CN115196665A

CN115196665A - Copper oxide nanosheet and preparation method thereof, and method for preparing ammonia by electrocatalysis of nitrate radical

Info

Publication number: CN115196665A
Application number: CN202210747594.7A
Authority: CN
Inventors: 邹雨芹; 王双印; 潘玉平
Original assignee: Hunan University
Current assignee: Hunan University
Priority date: 2022-06-29
Filing date: 2022-06-29
Publication date: 2022-10-18

Abstract

The invention relates to the technical field of catalysts, and particularly relates to a copper oxide nanosheet, a preparation method thereof, and a method for preparing ammonia by electrocatalysis of nitrate reduction. The preparation method of the copper oxide nanosheet 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 0.02-0.10 mol/L, and the molar concentration of the sodium hydroxide solution is 2-4 mol/L. The copper oxide nanosheet prepared by the preparation method has excellent conductivity and can improve the ammonia production rate and the ammonia production efficiency.

Description

Copper oxide nanosheet, preparation method thereof and method for preparing ammonia by electrocatalysis of nitrate radical

Technical Field

The invention relates to the technical field of catalysts, and particularly relates to a copper oxide nanosheet, a preparation method thereof, and a method for preparing ammonia by electrocatalysis of nitrate reduction.

Background

CuO nanosheet is a copper-based catalyst containing rich (100) crystal faces, and has been widely applied to the fields of carbon dioxide reduction, hydrogenation of aldehyde organic matters, catalytic hydrogenation or dehydrogenation reaction of nitrate nitrogen reducing ammonia and other various compounds and the like. At present, various capping agents (such as polyvinylpyrrolidone) are often required to be added for preparing 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. And the process of the synthesis method is relatively complex, and the production period is long.

The nitrate radical is electrochemically reduced, so that the content of the nitrate radical in the wastewater can be effectively reduced, the environment is prevented from being damaged more, and a high-value-added chemical, namely ammonia, with wide application can be obtained. However, at present, the catalyst commonly used for electrochemical reduction of nitrate is transition metal oxide, and the electrolyte is usually a neutral salt solution, and the added substrate nitrate content is low. However, transition metal catalysts generally have low faradaic efficiency, low selectivity to ammonia, and low ammonia production rates. In addition, the neutral electrolyte contains a small amount of nitrate, so that the pH value of the solution cannot be changed in the reduction process, the quantitative determination of ammonia has a large error at present, and when the content of nitrate (namely the yield of ammonia) is small, the relative error is larger. If the amount of nitrate is increased, a large amount of OH is accompanied in the reduction process ^- The formation of (a) results in a significant change in pH before and after electrolysis, which can result in a significant shift in the actual electrolysis potential during electrolysis. Therefore, the system is difficult to scale up.

Disclosure of Invention

Based on the above, it is necessary to provide a copper oxide nanosheet capable of improving conductivity and improving the ammonia production rate and ammonia production efficiency, a preparation method thereof, and a method for producing ammonia by electrocatalysis of nitrate reduction.

In one aspect of the invention, a preparation method of copper oxide nanosheets is provided, which includes 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 0.02-0.10 mol/L, and the molar concentration of the sodium hydroxide solution is 2-4 mol/L.

In one aspect of the invention, the copper oxide nanosheet prepared by the preparation method of the copper oxide nanosheet is also provided.

In another aspect of the present invention, there is further provided a method for producing ammonia by electrocatalysis nitrate reduction, wherein the method uses the copper oxide nanosheet as the catalyst, and comprises the following steps:

taking the conductive material loaded with the copper oxide nanosheets as a working electrode, and taking inorganic base as a raw material to prepare an electrolyte with pH of 13.5-14;

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

regulating the concentration of nitrate radical in the electrolyte to 100-1000 mmol/L, and electrocatalysis is carried out to reduce the nitrate radical to prepare ammonia.

According to the preparation method of the copper oxide nanosheet, the copper oxide (CuO) nanosheet with a clean surface and a good shape can be obtained without any capping agent under the guiding action of chloride ions by reasonably regulating and controlling the molar concentrations of the copper chloride solution and the sodium hydroxide solution. The good conductivity of the copper oxide nano-sheet is ensured, the active site of the copper oxide nano-sheet is fully exposed, and the whole synthesis process is simple and has short production period.

