A confined catalyst for electrocatalysis, its preparation method and application

Abstract

The present invention discloses a confined catalyst for electrocatalysis, its preparation method and application, which belong to the technical field of electrocatalysis. Among them, the preparation method includes the following steps: dissolving 2-aminoterephthalic acid and ferric chloride in a solvent to obtain a mixed solution; performing heat treatment on the mixed solution to obtain a reaction mixture; separating a precipitate from the reaction mixture, and performing washing and drying treatments to obtain a confined support material; dropping a saturated ferric chloride solution onto the confined support material, and performing grinding and drying, followed by high-temperature carbonization to obtain a carbonized powder; dropping the saturated ferric chloride solution onto the carbonized powder again, and performing drying, followed by a reduction reaction to obtain the confined catalyst. The present invention can improve the disadvantages of the cathode catalyst for electrocatalytic removal of nitrate, such as heavy metal pollution caused by the dissolution of the catalyst, few active sites, weak conductivity, and poor adsorption capacity.

Description

Technical Field

[0001] The present invention relates to the technical field of electrocatalysis, and particularly to a confined catalyst for electrocatalysis, a preparation method thereof, and an application thereof. Background Art

[0002] The main sources of groundwater nitrate pollution are the extensive use of chemical fertilizers in agriculture and the uncontrolled discharge of wastewater from various industries. Especially in irrigated agricultural areas, the farm drainage system will carry excessive fertilizers and other pesticide chemicals into water bodies through surface runoff or infiltration, resulting in an increase in nitrate concentration in water. Nearly 50% of the drinking water in human daily life directly comes from groundwater, and excessive nitrate concentration in drinking water will have various adverse effects on human health. Because the human body will reduce nitrate to nitrite, which may cause methemoglobinemia, and increase the risk of cancer. Therefore, in the face of groundwater nitrate pollution, there is an urgent need to develop new technologies for removing groundwater nitrate with low cost and high efficiency.

[0003] Currently, converting nitrate into nitrogen or ammonia by biological or chemical methods is a common method for repairing groundwater nitrate pollution. The biological method is one of the most effective methods for treating nitrate pollution, but its speed is quite slow and it is sensitive to changes in environmental variables. At the same time, hydrogen gas needs to be used as a reducing agent electron donor in the chemical reduction process, but the transmission of pressurized hydrogen gas will bring safety hazards. In addition, electrodialysis is a new electrochemical membrane method for concentrating and separating pollutant ions in wastewater, but due to its high resistance and high energy consumption, it is not suitable for low-concentration nitrate solutions. Therefore, there is an urgent expectation to study green, efficient, and low-energy-consuming nitrate pollution repair technologies.

[0004] In summary, currently, the degree of groundwater nitrate pollution has become increasingly serious over time due to human activities. Conventional nitrate removal methods include biological methods, chemical methods, and physical methods, but their high costs, low treatment efficiencies, and complex operations limit the further development and application of these technologies. Therefore, it is necessary to study environmentally friendly and sustainable nitrate pollution repair technologies.

[0005] Electrocatalysis is a newly emerging environmentally friendly and sustainable technology for nitrate pollution remediation. It shows great advantages and application potential due to its friendly environmental compatibility and the utilization of renewable energy sources such as solar energy and wind energy. However, the cathode catalysts for electrocatalytic reduction of nitrate reported currently often have problems such as few active sites and low kinetic efficiency. The fundamental reason is that the unreasonable morphology, size, and electronic structure of the catalyst are not conducive to nitrate adsorption and electron transfer during the electrocatalytic removal of nitrate reaction process. Especially in practical applications, electrocatalytic removal of nitrate faces various challenges, such as secondary heavy metal pollution of treated water caused by metal leaching, low efficiency of electrocatalytic removal of nitrate due to few active sites and easy inactivation, and weak conductivity, etc. Summary of the Invention

[0006] The purpose of the present invention is to provide a preparation method of a confined catalyst for electrocatalysis to solve the problems proposed in the above background technology.

[0007] To achieve the above purpose, the embodiments of the present invention provide the following technical solutions:

[0008] A preparation method of a confined catalyst for electrocatalysis, which includes the following steps:

[0009] Dissolve 2-aminoterephthalic acid and iron chloride in a solvent to obtain a mixed solution;

[0010] Perform heat treatment on the mixed solution to obtain a reaction mixture;

[0011] Separate the precipitate from the reaction mixture, and perform washing and drying treatments to obtain a confined carrier material;

[0012] Drop saturated iron chloride solution into the confined carrier material, and perform grinding and drying, and then perform high-temperature carbonization to obtain a carbonized powder;

[0013] Drop saturated iron chloride solution into the carbonized powder again, and perform drying, and then perform a reduction reaction to obtain the confined catalyst.

[0014] Preferably, the molar ratio of 2-aminoterephthalic acid to iron chloride is 1:(0.8 - 1.2).

[0015] Preferably, the solvent is N,N-dimethylformamide.

[0016] Preferably, the temperature of the heat treatment is 110 - 130 °C.

[0017] Preferably, the high-temperature carbonization is carried out in an inert gas atmosphere, and the temperature of the high-temperature carbonization is 500 - 600 °C.

[0018] Preferably, the reduction reaction is carried out in a mixed atmosphere of hydrogen and argon, and the temperature of the reduction reaction is 350 - 450 °C; the volume ratio of hydrogen to argon in the mixed atmosphere is 3:97 - 7:93.

[0019] Another object of the embodiments of the present invention is to provide a confined catalyst prepared by the above preparation method.

[0020] Another object of the embodiments of the present invention is to provide an application of the above-mentioned confined catalyst in electrocatalytic reduction of nitrate.

[0021] Another object of the embodiments of the present invention is to provide an application of the above-mentioned confined catalyst in removing nitrate from water.

[0022] Another object of the embodiments of the present invention is to provide an electrocatalytic reduction device, including a cathode and an anode, wherein the cathode is made of the above-mentioned confined catalyst.

[0023] The present invention uses the carrier material NH2-MIL-88B to confine sub-nanometer Fe 0 To prepare a green and efficient cathode-confined catalyst Fe@NH2-MIL-88B and establish a corresponding application technology method. The present invention can improve the disadvantages of the cathode catalyst for electrocatalytic removal of nitrate, such as heavy metal pollution caused by the dissolution of the cathode catalyst, few active sites, weak conductivity, and poor adsorption capacity. These improvements are beneficial to promoting the application of electrocatalysis in nitrate-contaminated groundwater and improving the efficiency of electrocatalytic removal of nitrate. Using this confined catalyst for electrocatalytic treatment of nitrate-contaminated groundwater effectively solves the following problems:

[0024] (1) When conventional metal catalysts are used for electrocatalytic removal of nitrate, they often face the difficulty of metal dissolution. These dissolved metals will cause secondary heavy metal pollution to the treated water, thereby reducing the water quality. By confining sub-nanometer Fe 0 in the metal-organic framework NH2-MIL-88B, Fe@NH2-MIL-88B is prepared as a new cathode material for electrocatalytic removal of nitrate. Fe 0 confined in the pores of NH2-MIL-88B can reduce the leaching of iron, and iron is also a trace element beneficial to the human body, solving the problem of secondary heavy metal pollution of the treated water caused by the dissolution of the current cathode material.

