Confined range catalyst for electro-catalysis as well as preparation method and application thereof

Abstract

The invention discloses a confinement catalyst for electrocatalysis and a preparation method and application thereof, and belongs to the technical field of electrocatalyst.The preparation method comprises the following steps that 2-aminoterephthalic acid and ferric chloride are dissolved in a solvent, and a mixed solution is obtained; performing heat treatment on the mixed solution to obtain a reaction mixture; separating the precipitate from the reaction mixture, washing and drying to obtain a confinement carrier material; dropwise adding a saturated ferric chloride solution into the confinement carrier material, grinding and drying, and then performing high-temperature carbonization to obtain carbonized powder; and dropwise adding the saturated ferric chloride solution into the carbonized powder again, drying, and then carrying out a reduction reaction to obtain the confined catalyst. According to the method, the defects of heavy metal pollution, few active sites, weak conductivity, poor adsorption capacity and the like caused by dissolution of a cathode catalyst for removing nitrate through electro-catalysis can be overcome.

Description

Technical Field

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

[0002] The main sources of groundwater nitrate pollution are the extensive use of fertilizers in agriculture and the uncontrolled discharge of wastewater from various industries. Especially in irrigated agricultural areas, farmland drainage systems will bring excess fertilizers and other pesticide chemicals into the water body through surface runoff or infiltration, resulting in increased nitrate concentrations in the water. Nearly 50% of drinking water in daily life comes directly from groundwater, and excessive nitrate concentrations in drinking water can have a variety of adverse effects on human health. Because the human body will reduce nitrates to nitrites that may cause hyperhemoglobinemia and increase the risk of cancer infection. Therefore, in the face of groundwater nitrate pollution, people urgently need to develop new technologies for groundwater nitrate removal with low cost and high efficiency.

[0003] At present, converting nitrates into nitrogen or ammonia using biological or chemical methods is a common method for remediating groundwater nitrate pollution. Biological methods are one of the most effective methods for treating nitrate pollution, but they are quite slow and sensitive to changes in environmental variables. At the same time, hydrogen is required as a reducing agent electron donor during the chemical reduction process, but the transmission of pressurized hydrogen poses a safety hazard. In addition, electrodialysis is an emerging electrochemical membrane method used to concentrate and separate pollutant ions in wastewater, but due to its high resistance and high energy consumption, it is not suitable for low-concentration nitrate solutions. Therefore, it is urgently desired to develop green, efficient and low-energy nitrate pollution remediation technologies.

[0004] In summary, at present, the degree of nitrate pollution in groundwater has become more and more serious due to human activities over time, and conventional nitrate removal methods include biological, chemical and physical methods, but the high cost, low treatment efficiency and complex operation have limited the further development and application of these technologies. Therefore, it is necessary to study environmentally friendly and sustainable nitrate pollution remediation technologies.

[0005] Electrocatalysis is an emerging environmentally friendly and sustainable nitrate pollution remediation technology. It shows great advantages and application potential due to its friendly environmental compatibility and the use of renewable energy (such as solar energy and wind energy). However, the cathode catalysts for electrocatalytic reduction of nitrates reported so far often have problems such as few active sites and low kinetic efficiency. The root cause 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 nitrates. In particular, electrocatalytic removal of nitrates faces many challenges in practical applications, such as secondary heavy metal pollution caused by metal dissolution in treated water, low efficiency of electrocatalytic removal of nitrates due to a small number of active sites, easy deactivation and weak conductivity. Summary of the invention

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

[0007] To achieve the above objectives, the embodiments of the present invention provide the following technical solutions: A method for preparing a confined catalyst for electrocatalysis comprises the following steps: Dissolving 2-aminoterephthalic acid and ferric chloride in a solvent to obtain a mixed solution; heat-treating the mixed solution to obtain a reaction mixture; 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, and then carbonized at high temperature to obtain a carbonized powder; 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.

[0008] Preferably, the molar ratio of 2-aminoterephthalic acid to ferric chloride is 1:(0.8-1.2).

[0009] Preferably, the solvent is N,N-dimethylformamide.

[0010] Preferably, the temperature of the heat treatment is 110-130°C.

[0011] Preferably, the high-temperature carbonization is performed in an inert gas atmosphere at a temperature of 500-600°C.

[0012] Preferably, the reduction reaction is carried out in a mixed atmosphere of hydrogen and argon, 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.

[0013] Another object of an embodiment of the present invention is to provide a confined catalyst prepared by the above preparation method.

[0014] Another object of an embodiment of the present invention is to provide an application of the above-mentioned confined catalyst in the electrocatalytic reduction of nitrates.

[0015] Another object of an embodiment of the present invention is to provide an application of the above-mentioned confined catalyst in removing nitrates from water.

[0016] Another object of an embodiment of the present invention is to provide an electrocatalytic reduction device, comprising a cathode and an anode, wherein the cathode is made of the above-mentioned confined catalyst.

[0017] The present invention uses the carrier material NH2-MIL-88B to confine sub-nanometer Fe 0 Prepare green and efficient cathode confined catalyst Fe@NH2-MIL-88B and establish corresponding application technology methods. The present invention can improve the shortcomings of cathode catalyst dissolution causing heavy metal pollution, few active sites, weak conductivity and poor adsorption capacity in electrocatalytic removal of nitrates. These improvements are conducive to promoting the application of electrocatalysis in nitrate-contaminated groundwater and improving the efficiency of electrocatalytic removal of nitrates. The confined catalyst is used for electrocatalytic treatment of nitrate-contaminated groundwater, which effectively solves the following problems: (1) Conventional metal catalysts often face difficulties in metal dissolution when performing electrocatalytic removal of nitrates. These dissolved metals will cause secondary heavy metal pollution to the treated water, thereby reducing water quality. 0 Fe@NH2-MIL-88B was confined in the metal organic framework NH2-MIL-88B to prepare a new cathode material for electrocatalytic removal of nitrate. 0 Confining in the pores of NH2-MIL-88B can reduce the leaching of iron, which is a trace element beneficial to the human body. It solves the current problem of secondary heavy metal pollution in treated water caused by the dissolution of cathode materials.

