Preparation method and application of nitrogen-doped carbon supported small-size nanoparticle and single-atom catalyst

Abstract

The application discloses a preparation method and application of a nitrogen-doped carbon-loaded small-size nanoparticle and single-atom catalyst. In a prototype system for synthesizing small-size palladium nanoparticles, metal titanium phthalocyanine is introduced while a palladium source is added, and the palladium metal is changed into small-size palladium nanoparticles through a polymerization reaction, and the metal titanium phthalocyanine is difficult to form alloy nanoparticle with the palladium metal, and the metal titanium phthalocyanine is uniformly dispersed on the carbon carrier and exists in the form of single atoms. The simultaneous introduction of small-size nanoparticles and different kinds of single-atom catalysts on the carbon carrier is original. A series of palladium nanoparticles and different metal single-atom catalysts are prepared as a catalyst main body and applied to the field of electrocatalytic reduction of nitrate to ammonia, so that efficient and directional conversion of nitrate is realized.

Description

Technical Field

[0001] This invention relates to methods for preparing nitrogen-doped carbon-supported small-sized palladium nanoparticles and single atoms (iron, copper, or cobalt single atoms), and their applications in the field of electrocatalysis. Specifically, by controlling the reaction process to introduce different single-atom metal sources, the electronic environment of the metal nanoparticles is altered, and new active sites are introduced, thereby exhibiting excellent catalytic performance in electrochemistry. Background Technology

[0002] Ammonia (NH3) is an important raw material for artificial fertilizers and various chemicals, and is also one of the most promising carbon-free energy carriers. Currently, the Haber-Bosch process remains the main route for NH3 production, but this process is characterized by high energy consumption and high carbon dioxide emissions. The electrochemical nitrate reduction reaction (NO3) driven by renewable energy sources... - (RR) utilizes renewable electricity as a power source for the green synthesis of ammonia. This process removes the toxic pollutant NO3. - Converting compounds into NH3 is a feasible method for turning waste into treasure and pollutants into high-value-added products. However, metal nanoparticles (NPs), as catalysts for electrochemical nitrate reduction, are prone to hydrogen evolution reaction and the generation of reduction intermediate NO2. - This limits catalytic activity. Therefore, various tandem catalytic strategies to enhance the catalytic activity of NPs and thus increase the electrochemical conversion rate of NH3 have attracted increasing interest.

[0003] Small-sized metal nanoparticles exhibit significant size effects relative to their bulky counterparts. Geometrically, in addition to their high surface area, smaller nanoparticles expose more low-coordination sites, and their different local geometries significantly alter the breaking and formation of chemical bonds during catalysis. Electronically, the quantum size effect also significantly alters the energy levels of the entire particle, thereby affecting orbital hybridization and overall charge transfer between the metal and reactants [Sci. Adv. 2019; 5: eaat6413]. Given the advantages of small-sized nanoparticles, loading active metal nanoparticles onto carbon substrates can fully utilize the conductivity of the carbon support and the catalytic activity of the nanoparticles, thereby improving the activity of electrocatalysis.

[0004] In addition, metal single atoms possess excellent electrocatalytic reduction performance and high atomic utilization, making them suitable for doping into carbon supports. Enhanced charge transfer occurs at the interface between the substrate and metal nanoparticles, effectively reshaping the electronic structure of the metal nanoparticles. Metal single atoms can also add new catalytic active sites [Angew. Chem. Int. Ed. 2021, 60, 16044–16050], and synergistic or tandem catalytic reactions with metal nanoparticles can significantly improve electrocatalytic activity. However, introducing new metal single atoms into the prototype system of carbon-supported metal nanoparticles involves complex experimental procedures, and care must be taken to avoid problems such as metal single atom aggregation and metal nanoparticle destruction.

