Method for detecting ultra-low concentration of nitrate and high-efficiency reduction of ammonia

Abstract

The application relates to the technical field of sewage treatment and discloses a method for detecting and efficiently reducing nitrate to prepare ammonia at an ultralow concentration, which comprises the following steps: synthesizing a Co-MOF precursor by a chemical synthesis method, and converting the Co-MOF into Cu-Co(OH)2HNS through a one-step hydrothermal method; specifically, adding cobalt nitrate hexahydrate and 2-methyl imidazole into a methanol solution, standing for one day, washing with methanol, drying, obtaining a cobalt-based zeolite imidazole acid ester framework (Co-MOF) purple powder, adding the Co-MOF, copper chloride dihydrate and sodium citrate into an ethanol solution to form a mixed solution, setting the oven temperature to 80-140 DEG C, setting the reaction time to 1-3 hours, washing the mixed solution after reaction with ethanol, drying, and obtaining a Cu-Co(OH)2HNS catalytic electrode material. The preparation method of the electrocatalyst is as follows: synthesizing a Co-MOF precursor by a chemical synthesis method, and converting the Co-MOF into Cu-Co(OH)2HNS through a one-step hydrothermal method; when the material is applied to the reduction of nitrate at an ultralow concentration to prepare ammonia, the material can exhibit high ammonia yield and high Faraday efficiency.

Description

Technical Field

[0001] The present invention relates to the technical field of sewage treatment, and in particular to a method for detecting ultra-low concentration nitrate and producing ammonia through efficient reduction. Background Art

[0002] Nitrate (NO3 - ) is a common nitrogen-containing pollutant in surface and subsurface water. Excessive nitrate can easily cause eutrophication and damage aquatic ecosystems. Nitrate can also harm human health, causing liver damage, methemoglobinemia, and even cancer. It is reported that the concentration of nitrate in commonly contaminated water is below 5 mM. Existing nitrate detection methods, including spectrophotometry, ion chromatography, and luminescence analysis, are difficult to adapt to the rapid analysis of low-concentration nitrate ions. Therefore, the development of a more sensitive and portable detection method is particularly important. Secondly, how to eliminate trace nitrate ions in water systems will be the biggest challenge in repairing aquatic ecosystems. Currently, commercial treatment technologies such as reverse osmosis, ion exchange, and electrodialysis are used to treat wastewater containing nitrates, but they cannot eliminate nitrates, and the resulting wastewater containing high concentrations of nitrates requires further treatment. Electrochemical reduction is an effective method for eliminating nitrates. However, traditional electrochemical methods reduce nitrate ions to non-toxic and harmless nitrogen gas, which is discharged into the atmosphere, preventing the reuse of waste resources. Ammonia, as one of the possible products of nitrate reduction, is not only an important raw material for agricultural nitrogen fertilizer, but also an important hydrogen energy carrier. Therefore, the selective reduction of nitrate to ammonia using electrochemical methods can not only alleviate the problem of water pollution, but also realize the reuse of waste nitrate resources. It is a "waste-to-treasure" strategy. The key to achieving ultra-low concentration nitrate detection and efficient electrocatalytic nitrate reduction to ammonia lies in the design of efficient electrocatalysts. Summary of the Invention

[0003] In order to solve the technical problems raised in the background technology, the present invention provides a method for detecting ultra-low concentration nitrate and producing ammonia with high efficiency through reduction.

[0004] The present invention is implemented by the following technical solution: A method for preparing an electrocatalyst comprises the following steps:

[0005] A Co-MOFs precursor was synthesized using a chemical synthesis method, and then converted to Cu-Co(OH)2HNS via a one-step hydrothermal method. Specifically, cobalt nitrate hexahydrate and 2-methylimidazole were added to a methanol solution, allowed to stand for one day, and the reaction product was washed with methanol and dried to obtain a purple powder of cobalt-based zeolite imidazolate framework (Co-MOFs). Co-MOFs, copper chloride dihydrate, and sodium citrate were then added to an ethanol solution to form a mixed solution. The oven temperature was set at 80-140°C, and the reaction time was 1-3 hours. The reaction mixture was washed with ethanol and dried to obtain the Cu-Co(OH)2HNS catalytic electrode material.

