Preparation method and application of Fe@S-1 catalytic material

Abstract

The present invention is applicable to the field of environmental engineering water treatment technology, and provides a preparation method and application of a novel Fe@S-1 catalytic material. The preparation method comprises the following steps: mixing 4.06g of tetrapropylammonium hydroxide with 32mL of ultrapure water; after continuous stirring for 10min, adding 2.6g of tetraethyl orthosilicate and stirring for 6h to obtain a mixture 1; mixing 0.909g of ferric nitrate nine hydrate, 0.4mL of ethylenediamine and 4mL of ultrapure water and stirring for 10 minutes to obtain a mixture 2; adding the mixture 2 dropwise to the mixture 1, stirring for 30min without precipitation, and then placing it in an autoclave and performing a hydrothermal synthesis at 170°C for 4 days; centrifuging and collecting the product, washing it with ethanol 3 times, washing it with ultrapure water 3 times, drying it at 80°C for 12h, and calcining the product in a hydrogen-argon mixed atmosphere at 550°C for 8h to fully reduce it. The catalytic material is green and pollution-free, and can improve the adsorption performance of nitrates, thereby improving the electrocatalytic removal rate of nitrates.

Description

Technical Field

[0001] The present invention belongs to the technical field of environmental engineering water treatment, and in particular relates to a preparation method and application of a novel Fe@S-1 catalytic material. Background Art

[0002] The diversification and extensive use of nitrogen-containing substances in industry and agriculture have disrupted the nitrogen balance in the environment, leading to the accumulation of nitrates (NO3 – Pollution is becoming increasingly serious. Surveys of nitrate levels in deep and shallow groundwater across 52 basin-scale groundwater systems nationwide show that 90% of surveyed points in 25 of the shallow aquifers in 36 major basins exceed national drinking water quality standards. Nitrate levels in 10 of the deep or karst aquifers in 37 major basins exceed standards. Increased nitrate concentrations severely impact aquatic ecosystems, leading to eutrophication and hypoxia. Furthermore, excessive nitrate concentrations in drinking water increase the risk of infections such as methemoglobinemia, diabetes, spontaneous abortion, thyroid disease, and gastric cancer. Therefore, water bodies with excessive nitrate levels require urgent treatment.

[0003] Electrocatalysis is an emerging technology for treating excessive nitrate in water, gaining popularity due to its mild operating conditions and excellent efficiency. The catalytic activity of the cathode catalyst significantly influences nitrate removal efficiency. Nano-iron-based catalysts, with their unique advantages (low cost, readily available, environmentally friendly, and excellent catalytic performance), are ideal catalytic materials for electrocatalytic nitrate reduction.

[0004] However, when nano-iron particles are directly used for electrocatalytic removal of nitrate, there are several problems:

[0005] (1) At room temperature, nano-iron particles will be oxidized in the air, and a dense oxide film will easily form on the surface, which will hinder the catalytic reaction.

[0006] (2) The surface energy of nano-iron particles is high and they are easy to agglomerate, resulting in a reduction in active sites and thus a decrease in catalytic activity.

[0007] (3) Conventional iron catalysts have poor activity and cannot compete with more active precious metals (such as ruthenium).

[0008] These problems are related to the size, valence state and d-band center of the iron particles. For example, the size of nano-iron particles is crucial to their performance. Smaller particles usually have a larger specific surface area and are therefore more likely to interact with other particles. Zero-valent iron exists in the form of pure elemental metal. Compared with the high-valent iron in conventional catalysts, it does not form chemical bonds with other elements. There are abundant active sites on the surface that can adsorb and catalyze reactions. These sites contribute to the electrocatalytic reduction and removal of nitrates. The closer the d-band center of iron is to the Fermi level, the stronger the adsorption of nitrates, thereby improving the efficiency of electrocatalytic removal of nitrates. Therefore, a new type of iron-based sub-nanocatalyst structure is proposed to solve the problems encountered when using nano-iron particles as catalytic materials by regulating the size, valence state and d-band center of iron particles. Summary of the Invention

[0009] The purpose of the embodiments of the present invention is to provide a preparation method of a novel Fe@S-1 catalytic material and its application, aiming to solve the problems raised in the above background technology.

[0010] The embodiment of the present invention is achieved by providing a method for preparing a novel Fe@S-1 catalytic material, wherein the preparation of the Fe@S-1 catalyst is carried out by a hydrothermal method, specifically comprising the following steps:

[0011] Step 1: Mix 4.06 g of tetrapropylammonium hydroxide with 32 mL of ultrapure water; after continuous stirring on a magnetic stirrer for 10 minutes, add 2.6 g of ethyl orthosilicate and stir for 6 hours to obtain a clear mixture 1;

[0012] Step 2, 0.909 g of ferric nitrate nonahydrate, 0.4 mL of ethylenediamine, and 4 mL of ultrapure water were mixed and stirred for 10 minutes to obtain a mixture 2;

[0013] Step 3: Add mixture 2 dropwise to mixture 1, stir for 30 minutes without precipitation, and then place in an autoclave for hydrothermal synthesis at 170° C. for 4 days;

[0014] Step 4: The obtained product was centrifuged and collected, washed with ethanol three times, washed with ultrapure water three times, dried at 80°C for 12 hours, and the collected product was calcined at 550°C in a hydrogen-argon mixed gas atmosphere for 8 hours for full reduction. The obtained powder was ground and named Fe@S-1.