[0018] (2) Conventional iron nanoparticles tend to agglomerate due to their high surface energy, resulting in a reduction in active sites and thus lower catalytic activity. 0 Sub-nano Fe with higher dispersion and smaller size can be obtained 0 particles, increasing the number of active sites involved in the electrocatalytic removal of nitrate, and Fe 0 Particle confinement in nanopores can prevent active sites from direct contact with the harsh external environment, solving the problem of low catalytic efficiency and poor stability of the cathode catalyst due to few active sites and easy deactivation of active sites, and improving the cathode material's ability to withstand environmental impact loads.

[0019] (3) The d-band center of bulk iron is far away from the Fermi level. When the d-band center is far away from the Fermi level, the p-band center of iron atoms moves downward, resulting in the aggregation of iron clusters, thereby reducing the catalytic performance of electrocatalytic removal of nitrates. 0 In the experiment, the d-band center of iron moved up closer to the Fermi level, which strengthened the effective overlap between the 3d orbital of Fe and the 2p orbital of O of nitrate, enhanced the adsorption capacity of Fe@NH2-MIL-88B for nitrate in groundwater, and solved the problem of insufficient activation of nitrate molecules due to weak adsorption of nitrate by cathode catalyst.

[0020] (4) The conductivity of the electrode material determines the efficiency of electron transfer within the electrode. Materials with poor conductivity will increase resistance and reduce the electron transfer rate. 0 Under the action of the unsaturated coordination field, the electrons from the iron 2g The orbital is rapidly transferred to the π* orbital of nitrate, which significantly accelerates the electron transfer in the electrocatalytic removal of nitrate reaction and solves the problem of low electron transfer efficiency in the electrocatalytic removal of nitrate reaction.

[0021] (5) The rate-limiting step of the electrocatalytic nitrate removal reaction, the formation of nitrite, has an extremely high reaction barrier, which limits the overall electrocatalytic nitrate removal efficiency. The new 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 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. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] Figure 1 Schematic diagram of the principle of electrocatalytic removal of nitrates from water using a confined catalyst provided in an embodiment of the present invention.

[0023] Figure 2 A schematic flow chart of a method for preparing a confined catalyst provided in an embodiment of the present invention.

[0024] Figure 3 A schematic structural diagram of an electrocatalytic reduction device provided in an 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; and ⑤ is a cathode.

[0025] Figure 4are 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.

[0026] Figure 5 This is the element distribution map of Fe@NH2-MIL-88B.

[0027] Figure 6 This is the XPS spectrum of Fe@NH2-MIL-88B.

[0028] Figure 7 These are the 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.

[0029] Figure 8 This is the Raman spectrum of Fe@NH2-MIL-88B.

[0030] Fig. 9 Electrochemical impedance spectra of Fe@NH2-MIL-88B, Fe / NH2-MIL-88B, NH2-MIL-88B and CC.

[0031] Fig.10 Linear sweep voltammetry curves (vs. RHE) of different materials.

[0032] Fig.11 is the Tafel slope (vs. RHE) of different materials.

[0033] Fig.12 This is the degradation curve of electrocatalytic nitrate removal by Fe@NH2-MIL-88B.

[0034] Fig.13 This is the cycle performance test result diagram of Fe@NH2-MIL-88B.

[0035] Fig.14 This is a comparison chart of the nitrate electrocatalytic degradation effects of different materials.

[0036] Fig.15 This is a graph showing the effect of different current densities on the electrocatalytic reduction of nitrate.

[0037] Fig.16 This is a graph showing the effect of different pH values ​​on the electrocatalytic reduction of nitrate.

[0038] Fig.17 This is a graph showing the effect of different initial nitrate concentrations on the electrocatalytic reduction of nitrate.

[0039] Fig.18 Graph showing the effect of different chloride ion concentrations on (a) electrocatalytic reduction of nitrate and (b) ammonia selectivity.

[0040] Fig.19 This is a graph showing the effect of different TBA concentrations on the electrocatalytic reduction of nitrate.

[0041] Fig. 20 The reaction pathway of Fe@NH2-MIL-88B electrocatalytic reduction of nitrate.

[0042] Fig.21 This is the reaction process and free energy change diagram of electrocatalytic reduction of nitrate.

[0043] Fig. 22 Comparison of the overall differential charge and adsorption energy of nitrate adsorbed by (a) Fe / NH2-MIL-88B and (b) Fe@NH2-MIL-88B. DETAILED DESCRIPTION

[0044] The following will be combined with the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative work are within the scope of protection of the present invention.

[0045] Electrocatalytic reduction of nitrate is an alternative method for treating nitrate pollution by combining electrochemical and catalytic technologies. Nitrate undergoes electrochemical reduction reaction at the cathode, converting it into harmless nitrogen or ammonia with economic value. In the electrocatalytic reduction of nitrate, the catalyst on the cathode surface first adsorbs nitrate ions, and then nitrate as a reactant undergoes reduction reaction on the catalyst surface. The reduction process also requires the participation of active hydrogen and the transfer of electrons. The electrons from the external power source are transferred to nitrate on the cathode surface, and the active hydrogen can participate in the reaction through proton reduction on the cathode surface, thereby promoting the reduction of nitrate. The composition, surface structure, number of active sites and electronic conductivity of the cathode material determine the progress and effect of the electrocatalytic reduction of nitrate. The cathode catalysts reported so far 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.

