Transition metal single atom catalysts, methods of making and use

Abstract

The disclosure provides a transition metal single-atom catalyst, a preparation method and application, wherein the method for preparing the transition metal single-atom catalyst by using a heteroatom substitution method comprises the following steps: adding zinc nitrate hexahydrate and a phosphorus source into an alcohol solvent, then adding an alcohol solution of an imidazole organic matter, and uniformly mixing to obtain a first suspension liquid; washing, purifying and drying a first precipitate obtained by standing the first suspension liquid to obtain a first precursor; dispersing the first precursor, a surfactant and an iron salt into an alcohol solvent, mixing and stirring to obtain a second suspension liquid; adding an alcohol solution of triethylamine into the second suspension liquid to perform a reaction, and collecting a second precipitate obtained by the reaction; washing and drying the second precipitate to obtain a second precursor; and calcining the second precursor to obtain a first transition metal single-atom catalyst, wherein the calcining is performed in an inert gas atmosphere.

Description

Technical Field

[0001] This disclosure belongs to the field of electrocatalytic materials technology, specifically relating to a transition metal single-atom catalyst, its preparation method, and its application. Background Technology

[0002] Due to the combustion of fossil fuels, excessive use of nitrogen-rich fertilizers, and the discharge of industrial wastewater, large amounts of nitrogenous waste accumulate in the environment in the form of nitrates, leading to severe nitrate pollution of surface and groundwater. Ion exchange, reverse osmosis, and electrodialysis have been reported as methods for removing nitrate pollutants from water; however, these methods are merely physicochemical approaches to separate and concentrate nitrates, without completely destroying the nitrate structure, resulting in large amounts of secondary brine waste requiring expensive post-treatment. Furthermore, biological denitrification technologies to remove nitrates into harmless nitrogen dioxide require continuous monitoring, generate sludge, and have long processing times, limiting their widespread application. Multiphase catalytic systems offer high efficiency for catalytic hydrogenation removal of nitrates, but the continuous feeding of hydrogen poses a potential explosion hazard and operational complexity.

[0003] Currently, the complex electron transfer pathways and competitive hydrogen evolution reactions in the electrocatalytic reduction of nitrates severely affect the adsorption of nitrate reactants on the catalyst surface, making the development of high-performance catalysts to improve the yield of ammonia from nitrate reduction a challenging task. Summary of the Invention

[0004] In view of the above-mentioned technical problems, the main objective of this disclosure is to provide a method for electrocatalytic reduction of nitrate to produce ammonia, so as to at least partially solve the aforementioned technical problems.

[0005] To solve the above-mentioned technical problems, the technical solution provided in this disclosure is as follows:

[0006] As a first aspect of this disclosure, a method for preparing transition metal single-atom catalysts using a heteroatom substitution method is provided, comprising:

[0007] Zinc nitrate hexahydrate and phosphorus source were added to an alcohol solvent, followed by an alcohol solution of imidazole organic compounds. The mixture was stirred until homogeneous to obtain the first suspension.

[0008] The first precipitate obtained after the first suspension has been allowed to stand is washed, purified, and dried to obtain the first precursor.

[0009] The first precursor, surfactant, and iron salt are dispersed in an alcohol solvent, mixed and stirred to obtain a second suspension;

[0010] A triethylamine alcohol solution was added to the second suspension to carry out the reaction, and the second precipitate obtained from the reaction was collected.

[0011] The second precipitate was washed and dried to obtain the second precursor.

[0012] The second precursor is calcined to obtain the first transition metal single-atom catalyst, wherein the calcination is carried out in an inert gas atmosphere.

[0013] In one embodiment, the phosphorus source includes any one of triphenylphosphine, potassium dihydrogen phosphate, sodium dihydrogen phosphate, phytic acid, and sodium hypophosphite.

[0014] In one embodiment, the amount of phosphorus source added is 0.5~2 g;

[0015] The dosage of zinc nitrate hexahydrate is 2-4 g;

[0016] The amount of imidazole organic compounds added is 2~4 g.

[0017] Imidazole organic compounds include any one or more of 2-methylimidazole, 1,2-dimethylimidazole, and 1,3-dimethylimidazole;

[0018] The above-mentioned alcohol solutions and alcohol solvents belong to alcohol reagents, wherein the alcohol reagents are selected from any one or more of methanol, ethanol, ethylene glycol, isopropanol, and hexanediol;

[0019] Iron salts include any one of nitrates, chlorides, sulfates, or organic salts;

[0020] Surfactants include any one or more of cationic surfactants, anionic surfactants, organic weak acids, and organic weak base salts, preferably including any one or more of polyvinylpyrrolidone, sodium polystyrene sulfonate, tetradecylammonium bromide, hexadecyltrimethylammonium bromide, citric acid, and ascorbic acid;

[0021] The mass ratio of the first precursor to the iron salt is 1:1 to 20:1;

[0022] The mass ratio of the first precursor to the surfactant is 1:1 to 50:1;

[0023] The amount of triethylamine added is 2-8 ml;

[0024] In one embodiment, the standing method is to let it stand at 30-40°C for 2-3 hours;

[0025] The purification method is centrifugation, with a preferred centrifugation speed of 7000~9000 r / min for 20~30 min;

[0026] The drying method is vacuum drying, the drying temperature is preferably 60~70 ℃, and the vacuum degree is preferably <10 Pa.

[0027] In one embodiment, the calcination temperature is preferably 800~1000 ℃, and the calcination time is preferably 3~4 h.

[0028] As a second aspect of this disclosure, a method for preparing transition metal single-atom catalysts using a metal coupling method is provided, comprising:

[0029] The alcoholic solution of zinc nitrate hexahydrate was mixed evenly with the alcoholic solution of imidazole organic compounds to obtain the third suspension.

[0030] The third precipitate obtained after the third suspension has been allowed to stand is washed, purified, and dried to obtain the third precursor.