[0015] A further technical solution is that in step 4, the volume ratio of hydrogen to argon in the hydrogen-argon mixture is V H2 :V Ar =5:95.

[0016] Another object of an embodiment of the present invention is to apply a new Fe@S-1 catalytic material to the electrocatalytic treatment of nitrates, using an electrode sheet prepared using Fe@S-1 and a platinum sheet as a cathode and anode, respectively.

[0017] A further technical solution is a method for preparing an electrode sheet based on Fe@S-1, comprising the following steps:

[0018] First, Fe@S-1 composite material, conductive carbon black, and polyvinylidene fluoride in a mass ratio of 8:1:1 were weighed, and then the mixture was dissolved in N,N-dimethylformamide. The concentration of the mixture was controlled to 10g / L to prepare a catalyst ink. The ink was then coated on a 1.0cm×2.0cm carbon cloth, ensuring that both sides were evenly coated. The cloth was then dried at 60°C for 30 minutes to obtain a working electrode sheet.

[0019] The present invention provides a method for preparing a novel Fe@S-1 catalytic material and its application. This method confines subnanometer zero-valent Fe in a silicon molecular sieve, reducing iron metal leaching and ensuring an extremely low iron ion content in the treated water, thereby preparing a green and pollution-free material. Direct contact between zero-valent Fe and a highly corrosive environment is blocked, thereby delaying and preventing the deactivation of the catalyst. Furthermore, nano-confinement can prevent the agglomeration of small-sized iron particles, shifting the d-band center of Fe upwards. The 3d orbital of Fe effectively overlaps with the 2p orbital of O in nitrate, improving the adsorption performance of nitrates and thereby increasing the nitrate removal rate. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] Figure 1 A flow chart for preparing a novel Fe@S-1 catalytic material provided in an embodiment of the present invention;

[0021] Figure 2 This is a technical schematic diagram of the electrocatalytic removal of nitrate from water based on Fe@S-1 catalytic material;

[0022] Figure 3 Diagram of the reaction device based on Fe@S-1 catalytic material;

[0023] Figure 4 SEM images of Fe@S-1 and Fe / S-1, where a is Fe@S-1 and b is Fe / S-1;

[0024] Figure 5 TEM images of Fe@S-1 and Fe / S-1, where a is Fe@S-1 and b is Fe / S-1;

[0025] Figure 6 is the EDX image of Fe@S-1;

[0026] Figure 7is the XPS spectrum of Fe@S-1;

[0027] Figure 8 are the BET results of Fe@S-1 and S-1, where a is Fe@S-1 and b is S-1;

[0028] Figure 9 Electrochemical impedance spectra of Fe@S-1, Fe / S-1 and S-1;

[0029] Figure 10 Linear sweep voltammetry curves (vs. RHE) of different materials;

[0030] Figure 11 is the Tafel slope (vs. RHE) of different materials;

[0031] Figure 12 The nitrate degradation effect and ammonia nitrogen production and conversion rate during the electrocatalytic reduction of nitrate;

[0032] Figure 13 is the change in nitrite concentration during the degradation process;

[0033] Figure 14 is the cycling performance of Fe@S-1 catalyst;

[0034] Figure 15 The electrocatalytic degradation effect of nitrate on different materials;

[0035] Figure 16 The effect of different current densities on the electrocatalytic reduction of nitrate;

[0036] Figure 17 The effect of different pH on the electrocatalytic reduction of nitrate;

[0037] Figure 18 The effect of different initial nitrate concentrations on the electrocatalytic reduction of nitrate;

[0038] Figure 19 The effects of different ions and concentrations on the electrocatalytic reduction of nitrate;

[0039] Figure 20 Fe@S-1 electrocatalytic reduction of NO3 - The reaction path;

[0040] Figure 21 is the reaction process and the change of Gibbs free energy;

[0041] Figure 22 Fe@S-1 and Fe / S-1 adsorb NO3 - The overall differential charge and adsorption energy of , where a is Fe@S-1 and b is Fe / S-1;

[0042] Figure 23 is the Fe d orbital partial wave state density of Fe@S-1 and Fe / S-1, where a is Fe@S-1 and b is Fe / S-1. DETAILED DESCRIPTION

[0043] In order to make the purpose, technical solutions and advantages of the present invention more clearly understood, the present invention will be further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not intended to limit the present invention.

[0044] The specific implementation of the present invention is described in detail below with reference to specific embodiments.