[0006] The molar ratio of the cobalt nitrate hexahydrate to 2-methylimidazole is 1:4 to 1:6, and the molar ratio of the Co-MOFs, copper chloride dihydrate, and sodium citrate is 9:2:2 to 9:3:8.

[0007] The present invention proposes an electrode preparation method, which comprises weighing the electrocatalyst as claimed in claim 1, dispersing the electrocatalyst in a mixed solution containing anhydrous ethanol, deionized water, and naphthol in a volume ratio of 1.5:1:0.1, ultrasonically dispersing the solution for 30 minutes to obtain a uniformly dispersed mixed solution, and using a pipette with a range of 10uL to measure 10uL of the uniformly dispersed mixed solution, dripping the solution ten times on an area of ​​1x1cm -2 The carbon cloth coated with the catalyst was directly used as the working electrode catalyst after drying.

[0008] The present invention proposes the application of the above-mentioned electrocatalyst, which can be used as an electrocatalyst to detect ultra-low concentration nitrate and efficiently reduce it to produce ammonia.

[0009] The present invention provides a method for detecting ultra-low concentration nitrate and producing ammonia through efficient reduction, comprising the following steps:

[0010] Step 1: Use a simple and clean electrochemical method to detect and treat nitrate in wastewater;

[0011] Step 2: using the method according to claim 2 to prepare the electrode;

[0012] Step 3: Under a three-electrode system, a linear sweep voltammetry (LSV) test is performed to obtain a linear plot of nitrate concentration and reduction peak current value, thereby realizing the detection of ultra-low concentration nitrate;

[0013] Step 4: Electrocatalytically reduce nitrate to ammonia with high Faradaic efficiency and high ammonia selectivity in a low-concentration nitrate solution.

[0014] Preferably, the concentration of nitrate in the electrolyte is 1-10 mM KNO 3 .

[0015] Preferably, the voltage applied during the electrocatalytic ultra-low concentration nitrate reduction to produce ammonia is 0 V vs. RHE to -1.0 V vs. RHE (relative to the reversible hydrogen electrode).

[0016] Preferably, the nitrate reduction to ammonia is carried out in an H-type three-electrode electrolytic cell, which includes an anode region, a cathode region, three electrodes and an electrolyte solution, and the electrode prepared by coating the Cu-Co(OH)2HNS catalytic electrode material on a conductive carbon cloth serves as the working electrode in the three electrodes.

[0017] Preferably, the three electrodes further include a platinum sheet electrode and a Hg / HgO electrode.

[0018] Compared with the prior art, the present invention has the following beneficial effects:

[0019] The preparation method of the electrocatalyst proposed in the present invention uses a chemical synthesis method to synthesize the Co-MOFs precursor, and then converts the Co-MOFs into Cu-Co(OH)2HNS through a one-step hydrothermal method. When this material is used to reduce ultra-low concentration nitrate to produce ammonia, it shows a high ammonia yield and high Faradaic efficiency.

[0020] The Cu-Co(OH)2HNS catalytic electrode material with a hollow nanosheet multi-level structure designed in the present invention is low in cost, has a high active area, can effectively enrich nitrate, realizes the detection of ultra-low concentration nitrate and has high catalytic activity.

[0021] The present invention proposes a method for detecting ultra-low concentration nitrate ions and a clean and efficient electrochemical method for treating nitrate pollutants in sewage, turning waste into treasure. During the entire reaction process, the device and equipment are simple and have broad market application prospects. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] Figure 1 The following are scanning electron microscope (SEM) and transmission electron microscope (TEM) images of Co-MOFs prepared in Example 1 of the present invention. Figure 1 (a) is the SEM image, Figure 1 (b) is a TEM image;

[0023] Figure 2 The SEM and TEM images of Cu-Co(OH)2HNS prepared in Example 1 of the present invention are shown in FIG. Figure 2 (a) is the SEM image, Figure 2 (b) is a TEM image;

[0024] Figure 3 1 is the X-ray diffraction (XRD) pattern of Co-MOFs and Cu-Co(OH)2HNS prepared in Example 1 of the present invention;

