Modified iron-based metal organic framework electrocatalyst and preparation method and application thereof

Abstract

The present application relates to a kind of modified iron-based metal organic framework electrocatalyst and its preparation method and application, synthesis carbon quantum dots with rich functional groups using solvothermal method, and the synthesized carbon quantum dots are used to modify iron-based metal organic framework;Wherein, the existence of functional groups provides many favorable sites for constructing high-activity electrocatalyst;Carbon quantum dots as primitive auxiliary synthesis iron-based metal organic framework, no obvious structural evolution occurs in pyrolysis process, so as to provide larger spacing for active metal, and aggregation is avoided.The modified iron-based metal organic framework electrocatalyst prepared by the present application is not easy to agglomerate, has higher stability, and has superior reaction activity and product selectivity when used for catalyzing nitrate reduction reaction.

Description

Technical Field

[0001] This invention relates to the field of catalyst preparation technology, specifically to a modified iron-based metal-organic framework electrocatalyst, its preparation method, and its application. Background Technology

[0002] Faced with the increasing depletion of traditional fossil fuels, electrochemistry for sustainable energy conversion has become a promising technology. Electrochemical catalysis is commonly used in reactions such as hydrogen evolution reaction (HER), oxygen evolution reaction (OER), carbon dioxide reduction reaction (COD), and nitrate reduction reaction (NTD). Compared to other methods, electrochemical methods can be carried out under very mild temperature and pressure conditions, and can achieve higher conversion rates, energy efficiency, and stability through renewable electrical energy and a potentially abundant supply of catalysts. However, currently used precious metal catalysts are expensive and also suffer from stability and energy efficiency issues. Therefore, exploring advanced and efficient electrocatalyst preparation strategies is an important way to solve these problems.

[0003] Metal nanomaterials derived from metal-organic frameworks have attracted considerable attention for their application in nitrate reduction reactions due to their molecular structure and abundant active sites. However, most reaction sites have not exhibited the expected catalytic performance due to channel size limitations. Therefore, customizing material properties through rational design has become a key research focus.

[0004] CN 113403633A discloses a method for preparing a Cu-CN metal-organic framework electrocatalyst for the reduction of nitrate to ammonia, comprising: dissolving 2-methylimidazole in a methanol solution to form a first solution; dissolving zinc nitrate hexahydrate and copper acetylacetonate in a methanol solution to form a second solution; adding the second solution to the first solution and sonicating to form a suspension; transferring the suspension to a reaction vessel, reacting at a constant temperature, and then cooling to room temperature to obtain a dark blue precipitate; washing the collected dark blue precipitate with anhydrous ethanol and centrifuging, and drying it under vacuum to obtain a precursor; and calcining the precursor under vacuum to obtain the final product. However, this electrocatalyst still has the problems of easy aggregation and the need to improve its stability.

[0005] Low-cost carbon quantum dots are promising candidates for in-situ assisted synthesis. Firstly, carbon quantum dots are widely available, non-toxic, inexpensive, and biocompatible. Secondly, they are rich in surface functional groups (-OH, -COOH, -NH2, etc.), providing numerous favorable sites for constructing highly active electrocatalysts. Thirdly, compared to organic precursors, carbon quantum dots, as intermediate carbon supports, do not undergo significant structural evolution during pyrolysis, thus providing a larger stable spacing for active metals, preventing aggregation, and improving stability.

[0006] CN 112808313A discloses a nitrogen-doped carbon quantum dot / metal-organic framework (MOF-5) photocatalyst, its preparation method, and its application. Using coconut shells as a natural resource, nitrogen-doped carbon quantum dots are prepared via a hydrothermal method. Nitrogen-doped carbon quantum dots are introduced into the MOF-5 material during in-situ synthesis. During photocatalysis, the nitrogen-doped carbon quantum dot / MOF-5 photocatalyst utilizes the high charge transfer and molecular oxygen activation capabilities of the nitrogen-doped carbon quantum dots to improve the conductivity, stability, and photocatalytic performance of the MOF-5 material. However, the catalyst provided by this invention is only suitable for photocatalysis and has limitations in electrocatalytic applications.

[0007] Therefore, in view of the shortcomings of the existing technology, there is an urgent need to provide an electrocatalyst that is not prone to agglomeration, has high stability, and good reactivity. Summary of the Invention

[0008] The purpose of this invention is to provide a modified iron-based metal-organic framework electrocatalyst, its preparation method, and its application. Utilizing carbon quantum dots, which are rich in functional groups and have good biocompatibility, as building blocks, a modified iron-based metal-organic framework electrocatalyst suitable for nitrate reduction reactions was synthesized. The modified iron-based metal-organic framework electrocatalyst prepared by this invention is not prone to aggregation, exhibits high stability, and demonstrates good activity when used for catalytic nitrate reduction.

[0009] To achieve this objective, the present invention employs the following technical solution:

[0010] This invention provides a method for preparing a modified iron-based metal-organic framework electrocatalyst, the method comprising the following steps:

[0011] (1) Mix catechol and ethanol, and the resulting homogeneous solution is subjected to heat treatment and purification treatment in sequence to obtain carbon quantum dots;

[0012] (2) Dissolve the iron source, zinc source and carbon quantum dots obtained in step (1) in a first organic solvent to obtain a first solution; dissolve 2-methylimidazole in a second organic solvent to obtain a second solution;

[0013] (3) Mix the first solution and the second solution obtained in step (2), age them, and then centrifuge them. The resulting precipitate is washed and dried to obtain an iron-based metal-organic framework.