[0031] After uniformly mixing the third precursor with an alcoholic solution of ferric nitrate nonahydrate, the mixture was added to an alcoholic solvent and stirred to obtain the fourth suspension.

[0032] Add an alcoholic solution of triethylamine to the fourth suspension to carry out the reaction, and collect the fourth precipitate obtained from the reaction;

[0033] The fourth precipitate was washed and dried to obtain the fourth precursor;

[0034] After grinding and first calcination of the fourth precursor, it was uniformly dispersed with the nano metal salt solution in an alcohol solvent and then ultrasonically treated to obtain a black suspension.

[0035] The black suspension was evaporated and dried to obtain a black powder, which was then subjected to a second calcination treatment in a gas atmosphere of mixed argon and hydrogen to obtain a second transition metal single-atom catalyst.

[0036] In one embodiment, the nano-metal salt includes any one of ruthenium chloride, palladium chloride, iridium chloride, rhodium chloride, silver chloride, copper chloride, nickel chloride, or cobalt chloride.

[0037] In one embodiment, the amount of the fourth precursor added is 190-210 mg;

[0038] The preferred concentration of the nano-metal salt solution is 8-12 mg / L;

[0039] The preferred amount of nano-metal salt solution to be added is 1~20 ml.

[0040] In one embodiment, the temperature of the first calcination treatment is preferably 800~1000 ℃, the temperature of the second calcination treatment is preferably 300~500 ℃, the calcination time of the first calcination treatment and the second calcination treatment is preferably 1~4 h, and the heating rate is 1~20 ℃ / min.

[0041] As a third aspect of this disclosure, a transition metal single-atom catalyst is provided, which is prepared by the method described above. The transition metal single-atom catalyst is a catalyst with asymmetric coordinated Fe active sites; or the transition metal single-atom catalyst is a synergistic porous catalyst in which Fe active sites are coupled with metal nanoparticles, clusters, or non-ferrous metal centers, wherein the metal nanoparticles and iron single atoms are uniformly dispersed on a carbon-nitrogen support.

[0042] As a fourth aspect of this disclosure, a method for preparing an activated electrode is provided, comprising:

[0043] A homogeneous mixture was prepared by dispersing a transition metal single-atom catalyst in a solvent containing a binder.

[0044] The mixture is coated onto the surface of a carrier to form an activated electrode.

[0045] In one embodiment, the solvent containing the binder is a mixture of isopropanol and naphthol, wherein the volume ratio of isopropanol to naphthol is 10:1 to 50:1.

[0046] The loading of transition metal single-atom catalysts is 0.1~1 mg / cm³. -2 ;

[0047] In one embodiment, the carrier is preferably activated carbon, including any one of dispersed carbon paper, porous carbon rod and glassy carbon electrode;

[0048] The coating method can be any one of drip coating, spray coating, or brush coating.

[0049] As a fifth aspect of this disclosure, an activation electrode is provided, which is prepared by the method described above.

[0050] As a sixth aspect of this disclosure, a method for electrocatalytic reduction of nitrate to produce ammonia is provided, comprising:

[0051] After being vacuum dried, the activated electrode is used as the cathode and connected to an electrolyte containing nitrate. A voltage is applied to form an electrolytic cell, which reduces the nitrate in the electrolyte to produce ammonia.

[0052] In one embodiment, the electrolytic cell employs a three-electrode system, with a saturated calomel electrode as the reference electrode, and the counter electrode comprising any one of a graphite carbon rod electrode, a platinum sheet electrode, a platinum mesh electrode, a gold electrode, and a titanium electrode.

[0053] The electrolyte consists of potassium hydroxide and potassium nitrate;

[0054] The molar concentrations of potassium hydroxide and potassium nitrate are both 0.05~0.15 mol / L.

[0055] In one embodiment, the applied voltage of the electrolytic cell is -0.3 to -0.8 V vs. RHE (reversible hydrogen electrode), and the test duration is 10 to 120 minutes.

[0056] Based on the above technical solution, the transition metal single-atom catalyst, preparation method, and application provided in this disclosure have at least one of the following beneficial effects:

[0057] (1) The transition metal single-atom catalyst provided in this disclosure directly replaces the N atom partially coordinated in the first shell of Fe-N4 with other heteroatoms, breaking the local charge symmetry of the iron single atom and constructing a catalyst with asymmetric coordinated Fe active sites, thereby enhancing the catalytic activity of the transition metal single atom. Non-metallic heteroatoms are chemically substituted and coordinated on the carbon support, providing great potential for non-metallic heteroatoms to act as electron acceptors (electron withdrawers) / electron donors (electron donors) on the carbon support.

[0058] (2) According to the embodiments of this disclosure, the metal-organic framework ZIF-8, which is self-assembled by coordination of zinc ions and imidazole organic compounds, is used as the carbon source and nitrogen source, and triphenylphosphine is used as the phosphorus source. Iron salt is used as the metal precursor, and a transition metal single-atom catalyst is prepared by high-temperature calcination and pyrolysis. The raw materials required by this disclosure are low in cost, and the preparation method is simple and convenient, which is conducive to large-scale production and application.

[0059] (3) The method for preparing transition metal single-atom catalysts by metal coupling provided in this disclosure utilizes the phase coupling between Fe-N4 and nano-metal particles, clusters or other non-ferrous metal centers to form a transition metal single-atom catalyst with a hierarchical porous structure and a large specific surface area, which is conducive to proton transfer and charge transport, and promotes the adsorption and desorption of the catalyst. Compared with simple iron single-atom or ruthenium nanoparticle catalysts, it further improves the catalytic efficiency and reaction activity of electrocatalytic reduction of nitrate to ammonia.

