Iron-based catalyst as well as preparation method and application thereof

Abstract

The invention relates to an iron-based catalyst and a preparation method and application thereof. The iron-based catalyst provided by the invention comprises ferroferric oxide nanoparticles and a porous graphite carbon layer coating the ferroferric oxide nanoparticles. The iron-based catalyst provided by the invention has the characteristics of high catalytic performance, good cycle stability, economy, low cost and environmental friendliness.

Description

Technical Field

[0001] This application belongs to the technical field of electrocatalysis, and particularly relates to an iron-based catalyst, a preparation method thereof, and an application thereof. Background Art

[0002] With the in-depth development of global industrialization and the continuous growth of the population, water resources are facing serious pollution and shortage problems. Among them, the emissions of nitrogen compounds in industrial activities and the excessive use of nitrogen fertilizers in agriculture are the main factors leading to this problem. This phenomenon poses a major threat to human health and the integrity of the ecosystem. In severely polluted areas, eutrophication of water bodies has occurred, giving rise to phenomena such as red tides and algal blooms, which have greatly damaged the ecological balance. Currently, nitrate has become the main water pollutant in various regions of the world, triggering extensive academic research and public attention.

[0003] To promote the nitrate reduction reaction (NO3RR), various noble metals and transition metal catalysts have been synthesized in existing work. Currently, noble metals (such as palladium and platinum) are the most commonly used denitrification electrocatalysts, and these catalysts exhibit good catalytic activity in NO3RR. However, noble metals are costly and produce toxic by-products such as ammonia (NH4 + ) and nitrite (NO2 - ). Therefore, the reconstruction of transition metal catalysts - iron is an important direction because iron has the characteristics of rich reserves, low cost, and low toxicity. It is used as an economical, efficient, and environmentally friendly electron donor or reducing agent for waste treatment and environmental remediation. However, existing iron catalysts have problems such as rapid deactivation in water and metal leaching, and research on improving the long-term stability of iron catalysts is relatively scarce, seriously hindering the large-scale application of electrocatalysts in water purification and environmental remediation. Summary of the Invention

[0004] To solve the problems in the prior art, this application provides an iron-based catalyst, a preparation method thereof, and an application thereof. The iron-based catalyst of this application has high catalytic performance, good cycle stability, low economic cost, and environmental friendliness.

[0005] In a first aspect, this application provides an iron-based catalyst, which includes magnetite nanoparticles and a porous graphite carbon layer coating the magnetite nanoparticles.

[0006] The high conductivity of the porous graphite carbon layer (carbon shell) in the iron-based catalyst provided by this application provides an efficient path for electron transport, significantly reducing the charge transfer resistance, thereby improving the kinetic efficiency of the catalytic reaction. Secondly, the coating of the carbon shell effectively inhibits the aggregation and shedding of Fe3O4 nanoparticles, increasing the exposed area and stability of the active sites. In addition, the carbon shell enhances the adsorption and activation ability of Fe3O4 nanoparticles towards nitrate ions by adjusting the surface electronic structure, thereby further improving the catalytic efficiency. Meanwhile, the uniformly distributed nanoparticles provide more active sites.

[0007] In some embodiments, in the iron-based catalyst, the mass content of the iron element is 3% - 20%, such as 4%, 5%, 6%, 7%, 7.5%, 8%, 8.5%, 9%, 9.5%, 10%, 10.5%, 11%, 11.5%, 12%, 12.5%, 13%, 14%, 15%, 16%, 17%, 18%, 19% or any value therebetween. In some embodiments, the mass content of the iron element is 5% - 15%. In some embodiments, the mass content of the iron element is 8% - 12%.

[0008] When the content of the iron element is relatively low, the number of active sites on the catalyst surface is relatively small, resulting in limited nitrate ions participating in the reaction, so the denitrification performance is relatively low. When the content of the iron element is too high, the aggregation of Fe3O4 nanoparticles is aggravated, resulting in a decrease in the effective utilization rate of the active sites and a decrease in the denitrification efficiency per unit mass of the catalyst.

[0009] In some embodiments, the I D / I G ratio of the iron-based catalyst is 0.7 - 0.98, such as 0.8, 0.9, 0.91, 0.92, 0.93 or 0.94. In some embodiments, the I D / I G ratio is 0.9 - 0.95.

[0010] The intensity ratio I D / I G of the D band to the G band can reflect the structural defects of the graphite carbon layer. The larger the I D / I G ratio, the more structural defects there are in the graphite carbon layer.

[0011] In some embodiments, the average particle size of the magnetite nanoparticles is 5 - 30 nm, such as 7 nm, 9 nm, 10 nm, 11 nm, 12 nm, 13 nm, 14 nm, 15 nm, 16 nm, 18 nm, 20 nm, 22 nm, 24 nm, 26 nm, 28 nm, or any value therebetween. In some embodiments, the average particle size of the magnetite nanoparticles is 10 - 15 nm.

[0012] In some embodiments, the average thickness of the porous graphite carbon layer is 5 - 20 nm, such as 7 nm, 8 nm, 9 nm, 10 nm, 11 nm, 12 nm, 13 nm, 14 nm, 15 nm, 16 nm, 18 nm, or any value therebetween. In some embodiments, the average thickness of the porous graphite carbon layer is 8 - 12 nm.

[0013] In a second aspect, the present application provides a method for preparing an iron-based catalyst, which comprises the following steps:

[0014] S1: Provide a biomass carbon aerogel;

[0015] S2: Immerse the biomass carbon aerogel obtained in step S1 in an iron salt solution to obtain an impregnated product;

[0016] S3: Heat-treat the impregnated product obtained in step S2 to obtain a heat-treated product;

[0017] Optionally, step S4: Wash and dry the heat-treated product to obtain the iron-based catalyst.

[0018] The present application synthesizes an iron-based catalyst through a simple heat treatment process. During the heat treatment process, the iron salt undergoes a thermal decomposition reaction, and iron ions gradually aggregate and interact with the carbon source, thereby forming Fe3O4 nanoparticles uniformly loaded on the biomass carbon aerogel matrix.

