Preparation method and application of iron-cobalt bimetallic hollow cube material

Abstract

The application relates to a preparation method and application of a cobalt-iron bimetal hollow cube material, and the preparation method comprises the following steps: firstly, mixing ZIF-67 and tannic acid in a solution, and drying to obtain TA-Co; then, mixing the TA-Co and an iron source in a solution, drying, and calcining to obtain the cobalt-iron bimetal hollow cube material. Compared with the prior art, the preparation method is simple in process and low in cost. The prepared catalyst has a hollow cube structure, can provide more active sites, promotes the ammonia reduction reaction kinetics of nitrate through the synergistic catalytic mechanism of the active sites, and is high in catalytic activity. The catalyst product is mainly ammonia, is good in stability, and can be repeatedly used.

Description

Technical Field

[0001] This invention belongs to the field of electrocatalyst technology, and relates to a method for preparing and applying an iron-cobalt bimetallic hollow cubic material, particularly to a method for preparing an iron-cobalt bimetallic hollow cubic material and its application in the electrocatalytic reduction of nitrates to ammonia in water. Background Technology

[0002] Nitrate (NO3) - Nitrate pollution has become one of the most serious water environment challenges of the 21st century. Its sources are widespread and difficult to control: excessive application of agricultural fertilizers leads to approximately 70% of nitrates seeping into groundwater; nitrogen-containing wastewater from intensive aquaculture is increasing year by year; industrial emissions (such as explosives manufacturing, electroplating, and dyeing) and urban sewage are continuously exacerbating the pollution load. According to the World Health Organization (WHO), the nitrate concentration in the drinking water sources of over 2 billion people worldwide is approaching or exceeding the safety limit of 50 mg / L, triggering a serious public health crisis—nitrates are reduced to nitrites (NO2) by microorganisms in the human body. - Nitrate not only blocks the oxygen-carrying capacity of hemoglobin (causing "blue baby syndrome"), but it can also be converted into the potent carcinogen N-nitrosamine in the acidic environment of the stomach. Simultaneously, eutrophication of water bodies is rampant due to excessive nitrate input, algal blooms consume dissolved oxygen, causing ecological collapse in "dead seas," resulting in global economic losses of tens of billions of dollars annually. Traditional treatment technologies face fundamental limitations: physicochemical methods such as ion exchange and reverse osmosis only achieve phase transfer of pollutants, producing secondary pollution from high-salinity wastewater; while biological denitrification is widely used, it relies on strict carbon-to-nitrogen ratio (C / N) control, is easily affected by water quality fluctuations, and faces challenges such as nitrite accumulation, N2O emission, and sludge disposal. Based on the biological characteristics of nitrate, my country has set a nitrate concentration limit of no more than 10 mg / L. However, currently, the nitrate pollution concentration in drinking water in many domestic and international regions exceeds this standard limit.

[0003] Ammonia (NH3) is an important chemical product with wide applications in many fields such as fertilizer synthesis, pharmaceuticals, and the textile industry, and is closely related to human production and daily life. Furthermore, NH3 has a high hydrogen density (4.3 kWh / kg⁻¹). -1With its advantages of low liquefaction pressure and zero carbon emissions, ammonia is expected to become the next generation of energy carrier. Typical ammonia synthesis methods mainly include biological nitrogen fixation, the Haber-Bosch process, and electrocatalysis. However, although biological nitrogen fixation catalyzed by nitrogenase can achieve the highest electronic efficiency, its low yield cannot meet the huge demand of the current fertilizer industry. The industrial Haber-Bosch process makes large-scale ammonia production possible, but the Haber-Bosch process, carried out under high temperature and pressure, suffers from drawbacks such as high energy consumption, low energy efficiency, and stringent equipment requirements. In addition, the hydrogen required in the Haber-Bosch process mainly comes from the catalytic reforming of fossil fuels such as natural gas, a process that consumes huge amounts of energy and emits serious greenhouse gases. Therefore, research on green and sustainable ammonia synthesis methods is of great significance in terms of energy and environment.

