-1200 rpm and the stirring reaction time is 12-30 h. Step 2, coating of iron-containing polydopamine precursor on template surface and construction of mesoporous structure: The SiO2 hard template prepared in Step 1 is ultrasonically dispersed in a mixed solvent of ethanol and deionized water. Block copolymer Pluronic F127 and dopamine hydrochloride are added and ultrasonically dissolved until fully dissolved. Iron source is added under stirring at 200-800 rpm. After uniform mixing, pore-forming agent is gradually added and stirring is continued for 1-6 h. Then, ammonia is added dropwise to polymerize dopamine, forming an iron-containing polydopamine coating layer on the surface of SiO2 hard template, obtaining SiO2@PDA-Fe composite. The composite is washed and dried to obtain SiO2@PDA-Fe powder. Step 3, carbonization and template removal: The SiO2@PDA-Fe powder obtained in Step 2 is subjected to programmed heating heat treatment under an inert atmosphere to carbonize it and obtain a carbon / SiO2 composite intermediate; then the carbon / SiO2 composite intermediate is heated in sodium hydroxide solution to etch and remove the SiO2 template, washed until neutral and dried to obtain the cavity-confined carbon-based iron single-atom catalytic material Fe-NC.

2. The preparation method according to claim 1, characterized in that, In step 1, the amount of ammonia added is 5-20 mL; the volume ratio of ethanol to deionized water is 0.3-1.5; and the tetraethoxysilane ethanol solution is a mixed solution with a volume ratio of tetraethoxysilane:ethanol = 5-20:80-100.

3. The preparation method according to claim 1, characterized in that, In step 2, the amount of SiO2 hard template used is 0.2–2.0 g; the total volume of ethanol and deionized water is 60–150 mL; the amount of Pluronic F127 added is 0.2–2.0 g; the amount of dopamine hydrochloride added is 0.3–3.0 g; the iron source is ferrous salt or ferric salt, and the amount of iron source added is 1–20 mg; the stirring speed is 200–800 rpm; the porogen is 1,3,5-trimethylbenzene; the volume of the porogen added is 1.0–10.0 mL, and stirring is continued for 1–6 h after addition; the volume of ammonia water added dropwise is 1–10 mL, and stirring is continued for 4–12 h after dropwise addition; the composite is washed with deionized water and ethanol, and then dried at 40–80 °C for 6–24 h to obtain SiO2@PDA-Fe powder.

4. The preparation method according to claim 3, characterized in that, The iron source is (NH4)2Fe(SO4)2·6H2O.

5. The preparation method according to claim 1, characterized in that, In step 3, the heating rate of the heat treatment is 0.5–5 °C / min; the heat treatment includes a first stage of heating to 250–450 °C and holding for 1–6 h, and a second stage of heating to 650–950 °C and holding for 0.5–5 h; the concentration of the sodium hydroxide solution used for etching to remove the SiO2 hard template is 1–8 mol / L; the etching temperature is 50–100 °C, and the etching time is 4–24 h.

6. A cavity-confined carbon-based iron single-atom catalytic material, characterized in that, The material is prepared by the method according to claims 1-5. The catalytic material Fe-NC has a cavity-confined structure, the structure including a hollow spherical shell made of nitrogen-doped carbon; and atomically dispersed iron-nitrogen active sites Fe-Nx uniformly anchored on the inner and outer surfaces of the shell; wherein the shell has a mesoporous structure that extends through the inside and outside, and the pore size of the mesopores ranges from 2 to 50 nm.

7. The application of the cavity-confined carbon-based iron single-atom catalytic material according to claim 6, characterized in that, The catalytic material is used as a catalyst in the degradation of organic pollutants by activated persulfate PMS, which is used to treat water bodies containing phenolic organic matter.

8. The application according to claim 7, characterized in that, The application includes adding the catalytic material and persulfate to rural domestic sewage containing phenolic organic pollutants to catalytically degrade the phenolic organic pollutants.

9. A method for degrading phenolic organic pollutants in water, characterized in that, include: Add the catalytic material described in any one of claims 1-5, along with persulfate, to rural domestic sewage containing phenolic organic pollutants, and carry out a catalytic reaction. The specific method is as follows: First, prepare 100 mL of a 10 mg / L MeP solution and place it in a 150 mL beaker. Add a predetermined concentration of quencher / scavenger to the MeP solution and mix well. The quencher / scavenger includes tert-butanol, methanol, p-benzoquinone, L-histidine, and dimethyl sulfoxide, which are used to scavenge hydroxyl radicals, sulfate radicals, superoxide anion radicals, singlet oxygen, and high-valence ferrooxide species, respectively. Then, add 0.005 g of Fe-NC material to the system and sonicate for 10 s. Next, add 0.2 mL of 100 mmol / L PMS solution, place the beaker on a magnetic stirrer and stir the reaction at 600 rpm. Take samples at 0, 1, 2, 4, 6, 8, and 10 min of reaction, 1.0 mL each time. After filtering the sample solution through a 0.22 μm filter membrane, add 0.5 mL of methanol to terminate the reaction.

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