[0046] Iron-based materials have become promising catalytic materials for the electrochemical reduction of nitrates due to their abundant reserves on earth, strong conductivity and excellent catalytic performance. The researchers used polarography to study the reduction mechanism of nitrates and concluded that the d-orbital electrons of the cathode material help inject charge into the lowest unoccupied molecular π* orbital of nitrates. Therefore, metal catalysts with high d-orbital occupancy and unclosed d-orbital shells (such as precious metals, copper, iron, etc.) have become the research focus of cathode materials. However, precious metal materials are expensive and difficult to use on a large scale. Copper-based materials are also easily dissolved during the reaction process, and are highly toxic after dissolution, which can easily cause secondary pollution. However, the use of iron-based materials as cathodes for the electrocatalytic reduction of nitrates has excellent application potential, because iron-based materials are cheap, easy to obtain, green and harmless, and even after dissolution, they are very easy to react with coagulants to form insoluble complexes for removal. However, iron-based materials often have problems such as few active sites, low conductivity and poor adsorption capacity. Moreover, although nano-iron particles have a large specific surface area, they are also more likely to agglomerate and are easily oxidized in the air to form a dense oxide film on the surface, which hinders the electrocatalytic removal of nitrates. Therefore, it is necessary to make reasonable combinations and structural designs to combine iron with other metals or non-metals to prepare excellent new iron-based catalytic materials, reduce iron agglomeration and oxidation, increase active sites, and enhance conductivity and adsorption capacity.

[0047] Confinement is an efficient solution to improve the shortcomings of the above-mentioned iron-based catalysts. Confining iron-based materials in microstructures can change the electron cloud density, d-band center, nitrate adsorption capacity, etc. of the catalyst, thereby 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 improved, which can significantly increase the exposure of surface active sites and improve the activity of the catalyst. In addition, the confinement effect can also accelerate the reaction kinetics by improving the charge transfer ability and improve the conductivity of the material. Metal organic framework (MOF) is a kind of confined support material. Its pores can act as a separate nanoscale reactor to enhance the host-guest interaction and promote the activation of the catalytic substrate. In addition, the confinement effect of the nano / sub-nanopores or specific binding sites of MOF can effectively limit the agglomeration and growth of metal nanoparticles, thereby stabilizing the resulting ultrafine nanostructure and regulating its electronic structure and three-dimensional structure in catalysis. MOF-derived porous carbon materials usually exhibit high conductivity while retaining the porous topological structure, thereby reducing the charge transfer energy barrier. MOF is suitable as a confined support material due to its high porosity, rich functionalized surface and good conductivity.

[0048] NH2-MIL-88B is a MOF material with high surface area and porosity, in which one-dimensional hexagonal channels and bipyramidal cages produce multiple micropores, which allow more nitrates to contact the catalyst, thereby improving the reaction efficiency. The carbonized NH2-MIL-88B is an ordered mesoporous carbon material that provides a highly dispersed surface area for active sites, appropriate and interconnected channels for nitrate diffusion, and good conductivity for promoting electronic pathways. The porous carbon substrate partially retains an open-pore structure, which promotes nitrate diffusion and alleviates mass transfer limitations. Due to the above outstanding structural advantages, NH2-MIL-88B is suitable for use as a confined carrier material for cathode catalysis in electrocatalytic removal of nitrates.

[0049] In summary, the present invention uses NH2-MIL-88B as a carrier material to 0 The cathode catalyst material is prepared in a limited area to solve the problem that the cathode catalyst has poor conductivity, few active sites and unreasonable electronic structure, which are not conducive to nitrate adsorption and electrocatalytic reduction of nitrate, thus limiting the application of electrocatalysis in nitrate pollution. The embodiment of the present invention prepares a new Fe@NH2-MIL-88B catalytic material for electrocatalytic treatment of nitrate-contaminated groundwater (such as Figure 1 As shown), the following problems are effectively solved: (1) By converting sub-nanometer Fe 0 Fe@NH2-MIL-88B was confined in the metal organic framework NH2-MIL-88B to prepare a new cathode material for electrocatalytic removal of nitrate. 0 Confining in the pores of NH2-MIL-88B can reduce the leaching of iron, which is a trace element beneficial to the human body. It solves the current problem of secondary heavy metal pollution in treated water caused by the dissolution of cathode materials.

[0050] (2) NH2-MIL-88B limited Fe 0 Sub-nano Fe with higher dispersion and smaller size can be obtained 0 particles, increasing the number of active sites involved in the electrocatalytic removal of nitrate, and Fe 0 Particle confinement in nanopores can prevent active sites from direct contact with the harsh external environment, solving the problem of low catalytic efficiency and poor stability of the cathode catalyst due to few active sites and easy deactivation of active sites, and improving the cathode material's ability to withstand environmental impact loads.

[0051] (3) Sub-nanometer Fe confined in NH2-MIL-88B 0In the experiment, the d-band center of iron moved up closer to the Fermi level, which strengthened the effective overlap between the 3d orbital of Fe and the 2p orbital of O of nitrate, enhanced the adsorption capacity of Fe@NH2-MIL-88B for nitrate in groundwater, and solved the problem of insufficient activation of nitrate molecules due to weak adsorption of nitrate by cathode catalyst.

[0052] (4) In the octahedral coordination field of NH2-MIL-88B and Fe 0 Under the action of the unsaturated coordination field, the electrons from the iron 2g The orbital is rapidly transferred to the π* orbital of nitrate, which significantly accelerates the electron transfer in the electrocatalytic removal of nitrate reaction and solves the problem of low electron transfer efficiency in the electrocatalytic removal of nitrate reaction.

