Preparation method and application of mononuclear nickel metal molecular catalyst

Abstract

The invention discloses a preparation method and application of a mononuclear nickel metal molecular catalyst, and belongs to the technical field of catalysis. The preparation method comprises the following steps: carrying out amidation reaction on salicylaldehyde and ethylenediamine to prepare a ligand N, N '-bis (salicylidene) ethylenediamine, namely a salen ligand; the ligand and nickel acetate tetrahydrate (Ni (OAc) 2.4 H2O) are refluxed in absolute ethyl alcohol to obtain a mononuclear nickel metal molecular catalyst, namely the Ni (salen) complex. A catalyst, carbon powder and a binder are fully dispersed in isopropanol and then uniformly dispensed on the surface of a glassy carbon electrode to prepare a working electrode. The molecular catalyst shows excellent catalytic performance and high selectivity in electro-catalysis NH3 oxidation synthesis of nitrate and nitrite, the synthesis method of the molecular catalyst is simple, and raw materials are cheap and easy to obtain.

Description

Technical Field

[0001] The present invention belongs to the field of electrocatalysis in the chemical energy industry, and particularly relates to a preparation method of a mononuclear nickel metal molecular catalyst and its application in the electrocatalytic oxidation of NH3 to synthesize nitrate and nitrite. Background Art

[0002] Nitrate is one of the most important nitrogen-containing chemicals, with an annual output of 50 million tons, playing a key role in the natural nitrogen cycle and can be produced through nitrification. The 8-electron oxidation of NH3 to produce nitrate and the 6-electron oxidation of NH3 to produce nitrite have been industrially realized through the Ostwald process, which is the second largest nitrogen-based process in the world. However, this process requires a large amount of precious metal catalysts (Pt and Rh) and is carried out under high pressure and high temperature. This means high consumption costs, as well as a considerable amount of energy input and carbon dioxide generation, and even the production of nitrous oxide gas, which exacerbates the greenhouse effect to some extent.

[0003] Converting NH3 to nitrate and nitrite through electrocatalysis is undoubtedly an attractive option. In electrocatalytic ammonia oxidation, carbon-free fuel H2 can be released at the cathode, and environmentally friendly N2 and nitrate and nitrite as a fertilizer source can be released at the anode, and the whole process is green. Therefore, how to develop a molecular catalyst with low cost, high efficiency and no pollution is the key to solving this problem. Summary of the Invention

[0004] The purpose of the present invention is to overcome the above technical problems, aiming to provide a preparation method of a mononuclear nickel metal molecular catalyst and apply it to the electrocatalytic oxidation of NH3 to synthesize nitrate and nitrite. This catalyst uses non-precious metal nickel as the metal center, greatly saving the cost of the catalyst. The present invention realizes the conversion of NH3 to nitrate and nitrite through electrocatalysis, and the whole process is green and pollution-free, avoiding a large amount of energy input and pollution in the thermal catalytic process. The catalyst provided by the present invention can highly selectively generate nitrate and nitrite, rather than low-value nitrogen or oxygen, and at the same time, the catalyst also has good stability. This also provides a good strategy for studying non-precious metal complexes for the electrocatalytic oxidation of NH3 to synthesize nitrate and nitrite.

[0005] In order to achieve the above purpose, the present invention adopts the following specific technical solutions: A preparation method of a mononuclear nickel metal molecular catalyst, comprising the following steps: (a) Under anhydrous and anaerobic conditions, mix a salicylaldehyde compound solution and an ethylenediamine solution, heat and react, and after the reaction is completed, filter, wash and dry to obtain a salen ligand; the molar ratio of the salicylaldehyde compound to ethylenediamine is 2:1; (b) The nickel salt solution is added dropwise to the salen ligand solution, and under anhydrous and anaerobic conditions, the reaction is heated. After the reaction is completed, filtration and purification are carried out to obtain the Ni(salen) complex, that is, the mononuclear nickel molecular catalyst; The molar ratio of the salen ligand to the nickel salt is 1:1.

[0006] Further, the salicylaldehyde compound is salicylaldehyde or a derivative of salicylaldehyde, and the salicylaldehyde derivative is 3,5 - di - tert - butylsalicylaldehyde; preferably, the salicylaldehyde compound is salicylaldehyde.

[0007] The nickel salt is nickel acetate tetrahydrate or nickel perchlorate hexahydrate.

