Manufacturing method and application of electrode for electrocatalytic reduction of nitrate

Abstract

The invention discloses a manufacturing method of an electrode for electrocatalytic reduction of nitrate, an electrode product and application, and belongs to the technical field of electrochemical water treatment in environmental engineering. The titanium-based cobalt-doped tin dioxide electrode with the double-layer structure is prepared by adopting a sol-gel-step heat treatment method, the bottom layer is an antimony-containing cobalt tin oxide conductive layer, the surface layer is an antimony-free cobalt tin oxide catalytic layer, and the molar ratio of tin to cobalt in the two layers is (5: 1)-(7: 1). The long-standing technical prejudice that a titanium-based antimony-doped tin dioxide electrode can only be used for anodic oxidation and cannot be used for cathode nitrate reduction in the field is overcome, the Faraday efficiency of the prepared electrode in high-concentration nitrate wastewater of 1000 mg / L is as high as 97.2%, the ammonia nitrogen selectivity is 99.3%, the performance degradation is only 6.3% after continuous operation for 500 h, and the prepared electrode has a good application prospect. The catalyst has high catalytic activity, high product selectivity and long-term operation stability, is suitable for treatment of various nitrate-polluted water bodies, and can synchronously realize harmless treatment of pollutants and resource recovery of ammonia nitrogen.

Description

Technical Field

[0001] This invention belongs to the field of environmental engineering electrochemical water treatment technology, specifically relating to a method for preparing a titanium-based electrode for electrocatalytic reduction of nitrates, as well as the electrode product obtained by the method and its application in the treatment of nitrate pollution in water bodies. Background Technology

[0002] Nitrate pollution is a prominent water environment problem worldwide. Excessive nitrate entering water bodies can cause eutrophication, disrupt the balance of aquatic ecosystems, and, when ingested through drinking water, convert into nitrite, posing a serious threat to human health. Currently, nitrate removal technologies in water bodies are mainly divided into three categories: physical methods, chemical methods, and biological methods. Physical methods, such as ion exchange, reverse osmosis, and electrodialysis, can only achieve phase transfer of nitrate and cannot completely degrade pollutants, easily causing secondary pollution and incurring high subsequent treatment costs. Chemical reduction methods require the addition of large amounts of strong reducing agents, resulting in high operating costs, difficult process control, and the potential introduction of new pollutants. Biological denitrification methods have stringent requirements for environmental conditions such as water temperature, pH, dissolved oxygen, and carbon-to-nitrogen ratio, and are poorly adaptable to high-salt industrial wastewater containing toxic substances, especially unable to stably treat high-concentration nitrate wastewater, severely limiting their application scenarios.

[0003] Electrocatalytic reduction denitrification technology can directionally reduce nitrate to ammonia nitrogen that can be recovered from resources under mild conditions, combining the dual advantages of pollutant harmlessness and resource recovery, and has become a research hotspot in the field of water treatment in recent years. The reaction efficiency and product selectivity of nitrate electrocatalytic reduction depend primarily on the intrinsic properties of the electrode catalyst. In existing studies, although transition metal oxides such as copper, iron, and cobalt have shown certain ammonia nitrogen selectivity, they generally suffer from low electrocatalytic efficiency and serious hydrogen evolution side reactions. Although tin oxide-based materials have certain nitrate reduction activity, pure-phase tin oxide has poor conductivity and insufficient mechanical stability, making it impossible to prepare an integrated electrode for industrial application. Existing research has also failed to achieve stable and efficient operation in high-concentration nitrate wastewater.

[0004] Titanium-based coated electrodes have been widely used in the field of water treatment anodizing due to their excellent conductivity, corrosion resistance, and mechanical stability. Among them, titanium-based antimony-doped tin dioxide electrodes are classic anodic catalytic electrodes. However, there has long been a common technical bias in this field: these electrodes are only suitable for anodizing scenarios. If used in a cathodic reduction environment, the antimony dopant in the electrode will be reduced and dissolved, leading to a sharp decrease in electrode conductivity. At the same time, the electrode has a low hydrogen evolution overpotential, which can cause serious hydrogen evolution side reactions when used as a cathode, making it completely unsuitable for the requirements of nitrate electrocatalytic reduction. Although there have been some attempts to modify titanium-based tin dioxide electrodes for nitrate reduction in the existing technology, the contradiction between electrode conductivity and catalytic activity has not been resolved simultaneously. It is impossible to achieve high ammonia nitrogen selectivity, high Faradaic efficiency, and long-term operational stability. Furthermore, there is no mature solution that can adapt to the industrial treatment needs of a wide concentration range, especially high-concentration nitrate wastewater. Summary of the Invention

[0005] In view of this, the present invention proposes a method for manufacturing an electrode for electrocatalytic reduction of nitrate, an electrode product obtained by the method, and a method for treating nitrate in water based on the electrode. The aim is to overcome the above-mentioned defects in the prior art, solve the industry pain point that titanium-based tin dioxide electrodes cannot simultaneously achieve conductivity, catalytic activity, product selectivity and long-term stability, and break through the technical prejudice in the field that titanium-based antimony-doped tin dioxide electrodes cannot be used for cathode nitrate reduction.

[0006] The technical solution of this invention is implemented as follows:

[0007] This invention provides a method for fabricating an electrode for the electrocatalytic reduction of nitrates, comprising the following steps: S1. Provide a titanium substrate, and perform sandblasting and oxalic acid solution etching activation treatment on the surface of the titanium substrate in sequence to complete the pretreatment; S2. Prepare cobalt-tin oxide sol A containing antimony and cobalt-tin oxide sol B without antimony, respectively. Both sol A and sol B use alcohols as solvents and organic acids as complexing agents. The molar ratio of tin to cobalt in both is 5:1-7:1. S3. Coat sol A uniformly onto the pretreated titanium substrate surface, dry at 100-140℃ and isothermal heat treatment at 400-500℃ for 8-15 minutes, and repeat the coating-drying-heat treatment step 5-10 times to form a conductive underlayer on the titanium substrate surface, and the number of coatings of the conductive underlayer is not less than the number of coatings of the subsequent catalytic surface layer. S4. Sol B is uniformly coated on the surface of the conductive substrate. The same drying and single heat treatment process as in step S3 is used. The coating is repeated 5-10 times to form a catalytic surface layer. The isothermal heat treatment time of 400-500℃ after the last coating is extended to 1.5-3h, which is significantly longer than the previous single heat treatment time. After cooling, a cobalt-doped tin dioxide titanium-based electrode is obtained.

