MgO / CoOheterojunction catalyst for catalyzing ozone to selectively oxidize ammonia nitrogen as well as preparation method and application of MgO / CoOheterojunction catalyst

Abstract

The invention discloses an MgO / CoO heterojunction catalyst for catalyzing ozone to selectively oxidize ammonia nitrogen and a preparation method and application thereof, the catalyst is prepared by a hydrothermal-calcination two-step method, and a heterojunction structure of MgO lamellar crystals and CoO spinel is formed by controlling the molar ratio of cobalt to magnesium, the hydrothermal temperature and the calcination temperature. The technical problems that in a traditional ozone catalytic oxidation technology, ammonia nitrogen is excessively oxidized into nitrate, the nitrogen selectivity is low, and the catalyst stability is poor are solved. The catalyst is used for treating low-concentration ammonia-nitrogen wastewater under the conditions that the pH value is 9 and the room temperature is achieved, high ammonia-nitrogen removal rate and gaseous nitrogen selectivity are achieved with low dosage, the activity is not obviously reduced after the catalyst is recycled, and the catalyst is suitable for advanced nitrogen removal treatment of urban sewage.

Description

Technical Field

[0001] This invention relates to the field of wastewater treatment, and in particular to a MgO / Co3O4 heterojunction catalyst for the selective oxidation of ammonia nitrogen by ozone, its preparation method, and its application. Background Technology

[0002] Excessive ammonia nitrogen discharge in industrial wastewater and urban sewage is a core factor causing eutrophication of water bodies. According to China's "Surface Water Environmental Quality Standard," the ammonia nitrogen limit for Class V surface water is 2.0 mg / L, but many regions (such as coastal urban sewage treatment plants) require discharge concentrations below 1.0 mg / L to ensure the safety of water reuse. Traditional biological nitrogen removal technologies have limitations such as long process flows, large land areas, and sensitivity to low temperatures and toxic substances; while physicochemical methods (such as stripping and ion exchange) are difficult to promote due to high costs and the potential for secondary pollution. Especially for low-concentration ammonia nitrogen wastewater, existing technologies generally suffer from insufficient efficiency or poor operational economics, becoming a common problem in the water treatment field.

[0003] Ozone catalytic oxidation technology, with its strong oxidation capacity (synergistic effect of direct ozone oxidation and indirect oxidation by ·OH radicals), fast reaction rate, and lack of secondary pollution, has been listed as a potential solution for deep nitrogen removal. However, the practical application of this technology faces several challenges: single metal oxide catalysts have inherent defects. Studies have shown that MgO has a high ammonia nitrogen removal rate (>85%), but excessive oxidation produces nitrates (NO3⁻), and the nitrogen (N2) selectivity is less than 10%; while Co3O4 can achieve a N2 selectivity of up to 40%, its ammonia nitrogen degradation efficiency is only about 50%. Composite catalysts are prone to metal ion dissolution (such as Mg²⁺) during the reaction, leading to structural collapse. The activity of MgO-Co3O4 prepared by co-precipitation decreases by more than 20% after five consecutive uses. Homogeneous catalytic systems (such as Fe²⁺ / O3) suffer from difficulties in catalyst recovery and strong pH dependence; heterogeneous catalysts, if they have a small specific surface area or uneven distribution of active sites, will significantly reduce ozone mass transfer efficiency and increase energy consumption.

[0004] To balance activity and selectivity, researchers have attempted to develop bimetallic catalysts. While supported catalysts (such as CuO-Co2O3 / γ-Al2O3) improve dispersion through supports, the active component's bonding to the support is weak, leading to easy detachment. A patent shows that a copper-cobalt catalyst supported on γ-Al2O5 experienced a 15% loss of active component in a shaking experiment. Emerging cobalt-cerium catalysts utilize Ce... 4 The ⁺ / Ce³⁺ redox pair enhances stability, but its preparation requires a carbonate precipitant, making the process complex, and it has poor adaptability to low concentrations of ammonia nitrogen.