Correspondingly, in the process of preparing ammonia by electrocatalysis nitrate radical reduction by taking the copper oxide nanosheet as the catalyst, a large number of protrusions can be formed on the surface of the copper oxide nanosheet through the reduction process under a fixed potential, enrichment and reduction of nitrate radicals are facilitated, the number and activity of reaction sites of the catalyst are increased, and high selectivity to ammonia can be maintained while high Faraday efficiency and high ammonia yield are achieved. In addition, the use of alkaline electrolyte with pH close to 14 can effectively avoid OH in the electrolytic process ^- The influence on pH can maintain the stability of electrolytic potential for a long time. Moreover, the concentration of nitrate radical is regulated and controlled at a higher value, thereby effectively avoiding the generation of ammonia on the basis of ensuring the ammonia productionThe influence of trace ammonia in the environment improves the accuracy of detection and is beneficial to industrial application.

Drawings

Fig. 1 is an XRD pattern of CuO nanoplates prepared in example 1;

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

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

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

fig. 5 is an SEM image of CuO nanoplates produced in example 4;

FIG. 6 is a linear sweep voltammogram for the electrocatalytic production of ammonia in example 8;

FIG. 7 is a graph of the electrocatalytic ammonia production rate at different potentials in example 8;

FIG. 8 shows the Faraday efficiencies for electrocatalytic ammonia production at different potentials in example 8;

FIG. 9 is a standard curve for nitrate content in electrolyte before and after electrolysis in example 8;

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

fig. 11 is a graph of ammonia production rates for different loading of CuO nanoplates in example 9;

FIG. 12 is a graph of ammonia production process efficiencies for different loadings of CuO nanosheets in example 9;

FIG. 13 is a linear sweep voltammogram of electrocatalytic ammonia production in example 10;

FIG. 14 is a graph of the ammonia production rates of CuO nanoplates produced at different reaction times in example 11;

FIG. 15 is a linear sweep voltammogram of electrocatalytic ammonia production in comparative example 2;

FIG. 16 is a graph showing the electrocatalytic ammonia production rate in comparative example 2.

Detailed Description

In order that the invention may be more fully understood, reference will now be made to the following more detailed description. 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.

"electrocatalysis" refers to a catalytic action that accelerates the charge transfer at the electrode-electrolyte interface.

The first purpose of the present invention is to provide a method for preparing copper oxide nanosheets, which 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; wherein, the molar concentration of the copper chloride solution is 0.02-0.10 mol/L, and the molar concentration of the sodium hydroxide solution is 2-4 mol/L.

According to the preparation method of the copper oxide nanosheet, the copper oxide (CuO) nanosheet with a clean surface and a good shape can be obtained without any capping reagent under the guiding action of chloride ions by reasonably regulating and controlling the molar concentrations of the copper chloride and the sodium hydroxide solution. The good conductivity of the copper oxide nano-sheet is ensured, the active site of the copper oxide nano-sheet is fully exposed, and the whole synthesis process is simple and has short production period.

In some embodiments, the molar concentration of the cupric chloride solution can be any value 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, 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 ranges, it is possible to ensure that a dark blue complex solution containing tetrahydroxy copper complex ions is formed, rather than the copper hydroxide precipitate. Therefore, the reactants are mixed more uniformly, and the size of the CuO nano-sheet generated in situ in the hydrothermal process is more uniform.

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

In some embodiments, the stirring speed 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 6h to 16h. 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 method comprises the following specific steps: and (3) carrying out centrifugal washing on the product obtained after heating for multiple times by using water and absolute ethyl alcohol in sequence, and drying. For example, the washing may be performed by centrifugation with water twice and then with anhydrous 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 drying mode is not limited, for example, the drying can be carried out in a vacuum oven, and the drying temperature can be 30 ℃ to 80 ℃, preferably 60 ℃; the drying time can be 8-12 h.

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

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

A second object of the present invention is to provide a copper oxide nanosheet produced by the above-described method for producing a copper oxide nanosheet.

A third object of the present invention is to provide a method for producing ammonia by electrocatalysis nitrate reduction, which uses the copper oxide nanosheet as a catalyst, comprising the steps of:

preparing electrolyte with pH of 13.5-14 by taking a conductive material loaded with copper oxide nanosheets as a working electrode and inorganic base as a raw material;

electrocatalysis is carried out at a fixed potential to reduce the copper oxide nanosheet; and

regulating the concentration of nitrate radical in electrolyte to 100-1000 mmol/L and electrocatalysis of nitrate radical to reduce to prepare ammonia.