[0005] Based on the above research, a few studies have reported on small-sized metal nanoparticles and metal single atoms loaded on carbon-based nanomaterials, exploring the effect of electron transfer between nanoparticles and single atoms on improving the electrocatalytic nitrate reduction performance and the different roles played by the two in the catalytic process. Zhou's group studied the growth of palladium nanorod arrays on porous nickel foam for electrocatalytic nitrate reduction. The palladium nanorods can generate a large amount of active hydrogen, accelerating the hydrogenation reaction of nitrate reduction intermediates. However, this catalyst also has disadvantages such as high electrocatalytic reduction potential and low Faraday efficiency in ammonia production [Small 2023,19,2207743]. Yu's group synthesized an iron single-atom gel for electrocatalytic reduction of nitrate to ammonia. This method has a good effect on reducing nitrate, but the synthesis process of single atoms is relatively complex, requiring many controlled synthesis steps and the removal of nanoparticles by acid washing, which will cause some metal waste [EnergyEnviron.Sci.,2021,14,3522–3531]. Kim's group studied the loading of copper nanoparticles and palladium single-atom materials on a carbon substrate for electrocatalytic nitrate reduction. However, the catalytic product is nitrogen gas, and the high-value-added product NH3 is not obtained. Compared with industrial production, it cannot achieve higher economic benefits [ACS Catal.2023,13,6804-6812].

[0006] Therefore, developing a controllable and simple preparation method that can simultaneously introduce metal nanoparticles and metal single atoms onto a carbon substrate, combining the advantages of metal nanoparticles and single-atom catalysts in the electrocatalytic reduction of nitrates, and further optimizing the efficient electron transfer between metal single atoms and metal nanoparticles within a small size range to achieve the directional and efficient conversion of reactants into NH3 is an urgent problem to be solved. Summary of the Invention

[0007] The purpose of this invention is to address the shortcomings of current technologies by providing a method for preparing nitrogen-doped carbon-supported palladium nanoparticles and single-atom (iron, copper, or cobalt) catalysts, as well as their applications. In a prototype system for synthesizing small-sized palladium nanoparticles, this method introduces titanium cyanide simultaneously with the palladium source. The palladium metal undergoes a polymerization reaction to become small-sized palladium nanoparticles, while the titanium cyanide is difficult to alloy with palladium metal into nanoparticles. The titanium cyanide is uniformly dispersed on a carbon support and exists in single-atom form. This method of simultaneously introducing small-sized nanoparticles and different types of single-atom catalysts onto a carbon support is groundbreaking. This invention prepares a series of palladium nanoparticles and different metal single-atom catalysts as catalyst substrates for application in the electrocatalytic reduction of nitrate to ammonia, achieving efficient and directional conversion of nitrate.

[0008] The technical solution of this invention:

[0009] A method for preparing nitrogen-doped carbon-supported small-sized palladium nanoparticles and a single-atom catalyst, the method comprising the following steps:

[0010] (1) K2PdCl4, titanium cyanide, 3-aminophenol (3-AP), and dicyandiamide (DCD) were added to water and stirred to disperse evenly, resulting in solution A. At the same time, hexamethylenetetramine (HMT) and F127 were added to water and stirred to mix, resulting in solution B. Subsequently, solution A and solution B were mixed and stirred for 30-60 min, then transferred to a hydrothermal reactor and hydrothermally reacted at 60-120℃ for 24-48 h. Afterward, the hydrothermal reaction was allowed to cool naturally to room temperature, and the product was washed by vacuum filtration using a large amount of deionized water. The solid product obtained by vacuum filtration was freeze-dried to obtain the product PdM-NC, where M is Fe, Cu, or Co.

[0011] The titanium cyanide mentioned is titanium cyanide iron, titanium cyanide copper, or titanium cyanide cobalt;

[0012] In solution A, the mass ratio of dicyandiamide (DCD), 3-aminophenol (3-AP), K2PdCl4, and titanium cyanide is 1:1-3:0.15-0.55:0.03-0.2.

[0013] In solution B, the mass ratio of HMT to F127 is 3:2 to 2:1.

[0014] After mixing solutions A and B, the mass ratio of K2PdCl4 to HMT is 1:16-6.

[0015] The concentration range of K2PdCl4 in solution A is 0.5-1.0 mg / ml;

[0016] The volume ratio of solution A to solution B is 5:3 to 5:1;

[0017] The freeze-drying temperature is -30℃ to 50℃; the vacuum degree is 15-20 kPa.