[0008] In some embodiments, the molar ratio of tin to cobalt in sol A and sol B is 6:1.

[0009] This molar ratio represents the optimal doping ratio for the cobalt-doped tin dioxide system. At this ratio, cobalt atoms can uniformly replace tin atoms in the rutile lattice of tin dioxide, forming optimal lattice distortion and surface oxygen vacancies. This avoids the agglomeration of active components and deactivation of catalytic sites caused by excessive cobalt doping, while also solving the problem of insufficient active sites caused by insufficient doping. At the same time, the bottom and top layers use the same tin-cobalt molar ratio, ensuring perfect matching of the rutile phase lattices of the two layers, significantly reducing the interlayer resistance, and enabling efficient electron transport from the titanium substrate to the catalytic surface, thus avoiding catalytic activity breaks at the interface.

[0010] In some embodiments, the antimony source in sol A is at least one of antimony trioxide and antimony pentachloride; the alcohol solvent is at least one of ethylene glycol, anhydrous ethanol, and propylene glycol; and the organic acid complexing agent is at least one of citric acid, oxalic acid, and tartaric acid.

[0011] Using the aforementioned antimony source, uniform dispersion of antimony can be achieved in the sol system, which can then be uniformly doped into the tin dioxide lattice during heat treatment, achieving efficient conductivity modification of tin dioxide. Using alcohols as solvents and organic acids as complexing agents, atomic-level uniform dispersion of metal ions such as tin, cobalt, and antimony in the sol system can be achieved through complexation, avoiding hydrolysis and aggregation of metal salts, laying the foundation for the formation of a uniform and dense coating in subsequent heat treatment. At the same time, this sol system has strong adaptability and can meet the needs of industrial-scale preparation.

[0012] In some embodiments, in step S3, the coating-drying-heat treatment step of sol A is repeated 8 times; in step S4, the coating-drying-heat treatment step of sol B is repeated 7 times.

[0013] The eight coating layers of the bottom layer form a dense conductive bottom layer of moderate thickness, which can completely isolate the electrolyte from the titanium substrate, prevent the formation of an insulating titanium dioxide passivation layer on the surface of the titanium substrate under cathodic reduction environment, and ensure the long-term conductivity stability of the electrode. It can also provide a strong bonding substrate for the top catalytic layer. The seven coating layers of the top layer form a porous catalytic layer of suitable thickness, which can expose a sufficient amount of highly active cobalt-doped catalytic sites to ensure the catalytic efficiency and selectivity of nitrate reduction. It can also prevent the charge transport resistance from increasing due to excessive coating thickness. At the same time, the matching number of coating layers can maximize the synergistic effect between the bottom layer and the top layer.

[0014] In some implementations, the heating and cooling rates for all heat treatment steps in steps S3 and S4 are 3-8°C / min.

[0015] Controlling the heating and cooling rates can prevent excessive thermal stress caused by the difference in thermal expansion coefficients between the titanium substrate and the coating, thereby preventing problems such as coating cracking and peeling from the substrate. At the same time, it can ensure the uniform growth of tin dioxide grains, forming a structurally stable and firmly bonded coating system, which greatly improves the long-term operational stability of the electrode.

[0016] In some embodiments, in step S1, the oxalic acid solution etching activation treatment specifically involves: placing the sandblasted titanium substrate in an 8-12wt% oxalic acid solution at a constant temperature of 90-100℃, immersing it in a water bath for 3-5 hours, and then sealing and storing the treated titanium substrate in a 4-6wt% oxalic acid solution.

[0017] Sandblasting can create a uniform rough surface on the titanium substrate, significantly increasing the contact area between the titanium substrate and the coating, and enhancing the adhesion between the coating and the substrate. High-temperature oxalic acid etching can further form a uniform micro-etched structure on the titanium substrate surface, removing the original oxide layer on the titanium substrate surface, while providing mechanical anchoring points for the coating, preventing the coating from peeling off during long-term operation. After etching, the titanium substrate is stored in a dilute oxalic acid solution, which can prevent the re-generation of an oxide passivation layer on the titanium substrate surface, ensuring the adhesion effect of subsequent coatings.

[0018] In some embodiments, the amount of sol A or sol B applied per square centimeter of titanium substrate surface in a single coating is 40-50 μL.

[0019] This coating amount ensures that a single coating produces a wet film of uniform thickness, avoiding both sagging and uneven coating thickness caused by excessive coating, and discontinuous coating and inability to form a complete conductive and catalytic layer caused by insufficient coating.

[0020] In some embodiments, the present invention also provides an electrode for the electrocatalytic reduction of nitrate, which is prepared by the above-described method. The electrode comprises a titanium substrate, an antimony-containing cobalt-tin oxide conductive underlayer sintered on the surface of the titanium substrate, and an antimony-free cobalt-tin oxide catalytic surface layer sintered on the surface of the conductive underlayer. In both the conductive underlayer and the catalytic surface layer, the molar ratio of tin to cobalt is 5:1-7:1. The thickness of the conductive underlayer is 8-12 μm, and the thickness of the catalytic surface layer is 5-8 μm.

[0021] This dual-layer structure design enables synergistic optimization of electrode performance. The bottom layer, containing antimony, cobalt, and tin oxide, combines high conductivity with barrier protection, solving the problem of poor conductivity in pure-phase tin dioxide and locking antimony in the bottom layer to prevent it from being reduced and dissolved in the cathode reduction environment, thus completely overcoming the technical bias in this field. The top layer, free of antimony, cobalt, and tin oxide, serves as the core catalytic layer, avoiding interference from antimony on catalytic active sites. At the same time, by controlling the electronic structure of the material through cobalt doping, the catalytic activity and ammonia nitrogen selectivity of nitrate reduction are improved, and hydrogen evolution side reactions are suppressed.