[0005] The challenges in the field of ammonia nitrogen catalytic ozone oxidation are: single-component catalysts cannot simultaneously achieve high ammonia nitrogen conversion rates and high N2 selectivity, while composite catalysts are limited by the unclear inter-metal synergistic mechanism. Furthermore, catalysts deactivate during long-term operation due to metal dissolution, crystal phase transformation, or pore blockage, especially in high-salt or acidic wastewater, where their lifespan is drastically reduced. Advanced oxidation technologies rely on high-concentration ozone addition, with electricity consumption accounting for over 60% of treatment costs, and catalyst regeneration processes are complex. Summary of the Invention

[0006] The purpose of this invention is to provide a MgO / Co3O4 heterojunction catalyst for the selective oxidation of ammonia nitrogen by ozone. Magnesium chloride is used as the magnesium source, ammonia water is used as the precipitant, and the catalyst is hydrothermally calcined with a cobalt metal solution to obtain a highly efficient and stable heterojunction catalyst. This method is simple and easy to control, and the resulting catalytic material is stable, highly pure, and has high catalytic activity and high selectivity.

[0007] The above objectives are achieved through the following technical solutions: A method for preparing a MgO / Co3O4 heterojunction catalyst for the selective oxidation of ammonia nitrogen by ozone includes the following steps: (1) Prepare a mixed solution of MgCl2 and CoCl2 with a molar ratio of Co to Mg of 1:2-2:1; (2) Add ammonia to the mixed solution to adjust the pH to 7-12, stir the reaction at room temperature for 10-30 min to obtain a suspension mixture; (3) The suspension mixture is transferred to a hydrothermal reactor and hydrothermally reacted at 100-220°C for 3-21 h; (4) Centrifuge the hydrothermal reaction products at a speed of 6000-10000 r / min for 1-5 min, and wash them with anhydrous ethanol and deionized water 3-10 times in sequence. (5) Dry the cleaned product at 60-105℃ for 8-24 h to obtain the precursor material; (6) The precursor material is calcined at 300-600℃ for 1-6 h with a heating rate of 5-10℃ / min. After cooling, it is ground to obtain the MgO / Co3O4 heterojunction catalyst.

[0008] Furthermore, the molar ratio of Co to Mg in step (1) is 1:2.

[0009] Furthermore, in step (2), the pH is 9 and the stirring time is 10 min.

[0010] Furthermore, the hydrothermal reaction temperature in step (3) is 200°C and the hydrothermal time is 12 h.

[0011] Furthermore, in step (4), the centrifugation speed is 6000 r / min, the centrifugation time is 5 min, and the number of cleaning cycles is 5.

[0012] Furthermore, the drying temperature in step (5) is 60°C and the drying time is 12 h.

[0013] Furthermore, the calcination temperature in step (6) is 500℃, the calcination time is 2 h, and the heating rate is 5℃ / min.

[0014] Another objective of this invention is to successfully prepare a MgO / Co3O4 heterojunction catalyst for the selective oxidation of ammonia nitrogen by ozone using a hydrothermal-calcination two-step method. This catalyst enables deep denitrification of secondary wastewater effluent from urban sewage through ozone catalytic oxidation technology. The denitrification performance of this technology in a low-concentration ammonia nitrogen system was systematically investigated. The aim is to enhance the treatment efficiency of secondary wastewater effluent from sewage treatment plants through this technology, thereby addressing the challenges of improving quality and efficiency, achieving deep pollution reduction in wastewater discharge, and solving the problems of high preparation cost, secondary pollution, low denitrification efficiency, and over-oxidation associated with traditional ozone catalysts. The heterojunction catalyst structure is a heterojunction formed by MgO plate-like crystals and Co3O4 spinel, with interplanar spacings including 0.2410 nm (Co3O4(311)) and 0.2131 nm (MgO(200)).

