Defective cobalt oxide catalyst with reversibly reconstructed structure as well as preparation method and application of defective cobalt oxide catalyst

Abstract

The invention provides a defect type cobalt oxide catalyst with a reversibly reconstructed structure as well as a preparation method and application of the defect type cobalt oxide catalyst. The catalyst is prepared from cobalt chloride hexahydrate through electro-deposition. The catalyst prepared by the method provided by the invention can realize uniform deposition of defective cobalt oxide on a foam titanium substrate, and has excellent performance of electro-catalysis nitrate reduction to produce ammonia and excellent stability under a neutral condition due to reversible reconstruction of the structure of the catalyst in a reaction process; the cost of raw materials needed for synthesizing the catalyst is low, the preparation method is simple, the practical application potential is achieved, and a new thought is provided for more efficient and practical preparation of the catalyst for ammonia preparation through nitrate reduction.

Description

Technical Field

[0001] The present invention relates to the field of metal oxide electrocatalysis, and in particular to a defective cobalt oxide catalyst with reversible structural reconstruction, and a preparation method and application thereof. Background Art

[0002] Nitrate pollution mainly comes from the abuse of agricultural fertilizers, industrial emissions and urban sewage. Its excessive accumulation will lead to eutrophication of water bodies, threatening the ecosystem and human health. - ) to ammonia as a green technology in the field of wastewater treatment and sustainable ammonia synthesis. However, nitrate reduction involves multiple electron transfers and complex intermediates. At the same time, the insufficient supply of active hydrogen under neutral conditions close to the actual wastewater conditions places stringent requirements on the activity, selectivity and stability of the catalyst. Cobalt (Co) oxide has become a research hotspot for nitrate reduction catalysts in recent years due to its unique electronic structure, tunable oxidation state and abundant oxygen vacancy defects. According to the theory of soft and hard acids and bases, high-valent Co 3+ Site optimization of NO3 ⁻ Adsorption capacity, and Co 2+ Species (such as Co(OH)2) have better water dissociation activity, thus providing abundant active hydrogen for nitrate reduction. 2+ / Co 3+ Redox couple, whose reversible electron cycle process is Co 3+ / Co 2+ The synergistic relay catalytic system provides an ideal platform. However, the dynamic surface reconstruction mechanism of cobalt active sites still needs further exploration. Establishing the structure-activity relationship of cobalt oxides and the reaction pathway control strategy will provide a scientific basis for the development of efficient and low-cost nitrate electrocatalytic conversion technology, helping to achieve the sustainable development goal of "turning waste into treasure." Summary of the Invention

[0003] In view of this, the object of the present invention is to provide a structurally reversibly reconstructed defective cobalt oxide catalyst with high stability and high catalytic activity, as well as a preparation method and application thereof. The defective cobalt oxide catalyst provided by the present invention can be used for the electrocatalytic reduction of nitrate pollutants to produce ammonia.

[0004] To achieve the purpose, the present invention adopts the following technical solutions: The invention provides a defective cobalt oxide catalyst with reversible structural reconstruction. The catalyst is a cobalt oxide having a layer of oxygen vacancy defects rich in the surface of a titanium foam substrate.

[0005] The structurally reversibly reconstructable defective cobalt oxide catalyst of the present invention is prepared by electrochemical deposition of cobalt chloride hexahydrate. The specific steps are: cobalt chloride hexahydrate and sodium citrate dihydrate are ultrasonically mixed in water to form an electrolyte, and then electrodeposition is performed in a two-electrode system using a foam titanium substrate as a working electrode. The substrate after deposition is then washed and vacuum-dried, and finally calcined in an air atmosphere to obtain the structurally reversibly reconstructable defective cobalt oxide catalyst.

[0006] Preferably, in the electrolyte, the molar concentration of sodium citrate dihydrate is 0.01-0.02 mol L -1 The molar concentration of cobalt chloride hexahydrate is 0.05 mol L -1 .