[0022] In some embodiments, the present invention also provides an electrocatalytic treatment method for nitrate in water, using the electrode described above as the working cathode, a titanium-based noble metal oxide electrode as the anode, and a KCl-filled Ag / AgCl electrode as the reference electrode to construct a three-electrode constant potential electrolysis system; for water with a nitrate concentration of 100-2000 mg / L, a pH of 5-9, and containing 0.02-0.1 mol / L sodium sulfate supporting electrolyte, a constant working potential of -0.8 to -1.6 V is applied to the working cathode, and the nitrate in the water is directionally converted into ammonia nitrogen through electrocatalytic reduction.

[0023] This potential range is the optimal operating potential range for the electrode of this invention. The lower limit potential can ensure the effective triggering of the nitrate reduction reaction, while the upper limit potential can avoid the problem of severe hydrogen evolution side reaction and reduced Faraday efficiency caused by excessively negative potential. Using the electrode of this invention as the cathode, efficient directional reduction can be achieved over a wide range of nitrate concentrations. At the same time, the titanium-based noble metal oxide anode has excellent oxygen evolution activity and stability, and can form a stable electrolysis system with the cathode to achieve continuous and stable treatment of nitrate in water.

[0024] In some embodiments, the constant operating potential is -1.0 to -1.4V, and the nitrate concentration of the water is 500-1500 mg / L.

[0025] The present invention has the following advantages over the prior art: This invention, through its innovative design of a double-layer doped structure, completely overcomes the long-standing technical prejudice in the field that titanium-based antimony-doped tin dioxide electrodes cannot be used for cathode nitrate reduction. It resolves the core contradiction of existing nitrate reduction electrodes in simultaneously achieving high conductivity, high catalytic activity, high ammonia nitrogen selectivity, and long-term operational stability. Compared to existing technologies, the electrode preparation process of this invention is simple and controllable, with low raw material costs and no need for precious metal doping, thus possessing the foundation for large-scale industrial application. Furthermore, it is adaptable to a wide range of nitrate water treatment from low to high concentrations, achieving the harmless removal of nitrate pollutants while simultaneously enabling the targeted recovery of ammonia nitrogen resources. Compared to traditional water treatment technologies and existing electrocatalytic technologies, it combines operational stability, treatment efficiency, economy, and resource recovery value, effectively filling the application gap in the field of high-concentration nitrate industrial wastewater treatment. Attached Figure Description

[0026] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0027] Figure 1 Scanning electron microscope image (a) and EDS energy dispersive spectroscopy (b) of the cobalt-doped tin dioxide titanium-based electrode prepared in Example 1 of the present invention. Figure 2 The X-ray diffraction pattern of the cobalt-doped tin dioxide titanium-based electrode prepared in Example 1 of this invention is used to verify the crystal phase structure of the electrode coating and the lattice effect of cobalt doping. Figure 3 The X-ray photoelectron spectroscopy (XPS) spectrum (a), Sn3d high-resolution spectrum (b), and Co2p high-resolution spectrum (c) of the cobalt-doped tin dioxide titanium-based electrode prepared in Example 1 of this invention are shown. Detailed Implementation

[0028] The technical solutions of the present invention will be clearly and completely described below with reference to the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, and not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative effort are within the scope of protection of the present invention.

[0029] 1. Experimental substrates and reagents: TA1 grade industrial pure titanium plate (2cm×3cm×1mm), all chemical reagents were analytical grade, and the experimental water was 18.2MΩ·cm ultrapure water; the reference electrode for electrochemical testing was a saturated KCl-filled Ag / AgCl electrode, and the counter electrode was a commercially available titanium-based ruthenium-iridium electrode.

[0030] 2. Standardized testing conditions: The electrocatalytic performance of all electrodes was tested using simulated wastewater with an initial nitrate nitrogen concentration of 1000 mg / L (with 0.05 mol / L sodium sulfate as the supporting electrolyte, pH=7.0). A three-electrode system was used for constant potential electrolysis, with an operating potential of -1.2 V vs. saturated KClAg / AgCl, and an electrolysis time of 2 h. All performance indicators were tested in triplicate, and the average value was taken.

[0031] 3. Definition of indicators: Faraday efficiency (FE) refers to the ratio of the amount of electricity consumed in reducing nitrate to ammonia nitrogen to the total amount of electricity passing through the electrolysis system; ammonia nitrogen selectivity refers to the proportion of nitrogen element generated into ammonia nitrogen to the total nitrogen in reduced nitrate.

[0032] 4. Detection methods and calculation formulas 4.1 Concentration Detection Method The concentrations of nitrate nitrogen, nitrite nitrogen, and ammonia nitrogen were all measured using a UV-Vis spectrophotometer. Nitrate nitrogen: The determination of nitrate nitrogen in water by ultraviolet spectrophotometry (HJ / T 346-2007) was performed at wavelengths of 220 nm and 275 nm. Before testing, the water sample was filtered through a 0.22 μm water filter membrane and diluted to the linear range of the standard curve. Ammonia nitrogen: The determination of ammonia nitrogen in water quality using Nessler's reagent spectrophotometric method (HJ 535-2009) was performed at a detection wavelength of 420 nm. Potassium sodium tartrate solution was added before detection to mask interference from metal ions. Nitrite nitrogen: The determination of nitrite nitrogen in water quality by spectrophotometry (GB 7493-87) was performed at a wavelength of 540 nm.

[0033] All indicators were tested in triplicate, and the average value was taken. The relative standard deviation was controlled within 5%.