[0015] Another objective of this invention is to provide the application of the MgO / Co3O4 heterojunction catalyst in the treatment of ammonia nitrogen wastewater, comprising the following steps: adding the catalyst at a rate of 0.5-3 g / L to ammonia nitrogen-containing wastewater with a pH of 7-12, introducing an O3 / O2 mixed gas with an ozone concentration of 100.9-119.8 mg / L, and reacting at 10-30°C for 1 h.

[0016] The beneficial effects are as follows: The MgO / Co3O4 heterojunction catalyst of this invention solves the problems of high catalyst cost, secondary pollution, low ozone denitrification efficiency, and over-oxidation. It is prepared using a two-step hydrothermal-calcination method, exhibiting stable performance and a wide availability of raw materials. It possesses stable catalytic efficiency, a regular crystal structure, and abundant active sites. It can achieve a 92.2% ammonia nitrogen removal rate and a 47.7% nitrogen yield, realizing one-step denitrification. Attached Figure Description

[0017] Figure 1 This is a SEM image of the MgO / Co3O4 heterojunction catalyst.

[0018] Figure 2 This is a TEM image of the MgO / Co3O4 heterojunction catalyst.

[0019] Figure 3The efficiency of MgO / Co3O4 heterojunction catalysts in ozone catalysis for removing ammonia nitrogen under different hydrothermal conditions is shown in the graph.

[0020] Figure 4 The efficiency of MgO / Co3O4 heterojunction catalysts in ozone catalysis for removing ammonia nitrogen under different hydrothermal preparation times is shown in the figure.

[0021] Figure 5 The efficiency of MgO / Co3O4 heterojunction catalysts for ozone catalytic removal of ammonia nitrogen under different calcination temperatures is shown in the graph.

[0022] Figure 6 The efficiency of MgO / Co3O4 heterojunction catalysts in ozone catalytic removal of ammonia nitrogen under different calcination times is shown in the figure.

[0023] Figure 7 The effect of temperature on the ozone catalytic removal of ammonia nitrogen by MgO / Co3O4 heterojunction catalyst.

[0024] Figure 8 The effect of pH on the ozone catalytic removal of ammonia nitrogen by MgO / Co3O4 heterojunction catalyst.

[0025] Figure 9 The effect of ozone concentration on the ozone-catalyzed removal of ammonia nitrogen by MgO / Co3O4 heterojunction catalyst.

[0026] Figure 10 The effect of catalyst dosage on the ozone catalytic removal of ammonia nitrogen by MgO / Co3O4 heterojunction catalyst.

[0027] Figure 11 This is a flowchart of the method of the present invention. Detailed Implementation

[0028] The present invention will be further described in detail below through specific embodiments. However, those skilled in the art will understand that the following embodiments are for illustrative purposes only and should not be considered as limiting the scope of protection of the present invention. Furthermore, unless specific technical operation steps or conditions are specified in the embodiments, they are all performed according to the general techniques or conditions described in the literature in the field or according to the product instructions. Reagents or instruments whose manufacturers are not specified are all conventional products that can be obtained commercially.

[0029] Example 1: Preparation of MgO / Co3O4 heterojunction catalyst First, 4.76 g of CoCl₂·6H₂O and 8.132 g of MgCl₂·6H₂O were weighed and dissolved in 60 mL of deionized water under magnetic stirring at room temperature, forming a deep red transparent solution with a Co / Mg ratio of 1:2. Then, 4 mL of urea was added to the solution to adjust the pH to approximately 9, and the mixture was continuously stirred magnetically for 10 min to ensure uniform reaction, resulting in a deep green suspension. The resulting suspension was transferred to a stainless steel PTFE-lined high-pressure hydrothermal reactor, ensuring the reaction volume did not exceed 70% of the total volume. After sealing the reactor, it was transferred to an oven at 200℃ for 12 h, starting the timer when the temperature reached 200℃. After the reaction, the reactor was cooled to room temperature, and the resulting product was centrifuged (6000 r / min, 5 min) and washed several times with ethanol and deionized water. The obtained solid precipitate was then placed in an electrically heated drying oven at 60°C for 12 hours. Finally, it was ground in a mortar until no large particles remained, yielding a powder material. This powder material was then calcined in a muffle furnace at 500°C (heating rate of 5°C / min) for 2 hours. After calcination, a MgO / Co3O4 heterojunction catalyst material was obtained. This material was cooled to room temperature and transferred to a dry sample vial for later use.