[0007] Preferably, the positive electrode of the two-electrode system is connected to a graphite carbon rod, and the negative electrode is connected to a foam titanium substrate serving as a working electrode.

[0008] Preferably, the current density of the electrodeposition is 100 mA cm -2 , the constant current electrodeposition time is 5 min.

[0009] Preferably, the washing is performed by rinsing with deionized water, and the vacuum drying temperature is 40-60° C. and the drying time is 2-6 h.

[0010] Preferably, the calcination is carried out in a muffle furnace at a calcination temperature of 350° C., a holding time of 90 min, and a heating rate of 5° C. / min.

[0011] In general, the formation of cobalt oxide will go through a redox process from precursor to intermediate to final product. The present invention selects cobalt chloride hexahydrate as the precursor, wherein Co 2+ Due to electrostatic interaction, Co migrates to the cathode during the electrodeposition process. Under the high pH environment at the cathode, 2+ With OH - Combined to form Co(OH)2 precipitation. Subsequently, Co(OH)2 is converted into Co3O4 during dehydration or oxidation, where sodium citrate as an electrolyte additive can promote the uniform dispersion of Co3O4 nanoparticles and inhibit excessive crystallization. At the same time, during electrodeposition, electrons are injected into the Co3O4 lattice, which may lead to the reduction of lattice oxygen (O 2- →O - or O vacancies), thus forming defective Co3O 4-xThe present invention selects titanium foam as the substrate for electrodeposition. Commercial titanium foam has an oxide layer on its surface when exposed to air. Depositing Co oxide on the oxide can form Co-O-Ti electronic interactions, further weakening the strength of the Co-O bond in the Co oxide and inducing structural transformation that may occur during the electrocatalytic process.

[0012] The reversibly reconfigurable defective cobalt oxide (Co-Ti) catalyst prepared by this invention can be used in the electrocatalytic reduction of nitrates to ammonia, removing pollutants while producing high-value-added products. During the electrocatalytic nitrate reduction reaction, the cobalt oxide undergoes a reversible structural reconfiguration: when a reducing potential is applied, the Co₃O₄ crystalline phase in the catalyst gradually transforms into a Co(OH)₂ intermediate phase. Upon removal of the applied potential, the catalyst reversibly reverts to its initial Co₃O₄ crystalline phase. This reversible structural reconfiguration facilitates the reaction.

[0013] Compared with the prior art, the beneficial effects of the present invention are embodied in: The method provided by the present invention can easily prepare the Co-Ti catalyst, and it is observed that the catalyst undergoes structural reversible reconstruction in the electrocatalytic reduction of nitrate, so that it has excellent electrochemical ammonia production performance under neutral conditions, with a yield (11.6 mg / L) -1 cm -2 ) and Faraday efficiency (~100%) are significantly better than those of cobalt oxide catalysts supported on ordinary carbon paper. In addition, the defective cobalt oxide catalyst supported on the titanium foam substrate has excellent stability and can operate continuously for 150 hours under constant current density test, while the electrocatalytic nitrate reduction performance does not decay. Since the final product of the reaction, ammonia, is a high-value chemical, the present invention can achieve an effective recovery rate of 89.53% of NH4Cl through the hydrochloric acid stripping recovery device. The Co-Ti catalytic system has strong anti-interference ability in different water quality environments, and common anions (Cl - 、HCO3 - 、Na + , Ca 2+ Mg 2+ 、Cu 2+ ) did not inhibit the reduction of nitrate pollutants. The catalyst synthesized by the present invention requires low raw material costs and is simple to prepare, showing potential for practical applications. This provides a new approach for the preparation of more efficient and practical electrocatalytic nitrate reduction catalysts. BRIEF DESCRIPTION OF THE DRAWINGS

[0014] Figure 1 The SEM images of Co-Ti and Co-C prepared in Example 1 (SEM, Figure 1 ab in), high-resolution transmission electron microscopy images (HRTEM, Figure 1cd) and selected area electron diffraction patterns (SAED, Figure 1 ef in), where (a, c, e) correspond to sample Co-Ti, and (b, d, f) correspond to sample Co-C.

