Application of cobalt-nitrogen-carbon confinement cobalt nanoparticle catalyst in heterogeneous Fenton-like reaction

Abstract

The invention relates to a series of cobalt-nitrogen-carbon confinement cobalt nano-particle catalysts with high catalytic activity and high cycle stability and a preparation method thereof, wherein the cobalt-nitrogen-carbon confinement cobalt nano-particle catalysts are obtained by changing the ratio of cobalt-cobalt Prussian blue analogue to melamine in a precursor and calcining. Under the conditions of different ions, organic matters and complex water quality, excellent catalytic activity is still maintained, and efficient degradation of p-hydroxybenzoic acid is realized. The core-shell structure formed by the cobalt-nitrogen-carbon confinement cobalt nanoparticles in the catalyst plays a very important role in exposure of active sites and formation of local high instantaneous concentration, persulfate can be effectively activated to generate non-free radical dominated high-valence cobalt oxygen species, migration and interaction with pollutants are enhanced, and the pollution to the pollutants is reduced. And the characteristics of high activity, strong environmental resistance, high cycle stability and the like of the catalyst are realized. The catalyst has excellent removal efficiency on various pollutants, preparation and characteristics of the catalyst are not reported before, and the catalyst has a good prospect in the application field of complex matrix organic wastewater.

Description

Technical Field

[0001] The present invention relates to the technical field of catalyst preparation and application, and specifically to the preparation of a cobalt-nitrogen-carbon-confined cobalt nanoparticle catalyst with high activity and high cycle stability and a preparation method thereof. Specifically, the present invention comprises the following two steps: 1) preparing a Co-Co PBA precursor by a coprecipitation method; 2) mixing the synthesized Co-Co PBA precursor with melamine, and calcining at high temperature in a nitrogen atmosphere by changing the relative proportion of PBA and melamine to obtain a series of cobalt-nitrogen-carbon-confined cobalt nanoparticle catalysts with different compositions and curvatures. Compared with the reported cobalt-based catalysts, the cobalt-nitrogen-carbon-confined cobalt nanoparticle catalyst prepared by the method has a unique structure, superior performance, and good resistance to environmental conditions, and exhibits excellent cycle performance in organic pollutant degradation and water treatment applications. The present invention provides a confined catalyst with high activity and high cycle stability and a preparation method thereof. Background Art

[0002] Advanced oxidation processes (AOPs) have emerged as promising technologies to address these challenges due to their ability to generate highly reactive species that can degrade a wide range of pollutants. Advanced oxidation technologies based on persulfate activation achieve complete mineralization of pollutants by generating free radicals and non-radical reactive oxygen species, making them particularly effective in treating recalcitrant materials that are resistant to traditional methods. Single-atom catalysts with transition metals as active centers have been widely used in efficient and diverse catalytic processes. Among a series of heterogeneous transition metal catalysts, cobalt-based single-atom catalysts have gradually become a superior nanomaterial in recent years due to their maximized atomic efficiency, tunable geometry, and unique electronic properties. However, recent studies have shown that in AOPs systems, cobalt single-atom catalysts exhibit significantly reduced cyclic stability and are easily disturbed by wastewater matrices such as humic acid, pH, and different anions / cations; at the same time, due to the limited number of catalytic sites on single-atom catalysts, their electron transfer and catalytic activation abilities are inferior to those of structures in which single atoms coexist with nanoclusters. However, when coexisting with nanoclusters, there are also problems such as severe leaching of cobalt atoms, which severely limits their practical application in advanced oxidation.

[0003] At present, the research on improving the catalytic stability, activity and environmental resistance of transition metal-based advanced oxidation systems mainly focuses on improving the resistance to environmental matrices by regulating the formation of non-radicals and constructing single atom / cluster mixed systems, producing highly selective non-radicals such as singlet oxygen and the number of metal catalytic sites and other active sites. Although these methods have improved the catalytic activity to a certain extent, there are still some problems, such as the construction of multi-metal sites will significantly increase the preparation cost, lead to the agglomeration of metal sites, and how to effectively regulate the fine structure between multi-metal sites is also a significant challenge. By coexisting with nanoparticles or clusters, the preparation and application costs can be significantly reduced, and the catalytic activity can be effectively improved, and the utilization efficiency of metal sites can be improved. However, due to the large size of nanoparticles or clusters and the unstable structure, metal leaching is relatively serious. By constructing cobalt-nitrogen-carbon confined cobalt nanoparticles, not only can the leaching of cobalt nanoparticles be effectively protected, dissolution can be reduced, and stability can be significantly improved, but also the formation of highly active non-radical reactive oxygen species can be regulated by regulating the size of the confinement scale, changing the confinement structure and curvature, and improving the ability to resist environmental interference. Summary of the invention

[0004] The present invention aims to provide a cobalt-nitrogen-carbon confined cobalt nanoparticle catalyst with high activity, high durability and high environmental adaptability and a preparation method thereof.

[0005] Different cobalt-cobalt Prussian blue analogs (Co-Co PBA) were used as precursors, and the mixing ratio of melamine and Co-Co PBA was changed. Cobalt-nitrogen-carbon-confined cobalt nanoparticle catalysts with different structures were obtained by further calcination in a nitrogen atmosphere. The purpose of using Co-Co PBA as a precursor is that it has a highly ordered framework structure, high thermal stability, can achieve the distribution of metal ions and stabilize them within the framework, and regulate the dispersion and coordination environment of cobalt atoms. Melamine can provide a stable and abundant carbon and nitrogen source for the formation of the catalyst, enhance the coordination and anchoring effect with cobalt atoms, and improve the service life and anti-deactivation ability of the catalyst.

[0006] The study found that the cobalt-nitrogen-carbon confined cobalt nanoparticle catalyst obtained by calcining Co-Co PBA / melamine precursors with different mixing ratios showed great activity in a heterogeneous Fenton-like reaction with p-hydroxybenzoic acid (HBA) as a model pollutant. When the mass ratio of Co-Co PBA precursor to melamine was 1:1, the generated CN-1 catalyst showed the best catalytic activity and had a certain weak magnetism, which was conducive to the recovery and utilization of the catalyst. The confined cobalt nanoparticles had the best curvature effect, were wrapped by an outer layer of cobalt, nitrogen and carbon, had a stable structure, and had a specific surface area of ​​up to 78.92m 2 / g. More importantly, it is a high-valent cobalt Co in the heterogeneous Fenton-like degradation of p-hydroxybenzoic acid. IV=O species dominate, so that it can maintain high activity in high-concentration inorganic anions, natural organic matter and complex water quality environments, avoiding excessive consumption by background water quality. Secondly, the catalyst has low metal cobalt leaching and can still maintain high activity after several cycle experiments, which has the potential for practical engineering and large-scale application.

[0007] The present invention is achieved by the following technical means:

[0008] Catalyst preparation

[0009] 1) Prepare Co-Co PBA precursor using coprecipitation method: 80-120mL of K3[Co(CN)6] solution with a concentration of 0.05-0.3M is slowly dripped into 80-110mL of CoCl2·6H2O solution with a concentration of 0.05-0.25M at a rate of 5mL per minute, stirred for 30-60min, and then further ultrasonicated for 30-60min. The obtained homogeneous solution is placed at room temperature for 12-24 hours under light-proof conditions for aging. Then, the precipitate is separated by centrifugation, washed with deionized water 3-6 times, and dried at 60-80°C for 12-24 hours to obtain a cobalt-cobalt Prussian blue analog.

[0010] 2) Synthesis of cobalt-nitrogen-carbon-confined cobalt nanoparticle catalyst by thermal decomposition: First, melamine is mixed with the Co-Co PBA precursor (50-100 mg) prepared by the above coprecipitation in a mass ratio of 0.2-8. Then, the mixture is fully ground and placed in a quartz boat, heated to 400-900°C (preferably 500-900°C) at a heating rate of 3-8°C (preferably 4-6°C) per minute in a nitrogen atmosphere, and maintained for 1-4 hours (preferably 2-3h). After cooling, wash with 0.5-2.0M (preferably 1.0-1.5M) H2SO4 solution for 12-24 hours. Finally, the catalyst is separated by centrifugation and washed with deionized water several times until the pH of the washing solution is stabilized between 6-7, and then dried at 60-80°C for 12-24 hours. According to the mixing mass ratio of melamine and Co-Co PBA, a series of cobalt-nitrogen-carbon-confined cobalt nanoparticle catalysts were obtained and named CN-1, CN-2, CN-3, CN-4, CN-5, CN-6 (preferably CN-3).

[0011] The catalyst is used in a heterogeneous Fenton-like reaction in which an oxidant is catalyzed to oxidize and degrade organic pollutants.

[0012] The oxidant is one or more of potassium persulfate, hydrogen peroxide or potassium persulfate;

[0013] The catalyst performance was evaluated using para-hydroxybenzoic acid (HBA), bisphenol A (BPA) and phenol (PN) as model organic pollutants. The experiment was carried out at room temperature and under magnetic stirring. The reaction substrate was one or two of 50-200uM HBA, BPA and PN solutions, the catalyst concentration in the solution was 50-500mg / L (preferably 100-400mg / L), the oxidant concentration was 0.2-5mM (preferably 0.5-3mM), and the pH was 2-11.

