Efficient electrocatalyst for reducing nitrate as well as preparation method and application of efficient electrocatalyst

Abstract

The invention belongs to the technical field of environmental engineering and electrocatalysts, and discloses an efficient electrocatalyst for reducing nitrate as well as a preparation method and application of the efficient electrocatalyst for reducing nitrate. The efficient electrocatalyst is a flaky compound FeSxCeOy synthesized by cerium oxide and ferrous sulfide, the FeSx and CeOy heterogeneous interface has a plurality of nanoscale microcosmic wrinkles, is porous and has a large specific surface area. According to the preparation method, a two-step hydrothermal synthesis method is adopted, and by controlling the proportion of cerium, iron, sulfur and other elements and the process, the obtained catalyst has a unique microstructure, has excellent activity of electrically reducing nitrate into ammonia, has optimal catalytic selectivity on ammonia, can greatly increase electron transfer, regulation and surface adsorption, and can be used for preparing the catalyst for the industrial production. The efficiency and the yield of converting nitrate into ammonia are effectively improved, and the method can be widely applied to environmental engineering.

Description

Technical Field

[0001] The invention relates to the technical field of environmental engineering and electrocatalysts, and in particular to a high-efficiency electrocatalyst for reducing nitrates, a preparation method and application thereof. Background Art

[0002] Nitrates mainly come from industrial wastewater and automobile exhaust emissions, which are combined with nitrogen oxide pollutants to produce nitric acid pollutants. A large amount of nitrate pollutants enter the river and are absorbed by the human body, which can affect human health. Collecting nitrates in wastewater and directly reducing them to high-value-added chemicals such as ammonia under the action of electricity and light energy has pointed out a direction for scientists. In particular, electrocatalytic reduction of nitrate to ammonia, compared with the traditional Habor-Bosch high-temperature and high-pressure ammonia, uses other clean power generation methods such as wind energy and tidal energy to form a series circuit to enhance energy storage efficiency, mild and controllable conditions, reduce carbon emissions, achieve sustainable nitrogen cycle, and turn waste into treasure. Usually, transition metals copper and cobalt are considered to be promising catalysts for electrocatalytic reduction of nitrate to ammonia due to their unique d-zone electronic structure. In comparison, this type of catalyst still faces the disadvantages of low catalytic efficiency, poor activity, poor stability, and lack of variety. People continue to modify it, such as forming bimetallic, oxide, metal organic framework, single atom catalyst, etc., in order to efficiently realize nitrogen cycle and turn waste into treasure.

[0003] Composite catalysts refer to catalysts composed of two or more substances. Due to their special heterogeneous structures, they can adjust the physical, chemical or structural properties on the surface interface (surface reconstruction, crystal strain, electronic structure modulation, electron transfer ability, etc. at the interface) and have better catalytic activity. In comparison, so far, heterojunction catalysts have rarely been reported as electrocatalysts for the electrocatalytic reduction of nitrate to ammonia. For example, Cheng et al. [1] The CuO / Mn2O3 heterojunction catalyst was synthesized. This catalyst has a large specific surface area and can accelerate electron transfer at the interface, thereby accelerating the adsorption of nitrate and the desorption of ammonia. Li's research group [2] Two-dimensional CuO / Fe3O4 heterojunction nanoarrays were reported. The heterojunction interface improved electron transfer and enhanced the adsorption of nitrate and protons, showing excellent electrocatalytic reduction of nitrate to ammonia. The above results show that heterojunction catalysts will significantly change the adsorption of reactants, activation of reactants, conversion of intermediates, interfacial electron transfer and desorption of products at the interface, thereby affecting the kinetics, reaction pathways and related reaction mechanisms of the electrocatalytic reduction of nitrate. However, this type of catalyst has many problems, such as complex operation, difficult industrial scale-up, high catalytic cost and poor conductivity. Summary of the invention

[0004] The purpose of the present invention is to provide a high-efficiency electrocatalyst for reducing nitrates, a preparation method and its application in view of the above-mentioned deficiencies in the prior art. By simultaneously improving the raw material components, proportions and preparation processes, the product is controlled to have a special FexSy / CeO2 heterogeneous interface and micromorphology, and based on the heterogeneous interface effect and micromorphology, the electron transfer, regulation and surface adsorption are greatly increased, and the efficiency and yield of nitrate conversion to ammonia are effectively improved, thereby solving the above-mentioned technical problems.

[0005] In order to achieve the above object, the technical solution provided by the present invention is: A highly efficient electrocatalyst for reducing nitrates, characterized in that it is a sheet-like compound FeSx·CeOy synthesized from cerium oxide and ferrosulfide, on the surface of the compound, there are a large number of nano-scale microscopic wrinkled and porous FeSx and CeOy heterogeneous interfaces with a large specific surface area, wherein x and y represent non-stoichiometric and intermediate phase structures; the average size of the sheet-like compound FeSx·CeOy is 100-200 nm, and Fe, S, Ce, and O elements are evenly distributed on the surface of the compound to form a large number of active sites.

