阅读说明：本技术 一种多孔Fe/C复合催化剂及其通过碱金属盐限域的制备方法与应用 (Porous Fe/C composite catalyst and preparation method and application thereof through alkali metal salt confinement ) 是由 丁良鑫 罗荻 郭浩 滕浩 于 2020-12-24 设计创作，主要内容包括：本发明公开了一种多孔Fe/C复合催化剂及其通过碱金属盐限域的制备方法与应用。该方法包括：将铁源和对苯二甲酸溶解在N,N-二甲基甲酰胺中,升温进行溶剂热反应,冷却至室温并进行离心洗涤,干燥,得到铁基MOF粉体,将得到的铁基MOF粉体分散于碱金属溶液中,冷冻干燥后,在保护性气体氩气气氛下进行热处理,最后在洗去碱金属盐后,再进行酸洗,离心收集沉淀,干燥,得到具有多孔结构的Fe/C复合材料。该复合材料在电催化氮还原(NRR)领域表现出优异的催化活性,在0.1M Li-2SO-4电解液中,-0.5V(相对于标准氢电极)下取得最大产氨产率1.25μg h～-～1mg-(cat)～(-1),法拉第效率为0.59％。(The invention discloses a porous Fe/C composite catalyst, a preparation method and application thereof through an alkali metal salt confinement. The method comprises the following steps: dissolving an iron source and terephthalic acid in N, N-dimethylformamide, heating to perform a solvothermal reaction, cooling to room temperature, performing centrifugal washing, drying to obtain iron-based MOF powder, dispersing the obtained iron-based MOF powder in an alkali metal solution, performing freeze drying, performing heat treatment under the atmosphere of protective gas argon, finally washing with acid after alkali metal salt is removed, centrifugally collecting precipitate, and drying to obtain the Fe/C composite material with a porous structure. The composite material shows excellent catalytic activity in the field of electrocatalytic nitrogen reduction (NRR), and has high catalytic activity in the range of 0.1M Li 2 SO 4 In the electrolyte, the maximum value is obtained under-0.5V (relative to a standard hydrogen electrode)The yield of ammonia is 1.25 mu g h ‑ 1 mg cat ‑1 The Faraday efficiency was 0.59%.)

1. A method for preparing a porous Fe/C composite catalyst through alkali metal salt confinement, which is characterized by comprising the following steps:

(1) dissolving an iron-containing reagent and terephthalic acid in N, N-dimethylformamide, uniformly mixing, heating to perform solvothermal reaction, cooling to room temperature, centrifuging to obtain a precipitate, washing, and drying to obtain iron-based MOF powder;

(2) dispersing the iron-based MOF powder obtained in the step (1) in an alkali metal solution to obtain a mixed solution, freezing and drying, heating in a protective atmosphere, and performing heat treatment to obtain mixed powder of Fe/C and an alkali metal salt;

(3) and (3) washing the mixed powder of Fe/C and alkali metal salt in the step (2), washing off the alkali metal salt to obtain Fe/C powder, soaking the Fe/C powder in an acid solution, carrying out acid washing treatment, centrifuging to obtain a precipitate, washing, and drying to obtain the porous Fe/C composite catalyst.

2. The method for preparing the porous Fe/C composite catalyst through alkali metal salt confinement according to claim 1, wherein the iron-containing reagent in the step (1) is more than one of ferric chloride hexahydrate, ferric nitrate nonahydrate and ferric sulfate.

3. The method for preparing a porous Fe/C composite catalyst through alkali metal salt confinement according to claim 1, wherein the molar ratio of the iron-containing reagent to terephthalic acid in step (1) is 1-2: 1-1.2.

4. The method for preparing a porous Fe/C composite catalyst through alkali metal salt confinement as claimed in claim 1, wherein the temperature of the solvothermal reaction in step (1) is 100-150 ℃, and the time of the solvothermal reaction is 1-24 h.

5. The method for preparing a porous Fe/C composite catalyst through alkali metal salt confinement according to claim 1, wherein the alkali metal solution in the step (2) is one or more of a saturated sodium chloride solution and a saturated potassium chloride solution; the mass-volume ratio of the iron-based MOF powder to the alkali metal solution is 1-10: 1 mg/mL.

6. The method for preparing a porous Fe/C composite catalyst through alkali metal salt confinement according to claim 1, wherein the freeze-drying of the step (2) comprises: the mixed solution is frozen in a liquid nitrogen environment for 3-15min and then is frozen for 10-24h at the temperature of-48 to-52 ℃.

