PdCu@UiO-S@PDMS Core-Shell Structure Composite Catalyst and Its Preparation Method and Use

Abstract

The present invention relates to the technical field of composite catalysts, and particularly to a PdCu@UiO-S@PDMS core-shell structured composite catalyst, its preparation method and use. The catalyst has a core-shell structure, that is, PdCu nanoparticles are coated in UiO-66(SO3H), and the outer surface of UiO-66(SO3H) is coated with polydimethylsiloxane (PDMS). In the present invention, a PdCu precursor is introduced into the pores of UiO-66(SO3H), then reduced by H2, and finally PDMS is thermally deposited to obtain a PdCu@UiO-S@PDMS core-shell structured composite catalyst. The catalyst prepared by the present invention has excellent electrocatalytic nitrogen fixation activity, and uses Cu to optimize the electronic state of the Pd catalytic site; PDMS generates a hydrophobic microenvironment around the metal site to prevent water molecules from entering the catalytic site, thereby effectively inhibiting the competitive hydrogen evolution side reaction; in addition, the deprotonated functional groups on UiO-66(SO3H) can capture protons through the hydrophobic microenvironment. The above factors can effectively synergistically promote the progress of the reaction. The preparation process of the present invention is simple, the reaction conditions are mild, the metal loading is only about 2%, and the reuse rate is high, having high practical value and application prospects.

Description

Technical Field

[0001] The present invention relates to the technical field of composite catalysts, and particularly to a PdCu@UiO-S@PDMS core-shell structured composite catalyst, its preparation method and application. In this catalyst, PdCu alloy nanoparticles are coated into UiO-66(SO 3 H), and the surface of UiO-66(SO 3 H) is coated with polydimethylsiloxane (PDMS). Background Art

[0002] Ammonia (NH 3 ) is an important chemical and plays an important role in the economy, being widely used in fields such as dyes and fertilizers. Currently, ammonia is synthesized industrially by the Haber-Bosch method, which consumes 2% of the global energy and emits 1.5% of greenhouse gases, bringing serious energy and environmental problems. Therefore, developing new green and efficient ammonia synthesis technologies is of great significance. In recent years, bioenzymes, photocatalysis, and electrocatalysis have received attention because they can synthesize NH 3 under normal temperature and pressure. Among them, the electrocatalytic method can directly synthesize ammonia by reacting N 2 with H 2 O, and its power is provided by renewable energy, having the advantages of environmental protection and high efficiency. However, the current bottleneck faced by NRR is that N 2 is very stable and difficult to be activated, resulting in too high a reaction energy barrier; in addition, in aqueous electrocatalysis, a competitive hydrogen evolution reaction (HER) is likely to occur, and designing an efficient catalyst is expected to solve this problem and improve the NH 3 yield and Faraday efficiency.

[0003] Pd metal nanoparticles are considered to be a good electrocatalytic material. Among them, the regulation of the electronic state of the metal active site and its surrounding microenvironment is considered to play a key role in the catalytic performance. Unfortunately, molecular hydrogen is easily poisoned on the Pd surface because its binding to hydrogen adsorbed atoms is stronger than that of nitrogen.

[0004] In addition to surface electronic state regulation, the surface microenvironment of metal nanoparticles is usually adjusted by modifying various functional molecules on their surfaces, such as surfactants, which actually is not conducive to the accessibility and activity of active metal sites. Encapsulating metal nanoparticles into porous materials with customizable structures may be a good solution because metal nanoparticles with bare surfaces and small sizes can be confined in the pore space, and the interconnected pores make them easily accessible to the substrate. To inhibit the competitive HER reaction, creating a hydrophobic microenvironment around the metal sites will prevent water molecules from entering the catalytic sites. However, the electrochemical nitrogen reduction reaction (NRR) is a proton-involved process related to gradual hydrogenation steps, in which H +The participation of [[]] is crucial. The generation of a hydrophobic microenvironment around the above metal sites will inhibit the progress of the NRR reaction. Summary of the Invention

[0005] Aiming at the technical problems such as low ammonia production rate and efficiency of existing electrochemically synthesized ammonia catalysts, the present invention provides a preparation method and application of a PdCu@UiO-S@PDMS core-shell structure composite catalyst. The Pd-Cu precursor is introduced into the MOF pores by a double-solvent method, then reduced by hydrogen, and finally a layer of polydimethylsiloxane is physically deposited to obtain a PdCu@UiO-S@PDMS core-shell structure composite catalyst, which has good electrochemically synthesized ammonia activity.