[0006] A method for preparing a highly efficient electrocatalyst for reducing nitrates is characterized in that it is synthesized by a two-step method, wherein cerium salt and iron salt are dissolved in a solvent respectively, and the materials are added in a ratio of cerium to iron of (0.2-5):1, and the composition, proportion and micromorphology of the obtained compound product are controlled under the set addition order, reaction time, reaction temperature, and drying conditions after the catalyst is synthesized, and finally a flaky compound FeSx·CeOy is obtained, comprising the following steps: S1: Synthesis of cerium oxide The cerium salt is dissolved in water to form a liquid A, and the alkali is dissolved in a solvent to form a liquid B. The liquid B is slowly added dropwise to the liquid A under continuous magnetic stirring until the liquid is completely added, and then the stirring is continued to obtain a mixed liquid. The mixed liquid is transferred to a hydrothermal reactor, reacted at 50-150°C, cooled, and then solid-liquid separated. The solid reactant is washed by centrifugation for multiple times with a common solvent, and then freeze-dried to obtain cerium oxide CeOy powder. S2: Preparation of iron source and sulfur source solutions The materials are added in a molar ratio of Fe:Ce=(0.2-5):1, and the iron source is first dissolved in an organic solvent to obtain an iron source solution C liquid dissolved in an organic solvent; and the sulfur source is dissolved in the same organic solvent to obtain a sulfur source solution D liquid; S3: Synthetic catalyst Liquid C and liquid D are added to a reaction vessel at a volume ratio of (0.2-5):1, stirred, and the temperature is controlled to react for 24 hours. After cooling, the obtained suspension is transferred to a hydrothermal reactor, and then the cerium oxide CeOy powder prepared in step S1 is added at a molar ratio of cerium oxide to ferrosulfide (0.2-5):1. The reaction is carried out under solvent thermal reaction conditions of 50-150°C for 24 hours. After cooling, solid-liquid separation is performed, and the particles are centrifuged and washed multiple times with organic and inorganic solvents. After vacuum and sufficient drying, the catalyst FeSx·CeOy is obtained.

[0007] A use of the high-efficiency electrocatalyst for reducing nitrate in electrocatalytic reduction of nitrate to ammonia.

[0008] Compared with the prior art, the present invention has at least the following beneficial effects: 1. The efficient electrocatalyst, preparation method and application of nitrate reduction provided by the present invention, through the simultaneous improvement of raw material components, proportions and preparation processes, controls the product to have a special FeSx / CeOy heterogeneous interface and micromorphology, and based on the heterogeneous interface effect and micromorphology, greatly increases electron transfer, regulation and surface adsorption. This new type of heterojunction catalyst, due to the formation of an interface effect to increase electron transfer, regulation and surface adsorption, can efficiently promote the activity and selectivity of electroreduction of nitrate to ammonia products, can effectively improve the efficiency and yield of nitrate conversion to ammonia, thereby solving a variety of problems existing in the prior art.

[0009] 2. The method for preparing an efficient electrocatalyst provided by the present invention adopts a controlled two-step method to synthesize a catalyst with a heterojunction interface. Cerium oxide is first synthesized, and then the iron salt is reduced by a suitable proportion of sulfur source to form FeSx. Then, by adjusting the iron-cerium ratio, a heterojunction interface efficient catalyst is formed in a hydrothermal reactor. The process is simple and easy to control, and the raw materials are cheap and readily available. The prepared catalyst has excellent properties such as activity and selectivity, which is of great significance for the effective utilization of nitrate ions into ammonia. In addition, the preparation process is simple, easy to synthesize, and the raw materials are cheap and readily available, which is easy to promote and apply industrially. 3. The high-efficiency electrocatalyst provided by the present invention is applied to the electrochemical reduction of nitrate. After actual testing, it has the best catalytic selectivity for ammonia, and the selectivity can reach 91% at -0.5 V. The maximum ammonia yield is 753 μg·h-1·mg-1, which is much higher than that of single-component catalysts. Its performance is also in a leading position among current catalysts, and it has good application prospects. BRIEF DESCRIPTION OF THE DRAWINGS