[0025] Figure 4 1 is a Raman spectrum (Raman) graph of the Co-MOFs and Cu-Co(OH)2HNS catalytic electrode materials prepared in Example 1 of the present invention;

[0026] Figure 5 is an X-ray photoelectron spectroscopy (XPS) graph of the Cu-Co(OH)2HNS catalytic electrode material prepared in Example 1 of the present invention;

[0027] Figure 6 Linear sweep voltammetry (LSV) curves of the Cu-Co(OH)2HNS catalytic electrode material prepared in Example 1 of the present invention, the Co-MOFs in Example 1, and the substrate pure CC in a mixed solution of 1 M potassium hydroxide and 1 mM nitrate;

[0028] Figure 7 This is a graph showing the Faraday efficiency (FE) and ammonia yield of the Cu-Co(OH)2HNS catalytic electrode material for ammonia production prepared in Example 1 of the present invention, wherein Figure 7 (a) is ammonia-producing FE, Figure 7 (b) is a graph of ammonia production yield;

[0029] Figure 8 The Cu-Co(OH)2HNS catalytic electrode material prepared in Example 5 of the present invention is used for ultra-low concentration nitrate detection; the LSV diagram of Cu-Co(OH)2HNS in standard nitrate solutions of different concentrations and the linear relationship diagram between nitrate concentration and reduction peak current value are compared, wherein Figure 8 (a) is the LSV graph, Figure 8 (b) is the linear relationship between the reduction peak current value and the concentration;

[0030] Figure 9 The cyclic voltammogram (CV) and electrochemically active surface area (ECSA) of the Cu-Co(OH)2HNS catalytic electrode material prepared in Example 5 of the present invention in the non-Faraday range are shown in FIG. Figure 9 (a) is the CV plot, 9(b) is the ECSA plot;

[0031] Figure 10 This is a diagram of the product detection marking method during the electrochemical test process of the present invention, wherein Figure 10 (a) is a detection line diagram for nitrate, and 10(b) is a detection line diagram for ammonia;

[0032] Figure 11 The scanning electron microscope (SEM) and transmission electron microscope (TEM) images of the Cu-Co(OH)2HNS catalytic electrode obtained by shortening the hydrothermal reaction time in Example 2 of the present invention are shown in FIG. Figure 11 (a) is a scanning electron microscope image. Figure 11 (b) is a transmission electron microscope image;

[0033] Figure 12 The scanning electron microscope (SEM) and transmission electron microscope (TEM) images of the Cu-Co(OH)2HNS catalytic electrode obtained by extending the hydrothermal reaction time in Example 2 of the present invention are shown in FIG. Figure 12 (a) is a scanning electron microscope image. Figure 12 (b) is a transmission electron microscope image. DETAILED DESCRIPTION

[0034] The present invention will be further described below in conjunction with the accompanying drawings and specific implementation methods. It should be noted that, under the premise of no conflict, the various embodiments or technical features described below can be arbitrarily combined to form new embodiments.

[0035] Example 1:

[0036] The present invention provides a method for preparing an electrocatalyst, comprising the following steps:

[0037] A Co-MOFs precursor was synthesized using a chemical synthesis method, and then the Co-MOFs were converted into Cu-Co(OH)2HNS via a one-step hydrothermal method. Specifically, cobalt nitrate hexahydrate and 2-methylimidazole were added to a methanol solution, allowed to stand for one day, the reaction product was washed with methanol, and dried to obtain a purple powder of cobalt-based zeolite imidazolate framework (Co-MOFs). Co-MOFs, copper chloride dihydrate, and sodium citrate were added to an ethanol solution to form a mixed solution. The oven temperature was set at 80-140°C, and the reaction time was 1-3 hours. The mixed solution after the reaction was washed with ethanol and dried to obtain a Cu-Co(OH)2HNS catalytic electrode material.

[0038] The molar ratio of the cobalt nitrate hexahydrate to 2-methylimidazole is 1:4 to 1:6, and the molar ratio of the Co-MOFs, copper chloride dihydrate, and sodium citrate is 9:2:2 to 9:3:8.

[0039] Example 2:

[0040] Under the conditions of Example 1, the water bath heating reaction time was changed to 20 min, and the other conditions were the same as in Example 1. The precursor treatment was the same as in Example 1. The obtained catalyst material was analyzed by scanning electron microscopy and high-resolution transmission electron microscopy.

