An oxygen-vacancy-rich ZnO catalyst electrode, its preparation method and application in electrocatalytic reduction of nitrate nitrogen

Abstract

The present invention discloses an oxygen-vacancy-rich ZnO catalyst electrode, a preparation method thereof, and an application thereof in electrocatalytic reduction of nitrate nitrogen. In the present invention, a zinc carboxylate salt synthesized by a chemical precipitation method is used as a precursor to be calcined to generate ZnO, and then a large number of oxygen vacancies are generated by electroreduction activation, enhancing the adsorption of nitrate nitrogen and its intermediate products, and effectively improving the activity of electrocatalytic reduction of nitrate and the selectivity of ammonia nitrogen, with better ammonia production effect.

Description

Technical Field

[0001] The present invention belongs to the field of electrochemistry technology, and particularly relates to an oxygen-vacancy-rich ZnO catalyst electrode, a preparation method thereof, and an application thereof in electrocatalytic reduction of nitrate nitrogen. Background Art

[0002] Nitrate nitrogen widely exists in surface water and groundwater. The nitrate nitrogen in surface water mainly comes from industrial wastewater, domestic sewage, and agricultural sewage. High concentrations of nitrate in water can cause eutrophication of water bodies, reduce the oxygen available to aquatic organisms, and damage the aquatic ecosystem. Excessive nitrate nitrogen in drinking water may cause diseases such as methemoglobinemia and non-Hodgkin lymphoma. Among them, the electrocatalytic reduction of nitrate (NRR) technology has the characteristics of high efficiency, no pollution, and simple reaction conditions. More importantly, one of the main products of this reaction is ammonia nitrogen, which can realize the reduction of nitrate nitrogen to ammonia nitrogen and then carry out resource utilization. Ammonia is one of the most valuable chemical products in the world, and it is a good nitrogen source for fertilizers and a green hydrogen-rich material.

[0003] In the electrocatalytic reduction of nitrate nitrogen technology, the research on catalyst materials is the focus at present. The current research mainly focuses on copper-based catalysts, and there is less research on other metals and their metal oxides. Summary of the Invention

[0004] In view of this, the purpose of the present invention is to provide an oxygen-vacancy-rich ZnO catalyst electrode, a preparation method thereof, and an application thereof in electrocatalytic reduction of nitrate nitrogen, which can effectively improve the activity of electrocatalytic reduction of nitrate and the selectivity of ammonia nitrogen.

[0005] To achieve the above object, the present invention provides the following technical solutions:

[0006] The present invention provides a preparation method of an oxygen-vacancy-rich ZnO catalyst electrode, comprising the following steps:

[0007] (1) Dissolve a zinc salt in water to obtain a zinc salt solution, dissolve an organic carboxylic acid in water to obtain an organic carboxylic acid solution, and then dropwise add the organic carboxylic acid solution into the zinc salt solution to obtain a zinc carboxylate precipitate;

[0008] (2) Filter, wash, and dry the zinc carboxylate precipitate obtained in step (1), and then calcine it in an air atmosphere to obtain ZnO particles;

[0009] (3) Load the ZnO particles obtained in step (2) on an electrode substrate to prepare an electrode, and then perform electroreduction activation on it to make its surface rich in oxygen vacancies, thereby obtaining an oxygen-vacancy-rich ZnO catalyst electrode.

[0010] As a preferred technical solution, in step (1), the zinc salt is one or more of zinc chloride, zinc sulfate, and zinc nitrate.

[0011] As a preferred technical solution, in the step (1), the organic carboxylic acid is one or more of malic acid, citric acid, oxalic acid, and tartaric acid.

[0012] As a preferred technical solution, in the step (1), the molar ratio of the zinc salt to the organic carboxylic acid is 1.0 - 4.0:1.

[0013] As a preferred technical solution, in the step (2), the calcination temperature is 250 - 450 °C.

[0014] As a preferred technical solution, the specific method of the step (3) is as follows: Add ZnO particles and carbon powder to a mixed solution of isopropanol and ethanol, then add Nafion as a binder, and ultrasonicate until it forms a uniform ink-like state; Under the heating of an infrared lamp, evenly apply the catalyst ink on both sides of the carbon paper as the working electrode; Activate the working electrode at -0.65 V vs. the hydrogen reversible electrode for 15.0 min to make its surface rich in oxygen vacancies, and obtain a ZnO catalyst electrode with rich oxygen vacancies.

[0015] The present invention also provides a ZnO catalyst electrode with rich oxygen vacancies prepared by the above preparation method.

[0016] The present invention also provides the application of the ZnO catalyst electrode with rich oxygen vacancies in the electrocatalytic reduction of nitrate nitrogen, and the working voltage is preferably -0.65 V.

