CN115196665B

CN115196665B - Copper oxide nano-sheet and preparation method thereof, and method for preparing ammonia by electrocatalytic nitrate radical

Info

Publication number: CN115196665B
Application number: CN202210747594.7A
Authority: CN
Inventors: 邹雨芹; 王双印; 潘玉平
Original assignee: Hunan University
Current assignee: Hunan University
Filing date: 2022-06-29
Publication date: 2024-07-16
Anticipated expiration: 2042-06-29

Abstract

The invention relates to the technical field of catalysts, in particular to a copper oxide nano-sheet, a preparation method thereof and a method for preparing ammonia by electrocatalytic nitrate radical reduction. The preparation method of the copper oxide nano-sheet comprises the following steps: mixing and stirring a copper chloride solution and a sodium hydroxide solution to form a solution containing tetrahydroxy copper complex ions, and carrying out hydrothermal reaction; the molar concentration of the copper chloride solution is 0.02 mol/L-0.10 mol/L, and the molar concentration of the sodium hydroxide solution is 2 mol/L-4 mol/L. The copper oxide nano-sheet prepared by the preparation method has excellent conductivity and can improve the ammonia production rate and the ammonia production Faraday efficiency.

Description

Copper oxide nano-sheet and preparation method thereof, and method for preparing ammonia by electrocatalytic nitrate radical

Technical Field

The invention relates to the technical field of catalysts, in particular to a copper oxide nano-sheet, a preparation method thereof and a method for preparing ammonia by electrocatalytic nitrate radical reduction.

Background

CuO nanosheets are copper-based catalysts with abundant (100) crystal planes, and have been widely used in the fields of carbon dioxide reduction, hydrogenation of aldehyde organics, catalytic hydrogenation or dehydrogenation of nitrate reduction to produce ammonia and other various compounds. At present, various capping agents (such as polyvinylpyrrolidone) are often required to be added in the preparation of the CuO catalyst rich in the (100) crystal face, however, the addition of the capping agents often causes the surface of the CuO catalyst to be wrapped by the capping agents, so that the active sites on the surface of the CuO catalyst are reduced, and the conductivity of the CuO catalyst is reduced. The synthesis method has complex process and long production period.

The nitrate radical is subjected to electrochemical reduction, so that the content of the nitrate radical in the wastewater can be effectively reduced, the greater damage to the environment caused by the nitrate radical is avoided, and the high-added-value chemical product ammonia with wide application can be obtained. However, at present, a catalyst commonly used for electrochemical reduction of nitrate is a transition metal oxide, and an electrolyte is often a neutral salt solution, so that the content of nitrate as a substrate is low. However, transition metal catalysts generally have low faradaic efficiency, low selectivity to ammonia, and low ammonia production rate. In addition, the neutral electrolyte contains a small amount of nitrate, the pH value of the solution is not changed in the reduction process, and the relative error is larger when the nitrate content (namely the ammonia yield) is smaller because of larger error in the current ammonia quantification. If the nitrate amount is increased, a large amount of OH ^- is formed in the reduction process, so that the pH is obviously changed before and after electrolysis, and the actual electrolysis potential is obviously deviated in the electrolysis process. Therefore, it is difficult to conduct scale-up production under this system.

Disclosure of Invention

Accordingly, it is necessary to provide a copper oxide nanoplatelet which can improve conductivity and can improve ammonia production rate and ammonia production faraday efficiency, a method for producing the same, and a method for producing ammonia by electrocatalytic nitrate reduction.

In one aspect of the present invention, a method for preparing a copper oxide nanoplatelet is provided, comprising the steps of:

Mixing and stirring a copper chloride solution and a sodium hydroxide solution to form a solution containing tetrahydroxy copper complex ions, and carrying out hydrothermal reaction; the molar concentration of the copper chloride solution is 0.02 mol/L-0.10 mol/L, and the molar concentration of the sodium hydroxide solution is 2 mol/L-4 mol/L.

In one aspect of the present invention, there is also provided a copper oxide nano-sheet prepared by the method for preparing a copper oxide nano-sheet as described above.

In another aspect of the present invention, there is further provided a method for preparing ammonia by electrocatalytic nitrate radical reduction, which adopts the copper oxide nano-sheet as a catalyst, comprising the following steps:

Preparing an electrolyte with pH of 13.5-14 by taking a conductive material loaded with the copper oxide nano-sheets as a working electrode and inorganic alkali as a raw material;

performing electrolysis at a constant potential to reduce the copper oxide nanoplatelets; and

Regulating the concentration of nitrate radical in the electrolyte to be 100-1000 mmol/L, and electrocatalytic reduction of the nitrate radical to prepare ammonia.

According to the preparation method of the copper oxide nano-sheet, the copper oxide (CuO) nano-sheet with clean surface and good appearance can be obtained without any capping agent under the guiding action of chloride ions by reasonably regulating and controlling the molar concentration of the copper chloride solution and the sodium hydroxide solution. Ensures good conductivity of the copper oxide nano-sheet, fully exposes active sites of the copper oxide nano-sheet, and has simple whole synthesis process and short production period.