[0041] Under the conditions of Example 1, the water bath heating reaction time was changed to 120 min. Other conditions were the same as in Example 1, and the precursor treatment was the same as in Example 1. The obtained catalyst material was analyzed by scanning electron microscopy and high-resolution transmission electron microscopy.

[0042] Example 3:

[0043] This solution also proposes an electrode preparation method, which comprises weighing the electrocatalyst as claimed in claim 1, dispersing it in a mixed solution containing anhydrous ethanol, deionized water, and naphthol in a volume ratio of 1.5:1:0.1, and ultrasonically dispersing it for 30 minutes to obtain a uniformly dispersed mixed solution. 10uL of the uniformly dispersed mixed solution is measured with a 10uL pipette and dripped ten times on an area of ​​1x1cm -2 The carbon cloth coated with the catalyst was directly used as the working electrode catalyst after drying.

[0044] Example 4:

[0045] This proposal also proposes the application of electrocatalysts, which can detect ultra-low concentration nitrate and efficiently reduce it to produce ammonia.

[0046] Example 5:

[0047] This scheme proposes a method for ultra-low concentration nitrate detection and efficient reduction to ammonia production, comprising the following steps:

[0048] Step 1: Use a simple and clean electrochemical method to detect and treat nitrate in wastewater;

[0049] Step 2: Using the electrodes described in Example 3;

[0050] Step 3: Under the three-electrode system, obtain a linear graph of nitrate concentration and reduction peak current value through linear sweep voltammetry (LSV) test, so as to realize the detection of ultra-low concentration nitrate; keep the electrolyte in the anode area unchanged, configure the electrolyte in the cathode area with standard nitrate solutions of different concentrations, and test at room temperature and normal pressure. The main test method is LSV, and the applied voltage range is 0Vvs.RHE to -0.6Vvs.RHE. Figure 8 (a) shows the comparison of LSV curves of different nitrate concentrations. The reduction peak current value is the vertical axis and the concentration is the horizontal axis. The linear graph of nitrate concentration and current density of the above test results is drawn, as shown in Figure 8 (b) shown.

[0051] In an H-type three-electrode electrolytic cell, at room temperature and normal pressure, cyclic voltammogram (CV) was mainly used for testing. Figure 9 (a) is the CV curve diagram of the voltage range with different scan rates. Figure 9 (b) is the corresponding ECSA diagram.

[0052] Step 4: High Faradaic efficiency and high ammonia selectivity can be achieved in low-concentration nitrate solution to electrocatalytically reduce nitrate to ammonia.

[0053] In this scheme, an H-type three-electrode electrolytic cell is used, separated by a proton exchange membrane in the middle, with the anode and cathode regions on both sides respectively. The electrodes used are a counter electrode (platinum electrode), a working electrode (catalyst electrode), and a reference electrode (Hg / HgO electrode). The electrolyte solution in the anode region is 1M potassium hydroxide, and the electrolyte solution in the cathode region is 1M potassium hydroxide and 1mM nitrate solution. The test is carried out at room temperature and normal pressure. The main test method is linear sweep voltammetry (LSV), and the applied voltage range is 0V vs. RHE to -0.6V vs s. RHE. Figure 6 As shown, the LSV curves of different samples are compared. Compared with Co-MOFs and pure CC, the Cu-Co(OH)2HNS catalytic electrode material has a larger current density, indicating that it has good nitrate reduction activity. By controlling the constant voltage, its electrochemical performance is measured, and the same reaction time is controlled each time. The electrolyte after the reaction is diluted within the testable range, and the indophenol blue colorimetric method is used to determine the concentration of ammonia in the solution. The formula is used to calculate that its Faraday efficiency reaches a maximum value of 97.9% at -0.2Vvs.RHE, and the ammonia yield at -0.6Vvs.RHE is as high as 894.81μgh. -1 cm -2 ,like Figure 7 shown.

[0054] The concentration of nitrate in the electrolyte is 1-10 mM KNO3.