[0019] In some embodiments, in step S1, the specific surface area of the biomass carbon aerogel is 300 m 2 / g - 1200 m 2 / g, such as 400 m 2 / g, 500 m 2 / g, 600 m 2 / g, 650 m 2 / g, 700 m 2 / g, 750 m 2 / g, 800 m 2 / g, 850 m 2 / g, 900 m 2 / g, 950 m 2 / g, 1000 m 2 / g, 1100 m 2 / g or any value therebetween. In some embodiments, the specific surface area of the biomass carbon aerogel is 600 m 2 / g - 1000 m 2 / g. In some embodiments, the specific surface area of the biomass carbon aerogel is 700 m 2 / g - 800 m 2 / g.

[0020] In some embodiments, in step S1, the pore size distribution of the biomass carbon aerogel is 0.5 nm - 0.8 nm, such as 0.53 nm, 0.55 nm, 0.57 nm, 0.59 nm, 0.6 nm, 0.61 nm, 0.62 nm, 0.63 nm, 0.64 nm, 0.65 nm, 0.66 nm, 0.67 nm, 0.68 nm, 0.69 nm, 0.7 nm, 0.73 nm, 0.75 nm or any value therebetween. In some embodiments, the pore size distribution of the biomass carbon aerogel is 0.6 nm - 0.7 nm.

[0021] In some embodiments, in step S1, the providing of the biomass carbon aerogel includes: mixing biomass with a eutectic molten salt of chloride and then performing carbonization treatment.

[0022] In some embodiments, the biomass is selected from wood, and the wood is preferably selected from one or more of beech, basswood, locust, eucalyptus, pine, toona sinensis, poplar, willow, peach, apricot, apple, pear, banyan, camphor, sophora, maple, cypress, oak, oak, fir, phoenix tree.

[0023] In some embodiments, the mass ratio of the biomass to the eutectic molten salt of chloride is 1:1 - 1:3, such as 1:1.3, 1:1.5, 1:1.6, 1:1.7, 1:1.8, 1:1.9, 1:2.0, 1:2.1, 1:2.2, 1:2.3, 1:2.4, 1:2.5, 1:2.7, 1:2.9 or any value therebetween. In some embodiments, the mass ratio of the biomass to the eutectic molten salt of chloride is 1:1.5 - 1:2.5.

[0024] When the mass ratio of the biomass to the eutectic molten salt of chloride is too low, the ionic action in the eutectic molten salt of chloride weakens, and it cannot effectively support the development of the carbon framework. The material obtained after carbonization treatment lacks sufficient micropores and mesopores. When the mass ratio of the biomass to the eutectic molten salt of chloride is too high, it is easy to increase the brittleness of the material and reduce its toughness, making the material more likely to crack and break during subsequent processing or application, thereby further reducing the porosity and mechanical properties of the material.

[0025] In some embodiments, the melting point of the chloride eutectic molten salt is 200°C - 900°C, such as 300°C, 400°C, 500°C, 600°C, 700°C or 800°C.

[0026] In some embodiments, the chloride eutectic molten salt is a eutectic molten salt of zinc chloride and potassium chloride.

[0027] In some embodiments, the carbonization treatment is carried out in an inert atmosphere.

[0028] In some embodiments, the temperature of the carbonization treatment is 600°C - 900°C, such as 650°C, 700°C, 750°C, 800°C or 850°C. In some embodiments, the temperature of the carbonization treatment is reached in an ascending temperature manner. In some embodiments, the heating rate is 1°C / min - 10°C / min, such as 2°C / min, 3°C / min, 4°C / min, 5°C / min, 6°C / min, 7°C / min, 8°C / min or 9°C / min.

[0029] In some embodiments, in step S2, the iron salt solution is selected from one or more of ferric citrate solution, ferric acetate solution and ferric chloride solution.

[0030] In some embodiments, calculated by iron element, the mass of the iron salt in the iron salt solution is 3% - 20% of the mass of the biomass carbon aerogel, such as 4%, 5%, 6%, 7%, 7.5%, 8%, 8.5%, 9%, 9.5%, 10%, 10.5%, 11%, 11.5%, 12%, 12.5%, 13%, 14%, 15%, 16%, 17%, 18%, 19% or any value therebetween. In some embodiments, the mass of the iron salt in the iron salt solution is 5% - 15% of the mass of the biomass carbon aerogel. In some embodiments, the mass of the iron salt in the iron salt solution is 8% - 12% of the mass of the biomass carbon aerogel.

[0031] In some embodiments, the concentration of the iron salt solution is 0.1 mg / L - 1 mg / L, such as 0.2 mg / L, 0.25 mg / L, 0.3 mg / L, 0.35 mg / L, 0.4 mg / L, 0.45 mg / L, 0.5 mg / L, 0.55 mg / L, 0.6 mg / L, 0.65 mg / L, 0.7 mg / L, 0.8 mg / L, 0.9 mg / L or any value therebetween. In some embodiments, the concentration of the iron salt solution is 0.2 mg / L - 0.6 mg / L. In some embodiments, the concentration of the iron salt solution is 0.3 mg / L - 0.5 mg / L.

[0032] When the concentration of the iron salt solution is relatively low, the number of active sites on the surface of the prepared catalyst is relatively small, resulting in limited nitrate ions participating in the reaction. Therefore, the denitrification performance is relatively low. However, when the concentration of the iron salt solution is too high, the aggregation of Fe3O4 nanoparticles in the prepared catalyst is aggravated, resulting in a decrease in the effective utilization rate of active sites and a decrease in the denitrification efficiency per unit mass of the catalyst.

[0033] In some embodiments, the temperature of the impregnation is 60°C - 100°C, such as 70°C, 75°C, 80°C, 85°C, 90°C or 95°C.

[0034] In some embodiments, the impregnation is equal-volume impregnation.

[0035] In some embodiments, the time of the impregnation is 1h - 24h, such as 2h, 4h, 6h, 10h, 12h, 15h, 18h, 20h or 22h.

[0036] In some embodiments, in step S3, the heat treatment is carried out in an inert atmosphere. In this application, the inert atmosphere is a nitrogen atmosphere or an argon atmosphere.