[0008] Further, the solvent used for washing in step (a) is n - hexane, and in step (b), purification is carried out by flushing with a neutral alumina column. The eluent is dichloromethane and methanol with a volume ratio of 10:1.

[0009] A preparation method of a mononuclear molecule - carbon - based material heterogeneous catalyst is to fully mix the mononuclear nickel molecular catalyst, carbon black, isopropanol and a binder obtained by the above method through ultrasonic treatment to obtain the mononuclear molecule - carbon - based material heterogeneous catalyst.

[0010] The mass ratio of the mononuclear nickel molecular catalyst to carbon black is 1:2. The binder is Nafion solution, and the volume ratio of isopropanol to the binder is 1:40 - 50, and the ultrasonic time is 1 - 2 hours.

[0011] Further, the amounts of the mononuclear nickel molecular catalyst, carbon black, isopropanol and the binder are 2 mg:4 mg:980 μL:20 μL.

[0012] A mononuclear molecule - carbon - based material heterogeneous catalyst is prepared by the above method.

[0013] An electrocatalytic NH₃ oxidation electrode is prepared from the above heterogeneous catalyst.

[0014] The preparation method of the electrocatalytic NH₃ oxidation electrode: The above heterogeneous catalyst is drop - coated on the surface of a glassy carbon electrode and dried to obtain it. The loading amount of the heterogeneous catalyst is 0.25 mg / cm 2 .

[0015] An application of the electrocatalytic NH₃ oxidation electrode is that the electrocatalytic NH₃ oxidation electrode is applied to the electrocatalytic oxidation of NH₃ to synthesize nitrates and nitrites.

[0016] The method for applying the electrocatalytic NH3 oxidation electrode to the electrocatalytic synthesis of nitrates and nitrites from NH3 oxidation is as follows: A three-electrode system is adopted, where the electrocatalytic NH3 oxidation electrode serves as the working electrode, silver / silver chloride serves as the reference electrode, a platinum mesh serves as the counter electrode, and an ammonium sulfate aqueous solution serves as the electrolyte. Electrocatalytic NH3 oxidation is carried out in an electrolytic cell, and the target products are nitrates and nitrites. The electrolytic cell is an H-type electrolytic cell. The concentration of the ammonium sulfate aqueous solution is 0.137 mM, the pH of the electrolyte is 9.0, the reaction potential is 1.10 V - 1.30 V vs. Ag / AgCl, the reaction time is 1 - 3 hours, and the reaction temperature is 20 - 30 °C.

[0017] One of the objectives of the present invention is to provide a preparation method of a mononuclear nickel metal molecular catalyst, which includes the following steps: (a) Dissolve 20 mmol of salicylaldehyde and 10 mmol of ethylenediamine in methanol respectively. Under an anhydrous and anaerobic condition, reflux the mixed solution at a temperature of 70 °C for 2 hours. After filtration, washing, and drying, the salen ligand can be obtained. (b) Dissolve 2 mmol of nickel acetate tetrahydrate (Ni(OAc)2·4H2O) in methanol, and drop it into ethanol containing the salen ligand. Under an anhydrous and anaerobic condition, reflux the obtained mixed solution at 70 °C for 2 hours. Collect the red precipitate by filtration to obtain the crude Ni(salen) complex. The molar ratio of the salen ligand to nickel acetate tetrahydrate is 1:1. (c) Dissolve the crude Ni(salen) complex in a mixed solution of a small amount of methanol and dichloromethane. Wash the obtained solution through a neutral alumina column, rotary evaporation, filtration, and drying to obtain the pure Ni(salen) complex.

[0018] Preferably, in step (a), the amount of methanol is 10 mL, the solvent used for washing is n-hexane, and the amount of n-hexane used for each wash is 5 mL, for a total of three times.

[0019] Preferably, in step (b), the dropping rate is 2 - 3 drops per second.

[0020] Preferably, in step (c), the eluent required for the neutral alumina column is dichloromethane and methanol, and the volume ratio of the two is 10:1. The column loading method is wet column loading, and the drying time is 24 hours.

[0021] The second object of the present invention is to provide a method for preparing an electrocatalytic NH3 oxidation electrode. The specific steps are as follows: Mix the above-mentioned mononuclear nickel metal molecular catalyst (2.0 mg), isopropanol, carbon black (CB, 4.0 mg) and binder, ultrasonically disperse to make them evenly mixed and drop-coat on the surface of the treated glassy carbon electrode (GC). After irradiating with an infrared lamp for 5 minutes, cool to room temperature to obtain the electrocatalytic NH3 oxidation electrode.

