Preparation method of molecular catalyst based on flexible coordination theory and application of molecular catalyst in preparation of ammonia by electrocatalytic reduction of nitrate

Abstract

The invention discloses a preparation method of a molecular catalyst based on a flexible coordination theory and an application of the molecular catalyst in ammonia production through electrocatalytic reduction of nitrate. According to the invention, Co is selected as a coordination center, homopiperazine is taken as a ligand, a flexible coordination molecular catalyst is prepared, and the catalyst is applied to an electrocatalytic reduction nitrate reaction. The catalyst is easy to prepare, economical and high in catalytic activity, has the characteristics that metal sites are not prone to loss and the like, can achieve efficient conversion of nitrate radicals into ammonia gas in alkaline electrolyte, and has high Faraday efficiency and excellent cycling stability.

Description

Technical Field

[0001] This invention relates to a method for preparing a molecular catalyst based on flexible coordination theory and its application in the electrochemical reduction of nitrate to ammonia, belonging to the field of electrocatalytic ammonia synthesis technology. Background Technology

[0002] Ammonia (NH3) is a crucial and irreplaceable raw material in modern chemical industry and agriculture. With the emergence of "hydrogen energy," an ammonia-hydrogen energy roadmap has been proposed, viewing NH3 not only as a high-value-added chemical but also as a potential energy carrier and storage medium for hydrogen. Therefore, the production of "green ammonia" has attracted attention. The mainstream traditional Haber-Bosch process for NH3 synthesis requires harsh operating conditions, leading to significant energy consumption and environmental challenges. Among emerging technologies for "green ammonia" synthesis, the electrocatalytic nitrate reduction reaction (NO3RR) is considered promising due to its high conversion efficiency at ambient temperature and pressure. Given the actual NO3 in wastewater... － The concentration range can be from 0.88 mM to 1.95 M, and wastewater from fertilizer industries, metal smelters, and nuclear power plants often contains extremely high concentrations of NO3. － (≥0.6 M), NO3RR provides an alternative solution for treating excess nitrates and converting them into valuable NH3. Therefore, electrocatalytic NO3... － The transformation of NH3 into a valuable resource provides a viable pathway to "turning waste into treasure".

[0003] One of the main challenges in developing highly efficient NO3RR electrocatalysts is achieving a balance between strong nitrate adsorption and readily convertible intermediates, while simultaneously suppressing competing reactions such as the hydrogen evolution reaction. Cobalt-coordinated molecular structures offer a promising platform for the nitrate reduction reaction (NO3RR) due to their tunable metal ligand environment and well-defined active sites. In the multi-step nitrate reduction reaction, the structural flexibility of the coordination network can influence catalytic behavior by dynamically altering local geometry and electron density, thereby changing the reaction pathway for ammonia synthesis. Therefore, selecting Co as the coordination center and suitable ligands holds promise for preparing a highly efficient molecular catalyst for the synthesis of ammonia from nitrate. Summary of the Invention The purpose of this invention is to address the problems existing in the prior art by preparing a highly efficient NO3RR electrocatalyst with a flexible coordination structure based on the flexible coordination theory, selecting Co as the coordination center and piperazine as the ligand.

[0004] To achieve the above objectives, the present invention adopts the following technical solution: This invention provides a method for preparing a molecular catalyst, comprising the following steps: Step 1): Dissolve a cobalt salt in an alcohol solvent to obtain solution A, wherein the cobalt salt is cobalt nitrate and / or its hydrate; Step 2): Dissolve piperazine in an alcohol solvent to obtain solution B; Step 3): Pour solution B into solution A and mix thoroughly to obtain a mixed solution; Step 4): The mixed solution is allowed to stand at room temperature, and the precipitate is collected. The precipitate is then washed and dried to obtain the molecular catalyst Co-Pip.

[0005] Preferably, the alcohol solvent in steps 1) and 2) is selected from one or more combinations of methanol, ethanol, and isopropanol. Preferably, the concentration of solution A in step 1) is 0.1-1 M; and / or the concentration of solution B in step 2) is 0.1-1 M.

[0006] Preferably, the molar ratio of the cobalt salt to piperazine is 1:1 to 5.

[0007] Preferably, the solvent used for washing in step 4) is methanol, the number of washing cycles is 3 to 5, the drying method is vacuum drying, the drying time is 6 to 12 hours, and the temperature is 40 to 80 ℃.

[0008] The present invention also provides a molecular catalyst prepared by the above preparation method.

[0009] The present invention also provides the application of the molecular catalyst prepared by the above preparation method in the electrochemical reduction of nitrate to ammonia.

