A supported ZnS / ZIF-L-NC catalyst for electrochemical synthesis of ammonia and a preparation method and application thereof

Abstract

The application belongs to the technical field of electrochemical synthesis ammonia catalyst, and aims at the problem of lack of high-performance ZnS catalyst for synthesizing ammonia, and provides a preparation method and application of a supported ZnS / ZIF-L-NC catalyst for electrochemical synthesis ammonia. The metal organic coordination polymer ZIF-L is prepared by using zinc nitrate hexahydrate and 2-methyl imidazole as raw materials; the ZIF-L is mixed with dibenzyl disulfide, and then is subjected to ultrasonic treatment, stirring and drying to obtain a precursor; and the precursor is calcined under a nitrogen atmosphere to obtain the ZnS / ZIL-L-NC electrocatalyst. The preparation method is simple, fast and easy to operate. The obtained ZnS nanoparticles are loaded on the ZIF-L and nitrogen-carbon carrier, the ZnS nanoparticles have small size, uniform dispersion and high stability. In addition, the ZnS / ZIL-L-NC catalyst shows hydrophobic characteristics and can effectively inhibit hydrogen evolution. Therefore, the catalyst shows excellent catalytic performance and cycle stability in the electrochemical synthesis ammonia.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of electrochemical synthesis ammonia catalysts, and particularly relates to a supported ZnS / ZIF-L-NC catalyst for electrochemical synthesis of ammonia and a preparation method thereof.

[0002] The application belongs to the technical field of development of high-performance catalysts for electrochemical synthesis of ammonia, development of supported catalysts, and preparation of metal-organic complex derivative composite catalysts, and particularly relates to synthesis of metal-organic coordination polymers and preparation of supported small-size ZnS nanoparticles, aiming to explore a catalyst with excellent activity and selectivity for electrochemical synthesis of ammonia. BACKGROUND

[0003] Ammonia is a nitrogen hydrogen compound with wide applications, and has been widely used in traditional industries such as pharmaceutical production, explosive production, agriculture, and chemical product manufacturing. In recent years, ammonia has also been regarded as a storage carrier of clean energy. At present, the industrial production of ammonia still mainly relies on the Haber-Bosch process method with high temperature (400-500 ℃) and high pressure (100-300 atm) reaction conditions. The harsh reaction conditions of the process result in serious consumption of fossil energy and emission of a large amount of greenhouse gases, which greatly aggravates the environmental problems.

[0004] Electrochemical nitrogen reduction to ammonia has the characteristics of zero pollution emission and reaction under mild conditions, and is regarded as a process that can replace the traditional synthesis of ammonia. Once reported, it has attracted much attention. However, the high bond energy (941 kJ mol −1 ) of N2 in electrochemical nitrogen reduction to ammonia and the influence of hydrogen evolution reaction have become the root of the unsatisfactory ammonia yield and Faraday efficiency.

[0005] Therefore, developing an electrocatalyst with high activation inert N2 capacity is a long-term goal of electrochemical synthesis of ammonia. Among them, although noble metal catalysts (platinum, gold, silver, etc.) show excellent catalytic performance in the field of electrochemical synthesis of ammonia, the scarcity of resources and high price are the biggest stumbling blocks for further development. Non-metallic catalysts (graphene, carbon nanotubes, etc.) have attracted the attention of researchers because of their low price and abundant reserves, but non-metallic catalysts have fewer active sites for nitrogen adsorption, resulting in generally low catalytic activity. Compared with the former two, transition metals (iron, manganese, cobalt, etc.) are low in cost and abundant in reserves, and have been favored by researchers in recent years. Among them, zinc sulfide (ZnS) has strong photoelectric performance, high chemical stability, and environmental friendliness. In addition, the weak bond energy of Zn-H can effectively inhibit the adsorption of hydrogen. Based on the above analysis, ZnS is considered to be one of the excellent transition metal catalysts for electrochemical synthesis of ammonia, and its application in electrocatalytic synthesis of ammonia is relatively less.