[0010] The above and / or additional aspects and advantages of the present invention will become apparent and easily understood from the description of the embodiments in conjunction with the following drawings, in which: Figure 1This is a schematic diagram of the process for preparing FeSx·CeOy catalyst according to an embodiment of the present invention; Figure 2 A scanning electron microscope image and a schematic diagram of element distribution of the FeSx·CeOy catalyst with a heterogeneous interface prepared in Example 1 of the present invention; Figure 3 Schematic diagram of transmission electron microscopy analysis of the FeSx·CeOy catalyst prepared in Example 1 of the present invention, wherein (a) is a high-resolution transmission electron microscopy image, (b) is a selected area electron diffraction image, and (c) is a high-angle annular dark field scanning transmission electron microscopy and the corresponding energy dispersive X-ray (EDS) element mapping image; Figure 4 X-ray diffraction test diagrams of the FeSx·CeOy catalyst and the FeSx or CeOy catalyst prepared in the embodiment of the present invention; Figure 5 Linear sweep voltammetry proves that the FeSx·CeOy catalyst prepared in Example 1 of the present invention has excellent catalytic activity; Figure 6 Schematic diagram showing the comparison of selectivity and ammonia production rate of the FeSx·CeOy catalyst prepared in Example 1 of the present invention and single CeOy and FeSx catalysts, wherein (a) is FeS x CeO y Catalyst, (b) CeO y Catalyst, (c) FeS x catalyst; Figure 7 The FeS prepared in different proportions in the embodiments of the present invention is x CeO y Schematic diagram comparing ammonia selectivity and ammonia yield at the Fe:Ce atomic ratio in the catalyst at -0.5V. DETAILED DESCRIPTION

[0011] The present invention is further described in detail below in conjunction with the accompanying drawings and embodiments so that those skilled in the art can implement the invention with reference to the description.

[0012] It should be understood that the terms such as “having”, “including” and “comprising” used herein do not exclude the existence or addition of one or more other elements or combinations thereof.

[0013] Unless otherwise specified, various raw materials, reagents, instruments and equipment used in the present invention can be purchased from the market or prepared by existing methods.

[0014] Basic Example See attached Figures 1 to 7The present embodiment provides a highly efficient electrocatalyst for reducing nitrates, a highly efficient electrocatalyst for reducing nitrates, characterized in that it is a sheet-like compound FeS synthesized from cerium oxide and ferrosulfide. x CeO y On the surface of the compound, there are many nanoscale microscopic wrinkles and porous FeS with large specific surface area. x and CeO y Heterogeneous interface, wherein x and y represent non-stoichiometric and intermediate phase structures, and their values ​​are not integers (can be expressed as approximate integers), which are specifically determined by the components, concentrations and reaction process conditions of the feedstock, for example, x can be between 1 and 5, and y can be between 1.5 and 2; the sheet compound FeS x CeO y The average size is 100-200 nm, and the Fe, S, Ce, and O elements are evenly distributed on the surface of the compound to form a large number of active sites.

[0015] A method for preparing the aforementioned high-efficiency electrocatalyst for reducing nitrates is synthesized by a two-step method, wherein cerium salt and iron salt are dissolved in a solvent respectively, and the materials are added in a ratio of cerium to iron of (0.2-5):1, and the composition, proportion and micromorphology of the obtained compound product are controlled under the set addition order, reaction time, reaction temperature, and drying conditions after the catalyst is synthesized, and finally a flaky compound FeS is obtained. x CeO y , which comprises the following steps: S1: Synthesis of cerium oxide The cerium salt is dissolved in water to form a liquid A, and the alkali is dissolved in a solvent to form a liquid B. The liquid B is slowly added to the liquid A under continuous magnetic stirring until the liquid is completely added, and then the stirring is continued to obtain a mixed liquid. The mixed liquid is transferred to a hydrothermal reactor, reacted at 50-150°C, cooled, and then solid-liquid separated. The solid reactant is washed by centrifugation several times with a common solvent, and then freeze-dried to obtain cerium oxide CeO y Particles; specifically: S1-1 dissolve the cerium salt in water to form a solution A with a concentration of 0.05 M to 1.15 M; The cerium salt is one of cerium chloride, cerium sulfate and cerium nitrate; S1-2 Dissolve sodium hydroxide in water to form solution B with a concentration of 1 M to 20 M; S1-3: Under the condition of continuous magnetic stirring at 1600 rpm, solution B was slowly added to solution A until the solution was completely added, and then stirring was continued for 20-40 min to obtain a mixed solution; S1-4: Transfer the entire mixed solution to a hydrothermal reactor, react at 50-150°C for 10-24 hours, then cool to room temperature and separate the solid and liquid; The solid reactant separated from S1-5 is centrifuged, washed and freeze-dried using a conventional solvent to obtain cerium oxide CeOy powder with a size of 20-50 nm and a porous needle-like structure.

[0016] The conventional solvents include inorganic solvents and organic solvents; the inorganic solvents include water, and the organic solvents include acetone, ethanol, methanol, etc.