Accordingly, in the process of preparing ammonia by electrocatalytic nitrate radical reduction by taking the copper oxide nano-sheet as a catalyst, a large number of protrusions are formed on the surface of the copper oxide nano-sheet through the reduction process under a fixed potential, so that enrichment and reduction of nitrate radicals are facilitated, the number and activity of reaction sites of the catalyst are increased, and high Faraday efficiency and high ammonia production rate can be realized while high selectivity on ammonia is maintained. In addition, the alkaline electrolyte with the pH close to 14 can effectively avoid the influence of OH ^- on the pH in the electrolysis process, and can maintain the stable electrolysis potential for a long time. And the concentration of nitrate radical is regulated to be higher, so that the influence of trace ammonia in the environment can be effectively avoided on the basis of ensuring the ammonia yield, the detection accuracy is improved, and the method is beneficial to industrial application.

Drawings

FIG. 1 is an XRD pattern of the CuO nanoplatelets prepared in example 1;

fig. 2 is an SEM image of CuO nanoplatelets prepared in example 1;

fig. 3 is an SEM image of CuO nanoplatelets prepared in example 2;

fig. 4 is an SEM image of CuO nanoplatelets prepared in example 3;

fig. 5 is an SEM image of CuO nanoplatelets prepared in example 4;

FIG. 6 is a linear sweep voltammogram for electrocatalytic ammonia process in example 8;

FIG. 7 is a graph of electrocatalytic ammonia production rate at various potentials for example 8;

FIG. 8 is the Faraday efficiency of electrocatalytic ammonia production at different potentials in example 8;

FIG. 9 is a graph showing the standard curve of nitrate content in the electrolyte before and after electrolysis in example 8;

FIG. 10 is a graph showing the standard ammonia content in the electrolyte before and after electrolysis in example 8;

FIG. 11 is a graph of ammonia production rate for example 9 loaded with different amounts of CuO nanoplatelets;

FIG. 12 is a graph of the Faraday efficiency of ammonia production with different amounts of CuO nanoplatelets loaded in example 9;

FIG. 13 is a linear sweep voltammogram for electrocatalytic ammonia process of example 10;

FIG. 14 is a graph showing the ammonia production rate of the CuO nanoplatelets prepared at different reaction times in example 11;

FIG. 15 is a linear sweep voltammogram for electrocatalytic ammonia process of comparative example 2;

FIG. 16 is a graph of electrocatalytic ammonia production rate for comparative example 2.

Detailed Description

The present invention will be described more fully hereinafter in order to facilitate an understanding of the present invention. This invention may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete.

"Electrocatalytic" refers to a catalytic effect that accelerates the charge transfer at the electrode, electrolyte interface, reactions.

The first object of the present invention is to provide a method for preparing copper oxide nanoplatelets, comprising the steps of:

Mixing and stirring a copper chloride solution and a sodium hydroxide solution to form a solution containing tetrahydroxy copper complex ions, and carrying out hydrothermal reaction; wherein, the molar concentration of the copper chloride solution is 0.02 mol/L-0.10 mol/L, and the molar concentration of the sodium hydroxide solution is 2 mol/L-4 mol/L.

According to the preparation method of the copper oxide nano-sheet, the copper oxide (CuO) nano-sheet with clean surface and good appearance can be obtained without any capping agent under the guiding action of chloride ions by reasonably regulating and controlling the molar concentration of the copper chloride and the sodium hydroxide solution. Ensures good conductivity of the copper oxide nano-sheet, fully exposes active sites of the copper oxide nano-sheet, and has simple whole synthesis process and short production period.

In some embodiments, the molar concentration of the copper chloride solution may be anywhere between 0.02mol/L and 0.10mol/L, for example, 0.03mol/L, 0.04mol/L, 0.05mol/L, 0.06mol/L, 0.07mol/L, 0.08mol/L, 0.09 mol/L, preferably 0.05mol/L; the molar concentration of the sodium hydroxide solution may be any value between 2mol/L and 4mol/L, and may be, for example, 2.5mol/L, 3mol/L, or 3.5mol/L, and preferably 3mol/L. By controlling the molar concentrations of the copper chloride solution and the sodium hydroxide solution within the above-described ranges, it is possible to ensure that a deep blue solution of a complex containing a tetrahydroxy copper complex ion can be formed instead of copper hydroxide precipitation. Thus, the reactants can be mixed more uniformly, and the CuO nano-sheets generated in situ in the hydrothermal process are more uniform in size.

In some embodiments, the volume ratio of copper chloride solution to sodium hydroxide solution is 1:1.

In some embodiments, the speed of agitation may be 800rpm to 1200rpm and the time may be 0.5h to 1.5h. The formation of the complex can be promoted by rapid stirring.

In some embodiments, the hydrothermal reaction may be at a temperature of 80 ℃ to 120 ℃ for a time of 6 hours to 16 hours. Wherein the temperature is preferably 100 ℃, and the time can be 6h, 8h, 10h, 12h, 14h and 16h.

In some embodiments, after the hydrothermal reaction, a purification step is further included; the purification comprises the following specific steps: and (3) sequentially carrying out centrifugal washing on the product obtained after heating with water and absolute ethyl alcohol for a plurality of times, and drying. For example, the washing may be performed by centrifugation with water twice and then centrifugation with absolute ethanol. Preferably, the speed of each centrifugal washing can be 5000r/min to 8000r/min and the time can be 5min to 10min. The water may be deionized water. Preferably, the deionized water has a conductivity of 18.25 M.OMEGA.cm ^-1.

In some embodiments, the manner of drying is not limited, and for example, the drying may be performed in a vacuum oven, and the drying temperature may be 30 to 80 ℃, preferably 60 ℃; the drying time can be 8-12 h.