[0045] like Figure 1 As shown, a method for preparing a novel Fe@S-1 catalytic material provided by one embodiment of the present invention is provided. The preparation of the Fe@S-1 catalyst is carried out by a hydrothermal method, which specifically includes the following steps:

[0046] Step 1: Mix 4.06 g of tetrapropylammonium hydroxide with 32 mL of ultrapure water; after continuous stirring on a magnetic stirrer for 10 minutes, add 2.6 g of ethyl orthosilicate and stir for 6 hours to obtain a clear mixture 1;

[0047] Step 2, 0.909 g of ferric nitrate nonahydrate, 0.4 mL of ethylenediamine, and 4 mL of ultrapure water were mixed and stirred for 10 minutes to obtain a mixture 2;

[0048] Step 3: Add mixture 2 dropwise to mixture 1, stir for 30 minutes without precipitation, and then place in an autoclave for hydrothermal synthesis at 170° C. for 4 days;

[0049] Step 4: The obtained product was centrifuged and collected, washed with ethanol three times, washed with ultrapure water three times, dried at 80°C for 12 hours, and the collected product was calcined at 550°C in a hydrogen-argon mixed gas atmosphere for 8 hours for full reduction. The obtained powder was ground and named Fe@S-1.

[0050] Another embodiment of the present invention provides an application of a novel Fe@S-1 catalytic material, wherein the Fe@S-1 catalytic material is applied to the electrocatalytic treatment of nitrates. The technical principle of electrocatalytic removal of nitrates in water based on the Fe@S-1 catalytic material is as follows: Figure 2 As shown. Simultaneously, the electrocatalytic reduction test of nitrate was carried out in a single-chamber electrolytic cell, using a two-electrode system. The reaction device is as follows Figure 3 shown; specifically:

[0051] Electrodes prepared from Fe@S-1 and platinum sheets served as the cathode and anode, respectively, with a spacing of 0.5-2.0 cm between the two plates. The electrolyte consisted of a 50 mg / L (N) nitrate solution prepared using NaNO3. This concentration was chosen because the actual nitrate concentration in nitrate-contaminated groundwater collected was 55 mg / L, while in the simulation experiment it was approximately 50 mg / L. Ion chromatography was used to measure the nitrate content of the water samples at various time points, and the nitrate removal rate was calculated.

[0052] In an embodiment of the present invention, the electrocatalytic removal of nitrate was carried out in a single-chamber electrolytic cell (50 mL) using a GPS305D DC regulated power supply. In the two-electrode system, the platinum sheet served as the anode and the electrode sheet prepared from Fe@S-1 served as the cathode. 30 mL of the electrolyte contained 50 mg / L nitrate (in terms of N) and 0.21 g Na2SO4. A current of 50 mA was applied to the system, and samples were taken every 30 minutes. The nitrate content of the water samples at each time point was measured using ion chromatography, and the nitrate removal rate was calculated. Under optimal conditions, 94.6% of the nitrate could be degraded within 3 hours. At the same time, the Fe@S-1 catalyst had good cyclic stability and still had a degradation rate of 92.3% after being reused 5 times.

[0053] As shown in Table 1, a comparison of nitrate reduction performance of different electrocatalytic materials was conducted. The Fe@S-1 catalytic material prepared by this method achieved a nitrate removal rate of 94.6%, and within just 3 hours, far less than the time required by other catalytic materials. Its nitrate removal efficiency was significantly higher than that of other iron-based and metal catalysts, and its ammonia selectivity was 56%. This demonstrates the excellent performance and astonishing nitrate removal efficiency of Fe@S-1.

[0054] Table 1. NO3 reduction by different electrocatalytic materials - Performance Comparison

[0055]

[0056] As a preferred embodiment of the present invention, the preparation method of the working electrode based on Fe@S-1 includes the following steps:

[0057] First, Fe@S-1 composite material, conductive carbon black, and polyvinylidene fluoride in a mass ratio of 8:1:1 were weighed, and then the mixture was dissolved in N,N-dimethylformamide. The concentration of the mixture was controlled to 10g / L to prepare a catalyst ink. The ink was then coated on a 1.0cm×2.0cm carbon cloth (CC), ensuring that both sides were evenly coated. The ink was then dried at 60°C for 30 minutes to obtain a working electrode sheet.

[0058] Several specific examples are provided below to verify the feasibility of this method:

[0059] Example 1:

[0060] 1. Preparation of catalytic materials:

[0061] (1) Preparation of S-1 silicon molecular sieve:

[0062] The preparation of silicon molecular sieve S-1 is carried out by hydrothermal method. The specific operation process is as follows: 4.06g of tetrapropylammonium hydroxide is mixed with 32mL of ultrapure water. After continuous stirring on a magnetic stirrer for 10 minutes, 2.6g of ethyl orthosilicate is added and stirred for 6 hours to obtain a clear mixture. It is then placed in an autoclave and hydrothermally synthesized at 170°C for 4 days. The obtained product is centrifuged and collected, washed with ethanol 3 times, washed with ultrapure water 3 times, and dried at 80°C for 12 hours. The collected product is ground into powder and named S-1. S-1 is the prepared silicon molecular sieve reference material.