[0006] It is known that small size ZnS nanoparticles have high catalytic activity, but small size ZnS nanoparticles are prone to aggregation during the reaction, resulting in reduced catalytic activity. Therefore, building small size ZnS nanoparticles with uniform dispersion is the key to improving its catalytic performance. SUMMARY

[0007] The present application provides a kind of supported ZnS / ZIF-L-NC catalyst for electrochemical synthesis of ammonia and its preparation method to solve the problem of lack of existing high-performance synthetic ammonia ZnS-based catalyst.The preparation method provided by the present application is simple, fast and easy to operate.The prepared ZnS nanoparticles are loaded on ZIL-L and NC carrier, have good dispersity, and the prepared ZnS has smaller size, which is beneficial to improve the yield and catalytic stability of ammonia.The prepared ZnS / ZIF-L-NC shows hydrophobic characteristics, can effectively inhibit the hydrogen evolution reaction, and helps to improve the Faraday efficiency.

[0008] The present application is realized by the following technical solutions: a kind of supported ZnS / ZIF-L-NC catalyst for electrochemical synthesis of ammonia, with zinc nitrate hexahydrate and 2-methyl imidazole as raw materials, metal organic coordination polymer ZIF-L is prepared;Then ZIF-L is mixed with dibenzyl disulfide, and precursor is obtained by ultrasonic, stirring and drying;ZnS / ZIF-L-NC electrocatalyst is obtained by calcining the precursor under nitrogen atmosphere.

[0009] The method for preparing the supported ZnS / ZIF-L-NC catalyst for electrochemical synthesis of ammonia is as follows:

[0010] (1) zinc nitrate hexahydrate and 2-methyl imidazole are dissolved in deionized water respectively, and then mixed;

[0011] (2) the mixed solution obtained in step (1) is magnetically stirred at room temperature for 4 h, and then centrifuged at 8000 r / min for 3 min;Washed with anhydrous ethanol 3 times, vacuum dried at 120 ℃ overnight, to obtain metal organic coordination polymer ZIF-L;

[0012] (3) the ZIF-L obtained in step (2) is mixed with dibenzyl disulfide in equal mass, anhydrous ethanol is added, and then ultrasonic dispersion is carried out, followed by magnetic stirring at room temperature for 3 h, and then drying to obtain a precursor;

[0013] (4) the precursor obtained in step (3) is placed in a porcelain boat, and calcined in a tube furnace under nitrogen atmosphere at a heating rate of 5 ℃ / min to 350 ℃ for 1-5 h to obtain ZnS / ZIF-L-NC catalyst, which is named as ZnS / ZIF-L-NC-1 h, ZnS / ZIF-L-NC-3 h and ZnS / ZIF-L-NC-5 h respectively according to different reaction times.

[0014] The mass of the zinc nitrate hexahydrate in step (1) is 0.59 g, which is dissolved in 40 mL of deionized water; the mass of 2-methylimidazole is 1.3 g, which is dissolved in 40 mL of deionized water, and the mixing method of the two solutions is that the 2-methylimidazole aqueous solution is slowly added into the zinc nitrate hexahydrate aqueous solution.

[0015] In step (2), the anhydrous ethanol cleaning method is: 20 ml each time, and cleaning is performed for 3 times.

[0016] In step (3), the amount of ZIF-L is 200 mg, the amount of anhydrous ethanol is 5 mL, the ultrasonic time is 5 min, and the drying method is: drying at 60 DEG C, and then drying in a vacuum oven at 120 DEG C overnight to obtain the precursor.

[0017] The application also provides an application of the supported ZnS / ZIF-L-NC catalyst for electrochemical synthesis of ammonia in nitrogen electrocatalytic synthesis of ammonia.