[0015] Figure 2 (a) and Figure 2 (b) Raman spectra and Fourier transform infrared (FTIR) spectra of Co-Ti and Co-C prepared in Example 1, respectively.

[0016] Figure 3 (a) X-ray photoelectron spectroscopy (XPS) of Co element in Co-Ti and Co-C prepared in Example 1; Figure 3 (b) and (c) are the X-ray photoelectron spectra (XPS) of the O element in Co-C and Co-Ti prepared in Example 1, respectively.

[0017] Figure 4 (a) Performance diagram of the electrocatalytic reduction of neutral nitrate by the Co-Ti catalyst in Example 2 at different voltages; Figure 4 (b) Performance comparison of Co-Ti, Co-C and Ti substrates for electrocatalytic neutral nitrate reduction.

[0018] Figure 5 This is the electrocatalytic nitrate reduction stability diagram of the Co-Ti catalyst in Example 3 after continuous operation for 150 hours.

[0019] Figure 6 This is a graph showing the electrocatalytic reduction performance of the Co-Ti catalyst in Example 4 under different nitrate concentration conditions.

[0020] Figure 7 The anti-interference ability of the electrocatalytic performance of the Co-Ti catalyst in Example 4 in the presence of different coexisting ions.

[0021] Figure 8 (a) is the in-situ stripping device in Example 5; Figure 8 (b) XRD pattern of the product ammonia produced by in situ recovery of Co-Ti electrocatalytic nitrate reduction, and the inset is a sample photo.

[0022] Figure 9 (ab) are the in situ electrochemical Raman spectra of Co-Ti and Co-C in Example 6, where (a) corresponds to Co-Ti and (b) corresponds to Co-C. The blue area corresponds to when the reduction potential is applied, and the red area corresponds to when the potential is removed. DETAILED DESCRIPTION

[0023] The technical solutions in the embodiments of the present invention will be described clearly and completely below. Obviously, the embodiments described are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments improved or modified by ordinary technicians in this field fall within the scope of protection of the present invention. It should be understood that the embodiments of the present invention are only used to illustrate the technical achievements of the present invention, and are not used to limit the scope of protection of the present invention. In the embodiments, the methods used are all conventional methods unless otherwise specified.

[0024] The present invention provides a structurally reversibly reconstructable defective cobalt oxide catalyst with high stability and high catalytic activity, as well as a preparation method and application thereof. The method comprises: uniformly mixing cobalt chloride hexahydrate and sodium citrate dihydrate in water by ultrasonication as an electrolyte; then performing electrodeposition in a two-electrode system using a foamed titanium substrate as a working electrode; then washing and vacuum-drying the substrate after deposition; and finally calcining in an air atmosphere to obtain the structurally reversibly reconstructable defective cobalt oxide catalyst.

[0025] The present invention provides a method for verifying the performance of the obtained catalyst in the electrocatalytic nitrate reduction to ammonia reaction: in a three-electrode H-type electrolytic cell system, the prepared defective cobalt oxide catalyst (Co-Ti) supported on a foam titanium substrate is used as a working electrode, the counter electrode is a Pt mesh, the reference electrode is an Ag / AgCl electrode, and the working electrode area is 1×1 cm 2 In this experiment, a neutral electrolyte solution (e.g., 0.1M K₂SO₄) is used. Potassium nitrate, a pollutant, is added to the electrolyte to achieve a NO₃-N concentration between 140 and 7000 ppm. A reduction potential is applied and the reaction is continued for a period of time to reduce the target nitrate pollutant to the high-value-added product, ammonia. The reaction time is preferably 10 to 30 minutes. The reaction is preferably carried out under magnetic stirring at a speed of 300 to 600 rpm.