[0063] (2) Preparation of Fe@S-1 catalyst

[0064] The preparation of Fe@S-1 catalyst was carried out by hydrothermal method, and the specific operation process was as follows: 4.06g of tetrapropylammonium hydroxide was mixed with 32mL of ultrapure water. After continuous stirring on a magnetic stirrer for 10 minutes, 2.6g of ethyl orthosilicate was added and stirred for 6 hours to obtain a clear mixture 1. 0.909g of ferric nitrate nonahydrate, 0.4mL of ethylenediamine, and 4mL of ultrapure water were mixed and stirred for 10 minutes to obtain mixture 2, which was added dropwise to mixture 1, stirred for 30 minutes under precipitation-free conditions, and then placed in an autoclave for hydrothermal synthesis at 170°C for 4 days. The obtained product was centrifuged and collected, washed with ethanol 3 times, washed with ultrapure water 3 times, dried at 80°C for 12 hours, and the collected product was heated at 550°C in a hydrogen-argon mixture (V H2 :V Ar =5:95) atmosphere for 8 h to fully reduce the product, and the obtained powder was ground and named Fe@S-1.

[0065] (3) Preparation of Fe / S-1 catalyst

[0066] The Fe / S-1 catalyst was prepared using a similar method, except that the iron-ethylenediamine-water mixture was not added initially. After the S-1 molecular sieve was prepared, 0.1 g of the ground molecular sieve was mixed with the iron-ethylenediamine-water mixture by impregnation. The mixture was then dried at 80°C for 12 hours, ground again, and calcined at 550°C in a hydrogen-argon atmosphere for 8 hours to fully reduce the material. The product was then collected and further ground. The powdered product was named Fe / S-1. Fe / S-1 serves as the non-confined control material prepared.

[0067] 2. Catalyst Characterization

[0068] 1) Scanning electron microscopy (SEM)

[0069] SEM images of Fe@S-1 catalyst and Fe / S-1 catalyst are shown in Figure 2. Figure 4 a and Figure 4 As shown in b. Figure 4 a) The rounded and irregular surface of the Fe@S-1 catalyst can be clearly observed, with only a portion of the hexagonal structure remaining. This is due to the confinement of Fe within the S-1 zeolite, resulting in a rough surface, blurred edges, and distorted morphology. Figure 4 Figure b shows an SEM image of Fe / S-1, which clearly shows its sharp edges and regular shape, with a very standard hexagonal structure, highly similar to the SEM image of the S-1 zeolite, showing no effect of the interaction between iron and the zeolite. The SEM image also shows that the individual Fe@S-1 and individual Fe / S-1 are similar in size, ranging from approximately 200nm to 1μm.

[0070] 2) Transmission electron microscopy (TEM)

[0071] TEM images such as Figure 5 a and Figure 5 As shown in b. Figure 5 The surface of Fe@S-1 in a is clean and tidy, and there is no Fe residue on the surface of the zeolite, but the shape is round and irregular, just like in the SEM image. This proves that the Fe in Fe@S-1 has successfully entered the interior of the molecular sieve, and the interaction between the two causes morphological distortion. Figure 5 In b, the TEM image of Fe / S-1 clearly shows that there are clusters of particles on the surface of the molecular sieve. These are the Fe elements gathered on the surface of the molecular sieve. Because Fe cannot enter the molecular sieve and only stays on its surface, the internal structure of the molecular sieve is not affected, and a clear and angular regular hexagonal structure can still be seen.

[0072] 3) Energy Dispersive X-ray Spectroscopy (EDX)

[0073] The element distribution of Fe@S-1 sample was determined by EDX, such as Figure 6 As shown in the figure, O, Si, and Fe are evenly distributed in nearly identical locations, indicating that Fe is successfully confined within the S-1 zeolite. O and Si appear more densely packed, as they are components of the zeolite. Fe, on the other hand, is more dispersed than the other two, indicating that Fe confined within the zeolite does not aggregate into clusters, increasing the reaction contact area and enhancing catalyst activity.

[0074] 4) X-ray photoelectron spectroscopy (XPS)

[0075] The valence state of Fe was analyzed in detail by XPS. The results are as follows: Figure 7 As shown in the XPS Fe 2p spectrum, there are two peaks at 707.4 eV and 721.4 eV, corresponding to zero-valent Fe. This also proves that the Fe@S-1 catalyst with zero-valent Fe has been successfully synthesized.

[0076] 5) Fully automatic physical adsorption test (BET)

[0077] pass Figure 8 From the BET results, it can be seen that Fe@S-1 and S-1 are type IV isotherms, and an adsorption hysteresis loop appears in the middle section, which corresponds to a system where capillary condensation occurs in the porous adsorbent. The adsorption hysteresis loop is an H3 type hysteresis loop. The pores reflected by the H3 type include flat slit structures, cracks and wedge structures, etc., and no adsorption saturation is shown in the higher relative pressure area. The most probable pore size of Fe@S-1 is 38.260nm, while the most probable pore size of S-1 is 83.697nm. The pore size of Fe@S-1 is reduced, indicating that zero-valent iron is successfully confined into the silicon molecular sieve. And the BET area of ​​Fe@S-1 is 187.289m 2 / g is much higher than the BET area of ​​S-1 (26.668m 2 / g, Fe@S-1 has a higher specific surface area and more active sites, which is more conducive to improving catalytic activity.