[0025] (2) Conventional iron nanoparticles are prone to agglomeration due to their high surface energy, resulting in a reduction in active sites and thus a decrease in catalytic activity. However, NH2-MIL-88B-confined Fe 0 can obtain sub-nanometer Fe 0 particles with higher dispersion and smaller size, increasing the number of active sites participating in the electrocatalytic removal of nitrate reaction, and Fe 0The confinement of particles in nanopores can prevent the active sites from direct contact with the harsh external environment, solve the problems of few active sites and easy inactivation of active sites in the cathode catalyst, resulting in low catalytic efficiency and poor stability of the catalyst, and improve the ability of the cathode material to withstand environmental impact loads.

[0026] (3)The d-band center of bulk iron is far from the Fermi level. When the d-band center is far from the Fermi level, the p-band center of iron atoms moves downward, leading to the aggregation of iron clusters, thereby reducing the catalytic performance of electrocatalytic nitrate removal. In the sub-nanometer Fe confined by NH2-MIL-88B 0 , the d-band center of iron moves upward and is closer to the Fermi level, strengthening the effective overlap of the 3d orbit of Fe with the 2p orbit of O of nitrate, enhancing the adsorption capacity of Fe@NH2-MIL-88B for nitrate in groundwater, and solving the problem of insufficient activation of nitrate molecules due to too weak adsorption of nitrate by the cathode catalyst.

[0027] (4)The conductivity of the electrode material determines the electron transfer efficiency within the electrode. Materials with poor conductivity will increase the resistance and reduce the electron transfer rate. Under the action of the octahedral coordination field of NH2-MIL-88B and the 0 unsaturated coordination field of Fe, electrons rapidly transfer from the t 2g orbital of iron to the π* orbital of nitrate, significantly accelerating the electron transfer in the electrocatalytic nitrate removal reaction and solving the problem of low electron transfer efficiency in the electrocatalytic nitrate removal reaction process.

[0028] (5)The formation of nitrite, the rate-limiting step in the electrocatalytic nitrate removal reaction, has an extremely high reaction barrier, which limits the overall electrocatalytic nitrate removal efficiency. The novel confined catalyst Fe@NH2-MIL-88B prepared in the present invention changes the rate-limiting step to the formation of nitric oxide and significantly reduces the maximum reaction barrier, solving the problem of high reaction barrier of the rate-limiting step of conventional catalysts resulting in low kinetic efficiency of electrocatalytic nitrate removal and reducing the energy consumption of the electrocatalytic nitrate removal reaction. BRIEF DESCRIPTION OF THE DRAWINGS

[0029] Figure 1 It is a schematic diagram of the principle of electrocatalytic nitrate removal from water by the confined catalyst provided in the embodiment of the present invention.

[0030] Figure 2 It is a schematic flow diagram of the preparation method of the confined catalyst provided in the embodiment of the present invention.

[0031] Figure 3 It is a schematic structural diagram of an electrocatalytic reduction device provided in the embodiment of the present invention; in the figure, ① is an external power supply; ② is a single-chamber electrolytic cell; ③ is an electrolyte; ④ is an anode; ⑤ is a cathode.

[0032] Figure 4 SEM images of different materials; in the figure, (a) is the SEM image of NH2-MIL-88B; (b) is the SEM image of carbonized NH2-MIL-88B; (c) is the SEM image of Fe / NH2-MIL-88B; (d) is the SEM image of Fe@NH2-MIL-88B.

[0033] Figure 5 Element distribution map of Fe@NH2-MIL-88B.

[0034] Figure 6 XPS spectrum of Fe@NH2-MIL-88B.

[0035] Figure 7 BET result diagrams of different materials; in the figure, (a) is the BET result diagram of Fe@NH2-MIL-88B; (b) is the BET result diagram of NH2-MIL-88B.

[0036] Figure 8 Raman spectrum of Fe@NH2-MIL-88B.

[0037] Figure 9 Electrochemical impedance spectra of Fe@NH2-MIL-88B, Fe / NH2-MIL-88B, NH2-MIL-88B and CC.

[0038] Figure 10 Linear sweep voltammograms of different materials (vs. RHE).

[0039] Figure 11 Tafel slopes of different materials (vs. RHE).

[0040] Figure 12 Electrocatalytic nitrate removal degradation curve of Fe@NH2-MIL-88B.

[0041] Figure 13 Cyclic performance test result diagram of Fe@NH2-MIL-88B.

[0042] Figure 14 Comparison diagram of electrocatalytic nitrate degradation effects of different materials.

[0043] Figure 15 Influence result diagram of different current densities on electrocatalytic reduction of nitrate.

[0044] Figure 16 Influence result diagram of different pH values on electrocatalytic reduction of nitrate.

[0045] Figure 17It is a graph showing the influence of different initial nitrate concentrations on the electrocatalytic reduction of nitrate.

[0046] Figure 18 It is a graph showing the influence of different chloride ion concentrations on (a) the electrocatalytic reduction of nitrate and (b) the ammonia selectivity.

[0047] Figure 19 It is a graph showing the influence of different TBA concentrations on the electrocatalytic reduction of nitrate.

[0048] Figure 20 It is the reaction path of the electrocatalytic reduction of nitrate by Fe@NH2-MIL-88B.

[0049] Figure 21 It is a graph showing the reaction process and free energy change of the electrocatalytic reduction of nitrate.

[0050] Figure 22 It is a graph comparing the overall differential charge and adsorption energy of nitrate adsorbed on (a) Fe / NH2-MIL-88B and (b) Fe@NH2-MIL-88B. Specific Embodiments

[0051] Next, the technical solutions in the embodiments of the present invention will be clearly and completely described in conjunction with the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without making creative efforts belong to the scope of protection of the present invention.

[0052] Electrocatalytic reduction of nitrate is an alternative method that combines electrochemistry and catalysis technologies to treat nitrate pollution. Nitrate undergoes an electrochemical reduction reaction at the cathode, converting it into harmless nitrogen gas or ammonia with economic value. In the electrocatalytic reduction of nitrate reaction, the catalyst on the cathode surface first adsorbs nitrate ions, and then nitrate undergoes a reduction reaction on the catalyst surface. The participation of active hydrogen and the transfer of electrons are also required during the reduction process. The electrons from the external power source are transferred to nitrate on the cathode surface, and active hydrogen can participate in the reaction through proton reduction on the cathode surface, thus promoting the reduction of nitrate. The composition, surface structure, number of active sites, and electron conductivity of the cathode material determine the progress and effect of the electrocatalytic reduction of nitrate reaction. However, the reported cathode catalysts currently have obvious deficiencies in these aspects, which limits the application of electrocatalysis in nitrate pollution. Therefore, it is urgent to develop cathode materials with excellent performance and reasonable structure to promote the large-scale application of electrocatalytic removal of nitrate.