[0014] The reaction device of the Fenton-like pollutant degradation reaction is a dark reaction box, in which dark conditions are maintained and a temperature-controlled stirrer and a reaction container are placed.

[0015] In the reaction device, the catalyst can achieve efficient degradation of model pollutants such as p-hydroxybenzoic acid, bisphenol A and phenol; in the presence of humic acid and various anions, the catalyst can attack organic pollutants with non-free radical species (high-valent cobalt oxygen species) to achieve degradation of pollutants.

[0016] In the reaction solution for Fenton-like degradation of pollutants, the concentration of one or more of bicarbonate ions, carbonate ions, sulfate ions, and chloride ions should be <25 mM (preferably 1 mM-15 mM), or the concentration of humic acid should be <25 mg / L (preferably 1 mg / L-20 mg / L).

[0017] In the reaction solution of Fenton-like degradation of pollutants, the pH range of the reaction solution should be controlled at 2-11 (preferably 3-10). In the reaction process of Fenton-like degradation of pollutants, the reaction time should be greater than or equal to 3 minutes (preferably greater than or equal to 10 minutes).

[0018] The catalyst preparation method is simple and easy, and the prepared catalyst has high degradation activity for pollutants in heterogeneous Fenton-like reactions (95.1% HBA removal rate is achieved within 10 minutes), strong adaptability to water quality environments, and high cycle stability (after five consecutive cycles, about 91% of the original removal efficiency is maintained). The cobalt-nitrogen-carbon confined cobalt nanoparticle structure can not only make full use of the activation effect of the cobalt site to produce a non-free radical-dominated reaction system dominated by high-valent cobalt oxygen species, but the confined structure can also protect the filtration of cobalt metal and achieve high cycle stability. The present invention uses cobalt-cobalt Prussian blue as the skeleton structure of the carbon source of the cobalt source, melamine as the nitrogen source and carbon source for anchoring the cobalt metal site, and the obtained cobalt-nitrogen-carbon confined cobalt nanoparticle catalyst is prepared by high-temperature calcination under a nitrogen atmosphere. The catalyst directly produces high-valent cobalt oxygen species (Co IV=O), and showed excellent activity in the heterogeneous Fenton-like degradation reaction of hydroxybenzoic acid, bisphenol A and phenol. The catalyst is relatively simple to prepare, has high activity and cyclic stability, and can resist the interference of complex water matrix. The preparation and characteristics of the catalyst with this structure have not been reported before, and it has good practical application value and prospects.

[0019] This catalyst maintains relatively superior catalytic activity even under different ion, organic matter and complex water conditions, achieving efficient degradation of p-hydroxybenzoic acid. The core-shell structure formed by cobalt-nitrogen-carbon confined cobalt nanoparticles in the catalyst plays a very important role in exposing active sites and forming local high instantaneous concentrations. It can effectively activate persulfate to produce high-valent cobalt oxygen species dominated by non-free radicals, enhance migration and interaction with pollutants, and achieve the characteristics of high activity, strong environmental resistance and high cyclic stability of the catalyst. The preparation method of this catalyst is simple and easy, the preparation cost is low, and it has excellent removal efficiency for a variety of pollutants. The preparation and characteristics of this catalyst have not been reported before, and it has good prospects in the application field of complex matrix organic wastewater. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] Figure 1 Comparison of the activities of cobalt-nitrogen-carbon-confined cobalt nanoparticle catalysts prepared by calcination of cobalt-cobalt Prussian blue analogue / melamine at different mass ratios;

[0021] Figure 2 Comparison of the activity of CN-3 under different pH conditions (a); different inorganic anions (b) and different concentrations of humic acid (c); and different pollutants (d).

[0022] Figure 3 TEM images of CN-1(a) and CN-3(b). DETAILED DESCRIPTION

[0023] Example 1-77

[0024] 1. Study on the effect of different mass ratios of melamine and Co-Co PBA precursor on catalytic activity Example 3

[0025] 1) Preparation of cobalt-cobalt Prussian blue analog precursor by coprecipitation method:

[0026] First, Co3[Co(CN)6]2(Co-CoPBA) was synthesized by reacting CoCl2·6H2O with aqueous solution of K3[Co(CN)6]. 100mL of 0.1M K3[Co(CN)6] solution was slowly dripped into 100mL of 0.1M CoCl2·6H2O solution at a rate of 5mL per minute, and the mixed solution was further stirred for 0.5 hours under magnetic stirring to obtain a uniform solution. The obtained solution was aged at room temperature for 24 hours under light-proof conditions. Then, the precipitate was separated by centrifugation and washed three times with deionized water. Finally, the metal organic ligand precursor was dried at 60°C for 24 hours to obtain Co-Co PBA.

[0027] 2) Preparation of Cobalt-N-C Confined Cobalt Nanoparticle Catalyst:

[0028] The cobalt-nitrogen-carbon-confined cobalt nanoparticle catalyst was synthesized by thermal decomposition under a nitrogen atmosphere. First, 50 mg of the metal organic ligand precursor Co-Co PBA synthesized in step 1) was weighed and mixed with 50 mg of melamine. Then, the mixture was fully ground and placed in a quartz boat. The sample was heated to 600°C at a heating rate of 5°C per minute under a nitrogen atmosphere and maintained for 3 hours. After cooling to room temperature, the calcined product was taken out and placed in 100 mL of 1M sulfuric acid solution and pickled in a water bath at 80°C for 24 hours. After the pickling was completed, the catalyst was washed with water until the pH of the washing solution was stabilized at 6-7, and then dried at 60°C for 12 hours to obtain the cobalt-nitrogen-carbon-confined cobalt nanoparticle catalyst CN-3 (melamine: Co-Co PBA=1).

[0029] The processes and conditions of Examples 1, 2, 4, 5, and 6 are the same as those of Example 3, except that: in step 2), the amount of melamine added is different (note in the brackets after the name). According to the different mixing mass ratios of melamine and Co-Co PBA, they are named CN-1 (melamine: Co-Co PBA = 0.2), CN-2 (melamine: Co-Co PBA = 0.5), CN-4 (melamine: Co-Co PBA = 2), CN-5 (melamine: Co-Co PBA = 3) and CN-6 (melamine: PBA = 5), respectively.

[0030] The process and conditions of Examples 7 and 8 are the same as those of Example 3, except that: melamine is not added in step 2), and the catalyst is named CoNP (Example 7). Co-Co PBA is not added in step 2), and the catalyst is named CN (Example 8).

[0031] The process and conditions of Examples 9-12 are the same as those of Example 3, except that: in step 2), the concentration of sulfuric acid pickling is 0.5 M, named CN-7 (Example 9), the concentration of sulfuric acid pickling is 2.0 M, named CN-8 (Example 10), the pickling time is 12 h, named CN-9 (Example 11), and the pickling time is 18 h, named CN-10 (Example 12).

[0032] The process and conditions of Examples 13-15 are the same as those of Example 3, except that: in step 1), the mixed solution is further stirred for 1.0 hour under magnetic stirring to obtain a uniform solution, named CN-11 (Example 13); the obtained solution is aged at room temperature for 12 hours under light-proof conditions, and is named CN-12 (Example 14); the obtained solution is aged at room temperature for 18 hours under light-proof conditions, and is named CN-13 (Example 15).

[0033] The specific characteristics of the series of catalysts obtained according to the different addition ratios of melamine-Co-Co PBA are as follows:

[0034] The specific surface area of ​​CN-1 in Example 1 is 40-55m 2 / g, the main components are nitrogen-doped graphitic carbon shell-supported cobalt single atom confined spherical cobalt nanoparticles, the size of the shell-wrapped round (or elliptical or quasi-spherical) cobalt nanoparticles is 13-15nm, the structure is elliptical or round or quasi-spherical, the diameter of the shell nitrogen-doped graphitic carbon sheet is 200-550nm, the average diameter is 350nm, the shell thickness of the cobalt single atom supported by the graphitic carbon shell is 3-5nm, the total cobalt content is 15.3wt%, the nitrogen-carbon molar ratio is 0.153, the active ingredient in the CN-1 catalyst is Co nanoparticles (attached Figure 3 a).

[0035] The specific surface area of ​​CN-2 obtained in Example 2 is 50-65m 2 / g, the main components are spherical cobalt nanoparticles confined in shells of cobalt single atoms supported by nitrogen-doped graphitic carbon, wherein the size of the round cobalt nanoparticles is 15-20nm, the diameter of the nitrogen-doped graphitic carbon flakes is 150-500nm, the average diameter is 300nm, the shell thickness of the cobalt single atom supported by the graphitic carbon shell is 3-5nm, the total cobalt content is 13.1wt%, the nitrogen-carbon molar ratio is 0.161, and the active ingredients in the CN-2 catalyst are CoNC and Co nanoparticles, among which Co nanoparticles dominate (78-86%).

[0036] The specific surface area of ​​CN-3 obtained in Example 3 is 68-81m 2 / g, the main components are spherical cobalt nanoparticles confined in the shell of cobalt single atoms supported by nitrogen-doped graphitic carbon, wherein the size of the round cobalt nanoparticles is 20-25nm, the structure is uniformly round, the diameter of the nitrogen-doped graphitic carbon sheet is 150-460nm, the average diameter is 270nm, the shell thickness of the cobalt single atom supported by the graphitic carbon shell is 2-5nm, the total cobalt content is 10.4wt%, the nitrogen-carbon molar ratio is 0.165, the active components in the CN-3 catalyst are CoNC and Co nanoparticles, wherein CoNC occupies the leading position (88-98%) (Appendix Figure 3 b).