[0037] In some embodiments, the temperature of the heat treatment is 600°C - 900°C, such as 650°C, 700°C, 750°C, 800°C or 850°C. In some embodiments, the heat treatment temperature is reached in an ascending temperature manner. The heating rate is 1°C / min - 10°C / min, such as 2°C / min, 3°C / min, 4°C / min, 5°C / min, 6°C / min, 7°C / min, 8°C / min or 9°C / min.

[0038] In some embodiments, the time of the heat treatment is 0.5h - 24h, such as 2h, 4h, 6h, 10h, 12h, 15h, 18h, 20h or 22h.

[0039] In some embodiments, in step S4, the washing is carried out using commonly used acid solutions and water in the art, such as hydrochloric acid and water.

[0040] In some embodiments, in step S4, the drying temperature is 60°C - 100°C, such as 70°C, 75°C, 80°C, 85°C, 90°C or 95°C.

[0041] In some embodiments, in step S4, the drying time is 1h - 24h, such as 2h, 4h, 6h, 10h, 12h, 15h, 18h, 20h or 22h.

[0042] In some embodiments, the preparation method of the iron-based catalyst includes the following specific steps:

[0043] (1) Synthesis method of biomass carbon aerogel

[0044] The biomass material is thoroughly ground with the eutectic molten salt of zinc chloride and potassium chloride at a mass ratio of 1:1 to 1:3 to ensure uniform mixing. The mixture is placed in a porcelain boat and then put into a tube furnace. Under an argon protection atmosphere, it is heated to 600°C - 900°C, such as 700°C, at a heating rate of 1 - 10°C / min, such as 5°C / min, and maintained at this temperature for 1 - 10 hours, such as 2 hours, and then naturally cooled to room temperature.

[0045] After cooling, the sample is immersed in deionized water and thoroughly rinsed to remove excess soluble substances until the conductivity of the washing liquid drops below 10 μS / cm.

[0046] (2) Preparation of Fe3O4@C material

[0047] Ferric citrate solution, ferric acetate solution, and ferric chloride solution are respectively selected and placed in a beaker together with the carbon aerogel. Continuous stirring impregnation is carried out under a constant temperature condition of 60°C - 100°C, such as 80°C, and then drying treatment is carried out. Ensure that the precursor can be evenly distributed in the pore structure of the carbon aerogel, laying a foundation for the subsequent formation of a catalyst with excellent performance. Subsequently, the obtained mixture is heated to 600°C - 900°C, such as 700°C, at a heating rate of 1 - 10°C / min, such as 5°C / min, in an argon environment, and kept at this temperature for 0.5 - 10 hours, such as 1 hour. After cooling to room temperature, the product is thoroughly washed with acid and deionized water to remove unreacted raw materials and impurities, and finally dried at 60°C - 100°C, such as 80°C, for 5 - 20 hours, such as 12 hours, to obtain the iron-based catalyst.

[0048] In the third aspect, the present application provides the use of the iron-based catalyst described in the first aspect or the iron-based catalyst prepared by the preparation method described in the second aspect in nitrate reduction.

[0049] In the fourth aspect, the present application provides a method for electrocatalytic reduction of nitrate, which includes reducing nitrate in a three-electrode system, wherein the working electrode in the three-electrode system includes the iron-based catalyst described in the first aspect or the iron-based catalyst prepared by the preparation method described in the second aspect.

[0050] In some embodiments, the electrolyte in the electrolyte solution of the three-electrode system is selected from sodium chloride and / or sodium sulfate.

[0051] In some embodiments, the working voltage of the three-electrode system is -1.1V to -1.5V, such as -1.2V, -1.3V, or -1.4V.

[0052] In some embodiments, the initial concentration of nitrate is 50 mg / L - 200 mg / L, such as 70 mg / L, 100 mg / L, 130 mg / L, 150 mg / L or 170 mg / L.

[0053] Compared with the prior art, the beneficial effects of the present application are as follows:

[0054] 1) The iron-based catalyst Fe3O4 nanoparticles of the present application serve as an electron donor for nitrate reduction during the catalytic process. The excellent electrical conductivity of the carbon shell significantly improves the electron transfer rate during the reaction. At the same time, it has a high specific surface area and a rich micro-mesoporous structure. The microporous system realizes the selective adsorption of planar NO3 - (with a size of about 0.68 nm), significantly enhancing the contact between NO3 - and the catalyst surface.

[0055] 2) The encapsulation of the carbon shell in the iron-based catalyst of the present application effectively protects the iron active component, ensuring its long-term stability and excellent tolerance. Description of the Drawings

[0056] Figure 1 SEM images of the biomass carbon aerogels MSCA-1:1, MSCA-1:2 and MSCA-1:3 of the present application.

[0057] Figure 2 Specific surface area and pore size distribution diagrams of the biomass carbon aerogels MSCA-1:1, MSCA-1:2 and MSCA-1:3 of the present application.

[0058] Figure 3 Mechanical property diagrams of the biomass carbon aerogels MSCA-1:1, MSCA-1:2 and MSCA-1:3 of the present application.

[0059] Figure 4 SEM images of the iron-based catalysts in Examples 1-3.

[0060] Figure 5 TEM images of the iron-based catalysts in Examples 1-3.

[0061] Figure 6 Elemental mapping diagram of the iron-based catalyst in Example 1.

[0062] Figure 7 X-ray photoelectron spectroscopy diagram of the iron-based catalyst in Example 1.

[0063] Figure 8 XRD diagrams and Raman spectra of the iron-based catalysts in Examples 1-3.

[0064] Figure 9Shows the electrochemical performance of the iron-based catalyst in Example 1.

[0065] Figure 10 Shows the denitrification performance of the iron-based catalysts in Example 1, Example 4, and Example 5.

[0066] Figure 11 Shows the performance of the iron-based catalysts in Example 1-3 and Comparative Example 1-2 in the nitrate reduction reaction.

[0067] Figure 12 Shows the electrochemical performance of the iron-based catalysts in Example 1-3.

[0068] Figure 13 Shows the electrocatalytic performance of the iron-based catalyst in Example 1 in 5 cycles.