[0053] Iron-based materials have become promising catalytic materials for electrochemical nitrate reduction due to their abundant reserves on Earth, strong electrical conductivity, and excellent catalytic performance. Researchers used polarography to study the reduction mechanism of nitrate and concluded that the d-orbital electrons of the cathode material contribute to injecting charge into the lowest unoccupied molecular π* orbital of nitrate. Therefore, metal catalysts with a high d-orbital occupancy and an unclosed d-orbital shell (such as noble metals, copper, iron, etc.) have become the focus of research on cathode materials. However, noble metal materials are too expensive to be used on a large scale, and copper-based materials are also troubled by the problems of being easily dissolved out during the reaction, having strong toxicity after dissolution, and being prone to secondary pollution. Using iron-based materials as the cathode for electrocatalytic nitrate reduction has excellent application potential because iron-based materials are cheap and easy to obtain, green and harmless, and can easily react with coagulants to form insoluble complexes for removal even after dissolution. However, iron-based materials often have problems such as few active sites, weak electrical conductivity, and poor adsorption ability. Moreover, although nano-iron particles have a large specific surface area, they are more likely to agglomerate and are easily oxidized in the air to form a dense oxide film on the surface, hindering the electrocatalytic removal of nitrate. Therefore, reasonable combination and structural design are needed to compound iron with other metals or non-metals to prepare excellent new iron-based catalytic materials to reduce the agglomeration and oxidation of iron, increase active sites, enhance electrical conductivity, and adsorption ability.

[0054] Confinement is an efficient solution to improve the shortcomings of the above-mentioned iron-based catalysts. Confining iron-based materials in a microstructure can change the electron cloud density, d-band center, nitrate adsorption ability, etc. of the catalyst, thus significantly improving the catalytic performance. By designing the catalyst support material into a nanoscale or porous structure, the specific surface area of the catalyst surface is significantly increased, which can significantly increase the exposure of surface active sites and enhance the activity of the catalyst. In addition, the confinement effect can also accelerate the reaction kinetics by improving the charge transfer ability and increase the electrical conductivity of the material. Metal-organic frameworks (MOFs) are one type of confinement support material. Their pores can serve as individual nanoscale reactors, enhancing the host-guest interaction and promoting the activation of catalytic substrates. And the confinement effect of the nano / sub-nano pores or specific binding sites of MOFs can effectively limit the agglomeration and growth of metal nanoparticles, thus stabilizing the resulting ultrafine nanostructures and regulating their electronic structure and three-dimensional structure in catalysis. MOF-derived porous carbon materials usually exhibit high electrical conductivity while retaining the porous topological structure, thus reducing the charge transfer energy barrier. MOF is suitable as a confinement support material due to its high porosity, rich functionalized surface, and good electrical conductivity.

[0055] NH2-MIL-88B is a kind of MOF material with high surface area and porosity. The one-dimensional hexagonal channels and double-cone cages in it generate multiple micropores, which enables more nitrates to contact the catalyst, thus improving the reaction efficiency. Carbonized NH2-MIL-88B is an ordered mesoporous carbon material, providing a highly dispersed surface area for active sites, appropriate and interconnected channels for nitrate diffusion, and good electrical conductivity for promoting electron pathways. And the porous carbon basal plane partly retains the open pore structure, which promotes nitrate diffusion and alleviates the mass transfer limitation. Due to the above excellent structural advantages, NH2-MIL-88B is suitable to be selected as the confined carrier material for the cathode catalysis of electrocatalytic nitrate removal.

[0056] In summary, in the embodiments of the present invention, NH2-MIL-88B is used as the carrier material to confine sub-nanometer Fe 0 to prepare the cathode catalytic material, so as to solve the problems that the poor electrical conductivity, few active sites and unreasonable electronic structure of the cathode catalyst are not conducive to nitrate adsorption and the electrocatalytic reduction of nitrate reaction, thus restricting the application of electrocatalysis in nitrate pollution. The embodiments of the present invention prepare a novel Fe@NH2-MIL-88B catalytic material for electrocatalytic treatment of nitrate-contaminated groundwater (such as Figure 1 shown), and effectively solve the following problems:

[0057] (1) By confining sub-nanometer Fe 0 in the metal-organic framework NH2-MIL-88B, Fe@NH2-MIL-88B is prepared as a novel cathode material for electrocatalytic nitrate removal. The confinement of Fe 0 in the pores of NH2-MIL-88B can reduce the leaching of iron, and iron is also a trace element beneficial to the human body, solving the problem of secondary heavy metal pollution of the treated water caused by the dissolution of the current cathode material.

[0058] (2) Confining Fe 0 in NH2-MIL-88B can obtain sub-nanometer Fe 0 particles with higher dispersion and smaller size, increasing the number of active sites participating in the electrocatalytic nitrate removal reaction. And the confinement of Fe 0 particles in the nanopores can avoid the direct contact of the active sites with the harsh external environment, solving the problems of few active sites of the cathode catalyst and easy inactivation of the active sites, resulting in low catalytic efficiency and poor stability of the catalyst, and improving the environmental shock load resistance of the cathode material.

[0059] (3) In the sub-nanometer Fe confined in NH2-MIL-88B 0In it, the d-band center of iron moves upward closer to the Fermi level, strengthening the effective overlap between the 3d orbit of Fe and the 2p orbit of O in nitrate, enhancing the adsorption capacity of Fe@NH2-MIL-88B for nitrate in groundwater, and solving the problem of insufficient activation of nitrate molecules due to weak adsorption of nitrate by the cathode catalyst.

[0060] (4) Under the action of the octahedral coordination field of NH2-MIL-88B and the unsaturated coordination field of Fe 0 electrons rapidly transfer from the t 2g orbit of iron to the π* orbit of nitrate, significantly accelerating the electron transfer in the electrocatalytic nitrate removal reaction and solving the problem of low electron transfer efficiency in the electrocatalytic nitrate removal reaction.

[0061] (5) The novel confined catalyst Fe@NH2-MIL-88B prepared in the embodiment of the present invention changes the rate-limiting step to the generation of nitric oxide and significantly reduces the maximum reaction barrier, solving the problem of low kinetic efficiency of electrocatalytic nitrate removal due to the high reaction barrier of the rate-limiting step of conventional catalysts and reducing the energy consumption of the electrocatalytic nitrate removal reaction.