[0037] The specific surface area of ​​CN-4 obtained in Example 4 is 64-79m 2 / g, the main components are spherical cobalt nanoparticles confined in shells of cobalt single atoms supported by nitrogen-doped graphitic carbon, wherein the size of the round cobalt nanoparticles is 26-31nm, the diameter of the nitrogen-doped graphitic carbon flakes is 130-420nm, the average diameter is 240nm, the shell thickness of the cobalt single atoms supported by the graphitic carbon shell is 4-7nm, the total cobalt content is 9.1wt%, the nitrogen-carbon molar ratio is 0.168, and the active ingredients in the CN-4 catalyst are CoNC and Co nanoparticles, among which CoNC occupies a dominant position (85-91%).

[0038] The specific surface area of ​​CN-5 obtained in Example 5 is 58-78m 2 / g, the main components are spherical cobalt nanoparticles confined in shells of cobalt single atoms supported by nitrogen-doped graphene, wherein the size of the round cobalt nanoparticles is 13-15nm, the diameter of the nitrogen-doped graphitic carbon flakes is 100-350nm, the average diameter is 180nm, the shell thickness of the cobalt single atoms supported by the graphitic carbon shell is 5-10nm, the total cobalt content is 7.6wt%, the nitrogen-carbon molar ratio is 0.171, and the active ingredients in the CN-5 catalyst are CoNC and Co nanoparticles, among which the CoNC structure occupies a dominant position (72-86%).

[0039] The specific surface area of ​​CN-6 obtained in Example 6 is 63-80m 2 / g, the main components are cobalt single atoms supported by nitrogen-doped graphene, infinite domain encapsulated spherical cobalt nanoparticles, the diameter of nitrogen-doped graphitic carbon sheets is 50-500nm, the average diameter is 320nm, the cobalt content is 5.3wt%, the nitrogen-carbon molar ratio is 0.174, and the active ingredient in the CN-6 catalyst is CoNC.

[0040] The specific surface area of ​​CoNP obtained in Example 7 is 30-57 m 2 / g, the main components are cobalt nanoparticles and cobalt single atoms supported by nitrogen-free doped graphene, the size of the cobalt nanoparticles is 50-70nm, the cobalt content is 23.6wt%, the nitrogen-carbon molar ratio is 0.176, and the active ingredient in the CoNP catalyst is cobalt nanoparticles.

[0041] The specific surface area of ​​CN obtained in Example 8 is: 80-102m 2 / g, the main components are nitrogen-doped graphitic carbon, cobalt single atoms supported by non-nitrogen-doped graphitic carbon and confined encapsulated spherical cobalt nanoparticles, wherein the size of the nitrogen-doped graphitic carbon flakes is 100-300nm, the nitrogen-carbon molar ratio is 1.15, and the active component in the CN catalyst is the nitrogen-doped graphitic carbon flakes.

[0042] In the activity experiment of cobalt-nitrogen-carbon-confined cobalt nanoparticle catalyst synthesized by mixing different mass ratios of melamine and cobalt-cobalt Prussian blue analog (Co-Co PBA), the Fenton-like potassium persulfate (PMS) oxidation experiment with p-hydroxybenzoic acid as a model pollutant was used as a probe reaction. All experiments were carried out at room temperature and under magnetic stirring. 15 mg of catalyst was added to 50 mL of HBA solution and stirred for 30 minutes to reach adsorption-desorption equilibrium. Then 0.5 mL of 100 mM PMS solution was added to start the reaction. At preset time intervals (specific sampling time moments: 0 min, 1 min, 2 min, 3 min, 5 min, 7.5 min, 10 min, 15 min, 20 min), 1.5 mL of sample was taken from the reaction system and filtered through a 0.22 μm filter membrane. Subsequently, the obtained sample was injected into a 2 mL chromatographic bottle, and 0.2 mL of 2 mM Na2S2O3 solution was added to terminate the reaction. The reaction pH is the pH value of the reaction solution before the reaction is started (the pH value of the solution is adjusted using 0.1M hydrochloric acid and 0.1M sodium hydroxide solution). The concentration of HBA was detected using a high performance liquid chromatograph equipped with a C18 column. A 1% acetic acid solution and methanol with a volume ratio of 70 / 30 (v / v) were used as the mobile phase, and the flow rate was 1 mL min -1 , the detection wavelength is fixed at 280nm. For better comparison, the reaction kinetic constant of catalytic degradation is calculated by pseudo-first-order kinetic model. Specifically, the first-order reaction rate constant k is used as the evaluation index of catalytic reaction efficiency. k represents the speed of reaction in Fenton-like reaction. The formula of first-order reaction rate equation is: k t =-In C / C0, C is the HBA concentration at sampling time t, C0 is the HBA concentration in the solution at the beginning of the reaction (time 0), t is the reaction time, k tThe slope obtained by linear fitting is the first-order reaction rate constant, k is a constant. The larger the k value, the faster the reaction rate and the better the catalytic activity. For Example 3, the removal rate of p-hydroxybenzoic acid by CN-3 catalyst at 10 min is as high as 95.1%.

[0043] Table 1 Effect of different composite ratios on catalyst activity

[0044]

[0045] From Table 1 and Appendix Figure 1 It can be seen that a series of cobalt-nitrogen-carbon-confined cobalt nanoparticle catalysts prepared by adding different amounts of melamine and cobalt-cobalt Prussian blue analogs have a crucial influence on the reaction, as shown in Table 1 and Appendix. Figure 1 As shown, too much or too little melamine addition is not conducive to the reaction, while CN-3 (i.e., the cobalt-nitrogen-carbon-confined cobalt nanoparticle catalyst synthesized with melamine: Co-CoPBA = 3:1) with the best melamine addition ratio can achieve a large amount of pollutant degradation in a short time, showing the best catalytic degradation activity. Similarly, no melamine addition, no cobalt-cobalt Prussian blue analogs addition, and too much or too little melamine / Co-CoPBA addition ratio are not conducive to the reaction. Too much addition ratio will lead to insufficient cobalt sites and fail to effectively improve the catalytic activity. Too little addition ratio will lead to the generation of a large number of cobalt particles, which is not conducive to the formation of cobalt single atom sites and affects the utilization efficiency of active sites, which is not conducive to the reaction. Secondly, changing the synthesis conditions for too long or too short time, the pickling time for too long or too short time, and the pickling concentration for too high or too low will affect the catalytic activity.

[0046] 2. Investigate the effects of different reaction conditions on catalyst activity

[0047] In the test of the activity of the catalyst under different reaction conditions, the Fenton-like potassium persulfate oxidation experiment with p-hydroxybenzoic acid as a model pollutant was used as a probe reaction. The effects of different reaction conditions on the activity of the catalyst were compared and analyzed by plotting C / C0 as the ordinate and the corresponding t moment as the abscissa. The first-order reaction rate constant k was used as an evaluation index for the efficiency of the catalytic reaction. k represents the speed of the reaction in the Fenton-like reaction. The formula of the first-order reaction rate equation is: k t =-In C / C0, C is the HBA concentration at sampling time t, C0 is the HBA concentration in the solution at the beginning of the reaction, t is the reaction time, k tThe degree of degradation of pollutants when the reaction proceeds to time t, with -1n C / C0 as the ordinate and the corresponding time t as the abscissa, the slope obtained by linear fitting is the first-order reaction rate constant, and k is a constant. The larger the k, the faster the reaction rate and the faster the catalytic rate. All experiments were carried out at room temperature and under magnetic stirring. 15 mg of catalyst was added to 50 mL of HBA solution and stirred for 30 minutes to reach adsorption-desorption equilibrium. Then PMS solution was added to start the reaction. 1.5 mL of sample was taken out from the reaction system at preset time intervals (specific sampling time: 0 min, 1 min, 2 min, 3 min, 5 min, 7.5 min, 10 min, 15 min, 20 min) and filtered through a 0.22 μm filter membrane. Subsequently, the obtained sample was injected into a 2 mL chromatographic bottle, and 0.2 mL of 2 mM Na2S2O3 solution was added to terminate the reaction. The pH value of the solution was adjusted using 0.1 M NaOH solution and 0.1 M HCl solution. The concentration of HBA was detected by high performance liquid chromatography equipped with a C18 column. A 1% acetic acid solution and methanol in a volume ratio of 70 / 30 (v / v) were used as the mobile phase, and the flow rate was 1 mL min -1 , the detection wavelength is fixed at 280nm.