[0018] Compared with the prior art, the ZnS / ZIF-L-NC electrocatalyst synthesized by the method has the advantages of simple synthesis method and easy preparation; the prepared ZnS / ZIF-L-NC electrocatalyst is derived from a metal organic coordination polymer. In the pyrolysis process, the metal node Zn in ZIL-L reacts with a sulfur source (dibenzyl disulfide) to generate ZnS nanoparticles; and the ligand is converted into an NC carrier by pyrolysis. It is known that the pyrolysis conditions are related to the size and dispersity of the ZnS nanoparticles in the generated catalyst, which directly affect the catalytic performance of the electrocatalytic ammonia synthesis catalyst. Therefore, strict control of the pyrolysis reaction conditions is the key to successful preparation of a high-performance electrocatalytic ammonia synthesis catalyst. In the experiment, the size and dispersity of the ZnS nanoparticles are regulated by controlling the reaction time, and finally the preparation of a high-performance electrocatalytic ammonia synthesis ZnS / ZIF-L-NC catalyst is realized. The NC skeleton after pyrolysis and the ZIF-L that is not carbonized by pyrolysis provide a good carrier for the generation of small-size and highly dispersed ZnS nanoparticles, which helps to improve the ammonia yield and stability. The prepared ZnS / ZIF-L-NC has hydrophobic characteristics, which effectively inhibits the occurrence of the hydrogen evolution reaction, which is beneficial to the improvement of the Faraday efficiency. BRIEF DESCRIPTION OF DRAWINGS

[0019] Figure 1 is the XRD pattern of ZIF-L;

[0020] Figure 2 is the XRD pattern of ZnS / ZIF-L-NC-1 h in Example 1;

[0021] Figure 3Figure 6 is a TEM image of ZnS / ZIF-L-NC-1 h in Example 1, wherein the red mark is ZnS nanoparticles;

[0022] Figure 4 Figure 7 is an XRD image of ZnS / ZIF-L-NC-3 h in Example 2;

[0023] Figure 5 Figure 8 is a TEM image of ZnS / ZIF-L-NC-3 h in Example 2, wherein: a is a TEM image of ZnS / ZIF-L-NC-3 h with a scale of 50 nm, and b is a TEM image of ZnS / ZIF-L-NC-3 h with a scale of 20 nm;

[0024] Figure 6 Figure 9 is an XRD image of ZnS / ZIF-L-NC-5 h in Example 3;

[0025] Figure 7 Figure 10 is a TEM image of ZnS / ZIF-L-NC-5 h in Example 3, wherein the red mark is ZnS nanoparticles;

[0026] Figure 8 Figure 11 is a comparative performance diagram of electrocatalytic nitrogen reduction to ammonia of ZnS / ZIF-L-NC-1 h, ZnS / ZIF-L-NC-3 h and ZnS / ZIF-L-NC-5 h in Examples 1, 2 and 3;

[0027] Figure 9 Figure 12 is a current density stability test diagram of ZnS / ZIF-L-NC-3 h in Example 1;

[0028] Figure 10 Figure 13 is a hydrophobicity experiment diagram of ZnS / ZIF-L-NC-3 h in Example 1;

[0029] Figure 11 Figure 14 is a comparative performance diagram of electrocatalytic nitrogen reduction to ammonia of ZnS / ZIF-L-NC-3 h and commercial ZnS in Examples 1 and 4. DETAILED DESCRIPTION

[0030] In order to make the objects, technical solutions and advantages of the embodiments of the present application clearer, the technical solutions in the embodiments of the present application will be described clearly and completely below. Obviously, the described embodiments are only part of the embodiments of the present application, rather than all the embodiments of the present application. Based on the embodiments in the present application, all other embodiments obtained by a person of ordinary skill in the art without creative labor fall within the protection scope of the present application.

[0031] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this application belongs. References to materials, articles, or devices herein are specifically incorporated by reference in their entirety for all that they contain, except to the extent that the incorporated reference might contradict any statement in the specification.

[0032] All equivalents of the described specific embodiments as would be known to one skilled in the art upon a complete review of this disclosure and considered as equivalent are intended to be covered.

[0033] The experimental methods in the following examples are all routine methods unless otherwise specified. The instruments used in the following examples are all routine laboratory instruments unless otherwise specified; the experimental materials used in the following examples are all purchased from routine biochemical reagent stores unless otherwise specified.

[0034] The application will be further described in conjunction with the following examples. The following examples are only used to more clearly illustrate the structure and catalytic performance of the relevant materials of the application, and cannot be limited to the following examples. The specific implementation methods described in the examples are all routine methods. The reagents and materials are obtained from commercial channels.