[0017] S2: Preparation of iron source and sulfur source solutions The materials are added in a molar ratio of Fe:Ce=(0.2-5):1, and the iron source is first dissolved in an organic solvent to obtain an iron source solution C liquid dissolved in an organic solvent; in addition, the sulfur source is dissolved in the same organic solvent to obtain a sulfur source solution D liquid; specifically: S2-1 dissolving an iron source in an organic solvent to obtain an iron source solution C having a concentration of 0.05 M to 2.5 M; S2-2 dissolving the sulfur source in the same organic solvent to obtain a sulfur source solution D having a concentration of 0.03 M to 13.31 M; Wherein, the iron source is one of ferric sulfate, ferric nitrate and ferric chloride; The organic solvent is one of dimethyl sulfoxide, cyclohexane, ethylene glycol and dimethylformamide; The sulfur source is one of thioacetamide, thiourea, sodium sulfide or sodium thiosulfate; S3: Synthetic catalyst Liquid C and liquid D are added to a reaction vessel at a volume ratio of (0.2-5):1, stirred, and reacted for 24 hours under temperature control. After cooling, the obtained suspension is transferred to a hydrothermal reactor, and then the cerium oxide CeOy particles prepared in step S1 are added at a molar ratio of cerium oxide to ferrous sulfide (0.2-5):1, and reacted under solvent thermal reaction conditions of 50-150°C for 24 hours. After cooling, solid-liquid separation is performed, and the particles are centrifuged and washed multiple times with organic and inorganic solvents. After vacuum and sufficient drying, the catalyst FeSx·CeOy is obtained, which is specifically: S3-1: Add liquid C and liquid D into a glass container at a volume ratio of (0.2-5):1, continue stirring and mixing, react for 5 min, add an appropriate amount of organic solvent, continue stirring for another 5 min, and transfer the reaction solution to an oil bath; S3-2: Control the temperature within the range of 70-120 °C, react for 24 h under continuous stirring, and then cool to room temperature to obtain a suspension; S3-3: The obtained suspension is transferred to a hydrothermal reactor, and then the cerium oxide CeOy particles prepared in step S1 are added in a molar ratio of cerium oxide to ferrous sulfide (0.25-5):1, and the reaction is carried out under a solvothermal reaction condition of 50-150°C for 24 h, and then the solid-liquid separation is performed after cooling to room temperature; S3-4: The solid reactant is centrifugally washed multiple times with organic and inorganic solvents respectively, and after being fully dried in a vacuum drying oven, the catalyst FeSx·CeOy is obtained.

[0018] The invention relates to an application of the aforementioned high-efficiency electrocatalyst for reducing nitrate in the electrocatalytic reduction of nitrate to ammonia.

[0019] The following is combined with Figure 1-7 And a plurality of specific embodiments are further described in detail the present invention.

[0020] Example 1 The high-efficiency electrocatalyst, preparation method and application for reducing nitrate provided by the present invention are the specific implementation of the above basic embodiments, and provide a high-efficiency catalyst FeSx·CeOy, wherein x and y are determined by the sum of contributions of each compound. In this embodiment, the materials are fed according to the molar ratio of Fe:Ce=3:1, and the loss of Fe and the residue of Ce are controlled during the reaction. The components and acid dissolution ICP are actually measured for the solid sample of the catalyst finally prepared. As a result, the molar ratio of Fe:Ce in the final product is about 2.5:1, which includes four specific compound forms of FeS2, FeS, CeO2 and Ce2O3, and there are FeSx / CeOy composite heterogeneous interfaces formed between these four compounds respectively; the molar ratio of the four compounds is about FeS2:FeS=2:1, CeO2:Ce2O3=1:1, and its equivalent molecular formula is about 2FeS2·FeS·CeO2·Ce2O3, wherein the charge balance is achieved through Fe²⁺, CeO2:Ce2O3. 4 The synergistic effect of ⁺ / Ce³⁺ and S²⁻ / O²⁻ is achieved. In the final product of the catalyst, the overall molar ratio of the four compounds is approximately FeS2:FeS:CeO2:Ce2O3 =4:2:2:2.

[0021] See attached Figure 1 The components, formula and preparation method of the catalyst of the present invention are as follows: Step 1: Dissolve 0.88g of cerium nitrate hexahydrate in 20ml of deionized water (to obtain liquid A), dissolve 8.44g of NaOH in 15ml of deionized water (to obtain liquid B), add liquid B to liquid A, slowly drop liquid B under continuous magnetic stirring until it is completely added, and stir for 30 minutes to obtain a mixed solution; then transfer the entire mixed solution to a hydrothermal reactor, react at 100°C for 24 hours, cool, centrifuge and wash with deionized water and anhydrous ethanol for multiple times, and freeze-dry for 12 hours to obtain CeO2; See attached Figure 6 , the synthesized CeO2 is a white powder with a size of about 20-50 nm, and SEM shows a porous needle-like microstructure; Step 2: Dissolve 1.1g of ferric nitrate nonahydrate in 5ml DMF, and 2.04g of thioacetamide in 10ml DMF. Mix 3.2ml of the prepared ferric nitrate solution and 8ml of thioacetamide solution in a 50ml beaker, add 30ml DMF, stir for 10min, transfer to a 90℃ oil bath and react for 24h under continuous stirring. After cooling to room temperature, transfer the obtained suspension to a hydrothermal reactor, add 100mg of CeO2, and react at 180℃ for 24h. After cooling, wash with deionized water and anhydrous ethanol by centrifugation several times, and dry in a vacuum oven at 60℃ for 12h to obtain a FeSx·CeOy catalyst with an equivalent molecular formula of approximately 2FeS2·FeS·CeO2·Ce2O3.