[0018] (2) The solid powder obtained by freeze-drying is ground, then treated in an atmosphere of hydrogen and argon mixture, ground again, and then treated in an ozone atmosphere to obtain the final product Pd@M. SA -NC stands for small palladium nanoparticles and single-atom supported on nitrogen-doped carbon supports.

[0019] The carbonization conditions under the hydrogen and argon atmosphere are a heating rate of 3-5℃ / min, heating to 400-700℃ and holding for 3-5 hours;

[0020] The carbonization conditions under the ozone atmosphere are a heating rate of 1-3℃ / min, heating to 150-200℃ and holding for 1-3 hours;

[0021] The small palladium nanoparticles have a particle size of 2–5 nm; the single atom is a Fe single atom, a Cu single atom, or a Co single atom. The Pd loading in the catalyst is 5–15 wt%, and the Fe loading is 0.05–0.50 wt%.

[0022] The nitrogen-doped carbon-supported palladium nanoparticles and single-atom catalysts prepared by the method are used as working electrode materials for electrocatalytic ammonia synthesis.

[0023] Specifically, the steps include: adding nitrogen-doped carbon-supported small palladium nanoparticles and single-atom catalysts to a mixed solution, ultrasonically dispersing for 20-40 minutes to obtain a uniformly dispersed liquid, and then dropping the dispersion onto carbon paper and allowing it to dry naturally at room temperature;

[0024] Electrolysis was performed under three-electrode conditions: Hg / HgO was used as the reference electrode, platinum wire as the counter electrode, carbon paper with nitrogen-doped carbon-supported small palladium nanoparticles and single-atom catalysts as the working electrode, and an alkaline solution containing Na2SO4 and NaNO3 with a pH of 12 was used as the electrolyte; constant voltage electrolysis was performed for 0.1 to 0.5 h in the voltage range of -0.5 to -0.9 V vs. RHE.

[0025] The mixed solution is composed of anhydrous ethanol, deionized water and Nafion solution, with a volume ratio of 1:1:0.1 to 0.3.

[0026] 1–4 mg of nitrogen-doped carbon-supported small palladium nanoparticles and a single-atom catalyst are added to every 200–300 μL of the mixed solution;

[0027] The carbon paper is 0.3–0.7 cm thick. 280–120 μL of dispersion needs to be added dropwise;

[0028] The Nafion solution has a mass fraction of 0.4–0.6 wt.%; the electrolyte has a Na2SO4 concentration of 0.4–0.7 M and a NaNO3 concentration of 0.05–0.1 M; and the pH of the electrolyte is adjusted to 12 using a 1 M NaOH solution.

[0029] The essential features of this invention are:

[0030] In current technologies, when using other metal ion sources to synthesize single atoms and introduce them into nanoparticle prototype systems, the single-atom metal sources can easily agglomerate to form nanoparticles or clusters. Furthermore, improper experimental methods during the synthesis process can damage the structure of the nanoparticles, making the synthesis methods relatively complex.

[0031] The titanium cyanine used in this invention has a unique porphyrin-like structure, with one cavity within the phthalocyanine ring that can accommodate metal elements such as iron, copper, cobalt, tin, and nickel, forming metallic phthalocyanine molecules. The metallic phthalocyanine itself possesses a typical M-N4 single-atom structure. Using titanium cyanine directly as the single-atom metal source ensures the structural stability of the nanoparticles, requires fewer controlled conditions during synthesis, and simplifies the synthesis method. Furthermore, due to the versatility of titanium cyanine, small-sized palladium nanoparticles and various types of single atoms can be synthesized and loaded onto nitrogen-doped carbon supports, giving the material scalability.

[0032] This invention provides a method for loading small-sized palladium nanoparticles and single atoms onto a nitrogen-doped carbon support. By simultaneously introducing a palladium metal source and a single-atom metal source into the reaction, palladium polymerizes to form small-sized nanoparticles, while the single-atom metal is uniformly dispersed on the carbon substrate, without agglomeration to form particles or clusters. The small-sized palladium nanoparticles have a higher surface area, providing more active sites. The introduction of single atoms can modulate the electronic state of the palladium nanoparticles, making them more conducive to nitrate adsorption. At the same time, the highly dispersed single atoms provide more active sites, and the palladium particles and metal single atoms can catalyze nitrate reduction in tandem.