[0060] (4) The transition metal single-atom catalyst provided in this disclosure has high catalytic activity. It easily adsorbs nitrate ions and enriches the intermediate products of the reaction during the electrocatalytic reduction of nitrate to ammonia, thereby reducing the overpotential of the reaction. At the same time, the transition metal single-atom catalyst synthesized by this method has a uniform distribution of metal single atoms and metal nanoparticles, good dispersion, and easy control of loading. It exhibits superior catalytic activity and stability during the electrocatalytic reduction of nitrate. Moreover, the process is simple, energy-efficient, mild, and has strong universality, showing broad application prospects. Attached Figure Description

[0061] Figure 1 This is the X-ray diffraction pattern of the transition metal single-atom catalyst in Example 1 of this disclosure;

[0062] Figure 2These are transmission electron microscope images of the transition metal single-atom catalyst in Example 1 of this disclosure;

[0063] Figure 3 These are aberration-corrected electron micrographs of the transition metal single-atom catalyst in Example 1 of this disclosure;

[0064] Figure 4 This is a comparison chart of the Faraday efficiencies of the transition metal single-atom catalysts in the embodiments of this disclosure;

[0065] Figure 5 This is a cycle stability diagram of the electrocatalytic reduction of nitrate to ammonia by the transition metal single-atom catalyst in Example 2 of this disclosure;

[0066] Figure 6 This is the X-ray diffraction pattern of the transition metal single-atom catalyst in Example 8 of this disclosure;

[0067] Figure 7 These are transmission electron microscope images of the transition metal single-atom catalyst in Example 8 of this disclosure;

[0068] Figure 8 These are aberration-corrected electron micrographs of the transition metal single-atom catalyst in Example 8 of this disclosure;

[0069] Figure 9 This is a comparison chart of the Faraday efficiency of the transition metal single-atom catalyst under different voltage conditions in Example 9 of this disclosure;

[0070] Figure 10 This is a comparison chart of NH3 yield under different voltage conditions for the transition metal single-atom catalyst in Example 9 of this disclosure;

[0071] Figure 11 This is a cycle stability diagram of the electrocatalytic reduction of nitrate to ammonia by the transition metal single-atom catalyst in Example 9 of this disclosure. Detailed Implementation

[0072] To make the objectives, technical solutions, and advantages of this disclosure clearer, the following detailed description is provided in conjunction with specific embodiments and the accompanying drawings.

[0073] Electrocatalysis for the reduction of nitrate to ammonia offers mild operating conditions, environmental friendliness, and high controllability; however, the complex electron transfer pathway and the competitive hydrogen evolution reaction (HER) negatively impact reaction efficiency. Therefore, the development of high-performance electrocatalysts is needed to selectively reduce nitrate to NH3.

[0074] In the process of realizing this disclosure, it was discovered that single-atom catalysts (SACs) possess 100% theoretical atomic utilization and unique electronic structures compared to traditional nanocatalysts (metal nanoparticles, clusters, etc.). However, the coordination mechanism of existing single-atom catalysts mainly involves metal-nitrogen coordination, leading to agglomeration during the preparation process. Simultaneously, the symmetrical electronic structure distribution of the nitrogen ligand (M-N4) is unfavorable for the adsorption / activation / desorption of intermediate species, limiting the kinetics and performance of the catalytic reaction. In view of the technical problems existing in related technologies, this disclosure provides a transition metal single-atom catalyst, its preparation method, and its application from the perspective of optimizing the electronic structure. This catalyst is used for the efficient electrochemical catalytic reduction of nitrate to ammonia. The method features simple operation, strong process controllability, and high nitrate removal rate, ammonia formation rate, and cycle stability.

[0075] To achieve the above technical objectives, as a first aspect of this disclosure, a method for preparing transition metal single-atom catalysts using a heteroatom substitution method is provided, comprising:

[0076] Zinc nitrate hexahydrate and phosphorus source are added to an alcohol solvent, followed by an alcohol solution of imidazole organic compounds. The mixture is stirred until homogeneous to obtain the first suspension. In this suspension, zinc ions in zinc nitrate hexahydrate and imidazole organic compounds can coordinate self-assemble to form a porous crystalline material (ZIF8). The porous framework of ZIF8 can provide periodically dispersed hollow cages that can encapsulate phosphorus precursor molecules of suitable size.

[0077] The first precipitate obtained after the first suspension has been allowed to stand is washed, purified, and dried to obtain the first precursor.

[0078] The first precursor, surfactant, and iron salt are dispersed in an alcohol solvent and mixed and stirred to obtain a second suspension. The mixing and stirring are sufficient to completely dissolve and uniformly disperse the materials in the solvent. There is no specific limitation on the stirring rate, which can be adjusted according to actual needs. The surfactant acts as a porogen in the reaction, which can form a large number of large-diameter and interconnected pores inside and outside the dodecahedral framework structure of the precursor, resulting in a porous framework structure with low density, high specific surface area and porosity.

[0079] A solution of triethylamine in alcohol was added to the second suspension to carry out the reaction, and the second precipitate obtained from the reaction was collected.

[0080] The second precipitate was washed and dried to obtain the second precursor.

[0081] The second precursor was calcined to obtain the first transition metal single-atom catalyst. The calcination was carried out in an inert gas atmosphere, and the catalyst was naturally cooled to room temperature after calcination. When the temperature was above 800℃, the zinc in the ZIF-8 framework volatilized in gaseous form, and the metallic iron element in the added iron salt replaced the vacancies of zinc ions, thereby forming active sites of iron-nitrogen compounds.

[0082] In the embodiments of this disclosure, the precursor is rinsed with methanol 4 to 6 times to remove unreacted or incompletely reacted substances.

[0083] According to embodiments of this disclosure, the phosphorus source includes any one of triphenylphosphine, potassium dihydrogen phosphate, sodium dihydrogen phosphate, phytic acid, and sodium hypophosphite.