[0069] Figure 14 Shows the microstructure of the iron-based catalyst in Example 1 after cycling. Detailed implementation manners

[0070] To make the objectives, technical solutions, and advantages of this application clearer, the following further elaborates on this application in combination with examples and the accompanying drawings. The specific examples described herein are only used to explain this application and do not constitute any limitation to this application. In addition, in the following description, the description of well-known structures and technologies is omitted to avoid unnecessarily confusing the concepts of this disclosure. Such structures and technologies are also described in many publications.

[0071] The following further illustrates this application through specific examples and comparative examples.

[0072] In the following examples and comparative examples, the model of the scanning electron microscope (SEM) is Thermo Fisher, ApreoS. This scanning electron microscope is used to characterize the samples. Before the test, the sample is fixed on a standard SEM sample stage to ensure its stability and a flat surface facing the electron beam. During the test, the sample chamber is evacuated to below 10 -4 Pa to reduce the scattering of gas molecules on the electron beam. The acceleration voltage of the electron beam emitted by the electron gun is set to 15 kV. This voltage can not only ensure sufficient resolution to clearly present the microscopic morphology of the sample but also effectively control the damage of the electron beam to the sample. The working distance is set between 8 - 10 millimeters to obtain the best imaging effect. The secondary electron detector is used to collect signals. By adjusting parameters such as the detector gain and contrast, the image quality is optimized to ensure that the fine features on the sample surface, such as particle size, shape, distribution, and surface texture, can be clearly observed. For different regions and magnification requirements, multiple fields of view are photographed to comprehensively reflect the characteristics of the sample microstructure.

[0073] In the following examples and comparative examples, the model of the transmission electron microscope (TEM) is FEI Talos F200s. Before testing with this TEM, the sample to be analyzed was ultrasonically dispersed in absolute ethanol to form a uniform suspension. Subsequently, an appropriate amount of the suspension was aspirated with a pipette and dropped onto a double-sided grid, and dried under an infrared lamp to ensure that the sample was firmly attached to the copper grid. The prepared sample was placed on the sample rod of the TEM, carefully inserted into the microscope sample chamber, and the sample chamber was evacuated to below 10 -6 Pa to reduce electron scattering interference. The electron beam emitted by the electron gun was accelerated by an electric field with an acceleration voltage of 200 kV. The high-energy electron beam penetrated the sample and interacted with the sample atoms. By adjusting the condenser lens current and aperture size, the electron beam spot size and illumination uniformity were optimized to uniformly irradiate the sample with the electron beam. The objective aperture was used to select electrons with a specific scattering angle for imaging to highlight different structural information of the sample. For example, bright-field images were used to observe the overall morphology and general structure of the sample, and dark-field images could enhance the contrast of specific crystal structures or defects. By adjusting the magnification of the intermediate lens and projection lens, different resolution images could be obtained, ranging from low magnification to observe the overall distribution of the sample to high magnification for atomic-scale structural analysis. During the imaging process, the image quality was monitored in real time, and by adjusting parameters such as focus and astigmatism, clear and accurate TEM images reflecting the microscopic structural characteristics of the sample were obtained, including information such as lattice fringes, nanoparticle size and distribution, and crystal orientation, providing data support for in-depth study of the microscopic properties of the material.

[0074] In the following examples and comparative examples, the pore structure parameters were measured by a physical adsorption instrument (model ASAP 2020) to obtain the nitrogen physical adsorption and desorption isotherms, and then calculated by the BET method and t-plot method. First, an appropriate amount of the sample was pretreated. It was placed in a sample tube and connected to the instrument pretreatment station, and degassed at 150 - 300 °C for 4 - 8 h under vacuum according to the sample characteristics to make the vacuum degree stable below 10 -3 Pa to remove impurities and moisture. Then, the sample tube that had completed pretreatment and cooled to room temperature was transferred to the analysis station, evacuated to below 10 -5 Pa, the temperature of the analysis station was precisely maintained at liquid nitrogen temperature (77 K), and liquid nitrogen was continuously supplied using a Dewar flask. High-purity nitrogen with a purity of ≥99.999% was used as the adsorbate. Using the continuous flow method, starting from a relative pressure of 0.005, it was gradually increased to about 0.995, and the adsorption amount was recorded after stabilizing for 1 - 5 min at each relative pressure point until adsorption equilibrium was reached. After adsorption, the relative pressure was gradually decreased in the reverse direction for desorption and the desorption amount was recorded, thereby obtaining a complete nitrogen physical adsorption and desorption isotherm.

[0075] In the following examples and comparative examples, the model of the X-ray powder diffractometer is Rigaku, miniflex-600. The diffractometer was used to analyze the phase of the samples. A Cu target was selected as the X-ray source, and the characteristic X-ray wavelength it generated was The tube voltage was set at 40 kV and the tube current was set at 15 mA to ensure the generation of X-rays with sufficient intensity and stability for exciting the samples. The scanning mode was continuous scanning. The scanning range was set at 20° - 90°, the scanning speed was 10° / min, and the step size was 0.02° to comprehensively cover the main diffraction peaks of common crystalline materials and finely resolve adjacent peaks. The samples to be measured need to be first ground into uniform fine powder with a particle size less than 100 μm, and then evenly spread on the groove of a special sample holder and compacted with a glass slide to make the sample surface flat and flush with the surface of the sample holder. The test was carried out in an environment with a temperature of 25 ± 2°C.

[0076] In the following examples and comparative examples, the model of the Raman spectrometer is HORIBA, Lab RAM HR Evolution. The Raman spectra of the samples were analyzed before. The analysis conditions were as follows: a laser with a wavelength of 532 nm was used as the excitation light source, which could produce good Raman scattering on most samples and cause little damage to the samples. According to the characteristics and tolerance of the samples, the laser power was adjusted to 1 - 20 mW to avoid thermal effects or photolysis while ensuring the signal intensity. The scanning range of the spectrometer was set at 100 - 3000 cm -1 , which could cover the Raman shift regions of common chemical bond vibrations. The scanning resolution was set at 1 - 5 cm -1 to clearly resolve adjacent Raman peaks. The integration time was adjusted between 1 - 10 s according to the signal intensity of the samples. If the signal was weak, the integration time was appropriately extended to improve the signal-to-noise ratio. An appropriate amount of sample was placed on a special sample stage to ensure that the surface was flat and perpendicular to the laser beam. Solid samples could be directly placed, and liquid samples were loaded in a special sample cell. The test environment was maintained at a temperature of 25 ± 2°C and a relative humidity below 60% to ensure the stable operation of the instrument and the accuracy of the results.