7. The method for preparing a porous Fe/C composite catalyst through alkali metal salt confinement according to claim 1, wherein the protective atmosphere in the step (2) is an argon atmosphere or a nitrogen atmosphere; the temperature of the heat treatment is 200-800 ℃, the time of the heat treatment is 1-120min, and the rate of the temperature rise is 2-10 ℃/min.

8. The method for preparing a porous Fe/C composite catalyst through alkali metal salt confinement according to claim 1, wherein the acidic solution in the step (3) is one of sulfuric acid solution and hydrochloric acid solution; the concentration of the acid solution is 0.01-3mol/L, and the time of acid washing treatment is 0.25-12 h.

9. A porous Fe/C composite catalyst prepared by the preparation method according to any one of claims 1 to 8.

10. Use of the porous Fe/C composite catalyst according to claim 9 for electrocatalytic nitrogen reduction.

Technical Field

The invention relates to the technical field of nitrogen reduction, in particular to a porous Fe/C composite catalyst, and a preparation method and application thereof through alkali metal salt confinement.

Background

Ammonia as a nitrogen fertilizer makes a great contribution to maintaining the population growth in the world, and plays a very important role in national defense, medicine and industry. Because ammonia is readily liquefied at light pressures and low temperatures, it is easy to store, transport, and use, and can also be transported to a power plant to produce carbon-free electricity. Therefore, in the future, demand for ammonia will increase. The industrial synthetic ammonia is continuously used at presentThe law has triggered a green revolution in modern agriculture, but centralized ammonia production is not suitable for the distributed features of agriculture. Moreover, although the haber process is highly efficient, its harsh reaction conditions of high temperature and pressure consume about 2% of the world's energy, producing about 1% of the world's CO₂And (5) discharging. Although the ammonia industry has realized the co-production with new industrial technologies such as coal chemical industry, hydrogen production technology and the like, the current situation of high energy consumption is not changed. Therefore, people are continuously striving for mild production conditions to synthesize ammonia, thereby reducing energy consumption and protecting the environment.

The introduction of electrical energy into ammonia synthesis processes has been one of the areas of research that has received much attention. The electrochemical synthesis of ammonia is favorable for creating a nitrogen fixation environment which is similar to the nitrogen fixation enzyme and is synthesized by synergistic activation and reduction, and provides more than the prior artThe synthesis of ammonia is a cheaper ammonia. In the future, the price of raw materials for synthesizing ammonia such as petroleum, coal and the like is greatly increased due to energy crisisLong synthesis ammonia cost, electricityThe chemical synthesis of ammonia will not be lost as a beneficial choice, so the research of the electrochemical synthesis of ammonia has huge potential application prospect. However, the slow reaction kinetics and the ultra-low selectivity seriously hinder the development of the catalyst, so that the research on the nitrogen reduction catalyst with high selectivity and high stability has very important significance for further development of the synthetic ammonia industry.

According to a large number of literature reports, the transition metal is expected to become an ideal electrocatalyst for synthesizing ammonia by electrocatalytic nitrogen reduction due to the advantages of rich content, low price, no toxicity, easy control and the like. Iron, which is one of the main elements of nitrogenase, has attracted attention because of its excellent high activity that may be exhibited in synthesizing ammonia by electrocatalytic nitrogen reduction. However, at present, a heteroatom doping strategy is adopted for the iron element to participate in the preparation of the catalyst for synthesizing ammonia by electrochemical nitrogen reduction, the advantage of low cost of iron production is not well exerted, and meanwhile, the catalyst is still poor in nitrogen mass transfer. For example, in ACS Catal.2018,8,10, 9312-grade 9319, Fe/Fe is prepared by oxidizing Fe foil at 300 deg.C and then reducing in situ₃O₄Catalyst with maximum ammonia yield of 0.19 μ g h at-0.3V (relative to standard hydrogen electrode)^-1cm^-2The iron-based catalyst has nitrogen reduction performance, but the nitrogen reduction performance is still not good in mass transfer of nitrogen and exposed active sites, and needs to be further improved.

Disclosure of Invention

In order to overcome the defects in the prior art, the invention aims to provide a porous Fe/C composite catalyst, and a preparation method and application thereof by alkali metal salt confinement.