[0022] Preferably, the treatment method of the glassy carbon electrode is as follows: Prepare a glass plate, lay a layer of suede on the glass plate as a polishing pad, put a small amount of alumina polishing powder on the polishing pad, and add a small amount of deionized water to moisten it. Polish the glassy carbon electrode in a clockwise direction in a figure-eight shape for about 10 minutes. After polishing, ultrasonically clean with a 1:1 ethanol and deionized water mixture for 2 - 3 minutes. This can effectively remove surface impurities and obtain a smooth and uniform electrode surface.

[0023] Preferably, the ultrasonic dispersion time is 1 hour, the binder is Nafion solution, the volume ratio of isopropanol to the binder is 1:50, the dispersion liquid is evenly dropped on the surface of the glassy carbon electrode in 5 times, and the catalyst loading is 0.25 mg / cm 2 .

[0024] The third object of the present invention is a method for electrocatalytic synthesis of nitrate and nitrite by NH3 oxidation. The specific steps are as follows: Adopt a three-electrode system, with a silver / silver chloride electrode as the reference electrode, the above-mentioned electrocatalytic NH3 oxidation electrode as the working electrode, and a platinum mesh as the counter electrode. In the electrolytic cell, ammonia buffer solution is oxidized by electrocatalysis to generate nitrate and nitrite.

[0025] Preferably, the electrolyte is ammonia buffer solution. That is, the preparation of 0.1 M NH3 buffer solution is to adjust the pH of 0.137 mM (NH4)2SO4 to 9.0 with 1 M NaOH. The reaction potential is 1.10 V - 1.30 V vs. Ag / AgCl, the reaction time is 1 hour, and the reaction temperature is 25 °C.

[0026] Compared with the prior art, the present invention has the following beneficial effects: (1) In the preparation method of the mononuclear nickel metal molecular catalyst disclosed in the present invention, the mononuclear nickel molecular catalyst has good stability in the electrocatalytic NH3 oxidation system, which can avoid the decoordination phenomenon of the molecular catalyst during the electrolysis process, that is, the molecular structure remains unchanged before and after electrolysis.

[0027] (2) The molecular catalyst and inorganic material carbon powder were thoroughly mixed by a centrifuge to enhance its conductivity, and a heterogeneous catalyst of mononuclear molecule-carbon-based material was prepared. The current density of the catalyst Ni(salen)@CB can reach as high as 5.7 mA / cm 2 (for a glassy carbon electrode with a diameter of 3 mm), and during the electrolysis process lasting up to 4 hours, the current density did not show a significant decrease.

[0028] (3) The heterogeneous catalyst of mononuclear molecule-carbon-based material prepared in the present invention, in 0.1 M NH3 buffer solution, at a potential of 1.15 V vs. Ag / AgCl, the Faraday efficiency of the catalyst Ni(salen)@CB for producing nitrate and nitrite is as high as 84.15%, indicating that this catalyst has very high activity and selectivity for NH3.

[0029] (4) So far, there are only two reports on the electrocatalytic oxidation of NH3 to synthesize nitrate and nitrite using molecular catalysts, namely ruthenium (noble metal) and copper (non-noble metal). The present invention uses non-noble metal nickel as the center of the molecular catalyst for the first time, providing a brand-new idea for the future research on the electrocatalysis of nickel-based molecular catalysts to achieve high-efficiency production of nitrate and nitrite. Description of the Drawings

[0030] Figure 1 (a) Ultraviolet spectrum of Ni(salen) complex and salen ligand; (b) Infrared spectrum of Ni(salen) complex and ligand.

[0031] Figure 2 is the X-ray photoelectron spectrum of Ni(salen).

[0032] Figure 3 (a) Scanning electron micrographs of Ni(salen) and (b) Ni(salen)@CB uniformly dispersed on the surface of the glassy carbon electrode.

[0033] Figure 4 is the LSV curves of Ni(salen)@CB / GC, Ni(tbu-salen)@CB / GC, and GC in 0.1 M NH3 buffer solution.

[0034] Figure 5 is the impedance diagrams of Ni(salen)@CB / GC and Ni(tbu-salen)@CB / GC at a potential of 1.15 V vs. Ag / AgCl.

[0035] Figure 6Faraday efficiency diagrams of nitrate and nitrite for Ni(salen)@CB / GC and Ni(tbu-salen)@CB / GC at five potentials.