[0035] Example 1: A supported ZnS / ZIF-L-NC-1 h catalyst for electrochemical synthesis of ammonia, the specific preparation method is as follows:

[0036] (1) 0.59 g of zinc nitrate hexahydrate and 1.3 g of 2-methylimidazole were dissolved in 40 mL of deionized water, respectively, and then mixed;

[0037] (2) The mixed solution obtained in step (1) was magnetically stirred at room temperature for 4 h, and then centrifuged at 8000 r / min for 3 min; washed with 20 mL of anhydrous ethanol for 3 times, and dried in a vacuum oven at 120°C overnight to obtain a metal organic coordination polymer ZIF-L. Figure 1 The XRD spectrum of ZIF-L is shown in the figure, and the diffraction peak of the prepared ZIF-L matches well with the simulated diffraction peak of ZIF-L derived from the Cambridge database, which proves that the ZIF-L is successfully synthesized in the experiment.

[0038] (3) 200 mg of the prepared metal organic coordination polymer ZIF-L was mixed with 200 mg of dibenzyl disulfide in equal mass, 5 mL of anhydrous ethanol was added, and magnetically stirred at room temperature for 3 h, dried at 60°C, and then dried in a vacuum oven at 120°C overnight to obtain a precursor.

[0039] (4) The obtained precursor was placed in a porcelain boat and calcined in a tube furnace under a nitrogen atmosphere, the calcination temperature was 350°C, and the calcination time was 1 h, and was named ZnS / ZIF-L-NC-1 h,Figure 2 XRD pattern of ZnS / ZIF-L-NC-1 h. As shown in the figure, the diffraction peaks of ZnS / ZIF-L-NC-1 h not only retain the diffraction peaks of ZIF-L, but also retain the diffraction peaks of ZnS, which indicates that in the process of pyrolysis of ZIF-L and dibenzyl disulfide, part of the metal nodes in ZIF-L are pyrolyzed to react with dibenzyl disulfide to generate ZnS nanoparticles, and part of the ligand in ZIF-L is pyrolyzed to transform into NC carrier. The morphology of ZnS / ZIF-L-NC-1 h is shown in Figure 3 The figure shows that only a small amount of ZnS nanoparticles are generated, which indicates that only a small amount of Zn in ZIF-L reacts with the sulfur source to generate ZnS nanoparticles, further indicating that the ZnS nanoparticles involved in the NRR reaction are less, which is not conducive to the generation of ammonia.

[0040] Example 2: A supported ZnS / ZIF-L-NC-3 h catalyst for electrochemical synthesis of ammonia, the synthesis of ZIF-L and the preparation of the precursor are as described in Example 1, which will not be repeated here.

[0041] The obtained precursor was placed in a porcelain boat and calcined in a tube furnace under a nitrogen atmosphere, the calcination temperature was 350 ℃, and the calcination time was 3 h, which was named ZnS / ZIF-L-NC-3 h. Figure 4 XRD pattern of ZnS / ZIF-L-NC-3 h. As shown in the figure, the diffraction peaks of ZnS / ZIF-L-NC-3 h not only match the diffraction peaks of ZIF-L, but also match the diffraction peaks of ZnS, which indicates that the prepared ZnS / ZIF-L-NC-3 h catalyst not only retains the structure of metal-organic coordination polymer ZIF-L, but also has the structure of ZnS, which indicates that after 3 h of calcination and sulfurization, part of ZIF-L reacts with dibenzyl disulfide to transform into ZnS nanoparticles. The morphology of ZnS / ZIF-L-NC-3 h is shown in Figure 5 The figure shows that a large amount of ZnS nanoparticles are generated and uniformly dispersed. Figure 5 a can be observed, a large amount of ZnS nanoparticles are generated and uniformly dispersed. Figure 5 b can be observed, the size of ZnS nanoparticles is about 10 nm. The above results indicate that uniformly dispersed and small-sized ZnS nanoparticles are helpful to improve the ammonia yield and catalytic stability in NRR.

[0042] Example 3: A supported ZnS / ZIF-L-NC-5 h catalyst for electrochemical synthesis of ammonia, the synthesis of ZIF-L and the preparation of the precursor are as described in Example 1, which will not be repeated here.