[0084] According to the embodiments of this disclosure, the amount of phosphorus source added is 0.5~2 g, for example, it can be 0.6 g, 0.8 g, 1.0 g, 1.5 g, etc.;

[0085] The amount of zinc nitrate hexahydrate added is 2~4 g, for example, 2 g, 2.5 g, 3 g, 3.5 g, etc.;

[0086] The amount of imidazole organic compounds added is 2~4 g, for example, 2 g, 2.5 g, 3 g, 3.5 g, etc.;

[0087] According to embodiments of this disclosure, by controlling the amounts of phosphorus source, zinc nitrate hexahydrate, and imidazole organic compounds within a suitable range, it is beneficial to form a phosphorus-doped ZIF framework structure.

[0088] According to embodiments of this disclosure, imidazole organic compounds include any one or more of 2-methylimidazole, 1,2-dimethylimidazole, and 1,3-dimethylimidazole;

[0089] The above-mentioned alcohol solutions and alcohol solvents belong to alcohol reagents, wherein the alcohol reagents are selected from any one or more of methanol, ethanol, ethylene glycol, isopropanol, and hexanediol;

[0090] Iron salts include any one of nitrates, chlorides, sulfates, or organic salts;

[0091] Surfactants include any one or more of cationic surfactants, anionic surfactants, organic weak acids, and organic weak base salts, preferably including any one or more of polyvinylpyrrolidone, sodium polystyrene sulfonate, tetradecylammonium bromide, hexadecyltrimethylammonium bromide, citric acid, and ascorbic acid;

[0092] According to embodiments of this disclosure, the mass ratio of the first precursor to the iron salt is 1:1 to 20:1, for example, it can be 1:1, 5:1, 10:1, 15:1, 20:1, etc.

[0093] The mass ratio of the first precursor to the surfactant is 1:1 to 50:1, for example, it can be 1:1, 5, 10:1, 20:1, 30:1, 40:1, 50:1, etc.;

[0094] The amount of triethylamine added is 2-8 ml, for example, 2 ml, 4 ml, 6 ml, 8 ml, etc.

[0095] According to embodiments of this disclosure, triethylamine (TEA) can increase the pH of the system, promote the deprotonation process of imidazole organic compounds to improve the nucleation rate of ZIF-8, and is a commonly used crystallization promoter for adjusting the particle size of ZIF-8 framework.

[0096] According to embodiments of this disclosure, the method of settling is to settling at 30-40°C for 2-3 hours. For example, the settling temperature can be 30°C, 35°C, 40°C, etc., and the settling time can be 2 hours, 2.5 hours, 3 hours, etc.

[0097] The purification method is centrifugation, with a preferred centrifugation speed of 7000~9000 r / min for 20~30 min;

[0098] The drying method is vacuum drying, the drying temperature is preferably 60~70 ℃, and the vacuum degree is preferably <10 Pa.

[0099] According to embodiments of this disclosure, the calcination temperature is preferably 800~1000 ℃, and the calcination time is preferably 3~4 h.

[0100] In one specific embodiment of this disclosure, zinc nitrate hexahydrate is first dissolved in methanol to form a homogeneous solution. Triphenylphosphine is then added and mixed thoroughly under vigorous stirring. Next, it is rapidly added to a methanol solution of dimethylimidazole and mixed thoroughly. The mixture is then magnetically stirred for 10 minutes at room temperature to obtain a suspension. After standing at 35°C for 2 hours, a white precipitate is obtained. The white precipitate is washed with methanol and purified by centrifugation at 8000 r / min for 30 minutes. It is then vacuum dried to obtain the ZIF8-P precursor. The ZIF8-P precursor powder and a surfactant are dispersed in a methanol solution of ferric nitrate nonahydrate. The mixture is ultrasonically vibrated for 1 hour and magnetically stirred for 1 hour until homogeneous to obtain a uniform suspension. A methanol solution of triethylamine is then added to the suspension and mixed thoroughly. The mixture is stirred at 40°C for 4 hours to react. The resulting precipitate is collected and labeled as Fe / ZIF8-P. After washing with methanol, the precipitate is vacuum dried at 60°C for 12-24 hours and then calcined under an argon atmosphere to obtain a transition metal single-atom catalyst, which can be labeled Fe-N / PC.

[0101] As a second aspect of this disclosure, a method for preparing transition metal single-atom catalysts using a metal coupling method is provided, comprising:

[0102] The alcoholic solution of zinc nitrate hexahydrate was mixed evenly with the alcoholic solution of imidazole organic compounds to obtain the third suspension.

[0103] The third precipitate obtained after the third suspension has been allowed to stand is washed, purified, and dried to obtain the third precursor.

[0104] After uniformly mixing the third precursor with an alcoholic solution of ferric nitrate nonahydrate, the mixture was added to an alcoholic solvent and stirred to obtain the fourth suspension.

[0105] Add an alcoholic solution of triethylamine to the fourth suspension to carry out the reaction, and collect the fourth precipitate obtained from the reaction;

[0106] The fourth precipitate was washed and dried to obtain the fourth precursor;

[0107] After grinding and first calcination of the fourth precursor, it was uniformly dispersed with the nano metal salt solution in an alcohol solvent and then ultrasonically treated to obtain a black suspension.

[0108] The black suspension was evaporated and dried to obtain a black powder, which was then subjected to a second calcination treatment in a gas atmosphere of mixed argon and hydrogen to obtain a second transition metal single-atom catalyst.

[0109] In the embodiments of this disclosure, the above method is used to achieve uniform and stable loading of nano-metal particles in nitrogen-doped mesoporous carbon through an impregnation-calcination reduction method. Meanwhile, the reduction temperature has a certain influence on the morphology and particle size of the metal particles. After comprehensive evaluation, calcination reduction can be carried out at a lower temperature.

[0110] In the embodiments of this disclosure, the nano-metal particles are uniformly distributed in the Fe-N4 channels, which plays a synergistic catalytic role in the nitrate reduction reaction.

[0111] According to embodiments of this disclosure, the nano-metal salt includes any one of ruthenium chloride, palladium chloride, iridium chloride, rhodium chloride, silver chloride, copper chloride, nickel chloride, or cobalt chloride.