[0022] See attached Figure 2 , Figure 3 The catalyst with an equivalent molecular formula of about 2FeS2·FeS·CeO2·Ce2O3 is a nanosheet with a size of 100-200 nm. On the surface of the compound, there are many nanoscale microscopic wrinkles and porous, large specific surface area FeSx and CeOy heterogeneous interfaces; the average scale of the sheet compound 2FeS2·FeS·CeO2·Ce2O3 is 100-200 nm, and the Fe, S, Ce, and O elements are evenly distributed on the surface of the compound to form a large number of active sites; by constructing this microstructure of iron sulfide, its specific surface area can be increased. Through EDS scanning, we can see that the distribution of Fe, S, Ce, and O elements on the catalyst is relatively uniform, which proves that their interaction is relatively good and a large number of active sites are conducive to improving the catalytic efficiency.

[0023] have Figure 3(a) High-resolution transmission electron image (HRTEM) shows that the adjacent lattice spacing of 0.311 nm and 0.289 nm corresponds to the (111) crystal plane of FeSx and the (200) crystal plane of CeOy, respectively, indicating that FeSx and CeOy have abundant interfaces. The synthesized FeSx·CeOy electrocatalyst has large-area contact of FeSx and CeOy heterojunction surfaces, which can serve as key active sites for eNO3RR and promote the reaction. Figure 3 (b) Our selected area electron diffraction of FeSx·CeOy samples shows the polycrystalline characteristics of FeSx·CeOy catalysts. The 3.03 and 3.33 nm-1 are formed by the (111) reflections of FeSx and (200) of CeOy, respectively, which are consistent with the X-ray diffraction pattern and high-resolution transmission electron microscopy image. High-angle annular dark field (HADDF) images and corresponding elemental analysis ( Figure 3 c) It proves that Fe and Ce elements are evenly distributed in the sample.

[0024] The preparation method provided by the embodiment of the present invention has a simple operation process, and the prepared catalyst can effectively improve the selectivity of electroreduction of nitrate to ammonia product.

[0025] In the process of preparing each solution, the alkali solution and the sulfur source organic solution used for the synthesis of cerium oxide should be freshly prepared and used immediately. The aqueous solution of cerium salt, the organic solution of iron salt, etc. can be prepared in advance.

[0026] Preparation of electrode: Using carbon paper as a carrier, weigh 2-10 mg of the FeSx·CeOy catalyst prepared by the above two-step method into a centrifuge tube, and add 100-500 μL of organic solvent and 20-100 μL of Nafion solution, and then perform ultrasonic dispersion; take 50-100 μL of the evenly dispersed catalyst solution, evenly drop it on the carbon paper, and then dry it under an infrared lamp for use.

[0027] Electrochemical reduction of nitrate to ammonia performance test: The electrochemical properties of the catalyst were tested using an electrochemical workstation, which used a three-electrode working system, a graphite rod as the counter electrode, and Ag / AgCl as the reference electrode. In 0.1M KOH+0.1M KNO3 electrolyte, we conducted a linear sweep voltammetry test and a 1.5 h current-time electrolysis test. The ammonia product was detected using the indophenol blue method, and the Faraday efficiency and ammonia yield of ammonia can be quantitatively obtained.

[0028] In this embodiment, the indophenol blue method is used to detect the faradaic efficiency of ammonium ions, which is comparable to the results of other reported ammonia detection methods. The specific steps are: 1) Determination of ammonium ion standard curve: Prepare 10 μg·mL-1 ammonium ion standard solution, dissolve 0.3146 g ammonium chloride in a 100 mL volumetric flask, and then dilute it 100 times to obtain the target standard solution.

[0029] 2) Preparation of series of gradient standard solutions: Sample from the standard solution in a gradient increment of 20 μL from 20 μL to 200 μL, add to a 15 mL sample tube, add the corresponding electrolyte to dilute to 2 mL, to obtain standard solutions of 0.1, 0.2 to 1 μg·mL-1 ammonium ions.

[0030] 3) Color developer configuration: The color developer for the indigo blue method requires the use of three solutions: A1, B1, and C1. Solution A1: Dissolve 5g salicylic acid, 5g trisodium citrate trihydrate and 4g sodium hydroxide in deionized water and add to a 100mL volumetric flask. Solution B1: Pipette 8.9 mL of 4% available chlorine sodium hypochlorite solution and dilute to 100 mL with deionized water. Solution C1: Weigh 0.25 g of sodium nitrosoferrocyanide dihydrate and dilute to a 25 mL volumetric flask with deionized water.