In some embodiments, the method of preparing copper oxide nanoplatelets further comprises a step of preparing a copper chloride solution comprising the steps of: dissolving copper chloride in water, carrying out ultrasonic treatment until the copper chloride is completely dissolved, and adding water to ensure that the molar concentration is 0.02mol/L to 0.10mol/L. The parameters of the ultrasound are not limited, and for example, the ultrasound temperature may be 20 to 30℃and the frequency may be 30 to 50kHz, preferably 40kHz.

In some embodiments, the method of preparing copper oxide nanoplatelets further comprises a step of preparing a sodium hydroxide solution comprising the steps of: sodium hydroxide is dissolved in water, ultrasonic treatment is carried out until the sodium hydroxide is completely dissolved, and water is added to ensure that the molar concentration is 2mol/L to 4mol/L. The parameters of the ultrasound are not limited, and for example, the ultrasound temperature may be 20 to 30℃and the frequency may be 30 to 50kHz, preferably 40kHz.

The second object of the present invention is to provide a copper oxide nano-sheet prepared by the method for preparing a copper oxide nano-sheet as described above.

The third object of the present invention is to further provide a method for preparing ammonia by electrocatalytic nitrate radical reduction, which adopts the copper oxide nano-sheet as a catalyst, comprising the following steps:

Preparing electrolyte with pH of 13.5-14 by taking a conductive material loaded with copper oxide nano sheets as a working electrode and inorganic alkali as a raw material;

Electrocatalytic is carried out at a fixed potential to reduce the copper oxide nanoplatelets; and

Regulating the concentration of nitrate radical in the electrolyte to 100-1000 mmol/L, and preparing ammonia by electrocatalytic reduction of nitrate radical.

In the process of preparing ammonia by electrocatalytic nitrate radical reduction by taking the copper oxide nano-sheet as a catalyst, a large number of protrusions are formed on the surface of the copper oxide nano-sheet through the reduction process under a fixed potential, so that enrichment and reduction of nitrate radicals are facilitated, the number and activity of reaction sites of the catalyst are increased, and high Faraday efficiency and high ammonia production rate can be realized while high selectivity to ammonia is maintained. In addition, the alkaline electrolyte with the pH close to 14 can effectively avoid the influence of OH ^- on the pH in the electrolysis process, and can maintain the stable electrolysis potential for a long time. And the concentration of nitrate radical is regulated to be higher, so that the influence of trace ammonia in the environment can be effectively avoided on the basis of ensuring the ammonia yield, and the detection accuracy is improved. And the solvent used in the whole ammonia preparation process is mainly deionized water, so that the ammonia preparation method is environment-friendly, harmless to people, convenient, easy to obtain and store and low in cost. The method can explore the optimal catalyst loading and the optimal reaction electrolysis potential, and provides a theoretical basis for large-scale production.

In some embodiments, the copper oxide nanoplatelets are coated on the conductive material in the form of a slurry, and the copper oxide nanoplatelet slurry may be prepared as follows:

Mixing the copper oxide nano-sheets with a solution formed by water and alcohol, and adding Nafion film solution after ultrasonic treatment until the copper oxide nano-sheets are completely dispersed.

The mass percentage of the Nafion film solution is 5%, and the alcohol can be isopropanol.

In some embodiments, the conductive material may be carbon paper. Preferably, before loading the copper oxide nano-sheet, the method further comprises the step of calcining the carbon paper; the calcination temperature can be 400 ℃, the time can be 16 hours, and the heating rate can be 5 ℃/min. Still more preferably, the carbon paper is hydrophilic PN030 carbon paper.

In some embodiments, the method of preparing an electrolyte may include: inorganic alkali and water are mixed to prepare inorganic alkali solution with the molar concentration of 1mol/L, namely the electrolyte. Among them, the inorganic base is preferably potassium hydroxide. More preferably, the purity of potassium hydroxide is greater than or equal to 85%.

In some embodiments, the fixed potential may be any one of 0V _RHE～-0.6V_RHE.

In some embodiments, the electrolytic cell used for electrolysis is an H-type cell comprising the working electrode, the counter electrode and the reference electrode, i.e. the working electrode forms a three-electrode system with the counter electrode and the reference electrode; preferably, the counter electrode is a carbon rod and the reference electrode is a mercury|oxidized mercury electrode.

The present invention will be described in further detail with reference to specific examples.

Example 1 preparation of CuO nanoplatelets

1) Preparing a copper chloride aqueous solution with the concentration of 0.05 mol/L: weighing copper chloride solid, adding the copper chloride solid into a small amount of deionized water, carrying out ultrasonic treatment at 20 ℃ until the copper chloride solid is completely dissolved, adding deionized water to prepare copper chloride aqueous solution with the concentration of 0.05mol/L, shaking uniformly, and standing for 10min for later use;

2) Preparing a 3mol/L sodium hydroxide aqueous solution: weighing sodium hydroxide solid, adding the sodium hydroxide solid into a small amount of deionized water, carrying out ultrasonic treatment at 20 ℃ until the sodium hydroxide solid is completely dissolved, cooling to room temperature, adding deionized water to prepare sodium hydroxide aqueous solution with the concentration of 3mol/L, shaking uniformly, and standing for 10min for later use;

3) Preparing a dark blue tetrahydroxy copper complex solution: mixing the 0.05mol/L copper chloride aqueous solution prepared in the step 1) with the 3mol/L sodium hydroxide aqueous solution prepared in the step 2), and rapidly stirring for 0.5h at normal temperature to obtain a dark blue tetrahydroxy copper complex solution;

4) Preparing a crude CuO nano-sheet product: transferring the complexing solution prepared in the step 3) into a reaction kettle, and carrying out hydrothermal reaction for 6h at 100 ℃.