[0026] The NH3 generated in the cathode electrolytic cell is detected using the indigo blue colorimetric assay. Due to the high concentration of NH3 generated, the electrolyzed solution needs to be diluted before color development to ensure that the NH3 concentration in the detection solution is within the linear range of the indigo blue colorimetric assay. After electrolysis, 2 mL of the diluted cathode electrolyte is taken and 2 mL of 1 M NaOH (containing 5 wt% salicylic acid and 5 wt% sodium citrate), 1 mL of 0.05 M NaClO, and 0.2 mL of 1.0 wt% sodium nitroferricyanide are added to each solution. After mixing thoroughly, the solution is incubated at room temperature in the dark for 1 hour. The absorbance of the solution is measured at 655 nm using a UV-visible spectrophotometer. The NH3 concentration is calibrated and calculated using a standard ammonium chloride solution.

[0027] Electrocatalytic NO3 -The Faradaic efficiency of NH3 reduction is the ratio of the charge required to synthesize NH3 in the electrolysis process to the total charge, and the calculation formula is as follows: Where F represents the Faraday constant, Represents the measured NH3 concentration (mg mL -1 ), V represents the volume of the electrolyte (mL), and Q is the total charge passing through the electrodes during the electrolysis process.

[0028] Example 1 In this example, defective cobalt oxide catalyst (Co-Ti) was prepared as follows: 0.3 g of cobalt chloride hexahydrate and 0.116 g of sodium citrate dihydrate were ultrasonically dissolved in 30 mL of water as the electrolyte (the concentration of cobalt chloride hexahydrate was 0.05 mol L -1 , the concentration of sodium citrate dihydrate was 0.015 mol L -1 ), a two-electrode system was established using a DC power supply for electrodeposition in a single-chamber electrolytic cell, where titanium foam was used as the cathode (1×1 cm 2 ), graphite carbon rod as anode, current density is 100mAcm -2 The constant current electrodeposition time was 5 minutes. After deposition, the cathode was rinsed several times with deionized water to remove surface electrolyte ions, dried in a 60°C vacuum oven for 6 hours, and then calcined in a muffle furnace at 350°C in air for 90 minutes to obtain a defective cobalt oxide catalyst (Co-Ti) supported on a titanium foam substrate.

[0029] For comparison, a commercial carbon paper-supported cobalt oxide catalyst (Co-C) was also prepared in this example, that is, the electrodeposition substrate was replaced with a 1×1 cm 2 Carbon paper, the rest of the steps are the same as above.

[0030] The Co-Ti and Co-C prepared in this example were tested by scanning electron microscopy (SEM), high resolution transmission electron microscopy (HRTEM), inverse Fourier transform (IFFT) and selected area electron diffraction (SAED). The test results are as follows: Figure 1 As shown. Figure 1 (ab) It can be seen that Co-Ti and Co-C present the nanoflower morphology of multi-nanosheet stacking; Figure 1 (cd) It can be seen that the lattice fringes of Co-Ti show a distorted structure, indicating that the crystallinity of its supported cobalt oxide is destroyed, while the lattice fringes of Co-C are clearly visible, indicating that it maintains a well-ordered structure; Figure 1 The selected area electron diffraction of (ef) shows that the cobalt oxides supported by Co-Ti and Co-C are all Co3O4 spinel structures (d (220) =2.85nm, d(311) = 2.43 nm), while the diffraction fringes of Co-Ti tend to be diffuse rings, further proving its defective structure.