[0112] According to embodiments of this disclosure, the amount of zinc nitrate hexahydrate added is 2-4 g; the amount of imidazole organic compound added is 2-4 g. Controlling the amounts of zinc nitrate hexahydrate and imidazole organic compound within appropriate ranges is beneficial for forming the ZIF framework structure. According to embodiments of this disclosure, the amount of the fourth precursor added is 190-210 mg, for example, 195 mg, 200 mg, 205 mg, etc.

[0113] The concentration of the nano metal salt solution is preferably 8~12 mg / L, for example, it can be 9 mg / L, 10 mg / L, 11 mg / L, etc.

[0114] The preferred amount of nano-metal salt solution added is 1~20 ml, for example, 2 ml, 5 ml, 10 ml, 15 ml, etc.

[0115] According to the embodiments of this disclosure, the temperature of the first calcination treatment is preferably 800~1000 ℃, the temperature of the second calcination treatment is preferably 300~500 ℃, the calcination time of the first calcination treatment and the second calcination treatment is preferably 1~4 h, and the heating rate is 1~20 ℃ / min.

[0116] In one specific embodiment of this disclosure, zinc nitrate hexahydrate is first dissolved in a methanol solution to form a homogeneous solution, which is then rapidly added to a methanol solution of dimethylimidazole and mixed thoroughly. The mixture is then magnetically stirred for 10 min at room temperature to obtain a suspension. After standing at 35 °C for 2 h, a white precipitate is obtained. The white precipitate is washed with methanol solution, centrifuged at 8000 r / min for 30 min for purification, and then vacuum dried. Subsequently, it is ultrasonically vibrated with a methanol solution of ferric nitrate nonahydrate for 1 h and magnetically stirred for 1 h until a homogeneous solution is obtained. This homogeneous solution is then added to a methanol solution of bis(4-aminophenyl) ether and stirred for 10 min. Finally, a methanol solution of triethylamine is added and mixed thoroughly. The mixture is stirred at 40 °C for 4 h, and the resulting precipitate is collected. The precursor can be labeled as Fe. SA -NC; This will introduce the precursor Fe... SA -NC and 10 mg / mL ruthenium chloride solution were uniformly dispersed in methanol solvent (the solvent can be ethanol, methanol, isopropanol, or other organic solvents). After ultrasonic treatment for 1 h, a black suspension was formed. The black suspension was evaporated in an open container, dried, and the collected black powder was placed in a porcelain boat and calcined in an oxygen-free environment under a mixed gas of argon and 10% hydrogen (Ar / H2) to obtain a transition metal single-atom catalyst, which can be labeled as Ru. NP @Fe SA -NC.

[0117] As a third aspect of this disclosure, a transition metal single-atom catalyst is provided, which is prepared by the method described above.

[0118] Among them, the transition metal single-atom catalyst is a catalyst with asymmetric coordinated Fe active sites; or

[0119] Transition metal single-atom catalysts are synergistic porous catalysts in which Fe active sites are coupled with metal nanoparticles, clusters or non-ferrous metal centers, with metal nanoparticles and iron single atoms uniformly dispersed on carbon-nitrogen supports.

[0120] In one embodiment of this disclosure, by introducing environmental heteroatoms or nanometals into a carbon support, the d-band centers of transition metal single atoms can be adjusted, thereby optimizing NO3. - The adsorption and desorption free energy of the RR intermediate reduces the reaction energy and thus increases NO3. - Reaction kinetics and reactivity of RR.

[0121] As a fourth aspect of this disclosure, a method for preparing an activated electrode is provided, comprising:

[0122] A transition metal single-atom catalyst is dispersed in a solvent containing a binder to form a homogeneous mixture. This mixture is then coated onto a support surface to form an activated electrode. Atomic doping introduces atomic substances into the electrode material to optimize the electronic structure and enhance electrochemical performance.

[0123] According to embodiments of this disclosure, the solvent containing the binder is a mixture of isopropanol and naphthol, and the mixing volume ratio of isopropanol and naphthol is 10:1 to 50:1, for example, it can be 10:1, 20:1, 30:1, 40:1, etc.

[0124] The loading of transition metal single-atom catalysts ranged from 0.1 to 1 mg / cm³. -2 For example, it could be 0.1 mg / cm³. -2 0.5 mg / cm -2 0.8 mg / cm -2 wait;

[0125] According to embodiments of this disclosure, the carrier is preferably activated carbon, including any one of dispersed carbon paper, porous carbon rods, and glassy carbon electrodes.

[0126] As a fifth aspect of this disclosure, an activation electrode is provided, which is prepared by the method described above.

[0127] As a sixth aspect of this disclosure, a method for electrocatalytic reduction of nitrate to produce ammonia is provided, comprising:

[0128] After being vacuum dried, the activated electrode is used as the cathode and connected to an electrolyte containing nitrate. A voltage is applied to form an electrolytic cell, which reduces the nitrate in the electrolyte to produce ammonia.

[0129] According to embodiments of this disclosure, the electrolytic cell employs a three-electrode system, with a saturated calomel electrode as the reference electrode, and the counter electrode comprising any one of a graphite carbon rod electrode, a platinum sheet electrode, a platinum mesh electrode, a gold electrode, and a titanium electrode.

[0130] The electrolyte consists of potassium hydroxide and potassium nitrate;

[0131] The molar concentrations of potassium hydroxide and potassium nitrate are both 0.05~0.15 mol / L, for example, 0.05 mol / L, 0.1 mol / L, 0.15 mol / L, etc.

[0132] The applied reduction voltage range for the electrolytic cell is -0.3 to -0.8 V vs. RHE, and the test duration is 10 to 120 min.

[0133] To make the objectives, technical solutions, and advantages of this disclosure clearer, the technical solutions and principles of this disclosure are further illustrated below with reference to specific embodiments and accompanying drawings. It should be noted that the specific embodiments described below are merely illustrative examples, and the scope of protection of this disclosure is not limited thereto.