[0036] Figure 7 Cyclic voltammograms were recorded to investigate the presence or absence of Ni(salen)@CB / GC in 0.1 M NH3 buffer solution.

[0037] Figure 8 Among them, (a) i-t curve of Ni(salen)@CB / GC at a constant potential of 1.15 V vs. Ag / AgCl for 1 hour; (b) Cyclic voltammetry rinse experiment of Ni(salen)@CB / GC after 1 hour of constant potential electrolysis. Detailed implementation manners

[0038] To make the objectives, technical solutions and advantages of the present invention clearer, the present invention will be further described below in conjunction with the accompanying drawings and specific examples. Those skilled in the art should understand that the described embodiments are only for helping to understand the present invention and should not be regarded as specific limitations on the present invention. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative efforts fall within the scope of protection of the present invention.

[0039] For the process parameters not specified under specific conditions in the following examples, they are usually in accordance with conventional conditions, and all reagents, drugs, materials, and instruments can be obtained through commercial means. Example 1

[0040] Preparation of mononuclear nickel molecular catalyst, the specific steps are as follows: (1) Synthesis of N,N'-bis(salicylidene)ethylenediamine, namely salen ligand: First, dissolve salicylaldehyde (20 mmol) in 10 mL of methanol and place it in a 25 mL round-bottomed two-necked flask. In another separate 25 mL flask, dissolve ethylenediamine (10 mmol) in 10 mL of methanol. Under anhydrous and anaerobic conditions, drop the salicylaldehyde solution into the ethylenediamine solution, and reflux the mixed solution at a temperature of 70 °C for 2 hours. After the reflux is completed, it can be clearly observed that a yellow precipitate is formed. Then place the reaction flask in an ice-water bath for 10 minutes, and a large amount of yellow precipitate will be produced. At this time, filter it with a Buchner funnel. The obtained yellow precipitate is washed with a small amount of n-hexane several times, and then the precipitate is placed in a vacuum drying oven for drying to obtain a pure salen ligand.

[0041] (2) Synthesis of N,N'-bis(salicylidene)ethylenediamine-nickel(II), i.e., Ni(salen) complex: First, in a 25 mL two-necked round-bottom flask, dissolve the salen ligand (2 mmol) synthesized in the first step in 10 mL of ethanol. Take another 25 mL two-necked round-bottom flask and dissolve 2 mmol of Ni(OAc)2·4H2O in 10 mL of ethanol. Then, under anhydrous and anaerobic conditions, slowly add the ethanol solution of Ni(OAc)2·4H2O dropwise to the salen ligand, and reflux the resulting mixture at 70 °C for 2 hours. Precipitation will occur during the reaction. Filter and collect the red precipitate through a Buchner funnel. Dissolve the obtained red precipitate in a mixed solution of a small amount of methanol and dichloromethane. Wash the resulting solution through a neutral alumina column, rotary evaporate, filter, and dry to obtain the pure Ni(salen) complex.

[0042] Preparation of heterogeneous catalyst of mononuclear molecule-carbon-based material: Mix the above-mentioned mononuclear nickel metal molecule catalyst (2.0 mg), isopropanol (980 μL), carbon black (CB, 4.0 mg), and binder (20 μL), and ultrasonically disperse for 1 hour to make them evenly mixed to obtain Ni(salen)@CB. Comparative Example 1

[0043] To prove that the mononuclear nickel metal molecule catalyst without substituents has higher catalytic activity than the molecular catalyst with substituents, the present invention prepared a mononuclear nickel metal molecule catalyst with substituents by a similar method.

[0044] (1) Synthesis of N,N'-bis(3,5-di-tert-butylsalicylidene)ethylenediamine, i.e., tbu-salen ligand: First, in a 25 mL two-necked round-bottom flask, fully dissolve ethylenediamine (4.27 mmol) in 10 mL of methanol and stir the resulting solution at room temperature for half an hour. Take another 10 mL two-necked round-bottom flask and completely dissolve 8.54 mmol of 3,5-di-tert-butylsalicylaldehyde in methanol. Under anhydrous and anaerobic conditions, slowly add the 3,5-di-tert-butylsalicylaldehyde solution dropwise to the ethylenediamine solution, and react the resulting solution at room temperature for 4 hours. Remove a small amount of solvent from the reaction solution under vacuum to obtain the yellow solid of N,N'-bis(3,5-di-tert-butylsalicylidene)ethylenediamine. After washing with n-hexane multiple times, obtain the pure product.