[0031] The Co-Ti and Co-C prepared in Example 1 were subjected to Raman spectroscopy and Fourier transform infrared spectroscopy. The test results are as follows: Figure 2 As shown. Figure 2 (a) It can be seen that both Co-Ti and Co-C exhibit the characteristic peaks of Co3O4 spinel structure (A 1g 、E g 、F 2g ), where A 1g (682cm -1 ) corresponds to the tetrahedral site Co in the spinel sublattice 2+ Symmetrical stretching vibration with O is the most significant Raman characteristic peak of Co3O4. Compared with Co-C, the peak in Co-Ti is broader, indicating the presence of defects (such as oxygen vacancies) inside the crystal, which leads to uneven energy distribution of the vibration mode, thus significantly increasing the Raman peak width. Moreover, the shift of the peak position to lower wavenumbers indicates that the Co-O bond strength of the cobalt oxide in Co-Ti is weakened and the vibration frequency is reduced. Figure 2 (b) It can be seen that the two belong to Co 2+ -O(Co Td ) and Co 3+ -O(Co Oh ), and the shift to higher wavenumbers also indicates that the strength of the Co-O bond is weakened. Raman and infrared spectroscopy results show that the defective Co3O 4-x , where the Co-O interaction is weaker, which may be more conducive to the structural transformation of the active phase during the reaction.

[0032] The composition and structure of the catalyst Co-Ti prepared in Example 1 were further analyzed by X-ray photoelectron spectroscopy (XPS). Figure 3 As shown. Figure 3 (a) It can be seen that Co exists in the form of Co(Ⅱ) and Co(Ⅲ), and the proportion of Co(Ⅲ) in Co-Ti is higher due to the electron transfer between Co-O-Ti. Figure 3 (bc) It can be seen that the oxygen vacancies (O v ) peaks increased significantly, indicating that the cobalt oxides supported thereon were rich in oxygen defects.

[0033] Example 2 The Co-Ti prepared in Example 1 was used as a catalyst to conduct an electrocatalytic reduction experiment of nitrate pollutants to produce ammonia in an H-type electrolytic cell using a Shanghai Chenhua electrochemical workstation (CHI760E). 2 The Co-Ti prepared in Example 1 was used as the working electrode, the reference electrode was an Ag / AgCl electrode, and the counter electrode was 1×1 cm 2 The electrolyte solution was 40 mL of a neutral aqueous solution of 0.1 M K2SO4 and 1400 ppm NO3-N. The solution was stirred at 600 rpm and different reduction voltages were applied for 30 min to test its reduction performance to produce ammonia.

[0034] The experimental results are as follows Figure 4 As shown in the figure, it can be seen that Co-Ti exhibits better Faradaic efficiency and yield of electrocatalytic reduction of nitrate to produce ammonia than Co-C at different voltages, which is -0.3~-0.5V vs. The RHE exhibits a Faradaic efficiency close to 100% over a wide voltage range, and at -0.5V vs. RHE showed 11.6 mgh -1 cm -2 high yield.

[0035] Example 3 Good stability is an important indicator for evaluating the practical application ability of a catalyst. In this example, the electrocatalytic stability of the Co-Ti prepared in Example 1 for nitrate reduction to ammonia production was tested in an H-type electrolytic cell using a Shanghai Chenhua electrochemical workstation (CHI760E). First, a 1×1 cm 2 The Co-Ti prepared in Example 1 was used as the working electrode, the reference electrode was an Ag / AgCl electrode, and the counter electrode was 1×1 cm 2 The electrolyte solution was a neutral aqueous solution of 0.1 M K2SO4 and 1400 ppm NO3-N. A peristaltic pump (10 rpm) was used to keep the cathode electrolyte flowing at a constant current of 50 mA cm -2 The electrocatalytic reduction ammonia production performance was tested by continuous operation for 150 hours under the same conditions. 2 mL of diluted cathode electrolyte was taken at regular intervals to detect the NH3 concentration and calculate the corresponding yield and Faraday efficiency. The experimental results are shown in the figure. Figure 5 As shown in the figure, it can be seen that the catalyst maintains stable ammonia production performance during the long-term operation of 150 h.