[0014] (4) The iron-based metal-organic framework obtained in step (3) is subjected to carbonization calcination and oxidation calcination in sequence to obtain the modified iron-based metal-organic framework electrocatalyst.

[0015] The preparation method provided by this invention uses catechol and ethanol as raw materials to synthesize carbon quantum dots with abundant functional groups via a solvothermal method. These synthesized carbon quantum dots are then used to modify iron-based metal-organic frameworks. The presence of these functional groups provides numerous favorable sites for constructing highly active electrocatalysts. Using carbon quantum dots as building blocks, the iron-based metal-organic framework is synthesized via a room-temperature precipitation method, exhibiting no significant structural evolution during pyrolysis, thus providing ample spacing for the active metals and preventing aggregation. The modified iron-based metal-organic framework electrocatalyst prepared by this invention is not prone to aggregation, possesses high stability, and demonstrates superior reactivity and product selectivity when used to catalyze nitrate reduction reactions.

[0016] Preferably, the mass-to-volume ratio of catechol to ethanol in step (1) is 1g:(190-210)mL, for example, it can be 1g:190mL, 1g:195mL, 1g:200mL, 1g:205mL or 1g:210mL, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0017] Preferably, the mixing in step (1) includes ultrasonic mixing.

[0018] Preferably, the ultrasonic mixing time is 8-12 minutes, for example, it can be 8 minutes, 9 minutes, 10 minutes, 11 minutes or 12 minutes, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0019] Preferably, the heat treatment temperature in step (1) is 175-185℃, for example, it can be 175℃, 178℃, 180℃, 182℃ or 185℃, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0020] Preferably, the heat treatment time in step (1) is 10-14 hours, for example, 10 hours, 11 hours, 12 hours, 13 hours or 14 hours, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0021] Preferably, the heat treatment in step (1) is carried out in a 100 mL polytetrafluoroethylene-lined reactor.

[0022] Preferably, the purification process in step (1) includes: sequentially evaporating and concentrating the liquid phase obtained from the heat treatment, dialysis, and drying.

[0023] Preferably, the dialysis time is 6-8 days, for example, 6 days, 6.5 days, 7 days, 7.5 days or 8 days, but not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0024] Preferably, the dialysis is performed in a dialysis bag with a molecular weight cutoff of 3500 Da.

[0025] Preferably, the drying temperature is 75-85℃, for example, 75℃, 78℃, 80℃, 82℃ or 85℃, but not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0026] Preferably, the carbon quantum dots in step (1) are brownish-red carbon quantum dots.

[0027] Preferably, the iron source in step (2) includes any one or a combination of at least two of ferrous sulfate, ferrous chloride, or ferrous acetate. Typical but non-limiting combinations include a combination of ferrous sulfate and ferrous chloride, a combination of ferrous chloride and ferrous acetate, or a combination of ferrous sulfate, ferrous chloride, and ferrous acetate.

[0028] Preferably, the zinc source in step (2) includes any one or a combination of at least two of zinc nitrate, zinc sulfate, or zinc acetate. Typical but non-limiting combinations include a combination of zinc nitrate and zinc sulfate, a combination of zinc sulfate and zinc acetate, or a combination of zinc nitrate, zinc sulfate, and zinc acetate.

[0029] Preferably, the ferrous sulfate comprises FeSO4·7H2O, and the zinc nitrate comprises Zn(NO3)2·6H2O.

[0030] Preferably, in step (2), the first organic solvent and the second organic solvent each independently comprise methanol.

[0031] Preferably, the molar volume ratio of the iron source to the first organic solvent in step (2) is 1 mmol:(10-60) mL, for example, it can be 1 mmol:10 mL, 1 mmol:30 mL, 1 mmol:40 mL, 1 mmol:50 mL or 1 mmol:60 mL, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0032] Preferably, the molar volume ratio of the zinc source to the first organic solvent in step (2) is 1 mmol:(6-9) mL, for example, it can be 1 mmol:6 mL, 1 mmol:7 mL, 1 mmol:7.5 mL, 1 mmol:8 mL or 1 mmol:9 mL, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0033] Preferably, the mass-volume ratio of the carbon quantum dots to the first organic solvent in step (2) is 1 mg:(3-10) mL, for example, it can be 1 mg:3 mL, 1 mg:5 mL, 1 mg:6 mL, 1 mg:8 mL or 1 mg:10 mL, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0034] Preferably, before obtaining the first solution and the second solution in step (2), they are mixed independently for 8-12 minutes, for example, 8 minutes, 9 minutes, 10 minutes, 11 minutes or 12 minutes, but not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0035] Preferably, the molar ratio of 2-methylimidazole to zinc source in step (2) is 1:(0.22-0.28), for example, it can be 1:0.22, 1:0.23, 1:0.25, 1:0.26 or 1:0.28, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0036] Preferably, the mixing in step (3) involves adding the second solution to the first solution for mixing.

[0037] Preferably, the aging time in step (3) is 23-25 ​​hours, for example, 23 hours, 23.5 hours, 24 hours, 24.5 hours or 25 hours, but not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0038] Preferably, the washing in step (3) involves washing with methanol 3-4 times, for example, 3 or 4 times.