The invention provides a porous iron-based carbon material based on an alkali metal salt confinement effect and a preparation method thereof. According to the method, through the domain limiting effect of the alkali metal salt, the collapse of the structure of the iron-based MOF material is limited and the porous structure of the iron-based MOF material is kept while the agglomeration of the metal is prevented in the heat treatment process. The simple and rapid preparation method not only effectively retains the active site-iron site of nitrogen reduction, but also maintains the porous structure of the material to be more beneficial to the mass transfer of nitrogen, thereby providing a new guiding idea for the improvement of selectivity and yield of the electrocatalytic ammonia production.

The purpose of the invention is realized by at least one of the following technical solutions.

The invention provides a method for preparing a porous Fe/C composite catalyst through alkali metal salt confinement, which comprises the following steps:

(1) dissolving an iron-containing reagent and terephthalic acid in N, N-dimethylformamide, uniformly mixing to fully dissolve, heating in a reaction kettle to perform solvothermal reaction, cooling to room temperature, centrifuging to obtain a precipitate, washing, and drying to obtain iron-based MOF powder;

(2) dispersing the iron-based MOF powder obtained in the step (1) in an alkali metal solution to obtain a mixed solution, freezing and drying, heating in a protective atmosphere, and performing heat treatment to obtain mixed powder of Fe/C and an alkali metal salt;

(3) and (3) washing the mixed powder of Fe/C and alkali metal salt in the step (2), washing off the alkali metal salt to obtain Fe/C powder, soaking the Fe/C powder in an acid solution for acid washing, centrifuging to obtain a precipitate, washing, and drying to obtain the porous Fe/C composite catalyst.

Further, the iron-containing reagent in the step (1) is more than one of ferric chloride hexahydrate, ferric nitrate nonahydrate and ferric sulfate.

Preferably, the iron-containing reagent of step (1) is ferric chloride hexahydrate.

Further, the molar ratio of the iron-containing reagent to the terephthalic acid in the step (1) is 1-2: 1-1.2.

Preferably, the molar ratio of the iron-containing reagent to terephthalic acid in step (1) is 2: 1.1.

Further, the temperature of the solvothermal reaction in the step (1) is 100-150 ℃, and the time of the solvothermal reaction is 1-24 h.

Preferably, the temperature of the solvothermal reaction in the step (1) is 110-120 ℃, and the time of the solvothermal reaction is 2-5 h.

Further, the alkali metal solution in the step (2) is more than one of saturated sodium chloride solution and saturated potassium chloride solution; the mass-volume ratio of the iron-based MOF powder to the alkali metal solution is 1-10: 1 mg/mL.

Preferably, the alkali metal solution in the step (2) is a saturated sodium chloride solution, and the concentration of the saturated sodium chloride solution is more than or equal to 0.36 g/mL; the mass-volume ratio of the iron-based MOF powder to the alkali metal solution is 2-5: 1 mg/mL.

Further, the freeze-drying of step (2) comprises: freezing the mixed solution in a liquid nitrogen environment for 3-15min, and then transferring the mixed solution to a freeze dryer to freeze for 10-24h at the temperature of-48 to-52 ℃.

Preferably, the freeze-drying of step (2) comprises: freezing the mixed solution in a liquid nitrogen environment for 5-10min, and then transferring the mixed solution to a freeze dryer to freeze for 12-20h at the temperature of-48 to-52 ℃.

Further, the protective atmosphere in the step (2) is an argon atmosphere or a nitrogen atmosphere; the temperature of the heat treatment is 200-800 ℃, the time of the heat treatment is 1-120min, and the rate of the temperature rise is 2-10 ℃/min.

Preferably, the protective atmosphere in step (2) is an argon atmosphere.

Preferably, the temperature of the heat treatment in the step (2) is 300-600 ℃, the time of the heat treatment is 3-60min, and the rate of temperature rise is 5-10 ℃/min.

Further, the acid solution in the step (3) is one of a sulfuric acid solution and a hydrochloric acid solution; the concentration of the acid solution is 0.01-3mol/L, and the time of acid washing treatment is 0.25-12 h.

Preferably, the acidic solution in the step (3) is a hydrochloric acid solution, and the concentration of the acidic solution is 0.5-1 mol/L.

Preferably, the acid washing treatment time of the step (3) is 0.5h-2 h.

The invention provides a porous Fe/C composite catalyst prepared by the preparation method.

The porous Fe/C composite catalyst provided by the invention is applied to electrocatalysis nitrogen reduction.