[0043] The obtained precursor was placed in a ceramic boat and calcined in a tube furnace under a nitrogen atmosphere, the calcination temperature was 350 ℃, and the calcination time was 5 h, and was named ZnS / ZIF-L-NC-5h. Figure 6 The XRD pattern of ZnS / ZIF-L-NC-5h is shown in the figure, and it can be seen from the figure that the diffraction peaks of ZnS / ZIF-L-NC-5h have both diffraction peaks matched with ZIF-L and diffraction peaks matched with ZnS, which shows that after 5 h of calcination and sulfuration, part of ZIF-L is converted into ZnS nanoparticles by reacting with dibenzyl disulfide. The morphology of ZnS / ZIF-L-NC-5h is shown in Figure 7 The figure shows that the ZnS nanoparticles in ZnS / ZIF-L-NC-5h have a serious aggregation phenomenon and poor dispersion, and the catalytic active sites provided by the aggregated ZnS nanoparticles are less, which is not conducive to the NRR.

[0044] Experimental Example 1: Nitrogen electrocatalytic synthesis of ammonia performance test

[0045] The specific application method of the electrocatalytic nitrogen reduction of the ZnS / ZIF-L-NC-1h, ZnS / ZIF-L-NC-3h and ZnS / ZIF-L-NC-5h catalysts prepared in Example 1-3 is as follows: the electrocatalytic nitrogen reduction experiment is carried out in a two-chamber H-type electrolytic cell, the catalyst modified carbon paper is used as the working electrode, the silver / silver chloride electrode is used as the reference electrode, and the platinum sheet is used as the counter electrode. The voltage shown in the present application has been converted into the voltage of the relative reversible hydrogen electrode.

[0046] The preparation method of the working electrode is as follows: 5 mg of catalyst is dispersed in 960 μL (V 水 :V 乙醇 =1:2) mixed solution, and 40 μL of Nafion solution (5 wt.%) is ultrasonically treated for 30 min to form a homogeneous ink. Then, 40 μL of the catalyst ink is dropped onto the carbon paper with a mass of 0.2 mg. The area of the carbonized paper electrode is 1.0×1.5 cm 2 , and the actual immersion area in the electrolyte is 1.0×1.0 cm 2 . After natural drying, it is dried under a baking lamp for 1 h. Then it can be used after being immersed in the prepared electrolyte for 1 h.

[0047] The electrochemical test method is as follows: at room temperature, the electrochemical test is carried out in a three-electrode system in an electrochemical workstation (CS235OH). The volume of the two parts of the electrolyte in the H-type electrolytic cell is 70 mL. For electrocatalytic N2 reduction, the static potential test is carried out in N2 saturated 0.05 M sulfuric acid for 2 h, and the test is purified with N2 for 30 min before the test. During the experiment, pure N2 is continuously sent into the cathode chamber at a proper position. The catalytic performance of the three catalysts is as shown in Figure 8As shown in FIG. 6, the ammonia production rates of ZnS / ZIF-L-NC-1h, ZnS / ZIF-L-NC-3h and ZnS / ZIF-L-NC-5h are 34.51 μg·h -1 ·mg -1 , 46.11 μg·h -1 ·mg -1 , 33.31 μg·h -1 ·mg -1 , and the faradaic efficiencies are 4.85%, 15.27% and 5.89%, respectively.

[0048] The overall analysis shows that ZnS / ZIF-L-NC-3h has the best electrocatalytic performance for ammonia synthesis, which is mainly attributed to the fact that the ZnS nanoparticles in ZnS / ZIF-L-NC-3h are uniformly dispersed and the small-sized ZnS nanoparticles are helpful for the activation of nitrogen molecules in NRR, thereby improving the ammonia production rate.

[0049] In addition, the current density stability and hydrophobicity of ZnS / ZIF-L-NC-3h are characterized, and the results of the current density stability test are shown in FIG. 7. Figure 9 As shown in FIG. 7, the results show that the current density of ZnS / ZIF-L-NC-3h remains stable after 72 h of electrolysis, which indicates that the catalyst has good electrocatalytic stability, which is mainly attributed to the fact that the ZnS nanoparticles are supported on the ZIF-L and NC carriers, and the ZnS nanoparticles can stably exist during the reaction.