[0010] The present invention also provides a method for electrochemically reducing nitrate to produce ammonia, comprising: dispersing the above-mentioned electrocatalyst in a solution as a working electrode, using an alkaline solution of nitrate as an electrolyte, and performing an electrochemical reduction reaction to prepare ammonia gas or an aqueous solution of ammonia.

[0011] Preferably, the preparation method uses an H-type electrolytic cell, the cathode electrolyte is an alkaline solution containing electrolyte with a pH of 8-14, and the anolyte is an alkaline aqueous solution with a pH of 8-14; the cathode and anode of the H-type electrolytic cell are separated by a proton exchange membrane (Nafion 117).

[0012] Preferably, the concentration of the nitrate solution is 0.1~1 M, the amount of the molecular catalyst is 0.1~10% of the mass of the nitrate, the pH value of the electrolyte is 8~14, the electrochemical reduction reaction adopts the constant potential electrolysis method, and the electrolysis voltage relative to the reversible hydrogen electrode potential is -1~1 V, more preferably -0.7 V.

[0013] Preferably, the molecular catalyst is dispersed in a mixed solution of deionized water, ethanol and Nafion, wherein the volume ratio is 8~12:8~12:1; the nitrate is selected from potassium nitrate and / or sodium nitrate.

[0014] Compared with the prior art, the present invention has the following beneficial effects: 1. The molecular catalyst provided by this invention exhibits high stability and excellent cycle stability; in an alkaline electrolyte, this molecular catalyst can efficiently catalyze the synthesis of ammonia from nitrates, with an ammonia yield as high as 1.57 mmol·h⁻¹. －1 ·cm －2 Furthermore, the corresponding Faraday efficiency reached over 99%.

[0015] 2. The molecular catalyst of the present invention is inexpensive, readily available, non-toxic, environmentally friendly, and can be reused multiple times. The preferred solvent for the reaction system is green, renewable, and non-toxic, and the product selection is high. Attached Figure Description

[0016] Figure 1 This is a schematic diagram of the electrochemical reduction of nitrate and the recovery of ammonium chloride.

[0017] Figure 2 The image shown is a scanning electron microscope (SEM) image of the molecular catalyst prepared in Example 1.

[0018] Figure 3 The image shows a linear sweep voltammogram of the molecular catalyst prepared in the examples.

[0019] Figure 4 The graph shows the yield and Faraday efficiency of ammonia preparation in Example 2.

[0020] Figure 5 This is a cycle performance diagram of the electrocatalytic reduction of nitrate to ammonia using a molecular catalyst. Detailed Implementation

[0021] To make the present invention more apparent and understandable, preferred embodiments are described in detail below with reference to the accompanying drawings.

[0022] Example 1 Preparation of molecular catalysts: 1) Take a certain amount of Co(NO3)3·6H2O and place it in a beaker. Then add a certain volume of anhydrous methanol. Stir magnetically for 40 min at room temperature to form a pink transparent solution with a concentration of 0.4 M. 2) Weigh a certain amount of piperazine and place it in another beaker, then add a certain volume of anhydrous methanol. Stir magnetically for 20-50 minutes at room temperature to form a colorless and transparent solution with a concentration of 0.4 M. 3) Then, while stirring continuously, quickly pour the solution obtained in step 2 into the solution in step 1 (mixed according to the molar ratio of Co:piperazine = 1:1), stir at room temperature for 40 min, and sonicate for 40 min to make the two evenly mixed; 4) The solution obtained in step 3 was left to stand at room temperature for 24 h to obtain a brown precipitate.

[0023] 5) The brown precipitate obtained in step 4 was washed 3-5 times by centrifugation with methanol, and then dried overnight in a vacuum drying oven to obtain the target product, named Co-Pip. Its morphology was observed by scanning electron microscopy; it is a flower-like nanomaterial. Its layered petals endow the material with a large specific surface area and expose more active sites, such as... Figure 1 As shown.

[0024] Example 2 Nitrate to ammonia production: First, the molecular catalyst Co-Pip (5 mg) prepared in Example 1 was uniformly dispersed in a mixed solution of deionized water (500 µL), ethanol (500 µL), and Nafion (50 µL) to prepare the working electrode. At room temperature, an H-type electrolytic cell was used, with a mixed electrolyte of sodium nitrate (0.1 M) and sodium hydroxide (1.0 M) (pH 14). Cyclic voltammetry, linear sweep voltammetry, and AC impedance spectroscopy were performed. Electrochemical tests were conducted on a CHI 760E electrochemical workstation. The electrocatalytic reduction of nitrate to ammonia was carried out using a potentiostatic electrolysis method, with a running time of 2–5 h. After the reaction, the cathode electrolyte was collected for product analysis.