The porous Fe/C composite catalyst provided by the invention can be tested by the following method. The test comprises the following steps: carrying out an electro-catalytic nitrogen reduction performance test on a German Gamry electrochemical workstation, and carrying out a test by using a three-electrode system, wherein hydrophilic carbon cloth coated with a porous Fe/C composite material is used as a working electrode, a platinum sheet is used as a counter electrode, and an Ag/AgCl electrode is used as a reference electrode; taking 0.1mol/L lithium sulfate solution as electrolyte; an H-shaped glass electrolytic tank is used as an electrolytic reaction device.

Compared with the prior art, the invention has the following advantages and beneficial effects:

(1) the preparation method provided by the invention limits the aggregation of metal centers in the pyrolysis process by adopting the strategy of coating the MOF material by using alkali metal salt, and effectively retains the porous structure of the MOF material. The porous structure can accelerate mass transfer of nitrogen, has a larger active area, and can enable more catalytic active sites in the porous structure to be exposed and effectively contact with the nitrogen. The preparation method is simple, and the used materials are cheap and easy to obtain.

(2) The porous Fe/C composite catalyst prepared by the invention shows excellent catalytic activity in the field of electrocatalytic nitrogen reduction (NRR), and has the catalytic activity in the range of 0.1M Li₂SO₄In the electrolyte, the maximum ammonia yield of 1.25 mu g h is obtained under-0.5V (relative to a standard hydrogen electrode)^-1mg_cat ^-1The Faraday efficiency was 0.59%.

Drawings

FIG. 1 is a Scanning Electron Microscope (SEM) photomicrograph of the dried material prepared in step two of example 1;

FIG. 2 is a high power Scanning Electron Microscope (SEM) picture of the porous Fe/C composite prepared in example 1;

FIG. 3 is an energy spectrum of the porous Fe/C composite prepared in example 1;

FIG. 4 is a high power Scanning Electron Microscope (SEM) picture of the porous Fe/C composite prepared in example 2;

FIG. 5 is a high power Scanning Electron Microscope (SEM) picture of the porous Fe/C composite prepared in example 3.

Detailed Description

The following examples are presented to further illustrate the practice of the invention, but the practice and protection of the invention is not limited thereto. It is noted that the processes described below, if not specifically described in detail, are all realizable or understandable by those skilled in the art with reference to the prior art. The reagents or apparatus used are not indicated to the manufacturer, and are considered to be conventional products available by commercial purchase.

Example 1

The first step is as follows: dissolving 0.675g of ferric trichloride hexahydrate and 0.206g of terephthalic acid into 30mL of N, N-dimethylformamide, ultrasonically mixing for 30min, transferring into a 50mL polytetrafluoroethylene-lined stainless steel autoclave, carrying out hydrothermal treatment for 5h in a 110 ℃ oven, naturally cooling to room temperature, washing with N, N-dimethylformamide and methanol for three times respectively, and drying in an 80 ℃ oven to obtain the iron-based MOF powder.

The second step is that: dispersing 50mg of iron-based MOF material in a saturated sodium chloride solution with the volume of 20mL, freezing the iron-based MOF material in liquid nitrogen for 10min, and then transferring the solution to a freeze dryer for drying for 20h, wherein the temperature of the freeze dryer is adjusted to-50 ℃ to obtain a dried material, and as shown in figure 1, the iron-based MOF material can be well coated by alkali metal salt particles, so that the domain limiting effect is achieved; placing the dried material in a tubular heating furnace, heating to 600 ℃ in an argon atmosphere, calcining for 30min, wherein the heating rate is 10 ℃/min, and the argon flow rate is 30mL/min, so as to obtain mixed powder of Fe/C and alkali metal salt;

the third step: and washing the mixed powder of the Fe/C and the alkali metal salt, washing off the alkali metal salt to obtain Fe/C powder, dispersing the Fe/C powder in 10mL of 1mol/L hydrochloric acid solution, stirring at the speed of 600 revolutions per minute for 30min, centrifuging to obtain a precipitate, washing with pure water for three times, and drying in an oven at 80 ℃ to obtain the porous Fe/C composite catalyst. As shown in FIG. 2, the prepared Fe/C composite material has a remarkable porous structure; the spectral results of fig. 3 show that the composite material has only C, O, Fe elements, and no other elements are present.

The fourth step: and carrying out NRR performance test on the obtained porous Fe/C composite material catalyst.