[0077] In the following examples and comparative examples, the nitrate concentration was determined by ultraviolet-visible spectroscopy of the products using a Cary 5000 spectrophotometer.

[0078] Specifically, 0.2 mL of the sample solution was taken and diluted with 4.8 mL of deionized water. The absorbance was measured in the wavelength range of 220 to 275 nm (A = A220nm - A2×275nm), and the nitrate concentration was calculated through a standard calibration curve.

[0079] In the following examples and comparative examples, the ammonium ion concentration was determined by the phenate spectrophotometry method.

[0080] Specifically, take 0.2 mL of the sample solution and dilute it with 4.8 mL of deionized water. Subsequently, add 0.5 mL of phenolate-citrate reagent and 0.2 mL of chlorine buffer solution, and mix well. After the mixture stands for 90 minutes, measure its absorbance at 630 nm. Finally, calculate the concentration of ammonia nitrogen by referring to the standard calibration curve.

[0081] In the following examples and comparative examples, the diazo-coupling spectrophotometric method was used to quantitatively determine the nitrite concentration.

[0082] Specifically, the diazo-coupling spectrophotometric method was used to quantitatively determine the nitrite concentration. Take 0.2 mL of the sample solution and dilute it with 4.8 mL of deionized water. Then, add 0.1 mL of sulfanilamide solution, mix well and let it stand for 5 minutes. Subsequently, add 0.1 mL of N-(1-naphthyl)ethylenediamine hydrochloride solution. After reacting for 10 minutes, measure its absorbance at 540 nm. Finally, determine the concentration of nitrite by referring to the standard calibration curve.

[0083] In the following examples and comparative examples, the ability of iron per unit mass to remove nitrate nitrogen, the removal rate of NO3 - -N, and the N2 selectivity were calculated using the following formula:

[0084]

[0085]

[0086]

[0087] Where: C0(NO3 - -N) and C0(NO2--N) represent the initial concentrations of NO3 - -N and NO2 - -N, respectively;

[0088] C t (NO3 - -N) and C t (NO2 - -N) represent the concentrations of NO3 - -N and NO2 - -N at time t (h), respectively;

[0089] C t (NH4 + -N) represents the concentration of NH4 + -N in the electrolyte;

[0090] V (L) represents the volume of the nitrate solution;

[0091] m (g) represents the mass of the active sites of the catalyst attached to the cathode;

[0092] h is the reaction time of 24 h.

[0093] Synthesis of biomass carbon aerogel (MSCA)

[0094] Sawdust of beech wood was thoroughly ground with an eutectic molten salt of zinc chloride and potassium chloride (molar ratio 54:46) at mass ratios of 1:1, 1:2, and 1:3 to ensure uniform mixing. The mixture was placed in a porcelain boat and put into a tubular furnace. Under an argon protective atmosphere, it was heated to 700 °C at a heating rate of 5 °C / min and held at this temperature for 2 hours, followed by natural cooling to room temperature.

[0095] After cooling, the samples were immersed in deionized water and rinsed thoroughly to remove excess soluble substances until the conductivity of the washing liquid dropped below 10 μS / cm. Finally, the derived samples were named MSCA-1:1, MSCA-1:2, and MSCA-1:3.

[0096] The biomass carbon aerogels MSCA-1:1, MSCA-1:2, and MSCA-1:3 were subjected to structural characterization and mechanical property testing.

[0097] The microstructures of MSCA-1:1, MSCA-1:2, and MSCA-1:3 are as Figure 1 shown. Figure 1 In Figures (a), (b), and (c) respectively are the SEM images of MSCA-1:1, MSCA-1:2, and MSCA-1:3.

[0098] As can be seen from Figure 1 , MSCA-1:1 shows a compact rod-like morphology. The deficiency of zinc ions leads to insufficient cross-linking and aggregation of the carbon aerogel, thus limiting the generation of the porous structure. In MSCA-1:2, the presence of zinc chloride can lower the pyrolysis temperature of the biomass, resulting in a more uniform carbon network structure at a higher temperature, thus forming abundant micropores and mesopores. While in MSCA-1:3, when the zinc ion concentration is too high, the zinc ions in the molten salt will have too strong an interaction with the carbon material, thus triggering the collapse of the pore structure.

[0099] The specific surface areas and pore size distributions of MSCA-1:1, MSCA-1:2, and MSCA-1:3 are as Figure 2 shown. Figure 2 In Figures (a), (b), and (c) respectively are the BET and pore size distribution diagrams of MSCA-1:1, MSCA-1:2, and MSCA-1:3, and Figure (d) is a schematic diagram of the specific surface area of the catalyst.

[0100] The mechanical properties of MSCA-1:1, MSCA-1:2, and MSCA-1:3 are as Figure 3 shown.

[0101] From Figure 2 and Figure 3 it can be seen that MSCA-1:1 forms a relatively compact material structure, lacking sufficient micropores and mesopores. The specific surface area of MSCA-1:1 is only 656.6 m 2 g -1 . Although the compact morphology enhances the hardness and strength of the material, it also inhibits its specific surface area and reactivity as a catalyst. The specific surface area of MSCA-1:2 is 779.1 m 2 g -1 . The micropores are mainly concentrated in the range of 0.6 - 0. nm. The incorporation of an appropriate amount of zinc chloride in MSCA-1:2 not only enhances the formation of pores but also provides the necessary support, enabling the carbon framework to remain stable during the pyrolysis process. The specific surface area of MSCA-1:3 is 857.5 m 2 g -1 , but due to the excessive zinc ions increasing the brittleness of the material and reducing its toughness, during subsequent processing or application, the material is more prone to cracks and breakages, further reducing the porosity and mechanical properties of the MSCA-1:3 material.

[0102] In summary, considering the specific surface area, pore size distribution, and mechanical properties, MSCA-1:2 is selected as the carrier in the following examples.