[0062] Specifically, in an embodiment of the present invention, a preparation method of a confined catalyst for electrocatalysis is provided, which includes the following steps:

[0063] S1. Dissolve 5.53 mmol of 2-aminoterephthalic acid (NH2-BDC) and 4.424 - 6.636 mmol of ferric chloride (FeCl3·6H2O) in 50 - 70 mL of N,N-dimethylformamide (DMF), and stir vigorously for several hours to obtain a mixed solution;

[0064] S2. Transfer the above mixed solution to a high-pressure autoclave lined with polytetrafluoroethylene, heat it to 110 - 130 °C for heat treatment for several hours to obtain a reaction mixture;

[0065] S3. After the reaction mixture is cooled to room temperature, separate the brown precipitate from the reaction mixture by centrifugation, and wash it thoroughly with DMF and ethanol several times to remove any unreacted starting materials, and then dry it at a temperature of 50 - 70 °C to obtain the confined carrier material NH2-MIL-88B;

[0066] S4. Drop 4 - 6 mL of saturated FeCl3·6H2O solution into the above confined carrier material, grind and dry it, and then place it in an argon atmosphere at 500 - 600 °C for high-temperature carbonization for several hours to obtain a black carbonized powder;

[0067] S5. Add 4 - 6 mL of saturated FeCl3·6H2O solution to the above carbonized powder again, and conduct drying. Then, place it in a mixed atmosphere of hydrogen and argon at 350 - 450 °C (the volume ratio of hydrogen to argon is 3:97 - 7:93) for a reduction reaction for several hours to obtain the confined catalyst Fe@NH2-MIL-88B.

[0068] The key point of the technology of the present invention is to confine sub-nanometer Fe 0 in NH2-MIL-88B, reduce the metal dissolution of iron and lower the iron ion content in the treated water, and prepare a green, safe and pollution-free iron-based catalytic material. The present invention increases the number of active sites of the catalyst and avoids direct contact with the external harsh environment, optimizes the d-band center and enhances the adsorption capacity for nitrate, accelerates the electron transfer in the reaction, changes the rate-limiting step to the generation of nitric oxide and significantly reduces the maximum reaction barrier, thereby improving the efficiency of electrocatalytic removal of nitrate.

[0069] Example 1: As Figure 2 shown, this example provides a preparation method of a confined catalyst for electrocatalysis, which includes the following steps:

[0070] S1. Dissolve 5.53 mmol of 2-aminoterephthalic acid (NH2-BDC) and 5.53 mmol of iron chloride (FeCl3·6H2O) in 60 mL of N,N-dimethylformamide (DMF), and stir vigorously for 2 hours to obtain a mixed solution;

[0071] S2. Transfer the above mixed solution to a 100 mL autoclave lined with polytetrafluoroethylene, heat it to 120 °C for heat treatment for 20 hours to obtain a reaction mixture;

[0072] S3. After the reaction mixture is cooled to room temperature, use centrifugation to separate the brown precipitate from the reaction mixture, and wash it thoroughly with DMF and ethanol 7 times to remove any unreacted starting materials, and then place it at a temperature of 60 °C for drying treatment for 48 hours to obtain the confined carrier material NH2-MIL-88B;

[0073] S4. Add 5 mL of saturated FeCl3·6H2O solution to the above confined carrier material, and conduct grinding and drying. Then, place it in an argon atmosphere at 550 °C for high-temperature carbonization for 4 hours to obtain a black carbonized powder;

[0074] S5. Add 5 mL of saturated FeCl₃·6H₂O solution to the above carbonized powder again, and dry it. Then, place it in a mixed atmosphere of hydrogen and argon (the volume ratio of hydrogen to argon is 5:95) at 400 °C for a reduction reaction for 2 hours to obtain the confined catalyst Fe@NH₂-MIL-88B.

[0075] Example 2: This example provides a preparation method of a confined catalyst for electrocatalysis, which includes the following steps:

[0076] S1. Dissolve 5.53 mmol of 2-aminoterephthalic acid (NH₂-BDC) and 4.424 mmol of ferric chloride (FeCl₃·6H₂O) in 50 mL of N,N-dimethylformamide (DMF), and stir vigorously for 1 hour to obtain a mixed solution;

[0077] S2. Transfer the above mixed solution to a high-pressure autoclave lined with polytetrafluoroethylene, heat it to 110 °C for heat treatment for 15 hours to obtain a reaction mixture;

[0078] S3. After the reaction mixture is cooled to room temperature, use centrifugation to separate the brown precipitate from the reaction mixture, and wash it thoroughly with DMF and ethanol 6 times to remove any unreacted starting materials. Then, place it at 50 °C for drying treatment for 36 hours to obtain the confined support material NH₂-MIL-88B;

[0079] S4. Add 4 mL of saturated FeCl₃·6H₂O solution to the above confined support material, and grind and dry it. Then, place it in an argon atmosphere at 500 °C for high-temperature carbonization for 3 hours to obtain a black carbonized powder;

[0080] S5. Add 4 mL of saturated FeCl₃·6H₂O solution to the above carbonized powder again, and dry it. Then, place it in a mixed atmosphere of hydrogen and argon (the volume ratio of hydrogen to argon is 3:97) at 350 °C for a reduction reaction for 1 hour to obtain the confined catalyst Fe@NH₂-MIL-88B.

[0081] Example 3: This example provides a preparation method of a confined catalyst for electrocatalysis, which includes the following steps:

[0082] S1. Dissolve 5.53 mmol of 2-aminoterephthalic acid (NH₂-BDC) and 6.636 mmol of ferric chloride (FeCl₃·6H₂O) in 70 mL of N,N-dimethylformamide (DMF), and stir vigorously for 3 hours to obtain a mixed solution;

[0083] S2. Transfer the above mixed solution into a high-pressure autoclave lined with polytetrafluoroethylene, heat it to 130 °C and conduct heat treatment for 30 hours to obtain a reaction mixture;

[0084] S3. After the reaction mixture is cooled to room temperature, separate the brown precipitate from the reaction mixture by centrifugation, and wash it thoroughly 8 times with DMF and ethanol to remove any unreacted starting materials. Then place it at a temperature of 70 °C for drying treatment for 72 hours to obtain the confined carrier material NH2-MIL-88B;

[0085] S4. Drop 6 mL of saturated FeCl3·6H2O solution into the above-mentioned confined carrier material, grind and dry it, and then place it in an argon atmosphere at 600 °C for high-temperature carbonization for 5 hours to obtain a black carbonized powder;

[0086] S5. Drop 6 mL of saturated FeCl3·6H2O solution into the above carbonized powder again, and dry it. Then place it in a mixed atmosphere of hydrogen and argon (the volume ratio of hydrogen to argon is 7:93) at 450 °C for reduction reaction for 3 hours to obtain the confined catalyst Fe@NH2-MIL-88B.

[0087] Example 4: As Figure 3 shown, this example provides an electrocatalytic reduction device, including an external power supply ①, a single-chamber electrolytic cell ②, an anode ④ and a cathode ⑤; wherein, the single-chamber electrolytic cell ② is filled with an electrolyte ③; the anode ④ is a platinum sheet, and the cathode ⑤ is an electrode sheet made of the confined catalyst provided in Example 1.