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

In some embodiments, the copper oxide nanosheets are coated on the conductive material in the form of a slurry, and the method of preparing the copper oxide nanosheet slurry may be as follows:

mixing the copper oxide nanosheet with a solution formed by water and alcohol, performing ultrasonic treatment until the copper oxide nanosheet is completely dispersed, and adding a Nafion film solution.

The Nafion membrane solution is 5% by mass, and the alcohol may be isopropanol.

In some embodiments, the conductive material may be carbon paper. Preferably, before loading the copper oxide nanosheets, a step of calcining the carbon paper is further included; the temperature of calcination may be 400 ℃, the time may be 16h, and the temperature rise rate may be 5 ℃/min. Still more preferably, the carbon paper is a hydrophilic type PN030 carbon paper.

In some embodiments, the method of preparing the 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 the potassium hydroxide is greater than or equal to 85%.

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

In some embodiments, the electrolytic cell used for electrolysis is an H-type electrolytic 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 | oxide mercury electrode.

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

Example 1 preparation of CuO nanoplates

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

2) Preparing 3mol/L sodium hydroxide aqueous solution: weighing sodium hydroxide solid, adding the sodium hydroxide solid into a small amount of deionized water, performing ultrasonic treatment at 20 ℃ until the sodium hydroxide solid is completely dissolved, cooling to room temperature, adding deionized water to prepare a sodium hydroxide aqueous solution with the concentration of 3mol/L, shaking up, 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 CuO nanosheet crude product: transferring the complex solution prepared in the step 3) into a reaction kettle, and carrying out hydrothermal reaction for 6h at 100 ℃.

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

Example 2 preparation of CuO nanoplates

1) Preparing 0.05mol/L copper chloride aqueous solution: weighing copper chloride solid, adding into a small amount of deionized water, performing ultrasonic treatment at 25 deg.C to completely dissolve, adding deionized water to obtain 0.05mol/L copper chloride aqueous solution, shaking, and standing for 10 min;

2) Preparing 3mol/L sodium hydroxide aqueous solution: weighing sodium hydroxide solid, adding into a small amount of deionized water, performing ultrasonic treatment at 25 deg.C to completely dissolve, cooling to room temperature, adding deionized water to prepare 3mol/L sodium hydroxide aqueous solution, shaking, and standing for 10 min;

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 CuO nanosheet crude product: transferring the complex solution prepared in the step 3) into a reaction kettle, and carrying out hydrothermal reaction for 8 hours at 100 ℃.

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

Example 3 preparation of CuO nanoplates

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

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

Example 4 preparation of CuO nanoplates

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

2) Preparing 3mol/L sodium hydroxide aqueous solution: weighing sodium hydroxide solid, adding into a small amount of deionized water, performing ultrasonic treatment at 30 deg.C to completely dissolve, cooling to room temperature, adding deionized water to prepare 3mol/L sodium hydroxide aqueous solution, shaking, and standing for 10 min;

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

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

Example 5 preparation of CuO nanoplates

This example is prepared substantially identically to example 1, except that: the hydrothermal reaction time in the step 4) is 14h.

Example 6 preparation of CuO nanoplates

This example is prepared substantially identically to example 1, except that: the hydrothermal reaction time in the step 4) is 16h.

Example 7 preparation of CuO nanoplates

This example is substantially the same as 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 nanosheet

This comparative example was prepared substantially the same as 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 electrocatalysis of nitrate reduction to Ammonia

1) Preparing slurry: weighing 5mg of the CuO nanosheet prepared in the embodiment 1, adding the CuO nanosheet into a mixed solution formed by 500 muL of deionized water and 450 muL of isopropanol, performing ultrasonic treatment at 25 ℃ until the CuO nanosheet is completely and uniformly dispersed, adding 50 muL of a Nafion membrane solution with the mass fraction of 5%, and performing continuous ultrasonic treatment until the CuO nanosheet is uniformly dispersed to prepare 1mL of slurry;

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

3) Preparing electrolyte: adding potassium hydroxide solid into a small amount of deionized water, performing ultrasonic treatment at 25 deg.C to completely dissolve, cooling to room temperature, adding deionized water to obtain 1mol/L potassium hydroxide aqueous solution (pH 14), shaking, and standing for 10 min;

4) And (3) forming an electrolysis system together with the electrolyte by using carbon paper as a working electrode, a carbon rod as a counter electrode and a mercury | mercury oxide electrode as a reference electrode, and carrying out constant potential electrolysis to reduce the CuO nanosheets. Nitrate is then added to the electrolyte and electrocatalysis is carried out to reduce nitrate to produce ammonia.