[0055] The voltage applied during the electrocatalytic ultra-low concentration nitrate reduction to produce ammonia is 0 V vs. RHE to -1.0 V vs. RHE (relative to the reversible hydrogen electrode).

[0056] The nitrate reduction to ammonia is carried out in an H-type three-electrode electrolytic cell, which includes an anode region, a cathode region, three electrodes and an electrolyte solution. The working electrode in the three electrodes is a Cu-Co(OH)2HNS catalytic electrode material coated on a conductive carbon cloth.

[0057] The three electrodes also include a platinum electrode and a Hg / HgO electrode:

[0058] In this protocol, nitrate concentration was measured according to standard methods. The electrolyte was removed from the electrolytic cell and diluted. Then, 0.1 mL of HCl and 0.01 mL of aminosulfonic acid solution were added to the solution in sequence. The solution was allowed to stand at room temperature and the absorption spectrum was measured using a UV-visible spectrophotometer. Figure 10 (a)

[0059] Determination of NH4 by indophenol blue method +Concentration. Remove the electrolyte from the electrolytic cell and dilute it 2-5 times. Then, add 2 mL of 1 M NaOH solution containing salicylic acid and sodium citrate to the above solution, followed by 1 mL of NaClO and 0.2 mL of C5FeN6Na2O. After placing it in the dark for a period of time, use a UV-visible spectrophotometer to measure the absorption spectrum, as shown in Figure 2. Figure 10 (b) shows that the illustration in the lower right corner shows the color after adding the developer, containing different NH4 + Optical photograph of standard solution of different concentrations.

[0060] Result analysis:

[0061] Figure 1 The Co-MOFs prepared in Example 1 were analyzed by scanning electron microscopy. Figure 1 As shown in (a), Co-MOFs present a regular dodecahedron, such as Figure 1 As shown in (b), the high-resolution transmission electron microscopy test results show that it is a solid structure.

[0062] Figure 2 The Cu-Co(OH)2HNS catalytic electrode material prepared in Example 1 was analyzed by scanning electron microscopy. Figure 2 As shown in (a), the Cu-Co(OH)2HNS catalytic electrode material exhibits a multi-level flower structure that maintains a dodecahedron framework, such as Figure 2 As shown in (b), the high-resolution transmission electron microscopy test results also confirmed the existence of the hollow structure.

[0063] Figure 3 X-ray diffraction analysis was performed on the Co-MOFs prepared in Example 1 and the Cu-Co(OH)2HNS catalytic electrode material in Example 1. The results confirmed the successful preparation of Co-MOFs and the amorphous structure of the Cu-Co(OH)2HNS catalytic electrode material in Example 1.

[0064] Figure 4 Raman spectroscopy analysis was performed on the Co-MOFs prepared in Example 1 and the Cu-Co(OH)2HNS catalytic electrode material in Example 1. The Raman shift observed was 462 cm -1 (A1), 524cm -1 The two peaks of (A2) are attributed to Co(OH)2, which are completely different Raman peaks compared with Co-MOFs, indicating that the catalyst is completely converted into Cu-Co(OH)2HNS.

[0065] Figure 5In order to perform X-ray photoelectron spectroscopy analysis on the Cu-Co(OH)2HNS catalytic electrode material in Example 1 and study the surface composition and chemical state of the obtained product sample, the Co2p and Cu2p high-resolution XPS spectra of the Cu-Co(OH)2HNS catalyst are shown as follows: Figure 5 (a) and 5(b). Figure 5 In (a), the two peaks at 781.1e and 796.9eV are attributed to Co 2+ Co2p 3 / 2 and Co2p 1 / 2 , the signals at 785.2 eV and 802.6 eV correspond to Co2p 3 / 2 and Co2p 1 / 2 Satellite peaks. Figure 5 In (b), the two peaks at 934.6 eV and 954.4 eV are attributed to Cu 2+ Cu2p 3 / 2 and Cu2p 1 / 2 , where the signals at 941.9eV, 944eV, and 962.4eV correspond to Cu2p 3 / 2 and Cu2p 1 / 2 The above results confirm that the Cu-Co(OH)2HNS catalytic electrode material is Cu-doped amorphous Co(OH)2.