[0031] 4) Color reaction: Add 4 mL of color development solution A and 2 mL of color development solution C1 to 4 mL of blank electrolyte, then add 400 μL of color development solution C1 to make a blank control sample, then add 2 mL of color development solution A, 1 mL of color development solution B1 and 200 μL of color development solution C1 to 2 mL of standard ammonium ion solution to be tested. Let the reaction stand for 1 h at room temperature in the dark, and scan the wavelength with a UV-visible spectrophotometer with a scan width of 800 nm to 500 nm, where the maximum absorption wavelength of ammonia is 655 nm. Prepare a standard concentration curve based on the data obtained, and use the standard curve obtained by linear fitting to calculate the ammonia content in the sample solution to obtain the Faraday efficiency and ammonia yield of the reaction.

[0032] Ammonia Faraday efficiency and yield are calculated according to the following formula:

[0033] Where N is the number of transferred electrons to produce 1 mol of ammonia, F is the Faraday constant, t is the reaction time, n is the amount of substance producing ammonia, j is the current, V is the volume of the electrolyte, A is the electrode area, and is the concentration of ammonia produced.

[0034] See also Figure 6-Figure 7The catalyst has excellent electrocatalytic properties. The equivalent molecular formula of the synthesized catalyst is about 2FeS2·FeS·CeO2·Ce2O3. The catalyst has excellent electroreduction activity of nitrate to ammonia. The linear sweep voltammetry method in 0.1 MKOH solution proves that the catalyst has good catalytic activity (compared with a single catalyst) (results see Figure 4 ), at -0.4 V~-0.8 V (vs. RHE), when electrocatalytically reducing potassium nitrate in 0.1 M KOH solution, it has the best catalytic selectivity for ammonia, and the selectivity can reach 91% at -0.5 V. The maximum ammonia yield is 753 μg·h-1·mg-1, which is much better than the performance of a single-component catalyst and ranks among the top performances among the currently reported catalysts.

[0035] from Figure 4 From the X-ray diffraction pattern in the figure, it can be clearly seen that in the FeSx sample, it corresponds well to PDF# 42-1340, and the (110), (111), (200), and (210) crystal planes can be clearly seen, proving that it is a FeSx sample. In the CeOy sample, 28.8°, 33.3°, 47.7°, and 56.5° belong to the (111), (200), (220), and (311) crystal planes, respectively. It can be considered that the CeOy phase is its main component, corresponding to the standard card PDF# 34-0394. In addition to the above crystal planes, the (100) and (102) crystal planes of FeS can also be detected in the XRD spectrum of FeSx·CeOy, which may be due to the production of other iron-sulfur compounds during the crystal growth process.

[0036] Example 2 The present invention provides a high-efficiency electrocatalyst for reducing nitrate, a preparation method and an application thereof, which are substantially the same as those in the embodiment, except that, on the basis of embodiment 1, the configuration components and concentrations of the A, B, C and D solutions are changed accordingly, the cerium salt is cerium chloride, the iron source is ferric chloride, and the sulfur source is thiourea; dimethyl sulfoxide is selected as the organic solvent, and the materials are fed according to the Fe:Ce molar ratio of 1:1. The content distribution of the four compounds in the prepared high-efficiency catalyst FeSx·CeOy is different, and the specific FeSx / CeOy composite heterogeneous interface finally formed is slightly different.

[0037] In the final product of the catalyst, the overall molar ratio of the four compounds is about FeS2:FeS:CeO2:Ce2O3 =10:5:6:6. If the reaction conditions are adjusted, catalysts with other overall molar ratios can also be obtained (the proportion of each element in the whole changes), but the ratio of FeS2:FeS=2:1, CeO2:Ce2O3=1:1 in the product remains unchanged.

[0038] Example 3 The present invention provides a high-efficiency electrocatalyst for reducing nitrate, a preparation method and an application thereof, which are substantially the same as those in the embodiment, except that, on the basis of embodiment 1, the configuration components and concentrations of the A, B, C and D solutions are changed accordingly, the cerium salt is cerium sulfate, the iron source is ferric sulfate; the sulfur source is sodium sulfide; cyclohexane is selected as the organic solvent; and the materials are fed according to the Fe:Ce molar ratio of 1:3. The content distribution of the four compounds in the prepared high-efficiency catalyst FeSx·CeOy is different, and the specific FeSx / CeOy composite heterogeneous interface finally formed is slightly different.

[0039] In the final catalyst product, the overall molar ratio of the four compounds is approximately FeS2:FeS:CeO2:Ce2O3 =10:5:6:6.

[0040] Example 4 The high-efficiency electrocatalyst for reducing nitrates, the preparation method and the application thereof provided by the present invention are basically the same as those in the embodiment, except that, on the basis of Example 1, the configuration components and concentrations of the A, B, C and D solutions are changed accordingly, the materials are fed according to the Fe:Ce molar ratio of 1:5, the cerium salt is cerium sulfate, the iron source is ferric sulfate, the sulfur source is sodium thiosulfate, and ethylene glycol is selected as the organic solvent; in the final product of the catalyst, the overall molar ratio of the four compounds is approximately FeS2:FeS:CeO2:Ce2O3 = 10:5:30:30.