And (3) detecting the ammonia production effect:

1) Linear Sweep Voltammetry (LSV) test: 50mL of the aqueous potassium hydroxide solution prepared in step 3) was charged into a 100mL single-port electrolytic cell. And 3) taking the carbon paper in the step 2) as a working electrode, taking a carbon rod as a counter electrode, and taking a mercury | mercury oxide electrode as a reference electrode. Firstly, using Shanghai Chenhua CHI660 electrochemical workstation to pre-reduce for 0.5h under a fixed potential of-0.6V vs. Then adding a certain mass of potassium nitrate into the electrolytic cell to make the nitrate concentration be 100mmol/L, and carrying out a plurality of linear sweep 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, taking the carbon paper prepared in the step 2) as a working electrode, a carbon rod as a counter electrode, and a mercury | mercury oxide electrode as a reference electrode. Firstly, using Shanghai Chenghua CHI660 electrochemical workstation to pre-reduce for 0.5h under the fixed potential-0.6V vs. Then adding a certain mass of potassium nitrate into the cathode chamber to ensure that the nitrate concentration is 100mmol/L, and carrying out electrolysis for 1h at a potential selected every-0.1V vs. RHE within the potential range of 0-0.6V vs. RHE. The ammonia production rate and the ammonia production faradaic 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 high performance liquid chromatography, and a detection standard curve is shown in fig. 9; ammonium chloride was used as a standard sample, and the ammonia content in the electrolyte before and after electrolysis was measured by ultraviolet absorption spectrometry, and the detection standard curve is shown in fig. 10.

As shown in FIG. 7, the ammonia production rates at 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 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 。

As shown in FIG. 8, the efficiencies of ammonia generation at 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 24.93%, 32.03%, 54.88%, 77.89%, 94.45%, 93.43%, and 95.09%, respectively.

As shown in FIG. 9, the equation for the standard curve is y =188213x +206422, and the correlation coefficient R ² Is 0.99963. The high performance liquid chromatography test has good linearity under the nitrate concentration of 0 to 100mmol/L, and the test result is reliable.

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

EXAMPLE 9 electrocatalysis of nitrate reduction to ammonia

1) Preparing slurry: weighing 5mg of the CuO nanosheet prepared in example 1, adding the CuO nanosheet into a mixed solution formed by 500 muL of deionized water and 450 muL of isopropanol, performing ultrasonic treatment at 25 ℃ until the CuO nanosheet is completely and uniformly dispersed, adding 50 muL of a Nafion membrane solution with the mass fraction of 5%, and performing continuous ultrasonic treatment until the CuO nanosheet is uniformly dispersed, so as to prepare 1mL of slurry;

2) Preparing carbon paper: cutting the 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 serous fluid prepared in the step 1) to drip-coat on the 6 pieces of carbon paper, and airing;

3) Preparing electrolyte: adding potassium hydroxide solid into a small amount of deionized water, performing ultrasonic treatment at 25 deg.C to completely dissolve, cooling to room temperature, adding deionized water to prepare 1mol/L potassium hydroxide aqueous solution (pH 14), shaking, and standing for 10 min;

4) And (3) forming an electrolytic cell together with electrolyte by using carbon paper as a working electrode, a carbon rod as a counter electrode and a mercury | mercury oxide electrode as a reference electrode, and carrying out electrocatalysis to reduce the CuO nanosheet. Nitrate is then added to the electrolyte and electrocatalysis is carried out to reduce nitrate to produce ammonia.

And (3) detecting the ammonia production effect:

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

The ammonia generation efficiency of the carbon paper loaded with CuO nanosheets of different contents is shown in fig. 12. As can be seen from FIG. 12, the content of the supported CuO nanosheet was 0.1mg cm ² 、0.2mg cm ² 、0.3mg cm ² 、0.4mg cm ² 、0.5mg cm ² And 0.6mg cm ² The corresponding ammonia-producing faradaic efficiencies were 95.76%, 94.45%, 95.52%, 96.81%, 96.70% and 95.87%, respectively.