[0078] 6) Electrochemical impedance spectroscopy (EIS)

[0079] In order to obtain the electrochemical performance of Fe@S-1 catalyst, the catalytic materials Fe@S-1, Fe / S-1, and S-1 were applied to carbon cloth (1.0×2.0 cm) to obtain working electrodes (Fe@S-1 / CC, Fe / S-1 / CC, and S-1 / CC). The electrochemical characterization of Fe@S-1 catalyst and other catalysts was carried out using an electrochemical workstation. The EIS results are shown in Figure 2. Figure 9 As shown in the graph, the semicircle diameter of Fe@S-1 / CC in the high-frequency region is smaller than that of Fe / S-1 / CC, indicating that the electrochemical impedance of Fe@S-1 / CC is smaller, which means that the charge transfer rate of Fe@S-1 catalyst is higher, which is more conducive to the electrocatalytic reduction of nitrate.

[0080] 7) Linear Sweep Voltammetry (LSV)

[0081] The LSV curves of Fe@S-1 / CC, Fe / S-1 / CC, S-1 / CC and CC were measured by LSV. Figure 10 As shown. It can be seen that the Fe@S-1 catalyst has a higher linear sweep voltammetry current, combined with Figure 10From the electrochemical impedance spectroscopy, it can be found that the Fe@S-1 catalyst has a smaller impedance and a larger current, which indicates that the Fe@S-1 catalyst has excellent conductivity, which is conducive to the transfer of electrons from the cathode to the Fe@S-1 catalyst surface, thereby significantly improving the catalytic activity of the electrocatalytic reduction process.

[0082] 8) Tafel curve

[0083] The Tafel curves of Fe@S-1 / CC, Fe / S-1 / CC, S-1 / CC and CC were measured, and the results are as follows: Figure 11 The Tafel slope represents the change in overpotential required when the current changes tenfold, so the smaller the absolute value of the slope, the better. The smaller the slope, the smaller the voltage rise and the lower the energy consumption. Figure 11 It can be seen that the absolute value of the Tafel slope of the Fe@S-1 catalyst is the smallest, followed by Fe / S-1 and S-1, and the absolute value of the Tafel slope of the pure carbon cloth is the largest, which indicates that the reaction kinetics of the Fe@S-1 catalyst is faster and has the best catalytic activity.

[0084] Example 2:

[0085] (1) Nitrate degradation effect and ammonia nitrogen production and conversion rate during electrocatalytic reduction of nitrate

[0086] All electrocatalytic nitrate removal experiments were conducted in a single-chamber electrolytic cell (50 mL) using a GPS305D DC regulated power supply. In the two-electrode system, a platinum sheet served as the anode, and the prepared Fe@S-1 catalytic material was applied to a 1.0 × 2.0 cm carbon cloth as the cathode. The spacing between the two plates was controlled to be 0.5-2.0 cm. 30 mL of electrolyte contained 50 mg / L NO3 - (in terms of N) and 0.21g Na2SO4. Apply 50mA current to the system, take samples every 30min, and use ion chromatography to measure the NO3 - Content, and calculate NO3 - The ammonia nitrogen concentration during the reaction was determined by Nessler's reagent spectrophotometry, and the ammonia nitrogen conversion rate was calculated. The experimental conditions and operating steps were kept unchanged and the experiment was repeated three times to obtain the nitrate degradation effect and ammonia nitrogen production and conversion rate during the electrocatalytic reduction of nitrate.

[0087] pass Figure 12It can be seen that after 3 hours of reaction, the degradation rate of nitrate by Fe@S-1 catalyst has reached 94.6%, showing excellent nitrate removal ability. In addition, it can be seen that the ammonia nitrogen conversion rate has been fluctuating around 50%, indicating that 56% of the nitrate degraded at the end of the reaction is converted into ammonia nitrogen, and the remaining 44% is converted into N2. N2 contains two N atoms, so the generation reaction of N2 is a secondary reaction and is always limited by the secondary reaction. Since NO3 - Many intermediates (such as NO2 - ) carries a negative charge and is easily repelled by the electric field, making it difficult to accumulate on the cathode, thereby inhibiting the occurrence of the secondary reaction to generate N2, so more ammonia nitrogen is produced relative to N2.

[0088] (2) Changes in nitrite concentration during degradation

[0089] The rate of nitrate reduction to nitrite is central to the nitrate reduction process, controlling the overall nitrate reduction kinetics. However, nitrite is far more harmful than nitrate. Therefore, a qualified electrochemical nitrate reduction process should not produce residual nitrite. Figure 13 The change in nitrite concentration during electrochemical degradation, measured by ion chromatography, shows that the initial nitrite concentration at 0 hours and the final 3 hours is both zero, with only a very small amount of nitrite generated as an intermediate product during the process. This indicates that no nitrite remains after degradation is complete, preventing secondary pollution or other issues.