[0004] In recent years, electrochemical ammonia synthesis has been widely praised by researchers due to its ambient temperature and pressure conditions and the availability of ammonia production raw materials (such as N2, NO3, and NO2). Currently, there are three main feasible electrochemical ammonia synthesis methods: (1) direct nitrogen reduction (DNRR); (2) nitrate reduction (NO3RR); and (3) NO2 reduction (NO2)... x (e.g., NO and NO2) Gas Reduction (NORR). The DNRR process mainly involves electrocatalysis to synthesize ammonia through a series of complex reactions, including adsorption of N2, stepwise hydrogenation, and subsequent dissociation. However, due to the low solubility of N2 and the high dissociation energy of the N≡N triple bond (941 kJ / mol), the process is difficult. -1 Furthermore, the DNRR suffers from severe hydrogen evolution competition (HER) problems, resulting in low selectivity and Faraday efficiency. In contrast, the NO3RR, with its abundant nitrogen sources (groundwater, excessive nitrogen fertilizers, and animal manure, etc.), effectively avoids the problems of low solubility and difficult dissociation. The highly soluble nitrate component contains NO3... - The N=O bond energy is only 204 kJ / mol. -1 This provides a more effective strategy for efficient ammonia production processes.

[0005] The main challenges in the electrocatalytic reduction of nitrate to ammonia stem from the high kinetic energy barrier of octet and quintet transfer, low catalytic activity, low ammonia selectivity, and the competitive hydrogen evolution reaction (HER). Therefore, designing and developing electrocatalysts that can efficiently promote the electrocatalytic reduction of nitrate to ammonia are crucial. Co-based non-noble metal catalysts are highly regarded, exhibiting catalytic performance even superior to many noble metal-based catalysts. They demonstrate high ammonia yield and Faradaic efficiency at relatively low overpotentials, making them ideal catalysts for NO3RR ammonia synthesis. However, the conductivity and intrinsic activity of pure cobalt-based materials hinder their application. Therefore, the rational control and design of Co-based catalysts are of great significance for improving the electrocatalytic reduction of nitrate to ammonia. Studies have shown that by introducing Fe atoms for doping, the Fe sites typically enhance the ammonia production of nitrate (NO3RR). - The Fe and Co sites exhibit strong adsorption capacity for nitrogen-containing intermediates (such as *NO2, *NO), which is beneficial for the initial activation of nitrate ions and the stability of key intermediates, thus promoting the initiation of the electroreduction reaction. The Co sites have relatively moderate adsorption strength for key intermediates and the final product ammonia, which is conducive to further hydrogenation reduction of intermediates and promotes the desorption and release of NH3, preventing the active sites from being poisoned or excessive adsorption of products from hindering the reaction. The synergistic effect between Fe and Co sites optimizes the activation capacity of the active sites for reactants, promoting electron acceptance by nitrate ions and their interaction with protons (H+). + The effective combination of iron and cobalt enhances the efficiency of multi-electron / proton transfer processes. It significantly lowers the energy barrier of the rate-determining step in the entire reaction pathway and suppresses side reactions, thus achieving highly active, selective, and stable ammonia electrosynthesis under mild conditions (room temperature and pressure). Therefore, constructing iron-cobalt bimetallic materials is beneficial for improving the catalytic activity of the nitrate electroreduction reaction. Summary of the Invention

[0006] The purpose of this invention is to provide a method for preparing and applying an iron-cobalt bimetallic hollow cubic material to address the problems of low catalytic efficiency and poor stability of existing pure Co-based catalysts. This invention relates to electrocatalytic nitrate reduction to ammonia (NO3RR) technology, which promotes the hydrogenation reaction in the electrochemical reduction of nitrate in water, and has significant advantages in the following aspects: Compared with existing technologies, the preparation method of this invention is simple and low-cost. The prepared catalyst has a hollow cubic structure, which provides more active sites. Through the synergistic catalytic mechanism at these active sites, it promotes the kinetics of the electrocatalytic nitrate reduction to ammonia reaction, improves ammonia selectivity and yield, and exhibits high catalytic activity.