[0039] Preferably, the drying temperature in step (3) is 55-65°C, for example, it can be 55°C, 58°C, 60°C, 62°C or 65°C, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0040] Preferably, the carbonization and calcination temperature in step (4) is 890-910℃, for example, it can be 890℃, 895℃, 900℃, 905℃ or 910℃, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0041] Preferably, the heating rate of the carbonization calcination in step (4) is 4-6℃ / min, for example, it can be 4℃ / min, 4.5℃ / min, 5℃ / min, 5.5℃ / min or 6℃ / min, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0042] Preferably, the carbonization and calcination time in step (4) is 0.8-1.2h, for example, it can be 0.8h, 0.9h, 1h, 1.1h or 1.2h, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0043] Preferably, the carbonization and calcination in step (4) is carried out in an atmosphere of hydrogen-argon mixture.

[0044] Preferably, the hydrogen-argon mixture contains 10% hydrogen.

[0045] Preferably, the carbonization and calcination in step (4) is carried out in a tube furnace.

[0046] Preferably, the oxidation calcination temperature in step (4) is 440-460℃, for example, it can be 440℃, 445℃, 450℃, 455℃ or 460℃, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0047] Preferably, the heating rate of the oxidation calcination in step (4) is 4-6℃ / min, for example, it can be 4℃ / min, 4.5℃ / min, 5℃ / min, 5.5℃ / min or 6℃ / min, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0048] Preferably, the oxidation and calcination time in step (4) is 2.8-3.2h, for example, it can be 2.8h, 2.9h, 3h, 3.1h or 3.2h, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0049] Preferably, the oxidation calcination in step (4) is carried out in a muffle furnace.

[0050] The iron-based metal-organic framework obtained after carbonization and calcination is directly subjected to oxidation and calcination without any further treatment.

[0051] As a preferred embodiment of the preparation method described in this invention, the preparation method includes the following steps:

[0052] (1) Ultrasonic mixing of catechol and ethanol with a mass-to-volume ratio of 1g:(190-210)mL for 8-12min, the resulting homogeneous solution is heat-treated at 175-185℃ for 10-14h, the resulting liquid phase is successively evaporated and concentrated, dialyzed in a dialysis bag with a molecular weight cutoff of 3500Da for 6-8d, and dried at 75-85℃ to obtain brown carbon quantum dots;

[0053] (2) Dissolve the iron source, zinc source, and brown carbon quantum dots obtained in step (1) in a first organic solvent and mix for 8-12 min to obtain a first solution; dissolve 2-methylimidazole in a second organic solvent and mix for 8-12 min to obtain a second solution; the molar volume ratio of the iron source to the first organic solvent is 1 mmol:(10-60) mL; the molar volume ratio of the zinc source to the first organic solvent is 1 mmol:(6-9) mL; the mass-volume ratio of the brown carbon quantum dots to the first organic solvent is 1 mg:(3-10) mL; the molar ratio of 2-methylimidazole to the zinc source is 1:(0.22-0.28);

[0054] (3) The second solution obtained in step (2) is added to the first solution and mixed. After aging for 23-25 ​​hours, it is centrifuged. The resulting precipitate is washed with methanol 3-4 times and dried at 55-65℃ to obtain an iron-based metal-organic framework.

[0055] (4) The iron-based metal-organic framework obtained in step (3) is placed in a hydrogen-argon mixture in a tube furnace with a hydrogen content of 10%, and heated to 890-910℃ at 4-6℃ / min for carbonization and calcination for 0.8-1.2h; then it is directly placed in a muffle furnace and heated to 440-460℃ at 4-6℃ / min for oxidation and calcination for 2.8-3.2h to obtain the modified iron-based metal-organic framework electrocatalyst.

[0056] In a second aspect, the present invention provides a modified iron-based metal-organic framework electrocatalyst, which is prepared by the preparation method described in the first aspect.

[0057] The modified iron-based metal-organic framework electrocatalyst is not prone to aggregation and has high stability. When used to catalyze nitrate reduction reactions, it exhibits superior reactivity and product selectivity.

[0058] Thirdly, the present invention provides an application of the modified iron-based metal-organic framework electrocatalyst as described in the second aspect, wherein the modified iron-based metal-organic framework electrocatalyst is used in a nitrate reduction to ammonia production reaction, the nitrate reduction to ammonia production reaction comprising the following steps:

[0059] A modified iron-based metal-organic framework electrocatalyst was used as the working electrode, an Ag / AgCl electrode as the reference electrode, and a platinum sheet electrode as the counter electrode to form a three-electrode system. The nitrate reduction to ammonia reaction was carried out using the three-electrode system.

[0060] Preferably, in the nitrate reduction to ammonia reaction, at least one of the following methods is used to test the catalytic activity of the modified iron-based metal-organic framework electrocatalyst: constant potential electrolysis, linear sweep voltammetry, or cyclic voltammetry.

[0061] Preferably, the potential tested in the constant potential electrolysis method is -0.68V.

[0062] Preferably, the scanning rate in the linear scanning voltammetry is 50 mV / s, and the scanning potential range is -0.98 V to 0.62 V.

[0063] Preferably, the scan rate in the cyclic voltammetry is 20-160 mV / s, and the scan potential range is -0.98 V to 0.62 V.