[0045] (2) Synthesis of N,N'-bis(3,5-di-tert-butylsalicylidene)ethylenediamine-nickel(II), i.e., Ni(tbu-salen) complex: First, take a 100 mL two-necked round-bottom flask and completely dissolve 0.20 mmol of N,N'-bis(3,5-di-tert-butylsalicylidene)ethylenediamine in 30 ml of methanol. Then add 56 μL of triethylamine solution thereto. Take another 10 mL two-necked round-bottom flask and fully dissolve 0.20 mmol of Ni(ClO4)2·6H2O in 10 mL of methanol. Then, under anhydrous and anaerobic conditions, slowly add the Ni(ClO4)2·6H2O solution dropwise to the N,N'-bis(3,5-di-tert-butylsalicylidene)ethylenediamine solution, and react the mixed solution at 70 °C for 24 hours. Then, filter the reacted mixture while it is hot with a Buchner funnel, retain the dark green solid and wash it several times with n-hexane to obtain the pure product. Finally, dry the product in a vacuum drying oven for 24 hours to obtain a dark green powder.

[0046] Preparation of a heterogeneous catalyst of a mononuclear molecule-carbon-based material with substituents: As a comparative example to Example 1, the preparation process of the heterogeneous catalyst is similar to that of Example 1, except that the catalyst used is Ni(tbu-salen) prepared in Comparative Example 1 to obtain Ni(tbu-salen)@CB. Example 2

[0047] Preparation of the Ni(salen)@CB / GC electrode, the specific steps are as follows: The treatment method of the glassy carbon electrode is specifically as follows: Prepare a glass plate, place a layer of suede on the glass plate as a polishing pad, put a small amount of alumina polishing powder on the polishing pad, and add a small amount of deionized water to wet it. Polish the glassy carbon electrode in a clockwise direction in an "8" shape for about 10 minutes. After polishing, ultrasonically clean it with a 1:1 ethanol and deionized water mixture for 2 - 3 minutes. This can effectively remove surface impurities and obtain a smooth and uniform electrode surface.

[0048] Take 10 μL of the mononuclear molecule-carbon-based material heterogeneous catalyst prepared in Example 1, i.e., Ni(salen)@CB, and evenly drop-coat it on the surface of the glassy carbon electrode (GC) to prepare the Ni(salen)@CB / GC electrode. Comparative Example 2

[0049] As a comparative example to Example 2, the preparation process of the heterogeneous catalyst is similar to that of Example 2, except that the heterogeneous catalyst of the mononuclear molecule-carbon-based material with substituents used is Ni(tbu-salen)@CB prepared in Comparative Example 1 to obtain Ni(tbu-salen)@CB / GC. Example 3

[0050] Electrocatalytic oxidation of NH3 to synthesize nitrate and nitrite.

[0051] All electrochemical tests in this invention were carried out on a Shanghai Chenhua CHI 660E electrochemical workstation under room temperature conditions, using a standard three-electrode system. The working electrode was the Ni(salen)@CB / GC electrode prepared in Example 2, the reference electrode was an Ag / AgCl electrode (saturated KCl, 0.197 V vs. NHE) produced by Gaushieldt Co., Ltd., and the counter electrode was a platinum mesh electrode. The electrolytic cell was an H-type electrolytic cell, and the cathode chamber and the anode chamber were separated by a Nafion membrane. The electrolyte was 0.1 M NH3 buffer solution obtained by adjusting the pH of 0.137 mM (NH4)2SO4 to 9.0 with 1 M NaOH.

[0052] Linear sweep voltammetry (LSV) test: The scanning rate was 100 mV / s, and a linear voltammogram was made in the voltage range of 0 - 1.5 V vs. Ag / AgCl.

[0053] Calculation of the Faraday efficiency for electrocatalytic synthesis of nitrate and nitrite from NH3: Potentiostatic electrolysis was carried out for 1 hour in the voltage range of 1.10 - 1.30 vs. Ag / AgCl. The obtained current-time curve (i-t curve) was analyzed, and the nitrate and nitrite in the anode chamber after electrolysis were quantified by using a UV chromogenic agent for color development.

[0054] Impedance spectroscopy (ESI): The impedance of the Ni(salen)@CB / GC electrode was measured at a potential of 1.5 V vs. Ag / AgCl.