[0048] Before the reaction is started, the pH in the reaction system is adjusted by using 0.1M NaOH solution and 0.1M HCl solution, and the pH value of the solution after titration is measured in real time with a pH meter to fix the reaction pH. The preparation process of 100μM HBA solution with different pH is as follows: 5mL of high-concentration 1mM HBA solution is added to 45mL of deionized water and mixed thoroughly. The reaction pH is adjusted by titrating 0.1M NaOH solution and 0.1M HCl solution. After it stabilizes, 50mL of HBA solution (reaction substrate) with different buffer pH is obtained. In addition, the concentration of potassium persulfate (PMS) in the system can be changed to 0.5mM, 1.0mM, 2.0mM, 3.0mM, 4.0mM, the concentration of catalyst CN-3 is 50mg / L, 150mg / L, 200mg / L, 300mg / L, 400mg / L, and the concentration of HBA solution (50μM, 100μM, 150μM, 200μM, 250μM) can be controlled to achieve the regulation of PMS concentration, catalyst concentration and HBA concentration in the reaction system. Experiments were conducted, and all experiments were conducted at room temperature and under magnetic stirring. 1.5mL samples were taken out from the reaction system at preset time intervals (specific sampling time: 0min, 1min, 2min, 3min, 5min, 7.5min, 10min, 15min, 20min), and filtered through a 0.22μm filter membrane. Subsequently, the obtained sample was injected into a 2 mL chromatographic vial, and 0.2 mL of 2 mM Na2S2O3 solution was added to terminate the reaction. The pH value of the solution was adjusted using 0.1 M NaOH solution and 0.1 M HCl solution. The concentration of HBA was detected using a high performance liquid chromatograph equipped with a C18 column. A 1% acetic acid solution and methanol with a volume ratio of 70 / 30 (v / v) were used as the mobile phase, and the flow rate was 1 mL min -1 , the detection wavelength is fixed at 280nm.

[0049] Table 2 Effect of different reaction conditions on catalyst activity

[0050]

[0051] From Table 2 and Appendix Figure 2It can be seen that the CN-3 system can achieve efficient removal of pollutants in a wide pH range, among which the weakly acidic environment of pH = 5.1 has the best catalytic activity, showing high stability and strong resistance to a wide range of pH changes. This may be attributed to the high surface charge of the CoNC and CoNP structures and the efficient confined encapsulation structure under harsh conditions. The catalytic performance gradually deteriorates with the increase of HBA concentration, and gradually increases and then weakens with the increase of PMS concentration; while with the increase of catalyst concentration, there is a trend of first getting better and then gradually stabilizing. This is because when too much catalyst is added, the number of catalytic sites is no longer a factor restricting the speed of the reaction. At this time, the effect of adding more catalyst on the reaction rate can be ignored.

[0052] 3. Investigate the influence of different ions on the reaction activity of catalysts

[0053] In the activity test of different ion pairs, the degradation reaction of p-hydroxybenzoic acid was used as a probe reaction. The reaction rate constant k was used as an evaluation index of the catalytic reaction efficiency. k represents the speed of the reaction in the Fenton-like reaction. The formula of the first-order reaction rate equation is: k t =-ln C / C0, C is the HBA concentration at sampling time t, C0 is the HBA concentration in the solution at the beginning of the reaction, t is the reaction time, k tThe degree of degradation of pollutants when the reaction reaches time t, with -ln C / C0 as the ordinate and the corresponding time t as the abscissa, the slope obtained by linear fitting is the first-order reaction rate constant, and k is a constant. The larger the k, the faster the reaction rate and the better the activity. All experiments were carried out at room temperature and under magnetic stirring. 15 mg of catalyst CN-3 was added to 50 mL of HBA solution and stirred for 30 minutes to reach adsorption-desorption equilibrium. Then PMS solution was added to start the reaction. The water quality conditions were changed by adding different ion salts to the reaction system. The added ions were inorganic anion salts, including potassium bromide, sodium chloride, dipotassium hydrogen phosphate, potassium dihydrogen phosphate, sodium carbonate, sodium bicarbonate and sodium sulfate (KBr, NaCl, K2HPO4, KH2PO4, Na2CO3, NaHCO3 and NaSO4). The concentration of the ion salt in the reaction system after addition was uniformly controlled to be 20mmol / L. 1.5mL of sample was taken out from the reaction system at preset time intervals and filtered through a 0.22μm filter membrane (specific sampling time: 0min, 1min, 2min, 3min, 5min, 7.5min, 10min, 15min, 20min). Subsequently, the obtained sample was injected into a 2mL chromatographic bottle, and 0.2mL2mM Na2S2O3 solution was added to terminate the reaction. The pH value of the solution was adjusted using 0.1M NaOH solution and 0.1M HCl solution. The concentration of HBA was detected using a high performance liquid chromatograph C18 column. The mobile phase was a 1% acetic acid solution and methanol in a volume ratio of 70 / 30 (v / v) at a flow rate of 1 mL min -1 , the detection wavelength is fixed at 280nm.

[0054] Table 3 Effect of different ions on the reaction activity of catalysts

[0055]

[0056]

[0057] Br - :bromide ion; Cl - :Chloride ion; HPO4 2- :Hydrogen phosphate ion; H2PO4 2- :Dihydrogen phosphate ion; CO3 2- : carbonate ion; HCO 3- :Bicarbonate ion; SO4 2- :Sulfate ion

[0058] From Table 3 and Appendix Figure 2bIt can be seen that the introduction of inorganic anions has almost no obvious inhibitory effect on the reaction activity of CN-3 catalyst. In addition, the addition of some ions interacts with PMS in the reaction system to generate other active groups, which promote the reaction. This is mainly because the active groups in the reaction system are not free radical groups that are easily affected by ions, but active components mainly composed of non-free radical groups play an important role in the degradation of HBA.

[0059] 4. Investigate the catalytic performance of cobalt-nitrogen-carbon confined cobalt nanoparticle catalysts for different pollutants

[0060] In the activity test of sulfur-modified cobalt nitrogen carbon catalyst for different pollutants, the pollutants selected were mainly p-hydroxybenzoic acid, bisphenol A, phenol, doxycycline hydrochloride, nitrobenzene and benzoic acid organic pollutants, and the pollutant removal rates were calculated and compared. All experiments were carried out at room temperature and under magnetic stirring. 15 mg of catalyst CN-3 was added to 50 mL of pollutant solution and stirred for 30 minutes to reach adsorption-desorption equilibrium. Then PMS solution was added to start the reaction. The reaction substrate was 50 mL of solution containing different pollutants. 1.5 mL of sample was taken out from the reaction system at preset time intervals and filtered through a 0.22 μm filter membrane (specific sampling time: 0 min, 1 min, 2 min, 3 min, 5 min, 7.5 min, 10 min, 15 min, 20 min). Subsequently, the obtained sample was injected into a 2 mL chromatographic bottle, and 0.2 mL of 2 mM Na2S2O3 solution was added to terminate the reaction. The pH value of the solution was adjusted using 0.1 M NaOH solution and 0.1 M HCl solution. High performance liquid chromatography (HPLC) with a C18 column was used to detect the concentrations of various pollutants.

[0061] Table 4 Catalytic performance of catalysts for different pollutants

[0062]

[0063]

[0064] From Table 4 and Appendix Figure 2 d It can be seen that the CN-3 / PMS system has superior catalytic ability for pollutants with strong electron-donating ability such as hydroxybenzoic acid, bisphenol A, phenol, and doxycycline hydrochloride (reaching about 97.1%, 100%, 96.91%, and 78.36% within 20 min, respectively). However, the removal effects for electron-withdrawing group pollutants such as nitrobenzene and benzoic acid were only 40.35%, 26.1%, and 24.6%, respectively, which indicates that there is a certain selective degradation due to the attack characteristics of the reactive species in the CN-3 catalytic process.

[0065] 5. Investigate the effect of different concentrations of humic acid on the reaction activity of cobalt-nitrogen-carbon confined cobalt nanoparticle catalysts

[0066] In the test of the activity of humic acid at different concentrations on the catalyst, the potassium persulfate oxidation reaction of p-hydroxybenzoic acid was used as the probe reaction. The reaction rate constant k was used as the evaluation index of the catalytic reaction efficiency. k represents the speed of the reaction in the Fenton-like reaction. The formula of the first-order reaction rate equation is: k t =-lnC / C0, C is the HBA concentration at sampling time t, C0 is the HBA concentration in the solution at the beginning of the reaction, t is the reaction time, k t The degree of degradation of pollutants when the reaction proceeds to time t, with -lnC / C0 as the ordinate and the corresponding time t as the abscissa, the slope obtained by linear fitting is the first-order reaction rate constant, and k is a constant. The larger the k, the faster the reaction rate and the better the activity. All experiments were carried out at room temperature and under magnetic stirring. 15 mg of catalyst was added to 50 mL of HBA solution and stirred for 30 minutes to reach adsorption-desorption equilibrium. Then PMS solution was added to start the reaction. The water quality conditions were changed by adding natural organic matter such as humic acid (HA) to the reaction system. The concentration of humic acid was controlled at 1 mg / L, 5 mg / L, 10 mg / L, 15 mg / L, and 20 mg / L. 1.5 mL of sample was taken out from the reaction system at preset time intervals (specific sampling time: 0 min, 1 min, 2 min, 3 min, 5 min, 7.5 min, 10 min, 15 min, 20 min), and filtered through a 0.22 μm filter membrane. Subsequently, the obtained sample was injected into a 2 mL chromatographic bottle, and 0.2 mL of 2 mM Na2S2O3 solution was added to terminate the reaction. The pH value of the solution was adjusted using 0.1 M NaOH solution and 0.1 M HCl solution. The concentration of HBA was detected using a high performance liquid chromatograph equipped with a C18 column. A 1% acetic acid solution and methanol with a volume ratio of 70 / 30 (v / v) were used as the mobile phase, and the flow rate was 1 mL min -1 , the detection wavelength is fixed at 280nm.