[0033] The beneficial effects of this invention are:

[0034] (1) This invention provides a catalyst preparation method that modulates the electronic state of palladium nanoparticles by introducing different metal single atoms. The palladium nanoparticles with regulated electronic state are more likely to adsorb nitrates. The metal single atoms can provide more additional active sites and form a tandem catalytic reaction with the metal particles, which greatly improves the performance of the catalyst in electrocatalytic reduction of nitrates.

[0035] (2) The nitrogen-doped carbon-supported palladium nanoparticles and iron single-atom catalyst obtained in this invention exhibit excellent electrochemical performance in the electrocatalytic reduction of high-concentration nitrates. Traditional palladium nanoparticles supported on nitrogen-doped carbon have a Faraday efficiency of only 35% for ammonium ions. The composite catalyst Pd@Fe obtained in this invention… SA In the electroreduction process, palladium nanoparticles more readily adsorb nitrates and reduce them to the key intermediate nitrite. Iron single atoms can then catalyze the reduction of nitrite to ammonia, significantly improving catalyst selectivity; the Faraday efficiency for ammonium ions can reach 93%. Furthermore, the palladium nanoparticles in the catalyst obtained in this invention are coated with a carbon shell. It is precisely this carbon shell's confinement effect that prevents significant change in current density after eight cycles at a constant voltage. NH3 >90%, exhibiting excellent electrocatalytic stability. Attached Figure Description

[0036] Figure 1 The Pd@Fe obtained in Example 1 of this invention SA X-ray diffraction pattern of NC.

[0037] Figure 2 The Pd@Fe obtained in Example 1 of this invention SA - Transmission electron microscopy image of NC.

[0038] Figure 3 The Pd@Fe obtained in Example 1 of this invention SA -High-angle annular dark-field scanning transmission electron microscope image from NC.

[0039] Figure 4 The Pd@Fe obtained in Example 1 of this invention SA -NC X-ray photoelectron spectrum.

[0040] Figure 5 The Pd@Fe obtained in Example 1 of this invention SA - NC's electroreduction performance diagram for nitrates.

[0041] Figure 6 Fe obtained in Example 3 of the present invention SA - NC's performance diagram for the electroreduction of nitrite. Detailed Implementation

[0042] The specific embodiments of the present invention will be further described below with reference to the accompanying drawings and technical solutions.

[0043] Example 1

[0044] Solution A: Dissolve 270 mg of 3-AP, 90 mg of DCD, 31.4 mg of K₂PdCl₄, and 4.4 mg of FePc in 50 mL of H₂O. Solution B: Dissolve 280 mg of HMT and 160 mg of F127 in 30 mL of H₂O. Stir solutions A and B at room temperature for 30 min to ensure uniform dispersion. Then mix the two solutions and continue stirring at room temperature for 30 min to allow for complete reaction. Transfer the mixed solution to a 100 mL polytetrafluoroethylene-lined reactor and perform a closed hydrothermal reaction at 80 °C for 24 h. Cool the reactor to room temperature, wash the product obtained from the hydrothermal reaction with a large amount of deionized water, and freeze-dry the obtained solid at -50 °C and a vacuum of 15 kPa for 8 h to obtain PdFe-NC.

[0045] The freeze-dried solid was ground, and a small portion was carbonized. First, it was heated to 400℃ at 5℃ / min and held for 4 hours in an H2 / Ar mixed atmosphere (H2 to Ar volume ratio 1:19), then heated to 600℃ and held for 1 hour. After the temperature holding period, it was allowed to cool naturally to room temperature. The resulting powder was then ground and heated to 180℃ at 2℃ / min and held for 3 hours in an O3 atmosphere to obtain Pd@Fe. SA -NC.

[0046] Among these methods, carbonization in a reducing atmosphere (H2 / Ar mixture) can reduce high-valence metals. Treatment in an O3 atmosphere can thin the carbon layer in the catalyst structure, making the metal sites more prominent and improving the catalyst's catalytic activity.