[0066] Figure 6 The LSV curves of the Cu-Co(OH)2HNS catalytic electrode material in Example 1, the Co-MOFs in Example 1, and the substrate pure CC in 1M potassium hydroxide and 1mM nitrate solution are compared. Among them, the Cu-Co(OH)2HNS catalytic electrode material has the largest current density, indicating that it has excellent electrocatalytic performance in the reduction of ultra-low concentration nitrate to ammonia.

[0067] Figure 7 The FE graph and ammonia yield graph of the Cu-Co(OH)2HNS catalytic electrode material in Example 1 when electrolyzed for 20 min in 1M potassium hydroxide and 1mM nitrate electrolyte at different potentials are shown. Figure 7 As shown in (a), as the applied potential increases, the FE diagram of ammonia production shows a volcano diagram trend, reaching a maximum of 97.9% at -0.2V vs. RHE. The Faradaic efficiency decreases at high potentials, which is attributed to the gradual enhancement of the competing reaction (HER). Figure 7 As shown in (b), the ammonia production rate and potential show a positive correlation trend, and the maximum ammonia production rate is 894.81 μgh at -0.6 V vs. RHE. -1 cm -2 .

[0068] Figure 8The Cu-Co(OH)2HNS catalytic electrode material prepared in Example 5 was used for ultra-low concentration nitrate detection. By comparing the LSV curves of standard nitrate solutions containing different concentrations, as shown in FIG. Figure 8 As shown in (a), as the concentration of nitrate increases, the current density of LSV also increases, indicating that the Cu-Co(OH)2HNS catalytic electrode material has good reaction activity to nitrate. As the concentration of nitrate increases, its reduction peak current increases, indicating that the reduction peak current value is positively correlated with the nitrate concentration. The reduction peak current value is the vertical axis and the concentration is the horizontal axis to obtain the standard linear graph of nitrate concentration and current for the above test results, as shown in Figure 8 (b) is shown. Linear fitting is performed, and the equation after linear fitting is j=-4.11-5.95[NO3 - ], R 2 =0.98, the detection range is 1mM to 5mM, and the minimum detection limit is 1mM. Figure 8 (a) It can be seen that the Cu-Co(OH)2HNS catalytic electrode material catalyzes the reduction of nitrate ions of different concentrations during the electrochemical reaction, and a reduction peak appears in the range of -0.3 to -0.5 V vs. RHE. Therefore, it can be regarded as the characteristic peak formed by the catalytic reduction of nitrate ions by Cu-Co(OH)2HNS, indicating that the Cu-Co(OH)2HNS catalytic electrode material can be used for ultra-low concentration nitrate detection.

[0069] Figure 9 The cyclic voltammogram (CV) and electrochemically active surface area (ECSA) of the Cu-Co(OH)2HNS catalytic electrode material prepared in Example 5 of the present invention in the non-Faraday range were obtained. According to the CV curves obtained at different scan rates, the electrochemical double layer capacitance (Cdl) of Cu-Co(OH)2HNS was calculated to be 33.6 mF cm -2 , the Cdl value is positively correlated with ECSA, so a larger Cdl value indicates a larger ECSA, confirming that Cu-Co(OH)2HNS has a high catalytic active area.

[0070] Figure 10 The product detection method in the electrochemical test process of the present invention uses a UV-visible spectrophotometer to detect the concentration of nitrate and ammonia. The electrolyte after the reaction is taken and diluted to a detectable range. Figure 10 (a) is NO3 - -N concentration linear standard graph, Figure 10 (b) NH4 + -N concentration linear standard graph, the illustration in the lower right corner shows the concentration of different NH4 + Optical photograph of standard solution of -N concentration.

[0071] Figure 11 In order to reduce the reaction time in Example 2, the scanning electron microscope ( Figure 11 a), it can be clearly observed that the catalyst surface is smooth, and high-resolution transmission electron microscopy ( Figure 11 b) Confirm that the catalyst is still in a solid state.

[0072] Figure 12 In order to extend the reaction time in Example 2, the scanning electron microscope ( Figure 12 a), it can be clearly observed that the catalyst is in a flake state without a frame, and high-resolution transmission electron microscopy ( Figure 12 b) Only the trailer exists.