[0067] Table 5 Effect of different concentrations of humic acid on the reaction activity of catalyst

[0068]

[0069] From Table 5 and Appendix Figure 2c It can be seen that the addition of different concentrations of humic acid (1 mg / L, 5 mg / L, 10 mg / L, 15 mg / L and 20 mg / L) did not have a significant adverse effect on the degradation of HBA by catalyst CN-3. However, it should be noted that with the increase of humic acid concentration, the inhibition of the reaction first increased and then weakened, indicating that the catalyst can resist the influence of humic acid to a certain extent in the process of degrading pollutants, and has a certain tolerance to wastewater containing humic acid.

[0070] 6. Investigate the effects of different oxidants on the reaction activity of cobalt-nitrogen-carbon confined cobalt nanoparticle catalysts

[0071] In the test of the activity of different oxidants on the catalyst, the potassium persulfate oxidation reaction of p-hydroxybenzoic acid was used as a probe reaction. The reaction rate constant k was used as an evaluation index of the catalytic reaction efficiency. k represents the speed of the reaction in the Fenton-like reaction. The formula of the first-order reaction rate equation is: k t =-lnC / C0, C is the HBA concentration at sampling time t, C0 is the HBA concentration in the solution at the beginning of the reaction, t is the reaction time, k t The slope obtained by linear fitting is the first-order reaction rate constant, and k is a constant. The larger the k, the faster the reaction rate and the better the activity. All experiments were carried out at room temperature and under magnetic stirring. 15 mg of catalyst CN-3 was added to 50 mL of HBA solution and stirred for 30 minutes to reach adsorption-desorption equilibrium. The reaction was started by adding 0.5 mL of 100 mM potassium persulfate (PMS), hydrogen peroxide (H2O2), potassium persulfate (PDS), periodate (PI) and peracetic acid (PAA) solution respectively. 1.5 mL of sample was taken out from the reaction system at preset time intervals and filtered through a 0.22 μm filter membrane. Subsequently, the obtained sample was injected into a 2 mL chromatographic bottle, and 0.2 mL of 2 mM Na2S2O3 solution was added to terminate the reaction. The pH value of the solution was adjusted using 0.1 M NaOH solution and 0.1 M HCl solution. The concentration of HBA was detected by high performance liquid chromatography equipped with a C18 column. A 1% acetic acid solution and methanol in a volume ratio of 70 / 30 (v / v) were used as the mobile phase, and the flow rate was 1 mL min -1 , the detection wavelength is fixed at 280nm.

[0072] Table 6 Effect of different oxidants on the reaction activity of catalysts

[0073]

[0074] It can be seen from Table 6 that the addition of different types of oxidants has super high activity for CN-3 to degrade HBA, among which PAA as an oxidant shows the best activity for degrading HBA, followed by PI, H2O2, PMS, and PDS. This shows that the CN-3 catalyst can achieve efficient activation of different oxidants, show excellent HBA degradation ability, and has a relatively broad practical application potential.

[0075] 7. Investigate the effect of different reaction temperatures on the reaction activity of cobalt-nitrogen-carbon confined cobalt nanoparticle catalysts

[0076] In the test of the activity of the catalyst under different reaction temperatures, the Fenton-like potassium persulfate oxidation experiment with p-hydroxybenzoic acid as a model pollutant was used as a probe reaction. The effects of different reaction conditions on the catalyst activity were compared and analyzed by plotting C / C0 as the ordinate and the corresponding t moment as the abscissa. The first-order reaction rate constant k was used as an evaluation index for the catalytic reaction efficiency. k represents the speed of the reaction in the Fenton-like reaction. The formula of the first-order reaction rate equation is: k t =-In C / C0, C is the HBA concentration at sampling time t, C0 is the HBA concentration in the solution at the beginning of the reaction, t is the reaction time, k t The degree of degradation of pollutants when the reaction proceeds to time t, with -1n C / C0 as the ordinate and the corresponding time t as the abscissa, the slope obtained by linear fitting is the first-order reaction rate constant, and k is a constant. The larger the k, the faster the reaction rate and the faster the catalytic rate. All experiments were carried out at room temperature and under magnetic stirring. 15 mg of catalyst was added to 50 mL of HBA solution and stirred for 30 minutes to reach adsorption-desorption equilibrium. Then PMS solution was added to start the reaction. 1.5 mL of sample was taken out from the reaction system at preset time intervals (specific sampling time: 0 min, 1 min, 2 min, 3 min, 5 min, 7.5 min, 10 min, 15 min, 20 min) and filtered through a 0.22 μm filter membrane. Subsequently, the obtained sample was injected into a 2 mL chromatographic bottle, and 0.2 mL of 2 mM Na2S2O3 solution was added to terminate the reaction. The reaction temperatures were selected to be 277K, 298K and 308K, respectively, and the concentration of HBA was detected by high performance liquid chromatography equipped with a C18 column. A 1% acetic acid solution and methanol with a volume ratio of 70 / 30 (v / v) were used as the mobile phase, and the flow rate was 1 mL min -1 , the detection wavelength is fixed at 280nm.

[0077] Table 7 Effect of different reaction conditions on catalyst activity

[0078]

[0079] As can be seen from Table 7, the catalytic activity experiments of CN-3 catalyst at different reaction temperatures found that the catalytic efficiency increased significantly with the increase of temperature (0.165 min at 277 K, 298 K, 308 K and 323 K, respectively). -1 、0.217min -1 、0.292min -1 and 0.315min -1 ), which is because the thermal activation of PMS and the injection of exogenous energy reduce the difficulty of crossing the reaction energy barrier. According to the Arrhenius equation (lnk = -Ea / RT + lnA), the reaction activation energy of the CN-3 system was determined to be 14.83 kJ mol -1 , further verifying its significant PMS activation and HBA oxidation properties.

[0080] 8. Investigating the durability and cyclic stability of cobalt-nitrogen-carbon confined cobalt nanoparticle catalysts

[0081] In the experiment of investigating the cyclic catalytic properties of the catalyst, the Fenton-like potassium persulfate oxidation experiment with p-hydroxybenzoic acid as a model pollutant was used as a probe reaction. The effects of different reaction conditions on the catalyst activity were compared and analyzed by plotting C / C0 as the ordinate and the corresponding t time as the abscissa. The durability and reusability of the synthesized catalyst were evaluated by continuous batch experiments. The catalyst after the reaction was centrifuged, washed with deionized water and anhydrous ethanol, dried in an oven at 60°C for 12h, and used for the next reaction, which was repeated five times. In each cycle experiment, 15mg of catalyst was added to 50mL of HBA solution and stirred for 30 minutes to reach adsorption-desorption equilibrium. Then PMS solution was added to start the reaction. 1.5mL of sample was taken out from the reaction system at preset time intervals (specific sampling time: 0min, 1min, 2min, 3min, 5min, 7.5min, 10min, 15min, 20min) and filtered through a 0.22μm filter membrane. Subsequently, the obtained sample was injected into a 2 mL chromatographic vial, and 0.2 mL of 2 mM Na2S2O3 solution was added to terminate the reaction. The concentration of HBA was detected by high performance liquid chromatography equipped with a C18 column. A 1% acetic acid solution and methanol with a volume ratio of 70 / 30 (v / v) were used as the mobile phase, and the flow rate was 1 mL min -1 , the detection wavelength is fixed at 280nm.

[0082] Table 8 Durability and cycle stability of catalysts

[0083]

[0084] As can be seen from Table 8, after five consecutive cycles under the same conditions, CN-3 can still maintain about 90% of the original removal efficiency, with good cycle stability and repeated reuse. The CN-3 catalyst is composed of small-particle cobalt nanoparticles and Co-N configurations encapsulated in few-layer graphite nanoencapsulation, which can not only maximize the synergistic utilization efficiency of cobalt nanoparticles and Co-N, but also ensure the tight protection of the encapsulated nanoparticles to prevent the loss and aggregation deactivation of cobalt nanoparticles during the catalytic process, thereby showing unique organic decontamination efficiency and long-term durability. In addition, the larger core nanoparticles induce greater interfacial stress between the core and the shell, which significantly improves the cycle stability of the CN-3 catalyst.

[0085] 10. Comparison of the reaction activity of cobalt-nitrogen-carbon confined cobalt nanoparticle catalysts with different metal organic ligand precursors, cobalt-nitrogen-carbon catalysts prepared with different nitrogen sources, and different commercial CoO and Co3O4 catalysts

[0086] Add some copper, manganese and other metal materials

[0087] Comparative Example 1:

[0088] 1) Preparation of metal organic ligand precursor Co-ZIF-8 by coprecipitation method:

[0089] First, 0.01 mol of cobalt (II) acetate (Co(acac)3) and 0.05 mol of zinc nitrate hexahydrate (Zn(NO3)2·6H2O) were dissolved in 100 mL of methanol and stirred to form a uniform solution. Then, 0.1 g of 2-methylimidazole (2-MeIM) was dissolved in methanol (100 mL) and mixed thoroughly. The two solutions were mixed under ultrasonic conditions and stirred at room temperature for 12 hours. The solution was centrifuged, washed with methanol, and finally dried in a vacuum oven at 60°C overnight.