[0055] Cyclic voltammetry (CV) test: The scanning rate was 50 mV / s, and a cyclic voltammogram was made in the voltage range of 0 - 1.6 V vs. Ag / AgCl. Comparative Example 3

[0056] As a comparative example to Example 3, the electrochemical test method (LSV test) for electrocatalytic oxidation of NH3 to synthesize nitrate and nitrite was similar to that of Example 3, except that the working electrode used was the Ni(tbu-salen)@CB / GC prepared in Comparative Example 2. Comparative Example 4

[0057] As a comparative example to Example 3, the electrochemical test method (LSV test) for electrocatalytic oxidation of NH3 to synthesize nitrate and nitrite was similar to that of Example 3, except that the working electrode used was a glassy carbon electrode (GC). Comparative Example 5

[0058] As a comparative example in Example 3, the electrochemical test method (ESI test) for electrocatalytic oxidation of NH3 to synthesize nitrates and nitrites is similar to that in Example 3, except that the electrode used is Ni(tbu-salen)@CB / GC prepared in Comparative Example 2. Comparative Example 6

[0059] As a comparative example in Example 3, the electrochemical test method (Faraday efficiency calculation) for electrocatalytic oxidation of NH3 to synthesize nitrates and nitrites is similar to that in Example 3, except that the electrode used is Ni(tbu-salen)@CB / GC prepared in Comparative Example 2. Comparative Example 7

[0060] As a comparative example in Example 3, the electrochemical test method (CV test) for electrocatalytic oxidation of NH3 to synthesize nitrates and nitrites is similar to that in Example 3, except that the electrode used is a glassy carbon electrode (GC). Characterization and electrocatalytic analysis of the catalyst in the examples:

[0061] Figure 1 In (a), it is the UV spectrum of the Ni(salen) complex and the salen ligand prepared in Example 1. For the salen ligand, two broad peaks are observed at 255 nm and 318 nm, belonging to π - π * transitions. Compared with the salen ligand, the peak of the Ni(salen) complex at 406 nm is attributed to n - π * transitions. Relative to the absorption peak of the salen ligand at 318 nm, the UV absorption peak of the Ni(salen) complex shows an obvious shift, approximately at 330 nm. This is due to the coordination of nickel ions with the salen ligand, which also greatly reduces the π * orbital energy, so that π - π * transitions can occur at lower energies. Figure 1 In (b), it is the infrared spectrum of the Ni(salen) complex and the salen ligand prepared in Example 1. The Ni(salen) complex shows some low-intensity bands in the range of 2800 - 3100 cm −1 These bands correspond to the stretching bands of aromatic CH(=C-H) and aliphatic (-C-H). The bands observed in the region from 1450 cm −1 to 1600 cm −1 are the (C-C) stretching vibrations in the aromatic ring. The strong band at 1624 cm −1 is attributed to the stretching vibration of the imine group in the complex shifting to a higher wavenumber due to the coordination of nitrogen atoms with nickel ions. Compared with the salen ligand, at 1127 cm−1 The band at

[0062] Figure 2 is the X-ray photoelectron spectroscopy pattern of the Ni(salen) complex prepared in Example 1. Figure 2 In (d), the full XPS spectrum of the Ni(salen) complex shows that the Ni(salen) complex is composed of four elements, Ni, C, O, and N, and there are no other impurities. In the Ni 2p spectrum, as Figure 2 shown in (b), the peaks at electron binding energies of 855.25 eV and 872.43 eV are attributed to Ni 2p 3 / 2 and Ni 2p 1 / 2 respectively. Meanwhile, the corresponding satellite peaks are at 860.97 eV and 878.6 eV respectively. These results indicate that the Ni element exists in the form of Ni 2+ . The C 1s spectrum mainly consists of three peaks. As Figure 2 shown in (a), the peak at 284.8 eV is attributed to the C-C bond in the salen structure. The peak at an electron binding energy of 285.97 eV can be attributed to the C-N bond, and the peak at an electron binding energy of 289.40 eV can be attributed to the phenolic hydroxyl group, i.e., the C-O bond. In the high-resolution O 1s spectrum, as Figure 2 shown in (c), the peak at an electron binding energy of 531.14 eV is attributed to the oxygen coordinated with metal ions. The peak at 532.24 eV is the O-C bond, and the peak at an electron binding energy of 532.9 eV can be attributed to adsorbed water molecules. Based on the above analysis, it is further proved that the metal ion Ni center of the Ni(salen) complex is divalent and has a stable coordination environment.