[0017] The beneficial effects of the present invention are as follows:

[0018] In the present invention, the zinc carboxylate synthesized by the chemical precipitation method is used as a precursor to be calcined to generate ZnO, and then a large number of oxygen vacancies are generated by electroreduction activation, enhancing the adsorption of nitrate nitrogen and its intermediate products, and can effectively improve the activity of electrocatalytic reduction of nitrate and the selectivity of ammonia nitrogen, and the ammonia production effect is better. Description of the Drawings

[0019] In order to make the objectives, technical solutions, and beneficial effects of the present invention clearer, the present invention provides the following drawings for illustration:

[0020] Figure 1 It is the TEM image of the ZnO catalyst electrode with rich oxygen vacancies prepared in Example 1.

[0021] Figure 2 It is the XPS image of the ZnO catalyst electrode with rich oxygen vacancies prepared in Example 1 and the ZnO catalyst electrode prepared in Comparative Example 1.

[0022] Figure 3 It is the graph of the change of nitrate nitrogen and product distribution with time in the electrocatalytic reduction of nitrate by the ZnO catalyst electrode with rich oxygen vacancies prepared in Example 1.

[0023] Figure 4 It is a graph showing the changes in nitrate nitrogen and product distribution over time during the electrocatalytic reduction of nitrate by the ZnO catalyst electrode prepared in Comparative Example 1. Specific Embodiments

[0024] The present invention will be further described below in conjunction with the accompanying drawings and specific embodiments, so that those skilled in the art can better understand the present invention and be able to implement it, but the exemplified embodiments are not intended to limit the present invention.

[0025] Unless otherwise specified, the methods used in the following examples are all conventional methods. The materials or reagents required in the following examples are all commercially available unless otherwise specified.

[0026] Example 1: ZnO catalyst electrode with rich oxygen vacancies

[0027] (1) Dissolve 5 mmol of Zn(NO 3 ) 2 and 5 mmol of oxalic acid (H 2 C 2 O 4 ) in 100.0 mL of water and ultrasonically disperse until completely dissolved; at a rotation speed of 200 rpm, gradually add the oxalic acid solution dropwise and uniformly into the zinc nitrate solution. This process takes 20.0 min to obtain zinc oxalate precipitate, seal it with plastic wrap and let it stand for 2.0 h;

[0028] (2) Filter the zinc oxalate precipitate obtained in step (1) using a suction filtration device, wash the obtained material with absolute ethanol and evenly spread it in a crucible, dry it in an oven at 60 °C for 1.0 h, and then, in an air atmosphere, heat it at a rate of 5 °C / min to 350 °C and calcine it for 6 h to obtain ZnO particles;

[0029] (3) Weigh the ZnO particles obtained in step (2) and 3 mg of carbon powder, add a mixture of 0.4 mL of isopropanol and 3 mL of ethanol solution, add 20 μL of Nafion as a binder, and ultrasonically disperse it until it forms a uniform ink-like substance; under the heating of an infrared lamp, evenly apply the catalyst ink on both sides of the carbon paper as a working electrode; activate the working electrode at -0.65 V vs. RHE (hydrogen reversible electrode) for 15.0 min to make its surface rich in oxygen vacancies, thus obtaining a ZnO catalyst electrode with rich oxygen vacancies.

[0030] Comparative Example 1: ZnO catalyst electrode

[0031] (1) Dissolve 5 mmol of Zn(NO 3 ) 2 and 5 mmol of oxalic acid (H 2 C 2 O 4) It was dissolved in 100.0 mL of water and ultrasonicated until completely dispersed; at a rotation speed of 200 rpm, the oxalic acid solution was added dropwise and uniformly into the zinc nitrate solution, which took 20.0 min, to obtain zinc oxalate precipitate. It was sealed with plastic wrap and left standing for 2.0 h;

[0032] (2) The zinc oxalate precipitate in step (1) was filtered with a suction filtration device. The obtained material was washed with absolute ethanol and evenly spread in a crucible, and then dried in an oven at 60 °C for 1.0 h. Then, in an air atmosphere, it was calcined at 350 °C for 6 h at a heating rate of 5 °C / min to obtain ZnO particles;

[0033] (3) Weighed the ZnO particles in step (2) and 3 mg of carbon powder, added 0.4 mL of isopropanol and 3 mL of ethanol solution and mixed them, added 20 μL of Nafion as a binder, and ultrasonicated until it formed a uniform ink-like state; under the heating of an infrared lamp, the catalyst ink was evenly coated on both sides of the carbon paper to obtain a ZnO catalyst electrode.