[0090] 2) The oxygen-limited pyrolysis method is used to prepare cobalt nanoparticle catalysts:

[0091] The cobalt-nitrogen-carbon catalyst was synthesized by thermal decomposition. First, the synthesized metal organic ligand precursor Co-ZIF-8 was mixed with a certain amount of melamine (mass ratio of 1:1). Then, the mixture was fully ground and placed in a quartz boat. The sample was heated to 600°C at a heating rate of 5°C per minute under a nitrogen atmosphere and maintained for 3 hours. After cooling, it was washed with a 1M H2SO4 solution to remove the cobalt particles on the surface. Finally, the catalyst was separated by centrifugation and washed 3 times with deionized water, and then dried at 60°C for 12 hours to obtain the cobalt nanocatalyst CZ-1.

[0092] Due to the difference in metal-based organic ligand precursors, the cobalt nanocatalysts obtained by calcination under nitrogen atmosphere also have certain differences. The specific surface area of ​​the CZ-1 catalyst is smaller than that of the catalyst obtained in Example 3, which is 25-46 m 2 / g, the main component is nitrogen-doped graphene-supported cobalt single atoms, infinite domain encapsulated spherical cobalt nanoparticles, and the diameter of its two-dimensional layer structure is slightly larger than that of the CN-3 catalyst. The size range is 190nm-570nm, the average diameter is 360nm, and the layer thickness is between 5nm-35nm. The total cobalt content is 6.9wt%, the nitrogen-carbon molar ratio is 0.166, and the active ingredient in the CZ-1 catalyst is CoNC.

[0093] Comparative Example 2:

[0094] 1) Preparation of metal organic ligand precursor Co-PE by complexation method:

[0095] First, 0.01 mol of cobalt acetate (II) and 0.03 mol of 1,10-phenanthroline were dissolved in 40 mL of ethanol and ultrasonically treated for 10 minutes for complexation reaction, then 0.1 g of carbon black ECP600JD was added and ultrasonic dispersion was continued for 20 minutes. The mixed solution was evaporated in a 60 ° C water bath to remove ethanol and dried in an oven at 80 ° C overnight until it was completely evaporated.

[0096] 2) The oxygen-limited pyrolysis method is used to prepare cobalt nanoparticle catalysts:

[0097] The cobalt-nitrogen-carbon catalyst was synthesized by thermal decomposition. First, the complexed metal organic ligand precursor Co-PE was mixed with a certain amount of melamine (mass ratio of 1:1). Then, the mixture was fully ground and placed in a quartz boat. The sample was heated to 600°C at a heating rate of 5°C per minute under a nitrogen atmosphere and maintained for 3 hours. After cooling, it was washed with a 1M H2SO4 solution to remove the cobalt particles on the surface. Finally, the catalyst was separated by centrifugation and washed 3 times with deionized water, and then dried at 60°C for 12 hours to obtain the cobalt nanocatalyst CP-1.

[0098] Due to the different complexation methods used to prepare the metal-based organic ligand precursors, the cobalt nanocatalysts calcined under nitrogen atmosphere also have obvious differences. The specific surface area of ​​the CP-1 catalyst is smaller than that of the catalyst obtained in Example 3, which is 15-33 m 2 / g, the main component is nitrogen-doped graphene-supported cobalt single atoms, infinite domain encapsulated spherical cobalt nanoparticles, and the diameter of its two-dimensional layer structure is also slightly larger than that of the CN-3 catalyst. The size range is 220nm-640nm, and the average diameter is 405nm. The layer thickness is thicker than that of the CN-3 catalyst, between 8nm-43nm. The total cobalt content is 8.7wt%, the nitrogen-carbon molar ratio is 0.173, and the active ingredient in the CP-1 catalyst is CoNC.

[0099] Comparative Example 3:

[0100] 1) Preparation of metal organic ligand precursor by coprecipitation method:

[0101] First, Co3[Co(CN)6]2(Co-CoPBA) was synthesized by reacting CoCl2·6H2O with an aqueous solution of K3[Co(CN)6]. 100 mL of 0.1 M K3[Co(CN)6] solution was slowly dripped into 100 mL of 0.1 M CoCl2·6H2O solution at a rate of 5 mL per minute under magnetic stirring. The mixed solution was further stirred for 0.5 h. The obtained solution was placed in a dark environment for 24 h for aging. Then, the precipitate was separated by centrifugation and washed three times with deionized water. Finally, the metal organic ligand precursor was dried at 60 °C for 24 h.

[0102] 2) Using vitamin B12 as a nitrogen source for the preparation of cobalt nanoparticle catalysts:

[0103] The cobalt-nitrogen-carbon catalyst was synthesized by thermal decomposition. First, the synthesized metal organic ligand precursor was mixed with a certain amount of vitamin B12 (mass ratio of 1:1). Then, the mixture was fully ground and placed in a quartz boat. The sample was heated to 600°C at a heating rate of 5°C per minute under a nitrogen atmosphere and maintained for 3 hours. After cooling, it was washed with a 1M H2SO4 solution to remove the cobalt particles on the surface. Finally, the catalyst was separated by centrifugation and washed 3 times with deionized water, and then dried at 60°C for 12 hours to obtain the cobalt nanocatalyst CB-1.

[0104] Since the cobalt-nitrogen-carbon catalyst prepared by using different nitrogen sources and the CN-3 catalyst obtained in Example 3 both present a two-dimensional sheet structure at the microscopic level, but the diameter size, specific surface area and composition of the two-dimensional sheet structure are different. The specific surface area of ​​the CB-1 catalyst is 20-42 m 2 / g, the main components are nitrogen-cobalt nanoparticles, cobalt single atoms supported by nitrogen-free graphite carbon shells, the size of the round cobalt nanoparticles is 50-85nm, the structure is elliptical, the cobalt content is 10.8wt%, the nitrogen-carbon molar ratio is 0.173, and the active ingredient in the CB-1 catalyst is Co nanoparticles.

[0105] Comparative Example 4:

[0106] 1) Preparation of metal organic ligand precursor by coprecipitation method:

[0107] First, Co3[Co(CN)6]2(Co-CoPBA) was synthesized by reacting CoCl2·6H2O with aqueous solution of K3[Co(CN)6]. 100 mL of 0.1 M K3[Co(CN)6] solution was slowly dripped into 100 mL of 0.1 M CoCl2·6H2O solution at a rate of 5 mL per minute under magnetic stirring. The mixed solution was further stirred for 0.5 h. The obtained solution was placed for aging for 24 h in the dark. Then, the precipitate was separated by centrifugation and washed three times with deionized water. Finally, the metal organic ligand precursor was dried at 60 °C for 24 h.

[0108] 2) Using dicyandiamide as a nitrogen source for the preparation of cobalt nanoparticle catalysts:

[0109] The cobalt-nitrogen-carbon catalyst was synthesized by thermal decomposition. First, the synthesized metal organic ligand precursor was mixed with a certain amount of dicyandiamide (mass ratio of 1:1). Then, the mixture was fully ground and placed in a quartz boat. The sample was heated to 600°C at a heating rate of 5°C per minute under a nitrogen atmosphere and maintained for 3 hours. After cooling, it was washed with a 1M H2SO4 solution to remove surface particles. Finally, the catalyst was separated by centrifugation and washed 3 times with deionized water, and then dried at 60°C for 12 hours to obtain the cobalt nanocatalyst CD-1.

[0110] Since the cobalt-nitrogen-carbon catalysts prepared by using different nitrogen sources and the CN-3 catalyst obtained in Example 3 both present a two-dimensional lamellar structure at the microscopic level, there are differences in specific surface area and component content. For example, the specific surface area of ​​the CD-1 catalyst is 40-75 m 2 / g, the main components are cobalt single atoms supported by nitrogen-doped graphitic carbon, infinite domain encapsulated spherical cobalt nanoparticles, the diameter of its lamellar structure ranges from 130nm to 510nm, the average diameter is 310nm, the lamellar thickness is between 5nm and 45nm, the cobalt content is 4.8wt%, the nitrogen-carbon molar ratio is 0.174, and the active ingredient in the CD-1 catalyst is CoNC.

[0111] Comparative Example 5:

[0112] 1) Preparation of Fe-Fe PBA precursors with different transition metal organic ligands by coprecipitation method:

[0113] First, Fe4[Fe(CN)6]3(Fe-Fe PBA) was synthesized by reacting FeSO4 with aqueous solution of K3[Fe(CN)6]. 100 mL of 0.1 M K3[Fe(CN)6] solution was slowly dripped into 100 mL of 0.1 M FeSO4 solution at a rate of 5 mL per minute under magnetic stirring. The mixed solution was further stirred for 0.5 h. The obtained solution was placed for aging for 24 h in the dark. Then, the precipitate was separated by centrifugation and washed three times with deionized water. Finally, the metal organic ligand precursor was dried at 60 °C for 24 h.

[0114] 2) The limited oxygen pyrolysis method is used for the preparation of iron nitrogen carbon catalyst:

[0115] The iron-nitrogen-carbon catalyst was synthesized by thermal decomposition. First, the complexed metal organic ligand precursor Fe-Fe PBA was mixed with a certain amount of melamine (mass ratio of 1:1). Then, the mixture was fully ground and placed in a quartz boat. The sample was heated to 600°C at a heating rate of 5°C per minute under a nitrogen atmosphere and maintained for 3 hours. After cooling, it was washed with a 1M H2SO4 solution to remove iron particles on the surface. Finally, the catalyst was separated by centrifugation and washed with deionized water 3 times, and then dried at 60°C for 12 hours to obtain the iron-nitrogen-carbon catalyst CF-1.