[0007] The objective of this invention can be achieved through the following technical solutions: The first aspect of the present invention provides a method for preparing an iron-cobalt bimetallic hollow cubic material, comprising: S1: ZIF-67 and tannic acid were mixed in solution and dried to obtain TA-Co; S2: Mix TA-Co with iron source in solution, dry, and calcine to obtain iron-cobalt bimetallic hollow cubic material.

[0008] In some specific embodiments, in step S1, the mass ratio of ZIF-67 to tannic acid is 1:(4~6).

[0009] In some specific embodiments, the feed ratio of cobalt nitrate hexahydrate to tannic acid is (250~300) mg:(300~400) mg.

[0010] In some specific embodiments, in step S1, the mixing temperature is 20~30℃ and the mixing time is 10~30min; the solution is selected from ethanol.

[0011] In some specific embodiments, in step S1, the concentration of tannic acid in the solution obtained by mixing is 0.5~3.0 wt%.

[0012] In some specific embodiments, in step S1, the preparation method of ZIF-67 includes: reacting hexadecyltrimethylammonium bromide (CTAB), cobalt source, and dimethylimidazole in an organic solvent, and then drying to obtain the product.

[0013] In some specific embodiments, the cobalt source is cobalt nitrate hexahydrate; The feed ratio of the cobalt source, hexadecyltrimethylammonium bromide, and dimethylimidazole is (250~300) mg:(0.4~0.8) mmol:(30~60) mmol; In the reaction, the reaction temperature is 20~30 ℃ and the reaction time is 30-60 min.

[0014] In some specific embodiments, the preparation method of ZIF-67 includes: dissolving cobalt nitrate hexahydrate in a methanol solution containing hexadecyltrimethylammonium bromide, and then injecting it into an aqueous solution containing dimethylimidazole for reaction; In the methanol solution, the concentration of CTAB is 0.2~0.5wt%, and the concentration of methanol is 70~80wt%; the dissolution temperature is 20~30℃, and the dissolution time is 10~15 min. In the aqueous solution containing dimethylimidazole, the concentration of dimethylimidazole is 10-20 wt%, and the concentration of water is 60-80 wt%. In the reaction, the reaction temperature is 20~30 ℃ and the reaction time is 30~60 min.

[0015] In some specific embodiments, in step S2, the iron source is ferric chloride; the feeding ratio of TA-Co to the iron source is 8 mg:(7~8)×10 -7 mol, preferably 8 mg: 7.4 × 10 -7 mol.

[0016] In some specific embodiments, the feeding ratio of cobalt nitrate hexahydrate and ferric chloride hexahydrate is (250~300) mg:(10~20) mg.

[0017] In some specific embodiments, the ferric chloride hexahydrate is mixed in solution form, wherein the concentration of ferric chloride hexahydrate in the solution is 0.2~0.5wt% and the concentration of ethanol is 60~80wt%.

[0018] In some specific implementations, in step S2, the mixing temperature is 20~30 ℃ and the mixing time is 1~2 h.

[0019] In some specific embodiments, in step S2, the calcination temperature is 300~350℃, the calcination time is 2~4h, and the calcination atmosphere is air.

[0020] A second aspect of the present invention provides an iron-cobalt bimetallic hollow cubic material, which is prepared by the method described above.

[0021] A third aspect of the present invention provides an application of the iron-cobalt bimetallic hollow cubic material as described above, comprising: using the iron-cobalt bimetallic hollow cubic material as an electrode material for electrocatalytic nitrate denitrification reaction.

[0022] In the electrocatalytic nitrate denitrification reaction, a platinum sheet is used as the counter electrode, a mercury / mercury oxide electrode is used as the reference electrode, and the working electrode is an electrode sheet loaded with iron-cobalt bimetallic hollow cubic material, wherein the carrier is preferably carbon cloth; the electrolyte includes potassium hydroxide, and the concentration is preferably 1M.