[0034] The TEM image of the oxygen-rich vacancy ZnO catalyst electrode prepared in Example 1 is as Figure 1 shown. Uniform ZnO particles can be seen from the figure.

[0035] The XPS images of the oxygen-rich vacancy ZnO catalyst electrode prepared in Example 1 and the ZnO catalyst electrode prepared in Comparative Example 1 are as Figure 2 shown. A large number of oxygen vacancies appeared in the activated oxygen-rich vacancy ZnO catalyst as seen from the figure.

[0036] Example 2: Removal effect of the ZnO catalyst electrode on the pollutant nitrate

[0037] (1) The specific steps for setting up the denitrification reaction device are as follows:

[0038] a) The denitrification reaction electrolytic cell is an H-type electrolytic cell. The anode chamber and the cathode chamber are separated by a cation exchange membrane (Nafion-117). The volume of both chambers is 150 mL. Sodium sulfate (50 mM) is added as the electrolyte to both the anode chamber and the cathode chamber, with a volume of 100 mL each, and nitrogen needs to be passed through for 10 min before the reaction; then, 1 mL of a pipette is used to add a nitrate nitrogen stock solution with an initial concentration of 100 mg / L to the cathode electrolytic chamber, and a Type B magnetic stir bar is added for stirring;

[0039] b) According to the principle of the three-electrode system, the circuit of the electrocatalytic denitrification device was set up. The counter electrode is a platinum sheet electrode (30 mm × 30 mm), the reference electrode is a reversible hydrogen electrode, and the working electrodes are respectively the oxygen-rich vacancy ZnO catalyst electrode prepared in Example 1 and the ZnO catalyst electrode prepared in Comparative Example 1;

[0040] c) Evaluation of the denitrification reaction effect, detecting the concentrations of pollutants, intermediate products and end products by a gas ultraviolet spectrophotometer (UV1000).

[0041] (2) Operation of the denitrification reaction device is as follows:

[0042] a) Place the entire denitrification experimental device in a constant temperature water bath magnetic stirrer at 25 °C, with a stirring rate of 400 rpm, and maintain uniform stirring;

[0043] b) Set the parameters of the electrochemical workstation, select the chronoamperometry program, set the voltage to -0.65 V, and start the electrocatalytic reduction of nitrate reaction program.

[0044] (3) Determine the denitrification reaction activity as follows:

[0045] a) At 0 min, 60 min, 120 min, 180 min, and 240 min during the reaction, use a glass syringe to take a sample (about 2.5 ml) from the reaction solution in the cathode chamber, and transfer 1 ml of the sample to a volumetric flask and make up to 25 ml.

[0046] b) Use a ultraviolet spectrophotometer (UV1000) to measure the concentrations of nitrate nitrogen, ammonia nitrogen, and nitrite nitrogen at wavelengths of 220 nm, 420 nm, and 540 nm respectively;

[0047] c) The removal rate of nitrate (η) = (1 - C t / C 0 ) * 100%;

[0048] The selectivity S of ammonia nitrogen NH4+ is expressed as: S NH4+ = C NH4+ / (C 0 - C t ) * 100%;

[0049] Among them, C NH4+ represents the concentration of NH 4 + -N after electrolysis for t time (mg L -1 ), C0 represents the initial concentration of NO 3 - -N (mgL -1 ); Ct represents the concentration of NO 3 - -N at time t (mg L -1 ).

[0050] The working electrode uses the oxygen-rich vacancy ZnO catalyst electrode prepared in Example 1 and the ZnO catalyst electrode prepared in Comparative Example 1 respectively. The electrocatalytic reduction of nitrate reaction is carried out according to the above steps. The graph of the change of nitrate nitrogen and product distribution with time in the electrocatalytic reduction of nitrate reaction is as Figure 3 and Figure 4 shown. The results show that when the oxygen-rich vacancy ZnO catalyst electrode prepared in Example 1 is used as the working electrode, the removal rate of nitrate nitrogen continuously increases. After 4 hours of reaction, the removal rate reaches 99.6%, and the selectivity of NH 3 is 92.2%. Under the same conditions, when the oxygen-rich vacancy ZnO catalyst electrode prepared in Comparative Example 1 is used as the working electrode, the removal rate is 99.5%, and the selectivity of NH 3 is 62.2%. It is proved that the oxygen-rich vacancy ZnO catalyst electrode prepared in Example 1 has higher activity and ammonia production selectivity than the ZnO catalyst electrode prepared in Comparative Example 1.