[0053] (5) The new 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, thereby solving the problem of low kinetic efficiency of electrocatalytic nitrate removal due to high reaction barrier of the rate-limiting step of conventional catalysts and reducing the energy consumption of the electrocatalytic nitrate removal reaction.

[0054] Specifically, in one embodiment of the present invention, a method for preparing a confined catalyst for electrocatalysis is provided, which comprises the following steps: 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; S2, transferring the mixed solution to a tetrafluoroethylene-lined autoclave, heating to 110-130° C. for heat treatment for several hours to obtain a reaction mixture; S3, after the reaction mixture is cooled to room temperature, the color precipitate is separated from the reaction mixture by centrifugation, and washed thoroughly with DMF and ethanol several times to remove any unreacted starting material, and then dried at a temperature of 50-70°C to obtain the confined carrier material NH2-MIL-88B; S4, adding 4-6 mL of saturated FeCl3·6H2O solution dropwise to the above confined carrier material, grinding and drying, and then placing it in an argon atmosphere at 500-600°C for high-temperature carbonization for several hours to obtain a black carbonized powder; S5. 4-6 mL of saturated FeCl3·6H2O solution was added to the carbonized powder again, and the powder was dried. The powder was then placed in a mixed atmosphere of hydrogen and argon at 350-450 °C (the volume ratio of hydrogen to argon was 3:97-7:93) for reduction reaction for several hours to obtain the confined catalyst Fe@NH2-MIL-88B.

[0055] The key point of the present invention is to use sub-nanometer Fe 0 The confinement is in NH2-MIL-88B, reducing the metal dissolution of iron and the iron ion content in the treated water, and preparing 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 harsh external environment, optimizes the d-band center and enhances the adsorption capacity of nitrates, 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 nitrates.

[0056] Example 1: Figure 2 As shown, this embodiment provides a method for preparing a confined catalyst for electrocatalysis, which comprises the following steps: 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; S2, transferring the mixed solution into a 100 mL tetrafluoroethylene-lined autoclave, heating to 120°C for heat treatment for 20 hours to obtain a reaction mixture; S3, after the reaction mixture is cooled to room temperature, the color precipitate is separated from the reaction mixture by centrifugation, and washed thoroughly with DMF and ethanol for 7 times to remove any unreacted starting material, and then dried at 60°C for 48 hours to obtain the confined carrier material NH2-MIL-88B; S4, adding 5 mL of saturated FeCl3·6H2O solution dropwise to the above confined carrier material, grinding and drying, and then placing it in an argon atmosphere at 550°C for high temperature carbonization for 4 hours to obtain a black carbonized powder; S5. 5 mL of saturated FeCl3·6H2O solution was added to the carbonized powder again, and the powder was dried. The powder was then placed in a mixed atmosphere of hydrogen and argon (the volume ratio of hydrogen to argon was 5:95) at 400 °C for reduction reaction for 2 hours to obtain the confined catalyst Fe@NH2-MIL-88B.

[0057] Example 2: This example provides a method for preparing a confined catalyst for electrocatalysis, which comprises the following steps: 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; S2, transferring the mixed solution to a tetrafluoroethylene-lined autoclave, heating to 110° C. for heat treatment for 15 hours to obtain a reaction mixture; S3, after the reaction mixture is cooled to room temperature, the color precipitate is separated from the reaction mixture by centrifugation, and washed thoroughly with DMF and ethanol for 6 times to remove any unreacted starting material, and then dried at 50°C for 36 hours to obtain the confined carrier material NH2-MIL-88B; S4, adding 4 mL of saturated FeCl3·6H2O solution dropwise to the above confined carrier material, grinding and drying, and then placing it in an argon atmosphere at 500°C for high-temperature carbonization for 3 hours to obtain a black carbonized powder; S5. 4 mL of saturated FeCl3·6H2O solution was added to the carbonized powder again, and the powder was dried. The powder was then placed in a mixed atmosphere of hydrogen and argon (the volume ratio of hydrogen to argon was 3:97) at 350 °C for a reduction reaction for 1 hour to obtain the confined catalyst Fe@NH2-MIL-88B.

[0058] Example 3: This example provides a method for preparing a confined catalyst for electrocatalysis, which comprises the following steps: 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; S2, transferring the mixed solution to a tetrafluoroethylene-lined autoclave, heating to 130° C. for heat treatment for 30 hours to obtain a reaction mixture; S3, after the reaction mixture is cooled to room temperature, the color precipitate is separated from the reaction mixture by centrifugation, and washed thoroughly with DMF and ethanol for 8 times to remove any unreacted starting material, and then dried at 70°C for 72 hours to obtain the confined carrier material NH2-MIL-88B; S4, adding 6 mL of saturated FeCl3·6H2O solution dropwise to the above confined carrier material, grinding and drying, and then placing it in an argon atmosphere at 600°C for high-temperature carbonization for 5 hours to obtain a black carbonized powder; S5. 6 mL of saturated FeCl3·6H2O solution was added to the carbonized powder again, and the powder was dried. The powder was then placed in a mixed atmosphere of hydrogen and argon (the volume ratio of hydrogen to argon was 7:93) at 450 °C for reduction reaction for 3 hours to obtain the confined catalyst Fe@NH2-MIL-88B.

[0059] Example 4: Figure 3 As shown, this embodiment 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 contains 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.

[0060] Specifically, the preparation method of cathode ⑤ is as follows: first, Fe@NH2-MIL-88B, conductive carbon black and polyvinylidene fluoride provided in Example 1 are weighed in a mass ratio of 8:1:1 and mixed, and then the mixture is dissolved in N,N-dimethylformamide (concentration of 10 g / L) to prepare catalyst ink, and then the ink is coated on a carbon cloth (CC, 1.0×2.0 cm) to ensure that both sides are evenly coated, and dried at 60°C for 30 minutes to obtain an electrode sheet as cathode ⑤.