1. Weighing 2mg of porous Fe/C composite catalyst, adding into 200 mu L of absolute ethyl alcohol, simultaneously adding 20 mu L of Nafion solution, and carrying out ultrasonic treatment for 30min to obtain a uniform dispersion liquid. And (3) dripping 100 mu L of the dispersion liquid on dried treated hydrophilic carbon cloth, wherein the surface area of the hydrophilic carbon cloth is controlled to be 1cm multiplied by 1cm, and naturally airing.

2. A three-electrode system is adopted to carry out an electro-catalytic nitrogen reduction performance test on a German Gamry electrochemical workstation, hydrophilic carbon cloth coated with porous Fe/C composite materials is taken as a working electrode, a platinum sheet is taken as a counter electrode, Ag/AgCl is taken as a reference electrode, 0.1mol/L lithium sulfate solution is taken as electrolyte, and an H-shaped glass electrolytic tank is taken as a reaction device.

3. A hydrophilic carbon cloth coated with a porous Fe/C composite material is used as a working electrode, and a cyclic voltammetry test is carried out by scanning 50 circles at a speed of 5mV/s in a potential interval of 0 to-1.0V (relative to a standard hydrogen electrode) under a three-electrode system and an Ar atmosphere so as to activate the material.

4. After cyclic voltammetry, electrolysis is carried out for 2 hours under the atmosphere of Ar and the voltage of-0.3V (relative to a standard hydrogen electrode) for a plurality of times, and the electrolyte is detected and replaced to eliminate impurity interference until no ammonia is detected in the electrolyte.

5. And after impurity interference is eliminated, introducing nitrogen into the electrolyte for 30min, performing a nitrogen reduction test for 2h after the nitrogen is saturated, and setting the potential to be-0.3V, -0.4V, -0.5V, -0.6V and-0.7V (relative to a standard hydrogen electrode) respectively for 7200 s.

The fifth step: and (3) testing the yield of ammonia: respectively taking 2mL of electrolyte after running for 2 hours at each potential, adding the electrolyte into 2mL of 1mol/L sodium hydroxide solution (containing 5 wt% of salicylic acid and 5 wt% of sodium citrate dihydrate), uniformly mixing the electrolyte and standing for 30 s; then, 1mL of sodium hypochlorite solution with the concentration of 0.05mol/L is added into the mixture, and the mixture is fully mixed and stands for 1 min; finally, 200. mu.l of sodium nitrosoferricyanide dihydrate with a concentration of 1 wt.% were added. Standing for 2h at room temperature in a dark place, performing spectrum scanning within 400-800 nm by using an ultraviolet spectrum, recording an absorbance value at 650nm, and contrasting a working curve to finally obtain the concentration of ammonia.

Through the above tests, it was found that: preparation of porous Fe/C composite catalyst by alkali metal salt confinement (porous Fe/C composite catalyst prepared in example 1) applied to NRR excellence, maximum ammonia production rate of 1.25 mu is achieved at-0.5V (relative to standard hydrogen electrode)g h^-1mg_cat ^-1The Faraday efficiency was 0.59%.

Example 2

The first step is as follows: dissolving 0.3375g of ferric trichloride hexahydrate and 0.103g of terephthalic acid into 30mL of N, N-dimethylformamide, ultrasonically mixing for 30min, transferring into a 50mL stainless steel autoclave with a polytetrafluoroethylene lining, carrying out hydrothermal reaction for 24h in an oven at 100 ℃, naturally cooling to room temperature, washing with N, N-dimethylformamide and methanol for three times respectively, and drying in the oven at 80 ℃ to obtain the iron-based MOF powder.

The second step is that: dispersing 20mg of iron-based MOF material in a saturated sodium chloride solution with the volume of 20mL, freezing the iron-based MOF material in liquid nitrogen for 3min, then transferring the solution to a freeze dryer for drying for 10h, adjusting the temperature of the freeze dryer to-50 ℃, placing the dried material in a tubular heating furnace, heating to 400 ℃ in an argon atmosphere, calcining for 60min, wherein the heating rate is 5 ℃/min, and the argon flow rate is 30mL/min to obtain mixed powder of Fe/C and alkali metal salt;

the third step: and washing the mixed powder of the Fe/C and the alkali metal salt, washing off the alkali metal salt to obtain Fe/C powder, dispersing the Fe/C powder in 10mL of 2mol/L hydrochloric acid solution, stirring for 12 hours at the speed of 600 revolutions per minute, centrifuging to obtain a precipitate, washing with pure water for three times, and drying in an oven at the temperature of 80 ℃ to obtain the porous Fe/C composite catalyst. As shown in FIG. 4, the prepared porous Fe/C composite material has a distinct porous structure.