[0051] The working electrode uses the oxygen-rich vacancy ZnO catalyst electrode prepared in Example 1. The electrocatalytic reduction of nitrate reaction is carried out according to the above steps. In addition, the conditions of voltage setting are changed, and the voltage values are set to -0.5V, -0.55V, -0.6V, -0.65V, -0.7V, -0.75V, -0.8V respectively, and 7 reactions are carried out. The results of the electrocatalytic reduction of nitrate reaction by the oxygen-rich vacancy ZnO catalyst electrode at different voltages are shown in Table 1. As the voltage increases to -0.65V, the ammonia nitrogen selectivity gradually increases. However, as the voltage continues to increase, the enhancement of the hydrogen evolution side reaction leads to a certain decrease in the ammonia nitrogen selectivity.

[0052] Table 1 Nitrate removal rate and ammonia nitrogen selectivity of oxygen-rich vacancy ZnO catalyst electrode at different voltages

[0053] Voltage (V) -0.50 -0.55 -0.60 -0.65 -0.70 -0.75 -0.80 Removal efficiency (η) % 99.8 99.6 99.4 99.6 99.2 99.7 99.6 Ammonia nitrogen selectivity (S) % 52.1 77.9 85.46 92.2 95.8 93.3 89.1

[0054] The working electrode uses the oxygen-rich vacancy ZnO catalyst electrode prepared in Example 1. The electrocatalytic reduction of nitrate reaction is carried out according to the above steps, and the reaction is repeated 5 times. The results are shown in Table 2. The ammonia nitrogen selectivity in the 5 repeated reactions is about 90%, indicating that the oxygen-rich vacancy ZnO catalyst electrode has good stability and repeatability.

[0055] Table 2 Removal efficiency and ammonia selectivity of the same electrode in 5 repeated reactions

[0056] Nitrate concentration (mg / L) 1 2 3 4 5 Removal efficiency (η) % 84.71 97.26 97.40 95.70 91.47 Ammonia nitrogen selectivity (S) % 100 88.22 85.44 90.34 92.24

[0057] The above-described embodiments are only preferred embodiments given to fully illustrate the present invention, and the protection scope of the present invention is not limited thereto. Equivalent substitutions or transformations made by those skilled in the art on the basis of the present invention are all within the protection scope of the present invention. The protection scope of the present invention shall be subject to the claims.

Claims

1. Application of an oxygen vacancy-rich ZnO catalyst electrode in electrocatalytic reduction of nitrate nitrogen, characterized in that: The preparation method of the oxygen vacancy-rich ZnO catalyst electrode comprises the following steps: (1) Dissolve a zinc salt in water to obtain a zinc salt solution, dissolve an organic carboxylic acid in water to obtain an organic carboxylic acid solution, and then dropwise add the organic carboxylic acid solution into the zinc salt solution to obtain a zinc carboxylate precipitate; (2) Filter, wash, and dry the zinc carboxylate precipitate obtained in step (1), and then calcine it in an air atmosphere to obtain ZnO particles; (3) Load the ZnO particles obtained in step (2) on an electrode substrate to prepare an electrode, and then perform electroreduction activation on it to make its surface rich in oxygen vacancies, thereby obtaining an oxygen vacancy-rich ZnO catalyst electrode; The specific method of step (3) is: Add ZnO particles and carbon powder to a mixture of isopropanol and ethanol solution, then add Nafion as a binder, and ultrasonicate until it forms a uniform ink-like shape; Under infrared lamp heating, evenly apply the catalyst ink on both sides of the carbon paper as a working electrode; Activate the working electrode at -0.65V vs. hydrogen reversible electrode for 15.0 min to make its surface rich in oxygen vacancies, thereby obtaining an oxygen vacancy-rich ZnO catalyst electrode.

2. The application of the oxygen vacancy-rich ZnO catalyst electrode according to claim 1 in electrocatalytic reduction of nitrate nitrogen, characterized in that: In step (1), the zinc salt is one or more of zinc chloride, zinc sulfate, and zinc nitrate.

3. The application of the oxygen vacancy-rich ZnO catalyst electrode according to claim 1 in electrocatalytic reduction of nitrate nitrogen, characterized in that: In step (1), the organic carboxylic acid is one or more of malic acid, citric acid, oxalic acid, and tartaric acid.

4. The application of the oxygen vacancy-rich ZnO catalyst electrode according to claim 1 in electrocatalytic reduction of nitrate nitrogen, characterized in that: In step (1), the molar ratio of the zinc salt to the organic carboxylic acid is 1.0 - 4.0:

1.

5. The application of the oxygen vacancy-rich ZnO catalyst electrode according to claim 1 in electrocatalytic reduction of nitrate nitrogen, characterized in that: In step (2), the calcination temperature is 250 - 450 °C.

6. The application of the oxygen vacancy-rich ZnO catalyst electrode according to claim 1 in electrocatalytic reduction of nitrate nitrogen, characterized in that: The working voltage is -0.65V.

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