[0116] Due to the use of metal-based organic ligand precursors prepared with different metal ions, the iron nanocatalysts calcined under nitrogen atmosphere also have obvious differences. The specific surface area of ​​CF-1 catalyst is 19-45m 2 / g, the main components are nitrogen-doped graphitic carbon-supported iron single atom shell-confined spherical iron nanoparticles, wherein the size of the iron nanoparticles is 18-25nm, the structure is elliptical, the diameter of the nitrogen-doped graphitic carbon flakes is 140-450nm, the average diameter is 260nm, the shell thickness of the iron single atom supported by the graphitic carbon shell is 2-6nm, the total iron content is 9.8wt%, the nitrogen-carbon molar ratio is 0.168, and the active ingredients in the CF-1 catalyst are FeNC and Fe nanoparticles, among which FeNC occupies a dominant position (85-93%).

[0117] Comparative Example 6:

[0118] 1) Preparation of Fe-Co PBA precursors with different transition metal organic ligands by coprecipitation method:

[0119] First, Fe4[Fe(CN)6]3(Fe-Co PBA) was synthesized by reacting FeSO4 with aqueous solution of K3[Co(CN)6]. 100 mL of 0.1 M K3[Co(CN)6] solution was slowly dripped into 100 mL of 0.1 M FeSO4 solution at a rate of 5 mL per minute under magnetic stirring. The mixed solution was further stirred for 0.5 h. The obtained solution was placed for aging for 24 h in the dark. Then, the precipitate was separated by centrifugation and washed three times with deionized water. Finally, the metal organic ligand precursor was dried at 60 °C for 24 h.

[0120] 2) The limited oxygen pyrolysis method is used for the preparation of iron-cobalt-nitrogen-carbon catalyst:

[0121] The iron-cobalt-nitrogen-carbon catalyst was synthesized by thermal decomposition. First, the complexed metal-organic ligand precursor Fe-CoPBA was mixed with a certain amount of melamine (mass ratio of 1:1). Then, the mixture was fully ground and placed in a quartz boat. The sample was heated to 600°C at a heating rate of 5°C per minute under a nitrogen atmosphere and maintained for 3 hours. After cooling, it was washed with a 1M H2SO4 solution to remove the iron-cobalt particles on the surface. Finally, the catalyst was separated by centrifugation and washed with deionized water 3 times, and then dried at 60°C for 12 hours to obtain the iron-nitrogen-carbon catalyst CF-2.

[0122] Due to the use of metal-based organic ligand precursors prepared by mixed metal ions, the Fe-Co nanocatalysts calcined under nitrogen atmosphere also have obvious differences. The specific surface area of ​​CF-2 catalyst is 16-47m 2 / g, the main components are nitrogen-doped graphitic carbon-supported iron / cobalt diatomic shells confined to spherical iron / cobalt nanoparticles, wherein the size of the iron / cobalt nanoparticles is 22-29nm, the structure is elliptical, the diameter of the nitrogen-doped graphitic carbon flakes is 150-480nm, the average diameter is 280nm, the shell thickness of the iron / cobalt diatomic supported by the graphitic carbon shell is 2-6nm, the total iron content is 4.9wt%, the total cobalt content is 5.3wt%, the nitrogen-carbon molar ratio is 0.172, and the active components in the CF-2 catalyst are FeCoNC and Fe / Co nanoparticles, wherein FeCoNC occupies a dominant position (80-91%).

[0123] Comparative Example 7:

[0124] 1) Preparation of different transition metal organic ligand precursors Mn-Co PBA by co-precipitation method:

[0125] First, Mn4[Co(CN)6]3 (Mn-Co PBA) was synthesized by reacting MnCl2 with aqueous solution of K3[Co(CN)6]. 100 mL of 0.1 M K3[Co(CN)6] solution was slowly dripped into 100 mL of 0.1 M MnCl2 solution at a rate of 5 mL per minute under magnetic stirring. The mixed solution was further stirred for 0.5 h. The obtained solution was placed for 24 h for aging under light-proof conditions. Then, the precipitate was separated by centrifugation and washed three times with deionized water. Finally, the metal organic ligand precursor was dried at 60 °C for 24 h.

[0126] 2) The limited oxygen pyrolysis method is used for the preparation of manganese cobalt nitrogen carbon catalyst:

[0127] The manganese-cobalt-nitrogen-carbon catalyst was synthesized by thermal decomposition. First, the complexed metal organic ligand precursor Mn-CoPBA was mixed with a certain amount of melamine (mass ratio of 1:1). Then, the mixture was fully ground and placed in a quartz boat. The sample was heated to 600°C at a heating rate of 5°C per minute under a nitrogen atmosphere and maintained for 3 hours. After cooling, it was washed with a 1M H2SO4 solution to remove the cobalt particles on the surface. Finally, the catalyst was separated by centrifugation and washed 3 times with deionized water, and then dried at 60°C for 12 hours to obtain the manganese-cobalt-nitrogen-carbon catalyst CM-1.

[0128] Due to the use of metal-based organic ligand precursors prepared by mixed metal ions, the manganese-cobalt nanocatalysts calcined under nitrogen atmosphere also have obvious differences. The specific surface area of ​​CM-1 catalyst is 21-52m 2 / g, the main components are nitrogen-doped graphitic carbon-supported manganese-cobalt diatomic shells confined in spherical manganese-cobalt nanoparticles, wherein the size of the manganese-cobalt nanoparticles is 19-29nm, the structure is elliptical, the diameter of the nitrogen-doped graphitic carbon flakes is 150-480nm, the average diameter is 270nm, the shell thickness of the manganese-cobalt single atom supported by the graphitic carbon shell is 2-7nm, the total manganese content is 5.3wt%, the total cobalt content is 5.2wt%, the nitrogen-carbon molar ratio is 0.173, and the active components in the CM-1 catalyst are MnCoNC and Mn / Co nanoparticles, among which MnCoNC occupies a dominant position (80-91%).

[0129] Comparative example: 8:

[0130] Commercially purchased CoO and Co3O4 catalysts (both purchased from Sinopharm Group)

[0131] In the test comparison of the reaction activity of CN-3 catalyst, nitrogen-modified cobalt nitrogen-carbon catalyst prepared by using vitamin B12 or dicyandiamide as nitrogen source, and commercial CoO and Co3O4 catalysts, the degradation reaction of p-hydroxybenzoic acid was used as the probe reaction. The reaction rate constant k was used as the evaluation index of the catalytic reaction efficiency. k represents the speed of the reaction in the Fenton-like reaction. The formula of the first-order reaction rate equation is: k t =-ln C / C0, C is the HBA concentration at sampling time t, C0 is the HBA concentration in the solution at the beginning of the reaction, t is the reaction time, k t The slope obtained by linear fitting is the first-order reaction rate constant, and k is a constant. The larger the k, the faster the reaction rate and the better the activity. All experiments were carried out at room temperature and under magnetic stirring. 15 mg of catalyst CoO and Co3O4 were added to 50 mL of HBA solution and stirred for 30 minutes to reach adsorption-desorption equilibrium. Then PMS solution was added to start the reaction. 1.5 mL of sample was taken out from the reaction system at preset time intervals and filtered through a 0.22 μm filter membrane (specific sampling time: 0 min, 1 min, 2 min, 3 min, 5 min, 7.5 min, 10 min, 15 min, 20 min). Subsequently, the obtained sample was injected into a 2 mL chromatographic bottle, and 0.2 mL of 2 mM Na2S2O3 solution was added to terminate the reaction. The pH value of the solution was adjusted using a buffer reagent. The concentration of HBA was detected using a high performance liquid chromatograph C18 column. The mobile phase was a 1% acetic acid solution and methanol in a volume ratio of 70 / 30 (v / v) at a flow rate of 1 mL min -1 , the detection wavelength is fixed at 280nm.

[0132] Table 9 Effect of different synthesis methods of cobalt, nitrogen and carbon on catalyst activity

[0133]

[0134] It can be seen from Table 9 that compared with the nitrogen-modified cobalt-nitrogen-carbon catalysts prepared using different metal organic ligand precursors and different vitamin B12 / dicyandiamide as nitrogen sources, the nitrogen-modified cobalt-nitrogen-carbon catalysts prepared using melamine as a nitrogen source and Co-CoPBA as a metal precursor exhibit the best catalytic performance, which is mainly related to the physical and chemical properties of cobalt-cobalt Prussian blue and melamine. At the same time, the cobalt-nitrogen-carbon catalysts prepared using vitamin B12 and dicyandiamide as nitrogen sources lead to a decrease in the utilization rate of cobalt sites in the reaction, and the activity is significantly reduced compared to the nitrogen-doped cobalt single-atom catalysts synthesized using melamine. In addition, compared with commercial CoO and Co3O4 catalysts, the reaction efficiency of the nitrogen-modified cobalt-nitrogen-carbon catalyst is more outstanding, indicating that the catalysts prepared by this simple method have very bright application prospects.