The fourth step: and carrying out NRR performance test on the obtained porous Fe/C composite material catalyst.

1. Weighing 2mg of porous Fe/C composite material, adding into 200 mu L of absolute ethyl alcohol, simultaneously adding 20 mu L of Nafion solution, and carrying out ultrasonic treatment for 30min to obtain uniform dispersion liquid. And (3) dripping 100 mu L of the dispersion liquid on dried treated hydrophilic carbon cloth, wherein the surface area of the hydrophilic carbon cloth is controlled to be 1cm multiplied by 1cm, and naturally airing.

2. A three-electrode system is adopted to carry out an electro-catalytic nitrogen reduction performance test on a German Gamry electrochemical workstation, hydrophilic carbon cloth coated with porous Fe/C composite materials is taken as a working electrode, a platinum sheet is taken as a counter electrode, Ag/AgCl is taken as a reference electrode, 0.1mol/L lithium sulfate solution is taken as electrolyte, and an H-shaped glass electrolytic tank is taken as a reaction device.

3. A hydrophilic carbon cloth coated with a porous Fe/C composite material is used as a working electrode, and a cyclic voltammetry test is carried out by scanning 50 circles at a speed of 5mV/s in a potential interval of 0 to-1.0V (relative to a standard hydrogen electrode) under a three-electrode system and an Ar atmosphere so as to activate the material.

4. After cyclic voltammetry, electrolysis is carried out for 2 hours under the atmosphere of Ar and the voltage of-0.3V (relative to a standard hydrogen electrode) for a plurality of times, and the electrolyte is detected and replaced to eliminate impurity interference until no ammonia is detected in the electrolyte.

5. And after impurity interference is eliminated, introducing nitrogen into the electrolyte for 30min, performing a nitrogen reduction test for 2h after the nitrogen is saturated, and setting the potential to be-0.3V, -0.4V, -0.5V, -0.6V and-0.7V (relative to a standard hydrogen electrode) respectively for 7200 s.

And (5) fifth step ammonia yield test: 2mL of the electrolyte solution after running for 2 hours at each potential was taken, added to 2mL of a 1mol/L sodium hydroxide solution (containing 5 wt% of salicylic acid and 5 wt% of sodium citrate dihydrate), mixed well, and left to stand for 30 s. Thereafter, 1mL of a sodium hypochlorite solution having a concentration of 0.05mol/L was added thereto, and sufficiently mixed and allowed to stand for 1 min. Finally, 200. mu.l of sodium nitrosoferricyanide dihydrate with a concentration of 1 wt.% were added. Standing for 2h at room temperature in a dark place, performing spectrum scanning within 400-800 nm by using an ultraviolet spectrum, recording an absorbance value at 650nm, and contrasting a working curve to finally obtain the concentration of ammonia.

Through the above tests, it was found that: preparation of porous Fe/C composite catalyst by alkali metal salt confinement (porous Fe/C composite catalyst prepared in example 2) applied to NRR excellence in-0.5V (relative to standard hydrogen electrode) to achieve maximum ammonia production rate of 1.25 mu g h^-1mg_cat ^-1The Faraday efficiency was 0.59%.

Example 3

The first step is as follows: dissolving 0.675g of ferric trichloride hexahydrate and 0.206g of terephthalic acid into 30mL of N, N-dimethylformamide, ultrasonically mixing for 30min, transferring into a 50mL polytetrafluoroethylene-lined stainless steel autoclave, carrying out hydrothermal treatment for 1h in a 150 ℃ oven, naturally cooling to room temperature, washing with N, N-dimethylformamide and methanol for three times respectively, and drying in an 80 ℃ oven to obtain the iron-based MOF powder.