[0047] Inductively coupled plasma mass spectrometry (ICP-MS) analysis showed that the catalyst contained 11.09 wt% Pd and 0.32 wt% Fe.

[0048] Electrochemical performance testing:

[0049] Weigh out 2 mg of Pd@Fe SA Add NC, and then add 238 μL of anhydrous ethanol, 238 μL of deionized water and 25 μL of 0.5 wt.% Nafion solution. Sonicate for 30 min to form a uniform dispersion.

[0050] 100 μL of the resulting dispersion was dropped onto carbon paper (0.5 cm thick). 2 Let it air dry at room temperature.

[0051] All electrochemical tests in this invention were performed using a conventional three-electrode battery on a CHI660E electrochemical workstation. The electrolyte was an alkaline mixture of 0.5M Na₂SO₄ and 0.1M NaNO₃, with the pH adjusted to 12 using 1M NaOH solution. Hg / HgO was used as the reference electrode, platinum wire as the counter electrode, and catalyst-coated carbon paper as the working electrode. The conversion formula between electrode potential and RHE is: E(vs.RHE)=E(vs.Hg / HgO)+0.095V+0.0596×pH.

[0052] The test procedure involved electroreduction of nitrates in an Ar-saturated alkaline electrolyte of 0.5 M Na₂SO₄ and 0.1 M NaNO₃ (pH = 12). Electrolysis was performed at a constant voltage for 15 min within a voltage range of -0.5 to -0.9 V vs. RHE. Pd@Fe SA -NC can achieve a maximum Faraday efficiency of 93% in ammonia production. The nitrogen-doped carbon-supported palladium nanoparticles and iron single-atom catalyst prepared in this invention exhibit high selectivity and efficiency in the electrochemical reduction of nitrate.

[0053] To more clearly illustrate the technical solutions in the embodiments of the present invention, the accompanying drawings used in the description of Embodiment 1 will be briefly introduced below:

[0054] Figure 1 The Pd@Fe obtained in Example 1 SA - The X-ray diffraction pattern of NC. It can be seen from the figure that the broad diffraction peak is located at 23°, which is the (002) plane of graphitized carbon, and the diffraction peak at 40° corresponds to the metallic palladium, which corresponds to the Pd (111) plane. However, no diffraction peaks related to iron were detected, which confirms that iron is well dispersed on the carbon substrate and does not exist in the form of iron nanoparticles.

[0055] Figure 2 The Pd@Fe obtained in Example 1 SA The transmission electron microscope (TEM) image of the NC nanoparticles shows that palladium nanoparticles are uniformly distributed on the carbon substrate, and the size of the palladium nanoparticles is very small, all concentrated at around 3 nm.

[0056] Figure 3 The Pd@Fe obtained in Example 1 SA - A high-angle annular dark-field scanning transmission electron microscope (STEM) image from NC reveals small palladium nanoparticles surrounded by distinct single atoms. This result is consistent with the XRD pattern.

[0057] Figure 4 The Pd@Fe obtained in Example 1 SAThe X-ray photoelectron spectroscopy of -NC clearly shows that palladium shifts to a higher energy field after nitrogen is introduced into the carbon substrate, indicating that palladium metal loses electrons. After introducing iron single atoms, palladium continues to shift to a higher energy field, indicating that the addition of iron can make palladium lose more electrons. The loss of electrons in palladium nanoparticles makes it easier to adsorb nitrates, which is beneficial to improving the performance of the catalyst in electrocatalyzing nitrate reduction.

[0058] Figure 5 The Pd@Fe obtained in Example 1 SA Performance diagram of NC electrocatalytic nitrate. Pd@Fe SA -NC exhibits excellent electrocatalytic performance in ammonia synthesis, with high selectivity for the product ammonia gas. Its highest ammonia Faraday efficiency reaches 93%, and it also has high current density and good stability.