[0036] Example 4 The universality of the catalyst for nitrate pollutants of different concentrations and its ability to resist interference from different coexisting ions are also key indicators for evaluating its catalytic performance. In this example, the Co-Ti prepared in Example 1 was used to test the electrocatalytic reduction and ammonia production performance under conditions containing different nitrate concentrations, and its removal performance for target pollutants in different water quality environments was explored: In the experiment for universality of pollutants with different concentrations, the experimental process is the same as the experimental steps in Example 2, except that the 1400 ppm NO3-N is replaced with neutral nitrate electrolytes of different concentrations (140 ppm, 700 ppm, 7000 ppm). The experimental results are as follows: Figure 6 As shown, Co-Ti at -0.5V vs. Under RHE, a Faradaic efficiency close to 100% was achieved for different concentrations of nitrate, indicating that Co-Ti can be applied to a variety of water bodies containing different nitrate pollutants.

[0037] In the experiment to explore the interference ability of Co-Ti on different coexisting ions, 0.1 M K2SO4 and 1400 ppm NO3-N were selected as the initial electrolyte, and potassium chloride (KCl), potassium bicarbonate (KHCO3), sodium chloride (NaCl), calcium chloride (CaCl2), magnesium chloride (MgCl2), and copper chloride (CuCl2) were added to the reaction system respectively. The concentration of each salt was set to 100 ppm. The sampling and detection steps were consistent with Example 2. The experimental results are shown in Figure 2. Figure 7 As shown in the figure, various interfering ions have no obvious inhibition on the electrocatalytic nitrate reduction to ammonia production performance of Co-Ti, indicating that Co-Ti has excellent anti-interference ability.

[0038] Example 5 Ammonia, the product of electrocatalytic nitrate reduction, is a high-value chemical, so efficient recovery of the product is also an essential step in practical applications. Figure 8 As shown in (a), in addition to continuing the test method in Example 2, this example also constructed an in-situ stripping device for ammonium chloride (NH4Cl). The electrolyte on the cathode side after the reaction was poured into a two-necked flask, and argon (Ar) was introduced from one side of the flask. The other opening was connected to a glass tube, into which was inserted a 0.1 mol L -1 The hydrochloric acid solution was placed in a conical flask. The double-necked flask was placed in an oil bath at 80°C for 120 min. The ammonia gas evaporated by heating entered the 0.1 mol L -1 The collected NH4Cl liquid solution was poured into a 100 mL round-bottom flask and heated at 60°C until the solvent water was completely evaporated to obtain white NH4Cl powder. The effective recovery rate of NH4Cl was calculated to be 89.53%. Figure 8(b) shows the XRD patterns of the recovered white NH4Cl powder and pure NH4Cl. The highly consistent peaks indicate that the NH4Cl recovered by this method is high-purity and free of impurities. The NH4Cl powder produced according to the present method can be used in industrial production such as agricultural fertilizers, ceramics, and batteries, as well as in daily life as a food additive and toiletries.

[0039] Example 6 The study of the dynamic structural evolution mechanism of catalysts during electrocatalysis is of great significance. In this experiment, in-situ electrochemical Raman spectroscopy was used to monitor the structural changes of catalysts in the process of electrocatalytic nitrate reduction in real time. This study used the C031 electrochemical in-situ Raman cell of Gosler and equipped it with a LabRAM HR Evolution spectroscopy system for in-situ Raman spectroscopy testing. The experiment was configured with a laser light source with an excitation wavelength of 532 nm and a 50x microscope objective lens with NA=0.5 as an optical detection module. Co-Ti and Co-C catalysts were directly used as working electrodes, and the three-electrode system included an Ag / AgCl reference electrode and a platinum wire counter electrode. The test was carried out in a 0.1M K2SO4 electrolyte (containing 1400ppm NO3-N) system, and the 300-1000 cm -1 In-situ spectrum acquisition across a wide wavenumber range systematically tracks the structural evolution of catalysts in different media.