EXAMPLE 10 electrocatalysis of nitrate reduction to ammonia

2) Preparing carbon paper: cutting the carbon paper into 6 same carbon papers of 1cm multiplied by 2cm, respectively taking 20 microliter of the serous fluid prepared in the step 1) to drip and coat on the 6 same carbon papers, and airing;

And (3) detecting the ammonia production effect:

1) Linear Sweep Voltammetry (LSV) test: 50mL of the aqueous potassium hydroxide solution prepared in step 3) was charged into a 100mL single-port electrolytic cell. And 3) taking the carbon paper in the step 2) as a working electrode, a carbon rod as a counter electrode and a mercury | mercury oxide electrode as a reference electrode. Firstly, using Shanghai Chenghua CHI660 electrochemical workstation to pre-reduce for 0.5h under the fixed potential-0.6V vs. Then adding certain mass of potassium nitrate into the electrolytic cell to enable the nitrate concentration to be 100mmol/L, 150mmol/L, 200mmol/L, 250mmol/L, 500mmol/L and 1000mmol/L respectively, and carrying out a plurality of linear sweep 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 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-shaped electrolytic cell, and respectively carrying out electrolysis by using the 6 pieces of carbon paper prepared in the step 2) as a working electrode, a carbon rod as a counter electrode and a mercury | mercury oxide electrode as a reference electrode. Before each electrolysis, the electrochemical workstation of CHI660 in Shanghai province is used for pre-reduction for 0.5h under the fixed potential of-0.6V vs. Then, potassium nitrate was added to the cathode chamber in such a mass that the nitrate concentrations were 100mmol/L, 150mmol/L, 200mmol/L, 250mmol/L, 500mmol/L and 1000mmol/L, respectively, and electrolysis was carried out at-0.5V vs. RHE potential for 1 hour. In addition, the nitrate content in the electrolyte before and after electrolysis was measured by high performance liquid chromatography, and the ammonia content in the electrolyte before and after electrolysis was measured by ultraviolet absorption spectrometry.

EXAMPLE 11 electro-catalytic nitrate reduction to Ammonia

1) Preparing slurry: respectively weighing 5mg of CuO nanosheets prepared in the

embodiments

1, 2, 3, 4, 5 and 6, adding the CuO nanosheets into a mixed solution formed by 500 muL of deionized water and 450 muL of isopropanol, performing ultrasonic treatment at 25 ℃ until the CuO nanosheets are completely and uniformly dispersed, adding 50 muL of Nafion membrane solution with the mass fraction of 5%, and performing continuous ultrasonic treatment until the CuO nanosheets are uniformly dispersed to prepare 6 kinds of slurry of 1 mL;

2) Preparing carbon paper: cutting the 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 drip-coat the 6 pieces of carbon paper, and airing;

And (3) detecting the ammonia production effect:

1) Linear Sweep Voltammetry (LSV) test: 50mL of the aqueous potassium hydroxide solution prepared in step 3) was charged into a 100mL single-port electrolytic cell. And 3) taking the carbon paper in the step 2) as a working electrode, taking a carbon rod as a counter electrode, and taking a mercury | mercury oxide electrode as a reference electrode. Firstly, using Shanghai Chenhua CHI660 electrochemical workstation to pre-reduce for 0.5h under a fixed potential of-0.6V vs. And then adding a certain mass of potassium nitrate into the electrolytic cell to enable the nitrate concentration to be 1000mmol/L, and carrying out a plurality of linear sweep voltammetry curve tests within the potential range of 0.2-0.6V vs.

2) Constant potential electrolysisAnd (3) testing: 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-shaped electrolytic cell, and respectively carrying out electrolysis by using the 6 pieces of carbon paper prepared in the step 2) as a working electrode, a carbon rod as a counter electrode and a mercury | mercury oxide electrode as a reference electrode. Before each electrolysis, the electrochemical workstation of Shanghai Chenhua CHI660 is used for pre-reduction for 0.5h under the fixed potential of-0.6V vs. Then, potassium nitrate with a certain mass was added to the cathode chamber so that the nitrate concentrations were 1000mmol/L, respectively, and electrolysis was carried out for 1 hour at a potential of-0.5V vs. In addition, the nitrate content in the electrolyte before and after electrolysis was measured by high performance liquid chromatography, and the ammonia content in the electrolyte before and after electrolysis was measured by ultraviolet absorption spectroscopy. The ammonia production rates of CuO nanoplates made in the different examples are shown in fig. 14. As can be seen from FIG. 14, the ammonia production rates corresponding to the CuO nanosheets (6 h, 8h, 10h, 12h, 14h and 16h for the reaction times) prepared in examples 1 to 6 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 。