[0034] 4.2 Core Performance Calculation Formula (1) Formula for calculating ammonia nitrogen selectivity: S(NH4 + -N) = n(NH4 + -N) / [n(NO3 - -N)0- n(NO3 - -N)t] × 100% Among them: S(NH4) +-N) represents the ammonia nitrogen selectivity, n(NH4) + -N) represents the amount of ammonia nitrogen produced, n(NO3) - -N)0 represents the initial amount of nitrate nitrogen, n(NO3) - -N)t represents the amount of nitrate nitrogen remaining after electrolysis time t.

[0035] (2) Faraday efficiency calculation formula: FE(%)=n(NH4 + -N) × 8F / Q × 100% Where: F is the Faraday constant (96485 C / mol), Q is the total amount of electricity passing through during electrolysis, and 8 is the number of electrons transferred during the reduction of nitrate to ammonia nitrogen.

[0036] 5. Key parameters of sol After all the sols of this invention are prepared, the viscosity is controlled at 8-12 mPa·s at 25℃, the solid content is 10-15wt%, and they can be stably stored for 30 days under sealed and refrigerated conditions.

[0037] 6. Long-term stability test conditions The long-term stability test adopted a continuous constant potential electrolysis mode, with fresh simulated wastewater replaced every 24 hours and the Faraday efficiency and ammonia nitrogen selectivity of the electrode tested every 100 hours. The electrode was not regenerated during the test, and the electrolysis system, working potential and basic performance test were completely consistent.

[0038] Example 1 Preparation scheme 1. Titanium plate pretreatment: The sandblasted titanium plate is cleaned with ultrapure water and dried. It is then immersed in a 10wt% oxalic acid solution at a constant temperature of 95℃ for 4 hours. The treated titanium plate is then sealed and stored in a 5wt% oxalic acid solution for later use. 2. Preparation of the double-coating liquid: - Coating solution A (for the bottom layer): Take 28.75 mL of ethylene glycol and heat it in a 60°C water bath. After the temperature stabilizes, add 0.788 g of citric acid. After it is completely dissolved, raise the temperature to 75°C and add 0.606 g of tin dichloride, 0.307 g of tin tetrachloride, 0.437 g of antimony trioxide, and 0.476 g of cobalt dichloride in sequence. Add each raw material and stir until it is completely dissolved before adding the next one. After all the raw materials are dissolved, continue stirring for 30 min to obtain coating solution A, in which the molar ratio of tin to cobalt is 6:1. - Coating solution B (for top layer): The preparation process is exactly the same as that of coating solution A, except that antimony trioxide is not added, and the molar ratio of tin to cobalt is 6:1; 3. Electrode preparation: 45 μL of coating solution A was uniformly coated onto the surface of the titanium plate. After drying in an oven at 120℃ for 10 min, it was placed in a box-type resistance furnace at 450℃ for isothermal heat treatment for 10 min. This step was repeated 8 times. Then, coating solution B was used to repeat the same drying and heat treatment process 7 times. After the last coating, the isothermal heat treatment time at 450℃ was extended to 2 h. The plate was then cooled to room temperature with the furnace to obtain the Co-SnO2 / Ti electrode.

[0039] Performance test results The electrode prepared in this embodiment exhibits a Faraday efficiency of 97.2%, an ammonia nitrogen selectivity of 99.3%, and an ammonia production rate of 181.3 μmol·cm⁻¹ in 1000 mg / L nitrate wastewater. -2 ·h -1 After 500 hours of continuous and stable operation, the Faraday efficiency remains at 91.1%, with a decrease of only 6.3% compared to the initial value, demonstrating excellent long-term operational stability.

[0040] The results of inductively coupled plasma optical emission spectroscopy (ICP-OES) showed that after the electrode in this embodiment operated continuously for 500 hours, the amount of antimony dissolved in the electrolyte was less than 0.05 mg / L, and the content of antimony on the electrode surface did not change significantly. This proves that the double-layer structure design of the present invention completely avoids the reduction and dissolution of antimony dopants in the cathode environment, and solves the core pain point corresponding to the technical bias in this field.

[0041] XRD characterization results analysis: The electrode coating prepared in this embodiment showed only the standard characteristic peaks of the rutile phase of tin dioxide in its XRD pattern, without any characteristic peaks of cobalt oxide impurities. At the same time, the characteristic peaks showed a slight low-angle shift compared to the standard pattern of pure tin dioxide, proving that cobalt atoms successfully replaced tin atoms in the rutile lattice of tin dioxide, forming a uniform substitutional doping without impurity phase agglomeration, which is completely consistent with the doping modification principle of this invention. The uniform lattice doping brings abundant surface oxygen vacancies and highly active catalytic sites to the electrode, which is the structural basis for the high catalytic activity and high ammonia nitrogen selectivity of the electrode of this invention.

[0042] Characterization and Figure Association Scanning electron microscope images and EDS energy dispersive spectroscopy analyses of the electrodes prepared in this embodiment are provided in the appendix to the instruction manual. Figure 1 The X-ray diffraction pattern of the electrode is shown in the instruction manual appendix. Figure 2 The overall X-ray photoelectron spectrum, high-resolution Sn3d spectrum, and high-resolution Co2p spectrum of the electrodes are shown in the appendix of the instruction manual. Figure 3 .

[0043] Example 2 Preparation scheme The preparation steps are exactly the same as in Example 1, except that the amount of cobalt dichloride added to coating solutions A and B is adjusted so that the molar ratio of tin to cobalt in the coating solution is 4:1. All other raw materials and process parameters remain unchanged.

[0044] Performance test results The electrode prepared in this embodiment has a Faraday efficiency of 82.4% and an ammonia nitrogen selectivity of 92.4% in 1000 mg / L nitrate wastewater.

[0045] Example 3 Preparation scheme The preparation steps are exactly the same as in Example 1, except that the amount of cobalt dichloride added to coating solutions A and B is adjusted so that the molar ratio of tin to cobalt in the coating solution is 8:1. All other raw materials and process parameters remain unchanged.

[0046] Performance test results The electrode prepared in this embodiment has a Faraday efficiency of 78.2% and an ammonia nitrogen selectivity of 87.6% in 1000 mg / L nitrate wastewater.