[0061] 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 by Fe@NH2-MIL-88B and the platinum sheet are used as the cathode ⑤ and the anode ④, respectively, and the distance between the two plates is kept between 0.5-2.0 cm. The electrolyte ③ is NaNO3 and the prepared nitrate solution (concentration of 60 mg / L, in terms of N) or nitrate-contaminated groundwater. The concentration of 60 mg / L was selected because the nitrate concentration in the nitrate-contaminated groundwater actually collected was 55 mg / L, which was approximately 60 mg / L in the simulation experiment. The nitrate content of the water samples at each time point was determined using ion chromatography, and the nitrate removal rate was calculated.

[0062] Comparative Example 1: This comparative example provides a method for preparing a comparative catalyst, which comprises the following steps: 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; S2, transferring the mixed solution into a 100 mL tetrafluoroethylene-lined autoclave, heating to 120°C for heat treatment for 20 hours to obtain a reaction mixture; S3, after the reaction mixture is cooled to room temperature, the color precipitate is separated from the reaction mixture by centrifugation, and washed thoroughly with DMF and ethanol for 7 times to remove any unreacted starting material, and then dried at 60°C for 48 hours to obtain the carrier material NH2-MIL-88B; S4. Add 5 mL of saturated FeCl3·6H2O solution dropwise to the above-mentioned support material and dry it. Then, 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 (the volume ratio of hydrogen to argon is 5:95) at 400°C for reduction reaction for 2 hours to obtain a non-confined comparison catalyst Fe / NH2-MIL-88B.

[0063] Performance test: 1. The electrocatalytic degradation effect of Fe@NH2-MIL-88B, a confined catalyst prepared in the embodiment of the present invention, on nitrate removal is enhanced, and the efficiency is significantly improved; Specifically, the electrocatalytic nitrate removal experiment is carried out in a single-chamber electrolytic cell (50 mL), using a GPS305D DC regulated power supply. In the two-electrode system, a platinum sheet is used as the anode, and the electrode sheet prepared in Example 4 is used as the cathode, and the distance between the two plates is controlled to be maintained at 0.5-2.0 cm. 30 mL of the electrolyte contains 60 mg / L nitrate (in terms of N) and 0.21g Na2SO4 sulfate. A current of 50 mA is applied to the system, and samples are taken every 30 min. The nitrate content of the water sample at each time point is measured using ion chromatography, and the nitrate removal rate is calculated. Under optimal conditions, 91.87% of nitrate can be degraded within 3 h. At the same time, the Fe@NH2-MIL-88B catalyst has good cycle stability, and there is still a removal rate of about 90% after repeated use for 5 times.

[0064] 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.

[0065] Table 1 Comparison of cathode material performance for electrocatalytic nitrate removal ; As can be seen from Table 1, the catalytic material Fe@NH2-MIL-88B prepared by the present invention removes 91.87% of nitrate in just 3 hours, and the service time is much shorter than the time required for cathode materials reported in other literatures, and the nitrate removal efficiency is also much higher than other iron-based and metal catalysts. This shows the excellent electrocatalytic reduction performance and ultra-high electrocatalytic nitrate removal efficiency of Fe@NH2-MIL-88B.

[0066] Second, the morphology and microstructure of the NH2-MIL-88B, Fe@NH2-MIL-88B and Fe / NH2-MIL-88B prepared in Example 1 before and after carbonization were analyzed by SEM. Figure 4 As shown. Figure 4 (a) It was observed that the prepared NH2-MIL-88B material exhibited good dispersion and uniformity with a spindle morphology. Figure 4 (b) shows cracks in the carbonized NH2-MIL-88B material, which is due to the partial retention of open pore structure of the porous carbon substrate after pyrolysis, which promotes nitrate diffusion and alleviates mass transfer limitations. Figure 4 In (c), Fe / NH2-MIL-88B is also spindle-shaped, but a large number of iron clusters are gathered on the surface of the material. Figure 4 In (d), Fe@NH2-MIL-88B is still a spindle, but there are only a few iron clusters on the loading surface, which is because most of the Fe is encapsulated into the pores of NH2-MIL-88B. This proves that the successful confinement of Fe into NH2-MIL-88B can increase the active sites, so that more nitrate molecules have the opportunity to contact the catalyst surface to participate in the electrocatalytic reaction, thereby improving the electrocatalytic removal of nitrate catalytic activity.

[0067] The element distribution of Fe@NH2-MIL-88B sample was determined by X-ray spectroscopy (EDX), such as Figure 5 As shown. It can be seen that O, N, C and Fe elements are evenly distributed in almost the same position, indicating that Fe is successfully confined into NH2-MIL-88B. Among them, O, N and C elements appear more dense, because they are the components of MOF, while Fe elements are more dispersed than O, N and C elements. This shows that Fe confined in NH2-MIL-88B will not gather together to form a large number of clusters, which is beneficial to increase the reaction contact area, help reduce mass transfer resistance, and make it easier for nitrate to contact the catalyst surface, thereby promoting the electrocatalytic removal of nitrate reaction.

[0068] The surface properties and chemical states of Fe@NH2-MIL-88B were analyzed by X-ray photoelectron spectroscopy (XPS). Figure 6 shown in Figure 6 There are three pairs of peaks around 707.2 eV (Fe 2p 3 / 2 ) and 722.1 eV (Fe 2p 1 / 2 ) corresponds to Fe 0 , about 712.6 eV (Fe 2p 3 / 2 ) and 724.0 eV (Fe 2p 1 / 2 ) corresponds to Fe2+ , about 714.3 eV (Fe 2p 3 / 2 ) and 726.0 eV (Fe 2p 1 / 2 ) corresponds to Fe 3+ .Fe 2+ and Fe 3+ The carrier material is NH2-MIL-88B, in which the confined Fe 0 The peak area of ​​occupies most of the peak area, which indicates that the 0 Materials, Fe 0 There is no chemical bond formed with other elements, and there are abundant active sites on the surface, which enables more nitrate to be reduced per unit time, thus improving the overall reaction efficiency of electrocatalytic reduction of nitrate.