[0050] The results of the hydrophobicity experiment are shown in FIG. 8. Figure 10 As can be observed from the figure, the catalyst is dispersed in the upper layer of hydrophobic n-hexane, and no catalyst is observed in the lower layer of water, which indicates that the catalyst has good hydrophobicity, which is also the reason for its high faradaic efficiency.

[0051] Example 4: Performance test of commercial ZnS for nitrogen electrocatalytic synthesis of ammonia, the electrocatalytic performance test is as shown above, which will not be repeated here. The experimental results are shown in FIG. 9. Figure 11 As can be observed from the figure, the ammonia production rate of commercial ZnS is 29.35 μg·h -1 ·mg -1 , and the FE is 5.85%. In comparison, the ammonia production rate of the ZnS / ZIF-L-NC-3h synthesized by us is 46.11 μg·h -1 ·mg -1) is about 1.6 times that of commercial ZnS, and FE (15.27%) is about 2.6 times that of commercial ZnS. The above results indicate that the synthesized ZnS / ZIF-L-NC-3h has significant advantages over commercial ZnS in NRR.

[0052] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, rather than to limit it. Although the present invention has been described in detail with reference to the above embodiments, those skilled in the art should understand that they can still modify the technical solutions described in the above embodiments, or replace some or all of the technical features therein with equivalents. However, these modifications or replacements do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. A method for preparing a supported ZnS / ZIF-L-NC catalyst for electrochemical ammonia synthesis, characterized by: A metal-organic coordination polymer (ZIF-L) was prepared using zinc nitrate hexahydrate and 2-methylimidazole as raw materials. ZIF-L was then mixed with dibenzyl disulfide, subjected to sonication, stirring, and drying to obtain a precursor. The precursor was calcined under a nitrogen atmosphere to obtain the ZnS / ZIF-L-NC electrocatalyst. The specific preparation steps are as follows: (1) Zinc nitrate hexahydrate and 2-methylimidazole were dissolved in deionized water respectively and then mixed; (2) The mixed solution obtained in step (1) was subjected to magnetic stirring at room temperature for 4 h, and then centrifuged at 8000 r / min for 3 min; washed with anhydrous ethanol three times, and vacuum dried at 120 °C overnight to obtain the metal organic coordination polymer ZIF-L; (3) The ZIF-L obtained in step (2) was mixed with dibenzyl disulfide in equal amounts, anhydrous ethanol was added, ultrasonically dispersed, magnetically stirred at room temperature for 3 h, and dried to obtain a precursor; (4) The precursor obtained in step (3) was placed in a porcelain boat and calcined in a tube furnace under a nitrogen atmosphere at a heating rate of 5°C / min to 350°C for 3 h to obtain a ZnS / ZIF-L-NC catalyst, which was named ZnS / ZIF-L-NC-3h.

2. The method for preparing a supported ZnS / ZIF-L-NC catalyst for electrochemical ammonia synthesis according to claim 1, characterized in that: In step (1), the mass of zinc nitrate hexahydrate is 0.59 g, which is dissolved in 40 mL of deionized water; the mass of 2-methylimidazole is 1.3 g, which is dissolved in 40 mL of deionized water. The two solutions are mixed by slowly adding the aqueous solution of 2-methylimidazole to the aqueous solution of zinc nitrate hexahydrate.

3. The method for preparing a supported ZnS / ZIF-L-NC catalyst for electrochemical ammonia synthesis according to claim 1, characterized in that: The anhydrous ethanol cleaning method in step (2) is: 20 ml each time, and wash 3 times.

4. The method for preparing a supported ZnS / ZIF-L-NC catalyst for electrochemical ammonia synthesis according to claim 1, characterized in that: In step (3), the amount of ZIF-L and dibenzyl disulfide used is 200 mg, and the amount of anhydrous ethanol is 5 mL; the ultrasonic time is 5 min; and the drying method is: drying at 60 °C and then drying in a vacuum oven at 120 °C overnight to obtain the precursor.

5. Use of the supported ZnS / ZIF-L-NC catalyst for electrochemical ammonia synthesis obtained by the preparation method according to claim 1 in the electrocatalytic synthesis of ammonia from nitrogen.

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