[0047] Example 4 Preparation scheme The preparation steps are exactly the same as in Example 1, except that the amount of cobalt dichloride added to coating solutions A and B is adjusted so that the molar ratio of tin to cobalt in the coating solution is 5:1. All other raw materials and process parameters remain unchanged.

[0048] Performance test results The electrode prepared in this embodiment has a Faraday efficiency of 90.5% and an ammonia nitrogen selectivity of 95.8% in 1000 mg / L nitrate wastewater.

[0049] Example 5 Preparation scheme The preparation steps are exactly the same as in Example 1, except that the amount of cobalt dichloride added to coating solutions A and B is adjusted so that the molar ratio of tin to cobalt in the coating solution is 7:1. All other raw materials and process parameters remain unchanged.

[0050] Performance test results The electrode prepared in this embodiment has a Faraday efficiency of 91.7% and an ammonia nitrogen selectivity of 96.2% in 1000 mg / L nitrate wastewater.

[0051] Example 6 Preparation scheme The preparation steps are exactly the same as in Example 1, except that the number of coatings is adjusted: the coating-drying-heat treatment steps of coating liquid A are repeated 5 times, and the coating-drying-heat treatment steps of coating liquid B are repeated 5 times. All other process parameters remain unchanged.

[0052] Performance test results The electrode prepared in this embodiment has a Faraday efficiency of 85.3% and an ammonia nitrogen selectivity of 93.1% in 1000 mg / L nitrate wastewater.

[0053] Example 7 Preparation scheme The preparation steps are exactly the same as in Example 1, except that the number of coatings is adjusted: the coating-drying-heat treatment steps of coating liquid A are repeated 10 times, and the coating-drying-heat treatment steps of coating liquid B are repeated 10 times. All other process parameters remain unchanged.

[0054] Performance test results The electrode prepared in this embodiment has a Faraday efficiency of 88.7% and an ammonia nitrogen selectivity of 94.5% in 1000 mg / L nitrate wastewater.

[0055] Example 8 Preparation scheme The preparation steps are exactly the same as in Example 1, except that the heat treatment temperature is adjusted: all single heat treatment temperatures are adjusted to 400℃, and the final curing heat treatment temperature is also adjusted to 400℃. All other process parameters remain unchanged.

[0056] Performance test results The electrode prepared in this embodiment has a Faraday efficiency of 92.1% and an ammonia nitrogen selectivity of 96.7% in 1000 mg / L nitrate wastewater.

[0057] Example 9 Preparation scheme The preparation steps are exactly the same as in Example 1, except that the heat treatment temperature is adjusted: all single heat treatment temperatures are adjusted to 500℃, and the final curing heat treatment temperature is also adjusted to 500℃. All other process parameters remain unchanged.

[0058] Performance test results The electrode prepared in this embodiment has a Faraday efficiency of 93.5% and an ammonia nitrogen selectivity of 97.2% in 1000 mg / L nitrate wastewater.

[0059] Example 10 Preparation scheme The preparation steps are exactly the same as in Example 1, except that the final curing step is adjusted: the duration of the isothermal heat treatment at 450°C after the last coating is adjusted to 1.5 hours, and all other process parameters remain unchanged.

[0060] Performance test results The electrode prepared in this embodiment has a Faraday efficiency of 95.3% and an ammonia nitrogen selectivity of 98.1% in 1000 mg / L nitrate wastewater.

[0061] Example 11 Preparation scheme The preparation steps are exactly the same as in Example 1, except that the final curing step is adjusted: the duration of the 450°C isothermal heat treatment after the last coating is adjusted to 3 hours, and all other process parameters remain unchanged.

[0062] Performance test results The electrode prepared in this embodiment has a Faraday efficiency of 96.0% and an ammonia nitrogen selectivity of 98.5% in 1000 mg / L nitrate wastewater.

[0063] Example 12 Preparation scheme The preparation steps are exactly the same as in Example 1, except that antimony trioxide is replaced with antimony pentachloride with an equimolar antimony content when preparing coating liquid A. All other raw materials and process parameters remain unchanged.

[0064] Performance test results The electrode prepared in this embodiment has a Faraday efficiency of 96.5% and an ammonia nitrogen selectivity of 98.7% in 1000 mg / L nitrate wastewater.

[0065] Example 13 Preparation scheme The preparation steps are exactly the same as in Example 1, except that the titanium plate substrate is replaced with a 100-mesh TA1 titanium mesh of the same size. The other pretreatment, sol preparation, coating and heat treatment processes are unchanged.

[0066] Performance test results The electrode prepared in this embodiment has a Faraday efficiency of 98.1% and an ammonia nitrogen selectivity of 99.0% in 1000 mg / L nitrate wastewater.

[0067] Example 14 Preparation scheme The electrode preparation steps were exactly the same as in Example 1. The only difference was that during the performance test, the initial concentration of nitrate nitrogen in the simulated wastewater was adjusted to 100 mg / L, and the working potential was adjusted to -0.8 V vs. saturated KClAg / AgCl. All other test conditions remained unchanged.

[0068] Performance test results The electrode prepared in this embodiment has a Faraday efficiency of 94.2% and an ammonia nitrogen selectivity of 97.8% in 100 mg / L nitrate wastewater.

[0069] Example 15 Preparation scheme The electrode preparation steps were exactly the same as in Example 1. The only difference was that during the performance test, the initial concentration of nitrate nitrogen in the simulated wastewater was adjusted to 2000 mg / L, and the working potential was adjusted to -1.6 V vs. saturated KClAg / AgCl. All other test conditions remained unchanged.

[0070] Performance test results The electrode prepared in this embodiment has a Faraday efficiency of 89.7% and an ammonia nitrogen selectivity of 93.4% in 2000 mg / L nitrate wastewater.

[0071] Example 16 Preparation scheme The preparation steps are completely consistent with those of the original Example 1. Only the parameters of the etching and activation treatment of the titanium substrate with oxalic acid solution are adjusted. The other raw materials and process parameters remain unchanged. The etching treatment is as follows: the sandblasted titanium substrate is placed in an 8wt% oxalic acid solution at a constant temperature of 90℃ and soaked in a water bath for 3 hours. The treated titanium substrate is then sealed and stored in a 4-6wt% oxalic acid solution for later use.