[0059] Example 2

[0060] Solution A: Dissolve 270 mg of 3-AP, 90 mg of DCD, and 31.4 mg of K₂PdCl₄ in 50 mL of H₂O. Solution B: Dissolve 280 mg of HMT and 160 mg of F127 in 30 mL of H₂O. Stir solutions A and B at room temperature for 30 min to ensure uniform dispersion. Then mix the two solutions and continue stirring at room temperature for 30 min to allow for complete reaction. Transfer the mixed solution to a 100 mL polytetrafluoroethylene-lined reactor and perform a closed hydrothermal reaction at 80 °C for 24 h. Cool the reactor to room temperature, wash the product obtained from the hydrothermal reaction with a large amount of deionized water, and freeze-dry the obtained solid at -50 °C and a vacuum of 15 kPa for 8 h to obtain Pd-NC.

[0061] The freeze-dried solid was ground, and a small portion was carbonized. First, it was heated to 400℃ at 5℃ / min in an H2 / Ar atmosphere and held for 4h. Then, it was heated to 600℃ and held for 1h. After the temperature holding period, it was naturally cooled to room temperature. The resulting powder was ground and then heated to 180℃ at 2℃ / min in an O3 atmosphere and held for 3h to obtain Pd@NC.

[0062] The same electrochemical testing method as in Example 1 was used.

[0063] The maximum Faraday efficiency for ammonia production using Pd@NC can reach 35%. Compared to Example 1, in this example, since no single atom was introduced into the palladium nanoparticle prototype system, the electronic state of the palladium nanoparticles was not regulated, making it difficult to adsorb nitrates and perform electrocatalytic reduction. A large amount of hydrogen gas, a byproduct, was generated during the electrocatalytic reduction of nitrates.

[0064] Example 3

[0065] Solution A: Dissolve 270 mg of 3-AP, 90 mg of DCD, and 4.4 mg of FePc in 50 mL of H2O. Solution B: Dissolve 280 mg of HMT and 160 mg of F127 in 30 mL of H2O. Stir solutions A and B at room temperature for 30 min to ensure uniform dispersion. Then mix the two solutions and continue stirring at room temperature for 30 min to allow for complete reaction. Transfer the mixed solution to a 100 mL polytetrafluoroethylene-lined reactor and perform a closed hydrothermal reaction at 80 °C for 24 h. Cool the reactor to room temperature, wash the product obtained from the hydrothermal reaction with a large amount of deionized water, and freeze-dry the obtained solid at -50 °C and a vacuum of 15 kPa for 8 h.

[0066] The freeze-dried solid was ground, and a small portion was carbonized. First, the temperature was increased to 400℃ at 5℃ / min and held for 4 hours in an H2 / Ar atmosphere, then increased to 600℃ and held for 1 hour. After the temperature holding period, it was allowed to cool naturally to room temperature. The resulting powder was then ground and then heated to 180℃ at 2℃ / min and held for 3 hours in an O3 atmosphere to obtain Fe. SA -NC.

[0067] The same electrochemical testing method as in Example 1 was used.

[0068] Fe SA -NC electrocatalytic reduction of nitrate to ammonia can achieve a maximum Faraday efficiency of 65%; however, due to the absence of K2PdCl4, the catalyst lacks palladium nanoparticles, resulting in reduced nitrate adsorption and decreased performance.

[0069] Figure 6 Fe obtained in Example 3 SA -Performance diagram of electroreduction of nitrite by NC. Fe SA -NC exhibits excellent electrocatalytic reduction of nitrite to ammonia, showing high selectivity for the product ammonia, with a maximum ammonia Faradaic efficiency of 100%, high current density, and good stability. This indicates that iron single atoms possess excellent electrocatalytic reduction activity for nitrite. Introducing them into the palladium nanoparticle prototype system not only modulates the electronic state of the palladium nanoparticles to facilitate nitrate adsorption but also further reduces the nitrite generated during the nitrate reduction process, thus catalyzing the nitrate reduction in tandem with the palladium nanoparticles.

[0070] Example 4

[0071] Solution A: Dissolve 270 mg of 3-AP, 90 mg of DCD, 31.4 mg of K₂PdCl₄, and 4.4 mg of CuPc in 50 mL of H₂O. Solution B: Dissolve 280 mg of HMT and 160 mg of F127 in 30 mL of H₂O. Stir solutions A and B at room temperature for 30 min to ensure uniform dispersion. Then mix the two solutions and continue stirring at room temperature for 30 min to allow for complete reaction. Transfer the mixed solution to a 100 mL polytetrafluoroethylene-lined reactor and perform a closed hydrothermal reaction at 80 °C for 24 h. Cool the reactor to room temperature, wash the product obtained from the hydrothermal reaction with a large amount of deionized water, and freeze-dry the obtained solid at -50 °C and a vacuum of 15 kPa for 8 h.