[0088] Specifically, the preparation method of the cathode ⑤ is as follows: First, weigh Fe@NH2-MIL-88B, conductive carbon black and polyvinylidene fluoride provided in Example 1 in a mass ratio of 8:1:1 and mix them. Then dissolve the mixture in N,N-dimethylformamide (concentration: 10 g / L) to prepare a catalyst ink. Subsequently, coat the ink on a carbon cloth (CC, 1.0×2.0 cm), ensure that both sides are evenly coated, and dry it at 60 °C for 30 minutes to obtain an electrode sheet as the cathode ⑤.

[0089] The electrocatalytic reduction test of nitrate can be carried out in a single-chamber electrolytic cell ② using a two-electrode system. The electrode sheet prepared from Fe@NH2-MIL-88B and the platinum sheet are used as the cathode ⑤ and the anode ④ respectively, with the distance between the two plates maintained between 0.5 - 2.0 cm. The electrolyte ③ is NaNO3 and the prepared nitrate solution (concentration 60 mg / L, calculated as N) or nitrate-contaminated groundwater. The concentration of 60 mg / L is selected because the nitrate concentration in the actually collected nitrate-contaminated groundwater is 55 mg / L, which is approximately 60 mg / L in the simulation experiment. The ion chromatography is used to determine the nitrate content of the water samples at each time point, and the nitrate removal rate is calculated.

[0090] Comparative Example 1: This comparative example provides a preparation method of a comparative catalyst, which includes the following steps:

[0091] S1. Dissolve 5.53 mmol of 2-aminoterephthalic acid (NH2-BDC) and 5.53 mmol of ferric chloride (FeCl3·6H2O) in 60 mL of N,N-dimethylformamide (DMF), and stir vigorously for 2 hours to obtain a mixed solution;

[0092] S2. Transfer the above mixed solution to a 100 mL Teflon-lined autoclave, heat to 120 °C for heat treatment for 20 hours to obtain a reaction mixture;

[0093] S3. After the reaction mixture is cooled to room temperature, separate the brown precipitate from the reaction mixture by centrifugation, and wash it thoroughly 7 times with DMF and ethanol to remove any unreacted starting materials, and then place it at 60 °C for drying treatment for 48 hours to obtain the support material NH2-MIL-88B;

[0094] S4. Drop 5 mL of saturated FeCl3·6H2O solution into the above support material and dry it, then successively place it in an argon atmosphere at 550 °C for high-temperature carbonization for 2 hours, and place it in a mixed atmosphere of hydrogen and argon (volume ratio of hydrogen to argon is 5:95) at 400 °C for reduction reaction for 2 hours to obtain the non-confined comparative catalyst Fe / NH2-MIL-88B.

[0095] Performance test: I. The electrocatalytic removal of nitrate by the confined catalyst Fe@NH2-MIL-88B prepared in the embodiments of the present invention has enhanced degradation effect and significantly improved efficiency. Specifically, the electrocatalytic removal of nitrate experiment was carried out in a single-chamber electrolytic cell (50 mL) using a GPS305D DC regulated power supply. In a two-electrode system, a platinum sheet was used as the anode, and the electrode sheet prepared in Example 4 was used as the cathode, and the distance between the two electrodes was controlled to be 0.5 - 2.0 cm. The 30 mL electrolyte contained 60 mg / L nitrate (calculated as N) and 0.21 g Na2SO4 sulfate. A current of 50 mA was applied to the system, and samples were taken every 30 min. The nitrate content of the water samples at each time point was measured using ion chromatography, and the nitrate removal rate was calculated. Under the optimal conditions, 91.87% of the nitrate could be degraded within 3 h. At the same time, the Fe@NH2-MIL-88B catalyst had good cycle stability, and the removal rate was still about 90% after being reused 5 times.

[0096] In addition, the confined catalyst Fe@NH2-MIL-88B was compared with other similar cathode materials at home and abroad, and the results are shown in Table 1.

[0097] Table 1 Performance comparison of cathode materials for electrocatalytic removal of nitrate

[0098] ;

[0099] As can be seen from Table 1, the catalytic material Fe@NH2-MIL-88B prepared in the present invention removed 91.87% of the nitrate within just 3 hours, and the usage time was far lower than that required by the cathode materials reported in other literatures. Moreover, the nitrate removal efficiency was also much higher than that of other iron-based and metal catalysts. This all shows the excellent electrocatalytic reduction performance and ultra-high electrocatalytic nitrate removal efficiency of Fe@NH2-MIL-88B.

[0100] II. SEM was used to analyze the morphology and microstructure of NH2-MIL-88B, Fe@NH2-MIL-88B before and after carbonization of the catalyst prepared in Example 1 above, and Fe / NH2-MIL-88B prepared in Comparative Example 1, as Figure 4 shown. It can be observed in Figure 4 (a) that the prepared NH2-MIL-88B material showed good dispersion and uniformity and had a spindle-like morphology. Figure 4 (b) shows that cracks appeared in the carbonized NH2-MIL-88B material. This is because after the pyrolysis of the porous carbon base surface, some of the open pore structures were retained, which promoted nitrate diffusion and alleviated the mass transfer limitation. In Figure 4 (c), Fe / NH2-MIL-88B was also spindle-shaped, but a large number of iron clusters aggregated on the material surface.Figure 4 In (d), Fe@NH2-MIL-88B is still spindle-shaped, but there are only a small number of iron clusters on the loaded surface. This is because most of the Fe has been encapsulated into the pores of NH2-MIL-88B. This proves that Fe has been successfully confined into NH2-MIL-88B, which can increase the active sites, enabling more nitrate molecules to have the opportunity to contact the catalyst surface and participate in the electrocatalytic reaction, thus improving the electrocatalytic activity for nitrate removal.

[0101] The elemental distribution of the Fe@NH2-MIL-88B sample was determined by energy-dispersive X-ray spectroscopy (EDX), as Figure 5 shown. It can be seen from this that the O, N, C, and Fe elements are evenly distributed at almost the same positions, indicating that Fe has been successfully confined into NH2-MIL-88B. Among them, the O, N, and C elements are shown to be more dense because they are the constituent elements of the MOF, while the Fe element is more dispersed compared to the O, N, and C elements. This indicates that the Fe confined into NH2-MIL-88B does not aggregate together to form a large number of clusters, which is beneficial to increasing the reaction contact area and helping to reduce the mass transfer resistance, making it easier for nitrate to contact the catalyst surface and thus promoting the electrocatalytic reaction for nitrate removal.

[0102] The surface properties and chemical states of Fe@NH2-MIL-88B were analyzed by X-ray photoelectron spectroscopy (XPS), as Figure 6 shown; three pairs of peaks are shown in Figure 6 . The peaks at approximately 707.2 eV (Fe 2p 3 / 2 ) and 722.1 eV (Fe 2p 1 / 2 ) correspond to Fe 0 , the peaks at approximately 712.6 eV (Fe 2p 3 / 2 ) and 724.0 eV (Fe 2p 1 / 2 ) correspond to Fe 2+ , and the peaks at approximately 714.3 eV (Fe 2p 3 / 2 ) and 726.0 eV (Fe 2p 1 / 2 ) correspond to Fe 3+ . Fe 2+ and Fe 3+ belong to the carrier material NH2-MIL-88B, while the peak area of the confined Fe 0 occupies most, indicating that a material containing a large amount of Fe 0 has been successfully prepared. Fe 0 does not form chemical bonds with other elements, and there are abundant active sites on the surface, enabling more nitrate to be reduced per unit time and improving the overall reaction efficiency of electrocatalytic reduction of nitrate.