[0063] Figure 3 are the scanning electron microscopy images of (a) Ni(salen) and (b) Ni(salen)@CB uniformly dispersed on the surface of the glassy carbon electrode prepared in Example 1. Figure 3 In (a), it shows that the shape of the Ni(salen) complex presents a blocky shape. Figure 3 In (b), it can be clearly observed that the Ni(salen) complex and carbon black (CB) can be uniformly distributed on the surface of the glassy carbon electrode, and the shape also presents a blocky shape.

[0064] Figure 4It is the LSV curve of Example 3, Comparative Example 3 and Comparative Example 4 in 0.1 M NH3 buffer. When the surface of the glassy carbon electrode is not loaded with any catalyst, it can be clearly observed through the LSV image that its starting potential is much higher than that of the electrode loaded with the catalyst, and its catalytic current density is also much lower, which shows that the presence of the catalyst can significantly improve its catalytic activity. Then, by comparing the LSV images of the two catalysts of Example 3 and Comparative Example 3, it is found that Ni(salen)@CB / GC has a higher oxidation activity for NH3 than Ni(tbu-salen)@CB / GC. Compared with Comparative Example 3, Example 3 shows a very low starting potential. It is generally believed that the lower the starting potential, the better the catalytic activity. At the same time, Example 3 also shows a higher catalytic current. This once again shows that the Ni(salen) molecular catalyst without substituents exhibits better catalytic performance.

[0065] Figure 5 The impedance diagram of Example 3 and Comparative Example 5 at 1.15 V vs. Ag / AgCl potential. It is generally believed that the smaller the impedance value of the catalyst at the optimal performance, the lower the energy lost in the catalytic process, that is, the higher the catalytic efficiency. It can be clearly observed from the figure that the impedance radius of Ni(salen)@CB / GC is smaller, that is, the impedance is lower.

[0066] Figure 6 The Faraday efficiency diagram of nitrate and nitrite at 5 potentials for Example 3 and Comparative Example 6. When the concentration of the electrolyte is 0.1 M, both catalysts are more inclined to convert NH3 into nitrate (NO3 - ) and produces very little nitrite (NO2 - ), but Ni(salen)@CB / GC clearly exhibits a higher Faradaic efficiency. At a potential of 1.15 V vs. Ag / AgCl, Ni(salen)@CB / GC generates NO3 - The Faraday efficiency of the catalyst is as high as 78.01%, and the total Faraday efficiency is as high as 84.15%, which is 23% higher than the total Faraday efficiency of Ni(tbu-salen)@CB / GC. This also shows that the unsubstituted Ni(salen) molecular catalyst has a high selectivity in catalyzing the conversion of NH3 into nitrate and nitrite.

[0067] Figure 7The cyclic voltammetry curves of Example 3 and Comparative Example 7 in 0.1 M NH3 buffer were investigated. When only a glassy carbon electrode without a catalyst load was used, no obvious NH3 oxidation peak was observed. When the electrode selected was Ni(salen)@CB / GC prepared in Example 2, an obvious NH3 oxidation peak was observed at 1.4 V vs. Ag / AgCl, and its catalytic current density was also increased a lot, which also proved again that Ni(salen)@CB / GC has high catalytic activity.

[0068] Figure 8 In (a), it is the change curve of time with current density at a potential of 1.15 V vs. Ag / AgCl in Example 3, that is, the i-t curve. It can be clearly observed in the figure that its catalytic current density reached 5.7 mA / cm 2 , exceeding the catalytic current density of molecular catalysts for the oxidation of NH3 to nitrate and nitrite reported so far. Figure 8 In (b), it is the cyclic voltammetry rinsing experiment of the Ni(salen)@CB / GC electrode prepared in Example 2 after electrolysis. The stability of molecular catalysts is generally verified by the CV rinsing experiment. It can be clearly observed from Figure 8 (b) that after the electrode after electrolysis was rinsed many times, its CV curve was the same as that of the glassy carbon electrode itself. This proves that during electrolysis, the molecular catalyst did not undergo a decoordination phenomenon to generate metal oxides, indicating that the stability of Ni(salen)@CB / GC is also very high.