[0103] Example 1

[0104] Using a ferric citrate solution with a concentration of 0.4 mol / L as the precursor, it is placed in a beaker together with the biomass carbon aerogel MSCA-1:2. Continuous stirring impregnation (equal volume impregnation) is carried out under the constant temperature condition of 80 °C, and then drying treatment is performed. The drying temperature is 80 °C and the drying time is 12 h. Ensure that the precursor can be evenly distributed within the pore structure of the carbon aerogel, laying a foundation for the subsequent formation of a catalyst with excellent performance.

[0105] Subsequently, the obtained mixture is heated to 700 °C at a heating rate of 5 °C / min in an argon environment and held at this temperature for 1 hour. After cooling to room temperature, the product is thoroughly washed with acid and deionized water to remove unreacted raw materials and impurities, and finally dried at 80 °C for 12 hours to obtain the iron-based catalyst Fe3O4@C-1.

[0106] Example 2

[0107] The difference from Example 1 is only that the ferric citrate solution (0.4 mol / L) is replaced with an iron acetate solution to obtain the iron-based catalyst Fe3O4@C-2.

[0108] Example 3

[0109] It is only different from Example 1 in that the iron citrate solution is replaced with a ferric chloride solution (0.4 mol / L) to obtain the iron-based catalyst Fe3O4@C-3.

[0110] Example 4

[0111] It is only different from Example 1 in that the concentration of the iron citrate solution is 0.2 mol / L to obtain an iron-based catalyst.

[0112] Example 5

[0113] It is only different from Example 1 in that the concentration of the iron citrate solution is 0.6 mol / L to obtain an iron-based catalyst.

[0114] Comparative Example 1

[0115] Using the biomass carbon aerogel MSCA-1:2 directly as a catalyst.

[0116] Comparative Example 2

[0117] Using Fe3O4 nanoparticles (Macklin Reagent Company) directly as a catalyst.

[0118] Test Example

[0119] 1. Structure Characterization

[0120] (1) The SEM images of the iron-based catalysts in Examples 1-3 are as Figure 4 shown, where Figures (a), (b), and (c) are the SEM images of Fe3O4@C-1, Fe3O4@C-2, and Fe3O4@C-3, respectively.

[0121] As can be seen from Figure 4 it, the catalysts in Examples 1-3 exhibit a unique porous carbon structure, and the nanoparticles are evenly distributed on their surfaces. The porous structure in the catalyst can provide rich diffusion channels for the reactants, enabling them to quickly diffuse from the solution to the catalyst surface. At the same time, the evenly distributed nanoparticles provide more active sites, making the catalysts in Examples 1-3 have excellent electrochemical activity and being able to efficiently promote electron transfer and material transformation in electrochemical reactions.

[0122] (2) The TEM images of the iron-based catalysts in Examples 1-3 are as Figure 5 shown, where Figures (a)-(b), (c)-(d), and (e)-(f) are the TEM images of Fe3O4@C-1, Fe3O4@C-2, and Fe3O4@C-3, respectively.

[0123] As can be seen from Figure 5It can be seen that the nanoparticles in the catalysts of Examples 1-3 are uniformly coated with a graphite carbon layer, showing a good core-shell structure.

[0124] In Fe3O4@C-1, the lattice spacing of the graphite carbon layer is 0.338 nm, which is highly consistent with the (002) crystal plane of graphite carbon. At the same time, the crystal plane spacing of the iron nanoparticles is 0.24 nm, corresponding to the (222) crystal plane of iron oxide. This result confirms the interfacial matching between the graphite carbon layer and Fe3O4 nanoparticles, indicating that ferric citrate as a precursor can effectively promote the uniform growth of the graphite carbon layer.

[0125] In Fe3O4@C-2, the lattice spacing of the graphite carbon layer is 0.34 nm, also showing good matching with the (002) crystal plane of graphite carbon. The crystal plane spacing of the iron nanoparticles is 0.33 nm, corresponding to the (211) crystal plane of Fe3O4. Ferric acetate as a precursor can also achieve an effective combination between the graphite carbon layer and Fe3O4 nanoparticles.

[0126] In Fe3O4@C-3, the lattice spacing of the graphite carbon layer is 0.25 nm, highly matching the (002) crystal plane of graphite carbon. The crystal plane spacing of the iron nanoparticles is 0.148 nm, corresponding to the (440) crystal plane of Fe3O4. Compared with the previous two catalysts, the thickness distribution of its carbon shell is uneven.

[0127] (3) The elemental mapping diagram of the iron-based catalyst in Example 1 is as Figure 6 shown.

[0128] From Figure 6 it can be clearly observed that the elements of carbon (C), iron (Fe), and oxygen (O) are uniformly distributed in the nanoparticles, further confirming the successful synthesis of Fe3O4@C nanoparticles.

[0129] (4) The X-ray photoelectron spectroscopy (XPS) characterization results of the iron-based catalyst in Example 1 are as Figure 7 shown.

[0130] From Figure 7 it can be seen that the Fe element in the Fe3O4@C-1 material exists in the form of Fe3O4 phase, and the graphite carbon layer forms a stable interfacial bond with Fe3O4 nanoparticles through C-O bonds. This structure not only helps to improve the conductivity and structural stability of the material but also may enhance its performance in electrochemical applications.

[0131] In addition, not only sp exists in the graphite carbon layer 2The hybrid carbon structure also contains a certain amount of oxygen-containing functional groups, which may be due to partial oxidation of the carbon layer during the synthesis process. The oxygen-containing functional groups in the graphite carbon layer further improve the surface wettability and ion transport performance of the material, thereby enhancing its comprehensive performance as an electrode material.

[0132] (5) The XRD patterns and Raman spectra of the iron-based catalysts in Examples 1-3 are as Figure 8 shown.

[0133] From Figure 8 Figure (a), it can be seen that in the XRD patterns of the iron-based catalysts in Examples 1-3, diffraction peaks corresponding to the (002) crystal plane of the hexagonal structure of graphite carbon are observed at 2θ = 26.3°, indicating that graphitized carbon layers exist in all samples. Moreover, the samples Fe3O4@C-1, Fe3O4@C-2, and Fe3O4@C-3 all exhibit similar diffraction characteristics, with a significant diffraction peak appearing at 2θ = 36.1°, which belongs to magnetite.