[0041] Example 5 The present invention provides a high-efficiency electrocatalyst for reducing nitrates, a preparation method and an application thereof, which are basically the same as those in the embodiment, except that, on the basis of Example 1, the configuration components and concentrations of the A, B, C and D solutions are changed accordingly, the materials are fed according to a Fe:Ce molar ratio of 5:1, the cerium salt is cerium sulfate, the iron source is ferric sulfate, the sulfur source is sodium thiosulfate, and ethylene glycol is selected as the organic solvent; in the final product of the catalyst, the overall molar ratio of the four compounds is approximately FeS2:FeS:CeO2:Ce2O3 = 50:25:6:6.

[0042] Example 6 The high-efficiency electrocatalyst for reducing nitrate, the preparation method and the application thereof provided by the present invention are basically the same as those in the embodiment, except that, on the basis of Example 1, the configuration components and concentrations of the A, B, C and D solutions are changed accordingly, and the materials are fed according to the Fe:Ce molar ratio of 2:1; in the final product of the catalyst, the overall molar ratio of the four compounds is approximately FeS2:FeS:CeO2:Ce2O3 = 4:2:3:3.

[0043] Example 7 The high-efficiency electrocatalyst for reducing nitrate, the preparation method and the application thereof provided by the present invention are basically the same as those in the embodiment, except that, on the basis of Example 1, the configuration components and concentrations of the A, B, C and D solutions are changed accordingly, and the materials are added according to the Fe:Ce molar ratio of 1:2; in the final product of the catalyst, the overall molar ratio of the four compounds is approximately FeS2:FeS:CeO2:Ce2O3 = 2:1:2:2.

[0044] The catalyst prepared by the above-mentioned embodiments of the present invention has a uniform powder size distribution and rich FeSx and CeOy heterogeneous interfaces. By adjusting the feeding ratio of the iron source and the sulfur source, the molar amount of the cerium oxide and the ferrosulfide, and the selection of the cerium salt, the iron salt and the organic solvent, combined with the reaction sequence, the reaction time, the required temperature, the configuration method, the hydrothermal time, the amount of solution added, the reactant ratio, the drying treatment method when synthesizing the catalyst and other technical means, the components (compound forms), the proportion, the distribution of each element in the final product, the size and microscopic morphology of the powder, etc. are controlled. On the surface of the compound, a FeSx and CeOy heterogeneous interface with a large number of nanometer-scale microscopic wrinkles and pores is formed. The catalyst has excellent activity in electro-reducing nitrate to ammonia, has the best catalytic selectivity for ammonia, can greatly increase electron transfer, regulation and surface adsorption, and finally significantly improves its electrocatalytic performance, effectively improves the efficiency and yield of nitrate conversion to ammonia, and can be widely used in environmental engineering.

[0045] It should be particularly pointed out that within the scope of the components, ratios and process parameters recorded in the present invention, other technical solutions obtained by specific selection can achieve the technical effects of the present invention, so they are no longer listed one by one. At the same time, other catalyst technical solutions obtained by using the components, ratios, preparation methods and applications equivalent to those recorded in the present invention are all included in the protection scope of the present invention.

[0046] In the description of the present invention, the description of the terms "one embodiment", "some embodiments", "specific embodiments", etc. means that the specific features, structures, materials or characteristics described in conjunction with the embodiment or example are included in at least one embodiment or example of the present invention. In the present invention, the schematic representation of the above terms does not necessarily refer to the same embodiment or example. Moreover, the specific features, structures, materials or characteristics described can be combined in any one or more embodiments or examples in a suitable manner.

[0047] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. For those skilled in the art, the present invention may have various modifications and variations. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present invention shall be included in the protection scope of the present invention.

Claims

1. A highly efficient electrocatalyst for reducing nitrates, characterized in that: It is a flaky compound FeS synthesized from cerium oxide and ferrosulfide. x CeO y On the surface of the compound, there are many nanoscale microscopic wrinkles and porous FeS with large specific surface area. x and CeO y Heterojunction, where x and y represent non-stoichiometric and intermediate phase structures.

2. The high-efficiency electrocatalyst for reducing nitrate according to claim 1, characterized in that: The plate-like compound FeS x CeO y The average size is 100-200 nm, and the Fe, S, Ce, and O elements are evenly distributed on the surface of the compound to form a large number of active sites.

3. A method for preparing a high-efficiency electrocatalyst for reducing nitrate according to claim 1 or 2, characterized in that: The method adopts a two-step synthesis method, wherein cerium salt and iron salt are dissolved in a solvent respectively, and the materials are added in a ratio of cerium to iron (0.2-5):

1. The composition, proportion and micromorphology of the obtained compound product are controlled by setting the addition order, reaction time, reaction temperature, and drying conditions after the catalyst synthesis, and finally the flaky compound FeS is obtained. x CeO y .