[0090] (3) Catalyst cycle performance

[0091] During the use of Fe@S-1 catalyst, if it is affected by the internal and external environment and the catalyst is passivated, the reaction efficiency will be greatly reduced, affecting the continuous progress of the catalytic reaction. In order to further characterize the stability of the Fe@S-1 catalyst, 5 cycle experiments were carried out. During the experiment, the electrode sheet was carefully cleaned and dried after each electrocatalytic degradation, and then the electrode sheet was continued to be put into the electrocatalytic reduction system to repeatedly degrade new nitrate. The experimental conditions were 50mg / L (in terms of N) nitrate, 0.05A current (25mA / cm 2 The experimental results are as follows. Figure 14 As shown. Figure 14 As can be seen from the figure, the nitrate degradation rates of the Fe@S-1 catalyst within 3 h after five reuses were 94.6%, 91.8%, 93.2%, 93.1%, and 92.5%, respectively. There was no significant decrease in the degradation rate, indicating that the Fe@S-1 catalyst has good cyclic stability.

[0092] (4) Nitrate electrocatalytic degradation effect of different materials

[0093] In order to explore the influence of confinement effect on the electrocatalytic reduction degradation process of nitrate, the operation steps and other degradation conditions in Example 1 were kept unchanged. 1.0×2.0 cm Fe@S-1, Fe / S-1, S-1 and CC were used as cathodes to carry out degradation experiments. The degradation rate of nitrate when using different materials was measured after 4 hours of degradation. The experimental conditions and operation steps were kept unchanged and the experiment was repeated three times to obtain the degradation rate of nitrate when using different materials. The results are shown as follows. Figure 15 shown.

[0094] from Figure 15 It can be seen that the degradation rate of nitrate by Fe@S-1 catalyst has reached 94.6% after 3 hours of reaction, while the degradation rate of nitrate by Fe / S-1 catalyst is only 72.3% after 4 hours of reaction. As for the degradation rates of nitrate by S-1 molecular sieve alone and carbon cloth alone after 4 hours, they are 36.9% and 35.6% respectively. S-1 molecular sieve itself has no catalytic activity and, like carbon cloth, can only rely on cathode current to reduce nitrate in solution, so the degradation effect is highly similar. In just 3 hours, Fe@S-1 catalyst removed up to 94.6% of nitrate, showing an amazing removal efficiency, which is much higher than other similar catalysts, such as Fe(20%)@NC prepared by Duan, which can only remove 83% of nitrate in 24 hours. Therefore, Fe@S-1 is an excellent catalytic material for electrocatalytic reduction of nitrate.

[0095] Example 3:

[0096] In the electrocatalytic reduction process, current density is undoubtedly an important factor affecting the reaction. In order to verify the effect of current density on the electrocatalytic reduction of nitrate, the current density was controlled to 0 mA / cm 2 , 5mA / cm 2 , 15mA / cm 2 , 25mA / cm 2 、35mA / cm 2 , 50mA / cm 2 The degradation experiment was carried out by keeping the operation steps and other degradation conditions in Example 1 unchanged, and then the experiment was repeated three times with the experimental conditions and operation steps completely unchanged. The results are as follows: Figure 16 shown.

[0097] from Figure 16 It can be seen that the current density is 25mA / cm 2 The degradation effect is the best, and 94.6% of nitrate can be degraded after 3 hours of reaction. 2 Up to 25mA / cm 2During the process, the degradation effect continues to improve, but the improvement is getting smaller and smaller. 2 The degradation effect decreased slightly when the current was too high. This may be because the voltage in the reaction system was too high when the current was too high, resulting in a large number of side reactions. The higher current density may have accelerated the cathode hydrogen evolution reaction, thereby + The Fe@S-1 catalyst surface provides more active sites, which affects the electrocatalytic reduction of nitrate. Therefore, the optimal current density for electrocatalytic removal of nitrate using the catalytic material Fe@S-1 is 25 mA / cm 2 .

[0098] Example 4:

[0099] In order to explore the effect of the initial pH of the reaction system on the electrocatalytic reduction degradation of nitrate, in order to avoid side reactions, the pH of the reaction system was adjusted using appropriate amounts of H2SO4 and NaOH. Under different conditions of pH 3, 5, 7, 9, and 11, the degradation experiment was carried out while keeping the operating steps and other degradation conditions in Example 1 unchanged. The experiment was then repeated three times while keeping the experimental conditions and operating steps completely unchanged. The results are as follows: Figure 17 shown.

[0100] from Figure 17 As can be seen, the degradation effect is best at pH = 7, with 94.6% of nitrate degraded after 3 hours of reaction. While there is a slight effect at pH = 5, the degradation rate can ultimately reach the same level as at pH = 7 after 4 hours of degradation. At pH = 3, 9, and 11, the degradation rate drops to around 90% after 4 hours, indicating that pH has a certain influence on the degradation process. This may be because pH changes the existence form of the Fe@S-1 catalyst. However, Fe@S-1 is an iron-based catalyst confined on a molecular sieve, and the Fe therein is highly stable due to the protection of the molecular sieve. Therefore, the influence of pH on the degradation effect is not significant. This indicates that the electrocatalytic reduction system of the Fe@S-1 catalyst weakens the interference of pH. Therefore, the optimal pH value for electrocatalytic nitrate removal using the catalytic material Fe@S-1 is 7.