Comparative example 2

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

2) Preparing carbon paper: cutting the carbon paper into 1cm multiplied by 2cm, taking 20 microliter of the serous fluid prepared in the step 1) to drip-coat the carbon paper, and airing;

And (3) detecting the ammonia production effect:

1) Linear Sweep Voltammetry (LSV) test: 50mL of the aqueous potassium hydroxide solution prepared in step 3) was charged into a 100mL single-port electrolytic cell. And 3) taking the carbon paper in the step 2) as a working electrode, taking a carbon rod as a counter electrode, and taking a mercury | mercury oxide electrode as a reference electrode. Firstly, using Shanghai Chenghua CHI660 electrochemical workstation to pre-reduce for 0.5h under the fixed potential-0.6V vs. Then adding a certain mass of potassium nitrate into the electrolytic cell to ensure that the nitrate concentration is 1000mmol/L, and carrying out a plurality of linear sweep 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 results are 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-shaped electrolytic cell, taking the carbon paper prepared in the step 2) as a working electrode, taking a carbon rod as a counter electrode, and taking a mercury | mercury oxide electrode as a reference electrode. Firstly, using Shanghai Chenhua CHI660 electrochemical workstation to pre-reduce for 0.5h under a fixed potential of-0.6V vs. Then, potassium nitrate with a certain mass was added to the cathode chamber so that the nitrate concentration was 1000mmol/L, and 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 The corresponding faraday efficiency is 45.79%. In addition, the nitrate content in the electrolyte before and after electrolysis was measured by high performance liquid chromatography, and the ammonia content in the electrolyte before and after electrolysis was measured by ultraviolet absorption spectrometry.

Comparative example 3

This comparative example is essentially the same as the ammonia process of example 8, except that: using the CuO nanosheet prepared in comparative example 1 as a catalyst, the ammonia production rate was determined to be 1050mmol g ^-1 _cat h ^-1 The first efficiency of ammonia production process75.4％。

All possible combinations of the technical features of the above embodiments may not be described for the sake of brevity, but should be considered as within the scope of the present disclosure as long as there is no contradiction between the combinations of the technical features.

The above-mentioned embodiments only express several embodiments of the present invention, and the description thereof is more specific and detailed, but not construed as limiting the scope of the invention. It should be noted that various changes and modifications can be made by those skilled in the art without departing from the spirit of the invention, and these changes and modifications are all within the scope of the invention. Therefore, the protection scope of the present patent shall be subject to the appended claims.

Claims

1. A preparation method of copper oxide nanosheets is characterized by comprising the following steps:

2. A method for producing copper oxide nanoplates as claimed in claim 1, characterized in that the molar concentration of the copper chloride solution is 0.04 to 0.08mol/L and the molar concentration of the sodium hydroxide solution is 2.5 to 3.5mol/L.

3. A method for producing copper oxide nanosheets according to claim 1, wherein the stirring speed is from 800rpm to 1200rpm for from 0.5h to 1.5h.

4. A method for producing copper oxide nanoplates as in any one of claims 1 to 3, wherein the temperature of the hydrothermal reaction is 80 to 120 ℃ for 6 to 16 hours.

5. A method for producing copper oxide nanosheets according to any one of claims 1 to 3, further comprising a step of purification after the hydrothermal reaction; the purification method comprises the following specific steps: and (3) carrying out centrifugal washing on the product obtained by the hydrothermal reaction sequentially by using water and absolute ethyl alcohol for multiple times, and drying.

6. Copper oxide nanosheets produced by the method for producing copper oxide nanosheets according to any one of claims 1 to 5.

7. A method for producing ammonia by electrocatalysis nitrate reduction, which is characterized in that the copper oxide nanosheet of claim 6 is used as a catalyst, and the method comprises the following steps:

preparing electrolyte with pH of 13.5-14 by taking the conductive material loaded with the copper oxide nanosheets as a working electrode and inorganic base as a raw material;

regulating the concentration of nitrate radical in the electrolyte to 100-1000 mmol/L, and electrocatalysis is carried out on the nitrate radical to reduce to prepare ammonia.

8. The method of claim 7, wherein the electrically conductive material is carbon paper.

9. The method of claim 7, wherein the fixed potential is 0V _RHE ～-0.6V _RHE Any one of them.

10. The method of any one of claims 7 to 9, wherein the electrolysis cell is an H-type cell comprising the working electrode, a counter electrode and a reference electrode; preferably, the counter electrode is a carbon rod and the reference electrode is a mercury | oxide mercury electrode.