[0025] Example 3 Linear sweep voltammetry was used to evaluate whether the molecular catalyst prepared in Example 1 possessed the ability to electrocatalyze the reduction of nitrate to ammonia, and to preliminarily estimate the potential range in which the reaction would occur. The linear sweep voltammetry was performed within the range of 0.1 to -0.7 V (vs. RHE), with a scan rate of 5 mV / s. In the control experiment, the cathode electrolyte was changed to 50 mL of 1 M sodium hydroxide solution, i.e., without nitrate ions, while other test conditions remained unchanged. The test results are as follows: Figure 3 As shown, the current density with a flexible coordination structure is significantly higher when nitrates are present, demonstrating the superior activity of the molecular catalyst prepared in this invention for nitrate reduction.

[0026] Example 4 According to Example 2, the concentration of NH3 in the cathodic electrolyte after the reaction was determined using the indophenol blue method, and the corresponding ammonia yield and corresponding Faraday efficiency were calculated. Figure 4The catalyst with a flexible coordination structure achieved significantly higher ammonia yields at all applied potentials, reaching 1.57 mmol·h⁻¹ at a potential of -0.7 V (vs. RHE). －1 ·cm －2 The ammonia yield was high. Furthermore, it maintained an extremely high Faraday efficiency (>99%) across the entire potential range.

[0027] Example 5 The catalyst underwent multiple cycle performance tests under the conditions described in Example 4 above, such as... Figure 5 As shown, after 20 cycles, the ammonia yield and Faraday efficiency of this molecular catalyst still maintained good performance, indicating that the catalyst has good stability.

Claims

1. A method for preparing a molecular catalyst, characterized in that, Includes the following steps: Step 1): Dissolve the cobalt salt in an alcohol solvent to obtain solution A, wherein the cobalt salt is cobalt nitrate and / or cobalt nitrate hydrate; Step 2): Dissolve piperazine in an alcohol solvent to obtain solution B; Step 3): Pour solution B into solution A and mix thoroughly to obtain a mixed solution; Step 4): The mixed solution is allowed to stand at room temperature, and the precipitate is collected. The precipitate is then washed and dried to obtain the molecular catalyst Co-Pip.

2. The preparation method according to claim 1, characterized in that, The alcohol solvents in steps 1) and 2) are selected from one or more combinations of methanol, ethanol and isopropanol.

3. The preparation method according to claim 1, characterized in that, In step 1), the concentration of solution A is 0.1-1 M; and / or, in step 2), the concentration of solution B is 0.1-1 M.

4. The preparation method according to claim 1, characterized in that, The molar ratio of cobalt salt to piperazine is 1:1~5.

5. The preparation method according to claim 1, characterized in that, The solvent used for washing in step 4) is water or alcohol, and the number of washing cycles is 3 to 5. The drying method is vacuum drying, the drying time is 6 to 12 hours, and the temperature is 40 to 80 ℃.

6. The molecular catalyst prepared by any one of claims 1 to 5.

7. The application of the molecular catalyst prepared by the method according to any one of claims 1 to 5 in the electrochemical reduction of nitrate to ammonia.

8. A method for electrochemically reducing nitrates to produce ammonia, characterized in that, include: The molecular catalyst described in claim 6 is dispersed in solution and used as a working electrode; Ammonia gas or an aqueous solution of ammonia is prepared by using an alkaline solution of nitrate as an electrolyte and carrying out an electrochemical reduction reaction.

9. The method according to claim 8, characterized in that, The concentration of the nitrate solution is 0.1~1 M, the amount of the molecular catalyst is 0.1~10% of the mass of the nitrate, the pH of the electrolyte is 8~14, the electrochemical reduction reaction is carried out by constant potential electrolysis, and the electrolysis voltage is -1~1 V relative to the reversible hydrogen electrode potential.

10. The method according to claim 8, characterized in that, The method uses an H-type electrolytic cell or a continuous flow cell. The cathode electrolyte is an alkaline solution containing electrolytes with a pH of 8-14, and the anolyte is an alkaline aqueous solution with a pH of 8-14. The molecular catalyst is dispersed in a mixed solution of deionized water, ethanol, and Nafion, with a volume ratio of 8-12:8-12:

1. The nitrate is selected from potassium nitrate and / or sodium nitrate.

Images