[0103] To evaluate the specific surface area and pore structure of the catalysts, N2 adsorption-desorption isotherm tests were carried out on Fe@NH2-MIL-88B and NH2-MIL-88B, as Figure 7 shown. The N2 adsorption-desorption isotherms of Fe@NH2-MIL-88B all showed typical type-IV isotherms of mesoporous materials, with obvious H3-type hysteresis loops, indicating that Fe@NH2-MIL-88B should have a stronger adsorption capacity for nitrates. The type-IV isotherm of the N2 adsorption-desorption isotherm of NH2-MIL-88B did not show an obvious hysteresis loop, which means that the pore size of NH2-MIL-88B is relatively large, which will limit its performance in electrocatalytic nitrate removal. In addition, since the surface area and the most probable pore size of Fe@NH2-MIL-88B are 5.287 m 2 / g and 3.766 nm, respectively, which are smaller than the surface area and the most probable pore size of NH2-MIL-88B, it proves the successful preparation of the confined material Fe@NH2-MIL-88B, because confinement will cause pore channels to close or shrink and reduce particle size. Due to the high surface area and multiple active sites of the small-sized catalyst Fe@NH2-MIL-88B, it can adsorb nitrates more effectively, ensure that more reactants enter the active sites and undergo reduction reactions, thereby improving the reaction efficiency.

[0104] The carbon structure of Fe@NH2-MIL-88B was further analyzed using Raman spectroscopy, as Figure 8 shown; the prominent peaks at the D band (1341 cm −1 ) and G band (1588 cm −1 ) correspond to graphene defects and the coplanar vibration of sp 2 carbon atoms, respectively, and the I D / I G ratio indicates the degree of defect in the carbon material. The I D / I G ratio of Fe@NH2-MIL-88B is less than 1, indicating that its carbon material has a high crystallinity and few defects, the graphite structure is more ordered, and the carbon atoms are arranged more regularly, which can provide better conductivity and stability. The high conductivity of the catalyst helps to form a more uniform charge distribution on the catalyst surface, avoid excessive local charge accumulation, and thus optimize the reaction conditions and efficiency.

[0105] To obtain the electrochemical performance of the Fe@NH2-MIL-88B catalyst, the catalytic materials Fe@NH2-MIL-88B, Fe / NH2-MIL-88B, and NH2-MIL-88B were coated onto carbon cloth (CC, 1.0×2.0 cm) according to the method provided in Example 4 to obtain working electrodes (Fe@NH2-MIL-88B / CC, Fe / NH2-MIL-88B / CC, NH2-MIL-88B / CC).

[0106] The Fe@NH2-MIL-88B catalyst and other catalysts were electrochemically characterized by an electrochemical workstation. The EIS results are as Figure 9 shown. The semicircle diameter of Fe@NH2-MIL-88B / CC in the high-frequency region is smaller than that of Fe / NH2-MIL-88B / CC, indicating that Fe@NH2-MIL-88B has a lower charge transfer resistance than Fe / NH2-MIL-88B, NH2-MIL-88B, and CC, which is more conducive to the charge transfer of nitrate in the cathode and thus promotes the electrocatalytic removal of nitrate.

[0107] The LSV curves of Fe@NH2-MIL-88B, Fe / NH2-MIL-88B, and NH2-MIL-88B were measured by linear sweep voltammetry (LSV). The results are as Figure 10 shown. At the same potential, Fe@NH2-MIL-88B exhibits a higher current density, indicating that the Fe@NH2-MIL-88B catalyst has excellent electrical conductivity, which is beneficial to the transfer of electrons to the surface of the Fe@NH2-MIL-88B catalyst, thereby significantly enhancing the catalytic activity in the electrocatalytic reduction of nitrate process.

[0108] The Tafel slopes of Fe@NH2-MIL-88B, Fe / NH2-MIL-88B, and NH2-MIL-88B were calculated based on the LSV curves. The results are as Figure 11 shown. The Tafel slope represents the change in overpotential required when the current changes by a factor of ten. Therefore, the smaller the absolute value of the slope, the better, as a smaller slope means a smaller increase in voltage and lower energy consumption. From Figure 11 it can be seen that the value of the Tafel slope of the Fe@NH2-MIL-88B catalyst is also the smallest, which also indicates that the Fe@NH2-MIL-88B catalyst has a faster reaction kinetics for the electrocatalytic removal of nitrate and better catalytic activity.

[0109] III. Electro-catalytic nitrate removal experiment The electro-catalytic nitrate removal experiment was carried out in a single-chamber electrolytic cell (50 mL) using a GPS305D DC regulated power supply. In a two-electrode system, a platinum sheet was used as the anode, and the prepared Fe@NH2-MIL-88B catalytic material was coated on a carbon cloth of 1.0×2.0 cm as the cathode, controlling the distance between the two electrodes to be 0.5 - 2.0 cm. The 30 mL electrolyte contained 60 mg / L nitrate (calculated as N) and 0.21 g Na2SO4 sulfate radical. A current of 50 mA was applied to the system, and samples were taken every 30 min. The nitrate content of the water samples at each time point was measured using ion chromatography, and the nitrate removal rate was calculated. The ammonia nitrogen concentration during the reaction process was determined by the Nessler reagent spectrophotometry method, and the ammonia nitrogen conversion rate was calculated. Keeping the experimental conditions and operation steps unchanged, the experiment was repeated three times to obtain the nitrate degradation effect, ammonia nitrogen production and conversion rate during the electro-catalytic reduction of nitrate.

[0110] Figure 12 Describes the electro-catalytic nitrate removal degradation process of Fe@NH2-MIL-88B (the degradation conditions are as follows: 60 mg / L of NO3 - -N 30 mL, 0.21 g Na2SO4, current 50 mA, carbon cloth area 2.0 cm 2 , pH = 7), which is used to measure the nitrate, nitrite and ammonia gas content during the electrolysis process. At a constant current of 50 mA·cm 2 , the nitrate concentration of Fe@NH2-MIL-88B decreased from 100% to 8.89% within 3 hours, and the nitrate removal rate was 91.87%. The concentration of nitrite was 0 at both the initial 0 min and the end of 180 min, and only trace amounts of nitrite were generated as intermediate products during the intermediate process, indicating that there will be no nitrite residue after degradation and no problems such as secondary pollution will be caused. 71.25% of the removed nitrate was converted into ammonia gas, and the ammonia gas was then converted into harmless nitrogen gas through the Cl - reaction at the platinum sheet anode.