5) Purification of crude CuO nanoplatelets: cooling the crude CuO nanosheet product prepared in the step 4) to room temperature, transferring into a centrifuge tube, and centrifuging to obtain a first precipitate; adding a proper amount of deionized water into the first precipitate, stirring the precipitate uniformly, and centrifuging to obtain a second precipitate; adding a proper amount of deionized water into the second precipitate, stirring the precipitate uniformly, and centrifuging to obtain a third precipitate; adding a proper amount of absolute ethyl alcohol into the third precipitate, stirring the precipitate uniformly, and centrifuging to obtain a fourth precipitate; and (3) placing the fourth precipitate in a vacuum oven, drying at 60 ℃ for 8 hours, and cooling to room temperature to obtain the purified CuO nanosheets. The XRD pattern of the CuO nanoplatelets was tested as shown in fig. 1; fig. 2 is an SEM image of CuO nanoplatelets.

Example 2 preparation of CuO nanoplatelets

1) Preparing a copper chloride aqueous solution with the concentration of 0.05 mol/L: weighing copper chloride solid, adding the copper chloride solid into a small amount of deionized water, carrying out ultrasonic treatment at 25 ℃ until the copper chloride solid is completely dissolved, adding deionized water to prepare copper chloride aqueous solution with the concentration of 0.05mol/L, shaking uniformly, and standing for 10min for later use;

2) Preparing a 3mol/L sodium hydroxide aqueous solution: weighing sodium hydroxide solid, adding the sodium hydroxide solid into a small amount of deionized water, carrying out ultrasonic treatment at 25 ℃ until the sodium hydroxide solid is completely dissolved, cooling to room temperature, adding deionized water to prepare sodium hydroxide aqueous solution with the concentration of 3mol/L, shaking uniformly, and standing for 10min for later use;

3) Preparing a dark blue tetrahydroxy copper complex solution: mixing the 0.05mol/L copper chloride aqueous solution prepared in the step 1) with the 3mol/L sodium hydroxide aqueous solution prepared in the step 2), and rapidly stirring for 1h at normal temperature to obtain a dark blue tetrahydroxy copper complex solution;

4) Preparing a crude CuO nano-sheet product: transferring the complexing solution prepared in the step 3) into a reaction kettle, and carrying out hydrothermal reaction for 8 hours at the temperature of 100 ℃.

5) Purification of crude CuO nanoplatelets: cooling the crude CuO nanosheet product prepared in the step 4) to room temperature, transferring into a centrifuge tube, and centrifuging to obtain a first precipitate; adding a proper amount of deionized water into the first precipitate, stirring the precipitate uniformly, and centrifuging to obtain a second precipitate; adding a proper amount of deionized water into the second precipitate, stirring the precipitate uniformly, and centrifuging to obtain a third precipitate; adding a proper amount of absolute ethyl alcohol into the third precipitate, stirring the precipitate uniformly, and centrifuging to obtain a fourth precipitate; and (3) placing the fourth precipitate in a vacuum oven, drying at 60 ℃ for 10 hours, and cooling to room temperature to obtain the purified CuO nanosheets. SEM images of the CuO nanoplatelets were tested as shown in fig. 3.

Example 3 preparation of CuO nanoplatelets

4) Preparing a crude CuO nano-sheet product: transferring the complexing solution prepared in the step 3) into a reaction kettle, and carrying out hydrothermal reaction for 10h at 100 ℃.

5) Purification of crude CuO nanoplatelets: cooling the crude CuO nanosheet product prepared in the step 4) to room temperature, transferring into a centrifuge tube, and centrifuging to obtain a first precipitate; adding a proper amount of deionized water into the first precipitate, stirring the precipitate uniformly, and centrifuging to obtain a second precipitate; adding a proper amount of deionized water into the second precipitate, stirring the precipitate uniformly, and centrifuging to obtain a third precipitate; adding a proper amount of absolute ethyl alcohol into the third precipitate, stirring the precipitate uniformly, and centrifuging to obtain a fourth precipitate; and (3) placing the fourth precipitate in a vacuum oven, drying at 60 ℃ for 12 hours, and cooling to room temperature to obtain the purified CuO nanosheets. SEM images of the CuO nanoplatelets were tested as shown in fig. 4.

Example 4 preparation of CuO nanoplatelets

1) Preparing a copper chloride aqueous solution with the concentration of 0.05 mol/L: weighing copper chloride solid, adding the copper chloride solid into a small amount of deionized water, carrying out ultrasonic treatment at 30 ℃ until the copper chloride solid is completely dissolved, adding deionized water to prepare copper chloride aqueous solution with the concentration of 0.05mol/L, shaking uniformly, and standing for 10min for later use;

2) Preparing a 3mol/L sodium hydroxide aqueous solution: weighing sodium hydroxide solid, adding the sodium hydroxide solid into a small amount of deionized water, carrying out ultrasonic treatment at 30 ℃ until the sodium hydroxide solid is completely dissolved, cooling to room temperature, adding deionized water to prepare sodium hydroxide aqueous solution with the concentration of 3mol/L, shaking uniformly, and standing for 10min for later use;

4) Preparing a crude CuO nano-sheet product: transferring the complexing solution prepared in the step 3) into a reaction kettle, and carrying out hydrothermal reaction for 12h at 100 ℃.