[0134] Unless otherwise specified, all test materials and reagents used in the following examples are commercially available. Unless otherwise specified, specific techniques or conditions in the examples are conventional methods and can be performed according to the techniques or conditions described in the literature or the product instructions.

[0135] Example 1

[0136] A method for preparing transition metal single-atom catalysts using heteroatom substitution includes:

[0137] Weigh 3.08 g of 2-methylimidazole and add it to 75 ml of methanol solution. Stir thoroughly to form a methanol solution of dimethylimidazole. Under stirring conditions, add a mixture of 2.88 g of zinc nitrate hexahydrate (Zn(NO3)2·6H2O) and 0.50 g of triphenylphosphine to 75 mL of methanol solvent. Mix the mixture with the methanol solution of dimethylimidazole to obtain the first suspension. After standing at 35 °C for 2 h, centrifuge to collect the white precipitate. Wash the precipitate several times with methanol solvent and dry it under vacuum at 60 °C for 12 h to obtain the precursor powder.

[0138] Weigh 160 mg of hexadecyltrimethylammonium bromide and dissolve it in 50 mL of methanol. Mix well to form a transparent solution (hexadecyltrimethylammonium bromide-methanol solution). Weigh 200 mg of precursor powder and add it to 10 mL of methanol solution. Then add 4 mL of 2.5 mg / mL ferric nitrate nonahydrate (Fe(NO3)3·9H2O) methanol solution and mix well. After sonication for 60 minutes, add it to the above transparent solution and stir for 10 minutes to mix well to obtain a second suspension.

[0139] 4 mL of triethylamine and 25 mL of methanol were mixed to obtain a methanol solution of triethylamine, which was added to the second suspension. After stirring at 40 °C for 4 h, the mixture was filtered, and the precipitate was collected by centrifugation. After washing and drying, the precipitate was transferred to a ceramic crucible and heated to 950 °C at a rate of 5 °C / min under an argon atmosphere. The temperature was maintained at 950 °C for 3 h. After cooling, the product obtained was the nitrogen-phosphorus doped transition metal single-atom catalyst, labeled as Fe-N / PC.

[0140] The phase composition of the obtained transition metal single-atom catalyst was determined by X-ray diffraction (XRD), and the morphology of the sample was analyzed by transmission electron microscopy and spherical aberration electron microscopy.

[0141] Figure 1 This is the X-ray diffraction pattern of the transition metal single-atom catalyst in Example 1 of this disclosure; Figure 2 These are transmission electron microscope images of the transition metal single-atom catalyst in Example 1 of this disclosure; Figure 3 This is a spherical aberration electron microscope image of the transition metal single-atom catalyst in Example 1 of this disclosure.

[0142] The XRD pattern shows that the transition metal single-atom catalyst Fe-N / PC prepared in this embodiment only has a distinct carbon peak, and there are no absorption peaks from iron nanoparticles or other iron-containing compounds, which preliminarily proves that the catalyst prepared in this embodiment conforms to a single-atom structure; Figure 2 It can be seen that the transition metal single-atom catalysts prepared by the method provided in this disclosure are uniform in size, exhibiting a hollow rhombic dodecahedral morphology, and are uniformly distributed; Figure 3 It can be seen that iron single atoms are uniformly dispersed on the carbon support, further proving the formation of iron single atoms and obtaining a transition metal single-atom catalyst with optimized electronic structure.

[0143] Example 2

[0144] The transition metal single-atom catalyst obtained in Example 1 was dispersed in a mixed solution of isopropanol and naphthol (Nafion 117) to form a uniform mixture. Then, it was drop-coated onto the surface of 1×1 cm carbon paper to form an active electrode. After vacuum drying, it was used as the negative electrode and connected to an electrolyte containing nitrate to electrocatalyze the reduction of nitrate to ammonia.

[0145] Figure 4 This is a comparison chart of the Faraday efficiencies of the transition metal single-atom catalysts in the embodiments of this disclosure. For example... Figure 4As shown, the Faraday efficiency (NH3) of the transition metal single-atom catalyst Fe-N / PC is the highest at -0.4 V vs. RHE, approximately 90.3%, forming a volcano-shaped curve. It also maintains a high Faraday efficiency plateau over a wide potential range from -0.3 V vs. RHE to -0.8 V vs. RHE, exhibiting high activity. The activity of the nitrate reduction to ammonia production reaction (NO3-RR) is significantly better than that of other comparative samples.

[0146] The electrochemical durability of transition metal unit system catalysts in nitrate reduction was evaluated. The NH3 selectivity was optimal in a 0.1 M KOH / 0.1 M KNO3 mixed electrolyte. Under these reaction conditions, 20 consecutive electrolysis cycles were performed to test the Faraday efficiency and NH3 yield.

[0147] Figure 5 This is a graph showing the cyclic stability of the electrocatalytic reduction of nitrate to ammonia using a transition metal single-atom catalyst in Example 2 of this disclosure. Figure 5 As shown, although the NH3 yield and Faraday efficiency fluctuate slightly in each cycle, the Faraday efficiency is above 90% and the NH3 yield remains relatively stable.

[0148] Example 3

[0149] The same preparation method as in Example 1 was used, except that the amount of triphenylphosphine added in Example 1 was adjusted to 1.0 g.

[0150] Example 4

[0151] The same preparation method as in Example 1 was used, except that the amount of triphenylphosphine added in Example 1 was adjusted to 1.5 g, and the amount of ferric nitrate nonahydrate (Fe(NO3)3·9H2O) added was adjusted to 4 mL.

[0152] Example 5

[0153] The same preparation method as in Example 1 was used, except that the amount of triphenylphosphine added in Example 1 was adjusted to 2.0 g, and the amount of ferric nitrate nonahydrate (Fe(NO3)3·9H2O) added was adjusted to 4 mL.