[0111] During the use of the Fe@NH2-MIL-88B catalyst, if the catalyst is passivated due to internal and external environmental influences, the reaction efficiency will drop significantly, affecting the continuous progress of the catalytic reaction. To further confirm the stability of the Fe@NH2-MIL-88B catalyst, 5 cycle experiments were carried out (the degradation conditions are as follows: 60 mg / L of NO3 - -N 30 mL, 0.21 g Na2SO4, current 50 mA, carbon cloth area 2.0 cm 2, pH = 7), during the experiment, after each electrocatalytic degradation, the electrode was carefully cleaned and dried, and then the electrode was put back into the electrocatalytic reduction system to degrade new nitrate. As Figure 13 shown, the nitrate removal rate of Fe@NH2-MIL-88B was about 90% and the ammonia selectivity was about 70% after 5 cycles of use, showing strong comprehensive performance stability.

[0112] To investigate the effect of confinement effect on the electrocatalytic reduction of nitrate, the above operation steps and other degradation conditions were kept unchanged (degradation conditions are as follows: 60 mg / L of NO3 - -N 30 mL, 0.21 g of Na2SO4, current 50 mA, carbon cloth area 2.0 cm 2 , pH = 7). 1.0×2.0 cm of Fe@NH2-MIL-88B, Fe / NH2-MIL-88B and NH2-MIL-88B were used as anodes respectively for degradation experiments for 3 h. The degradation rates of nitrate when using different materials were measured. Keeping the experimental conditions and operation steps unchanged, the degradation rates of nitrate when using different materials were obtained by repeating the experiment three times. The results are as Figure 14 shown.

[0113] From Figure 14 it can be seen that the degradation rate of nitrate by Fe@NH2-MIL-88B catalyst reached 91.87% after 3 h of reaction, while the degradation rate of nitrate by Fe / NH2-MIL-88B catalyst was only 62.06% after 3 h of reaction. The degradation rate of nitrate by pure NH2-MIL-88B was 55.19% after 3 h. The Fe@NH2-MIL-88B catalyst removed up to 91.87% of nitrate in just 3 hours, showing extremely high removal efficiency, far higher than other reported catalysts of the same kind. Thus, Fe@NH2-MIL-88B is an excellent electrocatalytic reduction nitrate catalytic material.

[0114] IV. During the electrocatalytic reduction process, the current density is undoubtedly an important factor affecting the reaction. To verify the effect of current density on the electrocatalytic reduction of nitrate, the current density was controlled at 10 mA / cm 2 , 30 mA / cm 2 , 50 mA / cm 2 , 100 mA / cm 2 . Degradation experiments were carried out with the above operation steps and other degradation conditions unchanged, and then the experiment was repeated three times with the experimental conditions and operation steps completely unchanged. The results are as Figure 15 shown.

[0115] FromFigure 15 It can be seen that at 50 mA / cm 2 the nitrate degradation effect is the best, and 91.87% of the nitrate can be degraded after 180 min of reaction. When the current density increases from to 50 mA / cm 2 the degradation effect continuously improves, but the improvement amplitude becomes smaller and smaller, and the selectivity of ammonia reaches the highest. However, when the current exceeds 50 mA / cm 2 the degradation effect and the apparent rate constant decrease slightly. This may be because when the current is too large, the voltage in the reaction system is too high, resulting in a large number of side reactions. The higher current density may accelerate the hydrogen evolution reaction at the cathode, thus providing more active sites on the surface of the Fe@NH2-MIL-88B catalyst for H + and restricting the electrocatalytic removal of nitrate. It can be seen that the optimal current density for electrocatalytic removal of nitrate using the catalytic material Fe@NH2-MIL-88B is 50 mA / cm 2 .

[0116] V. The initial pH value of the solution is an important condition that cannot be ignored in the electrocatalytic removal of nitrate, which affects the proton concentration and the removal efficiency. In order to explore the influence of the initial pH of the reaction system on the electrocatalytic reduction and degradation of nitrate, the present invention sets different initial pH values (pH = 3, 5, 7, 9, 11) to analyze and evaluate the electrocatalytic nitrate removal performance of Fe@NH2-MIL-88B, and keeps the above operation steps and other degradation conditions unchanged for the degradation experiment, and then keeps the experimental conditions and operation steps completely unchanged for three repeated experiments, and the results are as Figure 16 shown.

[0117] From Figure 16 it can be seen that when the initial pH is 7, the nitrate removal ability and ammonia selectivity reach the maximum. The result data shows that the neutral condition is more favorable for the electrocatalytic reduction of nitrate, and the apparent rate constant also reaches the highest. In the acidic system, the generated hydrogen bubbles wrap around the surface of the active material, hindering the full contact between the active sites and the electrolyte, thus slightly reducing the electrocatalytic performance. A higher pH may cause the active surface to be occupied by adsorbed hydroxides, which is not conducive to the transfer of electrons from the electrocatalyst to nitrate. In the alkaline condition, on the one hand, the hydroxide ions in the solution will neutralize the protons during the reaction process, reducing the reaction efficiency. On the other hand, the hydroxide anions will occupy the active sites, which will also reduce the reaction efficiency. The electrocatalytic nitrate removal rate of Fe@NH2-MIL-88B reaches the maximum of 91.87% at pH = 7, and always maintains a good removal rate in a wide pH range, which indicates that the catalyst maintains stable catalytic activity in groundwater with different pH values and has a large application range because NH2-MIL-88B confines Fe0 It can buffer the impact of pH changes.

[0118] VI. To explore the effect of the initial nitrate concentration on the electrocatalytic removal of nitrate, in the embodiments of the present invention, under the condition that the remaining conditions remain unchanged, the electrocatalytic nitrate removal test is carried out under the conditions where the initial nitrate concentrations are 25 mg / L, 60 mg / L, 100 mg / L, and 150 mg / L (both are calculated as N), respectively. The degradation experiment is carried out while keeping the above operation steps and other degradation conditions unchanged, and then three repeated experiments are carried out while keeping the experimental conditions and operation steps completely unchanged, and the results are as Figure 17 shown.

[0119] From Figure 17 it can be seen that different initial concentrations within a certain range do not affect the nitrate degradation effect. After 180 min of reaction in the reaction systems with different concentrations, more than 85% of the nitrate is removed, and all have good nitrate degradation performance, proving that Fe@NH2-MIL-88B also has excellent removal ability for high-concentration nitrate-contaminated groundwater.

[0120] VII. To explore the effect of different initial chloride ion concentrations on the electrocatalytic removal of nitric acid, the electrocatalytic degradation test is carried out under the conditions where the chloride ion concentrations are 0, 10 mg / L, 30 mg / L, 50 mg / L, and 100 mg / L, respectively. The degradation experiment is carried out while keeping the above operation steps and other degradation conditions unchanged, and then three repeated experiments are carried out while keeping the experimental conditions and operation steps completely unchanged, and the results are as Figure 18 shown.