5) Purification of crude CuO nanoplatelets: cooling the crude CuO nanosheet product prepared in the step 4) to room temperature, transferring into a centrifuge tube, and centrifuging to obtain a first precipitate; adding a proper amount of deionized water into the first precipitate, stirring the precipitate uniformly, and centrifuging to obtain a second precipitate; adding a proper amount of deionized water into the second precipitate, stirring the precipitate uniformly, and centrifuging to obtain a third precipitate; adding a proper amount of absolute ethyl alcohol into the third precipitate, stirring the precipitate uniformly, and centrifuging to obtain a fourth precipitate; and (3) placing the fourth precipitate in a vacuum oven, drying at 60 ℃ for 12 hours, and cooling to room temperature to obtain the purified CuO nanosheets. SEM images of the CuO nanoplatelets were tested as shown in fig. 5.

Example 5 preparation of CuO nanoplatelets

The preparation method of this example is basically the same as that of example 1, except that: the hydrothermal reaction time in step 4) was 14h.

Example 6 preparation of CuO nanoplatelets

The preparation method of this example is basically the same as that of example 1, except that: the hydrothermal reaction time in step 4) was 16h.

Example 7 preparation of CuO nanoplatelets

The preparation method of this example is basically the same as that of example 1, except that: the concentration of the copper chloride aqueous solution was 0.10mol/L, and the concentration of the sodium hydroxide aqueous solution was 2mol/L.

Comparative example 1 preparation of CuO nanosheets

This comparative example was prepared in substantially the same manner as in example 1 except that: the concentration of the copper chloride aqueous solution was 0.5mol/L, and the concentration of the sodium hydroxide aqueous solution was 1mol/L.

Example 8 electrocatalytic reduction of nitrate to ammonia

1) Preparing slurry: weighing 5mg of the CuO nano-sheets prepared in the example 1, adding the CuO nano-sheets into a mixed solution formed by 500 mu L of deionized water and 450 mu L of isopropanol, carrying out ultrasonic treatment at 25 ℃ until the CuO nano-sheets are completely and uniformly dispersed, adding 50 mu L of Nafion film solution with the mass fraction of 5%, and continuing ultrasonic treatment until the Nafion film solution is uniform to prepare 1mL of slurry;

2) Preparing carbon paper: cutting carbon paper into 1cm multiplied by 2cm, dripping 40 mu L of the slurry prepared in the step 1) on the carbon paper, and airing;

3) Preparing an electrolyte: adding potassium hydroxide solid into a small amount of deionized water, performing ultrasonic treatment at 25 ℃ until the potassium hydroxide solid is completely dissolved, cooling to room temperature, adding deionized water to prepare 1mol/L potassium hydroxide aqueous solution (pH of 14), shaking uniformly, and standing for 10min for later use;

4) The carbon paper is used as a working electrode, a carbon rod is used as a counter electrode, a mercury|oxidized mercury electrode is used as a reference electrode, and the reference electrode and electrolyte together form an electrolysis system to carry out constant potential electrolysis to reduce the CuO nanosheets. Nitrate radical is then added into the electrolyte and the nitrate radical is electrocatalytic reduced to produce ammonia.

And (3) detecting the ammonia production effect:

1) Linear Sweep Voltammogram (LSV) test: 50mL of the aqueous potassium hydroxide solution prepared in step 3) was added to a 100mL single port cell. The carbon paper in the step 2) is used as a working electrode, a carbon rod is used as a counter electrode, and a mercury|mercury oxide electrode is used as a reference electrode. The electrochemical workstation of Shanghai Chenhua CHI660 is firstly used for pre-reduction for 0.5h under the fixed potential of-0.6V vs. RHE. And then adding potassium nitrate with a certain mass into the electrolytic cell to enable the concentration of nitrate to be 100mmol/L, and carrying out multiple linear scanning voltammetry curve tests within the potential range of 0.2-0.6V vs. RHE until the curves of two adjacent tests are completely the same, wherein the test result is shown in figure 6.

2) Constant potential electrolysis test: respectively adding 10mL of the potassium hydroxide aqueous solution prepared in the step 3) into a cathode chamber and an anode chamber of an H-type electrolytic cell, wherein the carbon paper prepared in the step 2) is used as a working electrode, a carbon rod is used as a counter electrode, and a mercury|mercury oxide electrode is used as a reference electrode. The electrochemical workstation of Shanghai Chenhua CHI660 is firstly used for pre-reduction for 0.5h under the fixed potential of-0.6V vs. RHE. Then adding potassium nitrate with a certain mass into the cathode chamber to enable the concentration of nitrate to be 100mmol/L, and selecting a potential at intervals of-0.1V vs. RHE within the potential range of 0-0.6V vs. RHE to carry out electrolysis for 1h. The ammonia production rate and the ammonia production faraday efficiency are shown in fig. 7 and 8, respectively. In addition, potassium nitrate is used as a standard sample, the nitrate content in the electrolyte before and after electrolysis is measured by adopting a high performance liquid chromatography, and a detection standard curve is shown in fig. 9; the ammonia content in the electrolyte before and after electrolysis was measured by ultraviolet absorption spectroscopy using ammonium chloride as a standard sample, and the detection standard curve is shown in fig. 10.