[0030] Example 2: Scanning electron microscopy (SEM) analysis of MgO / Co3O4 heterojunction catalyst like Figure 1 As shown, the MgO / Co3O4 heterojunction catalyst prepared by the hydrothermal-calcination two-step method exhibits a distinct crystal structure. Its main crystal structure is an octahedral spinel structure, with plate-like crystals attached to its surface. The spinel structure represents Co3O4 crystals, and the plate-like structures on the surface represent MgO crystals. The heterojunction structure formed by MgO and Co3O4 makes its structure more stable and results in good performance in the ozone catalytic oxidation of ammonia nitrogen.

[0031] Example 3: Transmission electron microscopy (TEM) analysis of MgO / Co3O4 heterojunction catalyst like Figure 2 As shown, TEM revealed two different types of crystal planes with interplanar spacings of 0.2410 nm and 0.2131 nm, respectively. These correspond to the Co3O4 (311) and MgO (200) crystal planes. The exposure of these crystal planes is the key reason for the excellent ammonia nitrogen removal efficiency of the MgO / Co3O4 heterojunction catalyst in the ozone catalytic oxidation system.

[0032] Example 4: Effect of different hydrothermal temperatures on the ozone catalytic removal of ammonia nitrogen by MgO / Co3O4 heterojunction catalyst like Figure 3 As shown, the effect of hydrothermal temperature on the ozone catalytic removal of ammonia nitrogen by the MgO / Co3O4 heterojunction catalyst was investigated. At a hydrothermal temperature of 140℃, the ammonia nitrogen removal rate of the MgO / Co3O4 heterojunction catalyst was only 43.0%, and there was no selectivity for gaseous nitrogen. When the hydrothermal temperature was increased from 160℃ to 200℃, the ammonia nitrogen removal rate of the MgO / Co3O4 heterojunction catalyst stabilized at around 92.2%, while the selectivity for gaseous nitrogen also increased from 14.8% to 47.7%. When the hydrothermal temperature was further increased to 220℃, the ammonia nitrogen removal rate of the MgO / Co3O4 catalyst dropped sharply to only 33.2%, and the gaseous nitrogen selectivity was only 9.0%. In summary, this invention determines that the MgO / Co3O4 heterojunction catalyst prepared at a hydrothermal temperature of 200℃ has the optimal performance, with an ammonia nitrogen removal rate of 92.2% and a gaseous nitrogen selectivity of 47.7%. This condition was identified as the optimal hydrothermal temperature for catalyst preparation.

[0033] Example 5: Effect of different hydrothermal times on the ozone catalytic removal of ammonia nitrogen by MgO / Co3O4 heterojunction catalyst The effect of different hydrothermal times on ozone catalytic denitrification is as follows: Figure 4 As shown, when the hydrothermal time is 3 h, the MgO / Co3O4 heterojunction catalyst only achieves a removal rate of approximately 50% for ammonia nitrogen, and the ammonia nitrogen is oxidized to nitrate nitrogen, lacking selectivity for gaseous nitrogen. When the hydrothermal time is extended from 6 h to 12 h, the ammonia nitrogen removal rate increases from 86.9% to over 90%, and the selectivity for gaseous nitrogen significantly improves, increasing from 1.2% to approximately 50%. With the hydrothermal time further extended to 21 h, the ammonia nitrogen removal efficiency stabilizes at around 89%, while the selectivity for gaseous nitrogen shows a slow decreasing trend, dropping from nearly 50% to 33.5%, a decrease of approximately 30%. In conclusion, the optimal hydrothermal time for the catalyst is considered to be 12 h.