[0073] The Faradaic efficiency and ammonia yield of the present invention were determined by the indoxyl blue colorimetric method, wherein the formula involved is as follows: The Faradaic efficiency formula of ammonia is: FE = (8 × F × C NH3 ×V) / (M NH3-N ×Q)×100%, the ammonia yield formula is: NH3yield=(C NH3 ×V / S×t, where 8 is the number of transferred electrons and F is the Faraday constant (96485 Cmol -1 ), C NH3 is the NH3 concentration measured by UV-visible absorption spectroscopy, V is the electrolyte volume (L), M NH3-N is the relative molecular mass of N (gmol -1 ), Q is the charge (C), S is the electrode area (cm -2 ), t is the reaction time (h).

[0074] The above embodiments are only preferred embodiments of the present invention and cannot be used to limit the scope of protection of the present invention. Any non-substantial changes and replacements made by technicians in this field on the basis of the present invention fall within the scope of protection required by the present invention.

Claims

1. A method for preparing an electrocatalyst, comprising the following steps: synthesizing a Co-MOFs precursor by a chemical synthesis method, and then converting the Co-MOFs into Cu-Co(OH)2HNS by a one-step hydrothermal method; specifically, adding cobalt nitrate hexahydrate and 2-methylimidazole to a methanol solution, letting it stand for one day, washing with methanol, and drying to obtain a purple powder of a cobalt-based zeolite imidazolate framework, and then adding Co-MOFs, copper chloride dihydrate, and sodium citrate to an ethanol solution to form a mixed solution, stirring evenly, and then sealing the solution in a reactor, placing it in an oven at a temperature of 80-140°C, and reacting for 1-3 hours. The reaction product is washed with ethanol and then dried to obtain a Cu-Co(OH)2HNS catalytic electrode material; The molar ratio of the cobalt nitrate hexahydrate to 2-methylimidazole is 1:4 to 1:6, and the molar ratio of the Co-MOFs, copper chloride dihydrate, and sodium citrate is 9:2:2 to 9:3:

8.

2. An electrode preparation method, comprising weighing the electrocatalyst prepared by the method of claim 1, dispersing the electrocatalyst in a mixed solution containing anhydrous ethanol: deionized water: naphthol in a volume ratio of 1.5:1:0.1, ultrasonically dispersing the solution for 30 min to obtain a uniformly dispersed mixed solution, and using a 10 uL pipette to measure 10 uL of the uniformly dispersed mixed solution, dripping the solution ten times onto an area of ​​1 x 1 cm -2 The carbon cloth coated with the catalyst was directly used as the working electrode after drying.

3. The electrocatalyst prepared by the method according to claim 1, characterized in that As an electrocatalyst, it can detect ultra-low concentration nitrate and efficiently reduce it to produce ammonia; the concentration of nitrate in the electrolyte solution is 1~10 mM KNO3.

4. A method for detecting ultra-low concentration nitrate, characterized in that: The steps include: A simple and clean electrochemical method is used to detect nitrate in wastewater; Step 1: preparing an electrode by the method according to claim 2; Step 2: Under a three-electrode system, a linear sweep voltammetry test is performed to obtain a linear plot of nitrate concentration and reduction peak current value, thereby realizing nitrate detection; The concentration of nitrate in the electrolyte solution is 1~10 mM KNO3.

5. A method for producing ammonia by efficient reduction of ultra-low concentration nitrate, characterized in that: A simple and clean electrochemical method is used to treat nitrate in wastewater, which is carried out in an H-type three-electrode electrolytic cell. The electrolytic cell includes an anode region, a cathode region, three electrodes, and an electrolyte solution. The working electrode of the three electrodes is an electrode prepared by the method according to claim 2. The three electrodes also include a platinum electrode and a Hg / HgO electrode, which achieves high Faradaic efficiency and high ammonia selectivity in the electrocatalytic reduction of nitrate to ammonia in a low-concentration nitrate solution. The concentration of nitrate in the electrolyte solution is 1-10 mM KNO3.

6. The method for producing ammonia by highly efficient reduction of ultra-low concentration nitrate according to claim 5, wherein: The voltage applied during the electrocatalytic reduction of nitrate to produce ammonia is 0 V vs. RHE to -1.0 V vs. RHE.

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