As shown in FIG. 7, the ammonia production rates at the potentials of 0V vs. RHE, -0.1V vs. RHE, -0.2V vs. RHE, -0.3V vs. RHE, -0.4V vs. RHE, -0.5V vs. RHE, and-0.6V vs. RHE were 47mmol g^-1 _cat h^-1、142mmol g^-1 _cat h^-1、583mmol g^-1 _cat h^-1、1463mmol g^-1 _cat h^-1、2668mmol g^-1 _cat h^-1、2822mmol g^-1 _cat h^-1 and 2659mmol g ^-1 _cat h^-1, respectively.

As shown in FIG. 8, the ammonia-generating Faraday efficiencies at the 0V vs. RHE, -0.1V vs. RHE, -0.2V vs. RHE, -0.3V vs. RHE, -0.4V vs. RHE, -0.5V vs. RHE, and-0.6V vs. RHE potentials were 24.93%, 32.03%, 54.88%, 77.89%, 94.45%, 93.43%, and 95.09%, respectively.

As shown in fig. 9, the standard curve corresponds to the equation y=188231x+206422, and the correlation coefficient R ² is 0.99963. The high performance liquid chromatography test has good linearity under the concentration of nitrate radical of 0 to 100mmol/L, and the test result is reliable.

As shown in fig. 10, the standard curve corresponds to the equation y=0.01297x+0.01103, and the correlation coefficient R ² is 0.99932. The ultraviolet absorption spectrum test has good linearity under the ammonia concentration of 0 to 100mmol/L, and the test result is reliable.

Example 9 electrocatalytic reduction of nitrate to ammonia

2) Preparing carbon paper: cutting carbon paper into 6 pieces of same carbon paper with the length of 1cm multiplied by 2cm, respectively taking 20 mu L, 40 mu L, 60 mu L, 80 mu L, 100 mu L and 120 mu L of the slurry prepared in the step 1), dripping the slurry on the 6 pieces of carbon paper, and airing;

4) The method is characterized in that carbon paper is used as a working electrode, a carbon rod is used as a counter electrode, a mercury|mercury oxide electrode is used as a reference electrode, and the counter electrode and electrolyte together form an electrolytic cell to perform electrocatalytic and reduce CuO nano-sheets. Nitrate radical is then added into the electrolyte and the nitrate radical is electrocatalytic reduced to produce ammonia.

And (3) detecting the ammonia production effect:

1) Constant potential electrolysis test: respectively adding 10mL of the potassium hydroxide aqueous solution prepared in the step 3) into a cathode chamber and an anode chamber of an H-type electrolytic cell, and respectively carrying out electrolysis by taking 6 pieces of carbon paper prepared in the step 2) as working electrodes, taking a carbon rod as a counter electrode and taking a mercury|mercury oxide electrode as a reference electrode. The electrochemical workstation of Shanghai Chenhua CHI660 is used for pre-reduction for 0.5h under the fixed potential of-0.6V vs. RHE before each electrolysis. Then, a certain amount of potassium nitrate was added to the cathode chamber so that the nitrate concentration became 100mmol/L, and the electrolysis was carried out at-0.4V vs. RHE potential for 1 hour. In addition, the nitrate content in the electrolyte before and after electrolysis is measured by adopting a high performance liquid chromatography, and the ammonia content in the electrolyte before and after electrolysis is measured by adopting an ultraviolet absorption spectrometry. The ammonia production rate of carbon paper loaded with CuO nanoplatelets of different contents is shown in fig. 11. As can be seen from FIG. 11, the content of the supported CuO nanoplatelets was 0.1mg cm ²、0.2mg cm²、0.3mg cm²、0.4mg cm²、0.5mg cm² and the ammonia production rate corresponding to 0.6mg cm ² were 4080mmol g^-1 _cat h^-1、2668mmol g^-1 _cat h^-1、2091mmol g^-1 _cat h^-1、1842mmol g^-1 _cath^-1、1497mmol g^-1 _cat h^-1 and 936mmol g ^-1 _cat h^-1, respectively.

The faradaic efficiency of ammonia production of carbon papers loaded with CuO nanoplatelets of varying content is shown in fig. 12. As can be seen from fig. 12, the ammonia-generating faradaic efficiencies corresponding to the CuO-loaded nanosheets content of 0.1mg cm ²、0.2mg cm²、0.3mg cm²、0.4mg cm²、0.5mg cm² and 0.6mg cm ² were 95.76%, 94.45%, 95.52%, 96.81%, 96.70% and 95.87%, respectively.

Example 10 electrocatalytic reduction of nitrate to ammonia

2) Preparing carbon paper: cutting carbon paper into 6 pieces of same carbon paper with the length of 1cm multiplied by 2cm, respectively taking 20 mu L of the slurry prepared in the step 1) to be dripped on the 6 pieces of carbon paper, and airing;

And (3) detecting the ammonia production effect:

1) Linear Sweep Voltammogram (LSV) test: 50mL of the aqueous potassium hydroxide solution prepared in step 3) was added to a 100mL single port cell. The carbon paper in the step 2) is used as a working electrode, a carbon rod is used as a counter electrode, and a mercury|mercury oxide electrode is used as a reference electrode. The electrochemical workstation of Shanghai Chenhua CHI660 is firstly used for pre-reduction for 0.5h under the fixed potential of-0.6V vs. RHE. Then adding a certain mass of potassium nitrate into the electrolytic cell to ensure that the nitrate concentration is 100mmol/L, 150mmol/L, 200mmol/L, 250mmol/L, 500mmol/L and 1000mmol/L respectively, and carrying out multiple linear scanning voltammetry curve tests within the potential range of 0.2 to-0.6V vs. RHE until the curves of two adjacent tests are completely the same, wherein the test result is shown in figure 13.