[0135] 11. Investigate the effect of changes in synthesis conditions on the performance of cobalt-nitrogen-carbon confined cobalt nanoparticle catalysts

[0136] 1) Synthesis of Co-Co PBA precursor:

[0137] First, Co3[Co(CN)6]2 (Co-CoPBA) was synthesized by reacting CoCl2·6H2O with an aqueous solution of K3[Co(CN)6]. The preparation method was similar to the previous one with slight improvements. 100mL of 0.1M K3[Co(CN)6] solution was slowly dripped into 100mL of 0.1M CoCl2·6H2O solution at a rate of 5mL per minute under magnetic stirring. The mixed solution was further stirred for 0.5 hours. The obtained solution was left to age for 24 hours in the dark. Then, the precipitate was separated by centrifugation and washed three times with deionized water. Finally, the Co-Co PBA was dried at 60°C for 24 hours.

[0138] 2) Cobalt-nitrogen-carbon confined cobalt nanoparticle catalysts were synthesized by thermal decomposition:

[0139] The cobalt-nitrogen-carbon confined cobalt nanoparticle catalyst was synthesized by thermal decomposition. First, the synthesized Co-Co PBA precursor was mixed with a certain amount of melamine (mass ratio of 1:1). Then, the mixture was fully ground and placed in a quartz boat. The sample was heated at a heating rate of 5°C per minute under a nitrogen atmosphere and maintained at a nitrogen atmosphere of 600-900°C (600°C, 700°C, 800°C, 900°C) for 1-4h (1h, 2h, 3h, 4h). After cooling, it was washed with 0.5-2M (0.5M, 1.0M, 2M) H2SO4 solution for 8-24h (8h, 12h, 24h) to remove the cobalt particles on the surface. Finally, the catalyst was separated by centrifugation and washed with deionized water several times, and then dried at 60°C for 12 hours. In the activity test of the catalyst under different synthesis conditions, the potassium persulfate oxidation reaction of p-hydroxybenzoic acid was used as a probe reaction. The reaction rate constant k is used as an evaluation index for the efficiency of the catalytic reaction. k represents the speed of the reaction in the Fenton-like reaction. The formula of the first-order reaction rate equation is: k t =-lnC / C0, C is the HBA concentration at sampling time t, C0 is the HBA concentration in the solution at the beginning of the reaction, t is the reaction time, k t The slope obtained by linear fitting is the first-order reaction rate constant, k is a constant. The larger the k, the faster the reaction rate and the better the activity.

[0140] All experiments were carried out at room temperature and under magnetic stirring. 15 mg of catalyst was added to 50 mL of HBA solution and stirred for 30 minutes to reach adsorption-desorption equilibrium. PMS solution was then added to start the reaction. 1.5 mL of sample was taken out from the reaction system at preset time intervals and filtered through a 0.22 μm filter membrane. Subsequently, the resulting sample was injected into a 2 mL chromatographic vial, and 0.2 mL of 2 mM Na2S2O3 solution was added to terminate the reaction. The pH value of the solution was adjusted using 0.1 M NaOH solution and 0.1 M HCl solution. The concentration of HBA was detected using a high performance liquid chromatography C18 column. A volume concentration of 1% acetic acid solution and methanol in a volume ratio of 70 / 30 (v / v) was used as the mobile phase, and the flow rate was 1 mL min -1 , the detection wavelength is fixed at 280nm.

[0141] Table 10 Effect of changes in synthesis conditions on catalyst performance

[0142]

[0143] It can be seen from Table 10 that the change of synthesis conditions during the synthesis process has a certain impact on the performance of the catalyst. For the calcination temperature, too high or too low calcination temperature has a negative effect on the catalytic activity. At the same time, the shortening of the calcination time will also make it difficult for the cobalt nitrogen carbon catalyst to be fully calcined, resulting in a decrease in catalytic activity, which is not conducive to the formation of active sites. By changing the concentration of the sulfuric acid solution for pickling and the pickling time, it can be seen that sufficient pickling is very necessary for efficient catalytic activity. This is mainly because sufficient pickling can wash away the metal particles that may exist therein, maximizing the chance of single-atom catalytic sites in the catalyst participating in the reaction.

Claims

1. A method for preparing a cobalt-nitrogen-carbon confined cobalt nanoparticle catalyst, characterized in that: A cobalt-nitrogen-carbon-confined cobalt nanoparticle catalyst is prepared by calcination in a nitrogen atmosphere: first, 50-100 mg of Co-CoPBA precursor is weighed, and it is mixed with a certain mass of melamine, the mass ratio of melamine to Co-Co PBA is 0.2-8, and calcined in a nitrogen atmosphere at 500-900°C for 1-4 hours, and the heating rate from room temperature to the calcination temperature is 3-8°C / min; after the temperature drops to room temperature, the calcined product is taken out, and pickled with 0.5-2.0M H2SO4 solution for 12-24 hours; after the pickling is completed, it is washed with water until the pH of the washing liquid is stabilized between 6-7, and dried in an oven at 60-80°C for 12-24 hours to obtain a cobalt-nitrogen-carbon-confined cobalt nanoparticle catalyst.

2. The method for preparing the catalyst according to claim 1, characterized in that: The Co-Co PBA precursor is prepared by a coprecipitation method: first, 80-120 mL of a 0.05-0.3 M K3[Co(CN)6] solution is dropped into 80-110 mL of a 0.05-0.25 M CoCl2·6H2O solution, and stirred for 30-60 min to obtain a solution; the homogeneous solution is placed at room temperature for 12-24 hours under light-proof conditions for aging, solid-liquid separation is performed, the solid is washed with water, and the obtained solid is dried in an oven at 60-80°C for 12-24 hours to obtain a Co-Co PBA precursor; or, preferably, The Co-Co PBA precursor was prepared by a coprecipitation method: 90-105 mL of a 0.05-0.15 M K3[Co(CN)6] solution was slowly dripped into 90-105 mL of a 0.05-0.15 M CoCl2·6H2O solution and stirred for 30-50 min to obtain a solution; the homogeneous solution was placed at room temperature for 12-20 hours under light-proof conditions for aging, the solid-liquid separation was performed, the solid was washed with water, and the obtained solid was dried in an oven at 60-80°C for 12-16 hours to obtain a Co-Co PBA precursor.

3. The method for preparing the catalyst according to claim 1 or 2, characterized in that: The specific process is: A cobalt-nitrogen-carbon-confined cobalt nanoparticle catalyst is prepared by calcination in a nitrogen atmosphere: first, 50-80 mg of Co-CoPBA precursor is weighed and mixed with a certain mass of melamine, the mass ratio of melamine to Co-Co PBA is 0.5-6, and the mixture is evenly ground and calcined in a nitrogen atmosphere at 600-900°C for 2-3 hours, and the heating rate from room temperature to the calcination temperature is 3-5°C / min; after the temperature drops to room temperature, the calcined product is taken out and pickled with a 1.0-1.5M H2SO4 solution for 12-18 hours; after pickling, the mixture is washed with water until the pH of the washing liquid is stabilized between 6-7, and dried in an oven at 60-80°C for 12-24 hours to obtain a cobalt-nitrogen-carbon-confined cobalt nanoparticle catalyst.

4. A cobalt-nitrogen-carbon confined cobalt nanoparticle catalyst prepared by the preparation method according to claim 1, 2 or 3.

5. The catalyst according to claim 4, characterized in that: The cobalt-nitrogen-carbon confined cobalt nanoparticle catalyst presents a two-dimensional layered stacking structure at the microscopic level, consisting of cobalt nanoparticles encapsulated by a nitrogen-doped graphitic carbon shell, in which the nitrogen-doped graphitic carbon shell carries atomically dispersed cobalt sites.

6. Use of the catalyst according to claim 4 or 5 in a Fenton-like reaction for oxidative degradation of organic pollutants by catalytic oxidants.

7. The use according to claim 6, characterized in that: The oxidant is one or more of potassium persulfate, hydrogen peroxide or potassium persulfate; the reaction substrate is one or more of 50-200uM p-hydroxybenzoic acid (HBA), bisphenol A (BPA) and phenol (PN) solution, doxycycline hydrochloride, nitrobenzene and benzoic acid; the concentration of the solution catalyst is 50-500mg / L, and the concentration of the oxidant is 0.2-5mM.

8. The use according to claim 6 or 7, characterized in that: The reaction device of the Fenton-like pollutant degradation reaction is a dark reaction box, in which darkness is maintained and a temperature-controlled stirrer and a reaction container are placed. The catalyst is in the reactor to achieve efficient degradation of the model pollutant p-hydroxybenzoic acid. The catalyst can attack organic pollutants with non-free radical species (singlet oxygen) in the presence of humic acid and various ions to achieve degradation of organic pollutants.

9. The use according to claim 6 or 7, characterized in that: In the reaction solution for Fenton-like degradation of pollutants, the concentration of one or more of bicarbonate ion, carbonate ion, sulfate ion, chloride ion, hydrogen phosphate ion, dihydrogen phosphate ion and bromide ion should be <25mM, or the concentration of humic acid should be <25mg / L.

10. Use of the catalyst according to claim 6 or 7, characterized in that: In the reaction solution of Fenton-like degradation of pollutants, the pH range of the reaction solution should be controlled at 2-11; in the reaction process of Fenton-like degradation of pollutants, the reaction time should be greater than or equal to 3 minutes.

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