[0034] Example 6: Effect of different calcination temperatures on the ozone catalytic removal of ammonia nitrogen by MgO / Co3O4 heterojunction catalyst The effect of different calcination temperatures on the ozone catalytic removal efficiency of MgO / Co3O4 heterojunction catalysts is as follows: Figure 5As shown, the calcination time was investigated through single-factor experiments. The results showed that at 300℃, the ammonia nitrogen removal efficiency of the catalyst was poor, only 32.3%, and there was no gaseous selectivity; ammonia nitrogen was completely oxidized to nitrate nitrogen. When the calcination temperature was increased to 400 and 500℃, the catalyst efficiency significantly improved due to the sufficient growth of crystals and the formation of a stable structure. At 400℃, the ammonia nitrogen removal efficiency reached 87.7%, and the gaseous selectivity also improved to approximately 48.0%. When the calcination temperature was increased to 500℃, the ammonia nitrogen removal rate reached 92.2%, with a gaseous nitrogen selectivity of 47.7%. However, when the calcination temperature was increased again to 600℃, the efficiency of the catalyst in catalyzing the oxidation of ammonia nitrogen by ozone showed a decreasing trend, with an ammonia nitrogen removal efficiency of 87.5% and a gaseous selectivity of 32.5%. In conclusion, the optimal calcination temperature for the catalyst is considered to be 500℃.

[0035] Example 7: Effect of different calcination times on the ozone catalytic removal of ammonia nitrogen by MgO / Co3O4 heterojunction catalyst like Figure 6 As shown, the investigation of calcination time revealed that at a calcination time of 1 h, the MgO / Co3O4 heterojunction catalyst only showed an ammonia nitrogen removal efficiency (43.8%), and did not exhibit selectivity for the reaction products. Compared to a calcination time of 1 h, at a calcination temperature of 1.5 h, although the ammonia nitrogen removal efficiency did not improve, selectivity for gaseous nitrogen began to appear (10.3%). At a calcination time of 2 h, the ammonia nitrogen removal efficiency and gaseous nitrogen selectivity were 92.2% and 47.7%, respectively. Further extension of the calcination time showed a slow decreasing trend in the selectivity for both ammonia nitrogen and gaseous nitrogen. At calcination times of 4 h and 6 h, the ammonia nitrogen removal rate and gaseous nitrogen selectivity were 90.3% and 52.9%, and 27.6% and 13.9%, respectively. Therefore, a calcination time of 2 h is the optimal calcination time.

[0036] Example 8: The effect of temperature on the ozone catalytic removal of ammonia nitrogen by MgO / Co3O4 heterojunction catalyst like Figure 7As shown, the effect of reaction temperature on the performance of the MgO / Co3O4 heterojunction catalyst in the ozone catalytic oxidation of ammonia nitrogen was studied through single-factor experiments. At 10℃, the catalyst achieved a 92.2% removal rate of ammonia nitrogen and a gaseous nitrogen selectivity of 52.6%. When the temperature increased to 20℃, the ammonia nitrogen removal rate remained high, but the gaseous nitrogen selectivity decreased to 47.7%. Further increasing the temperature to 30℃ significantly reduced the catalytic performance, with the ammonia nitrogen removal rate dropping to 83.3% and the gaseous nitrogen selectivity further decreasing to 15.8%. These performance changes mainly stem from the influence of temperature on the solubility of ozone in water: lower temperatures help increase ozone solubility, enhancing its mass transfer and utilization efficiency, thereby improving the catalytic reaction effect; while as the temperature rises, ozone solubility decreases, leading to a decrease in its concentration in the liquid phase, which in turn affects the catalytic reaction rate and nitrogen conversion pathway. Based on the above experimental results and mechanism analysis, this invention determines that the optimal operating temperature for the MgO / Co3O4 heterojunction catalyst in the ozone catalytic oxidation of ammonia nitrogen removal is 10℃. Under these conditions, the catalyst exhibits both high ammonia nitrogen removal rate and high gaseous nitrogen selectivity.