[0072] The freeze-dried solid was ground, and a small portion was carbonized. First, the temperature was increased to 400℃ at 5℃ / min and held for 4 hours in an H2 / Ar atmosphere, then increased to 600℃ and held for 1 hour. After the temperature holding period, it was allowed to cool naturally to room temperature. The resulting powder was then ground and then heated to 180℃ at 2℃ / min and held for 3 hours in an O3 atmosphere to obtain Pd@Cu. SA -NC.

[0073] The same electrochemical testing method as in Example 1 was used.

[0074] Pd@Cu SA -NC can achieve a maximum Faraday efficiency of 70% in ammonia production.

[0075] Example 5

[0076] Solution A: Dissolve 270 mg of 3-AP, 90 mg of DCD, 31.4 mg of K₂PdCl₄, and 4.4 mg of CoPc in 50 mL of H₂O. Solution B: Dissolve 280 mg of HMT and 160 mg of F127 in 30 mL of H₂O. Stir solutions A and B at room temperature for 30 min to ensure uniform dispersion. Then mix the two solutions and continue stirring at room temperature for 30 min to allow for complete reaction. Transfer the mixed solution to a 100 mL polytetrafluoroethylene-lined reactor and perform a closed hydrothermal reaction at 80 °C for 24 h. Cool the reactor to room temperature, wash the product obtained from the hydrothermal reaction with a large amount of deionized water, and freeze-dry the obtained solid at -50 °C and a vacuum of 15 kPa for 8 h.

[0077] The freeze-dried solid was ground, and a small portion was carbonized. First, the temperature was increased to 400℃ at 5℃ / min and held for 4 hours in an H2 / Ar atmosphere, then increased to 600℃ and held for 1 hour. After the temperature holding period, it was allowed to cool naturally to room temperature. The resulting powder was then ground and then heated to 180℃ at 2℃ / min and held for 3 hours in an O3 atmosphere to obtain Pd@Co. SA -NC.

[0078] The same electrochemical testing method as in Example 1 was used.

[0079] Pd@Co SA -NC can achieve a maximum Faraday efficiency of 85% in ammonia production.

[0080] As can be seen from the above embodiments, the present invention prepares nitrogen-doped carbon-supported palladium nanoparticles and single-atom catalyst Pd@M... SA -NC, in its synthesis, simultaneously introduces a palladium source and a single-atom metal source. Through polymerization, palladium atoms form uniform nanoparticles with a diameter of 3 nm. These single atoms are uniformly dispersed on a nitrogen-doped carbon substrate, achieving a simple process for simultaneously introducing nanoparticles and single atoms, with the single atoms being replaceable. The introduced single atoms affect the electronic state of the palladium nanoparticles, further optimizing the adsorption energy of palladium nanoparticles for nitrate. Simultaneously, the single atoms exhibit excellent reduction performance for nitrate reduction intermediates. The palladium nanoparticles and single atoms catalyze the reduction of nitrate in tandem, achieving efficient activation and directional transformation of the reactants.

[0081] Matters not covered in this invention are common knowledge.

Claims

1. A method for preparing nitrogen-doped carbon-supported small-sized palladium nanoparticles and a single-atom catalyst, characterized in that the method includes the following steps: (1) K2PdCl4, metal phthalocyanine, 3-aminophenol (3-AP), and dicyandiamide (DCD) were added to water and stirred to obtain solution A. At the same time, hexamethylenetetramine (HMT) and F127 were added to water and stirred to obtain solution B. Then, solution A and solution B were mixed and stirred for 30-60 min, and then transferred to a hydrothermal reactor and hydrothermally reacted at 60-120℃ for 24-48 h. After naturally cooling to room temperature, the solid product obtained by washing and filtration was freeze-dried to obtain the product PdM-NC, where M is Fe, Cu or Co. The metal phthalocyanine is iron phthalocyanine, copper phthalocyanine, or cobalt phthalocyanine; In solution A, the mass ratio of dicyandiamide (DCD), 3-aminophenol (3-AP), K2PdCl4, and metal phthalocyanine is 1:1~3:0.15~0.55:0.03~0.