2) Constant potential electrolysis test: respectively adding 10mL of the potassium hydroxide aqueous solution prepared in the step 3) into a cathode chamber and an anode chamber of an H-type electrolytic cell, and respectively carrying out electrolysis by taking 6 pieces of carbon paper prepared in the step 2) as working electrodes, taking a carbon rod as a counter electrode and taking a mercury|mercury oxide electrode as a reference electrode. The electrochemical workstation of Shanghai Chenhua CHI660 is used for pre-reduction for 0.5h under the fixed potential of-0.6V vs. RHE before each electrolysis. Then adding a certain mass of potassium nitrate into the cathode chamber to ensure that the nitrate concentration is 100mmol/L, 150mmol/L, 200mmol/L, 250mmol/L, 500mmol/L and 1000mmol/L respectively, and electrolyzing for 1h at-0.5V vs. RHE potential. In addition, the nitrate content in the electrolyte before and after electrolysis is measured by adopting a high performance liquid chromatography, and the ammonia content in the electrolyte before and after electrolysis is measured by adopting an ultraviolet absorption spectrometry.

Example 11 electrocatalytic reduction of nitrate to ammonia

1) Preparing slurry: 5mg of the CuO nano-sheets prepared in example 1, example 2, example 3, example 4, example 5 and example 6 are respectively weighed, added into a mixed solution formed by 500 mu L of deionized water and 450 mu L of isopropanol, and subjected to ultrasonic treatment at 25 ℃ until the CuO nano-sheets are completely and uniformly dispersed, and then 50 mu L of Nafion film solution with the mass fraction of 5% is added, and ultrasonic treatment is continued until the mixture is uniform, so that 1mL of 6 slurries are prepared;

And (3) detecting the ammonia production effect:

1) Linear Sweep Voltammogram (LSV) test: 50mL of the aqueous potassium hydroxide solution prepared in step 3) was added to a100 mL single port cell. The carbon paper in the step 2) is used as a working electrode, a carbon rod is used as a counter electrode, and a mercury|mercury oxide electrode is used as a reference electrode. The electrochemical workstation of Shanghai Chenhua CHI660 is firstly used for pre-reduction for 0.5h under the fixed potential of-0.6V vs. RHE. Then adding potassium nitrate with a certain mass into the electrolytic cell to make the concentration of nitrate be 1000mmol/L, and carrying out multiple linear scanning voltammetry curve tests within the potential range of 0.2-0.6V vs. RHE until the curves of two adjacent tests are completely the same.

2) Constant potential electrolysis test: respectively adding 10mL of the potassium hydroxide aqueous solution prepared in the step 3) into a cathode chamber and an anode chamber of an H-type electrolytic cell, and respectively carrying out electrolysis by taking 6 pieces of carbon paper prepared in the step 2) as working electrodes, taking a carbon rod as a counter electrode and taking a mercury|mercury oxide electrode as a reference electrode. The electrochemical workstation of Shanghai Chenhua CHI660 is used for pre-reduction for 0.5h under the fixed potential of-0.6V vs. RHE before each electrolysis. Then adding a certain mass of potassium nitrate into the cathode chamber to make the nitrate concentration be 1000mmol/L respectively, and electrolyzing for 1h at-0.5V vs. RHE potential. In addition, the nitrate content in the electrolyte before and after electrolysis is measured by adopting a high performance liquid chromatography, and the ammonia content in the electrolyte before and after electrolysis is measured by adopting an ultraviolet absorption spectrometry. The ammonia production rate of CuO nanoplatelets prepared in the different examples is shown in fig. 14. As can be seen from FIG. 14, the ammonia production rates corresponding to the CuO nanoplatelets prepared in examples 1 to 6 (reaction times of 6h, 8h, 10h, 12h, 14h and 16h, respectively) were 11372mmol g^-1 _cat h^-1、7268mmol g^-1 _cat h^-1、5176mmol g^-1 _cat h^-1、8052mmol g^-1 _cat h^-1、7932mmol g^-1 _cat h^-1 and 8444mmol g ^-1 _cat h^-1, respectively.

Comparative example 2

1) Preparing slurry: weighing 5mg of commercial copper catalyst (copper powder with the purity of 99.9%, the particle size of >325 meshes and CAS number: 7440-50-8), adding into a mixed solution formed by 500 mu L of deionized water and 450 mu L of isopropanol, carrying out ultrasonic treatment at 25 ℃ until the copper catalyst is completely and uniformly dispersed, adding 50 mu L of Nafion film solution with the mass fraction of 5%, and continuing ultrasonic treatment until the copper catalyst is uniformly dispersed to prepare 1mL of slurry;

2) Preparing carbon paper: cutting carbon paper into 1cm multiplied by 2cm, dripping 20 mu L of the slurry prepared in the step 1) on the carbon paper, and airing;

And (3) detecting the ammonia production effect:

1) Linear Sweep Voltammogram (LSV) test: 50mL of the aqueous potassium hydroxide solution prepared in step 3) was added to a 100mL single port cell. The carbon paper in the step 2) is used as a working electrode, a carbon rod is used as a counter electrode, and a mercury|mercury oxide electrode is used as a reference electrode. The electrochemical workstation of Shanghai Chenhua CHI660 is firstly used for pre-reduction for 0.5h under the fixed potential of-0.6V vs. RHE. Then adding potassium nitrate with a certain mass into the electrolytic cell to make the concentration of nitrate be 1000mmol/L, and carrying out multiple linear scanning voltammetry curve tests within the potential range of 0.2-0.6V vs. RHE until the curves of two adjacent tests are completely the same, wherein the test result is shown in figure 15.