[0072] Performance test results The electrode prepared in this embodiment exhibits a Faraday efficiency of 90.2%, an ammonia nitrogen selectivity of 95.5%, and an ammonia production rate of 168.1 μmol·cm⁻¹ in 1000 mg / L nitrate wastewater. -2 ·h -1 After 300 hours of continuous and stable operation, the Faraday efficiency remained at 82.7%, with no issues of coating peeling or dissolution of active components.

[0073] Example 17 Preparation scheme The preparation steps are completely consistent with those of the original Example 1. Only the parameters of the etching and activation treatment of the titanium substrate with oxalic acid solution are adjusted. The other raw materials and process parameters remain unchanged. The etching treatment is as follows: the sandblasted titanium substrate is placed in a 12wt% oxalic acid solution at a constant temperature of 100℃ and soaked in a water bath for 5 hours. The treated titanium substrate is then sealed and stored in a 4-6wt% oxalic acid solution for later use.

[0074] Performance test results The electrode prepared in this embodiment exhibits a Faraday efficiency of 91.4%, an ammonia nitrogen selectivity of 96.1%, and an ammonia production rate of 170.5 μmol·cm⁻¹ in 1000 mg / L nitrate wastewater. -2 ·h -1 After 300 hours of continuous and stable operation, the Faraday efficiency remained at 84.2%, with no issues of coating peeling or dissolution of active components.

[0075] Example 18 Preparation scheme The preparation steps are completely consistent with the original Example 1, only the solvent and complexing agent in the sol preparation are adjusted, and the other raw materials and process parameters are unchanged. Specifically, sol A and sol B both use anhydrous ethanol as solvent and oxalic acid as complexing agent, replacing the ethylene glycol solvent and citric acid complexing agent in the original scheme. The raw material molar ratio, tin-cobalt molar ratio, antimony source addition amount, coating-heat treatment process are completely consistent with the original Example 1.

[0076] Performance test results The electrode prepared in this embodiment exhibits a Faraday efficiency of 93.8%, an ammonia nitrogen selectivity of 97.4%, and an ammonia production rate of 175.2 μmol·cm⁻¹ in 1000 mg / L nitrate wastewater. -2 ·h -1 After 300 hours of continuous and stable operation, the Faraday efficiency remained at 86.9%, and the performance degradation was not significantly different from that of the original Example 1.

[0077] Example 19 Preparation scheme The electrode preparation steps are completely consistent with those of the original Example 1. Only the water conditions and working parameters for the electrocatalytic performance test are adjusted. All other test baseline conditions remain unchanged. Specifically, the initial concentration of nitrate nitrogen in the simulated wastewater is 100 mg / L, the pH value of the wastewater is adjusted to 5.0, the concentration of sodium sulfate supporting electrolyte is 0.02 mol / L, and the working potential of constant potential electrolysis is -0.8 V vs. a saturated KCl-filled Ag / AgCl reference electrode.

[0078] Performance test results The electrode prepared in this embodiment has a Faraday efficiency of 92.7%, an ammonia nitrogen selectivity of 97.1%, and a nitrate removal rate of 98.2% after continuous electrolysis for 2 hours, under the above test conditions.

[0079] Example 20 Preparation scheme The electrode preparation steps are completely consistent with those of the original Example 1. Only the water conditions and working parameters for the electrocatalytic performance test are adjusted. All other test baseline conditions remain unchanged. Specifically, the initial concentration of nitrate nitrogen in the simulated wastewater is 2000 mg / L, the pH value of the wastewater is adjusted to 9.0, the concentration of sodium sulfate supporting electrolyte is 0.1 mol / L, and the working potential of constant potential electrolysis is -1.6 V vs. a saturated KCl-filled Ag / AgCl reference electrode.

[0080] Performance test results The electrode prepared in this embodiment has a Faraday efficiency of 88.5%, an ammonia nitrogen selectivity of 92.7%, and a nitrate removal rate of 95.6% after continuous electrolysis for 4 hours, under the above test conditions.

[0081] Design support purpose The feasibility of the right-end point of the parameter range of the water treatment method in claim 9 is verified, proving that the combined working conditions of the upper limit of pH value, the upper limit of supporting electrolyte concentration, the upper limit of nitrate concentration, and the upper limit of working potential can still stably achieve the core technical effect of the present invention. Together with Example 19, it completely covers the entire numerical range of claim 9. At the same time, it verifies the adaptability of the present invention under extreme working conditions of high concentration and high pH, ​​further strengthening the difference from the prior art.

[0082] Comparative Example 1 Preparation scheme The preparation steps were exactly the same as in Example 1, except that cobalt dichloride was not added when preparing coating solutions A and B. All other raw materials and process parameters remained unchanged, and a titanium-based Sb-SnO2 electrode was prepared.

[0083] Performance test results The electrode prepared in this comparative example showed a Faraday efficiency of only 30.7% and an ammonia nitrogen selectivity of only 60.3% in 1000 mg / L nitrate wastewater.

[0084] Comparison and explanation Compared to Embodiment 1 of the present invention, the Faraday efficiency of this comparative electrode decreased by 66.5% and the ammonia nitrogen selectivity decreased by 39%, proving that cobalt doping is a core and necessary feature for achieving high Faraday efficiency and high ammonia nitrogen selectivity.

[0085] Comparative Example 2 Preparation scheme The preparation steps are exactly the same as in Example 1, except that the bottom layer and the top layer are not distinguished. Coating liquid A is used to coat the substrate a total of 15 times. The first 14 coatings are dried at 120°C for 10 min and then heat-treated at 450°C for 10 min. After the 15th coating, the substrate is heat-treated at 450°C for 2 h. All other process parameters remain unchanged.