[0023] Cobalt-based materials have significant advantages in electrocatalytic nitrate reduction (NO3RR), such as the d-electron structure of cobalt, which favors NO3. - While adsorption and NO bond breaking are advantageous and low-cost, their applications are limited by poor product selectivity (easily generating nitrite or nitrogen gas instead of the target product ammonia), low Faradaic efficiency due to the competitive hydrogen evolution reaction (HER), insufficient long-term stability, and high overpotential. Iron incorporation can effectively modulate the performance of cobalt-based materials. Through the synergistic catalytic effect of forming Fe-Co bimetallic active sites, it reduces the d-band center of cobalt to optimize NO3- adsorption. -Adsorption energy, along with the synergistic specialization of iron sites to preferentially activate nitrate and cobalt sites to accelerate the hydrogenation step, thereby improving reaction kinetics and reducing overpotential (down to 0.1 V vs. RHE). Furthermore, iron doping can suppress side reactions and competitive hydrogen evolution reactions, increasing ammonia selectivity to over 90%.

[0024] Compared with the prior art, the present invention has the following beneficial effects: This invention features a simple and low-cost process. The resulting hollow cubic structure provides more active sites, and Fe alters the electron distribution on the material surface, weakening *H adsorption and reducing the HER hydrogen evolution reaction. Through a synergistic catalytic mechanism at its active sites, Fe-Co synergistically promotes NO2 production. - Rapid hydrogenation to produce NH3 reduces NO2. - The accumulation of N2 formation promotes the kinetics of the electrocatalytic reduction of nitrate to ammonia, improving ammonia selectivity and yield. This catalyst exhibits good dispersion of its catalytic components, high catalytic activity, good stability, and reusability. Attached Figure Description

[0025] Figure 1 The image shows an SEM image (a) and a corresponding magnified view (b) of the electrocatalyst prepared in Example 1.

[0026] Figure 2 The images shown are TEM images (a) and corresponding magnified views (b) of the electrocatalyst prepared in Example 1.

[0027] Figure 3 SEM images of the electrocatalyst in Comparative Example 1 (a, b).

[0028] Figure 4 The images show (a) SEM image, (b) TEM image, and (c) performance graph of the prepared FeCo bimetallic hollow cubic material (FeCo-NC) electrode after 20 long cycles. Detailed Implementation

[0029] The present invention will now be described in detail with reference to the accompanying drawings and specific embodiments. These embodiments are based on the technical solution of the present invention and provide detailed implementation methods and specific operating procedures. However, the scope of protection of the present invention is not limited to the following embodiments.

[0030] The following are more detailed implementation examples, which further illustrate the technical solution of the present invention and the technical effects that can be obtained.

[0031] In the following embodiments, unless otherwise specified, the raw materials, reagents or processing techniques are all conventional commercial products or conventional processing techniques in the art.

[0032] In the following examples, cetyltrimethylammonium bromide, dimethylimidazole, cobalt nitrate hexahydrate, tannic acid, ferric chloride hexahydrate, methanol, and ethanol were purchased from Shanghai Titan Technology Co., Ltd.; potassium nitrate was purchased from Sinopharm Chemical Reagent Co., Ltd.