4. The method for preparing a high-efficiency electrocatalyst for reducing nitrate according to claim 3, characterized in that: It includes the following steps: S1: Synthesis of cerium oxide The cerium salt is dissolved in water to form a liquid A, and the alkali is dissolved in a solvent to form a liquid B. The liquid B is slowly added to the liquid A under continuous magnetic stirring until the liquid is completely added, and then the stirring is continued to obtain a mixed liquid. The mixed liquid is transferred to a hydrothermal reactor, reacted at 50-150°C, cooled, and then solid-liquid separated. The solid reactant is washed by centrifugation several times with a common solvent, and then freeze-dried to obtain cerium oxide CeO y Powder; S2: Preparation of iron source and sulfur source solutions The materials are added in a molar ratio of Fe:Ce=(0.2-5):1, and the iron source is first dissolved in an organic solvent to obtain an iron source solution C liquid dissolved in an organic solvent; and the sulfur source is dissolved in the same organic solvent to obtain a sulfur source solution D liquid; S3: Synthetic Catalyst Liquid C and Liquid D were added to the reaction vessel at a volume ratio of (0.2-5):1, stirred, and the temperature was controlled to continue the reaction for 24 hours. After cooling, the obtained suspension was transferred to a hydrothermal reactor, and then the cerium oxide CeO prepared in step S1 was added at a molar ratio of cerium oxide to ferrous sulfide (0.25-5):

1. y The powder was reacted under the solvent thermal reaction conditions of 50~150℃ for 24h, and then solid-liquid separation was performed after cooling. The particles were centrifuged and washed several times with organic and inorganic solvents, and then the catalyst FeS was obtained after vacuum and sufficient drying. x CeO y .

5. The method for preparing a high-efficiency electrocatalyst for reducing nitrate according to claim 4, characterized in that: Step S1 specifically includes the following steps: S1-1 dissolve the cerium salt in water to form a solution A with a concentration of 0.05 M to 1.15 M; The cerium salt is one of cerium chloride, cerium sulfate and cerium nitrate; S1-2 Dissolve sodium hydroxide in water to form solution B with a concentration of 1 M to 20 M; S1-3 at 1600 rpm -1 Under the condition of continuous magnetic stirring, slowly add solution B to solution A until it is finished, and continue stirring for 20-40 min to obtain a mixed solution; S1-4: Transfer the entire mixed solution to a hydrothermal reactor, react at 50-150°C for 10-24 hours, then cool to room temperature and separate the solid and liquid; The solid reactant separated from S1-5 was centrifuged, washed and freeze-dried using a conventional solvent to obtain cerium oxide CeO with a size of 20-50nm and a porous needle-like structure. y .

6. The method for preparing a high-efficiency electrocatalyst for reducing nitrate according to claim 5, characterized in that: Step S2 prepares an iron source and a sulfur source solution, and feeds the materials according to a molar ratio of Fe:Ce=(0.2-5):1, specifically comprising the following steps: S2-1 dissolving an iron source in an organic solvent to obtain an iron source solution C having a concentration of 0.05 M to 2.5 M; S2-2 dissolves the sulfur source in the same organic solvent to obtain a sulfur source solution D having a concentration of 0.03 M to 13.31 M.

7. The method for preparing a high-efficiency electrocatalyst for reducing nitrate according to claim 6, characterized in that: The iron source is one of ferric sulfate, ferric nitrate and ferric chloride; The organic solvent is one of dimethyl sulfoxide, cyclohexane, ethylene glycol and dimethylformamide; The sulfur source is one of thioacetamide, thiourea, sodium sulfide or sodium thiosulfate.

8. The method for preparing a high-efficiency electrocatalyst for reducing nitrate according to claim 5, characterized in that: Step S3 specifically includes the following steps: S3-1: Add liquid C and liquid D into a glass container at a volume ratio of (0.2-5):1, continue stirring and mixing, react for 5 minutes, add an appropriate amount of organic solvent, continue stirring for another 5 minutes, and then transfer the reaction solution to an oil bath; S3-2: Control the temperature within the range of 70-120 °C, react for 24 h under continuous stirring, and then cool to room temperature to obtain a suspension; S3-3: The obtained suspension is transferred to a hydrothermal reactor, and then the cerium oxide CeO prepared in step S1 is added in a molar ratio of cerium oxide to ferrous sulfide (0.2-5):

1. y The powder was reacted under solvent thermal reaction conditions of 50-150°C for 24 hours, and then cooled to room temperature for solid-liquid separation; S3-4: The solid reactant is centrifuged and washed several times with organic and inorganic solvents respectively, and then fully dried in a vacuum drying oven to obtain the catalyst FeS x CeO y .

9. Use of the high-efficiency electrocatalyst for reducing nitrate according to claim 1 or 2 in electrocatalytic reduction of nitrate to ammonia.

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