[0134] From Figure 8 Figure (b), it can be seen that the I D / I G ratio of the Fe3O4@C-1 sample is 0.94, which is higher than that of Fe3O4@C-2 and Fe3O4@C-3, indicating that its graphite carbon layer contains more structural defects. It is speculated that due to the high reactivity of the carbon source released during the decomposition of iron citrate, more structural disorder is generated during the growth of the carbon layer, improving the conductivity of the material and thus affecting the electrocatalytic performance of the subsequent material.

[0135] 2. Electrocatalytic performance test

[0136] The electrocatalytic denitrification performance of the catalyst was systematically evaluated using a CHI 660E electrochemical workstation in a three-electrode cell system.

[0137] In the experiment, a platinum electrode and a saturated calomel electrode (SCE) were used as the counter electrode (with a size of 2 cm × 2 cm) and the reference electrode, respectively, and the iron-based catalysts of the examples and comparative examples were used as the working electrode, with a size of 1.5 cm × 1.5 cm.

[0138] The electrolyte was composed of 50 mg / L of NaNO3, 0.1 M of Na2SO4, and 0.1 M of NaCl (pH = 7). At a constant potential (-1.3 V vs. SCE), the electrocatalytic nitrate reduction experiment was carried out using the time-current (i-t) method, and the reaction cycle was 24 hours. To ensure the reliability of the experimental data, all electrolysis experiments were repeated three times.

[0139] (1) As Figure 9As shown, the Fe3O4@C-1 catalyst prepared in Example 1 exhibited a stable current density (10 mA / cm 2 ) during the reaction, indicating its good electrochemical stability.

[0140] (2) Table 1 and Figure 10 show the denitrification performance of the catalysts in Example 1, Example 4, and Example 5

[0141] Table 1

[0142]

[0143] As can be seen from Table 1 and Figure 10 , as the concentration of iron citrate increased, the nitrate degradation rate gradually increased, reaching a maximum of 97.3%. However, according to the results of measuring the mass percentage of Fe in the sample by inductively coupled plasma (ICP) (Table 1), the catalytic ability of Fe per unit mass per unit time reached the optimal value when the concentration of iron citrate was 0.4 mg / L, which was 305.6 mg Nh -1 g -1 .

[0144] (3) Figure 11 shows the performance of the catalysts in Example 1-3 and Comparative Example 1-2 in the nitrate reduction reaction. The mass of Fe participating in the reaction in all catalysts was ensured to be the same by ICP testing.

[0145] As can be seen from Figure 11 , compared with the iron-based catalysts in Example 1-3, MSCA hardly had the ability to remove nitrate, while the nitrate removal rate of pure Fe3O4 nanoparticles was only 8.3%, and the catalytic efficiency of Fe per unit mass was 9.3 mg Nh -1 g -1 . In contrast, the iron-based catalysts in Example 1-3 showed a significant performance improvement, among which Fe3O4@C-1 had the best performance, with a nitrate degradation efficiency as high as 94.6%, and the catalytic efficiency of Fe per unit mass reached 305.6 mg N h -1 g -1 , which was nearly 30 times higher than that of pure Fe3O4 nanoparticles.

[0146] The high conductivity of the graphite carbon layer provides an efficient path for electron transport, significantly reducing the charge transfer resistance, thereby improving the kinetic efficiency of the catalytic reaction. Secondly, the coating of the carbon shell effectively inhibits the aggregation and shedding of Fe3O4 nanoparticles, increasing the exposure area and stability of the active sites. In addition, the carbon shell enhances the adsorption and activation ability of Fe3O4 nanoparticles towards nitrate ions by adjusting the surface electronic structure, thereby further improving the catalytic efficiency

[0147] (4)Figure 12 The electrochemical performance of the iron-based catalysts of Examples 1-3 is shown.

[0148] Figure 12 In (a), the test results of linear sweep voltammetry (LSV) are shown. It can be seen from the figure that the current density of Fe3O4@C-1 is significantly lower than that of Fe3O4@C-2 and Fe3O4@C-3, and the current density gradually increases with the increase of the applied potential. This phenomenon indicates that Fe3O4@C-1 has higher catalytic activity and can achieve efficient nitrate reduction reaction (NO3RR) at a lower potential.

[0149] Figure 12 In (b), the test results of the reaction kinetics of NO3RR are shown. It can be seen from the figure that the Tafel slopes of Fe3O4@C-1, Fe3O4@C-2 and Fe3O4@C-3 are 434.5, 685.8 and 789.2 mV dec -1 . The Tafel slope of Fe3O4@C-1 is the smallest, indicating that it has faster reaction kinetics and higher catalytic efficiency. This result is consistent with the LSV test results, further confirming the excellent performance of Fe3O4@C-1 in NO3RR.

[0150] Figure 12 In (c), the charge transfer characteristics are shown. It can be seen from the figure that Fe3O4@C-1, Fe3O4@C-2 and Fe3O4@C-3 all show semi-circles of different sizes in the high-frequency region, reflecting the differences in their charge transfer resistances. Among them, the resistance value of Fe3O4@C-1 is the lowest, and the lower resistance value can promote the rapid transfer of electrons on the catalyst surface, indicating that it has high conductivity and low electron transfer resistance.

[0151] In summary, the comprehensive analysis of LSV, Tafel curves and EIS shows that Fe3O4@C-1 has more superior electrocatalytic ability compared with Fe3O4@C-2 and Fe3O4@C-3.

[0152] 3. Cyclic stability test

[0153] The electrocatalytic denitrification performance of the catalyst was systematically evaluated using a CHI 660E electrochemical workstation in a three-electrode cell system.

[0154] In the experiment, a platinum electrode and a saturated calomel electrode (SCE) were used as the counter electrode (size: 2 cm × 2 cm) and the reference electrode respectively, and the iron-based catalyst of Example 1 was used as the working electrode, with a size of 1.5 cm × 1.5 cm.

[0155] The electrolyte consists of 50 mg / L of NaNO3, 0.1 M of Na2SO4, and 0.1 M of NaCl (pH = 7). At a constant potential (-1.3 V vs. SCE), the electrocatalytic nitrate reduction experiment was carried out using the time-current (i-t) method. The reaction cycle was 24 hours, and five consecutive cyclic electrolysis batch experiments were conducted, with each cycle lasting 24 hours, to simulate its long-term use in actual operation.