[0069] In order to evaluate the specific surface area and pore structure of the catalysts, N2 adsorption-desorption isotherms were performed on Fe@NH2-MIL-88B and NH2-MIL-88B, as shown in Figure 2. Figure 7 As shown. The N2 adsorption-desorption isotherms of Fe@NH2-MIL-88B all show the typical IV type isotherm of mesoporous materials, with obvious H3 type hysteresis loops, indicating that Fe@NH2-MIL-88B should have a stronger adsorption capacity for nitrate. The IV type isotherm of the NH2-MIL-88BN2 adsorption-desorption isotherm does not show obvious hysteresis loops, which means that the NH2-MIL-88BN2 pore size is large, which will limit the performance of electrocatalytic removal of nitrate. In addition, since the surface area and most probable pore size of Fe@NH2-MIL-88B are 5.287 m 2 / g and 3.766 nm are both smaller than the surface area and most probable pore size of NH2-MIL-88B, proving the successful preparation of the confined material Fe@NH2-MIL-88B, because confinement will lead to pore closure or shrinkage 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 more effectively adsorb nitrates, ensuring that more reactants enter the active sites and undergo reduction reactions, thereby improving the reaction efficiency.

[0070] The carbon structure of Fe@NH2-MIL-88B was further analyzed using Raman spectroscopy. Figure 8 As shown; among them, the D band (1341cm −1 ) and G band (1588 cm −1 ) correspond to graphene defects and sp 2 The coplanar vibration of carbon atoms and I D / I G The ratio indicates the degree of defects in the carbon material. D / I G A ratio less than 1 indicates that the carbon material has higher crystallinity and fewer defects, a more ordered graphite structure, and a more regular arrangement of carbon atoms, 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, avoiding excessive local charge accumulation, thereby optimizing reaction conditions and efficiency.

[0071] In order to obtain the electrochemical performance of Fe@NH2-MIL-88B catalyst, the catalytic materials Fe@NH2-MIL-88B, Fe / NH2-MIL-88B and NH2-MIL-88B were coated on 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).

[0072] The electrochemical characterization of Fe@NH2-MIL-88B and other catalysts was carried out by electrochemical workstation. Fig. 9 As 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.

[0073] The linear sweep voltammetry (LSV) was used to measure the LSV curves of Fe@NH2-MIL-88B, Fe / NH2-MIL-88B and NH2-MIL-88B. Fig.10 At the same potential, Fe@NH2-MIL-88B exhibits a higher current density, indicating that the Fe@NH2-MIL-88B catalyst has excellent conductivity, which is conducive to the transfer of electrons to the surface of the Fe@NH2-MIL-88B catalyst, thereby significantly improving the catalytic activity of the electrocatalytic reduction of nitrate.

[0074] The Tafel slopes of Fe@NH2-MIL-88B, Fe / NH2-MIL-88B and NH2-MIL-88B were calculated based on the LSV curves. Fig.11 The Tafel slope indicates the change in overpotential required when the current changes tenfold, so the smaller the absolute value of the slope, the better. The smaller the slope, the smaller the voltage rise and the lower the energy consumption. Fig.11It can be seen that the Tafel slope value of Fe@NH2-MIL-88B catalyst is also the smallest, which also indicates that the electrocatalytic nitrate removal reaction kinetics of Fe@NH2-MIL-88B catalyst is faster and has better catalytic activity.

[0075] 3. The electrocatalytic removal of nitrate was carried out in a single-chamber electrolytic cell (50 mL) using a GPS305D DC regulated power supply. In the two-electrode system, a platinum sheet was used as the anode, and the prepared Fe@NH2-MIL-88B catalytic material was coated on a 1.0×2.0 cm carbon cloth as the cathode, and the distance between the two plates was controlled to be 0.5-2.0 cm. The 30 mL electrolyte contained 60 mg / L nitrate (in terms of N) and 0.21g 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. The ammonia nitrogen concentration during the reaction was determined by Nessler's reagent spectrophotometry, and the ammonia nitrogen conversion rate was calculated. The experimental conditions and operating steps were kept unchanged, and the experiment was repeated three times to obtain the nitrate degradation effect and the yield and conversion rate of ammonia nitrogen during the electrocatalytic reduction of nitrate.

[0076] Fig.12 The electrocatalytic nitrate removal degradation process of Fe@NH2-MIL-88B is depicted (the degradation conditions are as follows: 60 mg / L NO3 - -N 30 mL, 0.21 g Na2SO4, current 50 mA, carbon cloth area 2.0 cm 2 , pH = 7), used to determine the nitrate, nitrite and ammonia content during electrolysis. 2 Under a constant current of , the nitrate concentration of Fe@NH2-MIL-88B dropped from 100% to 8.89% within 3 hours, and the nitrate removal rate was 91.87%. The concentration of nitrite was 0 at the initial 0min and the end 180min, and only a very small amount of nitrite was generated as an intermediate product in the middle process, which means that there will be no nitrite residue after the degradation is completed, and no secondary pollution will be caused. 71.25% of the removed nitrate was converted into ammonia, which was then converted into ammonia at the platinum anode through Cl - The reaction converts it into harmless nitrogen gas.