2. In solution B, the mass ratio of HMT to F127 is 3:2 to 2:

1. After mixing solutions A and B, the mass ratio of K2PdCl4 to HMT is 1:16~6; The concentration range of K2PdCl4 in solution A is 0.5-1.0 mg / ml; (2) The solid powder obtained by freeze drying is ground and then carbonized in an atmosphere of hydrogen and argon; it is ground again and then carbonized in an ozone atmosphere to obtain the final product Pd@M. SA -NC, which stands for small palladium nanoparticles and single-atom supported on nitrogen-doped carbon supports; The carbonization conditions under the ozone atmosphere are a heating rate of 1-3℃ / min, heating to 150-200℃ and holding for 1-3 h.

2. The method for preparing nitrogen-doped carbon-supported small-sized palladium nanoparticles and single-atom catalysts as described in claim 1, characterized in that: In step (1), the freeze-drying temperature is -30℃ to -50℃ and the vacuum degree is 15-20 kPa.

3. The method for preparing nitrogen-doped carbon-supported small-sized palladium nanoparticles and single-atom catalysts as described in claim 1, characterized in that... In step (1), the volume ratio of solution A to solution B is 5:3-5:

1.

4. The method for preparing nitrogen-doped carbon-supported small-sized palladium nanoparticles and single-atom catalysts as described in claim 1, characterized in that... In step (2), the carbonization conditions under the hydrogen and argon atmosphere are a heating rate of 3-5℃ / min, heating to 400-700℃ and holding for 3-5 h.

5. The method for preparing nitrogen-doped carbon-supported small-sized palladium nanoparticles and single-atom catalysts as described in claim 1, characterized in that... The small palladium nanoparticles have a particle size of 2-5 nm; the single atom is a Fe single atom, a Cu single atom, or a Co single atom.

6. The method for preparing nitrogen-doped carbon-supported small-sized palladium nanoparticles and single-atom catalyst as described in claim 1, characterized in that the loading of Pd in ​​the catalyst is 5~15wt%, and the loading of Fe is 0.05~0.50wt%.

7. The application of the nitrogen-doped carbon-supported palladium nanoparticles and single-atom catalyst prepared by the method of claim 1, characterized in that they are used as working electrode materials for electrocatalytic ammonia synthesis.

8. The application as described in claim 7, characterized by comprising the following steps: adding nitrogen-doped carbon-supported small-sized palladium nanoparticles and a single-atom catalyst to a mixed solution, ultrasonically dispersing for 20-40 min to obtain a uniformly dispersed liquid, and then dropping the dispersion onto carbon paper and allowing it to dry naturally at room temperature; Electrolysis was performed under three-electrode conditions: Hg / HgO was used as the reference electrode, platinum wire as the counter electrode, carbon paper with nitrogen-doped carbon-supported small palladium nanoparticles and single-atom catalysts as the working electrode, and an alkaline solution containing Na2SO4 and NaNO3 with a pH of 12 was used as the electrolyte; constant voltage electrolysis was carried out for 0.1 to 0.5 h in the voltage range of -0.5 to -0.9 V vs. RHE. The mixed solution is composed of anhydrous ethanol, deionized water, and Nafion solution, with a volume ratio of 1:1:0.1 to 0.

3. 1–4 mg of nitrogen-doped carbon-supported small palladium nanoparticles and a single-atom catalyst are added to every 200–300 μL of the mixed solution; The carbon paper is 0.3–0.7 cm thick. 2 80–120 μL of dispersion needs to be added dropwise; In the electrolyte, the concentration of Na2SO4 is 0.4–0.7 M; the concentration of NaNO3 is 0.05–0.1 M; and the pH of the electrolyte is adjusted to 12 using 1 M NaOH solution.

9. The application as described in claim 8, characterized in that the mass fraction of the Nafion solution is 0.4 to 0.6 wt.%.

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