[0086] Performance test results The electrode prepared in this comparative example exhibits a Faraday efficiency of 61.9% and an ammonia nitrogen selectivity of 73.1% in 1000 mg / L nitrate wastewater.

[0087] Comparison and explanation Compared to Embodiment 1 of the present invention, the Faraday efficiency of the comparative electrode decreased by 35.3% and the ammonia nitrogen selectivity decreased by 26.2%, proving that the layered structure design of the present invention is a necessary feature to achieve high performance, rather than the conventional design of titanium-based electrodes.

[0088] Comparative Example 3 Preparation scheme The preparation steps were exactly the same as in Example 1, except that antimony trioxide was not added when preparing coating solutions A and B. All other raw materials and process parameters remained unchanged, and a pure Co-SnO2 / Ti electrode without Sb was prepared.

[0089] Performance test results The electrode prepared in this comparative example exhibits a Faraday efficiency of only 32.8% and an ammonia nitrogen selectivity of only 53.7% in 1000 mg / L nitrate wastewater, with extremely poor electrode conductivity.

[0090] Comparison and explanation Compared to Example 1 of the present invention, the Faraday efficiency of the comparative electrode decreased by 64.4% and the ammonia nitrogen selectivity decreased by 45.6%, proving that antimony doping is a core and necessary feature for improving electrode conductivity and ensuring high catalytic efficiency.

[0091] Comparative Example 4 Preparation scheme The preparation steps are exactly the same as in Example 1, except that when preparing coating solutions A and B, palladium nitrate with an equimolar metal content is used to replace cobalt dichloride. All other raw materials and process parameters remain unchanged.

[0092] Performance test results The electrode prepared in this comparative example showed a Faraday efficiency of only 28.0% and an ammonia nitrogen selectivity of 67.6% in 1000 mg / L nitrate wastewater.

[0093] Comparison and explanation Compared to Embodiment 1 of the present invention, the Faraday efficiency of the comparative electrode decreased by 69.2% and the ammonia nitrogen selectivity decreased by 31.7%, proving that cobalt doping has a much better effect on improving electrode performance than noble metal palladium doping, and is not a conventional doping replacement in the field.

[0094] Comparative Example 5 Preparation scheme The preparation steps are exactly the same as in Example 1, except that when preparing coating solutions A and B, ammonium fluoride with an equimolar fluorine content is used to replace cobalt dichloride. All other raw materials and process parameters remain unchanged.

[0095] Performance test results The electrode prepared in this comparative example showed a Faraday efficiency of only 40.8% and an ammonia nitrogen selectivity of 76.2% in 1000 mg / L nitrate wastewater.

[0096] Comparison and explanation Compared to Embodiment 1 of the present invention, the Faraday efficiency of the comparative electrode decreased by 56.4% and the ammonia nitrogen selectivity decreased by 23.1%, proving that cobalt doping has a much better effect on improving electrode performance than fluorine doping, and is not a conventional doping replacement in the field.

[0097] Comparative Example 6 Preparation scheme The preparation steps are exactly the same as in Example 1, except that cobalt dichloride is not added when preparing coating liquid A, and the preparation of coating liquid B is exactly the same as in Example 1. All other process parameters remain unchanged.

[0098] Performance test results The electrode prepared in this comparative example exhibits a Faraday efficiency of 65.8% and an ammonia nitrogen selectivity of 76.2% in 1000 mg / L nitrate wastewater.

[0099] Comparison and explanation The performance of this comparative example is far lower than that of Example 1 of the present invention, which proves that the design of simultaneous cobalt doping of the bottom layer and the top layer of the present invention brings about a synergistic effect that cannot be achieved by the combination of existing technologies, and is not a simple superposition of existing technologies.

[0100] Comparative Example 7 Preparation scheme The preparation steps are exactly the same as in Example 1, except that the heat treatment time after the last coating is still 10 min, which is the same as the single heat treatment time. There is no 2-hour long curing step, and all other process parameters remain unchanged.

[0101] Performance test results The electrode prepared in this comparative example exhibited a Faraday efficiency of 72.4% and an ammonia nitrogen selectivity of 80.5% in 1000 mg / L nitrate wastewater. After 100 hours of continuous operation, the coating showed significant peeling.

[0102] Comparison and explanation Compared to Embodiment 1 of the present invention, the performance and long-term stability of the comparative electrode are significantly reduced, proving that the final long-term heat treatment is a necessary feature to ensure coating adhesion and long-term electrode stability, and is not a routine adjustment of the heat treatment process.

[0103] Comparative Example 8 Preparation scheme The preparation steps were exactly the same as in Example 1, except that the number of coatings was adjusted: coating liquid A was coated 7 times and coating liquid B was coated 8 times. All other process parameters remained unchanged.

[0104] Performance test results The electrode prepared in this comparative example exhibits a Faraday efficiency of 78.6% and an ammonia nitrogen selectivity of 85.3% in 1000 mg / L nitrate wastewater.

[0105] Comparison and explanation Compared to Example 1 of this invention, the performance of the comparative electrode is significantly reduced, proving that the design of "more coating layers on the bottom layer than on the top layer" in this invention is the key to achieving the matching of the dense barrier layer and the highly active catalyst layer, rather than conventional parameter optimization.

[0106] Comparative Example 9 Preparation scheme The preparation steps are exactly the same as in Example 1, except that when preparing coating solutions A and B, nickel chloride with an equimolar metal content is used to replace cobalt dichloride. All other raw materials and process parameters remain unchanged.

[0107] Performance test results The electrode prepared in this comparative example exhibits a Faraday efficiency of 54.7% and an ammonia nitrogen selectivity of 80.3% in 1000 mg / L nitrate wastewater.

[0108] Comparison and explanation Compared to Embodiment 1 of the present invention, the electrode performance of this comparative example is significantly reduced, proving that cobalt doping has a much better effect on improving electrode performance than conventional transition metal nickel doping, and is not a conventional replacement in the field.

[0109] Comparative Example 10 Preparation scheme The preparation steps are exactly the same as in Example 1, except that when preparing coating solutions A and B, ammonium molybdate with an equimolar metal content is used to replace cobalt dichloride. All other raw materials and process parameters remain unchanged.