[0037] Example 9: Effect of pH on the ozone catalytic removal of ammonia nitrogen by MgO / Co3O4 heterojunction catalyst Using pH as a variable, a single-factor experiment was conducted to investigate the catalytic oxidation efficiency of ammonia nitrogen by ozone. Figure 8 As shown, within the initial pH range of 3-9, both the ammonia nitrogen removal rate and the gaseous nitrogen selectivity generally increased with increasing pH. When the pH was greater than 9, the solution became strongly alkaline, and both the ammonia nitrogen removal rate and the gaseous nitrogen selectivity in the ozone catalytic oxidation system decreased slightly. It was found that the catalyst exhibited the best selectivity for ammonia nitrogen and gaseous nitrogen at pH 9, at 92.2% and 47.7%, respectively.

[0038] Example 10: Effect of Ozone Concentration on Ozone Catalytic Removal of Ammonia Nitrogen by MgO / Co3O4 Heterojunction Catalyst Heterogeneous catalytic ozone oxidation is a complex process involving gas, liquid, and solid phases, in which the inlet ozone concentration plays a crucial role. Increasing the ozone concentration enhances its oxidizing capacity, promotes pollutant decomposition, and simultaneously increases the free radical generation rate, thereby accelerating pollutant removal. Therefore, this study investigated the effect of ozone concentration on the removal of ammonia nitrogen by ozone catalytic oxidation under the conditions of varying ozone flow rate to control ozone concentration, using a MgO / Co3O4 heterojunction catalyst with a dosage of 1 g / L, an initial solution concentration of 10 mg / L, and a pH of 9. Figure 9As shown, with the ozone concentration increasing from 100.9 mg / L to 119.8 mg / L, the ammonia nitrogen degradation efficiency increased from 77.4% to 95.7%, an increase of 18.3 percentage points. Furthermore, the selectivity for gaseous nitrogen increased from 21.5% to 52.9%, an increase of 31.4 percentage points. Therefore, with the increase of ozone concentration, both the ammonia nitrogen removal rate and the selectivity for gaseous nitrogen in the ozone catalytic oxidation system are improved. In conclusion, increasing ozone concentration is beneficial for the removal of ammonia nitrogen pollutants and can also improve the selectivity for gaseous nitrogen.

[0039] Example 11: Effect of catalyst dosage on the ozone catalytic removal of ammonia nitrogen by MgO / Co3O4 heterojunction catalyst like Figure 10 As shown, when the catalyst dosage is 0.5 g / L, the low catalyst dosage results in insufficient ·OH generated by the ozone-activated catalyst, leading to a disappointing ammonia nitrogen removal rate of only 51.8%, and a gaseous nitrogen selectivity of only 2.9%. With increasing catalyst dosage, the number of active sites also increases. When the catalyst dosage is 1 g / L, the ammonia nitrogen removal rate reaches 92.2%, and the gaseous nitrogen selectivity also increases to 47.7%. However, when the catalyst dosage is further increased to 2 g / L and 3 g / L, the ammonia nitrogen removal rate decreases to 86.4% and 85.1%, respectively, and the gaseous nitrogen selectivity also decreases to 19.4% and 19.2%, respectively. This is because when the catalyst is in excess, it becomes unevenly dispersed and agglomerates. This agglomeration prevents pollutants from diffusing to the surface of the active sites, reducing the probability of the catalytic reaction. Furthermore, when the catalyst is in excess, the rate at which active species are generated exceeds the rate at which ammonia nitrogen is degraded. Therefore, the excess active species react with each other, leading to quenching and a decrease in catalytic efficiency. Thus, in the ozone catalytic oxidation of ammonia nitrogen system, a catalyst dosage of 1 g / L yields the optimal ammonia nitrogen removal efficiency and selectivity for gaseous nitrogen.