[0101] Example 5:

[0102] In order to explore the effect of initial nitrate concentration on the electrocatalytic reduction degradation process of nitrate, electrocatalytic degradation tests were carried out under the conditions of controlling nitrate concentrations of 5 mg / L, 10 mg / L, 20 mg / L, and 50 mg / L (all in terms of N). The degradation experiment was carried out while keeping the operating steps and other degradation conditions in Example 1 unchanged. The experiment was then repeated three times while keeping the experimental conditions and operating steps completely unchanged. The results are as follows: Figure 18 shown.

[0103] from Figure 18 It can be seen that varying initial concentrations within a certain range did not affect nitrate degradation. Approximately 95% of nitrate was removed after 3 hours of reaction in all reaction systems with different concentrations. Subsequent experiments were conducted with an initial concentration of 50 mg / L.

[0104] Example 6:

[0105] In order to explore the effects of different initial chloride ion concentrations, initial carbonate concentrations and initial carbonate concentrations on the electrocatalytic reduction degradation process of nitrate, electrocatalytic degradation tests were carried out under the conditions of controlled ion concentrations of 0, 5mmol / L and 20mmol / L, and the degradation experiments were carried out while keeping the operating steps and other degradation conditions in Example 1 unchanged. The experiments were then repeated three times while keeping the experimental conditions and operating steps completely unchanged. The results are as follows: Figure 19 shown.

[0106] from Figure 19 It can be seen that different initial chloride ion concentrations have a certain effect on the nitrate degradation effect. After 4 hours of reaction, the nitrate removal rates under the conditions of 5mmol / L and 20mmol / L chloride ions were 89.7% and 88.4% respectively. This may be because Cl - The presence of carbonate ions can cause side reactions in the reaction system, generating Cl₂, thereby affecting nitrate removal. Different initial carbonate concentrations have a certain impact on nitrate degradation. After 4 hours of reaction, nitrate removal rates at 5 mmol / L and 20 mmol / L carbonate concentrations were 92.1% and 80.4%, respectively. This may be because carbonate ions also induce side reactions, affecting nitrate degradation. Simultaneously, the addition of carbonate further affects the pH of the reaction system, thus having a greater impact on degradation than chloride ions. Different initial bicarbonate concentrations have very little effect on nitrate degradation. After 4 hours of reaction, nitrate removal rates at 5 mmol / L and 20 mmol / L carbonate concentrations were 94.5% and 92.8%, respectively. This may be because, although bicarbonate ions also induce side reactions, slightly affecting nitrate degradation, they act as a pH buffer, with minimal impact on pH. Furthermore, the added bicarbonate ions require very few electrons for the reaction, thus having minimal impact on degradation. Therefore, different ions and concentrations in water have little effect on the electrocatalytic removal of nitric acid by Fe@S-1.

[0107] Example 7:

[0108] Based on the previous characterization results, density functional theory (DFT) calculations were performed using Fe atoms as active sites in the model to study the reaction mechanism and reveal the reasons for the high performance of Fe@S-1 in nitrate reduction. -The reaction pathway, such as Figure 20 、 21 shown.

[0109] (1) Electrocatalytic reduction of NO3 - Reaction path and reaction Gibbs free energy change

[0110] The reduction of nitrate to ammonia is accompanied by the transfer of nine protons and eight electrons. Figure 20 The reaction pathway is used for the electrocatalytic reduction of nitrate to ammonia on Fe sites. The first step is NO3 - adsorption, which is NO3 - No electron transfer is required when NO3 is adsorbed onto the Fe@S-1 catalyst. The next step is the gradual reduction of NO3, which generates NO2, ON, ONH, ONH2, O, OH, and finally desorbs to obtain the catalyst. The specific reaction formula is as follows:

[0111] *+NO3 - +9H + →*NO3+9H +

[0112] *NO3+9H + →*NO2+7H + +H2O

[0113] *NO2+7H + +H2O→*NO+5H + +2H2O

[0114] *ON+5H + +2H2O→*ONH+4H + +2H2O

[0115] *ONH+4H + +2H2O→*ONH2+3H + +2H2O

[0116] *ONH2+3H + +2H2O→*O+2H + +NH3+2H2O

[0117] *O+2H + +NH3+2H2O→*OH+H + +NH3+2H2O

[0118] *OH+H + +NH3+2H2O→*+3H2O+NH3

[0119] Figure 21The following is the Gibbs free energy change diagram of Fe@S-1 and Fe / S-1 with nitrate reduction. The reaction path is further calculated to calculate the free energy of each intermediate on the Fe site. The lower energy indicates that NO3 - It is easier to adsorb on the Fe@S-1 site to form *NO3. Then the NO bond of *NO3 breaks and gradually reacts with H + Combined to form *NO2, *ON, *ONH, *ONH2, *O and *OH. The high energy of the lowest unoccupied molecular π* orbital of nitrate limits the charge injection, so the rate-limiting step in the electrocatalytic removal of nitrate reaction is the nitrite formation process, thereby limiting the overall electrocatalytic removal of nitrate efficiency. However, the rate of formation of *ON on Fe@S-1 is the rate-limiting step, and the energy barrier for the reduction of *ON to *ONH of Fe@S-1 is much smaller than that of Fe / S-1, so the removal efficiency of Fe@S-1 is higher. Based on DFT calculations, it can be seen that Fe@S-1 has a high adsorption capacity for nitrate ions, and for NO3 - The reduction reaction has no large energy barrier, thus having significant electrocatalytic activity.

[0120] Figure 22 a and Figure 22 b shows NO3 - The difference in charge between Fe / S-1 and Fe@S-1. Green represents a decrease in electron cloud density, and yellow represents an increase in electron cloud density. Fe@S-1 has a higher electron-accepting ability than Fe / S-1, and the Fe-O bond length of Fe@S-1 is shorter than that of Fe / S-1. The shorter the bond length, the stronger the bond and the more stable the molecule formed. The adsorption energies of Fe@S-1 and Fe / S-1 are also calculated. Fe@S-1 has a higher adsorption capacity for NO3 - The adsorption capacity of Fe@S-1 is also stronger than that of Fe / S-1. Therefore, Fe@S-1 is more likely to absorb NO3 - Adsorption occurs and electrocatalytic reduction reaction occurs.

[0121] The partial wave density of states of the Fe atomic d orbitals of Fe / S-1 and Fe@S-1 were calculated respectively. The energy band is projected along the vertical axis to obtain the density of states. The flatter the energy band, the sharper the peak of the density of states. The wider the energy band, the flatter the density of states and the stronger the delocalization. Figure 23 a and Figure 23b. It can be seen that the larger and flatter the span of the Fe@S-1 density of states, the wider the energy band, the stronger and more stable the bonding ability, which is consistent with the differential charge results. Furthermore, based on the d-orbital partial-wave density of states, the d-band centers of Fe / S-1 and Fe@S-1 are calculated to be -1.227 and -1.012 eV, respectively. The relationship between the position of the d-band center and the Fermi level can predict the reactivity of the catalyst. The closer to the Fermi level, the better the catalytic effect of the material. Therefore, the catalytic activity of Fe@S-1 is stronger than that of Fe / S-1, which is consistent with the actual experiments mentioned above.

[0122] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions and improvements made within the spirit and principles of the present invention should be included in the scope of protection of the present invention.

Claims

1. A method for preparing a Fe@S-1 catalytic material, characterized in that: The preparation of the Fe@S-1 catalyst is carried out by a hydrothermal method, which specifically includes the following steps: Step 1: Mix 4.06 g of tetrapropylammonium hydroxide with 32 mL of ultrapure water; after continuous stirring on a magnetic stirrer for 10 minutes, add 2.6 g of ethyl orthosilicate and stir for 6 hours to obtain a clear mixture 1; Step 2, 0.909 g of ferric nitrate nonahydrate, 0.4 mL of ethylenediamine, and 4 mL of ultrapure water were mixed and stirred for 10 minutes to obtain a mixture 2; Step 3: Add mixture 2 dropwise to mixture 1, stir for 30 minutes without precipitation, and then place in an autoclave for hydrothermal synthesis at 170° C. for 4 days; Step 4: The obtained product was centrifuged and collected, washed with ethanol three times, washed with ultrapure water three times, dried at 80°C for 12 hours, and the collected product was calcined at 550°C in a hydrogen-argon mixed gas atmosphere for 8 hours to fully reduce it. The obtained powder was ground and named Fe@S-1; In step 4, the volume ratio of hydrogen to argon in the hydrogen-argon mixture is V H2 :V Ar =5:

95.

2. An application of a Fe@S-1 catalytic material, wherein the Fe@S-1 catalytic material is prepared by the preparation method of the novel Fe@S-1 catalytic material according to claim 1, characterized in that: Applying the Fe@S-1 catalytic material to the electrocatalytic treatment of nitrate; Among them, the electrode sheet prepared by Fe@S-1 was used as the cathode and the platinum sheet was used as the anode.

3. The use of the Fe@S-1 catalytic material according to claim 2, characterized in that: The preparation method of the Fe@S-1-based electrode sheet includes the following steps: First, Fe@S-1 composite material, conductive carbon black, and polyvinylidene fluoride in a mass ratio of 8:1:1 were weighed, and then the mixture was dissolved in N,N-dimethylformamide. The concentration of the mixture was controlled to 10g / L to prepare a catalyst ink. The ink was then coated on a 1.0cm×2.0cm carbon cloth, ensuring that both sides were evenly coated. The cloth was then dried at 60°C for 30 minutes to obtain a working electrode sheet.

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