[0040] like Figure 9 As shown in (a), the Co-Ti catalyst exhibits a significant structural dynamic response due to its defective cobalt oxide structure: when a reduction potential is applied, the Co3O4 crystal phase gradually transforms into a Co(OH)2 intermediate phase, and when the applied potential is removed, it can reversibly restore the initial structure. This dynamic structural evolution has dual catalytic advantages: according to the theory of hard and soft acids and bases, the initial high-valent Co species can enhance the surface adsorption of nitrate ions, while the dynamically generated Co(OH)2 intermediate phase promotes the dissociation of water molecules, thereby creating favorable conditions for the hydrogenation of nitrate reduction reaction intermediates. In contrast, no obvious structural changes were observed in the high-crystallinity cobalt oxide in the Co-C catalyst during the reaction ( Figure 9 (b) ), resulting in limited catalytic activity. Experimental results indicate that the unique dynamic structural regulation capability of Co-Ti catalysts is the key mechanism for their excellent nitrate electrocatalytic reduction performance in neutral media.

[0041] In summary, the present invention provides a simple method for preparing a structurally reversibly reconfigurable defective cobalt oxide catalyst with high stability and catalytic activity for the electrocatalytic reduction of nitrate pollutants to ammonia under neutral conditions. This catalyst possesses abundant reactive sites, and the titanium foam support effectively modulates the Co-O interaction within the defective cobalt oxide, further enhancing catalytic activity and facilitating the dissociation of water to produce a large amount of active hydrogen, reducing nitrate pollutants to ammonia in a short period of time. Furthermore, the catalyst exhibits excellent stability and is reusable, demonstrating its great potential for practical application.

[0042] The above are merely exemplary embodiments of the present invention and are not intended to limit the present invention. Any modifications, equivalent replacements, 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 structurally reversibly reconstructible defective cobalt oxide catalyst, characterized in that: The catalyst is a cobalt oxide layer rich in oxygen vacancy defects formed on the surface of a foam titanium substrate.

2. A method for preparing the structurally reversibly reconstructible defective cobalt oxide catalyst according to claim 1, characterized in that: Cobalt chloride hexahydrate and sodium citrate dihydrate are ultrasonically mixed in water to form an electrolyte. Then, in a two-electrode system, electrodeposition is carried out using a titanium foam substrate as the working electrode. The substrate after deposition is washed and vacuum-dried, and finally calcined in an air atmosphere to obtain a defective cobalt oxide catalyst with reversible structural reconstruction.

3. The preparation method according to claim 2, wherein: In the electrolyte, the molar concentration of sodium citrate dihydrate is 0.01-0.02 mol L -1 The molar concentration of cobalt chloride hexahydrate is 0.05 mol L -1 .

4. The preparation method according to claim 2, wherein: The positive electrode of the two-electrode system is connected to the graphite carbon rod, and the negative electrode is connected to the foam titanium substrate.

5. The preparation method according to claim 2, wherein: The current density of the electrodeposition was 100 mA cm -2 , the constant current electrodeposition time is 5 min.

6. The preparation method according to claim 2, wherein: The washing step is performed by rinsing with deionized water; the vacuum drying step is performed at a temperature of 40 to 60° C. and a drying time of 2 to 6 hours.

7. The preparation method according to claim 2, characterized in that: The calcination is carried out in a muffle furnace at a calcination temperature of 350° C., a holding time of 90 min, and a heating rate of 5° C. / min.

8. Use of the reversibly reconstructable defective cobalt oxide catalyst according to claim 1 in electrocatalytic nitrate reduction to produce ammonia.

9. The use according to claim 8, characterized in that During the electrocatalytic nitrate reduction reaction to produce ammonia, cobalt oxide undergoes reversible structural reconstruction: when a reduction potential is applied, the Co3O4 crystal phase in the catalyst is gradually converted into a Co(OH)2 intermediate phase, and when the external potential is removed, it can reversibly recover to the initial Co3O4 crystal phase structure.

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