2) Constant potential electrolysis test: respectively adding 10mL of the potassium hydroxide aqueous solution prepared in the step 3) into a cathode chamber and an anode chamber of an H-type electrolytic cell, wherein the carbon paper prepared in the step 2) is used as a working electrode, a carbon rod is used as a counter electrode, and a mercury|mercury oxide electrode is used as a reference electrode. The electrochemical workstation of Shanghai Chenhua CHI660 is firstly used for pre-reduction for 0.5h under the fixed potential of-0.6V vs. RHE. Then, a certain amount of potassium nitrate was added to the cathode chamber so that the nitrate concentration became 1000mmol/L, and the electrolysis was carried out at-0.5V vs. RHE potential for 1 hour. As can be seen from FIG. 16, the ammonia production rate was 2703mmol g ^-1 _cat h^-1, and the Faraday efficiency was 45.79%. In addition, the nitrate content in the electrolyte before and after electrolysis is measured by adopting a high performance liquid chromatography, and the ammonia content in the electrolyte before and after electrolysis is measured by adopting an ultraviolet absorption spectrometry.

Comparative example 3

This comparative example is substantially the same as the ammonia production method of example 8, except that: the ammonia production rate was measured to be 1050mmol g ^-1 _cat h^-1 and the ammonia production faraday efficiency was measured to be 75.4% by using the CuO nanosheets prepared in comparative example 1 as a catalyst.

The technical features of the above-described embodiments may be arbitrarily combined, and all possible combinations of the technical features in the above-described embodiments are not described for brevity of description, however, as long as there is no contradiction between the combinations of the technical features, they should be considered as the scope of the description.

The above examples illustrate only a few embodiments of the invention, which are described in detail and are not to be construed as limiting the scope of the invention. It should be noted that it will be apparent to those skilled in the art that several variations and modifications can be made without departing from the spirit of the invention, which are all within the scope of the invention. Accordingly, the scope of protection of the present invention is to be determined by the appended claims.

Claims

1. A method for producing ammonia by electrocatalytic nitrate radical reduction, comprising the steps of:

Preparing an electrolyte with pH of 13.5-14 by taking a conductive material loaded with copper oxide nano-sheets as a working electrode and inorganic alkali as a raw material, wherein the load of the copper oxide nano-sheets in the conductive material is selected from 0.05mg/cm ²、0.1mg/cm²、0.15mg/cm²、0.2mg/cm²、0.25mg/cm² or 0.3mg/cm ²;

carrying out electrolysis under constant potential to reduce the copper oxide nano-sheet, wherein the constant potential is-0.4V vs. RHE to-0.6V vs. RHE; and

Regulating the concentration of nitrate radical in the electrolyte to be 100-1000 mmol/L, and electrocatalytic reduction of the nitrate radical to prepare ammonia;

The preparation method of the copper oxide nano sheet comprises the following steps:

Mixing and stirring a copper chloride solution and a sodium hydroxide solution to form a solution containing tetrahydroxy copper complex ions, and carrying out hydrothermal reaction; the molar concentration of the copper chloride solution is 0.04 mol/L-0.08 mol/L, the molar concentration of the sodium hydroxide solution is 2.5 mol/L-3.5 mol/L, the volume ratio of the copper chloride solution to the sodium hydroxide solution is 1:1, the temperature of the hydrothermal reaction is 100 ℃, and the time is 6 hours.

2. The method for producing ammonia by electrocatalytic nitrate radical reduction according to claim 1, wherein the molar concentration of the copper chloride solution is 0.05mol/L and the molar concentration of the sodium hydroxide solution is 3mol/L.

3. The method for producing ammonia by electrocatalytic nitrate radical reduction according to claim 1, wherein the stirring speed is 800-1200 rpm, and the stirring time is 0.5-1.5 h.

4. The method for producing ammonia by electrocatalytic nitrate radical reduction according to claim 1, further comprising a step of purification after the hydrothermal reaction; the specific steps of the purification are as follows: and (3) sequentially carrying out centrifugal washing on the product obtained by the hydrothermal reaction for multiple times by using water and absolute ethyl alcohol, and drying.

5. The method for producing ammonia by electrocatalytic nitrate radical reduction according to any one of claims 1-4, wherein the conductive material is carbon paper.

6. The method for producing ammonia by electrocatalytic nitrate radical reduction according to any one of claims 1 to 4, wherein the electrolytic cell used for electrolysis is an H-type electrolytic cell, and the H-type electrolytic cell comprises the working electrode, a counter electrode and a reference electrode.

7. The method for producing ammonia by electrocatalytic nitrate radical reduction according to claim 6, wherein the counter electrode is a carbon rod and the reference electrode is a mercury|mercury oxide electrode.