[0077] During the use of Fe@NH2-MIL-88B catalyst, if it is affected by the internal and external environment and the catalyst is passivated, the reaction efficiency will be greatly reduced, affecting the continuous progress of the catalytic reaction. In order to further confirm the stability of Fe@NH2-MIL-88B catalyst, 5 cycle experiments were carried out (degradation conditions are as follows: 60 mg / L NO3- -N 30 mL, 0.21 g Na2SO4, current 50 mA, carbon cloth area 2.0 cm 2 , pH = 7). During the experiment, the electrode sheet was carefully cleaned and dried after each electrocatalytic degradation, and then the electrode sheet was put into the electrocatalytic reduction system to repeatedly degrade new nitrates. Fig.13 As shown, the nitrate removal efficiency of Fe@NH2-MIL-88B after 5 cycles is about 90%, and the ammonia selectivity is about 70%, which has strong comprehensive performance stability.

[0078] In order to explore the influence of confinement effect on the electrocatalytic reduction degradation of nitrate, the above operation steps and other degradation conditions were kept unchanged (degradation conditions were as follows: 60 mg / L NO3 - -N 30 mL, 0.21 g Na2SO4, current 50 mA, carbon cloth area 2.0 cm 2 , pH = 7), using 1.0×2.0 cm Fe@NH2-MIL-88B, Fe / NH2-MIL-88B and NH2-MIL-88B as anodes, respectively, to carry out degradation experiments. After 3 h of degradation, the degradation rate of nitrate using different materials was measured. The experimental conditions and operating steps were kept unchanged, and the experiment was repeated three times to obtain the degradation rate of nitrate using different materials. The results are shown in Fig.14 shown.

[0079] from Fig.14 It can be seen that the degradation rate of nitrate by Fe@NH2-MIL-88B catalyst has reached 91.87% after 3 h of reaction, while the degradation rate of nitrate by Fe / NH2-MIL-88B catalyst is only 62.06% after 3 h of reaction. The degradation rate of nitrate by NH2-MIL-88B alone is 55.19% after 3 h. In just 3 hours, Fe@NH2-MIL-88B catalyst removed up to 91.87% of nitrate, showing an ultra-high removal efficiency, which is much higher than other similar catalysts reported. It can be seen that Fe@NH2-MIL-88B is an excellent catalytic material for electrocatalytic reduction of nitrate.

[0080] Fourth, in the process of electrocatalytic reduction, current density is undoubtedly an important factor affecting the reaction. In order to verify the effect of current density on the electrocatalytic reduction of nitrate, the current density was controlled to 10 mA / cm 2 , 30 mA / cm 2 , 50 mA / cm 2 , 100mA / cm 2The degradation experiment was carried out by keeping 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 follows Fig.15 shown.

[0081] from Fig.15 It can be seen that 50 mA / cm 2 The nitrate degradation effect was the best when the current density was increased from 50 mA / cm 2 During the process, the degradation effect continued to improve, but the increase became smaller and smaller, and the selectivity of ammonia reached the highest. However, when the current exceeded 50 mA / cm 2 When the current density is too high, the degradation effect and the apparent rate constant decrease slightly. This may be because the voltage in the reaction system is too high when the current is too large, resulting in a large number of side reactions. The higher current density may accelerate the cathode hydrogen evolution reaction, thereby + More active sites are provided on the surface of Fe@NH2-MIL-88B catalyst, which limits 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 .

[0082] 5. The initial pH value of the solution is an important condition that cannot be ignored in the electrocatalytic removal of nitrates, which affects the proton concentration and removal efficiency. In order to explore the effect of the initial pH of the reaction system on the electrocatalytic reduction and degradation of nitrates, the present invention sets different initial pH values ​​(pH = 3, 5, 7, 9, 11) to analyze and evaluate the electrocatalytic removal of nitrates by Fe@NH2-MIL-88B, and keeps the above-mentioned 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 follows: Fig.16 shown.

[0083] from Fig.16It can be seen that when the initial pH is 7, the nitrate removal capacity and ammonia selectivity reach the maximum value. The result data show that neutral conditions are more conducive to 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, thereby slightly reducing the electrocatalytic performance. 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. Under alkaline conditions, on the one hand, the hydroxide ions in the solution will neutralize the protons in the reaction process and reduce the reaction efficiency. On the other hand, the hydroxide anions will occupy the active sites and also reduce the reaction efficiency. The electrocatalytic nitrate removal rate of Fe@NH2-MIL-88B reached a maximum of 91.87% at pH 7, and it always maintained a good removal rate in a wide pH range, which shows that the catalyst maintains stable catalytic activity in groundwater with different pH values ​​and has a wide range of applications, because NH2-MIL-88B is used to confine Fe 0 Can buffer the effects of pH changes.

[0084] VI. In order to explore the effect of initial nitrate concentration on electrocatalytic removal of nitrate, the present invention conducted electrocatalytic removal of nitrate under the conditions of controlling initial nitrate concentrations to 25 mg / L, 60 mg / L, 100 mg / L, and 150 mg / L (all in terms of N) while keeping other conditions unchanged. The degradation experiment was conducted while keeping the above-mentioned operation steps and other degradation conditions unchanged. Then, the experiment was repeated three times while keeping the experimental conditions and operation steps completely unchanged. The results are as follows: Fig.17 shown.

[0085] from Fig.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, more than 85% of the nitrate was removed in the reaction systems with different concentrations. They all have good nitrate degradation performance, which proves that Fe@NH2-MIL-88B also has excellent removal ability for high-concentration nitrate-contaminated groundwater.

[0086] VII. In order to explore the effect of different initial chloride ion concentrations on the electrocatalytic removal of nitric acid, electrocatalytic degradation tests were carried out under the conditions of controlling chloride ion concentrations of 0, 10 mg / L, 30 mg / L, 50 mg / L, and 100 mg / L, and the degradation experiment was carried out while keeping the above operation steps and other degradation conditions unchanged. Then, the experiment was repeated three times while keeping the experimental conditions and operation steps completely unchanged. The results are as follows: Fig.18 shown.