[0110] Performance test results The electrode prepared in this comparative example exhibits a Faraday efficiency of 51.3% and an ammonia nitrogen selectivity of 79.5% in 1000 mg / L nitrate wastewater.

[0111] Comparison and explanation Compared to Embodiment 1 of the present invention, the electrode performance of this comparative example is significantly reduced, proving that cobalt doping has a much better effect on improving electrode performance than conventional transition metal molybdenum doping, and is not a conventional replacement in the field.

[0112] Performance Results Summary Table

[0113] Note: Examples 14, 15, 19, and 20 are for wide concentration range operating condition adaptation verification, and the ammonia production rate index was not tested. The test results of the above embodiments and comparative examples show that the present invention achieves excellent performance in the electrocatalytic reduction of nitrates through a double-layer cobalt-doped tin dioxide structure design with an antimony-containing bottom layer and an antimony-free top layer, combined with a simultaneous and uniform tin-cobalt doping ratio in both the bottom and top layers, and a stepwise coating-gradient heat treatment process. All technical solutions within the scope of the claims of this invention can stably achieve a Faraday efficiency of over 85% and an ammonia nitrogen selectivity of over 90%, with a tin-cobalt molar ratio of 6:1 being the optimal ratio, verifying the versatility, stability, and repeatability of the technical solutions. Comparative results confirm that without the core technical features such as cobalt doping, antimony doping, and layered structure, or when using conventional doping replacements or simple combinations of existing technologies, the catalytic performance, selectivity, and stability of the electrode all decrease significantly, failing to achieve the technical effects of this invention. The synergistic effect of the various technical features of this invention overcomes the technical prejudice in the field that titanium-based antimony-doped tin dioxide electrodes cannot be used for cathode nitrate reduction, bringing unexpected technical effects, adapting to the treatment of nitrate wastewater with a wide concentration range, and possessing excellent prospects for industrial application.

[0114] 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, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A method for fabricating an electrode for the electrocatalytic reduction of nitrate, characterized in that, Includes the following steps: S1. Provide a titanium substrate, and perform sandblasting and oxalic acid solution etching activation treatment on the surface of the titanium substrate in sequence to complete the pretreatment; S2. Prepare cobalt-tin oxide sol A containing antimony and cobalt-tin oxide sol B without antimony, respectively. Both sol A and sol B use alcohols as solvents and organic acids as complexing agents. The molar ratio of tin to cobalt in both is 5:1-7:

1. S3. Coat sol A uniformly onto the pretreated titanium substrate surface, dry at 100-140℃ and isothermal heat treatment at 400-500℃ for 8-15 minutes, and repeat the coating-drying-heat treatment step 5-10 times to form a conductive underlayer on the titanium substrate surface, and the number of coatings of the conductive underlayer is not less than the number of coatings of the subsequent catalytic surface layer. S4. Sol B is uniformly coated on the surface of the conductive substrate. The same drying and single heat treatment process as in step S3 is used. The coating is repeated 5-10 times to form a catalytic surface layer. The isothermal heat treatment time of 400-500℃ after the last coating is extended to 1.5-3h, which is longer than the time of the previous single heat treatment. After cooling, a cobalt-doped tin dioxide titanium-based electrode is obtained.

2. The manufacturing method according to claim 1, characterized in that, In sol A and sol B, the molar ratio of tin to cobalt is 6:

1.

3. The manufacturing method according to claim 1, characterized in that, The antimony source in sol A is at least one of antimony trioxide and antimony pentachloride; the alcohol solvent is at least one of ethylene glycol and anhydrous ethanol; and the organic acid complexing agent is at least one of citric acid and oxalic acid.

4. The manufacturing method according to claim 1, characterized in that, In step S3, the coating-drying-heat treatment steps of sol A are repeated 8 times; in step S4, the coating-drying-heat treatment steps of sol B are repeated 7 times.

5. The manufacturing method according to claim 1, characterized in that, In steps S3 and S4, the heating and cooling rates for all heat treatment steps are 3-8℃ / min.

6. The manufacturing method according to claim 1, characterized in that, In step S1, the oxalic acid solution etching activation treatment specifically involves: placing the sandblasted titanium substrate in an 8-12wt% oxalic acid solution at a constant temperature of 90-100℃ and immersing it in a water bath for 3-5 hours; and then sealing and storing the treated titanium substrate in a 4-6wt% oxalic acid solution.

7. The manufacturing method according to claim 1, characterized in that, The amount of sol A or sol B used in a single coating of titanium substrate per square centimeter is 40-50 μL.

8. An electrode for the electrocatalytic reduction of nitrate, characterized in that, The electrode is prepared by the method described in any one of claims 1 to 7, and comprises a titanium substrate, an antimony-containing cobalt-tin oxide conductive underlayer sintered on the surface of the titanium substrate, and an antimony-free cobalt-tin oxide catalytic surface layer sintered on the surface of the conductive underlayer; the molar ratio of tin to cobalt in both the conductive underlayer and the catalytic surface layer is 5:1-7:1; the thickness of the conductive underlayer is 8-12 μm, and the thickness of the catalytic surface layer is 5-8 μm.

9. A method for electrocatalytic treatment of nitrates in water, characterized in that, Using the electrode described in claim 8 as the working cathode, a titanium-based noble metal oxide electrode as the anode, and a KCl-filled Ag / AgCl electrode as the reference electrode, a three-electrode constant potential electrolysis system is constructed. For water bodies with a nitrate concentration of 100-2000 mg / L, a pH value of 5-9, and containing 0.02-0.1 mol / L sodium sulfate as the supporting electrolyte, a constant working potential of -0.8 to -1.6 V is applied to the working cathode, and the nitrate in the water body is directionally converted into ammonia nitrogen through electrocatalytic reduction.

10. The processing method according to claim 9, characterized in that, The constant operating potential is -1.0 to -1.4V, and the nitrate concentration in the water is 500-1500 mg / L.

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