[0033] Example 1: A hollow cubic iron-cobalt bimetallic material is prepared by the following steps: (1) Dissolve 4 mg of hexadecyltrimethylammonium bromide (CTAB) in 10 mL of methanol solution, and label this solution A. Weigh 292 mg of Co(NO3)2·6H2O and add it to solution A to dissolve. Weigh 4.54 g of dimethylimidazole and dissolve it in 70 mL of deionized water. Stir at room temperature for 10 min until completely dissolved, and label this solution B. Then quickly pour the completely dissolved solution A into solution B, seal it with aluminum foil, and stir at room temperature for 40 min. Finally, collect the product by centrifugation, wash it repeatedly with ethanol, and then dry it in a vacuum oven at 70 °C. The resulting product is labeled ZIF-67. (2) Dissolve 60 mg ZIF-67 in 20 mL of ethanol and sonicate for 10 min to mix thoroughly. Then add 300 mL of ethanol containing 1 mg / mL tannic acid (TA) and stir for 10 min at room temperature. Collect the product by centrifugation, wash repeatedly with ethanol, and dry in a vacuum oven at 70 °C. The resulting product is denoted as TA-Co. (3) First, dissolve 8 mg TA-Co in 6 mL of ethanol and sonicate for 10 min to mix it evenly. Then, add 4 mL of deionized water containing 0.05 mg / mL FeCl3·6H2O to the above mixture and stir at room temperature for 1 h. Collect the product by centrifugation, wash it repeatedly with ethanol, and dry it in a vacuum oven at 70 °C. The obtained product is denoted as TA-FeCo. (4) Place the prepared TA-FeCo in the middle of a tube furnace, raise the temperature to 300 ℃ at a heating rate of 1 ℃ / min under air atmosphere and hold for 120 min to completely burn the organic ligands and convert the metal ions into metal oxides. The product is naturally cooled to room temperature under air atmosphere to obtain the final product FeCo-NC.

[0034] Figure 1 , Figure 2 The images show the SEM and TEM images of the catalyst prepared in this embodiment, which reveal its hollow cubic structure. Figure 4The images show (a) SEM image, (b) TEM image, and (c) performance graph of the prepared FeCo bimetallic hollow cubic material (FeCo-NC) electrode after 20 cycles. As can be seen from the figures, the catalyst has good cycling stability.

[0035] Comparative Example 1 A cobalt-based hollow cubic material, the preparation method of which differs from that in Example 1 only in that: Step (3) is omitted, meaning that the product is obtained directly by calcination in air without dissolving it in ethanol or adding a deionized aqueous solution containing 0.05 mg / mL FeCl3·6H2O. The rest is the same as in Example 1.

[0036] Figure 3 The image shows a SEM image of the prepared catalyst, revealing its hollow cubic structure.

[0037] Application Examples According to Example 1 and Comparative Example 1, electrocatalytic nitrate nitrogen removal tests were conducted under the same conditions, specifically as follows: A three-electrode testing system was used, with a platinum sheet as the counter electrode, a mercury / mercury oxide electrode as the reference electrode, and the prepared catalyst electrode sheet as the working electrode (loading method: 4 mg of catalyst was dissolved in 500 μL of deionized water and 500 μL of ethanol, then 30 μL of Nafion solution was added, and the mixed catalyst solution was dropped onto a 1×1 cm⁻¹ plate). 2 The electrode pads were dried on carbon cloth. The test electrolyte was 30 mL of a 1 M potassium hydroxide solution containing 1400 mg / L nitrate nitrogen. The test method was time-current (TFC). i - t The reaction curve was tested for 1 hour, and the electrolyte was collected after the test.

[0038] The electrocatalysts prepared in Example 1 and Comparative Example 1 were subjected to laboratory electrocatalytic denitrification experiments for comparison. The electrolytes were a mixed solution of potassium nitrate containing 1400 mg / L nitrate nitrogen and potassium hydroxide containing 1 M, respectively. Time-current curves were measured for one hour using an electrochemical workstation. The concentration of ammonium ions in the electrolyte after the reaction was determined using the indophenol blue method. Specifically, 2 mL of the electrolyte after the reaction was taken, and 2 mL of solution A (4 g of sodium hydroxide, 5.42 g of salicylic acid, and 5.42 g of sodium citrate solution were dissolved and mixed in 100 mL of deionized water, and then diluted to volume in a volumetric flask) was added to the above solution. Then, 1 mL of solution B (9 mL of sodium hypochlorite was added to 96.93 g of deionized water) and 200 μL of solution C (0.997 g of sodium ferrocyanide dihydrate was dissolved in 100 mL of deionized water) were added to the above solution. After waiting for 40 min of reaction, the reaction product mixture was measured in a UV-Vis spectrophotometer. Ammonium ions exhibit a visible light absorption peak at 655 nm, and their absorbance can be used to determine the concentration of ammonium ions in the reaction solution. The experimental results of the ammonia production rate and Faraday efficiency of the above electrocatalyst over 1 hour are shown in Table 1.