[0087] from Fig.18It can be seen that different initial chloride ion concentrations have a certain effect on the nitrate degradation effect. As the chloride ion concentration in the electrolyte increases, the nitrate degradation effect is reduced. This may be because: the presence of chloride ions will cause a side reaction 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 - The 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 inflection point chlorination theory, in which chloride can be oxidized to form HClO and further oxidized to form nitrogen, as shown in equations (1)-(3).

[0088] ; 8. Electrocatalytic nitrate reduction involves two pathways: one is the regulation of active hydrogen produced at the active site, and the other is the reduction of cathode electrons. Tert-butyl alcohol (TBA) is a specific hydrogen radical quencher, which is used to determine whether the electrocatalytic nitrate reduction process is mediated by active hydrogen or dominated by electron regulation. Electrocatalytic degradation tests were carried out under the conditions of controlling TBA concentrations of 0, 10 mg / L, 30 mg / L, and 50 mg / L, respectively. The degradation experiment was carried out keeping the above operating steps and other degradation conditions unchanged, and then the experimental conditions and operating steps were kept completely unchanged for three repeated experiments. The results are as follows Fig.19 shown.

[0089] like Fig.19 As shown in Figure 2, the residual nitrate gradually increases with the increase of TBA concentration. The electrocatalytic removal of nitrate by Fe@NH2-MIL-88B 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.

[0090] IX. Based on the previous characterization results of the present embodiment, using Fe atoms as active sites in the model, density functional theory (DFT) calculations were performed to study the reaction mechanism and reveal the reasons for the high performance of Fe@NH2-MIL-88B in nitrate reduction. The reaction path and reaction free energy change of the electrocatalytic removal of nitrate by the catalytic material Fe@NH2-MIL-88B, such as Fig. 20 , Fig.21 shown.

[0091] The reduction of nitrate to ammonia is accompanied by the transfer of nine protons and eight electrons. Fig. 20The reaction pathway is used to electrocatalytically reduce nitrate to ammonia on the Fe site. The first step is the adsorption of nitrate, which is the adsorption of nitrate onto the catalyst Fe@NH2-MIL-88B without the need for electron transfer, followed by the gradual reduction of *NO3, which generates *NO2, *ON, *ONH, *ONH2, *O, *OH in sequence and finally desorbs to obtain the catalyst *. The specific reaction formula is as follows: ; Fig.21 The free energy change diagram of Fe@NH2-MIL-88B and Fe / NH2-MIL-88B with nitrate reduction is shown in Figure 2. The reaction path is further calculated to calculate the free energy of each intermediate on the Fe site. The lower energy indicates that nitrate is more easily adsorbed on the Fe@NH2-MIL-88B site to form *NO3. Then the NO bond of *NO3 breaks and gradually reacts with H + Combined to generate *NO2, *ON, *ONH, *ONH2, *O and *OH. When using conventional iron-based materials for electrocatalytic removal of nitrate, the rate-limiting step of the electrocatalytic removal of nitrate is the nitrite generation process, because the high energy of the lowest unoccupied molecular π* orbital of nitrate limits the charge injection, thus limiting the overall electrocatalytic removal of nitrate efficiency.

[0092] However, the rate of formation of *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, so 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 having significant electrocatalytic activity for nitrate removal.

[0093] like Fig. 22As shown in (a) and (b), in order to compare the nitrate adsorption capacity and interaction of the catalyst, the differential charge of nitrate and Fe / NH2-MIL-88B and Fe@NH2-MIL-88B was calculated, where green represents a decrease in electron cloud density and 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 energy of Fe@NH2-MIL-88B and Fe / NH2-MIL-88B was calculated. The charge density of *NO3 accumulated on Fe@NH2-MIL-88B is 2.18 eV higher than that of 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 promote the removal of electrocatalytic reduction reaction.

[0094] In summary, the confined catalyst Fe@NH2-MIL-88B prepared in the embodiment of the present invention can solve the problem of low efficiency of electrocatalytic nitrate removal by increasing active sites, enhancing adsorption capacity and accelerating electron transfer at multiple levels and dimensions, and can provide a more economical and effective cathode material and process technology for the treatment of electrocatalytic nitrate-contaminated groundwater.

[0095] Based on the above-mentioned ideal embodiments of the present invention, the relevant staff can make various changes and modifications without departing from the technical concept of the present invention through the above-mentioned description. The technical scope of the present invention is not limited to the contents of 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; heat-treating the mixed solution to obtain a reaction mixture; 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, and then carbonized at high temperature to obtain a carbonized powder; 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.

2. The method for preparing a confined catalyst for electrocatalysis according to claim 1, characterized in that: The molar ratio of the 2-aminoterephthalic acid to ferric chloride is 1:(0.8-1.2).

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

4. The method for preparing a confined catalyst for electrocatalysis according to claim 1, characterized in that: The temperature of the heat treatment is 110-130°C.

5. The method for preparing a confined catalyst for electrocatalysis according to claim 1, characterized in that: The high-temperature carbonization is carried out in an inert gas atmosphere at a temperature of 500-600°C.

6. The method for preparing a confined catalyst for electrocatalysis according to claim 1, characterized in that: 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.

7. A confined catalyst prepared by the preparation method according to any one of claims 1 to 6.

8. Use of the confined catalyst as claimed in claim 7 in the electrocatalytic reduction of nitrates.

9. Use of the confined catalyst as claimed in claim 7 in removing nitrates from water.

10. An electrocatalytic reduction device, comprising a cathode and an anode, characterized in that: The cathode is made of the confined catalyst described in claim 7.

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