[0064] Preferably, the nitrate reduction to ammonia reaction specifically includes: coating a modified iron-based metal-organic framework electrocatalyst onto carbon paper, drying it, and then placing the carbon paper as the working electrode in 50 mL of electrolyte, wherein the electrolyte is 0.1 M Na2SO4 as the electrolyte, and then using an Ag / AgCl electrode as the reference electrode and a platinum sheet electrode as the counter electrode to form a three-electrode system, and using the three-electrode system to carry out the nitrate reduction to ammonia reaction.

[0065] Compared with the prior art, the present invention has the following beneficial effects:

[0066] The present invention provides a method for preparing modified iron-based metal-organic framework electrocatalysts. Using catechol and ethanol as raw materials, carbon quantum dots with abundant functional groups are synthesized via a solvothermal method. These synthesized carbon quantum dots are then used to modify the iron-based metal-organic framework. The presence of functional groups provides numerous favorable sites for constructing highly active electrocatalysts. Carbon quantum dots, as building blocks, are used in the room-temperature precipitation method to assist in the synthesis of the iron-based metal-organic framework. During pyrolysis, they do not undergo significant structural evolution, thus providing a large spacing between the active metals and preventing aggregation. The modified iron-based metal-organic framework electrocatalyst prepared by this invention is not prone to aggregation and exhibits high stability. When used to catalyze nitrate reduction reactions, it demonstrates superior reactivity and product selectivity, achieving a removal capacity of 919 mg N / g cat. in 6 hours. Attached Figure Description

[0067] Figure 1 This is a SEM image of the modified iron-based metal-organic framework electrocatalyst provided in Example 1 of this invention;

[0068] Figure 2 This is a 100nm-TEM image of the modified iron-based metal-organic framework electrocatalyst provided in Example 1 of this invention;

[0069] Figure 3 This is a 20nm-TEM image of the modified iron-based metal-organic framework electrocatalyst provided in Example 1 of this invention;

[0070] Figure 4 This is a 5nm-TEM image of the modified iron-based metal-organic framework electrocatalyst provided in Example 1 of this invention;

[0071] Figure 5 This is a comparison diagram of the catalytic activity of the modified iron-based metal-organic framework electrocatalysts provided in Example 1 and Comparative Examples 1-3 of the present invention;

[0072] Figure 6 This is a graph showing the electrocatalytic performance of the modified iron-based metal-organic framework electrocatalyst for nitrate reduction to ammonia production provided in Example 1 of this invention.

[0073] Figure 7 This is a graph showing the electrocatalytic performance of the modified iron-based metal-organic framework electrocatalyst for nitrate reduction to ammonia production provided in Comparative Example 1 of this invention.

[0074] Figure 8 This is a graph showing the electrocatalytic performance of the modified iron-based metal-organic framework electrocatalyst for nitrate reduction to ammonia production provided in Comparative Example 2 of this invention.

[0075] Figure 9 This is a graph showing the electrocatalytic performance of the modified iron-based metal-organic framework electrocatalyst for nitrate reduction to ammonia production provided in Comparative Example 3 of this invention. Detailed Implementation

[0076] The technical solution of the present invention will be further illustrated below through specific embodiments. Those skilled in the art should understand that the embodiments described are merely illustrative of the present invention and should not be construed as limiting the invention in any way.

[0077] Example 1

[0078] This embodiment provides a modified iron-based metal-organic framework electrocatalyst, the preparation method of which includes the following steps:

[0079] (1) Ultrasonic mixing of catechol and ethanol at a mass-volume ratio of 1g:200mL for 10min, the resulting homogeneous solution was heat-treated at 180℃ for 12h, the resulting liquid phase was successively evaporated and concentrated, dialyzed in a dialysis bag with a molecular weight cutoff of 3500Da for 7d, and then dried at 80℃ to obtain brown carbon quantum dots.

[0080] (2) FeSO4·7H2O, Zn(NO3)2·6H2O and the brown carbon quantum dots obtained in step (1) were dissolved in methanol and mixed for 10 min to obtain the first solution; 2-methylimidazole was dissolved in methanol and mixed for 10 min to obtain the second solution; the molar volume ratio of FeSO4·7H2O to methanol was 1 mmol:30 mL; the molar volume ratio of Zn(NO3)2·6H2O to methanol was 1 mmol:7.5 mL; the mass-volume ratio of brown carbon quantum dots to methanol was 1 mg:6 mL; the molar ratio of 2-methylimidazole to Zn(NO3)2·6H2O was 1:0.25;

[0081] (3) The second solution obtained in step (2) was added to the first solution and mixed. After aging for 24 hours, it was centrifuged. The precipitate was washed three times with methanol and dried at 60°C to obtain an iron-based metal-organic framework.

[0082] (4) The iron-based metal-organic framework obtained in step (3) is placed in a hydrogen-argon mixture in a tube furnace, with hydrogen accounting for 10%, and heated to 900°C at 5°C / min for carbonization and calcination for 1 hour; then it is directly placed in a muffle furnace and heated to 450°C at 5°C / min for oxidation and calcination for 3 hours to obtain the modified iron-based metal-organic framework electrocatalyst. The SEM image of the modified iron-based metal-organic framework electrocatalyst is shown below. Figure 1 As shown, the 100nm-TEM image, 20nm-TEM image, and 5nm-TEM image are respectively as follows: Figure 2 , Figure 3 , Figure 4 As shown in the figure, the prepared material is a porous nanosphere, and the presence of pores is more conducive to the contact between the active substance and the reactant.