[0121] From Figure 18 it can be seen that different initial chloride ion concentrations have a certain impact on the nitrate degradation effect. As the chloride ion concentration in the electrolyte increases, the nitrate degradation effect decreases. This may be because: the presence of chloride ions will cause side reactions in the reaction system to generate chlorine gas, thus affecting the removal effect of nitrate. A large number of studies at home and abroad have shown that appropriately increasing the Cl - concentration can effectively improve the nitrogen selectivity of the catalyst, which is consistent with the experimental results of the present invention. This significant increase can be attributed to the breakpoint chlorination theory, in which chloride can be oxidized to form HClO and further oxidize ammonia to form nitrogen, as shown in equations (1)-(3).

[0122] ;

[0123] VIII. Electrochemical nitrate reduction involves two pathways: one is the regulation of active hydrogen generated at the active sites, and the other is the reduction of cathode electrons. Tert-butanol (TBA) is a specific hydrogen radical quencher used to determine whether the electrochemical nitrate reduction process is mediated by active hydrogen or dominated by electron regulation. Electrochemical degradation tests were carried out under the conditions of controlling the TBA concentration at 0, 10 mg / L, 30 mg / L, and 50 mg / L respectively, keeping the above operation steps and other degradation conditions unchanged for the degradation experiment, and then keeping the experimental conditions and operation steps completely unchanged for three repeated experiments, and the results are as Figure 19 shown.

[0124] As Figure 19 shown, as the TBA concentration increases, the nitrate residue also gradually increases. The electrocatalytic performance of Fe@NH2-MIL-88B for nitrate removal was measured by adding different amounts of TBA (from 0 to 50 mg / L) to the electrolyte. The results show that active hydrogen plays a dominant role in promoting the electrocatalytic removal of nitrate by Fe@NH2-MIL-88B.

[0125] IX. According to the previous characterization results of the embodiments of the present invention, using Fe atoms as the active sites in the model, density functional theory (DFT) calculations were carried out to study the reaction mechanism and reveal the reason for the high performance of Fe@NH2-MIL-88B in nitrate reduction. The reaction path and the change of reaction free energy for the electrocatalytic removal of nitrate by the catalytic material Fe@NH2-MIL-88B are as Figure 20 , Figure 21 shown.

[0126] The reduction of nitrate to ammonia is accompanied by the transfer of nine protons and eight electrons. Figure 20 The reaction path of

[0127] ;

[0128] Figure 21 is the reaction free energy change diagram of Fe@NH2-MIL-88B and Fe / NH2-MIL-88B with the reduction of nitrate. By further calculating the reaction path, the free energy of each intermediate at the Fe site was calculated. The lower energy indicates that nitrate is more likely to be adsorbed on the Fe@NH2-MIL-88B site to form *NO3. Then the N-O bond of *NO3 breaks and reacts with H step by step +Combination generates *NO2, *ON, *ONH, *ONH2, *O, and *OH. When electrocatalytically removing nitrate using conventional iron-based materials, the rate-limiting step in the electrocatalytic nitrate removal reaction process is the nitrite generation process because the high energy of the lowest unoccupied molecular π* orbital of nitrate limits charge injection, thus restricting the overall electrocatalytic nitrate removal efficiency.

[0129] However, the rate of forming *ON on Fe@NH2-MIL-88B is the rate-limiting step, and the energy barrier for the reduction of *ON to *ONH on Fe@NH2-MIL-88B is much smaller than that on Fe / NH2-MIL-88B. Therefore, the removal efficiency of Fe@NH2-MIL-88B is higher. Based on DFT calculations, it can be seen that Fe@NH2-MIL-88B has a high adsorption capacity for nitrate ions and has no large reaction energy barrier for nitrate reduction, thus showing significant electrocatalytic nitrate removal activity.

[0130] As Figure 22 shown in (a) and (b), in order to compare the nitrate adsorption capacity and interaction of the catalysts, the differential charges of nitrate with Fe / NH2-MIL-88B and Fe@NH2-MIL-88B were calculated respectively. The green represents a decrease in electron cloud density, and the yellow represents an increase in electron cloud density. The Fe-O bond length of Fe@NH2-MIL-88B is shorter than that of Fe / NH2-MIL-88B, indicating that the connection between Fe@NH2-MIL-88B and nitrate is more stable. In addition, the adsorption energies of Fe@NH2-MIL-88B and Fe / NH2-MIL-88B were calculated. The charge density accumulated by *NO3 on Fe@NH2-MIL-88B is 2.18 eV higher than that on Fe / NH2-MIL-88B, indicating that Fe@NH2-MIL-88B has a stronger electronic interaction with nitrate. Therefore, Fe@NH2-MIL-88B is more likely to adsorb nitrate and thus promote the electrocatalytic reduction reaction for removal.

[0131] In summary, the confined catalyst Fe@NH2-MIL-88B prepared in the embodiments of the present invention can solve the problem of low electrocatalytic nitrate removal efficiency through multiple levels and dimensions such as increasing active sites, enhancing adsorption capacity, and accelerating electron transfer, and can provide a more economical and effective cathode material and process technology for the treatment of electrocatalytic nitrate-contaminated groundwater.

[0132] Inspired by the above ideal embodiments of the present invention, through the above description, relevant staff can completely make various changes and modifications without departing from the technical idea of this invention. The technical scope of this invention is not limited to the content in the specification.

Claims

1. A method for preparing a confined catalyst for electrocatalysis, characterized in that: The following steps are involved: Dissolving 2-aminoterephthalic acid and ferric chloride in a solvent to obtain a mixed solution; the molar ratio of the 2-aminoterephthalic acid to the ferric chloride is 1:(0.8-1.2); The mixed solution is subjected to heat treatment to obtain a reaction mixture; the heat treatment temperature is 110-130°C; Separating the precipitate from the reaction mixture, washing and drying the precipitate to obtain a confined carrier material; A saturated ferric chloride solution is added dropwise to the confined carrier material, and the material is ground and dried, followed by high-temperature carbonization to obtain a carbonized powder; the high-temperature carbonization is carried out in an inert gas atmosphere at a temperature of 500-600°C; A saturated ferric chloride solution is added dropwise to the carbonized powder again, and the powder is dried, followed by a reduction reaction to obtain the confined catalyst; the reduction reaction is carried out in a mixed atmosphere of hydrogen and argon at a temperature of 350-450° C.; the volume ratio of hydrogen to argon in the mixed atmosphere is 3:97-7:

93.

2. The method for preparing a confined catalyst for electrocatalysis according to claim 1, characterized in that: The solvent is N,N-dimethylformamide.

3. A confined catalyst prepared by the preparation method according to any one of claims 1 to 2.

4. Use of the confined catalyst as claimed in claim 3 in the electrocatalytic reduction of nitrates.

5. Use of the confined catalyst as claimed in claim 3 in removing nitrates from water.

6. An electrocatalytic reduction device, comprising a cathode and an anode, characterized in that: The cathode is made of the confined catalyst described in claim 3.

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