[0154] Example 6

[0155] The same preparation method as in Example 1 was used, except that the amount of triphenylphosphine added in Example 1 was adjusted to 2.0 g, and the amount of ferric nitrate nonahydrate (Fe(NO3)3·9H2O) added was adjusted to 8 mL.

[0156] Example 7

[0157] The transition metal single-atom catalysts prepared in Examples 3-6 were dispersed in a mixed solution of isopropanol and naphthol (Nafion 117) to form a uniform mixture. Then, the mixture was drop-coated onto the surface of 1×1 cm carbon paper to form an active electrode. After vacuum drying, the active electrode was connected to an electrolyte containing nitrate ions as a negative electrode and could be used in the electrocatalytic reduction of nitrate ions to produce ammonia.

[0158] Comparative Example 1

[0159] The same preparation method as in Example 1 was used, the only difference being that triphenylphosphine was not added, and the prepared catalyst material was labeled as Fe-NC.

[0160] Figure 4 This is a comparison chart of the Faraday efficiencies of the transition metal single-atom catalysts in the embodiments of this disclosure. For example... Figure 4 As shown, the performance of the Fe-NC catalyst obtained without the addition of a phosphorus source is reduced. Due to the strong electronegativity of the symmetrically adjacent nitrogen atoms around the active metal sites of Fe-NC, the Gibbs free energy of the reaction of intermediate adsorption on the surface of the active metal sites is large, which slows down the kinetic process of the catalytic reaction. Therefore, the Faraday efficiency of Fe-NC is low.

[0161] Example 8

[0162] A method for preparing transition metal single-atom catalysts using a metal coupling method includes:

[0163] Weigh 6.5 g of 2-methylimidazole and add it to 80 mL of methanol solution. Stir thoroughly to dissolve and form a methanol solution of dimethylimidazole. Under stirring conditions, mix 3 g of zinc nitrate hexahydrate (Zn(NO3)2·6H2O) with the methanol solution of dimethylimidazole to obtain a suspension. After standing at 35 °C for 2 h, centrifuge to collect the precipitate. Wash it several times with methanol solvent, centrifuge to purify it, and dry it under vacuum at 60 °C for 12 h to obtain the precursor powder.

[0164] Weigh 160 mg of hexadecyltrimethylammonium bromide and dissolve it in 50 mL of methanol. Mix well to form a homogeneous and transparent hexadecyltrimethylammonium bromide-methanol solution. Add 4 mL of 2.5 mg / mL ferric nitrate nonahydrate (Fe(NO3)3·9H2O) methanol solution to the precursor and mix well. After sonication for 60 minutes, add the mixture to the hexadecyltrimethylammonium bromide-methanol solution and stir for 10 minutes to mix well to obtain a suspension.

[0165] 4 mL of triethylamine and 50 mL of methanol were mixed to obtain a methanol solution of triethylamine. The solution was added to the suspension and stirred vigorously for 24 h. The precipitate was collected by centrifugation, washed and dried with methanol, and then uniformly ground and transferred to a quartz boat. The boat was then placed in a tube furnace and heated at a rate of 5 °C / min under flowing argon gas. After heating to 950 °C, the calcination was performed for 3 h. After the tube furnace cooled down, the black powder sample was collected.

[0166] 30 g of black powder and 600 μL of ruthenium chloride solution (10 mg / L) were dispersed in 20 mL of methanol solution. After sonication for 1 hour to form a black suspension, the suspension was placed open on a magnetically heated stirring table at a constant temperature of 40 °C and stirred vigorously until the solvent was completely evaporated. Then, it was dried under vacuum. The suspension was then pyrolyzed at 350 °C for 1 hour in an Ar / H2 (10%) mixed gas in a tube furnace. After the tube furnace cooled naturally to room temperature, the sample was collected to obtain a transition metal single-atom catalyst loaded with nano-metal.

[0167] The transition metal single-atom phase structure obtained in Example 8 was obtained using X-ray diffraction. Figure 6 This is the X-ray diffraction pattern of the transition metal single-atom catalyst in Example 8 of this disclosure. Figure 6 As shown, there are two distinct peaks at 25° and 44°, which are attributed to the (002) and (100) planes of graphite carbon; meanwhile, due to the low Ru content and the partial overlap between the Ru main peak and the carbon peak, the detected Ru XRD signal is not obvious.

[0168] The morphology of the samples was analyzed using transmission electron microscopy and aberration-corrected electron microscopy. Figure 7 These are transmission electron microscope images of the transition metal single-atom catalyst in Example 8 of this disclosure; Figure 8 This is a spherical aberration electron microscope image of the transition metal single-atom catalyst in Example 8 of this disclosure.

[0169] from Figure 7 and Figure 8 It can be observed that the synthesized transition metal single-atom catalyst is uniform in size and exhibits a regular dodecahedral shape, with ruthenium nanoparticles uniformly dispersed in the cavities of the transition metal single-atom catalyst. In high-resolution high-angle annular dark-field scanning transmission (HAADF-STEM) images, many highly dispersed bright spots and multinucleated nanoparticles can be observed on nitrogen-doped carbon.

[0170] Example 9

[0171] The transition metal single-atom catalyst obtained in Example 8 was dispersed in a mixed solution of isopropanol and naphthol (Nafion 117) to form a uniform mixture. Then, it was drop-coated onto the surface of 1×1 cm carbon paper to form an active electrode. After vacuum drying, it can be used as a negative electrode in an electrolyte containing nitrate ions for electrocatalytic reduction of nitrate ions to produce ammonia.

[0172] Figure 9 This is a comparison chart of the Faraday efficiency of the transition metal single-atom catalyst under different voltage conditions in Example 9 of this disclosure; Figure 10 This is a comparison chart of NH3 yield under different voltage conditions for the transition metal single-atom catalyst in Example 9 of this disclosure. From... Figure 9 and Figure 10 It can be seen that the transition metal single-atom catalyst can achieve a Faraday efficiency of 96% and a high ammonia yield at a low overpotential of 0 V vs. RHE.