[0083] A modified iron-based metal-organic framework electrocatalyst was coated onto carbon paper. After drying, the carbon paper was placed in 50 mL of electrolyte, using 0.1 M Na₂SO₄ as the working electrode. An Ag / AgCl electrode was used as the reference electrode, and a platinum sheet electrode was used as the counter electrode, forming a three-electrode system. The nitrate reduction to ammonia reaction was performed using this three-electrode system. The catalytic activity of the modified iron-based metal-organic framework electrocatalyst was tested using linear sweep voltammetry at a scan rate of 50 mV / s and a scan potential range of -0.98 V to 0.62 V. The results are shown below. Figure 5 As shown in the figure, the current density and Tafel slope are significantly better than those of the electrocatalysts provided in the other comparative examples; the electrocatalytic nitrate reduction performance of the modified iron-based metal-organic framework electrocatalyst was tested, and the results are as follows. Figure 6 As shown in the figure, the removal capacity can reach 919 mg N / gcat. after 6 hours.

[0084] Example 2

[0085] This embodiment provides a modified iron-based metal-organic framework electrocatalyst, the preparation method of which includes the following steps:

[0086] (1) Ultrasonic mixing of catechol and ethanol with a mass-to-volume ratio of 1g:190mL for 8min, the resulting homogeneous solution was heat-treated at 175℃ for 14h, the resulting liquid phase was successively evaporated and concentrated, dialyzed in a dialysis bag with a molecular weight cutoff of 3500Da for 6d, and then dried at 75℃ to obtain brown carbon quantum dots.

[0087] (2) FeSO4·7H2O, Zn(NO3)2·6H2O and the brown carbon quantum dots obtained in step (1) were dissolved in methanol and mixed for 8 min to obtain the first solution; 2-methylimidazole was dissolved in methanol and mixed for 8 min to obtain the second solution; the molar volume ratio of FeSO4·7H2O to methanol was 1 mmol:30 mL; the molar volume ratio of Zn(NO3)2·6H2O to methanol was 1 mmol:6 mL; the mass-volume ratio of brown carbon quantum dots to methanol was 1 mg:6 mL; the molar ratio of 2-methylimidazole to Zn(NO3)2·6H2O was 1:0.28;

[0088] (3) The second solution obtained in step (2) was added to the first solution and mixed. After aging for 23 hours, it was centrifuged. The precipitate was washed three times with methanol and dried at 55°C to obtain an iron-based metal-organic framework.

[0089] (4) The iron-based metal-organic framework obtained in step (3) is placed in a hydrogen-argon mixture in a tube furnace with a hydrogen content of 10%, and heated to 890°C at 4°C / min for carbonization and calcination for 1.2 h; then it is directly placed in a muffle furnace and heated to 440°C at 4°C / min for oxidation and calcination for 3.2 h to obtain the modified iron-based metal-organic framework electrocatalyst.

[0090] The modified iron-based metal-organic framework electrocatalyst was coated onto carbon paper and dried. The carbon paper was then placed in 50 mL of electrolyte, using 0.1 M Na₂SO₄ as the electrolyte. An Ag / AgCl electrode was used as the reference electrode, and a platinum sheet electrode as the counter electrode, forming a three-electrode system. This three-electrode system was used for the nitrate reduction to ammonia reaction. The catalytic activity of the modified iron-based metal-organic framework electrocatalyst was tested using linear sweep voltammetry at a scan rate of 50 mV / s and a scan potential range of -0.98 V to 0.62 V. The current density and Tafel slope showed good performance. The electrocatalytic nitrate reduction performance of the modified iron-based metal-organic framework electrocatalyst was tested, and the removal capacity reached 870 mg N / g cat. after 6 h.

[0091] Example 3

[0092] This embodiment provides a modified iron-based metal-organic framework electrocatalyst, the preparation method of which includes the following steps:

[0093] (1) The catechol and ethanol were ultrasonically mixed at a mass-volume ratio of 1g:210mL for 12min. The resulting homogeneous solution was heat-treated at 185℃ for 10h. The resulting liquid phase was then evaporated and concentrated, dialyzed in a dialysis bag with a molecular weight cutoff of 3500Da for 8d, and then dried at 85℃ to obtain brown carbon quantum dots.

[0094] (2) FeSO4·7H2O, Zn(NO3)2·6H2O and the brown carbon quantum dots obtained in step (1) were dissolved in methanol and mixed for 12 min to obtain the first solution; 2-methylimidazole was dissolved in methanol and mixed for 12 min to obtain the second solution; the molar volume ratio of FeSO4·7H2O to methanol was 1 mmol:30 mL; the molar volume ratio of Zn(NO3)2·6H2O to methanol was 1 mmol:9 mL; the mass-volume ratio of brown carbon quantum dots to methanol was 1 mg:6 mL; the molar ratio of 2-methylimidazole to Zn(NO3)2·6H2O was 1:0.22;

[0095] (3) The second solution obtained in step (2) was added to the first solution and mixed. After aging for 25 hours, it was centrifuged. The precipitate was washed with methanol four times and dried at 65°C to obtain an iron-based metal-organic framework.

[0096] (4) The iron-based metal-organic framework obtained in step (3) is placed in a hydrogen-argon mixture in a tube furnace with a hydrogen content of 10%, and heated to 910°C at 6°C / min for carbonization and calcination for 0.8h; then it is directly placed in a muffle furnace and heated to 460°C at 6°C / min for oxidation and calcination for 2.8h to obtain the modified iron-based metal-organic framework electrocatalyst.