[0156] (1) Figure 13 Figure 5 shows the electrocatalytic performance of the iron-based catalyst in Example 1 during 5 cycles.

[0157] From Figure 13 Figure 5(a), it can be seen that within the 95% confidence interval, the Fe3O4@C-1 catalyst can still maintain a nitrate removal rate of over 90% after 5 cycles, demonstrating the stability of its structure.

[0158] From Figure 13 Figure 5(b), it can be seen that after the Fe3O4@C-1 cathode is cycled 5 times at a constant voltage of -1.3 V, the current density still remains stable, indicating that its electrochemical performance has not significantly decayed during long-term operation.

[0159] (2) Figure 14 Figure 6 shows the microstructure of the iron-based catalyst after cycling.

[0160] From Figure 14 Figures 6(a) and 6(b), it can be seen that no obvious agglomeration phenomenon appears in the TEM image of the Fe3O4@C-1 catalyst after cycling, demonstrating its good dispersibility and stability. In addition, the core-shell structure of this catalyst remains intact, and the iron (Fe) nanoparticles are still tightly wrapped by the graphite carbon layer, indicating that the catalyst can still effectively resist the erosion of the reaction medium and the degradation of the material after five cycles.

[0161] From Figure 14 Figures 6(c)-(f), it can be seen that the distributions of Fe, O, and C elements have not changed significantly after cycling, indicating that no obvious element migration or loss has occurred in Fe3O4@C-1 during the reaction process. This phenomenon further verifies the good structural stability of the catalyst during electrolysis.

[0162] The preferred embodiments of the present application have been described in detail above. However, the present application is not limited thereto. Within the scope of the technical concept of the present application, various simple modifications can be made to the technical solutions of the present application, including any other suitable combination of each technical feature. These simple modifications and combinations should also be regarded as the content disclosed by the present application and fall within the protection scope of the present application.

Claims

1. An iron-based catalyst, which comprises magnetite nanoparticles and a porous graphite carbon layer coating the magnetite nanoparticles.

2. The iron-based catalyst according to claim 1, wherein Based on the iron-based catalyst, the mass content of iron element is 3%-20%, preferably 5%-15%, more preferably 8%-12%; and / or The I of the iron-based catalyst D / I G The ratio is 0.7 - 0.98, preferably 0.9 - 0.95; and / or The average particle size of the magnetite nanoparticles is 5-30 nm, preferably 10-15 nm; The average thickness of the porous graphite carbon layer is 5-20 nm, preferably 8-12 nm.

3. A preparation method of an iron-based catalyst, which comprises the following steps: S1: Provide a biomass carbon aerogel; S2: Immerse the biomass carbon aerogel obtained in step S1 in an iron salt solution to obtain an immersion product; S3: Heat-treat the immersion product obtained in step S2 to obtain a heat-treated product; Optionally, step S4: Wash and dry the heat-treated product to obtain the iron-based catalyst.

4. The preparation method according to claim 3, characterized in that, In step S1, the specific surface area of the biomass carbon aerogel is 300 m 2 / g - 1200 m 2 / g, preferably 600 m 2 / g - 1000 m 2 / g, more preferably 700 m 2 / g - 800 m 2 / g; and / or The pore size distribution of the biomass carbon aerogel is 0.5 nm - 0.8 nm, preferably 0.6 nm - 0.7 nm.

5. The preparation method according to claim 3 or 4, characterized in that, In step S1, the providing of the biomass carbon aerogel includes: mixing biomass with a eutectic molten salt of chloride and then performing carbonization treatment; Preferably, the biomass is selected from wood, and the wood is preferably selected from one or more of beech, basswood, locust, eucalyptus, pine, toona, poplar, willow, peach, apricot, apple, pear, banyan, camphor, locust, maple, cypress, oak, fir, paulownia; Preferably, the mass ratio of the biomass to the eutectic molten salt of chloride is 1:1 - 1:3, preferably 1:1.5 - 1:2.5; Preferably, the melting point of the eutectic molten salt of chloride is 200°C - 900°C; Preferably, the eutectic molten salt of chloride is a eutectic molten salt of zinc chloride and potassium chloride; Preferably, the carbonization treatment is carried out in an inert atmosphere; Preferably, the temperature of the carbonization treatment is 600°C - 900°C, and preferably reaches the temperature of the carbonization treatment in an ascending temperature manner, and the heating rate is 1°C / min - 10°C / min.

6. The preparation method according to any one of claims 3-5, characterized in that, In step S2, the iron salt solution is selected from one or more of ferric citrate solution, ferric acetate solution and ferric chloride solution; and / or Calculated by iron element, the mass of the iron salt in the iron salt solution is 3%-20% of the mass of the biomass carbon aerogel, preferably 5%-15%, more preferably 8%-12%; and / or The concentration of the iron salt solution is 0.1 mg / L - 1 mg / L, preferably 0.2 mg / L - 0.6 mg / L, more preferably 0.3 mg / L - 0.5 mg / L; and / or The temperature of the immersion is 60°C - 100°C.

7. The preparation method according to any one of claims 3-6, characterized in that, In step S3, the heat treatment is carried out in an inert atmosphere; and / or The temperature of the heat treatment is 600°C - 900°C, and preferably reaches the temperature of the heat treatment in an ascending temperature manner, and the heating rate is 1°C / min - 10°C / min.

8. Application of the iron-based catalyst according to any one of claims 1 or 2 or the iron-based catalyst prepared by the preparation method according to any one of claims 3-7 in nitrate reduction.

9. A method for electrocatalytic reduction of nitrate, which includes reducing nitrate in a three - electrode system, wherein the working electrode in the three - electrode system comprises the iron - based catalyst described in any one of claims 1 or 2 or the iron - based catalyst prepared by the preparation method described in any one of claims 3 - 7.

10. The method according to claim 9, wherein The electrolyte in the electrolyte solution of the three - electrode system is selected from sodium chloride and / or sodium sulfate; and / or The working voltage of the three - electrode system is - 1.1V to - 1.5V; and / or The initial concentration of nitrate is 50mg / L - 200mg / L.

Images