The second step is that: dispersing 200mg of iron-based MOF material in a saturated sodium chloride solution with the volume of 20mL, freezing the iron-based MOF material in liquid nitrogen for 15min, then transferring the solution to a freeze dryer for drying for 24h, adjusting the temperature of the freeze dryer to-50 ℃, placing the dried material in a tubular heating furnace, heating to 800 ℃ in an argon atmosphere, calcining for 1min, wherein the heating rate is 8 ℃/min, and the argon flow rate is 30mL/min, so as to obtain mixed powder of Fe/C and alkali metal salt;

the third step: and washing the mixed powder of the Fe/C and the alkali metal salt, washing off the alkali metal salt to obtain Fe/C powder, dispersing the Fe/C powder in 10mL of hydrochloric acid solution with the concentration of 3mol/L, stirring for 15min at the speed of 600 revolutions/min, centrifuging to obtain precipitate, washing with pure water for three times, and drying in an oven at 80 ℃ to obtain the porous Fe/C composite catalyst. As shown in FIG. 5, the prepared porous Fe/C composite material has a distinct porous structure.

The fourth step: and carrying out NRR performance test on the obtained porous Fe/C composite material catalyst.

1. Weighing 2mg of porous Fe/C composite catalyst, adding into 200 mu L of absolute ethyl alcohol, simultaneously adding 20 mu L of Nafion solution, and carrying out ultrasonic treatment for 30min to obtain a uniform dispersion liquid. And (3) dripping 100 mu L of the dispersion liquid on dried treated hydrophilic carbon cloth, wherein the surface area of the hydrophilic carbon cloth is controlled to be 1cm multiplied by 1cm, and naturally airing.

2. A three-electrode system is adopted to carry out an electro-catalytic nitrogen reduction performance test on a German Gamry electrochemical workstation, hydrophilic carbon cloth coated with porous Fe/C composite materials is taken as a working electrode, a platinum sheet is taken as a counter electrode, Ag/AgCl is taken as a reference electrode, 0.1mol/L lithium sulfate solution is taken as electrolyte, and an H-shaped glass electrolytic tank is taken as a reaction device.

3. A hydrophilic carbon cloth coated with a porous Fe/C composite material is used as a working electrode, and a cyclic voltammetry test is carried out by scanning 50 circles at a speed of 5mV/s in a potential interval of 0 to-1.0V (relative to a standard hydrogen electrode) under a three-electrode system and an Ar atmosphere so as to activate the material.

4. After cyclic voltammetry, electrolysis is carried out for 2 hours under the atmosphere of Ar and the voltage of-0.3V (relative to a standard hydrogen electrode) for a plurality of times, and the electrolyte is detected and replaced to eliminate impurity interference until no ammonia is detected in the electrolyte.

5. And after impurity interference is eliminated, introducing nitrogen into the electrolyte for 30min, performing a nitrogen reduction test for 2h after the nitrogen is saturated, and setting the potential to be-0.3V, -0.4V, -0.5V, -0.6V and-0.7V (relative to a standard hydrogen electrode) respectively for 7200 s.

And (5) fifth step ammonia yield test: 2mL of the electrolyte solution after running for 2 hours at each potential was taken, added to 2mL of a 1mol/L sodium hydroxide solution (containing 5 wt% of salicylic acid and 5 wt% of sodium citrate dihydrate), mixed well, and left to stand for 30 s. Thereafter, 1mL of a sodium hypochlorite solution having a concentration of 0.05mol/L was added thereto, and sufficiently mixed and allowed to stand for 1 min. Finally, 200. mu.l of sodium nitrosoferricyanide dihydrate with a concentration of 1 wt.% were added. Standing for 2h at room temperature in a dark place, performing spectrum scanning within 400-800 nm by using an ultraviolet spectrum, recording an absorbance value at 650nm, and contrasting a working curve to finally obtain the concentration of ammonia.

Through the above tests, it was found that: preparation of porous Fe/C composite catalyst by alkali metal salt confinement (porous Fe/C composite catalyst prepared in example 3) applied to NRR excellence in-0.5V (relative to standard hydrogen electrode) to achieve maximum ammonia production rate of 1.25 mu g h^-1mg_cat ^-1The Faraday efficiency was 0.59%.

Example 4

The first step is as follows: dissolving 0.675g of ferric trichloride hexahydrate and 0.206g of terephthalic acid into 30mL of N, N-dimethylformamide, ultrasonically mixing for 30min, transferring into a 50mL stainless steel autoclave with a polytetrafluoroethylene lining, carrying out hydrothermal treatment for 12h in an oven at 125 ℃, naturally cooling to room temperature, washing with N, N-dimethylformamide and methanol for three times respectively, and drying in the oven at 80 ℃ to obtain the iron-based MOF powder.