[0097] The modified iron-based metal-organic framework electrocatalyst was coated onto carbon paper and dried. The carbon paper was then placed in 50 mL of electrolyte, using 0.1 M Na₂SO₄ as the electrolyte. An Ag / AgCl electrode was used as the reference electrode, and a platinum sheet electrode as the counter electrode, forming a three-electrode system. This three-electrode system was used for the nitrate reduction to ammonia reaction. The catalytic activity of the modified iron-based metal-organic framework electrocatalyst was tested using linear sweep voltammetry at a scan rate of 50 mV / s and a scan potential range of -0.98 V to 0.62 V. The current density and Tafel slope showed good performance. The electrocatalytic nitrate reduction performance of the modified iron-based metal-organic framework electrocatalyst was tested, and the removal capacity reached 853 mg N / g cat. after 6 h.

[0098] Example 4

[0099] This embodiment provides a modified iron-based metal-organic framework electrocatalyst. The difference from Example 1 is that, except that the mass-volume ratio of brown carbon quantum dots to methanol in step (2) is adjusted to 1 mg: 3 mL, the rest is the same as in Example 1.

[0100] The electrocatalytic nitrate reduction performance of the modified iron-based metal-organic framework electrocatalyst was tested, and the removal capacity reached 736 mg N / g cat. after 6 h.

[0101] Example 5

[0102] This embodiment provides a modified iron-based metal-organic framework electrocatalyst. The difference from Example 1 is that, except that the mass-volume ratio of brown carbon quantum dots to methanol in step (2) is adjusted to 1 mg: 10 mL, the rest is the same as in Example 1.

[0103] The electrocatalytic nitrate reduction performance of the modified iron-based metal-organic framework electrocatalyst was tested, and the removal capacity reached 828 mg N / g cat. after 6 h.

[0104] Example 6

[0105] This embodiment provides a modified iron-based metal-organic framework electrocatalyst. The difference from Example 1 is that, except that the mass-volume ratio of brown carbon quantum dots to methanol in step (2) is adjusted to 1 mg: 2 mL, everything else is the same as in Example 1.

[0106] Adding too much carbon quantum dots can actually reduce the catalytic activity of the modified iron-based metal-organic framework electrocatalyst. The electrocatalytic nitrate reduction performance of the modified iron-based metal-organic framework electrocatalyst was tested, and the removal capacity reached 680 mg N / g cat. after 6 hours.

[0107] Example 7

[0108] This embodiment provides a modified iron-based metal-organic framework electrocatalyst. The difference from Example 1 is that, except that the mass-volume ratio of brown carbon quantum dots to methanol in step (2) is adjusted to 1 mg: 12 mL, everything else is the same as in Example 1.

[0109] The addition of too few carbon quantum dots has limited effect on the modification of the material. The electrocatalytic nitrate reduction performance of the modified iron-based metal-organic framework electrocatalyst was tested, and the removal capacity reached 521 mg N / g cat. after 6 h.

[0110] Example 8

[0111] This embodiment provides a modified iron-based metal-organic framework electrocatalyst. The difference from Example 1 is that, except that the molar volume ratio of FeSO4·7H2O to methanol in step (2) is adjusted to 1 mmol:60 mL, everything else is the same as in Example 1.

[0112] The electrocatalytic nitrate reduction performance of the modified iron-based metal-organic framework electrocatalyst was tested, and the removal capacity was 312 mg N / g cat. after 6 h.

[0113] Example 9

[0114] This embodiment provides a modified iron-based metal-organic framework electrocatalyst. The difference from Example 1 is that, except that the molar volume ratio of FeSO4·7H2O to methanol in step (2) is adjusted to 1 mmol:10 mL, the rest is the same as in Example 1.

[0115] The electrocatalytic nitrate reduction performance of the modified iron-based metal-organic framework electrocatalyst was tested, and the removal capacity was 612 mg N / g cat. after 6 h.

[0116] Example 10

[0117] This embodiment provides a modified iron-based metal-organic framework electrocatalyst. The difference from Example 1 is that, except that the molar volume ratio of FeSO4·7H2O to methanol in step (2) is adjusted to 1 mmol:65 mL, everything else is the same as in Example 1.

[0118] The reduction of iron source will reduce the number of active sites. The electrocatalytic nitrate reduction performance of the modified iron-based metal-organic framework electrocatalyst was tested, and the removal capacity was 190 mg N / g cat. after 6 h.

[0119] Example 11

[0120] This embodiment provides a modified iron-based metal-organic framework electrocatalyst. The difference from Example 1 is that, except that the molar volume ratio of FeSO4·7H2O to methanol in step (2) is adjusted to 1 mmol:8 mL, the rest is the same as in Example 1.

[0121] Increasing the iron source increases the aggregation of the material and reduces the reactivity. The electrocatalytic nitrate reduction performance of the modified iron-based metal-organic framework electrocatalyst was tested, and the removal capacity was 403 mg N / g cat. after 6 h.

[0122] Example 12

[0123] This embodiment provides a modified iron-based metal-organic framework electrocatalyst. The difference between this embodiment and Example 1 is that, except that the carbonization and calcination temperature in step (4) is adjusted to 850°C, everything else is the same as in Example 1.

[0124] Because the carbonization and calcination temperature is too low, the degree of carbonization will be reduced, resulting in a low current density; the electrocatalytic nitrate reduction performance of the modified iron-based metal-organic framework electrocatalyst was tested, and the removal capacity after 6 hours was 796 mg N / g cat.