[0039] Table 1 As can be seen from Table 1, by comparing the electrocatalytic performance of Example 1 and Comparative Example 1, it can be seen that Example 1 has a better ammonia yield and faradaic efficiency than Comparative Example 1, and is more conducive to conducting electrocatalytic nitrate denitrification experiments.

[0040] In summary, this invention provides an iron-cobalt bimetallic hollow cubic material. The process is simple and low-cost, and the resulting hollow cubic structure provides more active sites. Fe alters the electron distribution on the material surface, weakening *H adsorption and reducing the HER hydrogen evolution reaction. Through a synergistic catalytic mechanism at its active sites, Fe-Co synergistically promotes NO2 production. - Rapid hydrogenation to produce NH3 reduces NO2. - The accumulation of N2 formation promotes the kinetics of the electrocatalytic reduction of nitrate to ammonia, improving ammonia selectivity and yield. This catalyst exhibits good dispersion of its catalytic components, high catalytic activity, good stability, and reusability.

[0041] The above description of the embodiments is provided to enable those skilled in the art to understand and use the invention. It will be apparent to those skilled in the art that various modifications can be made to these embodiments, and the general principles described herein can be applied to other embodiments without inventive effort. Therefore, the present invention is not limited to the above embodiments, and any improvements and modifications made by those skilled in the art based on the disclosure of the present invention without departing from the scope of the invention should be within the protection scope of the present invention.

Claims

1. A method for preparing an iron-cobalt bimetallic hollow cubic material, characterized in that, include: S1: ZIF-67 and tannic acid were mixed in solution and dried to obtain TA-Co; S2: Mix TA-Co with iron source in solution, dry, and calcine to obtain iron-cobalt bimetallic hollow cubic material.

2. The method for preparing the iron-cobalt bimetallic hollow cubic material according to claim 1, characterized in that, In step S1, the mass ratio of ZIF-67 to tannic acid is 1:(4~6).

3. The method for preparing the iron-cobalt bimetallic hollow cubic material according to claim 1, characterized in that, In step S1, the mixing temperature is 20~30℃ and the mixing time is 10~30min; the solution is selected from ethanol.

4. The method for preparing the iron-cobalt bimetallic hollow cubic material according to claim 1, characterized in that, In step S1, the preparation method of ZIF-67 includes: reacting hexadecyltrimethylammonium bromide, cobalt source, and dimethylimidazole in an organic solvent and drying the mixture.

5. The method for preparing the iron-cobalt bimetallic hollow cubic material according to claim 4, characterized in that, The cobalt source is cobalt nitrate hexahydrate; The feed ratio of the cobalt source, hexadecyltrimethylammonium bromide, and dimethylimidazole is (250~300) mg:(0.4~0.8) mmol:(30~60) mmol; In the reaction, the reaction temperature is 20~30 ℃ and the reaction time is 30-60 min.

6. The method for preparing the iron-cobalt bimetallic hollow cubic material according to claim 1, characterized in that, In step S2, the iron source is ferric chloride; the feed ratio of TA-Co to the iron source is 8 mg:(7~8)×10. -7 mol, preferably 8 mg: 7.4 × 10 -7 mol.

7. The method for preparing the iron-cobalt bimetallic hollow cubic material according to claim 1, characterized in that, In step S2, the mixing temperature is 20~30 ℃ and the mixing time is 1~2 h.

8. The method for preparing the iron-cobalt bimetallic hollow cubic material according to claim 1, characterized in that, In step S2, the calcination temperature is 300~350℃, the calcination time is 2~4h, and the calcination atmosphere is air.

9. A hollow cubic iron-cobalt bimetallic material, characterized in that, It is prepared by the method described in any one of claims 1 to 8.

10. An application of the iron-cobalt bimetallic hollow cubic material as described in claim 9, characterized in that, The iron-cobalt bimetallic hollow cubic material is used as an electrode material for the electrocatalytic denitrification reaction of nitrate nitrogen.

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