The second step is that: dispersing 100mg of iron-based MOF material in a saturated sodium chloride solution with the volume of 20mL, freezing the iron-based MOF material in liquid nitrogen for 7min, and then transferring the iron-based MOF material to a freeze dryer for drying for 15h, wherein the temperature of the freeze dryer is adjusted to-50 ℃; placing the dried material in a tubular heating furnace, heating to 200 ℃ in an argon atmosphere, calcining for 120min, wherein the heating rate is 2 ℃/min, and the argon flow rate is 30mL/min, so as to obtain mixed powder of Fe/C and alkali metal salt;

the third step: and washing the mixed powder of the Fe/C and the alkali metal salt, washing off the alkali metal salt to obtain Fe/C powder, dispersing the Fe/C powder in 10mL of hydrochloric acid solution with the concentration of 0.01mol/L, stirring for 10 hours at the speed of 600 revolutions per minute, centrifuging to obtain a precipitate, washing with pure water for three times, and drying in an oven at 80 ℃ to obtain the porous Fe/C composite catalyst. The porous Fe/C composite material prepared in example 4 also has a porous structure under the observation of an electron microscope, and can be seen in FIG. 5.

The fourth step: and carrying out NRR performance test on the obtained porous Fe/C composite material catalyst.

1. Weighing 2mg of porous Fe/C composite catalyst, adding into 200 mu L of absolute ethyl alcohol, simultaneously adding 20 mu L of Nafion solution, and carrying out ultrasonic treatment for 30min to obtain uniform dispersion liquid. And (3) dripping 100 mu L of the dispersion liquid on dried treated hydrophilic carbon cloth, wherein the surface area of the hydrophilic carbon cloth is controlled to be 1cm multiplied by 1cm, and naturally airing.

2. A three-electrode system is adopted to carry out an electro-catalytic nitrogen reduction performance test on a German Gamry electrochemical workstation, hydrophilic carbon cloth coated with porous Fe/C composite materials is taken as a working electrode, a platinum sheet is taken as a counter electrode, Ag/AgCl is taken as a reference electrode, 0.1mol/L lithium sulfate solution is taken as electrolyte, and an H-shaped glass electrolytic tank is taken as a reaction device.

3. A hydrophilic carbon cloth coated with a porous Fe/C composite material is used as a working electrode, and a cyclic voltammetry test is carried out by scanning 50 circles at a speed of 5mV/s in a potential interval of 0 to-1.0V (relative to a standard hydrogen electrode) under a three-electrode system and an Ar atmosphere so as to activate the material.

4. After cyclic voltammetry, electrolysis is carried out for 2 hours under the atmosphere of Ar and the voltage of-0.3V (relative to a standard hydrogen electrode) for a plurality of times, and the electrolyte is detected and replaced to eliminate impurity interference until no ammonia is detected in the electrolyte.

5. And after impurity interference is eliminated, introducing nitrogen into the electrolyte for 30min, performing a nitrogen reduction test for 2h after the nitrogen is saturated, and setting the potential to be-0.3V, -0.4V, -0.5V, -0.6V and-0.7V (relative to a standard hydrogen electrode) respectively for 7200 s.

And (5) fifth step ammonia yield test: 2mL of the electrolyte solution after running for 2 hours at each potential was taken, added to 2mL of a 1mol/L sodium hydroxide solution (containing 5 wt% of salicylic acid and 5 wt% of sodium citrate dihydrate), mixed well, and left to stand for 30 s. Thereafter, 1mL of a sodium hypochlorite solution having a concentration of 0.05mol/L was added thereto, and sufficiently mixed and allowed to stand for 1 min. Finally, 200. mu.l of sodium nitrosoferricyanide dihydrate with a concentration of 1 wt.% were added. Standing for 2h at room temperature in a dark place, performing spectrum scanning within 400-800 nm by using an ultraviolet spectrum, recording an absorbance value at 650nm, and contrasting a working curve to finally obtain the concentration of ammonia.

Through the above tests, it was found that: preparation of porous Fe/C composite catalyst by alkali metal salt confinement (porous Fe/C composite catalyst prepared in example 4) applied to NRR excellence in-0.5V (relative to standard hydrogen electrode) to achieve maximum ammonia production rate of 1.25 mu g h^-1mg_cat ^-1The Faraday efficiency was 0.59%.

The above examples are only preferred embodiments of the present invention, which are intended to be illustrative and not limiting, and those skilled in the art should understand that they can make various changes, substitutions and alterations without departing from the spirit and scope of the invention.