[0125] Example 13

[0126] This embodiment provides a modified iron-based metal-organic framework electrocatalyst. The difference between this embodiment and Example 1 is that, except that the carbonization and calcination temperature in step (4) is adjusted to 950°C, everything else is the same as in Example 1.

[0127] Excessive carbonization and calcination temperature can lead to structural collapse and particle agglomeration. The electrocatalytic nitrate reduction performance of the modified iron-based metal-organic framework electrocatalyst was tested, and the removal capacity was 733 mg N / g cat. after 6 hours.

[0128] Example 14

[0129] This embodiment provides a modified iron-based metal-organic framework electrocatalyst. The difference between this embodiment and Example 1 is that, except that the oxidation and calcination temperature in step (4) is adjusted to 350°C, everything else is the same as in Example 1.

[0130] Because the oxidation calcination temperature is too low, the oxidation will be incomplete; the electrocatalytic nitrate reduction performance of the modified iron-based metal-organic framework electrocatalyst was tested, and the removal capacity was 740 mg N / g cat. after 6 h.

[0131] Example 15

[0132] This embodiment provides a modified iron-based metal-organic framework electrocatalyst. The difference between this embodiment and Example 1 is that, except that the oxidation and calcination temperature in step (4) is adjusted to 550°C, everything else is the same as in Example 1.

[0133] Excessive oxidation and calcination temperature can lead to sintering and agglomeration; the electrocatalytic nitrate reduction performance of the modified iron-based metal-organic framework electrocatalyst was tested, and the removal capacity after 6 hours was 854 mg N / g cat.

[0134] Comparative Example 1

[0135] This comparative example provides a modified iron-based metal-organic framework electrocatalyst. The difference from Example 1 is that step (4) does not involve oxidation and calcination, while the rest is the same as Example 1.

[0136] The absence of an oxidation-calcination step leads to the loss of oxygen vacancies, reducing the adsorption of surface lattice oxygen species. The catalytic activity of the modified iron-based metal-organic framework electrocatalyst was tested using linear sweep voltammetry, and the results are as follows: Figure 5 As shown in the figure, the current density is relatively low; the electrocatalytic nitrate reduction performance of the modified iron-based metal-organic framework electrocatalyst was tested, and the results are as follows. Figure 7 As shown, the removal capacity after 6 hours was 453 mg N / g cat.

[0137] Comparative Example 2

[0138] This comparative example provides an iron-based metal-organic framework electrocatalyst, which differs from Example 1 in that step (1) is omitted and carbon quantum dots are not present in step (2), while the rest is the same as Example 1.

[0139] Since carbon quantum dots were not used to modify the iron-based metal-organic framework, the morphology was not modified. The catalytic activity of the iron-based metal-organic framework electrocatalyst was tested using linear sweep voltammetry, and the results are as follows: Figure 5 As shown in the figure, the current density is relatively low; the electrocatalytic nitrate reduction performance of the iron-based metal-organic framework electrocatalyst was tested, and the results are as follows. Figure 8 As shown, the removal capacity after 6 hours was 821 mg N / g cat.

[0140] Comparative Example 3

[0141] This comparative example provides an iron-based metal-organic framework electrocatalyst, which differs from Example 1 in that step (1) is omitted, carbon quantum dots are omitted in step (2), and the oxidation calcination step is omitted in step (4). All other steps are the same as in Example 1.

[0142] Because the iron-based metal-organic framework was not modified with carbon quantum dots, and no oxidation-calcination step was performed, there was neither an increase in oxygen vacancies nor any morphological modification, resulting in the lowest catalytic performance. The catalytic activity of the iron-based metal-organic framework electrocatalyst was tested using linear sweep voltammetry, and the results are as follows: Figure 5 As shown, its current density is the lowest; the electrocatalytic nitrate reduction performance of the iron-based metal-organic framework electrocatalyst was tested, and the results are as follows. Figure 9 As shown, the removal capacity after 6 hours was only 256 mg N / gcat.

[0143] In summary, the method for preparing the modified iron-based metal-organic framework electrocatalyst provided by this invention uses catechol and ethanol as raw materials, and synthesizes carbon quantum dots with abundant functional groups using a solvothermal method. The synthesized carbon quantum dots are then used to modify the iron-based metal-organic framework. The presence of functional groups provides many favorable sites for constructing highly active electrocatalysts. As building blocks, carbon quantum dots, through room-temperature precipitation to assist in the synthesis of the iron-based metal-organic framework, do not undergo significant structural evolution during pyrolysis, thus providing a large spacing for the active metal and preventing aggregation. The modified iron-based metal-organic framework electrocatalyst prepared by this invention is not prone to aggregation and has high stability. When used to catalyze nitrate reduction reactions, it exhibits superior reactivity and product selectivity, with a removal capacity of up to 919 mg N / g cat. in 6 hours.

[0144] The above description is only a specific embodiment of the present invention, but the protection scope of the present invention is not limited thereto. Those skilled in the art should understand that any changes or substitutions that can be easily conceived by those skilled in the art within the technical scope disclosed in the present invention fall within the protection and disclosure scope of the present invention.