[0069] The Ni(salen)@CB / GC electrode prepared in the present invention is used for the electrocatalytic oxidation of NH3 to synthesize nitrate and nitrite, and can maintain the stability of its molecular structure before and after the reaction. At the same time, it can efficiently and solely produce nitrate (almost no nitrite), and its total Faraday efficiency can reach 84.15%. It is worth noting that the Ni(salen)@CB / GC electrode has a small impedance at a potential of 1.15 V vs. Ag / AgCl, which also results in a small initial potential of the catalyst during the catalytic process, and its catalytic current density can be as high as 5.7 mA / cm 2 . The present invention uses non-precious metal nickel as the center of the molecular catalyst for the first time, providing a new idea for the future research on the electrocatalytic production of nitrate and nitrite with high efficiency by nickel-based molecular catalysts.

Claims

1. A preparation method of a mononuclear nickel metal molecular catalyst, characterized in that, The following steps are involved: (a) under anhydrous and oxygen-free conditions, mixing a salicylaldehyde compound solution and an ethylenediamine solution, heating for reaction, filtering, washing, and drying after the reaction to obtain a salen ligand; the molar ratio of the salicylaldehyde compound to the ethylenediamine is 2:1; (b) adding a nickel salt solution dropwise to a salen ligand solution, and heating the solution under anhydrous and oxygen-free conditions for reaction; filtering and purifying the solution after the reaction is completed to obtain a Ni(salen) complex, i.e., a mononuclear nickel molecular catalyst; The molar ratio of the salen ligand to the nickel salt is 1:

1.

2. The preparation method of a mononuclear nickel metal molecular catalyst according to claim 1, characterized in that: The salicylaldehyde compound is salicylaldehyde or a derivative of salicylaldehyde, and the salicylaldehyde derivative is 3,5-di-tert-butyl salicylaldehyde; The nickel salt is nickel acetate tetrahydrate or nickel perchlorate hexahydrate.

3. The preparation method of a mononuclear nickel metal molecular catalyst according to claim 1, wherein: The solvent used for washing in step (a) is n-hexane, and the purification in step (b) is performed by washing and purifying with a neutral alumina column, and the eluent is a mixture of dichloromethane and methanol in a volume ratio of 10:

1.

4. A method for preparing a heterogeneous catalyst of a mononuclear molecule-carbon-based material, characterized in that: The mononuclear nickel molecule catalyst prepared by the method of any one of claims 1 to 3 is fully mixed with carbon black, isopropanol and a binder by ultrasonication to obtain a mononuclear molecule-carbon-based material heterogeneous catalyst; The mass ratio of the mononuclear nickel molecule catalyst to carbon black is 1:

2.

5. The preparation method of a heterogeneous catalyst of mononuclear molecule-carbon-based material according to claim 4, characterized in that: The binder is a Nafion solution, the volume ratio of isopropanol to the binder is 1:(40-50), and the ultrasonic time is 1-2 hours.

6. A heterogeneous catalyst of mononuclear molecule-carbon-based material, characterized in that: The heterogeneous catalyst is prepared by the method according to claim 4 or 5.

7. An electrocatalytic NH3 oxidation electrode, characterized in that: The electrocatalytic NH3 oxidation electrode is made from the heterogeneous catalyst described in claim 6.

8. The electrocatalytic NH3 oxidation electrode according to claim 7, wherein: The preparation method of the electrocatalytic NH3 oxidation electrode: Drop the heterogeneous catalyst described in Claim 6 on the surface of a glassy carbon electrode, and after drying, obtain the electrocatalytic NH3 oxidation electrode, with the loading amount of the heterogeneous catalyst being 0.25 mg / cm 2 .

9. Use of an electrocatalytic NH3 oxidation electrode according to claim 7, characterized in that: The electrocatalytic NH3 oxidation electrode is used in the electrocatalytic NH3 oxidation to synthesize nitrate and nitrite.

10. The application of an electrocatalytic NH3 oxidation electrode according to claim 7, characterized in that: The method for using the electrocatalytic NH3 oxidation electrode in electrocatalytic NH3 oxidation to synthesize nitrate and nitrite is as follows: a three-electrode system is used, the electrocatalytic NH3 oxidation electrode is a working electrode, silver / silver chloride is a reference electrode, a platinum mesh is a counter electrode, and the electrolyte is an ammonium sulfate aqueous solution, and electrocatalytic NH3 oxidation is carried out in an electrolytic cell, and the target products are nitrate and nitrite; The electrolytic cell is an H-type electrolytic cell, the concentration of the ammonium sulfate aqueous solution is 0.137 mM, the pH of the electrolyte is 9.0, the reaction potential is 1.10 V - 1.30 V vs. Ag / AgCl, the reaction time is 1-3 hours, and the reaction temperature is 20-30°C.

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