[0040] This invention employs a two-step hydrothermal-calcination method to construct a heterojunction structure by controlling the Co / Mg molar ratio, hydrothermal temperature, hydrothermal time, calcination temperature, and calcination time. Urea is used as a precipitant to adjust the pH to 9, ensuring the composite of Co3O4 spinel and MgO plate-like crystals to form a stable heterojunction. This method is simple and controllable, superior to traditional co-precipitation methods, avoiding metal leaching problems and the issues of poor selectivity or low activity associated with single MgO or Co3O4. SEM shows that plate-like MgO adheres to the surface of octahedral Co3O4 spinel, forming a heterojunction interface. TEM confirms that the interplanar spacing is 0.2410 nm (Co3O4 (311)) and 0.2131 nm (MgO (200)), exposing highly active crystal faces. The alkaline surface of MgO promotes the Co²⁺ / Co³⁺ redox cycle, accelerating ozone decomposition to generate active species such as ·OH and SO4•⁻. The heterojunction inhibits the excessive oxidation of ammonia nitrogen to NO3⁻ and directs its conversion to N2. This invention presents a MgO / Co3O4 heterojunction catalyst that achieves highly efficient removal (>92%) of ammonia nitrogen and highly selective conversion to N2 (>47%), solving the industry problem of over-oxidation in ozone catalytic oxidation. Structural characterization and parameter optimization reveal the core roles of oxygen vacancies, crystal facet exposure, and electron transfer, providing a low-cost and highly stable solution for wastewater denitrification.

Claims

1. A method for preparing a MgO / Co3O4 heterojunction catalyst for catalyzing selective oxidation of ammonia nitrogen by ozone, characterized in that, Includes the following steps: (1) Prepare a mixed solution of MgCl2 and CoCl2 with a molar ratio of Co to Mg of 1:2-2:1; (2) Add ammonia to the mixed solution to adjust the pH to 7-12, stir the reaction at room temperature for 10-30 min to obtain a suspension mixture; (3) The suspension mixture is transferred to a hydrothermal reactor and hydrothermally reacted at 100-220°C for 3-21 h; (4) Centrifuge the hydrothermal reaction products at a speed of 6000-10000 r / min for 1-5 min, and wash them with anhydrous ethanol and deionized water 3-10 times in sequence. (5) Dry the cleaned product at 60-105℃ for 8-24 h to obtain the precursor material; (6) The precursor material is calcined at 300-600℃ for 1-6 h with a heating rate of 5-10℃ / min. After cooling, it is ground to obtain the MgO / Co3O4 heterojunction catalyst.

2. The method of claim 1, wherein: The molar ratio of Co to Mg in step (1) is 1:

2.

3. The method of claim 1, wherein: The pH in step (2) is 9 and the stirring time is 10 min.

4. The method of claim 1, wherein: The hydrothermal reaction temperature in step (3) is 200℃ and the hydrothermal time is 12 h.

5. The preparation method according to claim 1, characterized in that: In step (4), the centrifugation speed is 6000 r / min, the centrifugation time is 5 min, and the number of cleaning cycles is 5.

6. The preparation method according to claim 1, characterized in that: The drying temperature in step (5) is 60°C and the drying time is 12 h.

7. The preparation method according to claim 1, characterized in that: The calcination temperature in step (6) is 500℃, the calcination time is 2 h, and the heating rate is 5℃ / min.

8. A MgO / Co3O4 heterojunction catalyst for the selective oxidation of ammonia nitrogen by ozone, characterized in that: The preparation method described in any one of claims 1-7 is used to obtain the structure of a heterojunction formed by MgO plate crystals and Co3O4 spinel, with interplanar spacing including 0.2410 nm (Co3O4 (311)) and 0.2131 nm (MgO (200)).

9. The application of the MgO / Co3O4 heterojunction catalyst according to claim 8 in the treatment of ammonia nitrogen wastewater, characterized in that... Includes the following steps: In ammonia nitrogen-containing wastewater with a pH of 7-12, the catalyst is added at a rate of 0.5-3 g / L, and an O3 / O2 mixed gas with an ozone concentration of 100.9-119.8 mg / L is introduced, and the reaction is carried out at 10-30℃ for 1 h.

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