[0173] Figure 11 This is a cycle stability graph of the electrocatalytic reduction of nitrate to ammonia using a transition metal single-atom catalyst in Example 9 of this disclosure. Under 0V vs. RHE conditions, 20 consecutive electrolysis cycles were tested in a 0.1 M KOH / 0.1 M KNO3 mixed electrolyte. The graph shows that the NH3 yield and Faraday efficiency generally remained stable, exhibiting high stability.

[0174] Example 10

[0175] The same preparation method as in Example 4 was used, except that the amount of ruthenium chloride solution added in Example 4 was adjusted to 100 μL.

[0176] Example 11

[0177] The same preparation method as in Example 4 was used, except that the pyrolysis temperature under the Ar / H2 (10%) mixed gas in Example 4 was adjusted to 450°C.

[0178] Example 12

[0179] The same preparation method as in Example 4 was used, except that the amount of methanol solution of ferric nitrate nonahydrate (Fe(NO3)3·9H2O) added in Example 4 was adjusted to 16 mL.

[0180] Example 13

[0181] The same preparation method as in Example 4 was used, except that the black suspension obtained in Example 4 was placed in a rotary evaporator and dried at 55°C. After drying, it was ground together with 3.0g of urea and then placed in a tube furnace for pyrolysis.

[0182] The electronic structure of the transition metal single-atom catalyst disclosed herein is optimized, the preparation process is simple and controllable, and it exhibits high Faraday efficiency, NH3 yield and cycle stability when used as an electrode for nitrate reduction to ammonia, with good nitrate removal effect.

[0183] The specific embodiments described above further illustrate the purpose, technical solutions, and beneficial effects of this disclosure. It should be understood that the above descriptions are merely specific embodiments of this disclosure and are not intended to limit this disclosure. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of this disclosure should be included within the protection scope of this disclosure.

Claims

1. A method for preparing transition metal single-atom catalysts using a metal coupling method, comprising: A zinc nitrate hexahydrate alcohol solution was mixed thoroughly with an imidazole organic compound alcohol solution to obtain a third suspension. The third precipitate obtained after the third suspension has been allowed to stand is washed, purified, and dried to obtain the third precursor. After uniformly mixing the third precursor with an alcohol solution of ferric nitrate nonahydrate, the mixture is added to an alcohol solvent and stirred to obtain a fourth suspension. A triethylamine alcohol solution was added to the fourth suspension to carry out the reaction, and the fourth precipitate obtained from the reaction was collected. The fourth precipitate was washed and dried to obtain the fourth precursor; After grinding and first calcination, the fourth precursor is uniformly dispersed in an alcohol solvent along with a metal salt solution, and then ultrasonically treated to obtain a black suspension. The black suspension was evaporated and dried to obtain a black powder, which was then subjected to a second calcination treatment in a gas atmosphere of mixed argon and hydrogen to obtain a second transition metal single-atom catalyst. The temperature of the first calcination treatment is 800~1000℃, and the temperature of the second calcination treatment is 300~500℃; The metal salt in the metal salt solution includes any one of ruthenium chloride, palladium chloride, iridium chloride, and rhodium chloride; The imidazole organic compounds are selected from any one or more of 2-methylimidazole, 1,2-dimethylimidazole, and 1,3-dimethylimidazole; The alcohol solution and alcohol solvent are selected from any one of methanol, ethanol, ethylene glycol, isopropanol, and hexanediol.

2. The method according to claim 1, wherein, The amount of the fourth precursor added is 190~210 mg; The concentration of the metal salt solution is 8~12 mg / L; The amount of the metal salt solution added is 1~20 ml; The calcination time for the first calcination treatment and the second calcination treatment is 1~4 h, and the heating rate is 1~20℃ / min.

3. A transition metal single-atom catalyst, wherein the transition metal single-atom catalyst is prepared by the method of claim 1 or 2; The transition metal single-atom catalyst is a synergistic porous catalyst in which Fe active sites are coupled with metal nanoparticles, clusters or non-ferrous metal centers, and the metal nanoparticles and iron single atoms are uniformly dispersed on a carbon-nitrogen support.

4. A method for preparing an activated electrode using a transition metal single-atom catalyst as described in claim 3, comprising: The transition metal single-atom catalyst was dispersed in a solvent containing a binder to form a homogeneous mixture; The mixture is coated onto the surface of a carrier to form an activated electrode.

5. The preparation method according to claim 4, wherein, The solvent containing the binder is a mixture of isopropanol and naphthol; The volume ratio of isopropanol to naphthol is 10:1 to 50:1; The loading of the transition metal single-atom catalyst is 0.1~1 mg / cm³. -2 ; The carrier is activated carbon, including any one of dispersed carbon paper, porous carbon rod and glassy carbon electrode; The coating method can be any one of drip coating, spray coating, or brush coating.

6. An activation electrode, wherein the activation electrode is prepared by the method of any one of claims 4 or 5.

7. A method for electrocatalytic reduction of nitrate to ammonia production using an activated electrode as described in claim 6, comprising: The activated electrode is dried under vacuum and then connected to an electrolyte containing nitrate as a cathode. A voltage is applied to form an electrolytic cell, and the nitrate in the electrolyte is reduced to produce ammonia.

8. The method according to claim 7, wherein, The electrolytic cell adopts a three-electrode system, with a saturated calomel electrode as the reference electrode, and the counter electrode including any one of graphite carbon rod electrode, platinum sheet electrode, platinum mesh electrode, gold electrode, and titanium electrode; The electrolyte includes potassium hydroxide and potassium nitrate; The molar concentrations of potassium hydroxide and potassium nitrate are both 0.05~0.15 mol / L; The applied reduction voltage range of the electrolytic cell is -0.3 to -0.8 V vs. RHE, and the test duration is 10 to 120 minutes.

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