Claims

1. A method for preparing a modified iron-based metal-organic framework electrocatalyst, characterized in that, The preparation method includes the following steps: (1) The homogeneous solution obtained by mixing catechol and ethanol is subjected to heat treatment and purification treatment in sequence to obtain carbon quantum dots; (2) Dissolve the iron source, zinc source and carbon quantum dots obtained in step (1) in a first organic solvent to obtain a first solution; dissolve 2-methylimidazole in a second organic solvent to obtain a second solution; (3) The first solution and the second solution obtained in step (2) are mixed, aged and then centrifuged. The resulting precipitate is washed and dried to obtain an iron-based metal-organic framework. (4) The iron-based metal-organic framework obtained in step (3) is subjected to carbonization calcination and oxidation calcination in sequence to obtain the modified iron-based metal-organic framework electrocatalyst. The carbonization and calcination temperature in step (4) is 890-910℃; The oxidation and calcination temperature in step (4) is 440-460℃.

2. The preparation method according to claim 1, characterized in that, In step (1), the mass-to-volume ratio of catechol to ethanol is 1 g:(190-210) mL.

3. The preparation method according to claim 1, characterized in that, The mixing in step (1) includes ultrasonic mixing.

4. The preparation method according to claim 3, characterized in that, The ultrasonic mixing time is 8-12 minutes.

5. The preparation method according to claim 1, characterized in that, The heat treatment temperature in step (1) is 175-185℃.

6. The preparation method according to claim 1, characterized in that, The heat treatment time in step (1) is 10-14 hours.

7. The preparation method according to claim 1, characterized in that, The purification process in step (1) includes: evaporating and concentrating the liquid phase obtained from the heat treatment, dialysis, and drying it in sequence.

8. The preparation method according to claim 7, characterized in that, The dialysis duration is 6-8 days.

9. The preparation method according to claim 7, characterized in that, The drying temperature is 75-85℃.

10. The preparation method according to claim 1, characterized in that, The iron source in step (2) includes any one or a combination of at least two of ferrous sulfate, ferrous chloride, or ferrous acetate.

11. The preparation method according to claim 1, characterized in that, The zinc source in step (2) includes any one or a combination of at least two of zinc nitrate, zinc sulfate, or zinc acetate.

12. The preparation method according to claim 1, characterized in that, Step (2) The first organic solvent and the second organic solvent each independently comprise methanol.

13. The preparation method according to claim 1, characterized in that, The molar volume ratio of the iron source to the first organic solvent in step (2) is 1 mmol:(10-60) mL.

14. The preparation method according to claim 1, characterized in that, The molar volume ratio of the zinc source to the first organic solvent in step (2) is 1 mmol:(6-9) mL.

15. The preparation method according to claim 1, characterized in that, The mass-to-volume ratio of the carbon quantum dots to the first organic solvent in step (2) is 1 mg:(3-10) mL.

16. The preparation method according to claim 1, characterized in that, Before obtaining the first solution and the second solution in step (2), mix them independently for 8-12 minutes.

17. The preparation method according to claim 1, characterized in that, The molar ratio of 2-methylimidazole to zinc source in step (2) is 1:(0.22-0.28).

18. The preparation method according to claim 1, characterized in that, The mixing in step (3) involves adding the second solution to the first solution and mixing them.

19. The preparation method according to claim 1, characterized in that, The aging time in step (3) is 23-25 ​​hours.

20. The preparation method according to claim 1, characterized in that, The washing process in step (3) involves washing with methanol 3-4 times.

21. The preparation method according to claim 1, characterized in that, The drying temperature in step (3) is 55-65℃.

22. The preparation method according to claim 1, characterized in that, The heating rate of carbonization calcination in step (4) is 4-6℃ / min.

23. The preparation method according to claim 1, characterized in that, The carbonization and calcination time in step (4) is 0.8-1.2h.

24. The preparation method according to claim 1, characterized in that, The carbonization and calcination in step (4) is carried out in an atmosphere of hydrogen-argon mixture.

25. The preparation method according to claim 1, characterized in that, The heating rate of the oxidation calcination in step (4) is 4-6℃ / min.

26. The preparation method according to claim 1, characterized in that, The oxidation and calcination time in step (4) is 2.8-3.2h.

27. A modified iron-based metal-organic framework electrocatalyst, characterized in that, The modified iron-based metal-organic framework electrocatalyst was prepared by the preparation method according to any one of claims 1-26.

28. The application of the modified iron-based metal-organic framework electrocatalyst as described in claim 27, characterized in that, The modified iron-based metal-organic framework electrocatalyst is used in the nitrate reduction to ammonia production reaction, which includes the following steps: A modified iron-based metal-organic framework electrocatalyst was used as the working electrode, an Ag / AgCl electrode as the reference electrode, and a platinum sheet electrode as the counter electrode to form a three-electrode system. The nitrate reduction to ammonia reaction was carried out using the three-electrode system.

29. The application according to claim 28, characterized in that, In the nitrate reduction to ammonia reaction, at least one of the following methods—constant potential electrolysis, linear sweep voltammetry, or cyclic voltammetry—is used to test the catalytic activity of the modified iron-based metal-organic framework electrocatalyst.

30. The application according to claim 29, characterized in that, The potential measured in the constant potential electrolysis method is -0.68V.

31. The application according to claim 29, characterized in that, The linear scanning voltammetry method uses a scanning rate of 50 mV / s and a scanning potential range of -0.98 V to 0.62 V.

32. The application according to claim 29, characterized in that, The scanning rate in the cyclic voltammetry is 20-160 mV / s, and the scanning potential range is -0.98 V to 0.62 V.

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