[0006] To achieve the above object, the present invention adopts the following technical solutions: A preparation method of a PdCu@UiO-S@PDMS core-shell structure composite catalyst, comprising the following steps:

[0007] Step S1: Add the zirconium-based MOF catalyst UiO-66(SO 3 H), abbreviated as UiO-S, with a size of 120-460 nanometers into n-hexane, ultrasonically disperse for 1-2 h to obtain a dispersion liquid, and then sequentially dropwise add the potassium palladium chloride / dichloride copper dihydrate precursor mixture into the above dispersion liquid. The molar concentration ratio of potassium palladium chloride to dichloride copper dihydrate in the potassium palladium chloride / dichloride copper dihydrate precursor mixture is 9:1, and the molar concentration of potassium palladium chloride is 0.4-0.6 mol / L. Keep stirring during the dropping process until the total molar amount of palladium and copper in the dispersion liquid reaches 0.01-0.02 mmol / 50 mg UiO-S. Continue ultrasonicating for a period of time and then centrifuge and dry to obtain a precursor precipitate;

[0008] Step S2: Place the precursor precipitate in a quartz tube, introduce nitrogen or an inert gas or a H 2 / Ar mixed gas with a gas flow rate of 30-50 mL / min, and react at 200-300 °C for 3-5 h to prepare a precursor in which Pd and Cu alloy nanoparticles are coated in UiO-66(SO 3 H), abbreviated as PdCu@UiO-S precursor;

[0009] Step S3: Place the solidified polydimethylsiloxane, i.e., PDMS, and the above PdCu@UiO-S precursor in a porcelain boat, then put them together into a tube furnace, introduce nitrogen or an inert gas or a H 2 / Ar mixed gas, perform heat treatment on the tube furnace, and after cooling, prepare a product in which Pd and Cu alloy nanoparticles are coated in UiO-66(SO 3 H) and UiO-66(SO 3H) A core-shell structured composite catalyst with a PDMS coating, simply referred to as the PdCu@UiO-S@PDMS core-shell structured composite catalyst.

[0010] As a further improvement to the preparation method of the PdCu@UiO-S@PDMS core-shell structured composite catalyst:

[0011] Preferably, the preparation method of UiO-S in step S1 is as follows:

[0012] Step S11: Dissolve zirconium chloride in N,N-dimethylformamide (DMF), stir at room temperature, and use it as solution a; dissolve terephthalic acid and sodium 2-sulfoterephthalate in DMF, stir at room temperature, and use it as solution b; mix solution a and solution b and continue stirring at room temperature, and adjust with acetic acid to obtain a mixed precursor solution;

[0013] Step S12: Place the mixed precursor solution from step S11 in a reaction at 110 - 130 °C for 18 - 36 h, cool to room temperature, centrifuge the obtained product, add the precipitate to H 2 SO 4 solution for dispersion and immersion to completely acidify the sulfonate groups, then wash with DMF and methanol and dry overnight under vacuum at 60 - 80 °C to obtain the zirconium-based MOF catalyst UiO-66(SO 3 H), simply referred to as UiO-S.

[0014] Preferably, in the mixed precursor solution in step S11, the molar ratio of zirconium chloride, terephthalic acid, sodium 2-sulfoterephthalate, and acetic acid is 1:1:2:(30 - 130), and the concentration of acetic acid is 0.525 - 2.1 mol / L.

[0015] Preferably, when preparing solution a, preparing solution b, and mixing solution a and solution b in step S11, the stirring speed at room temperature is 500 revolutions per minute, and the stirring time is 30 - 60 minutes.

[0016] Preferably, in step S12, the concentration of the H 2 SO 4 solution is 0.01 - 0.05 M, and the time for the precipitate to be dispersed and immersed in the H 2 SO 4 solution is 30 - 60 minutes.

[0017] Preferably, in step S3, the heat treatment temperature is 200 - 300 °C and the heat treatment time is 0.5 - 2 h.

[0018] Preferably, the solidified PDMS in step S3 is placed upstream of the inlet of the tube furnace in the porcelain boat, and the PdCu@UiO-S precursor is placed downstream of the inlet of the tube furnace in the porcelain boat, and a glass sheet is covered on the upper part of the porcelain boat, leaving only a small gap.

[0019] The second object of the present invention is to provide a PdCu@UiO-S@PDMS core-shell structure composite catalyst prepared by the above preparation method.

[0020] The third object of the present invention is to provide an application of the above PdCu@UiO-S@PDMS core-shell structure composite catalyst in the electrochemical synthesis of ammonia reaction.

[0021] As a further improvement of the application of the PdCu@UiO-S@PDMS core-shell structure composite catalyst in the electrochemical synthesis of ammonia reaction:

[0022] Preferably, the PdCu@UiO-S@PDMS core-shell structure composite catalyst is dispersed in water, ethanol or Nafion solution, and after ultrasonic treatment, it is dropped on a hydrophobic carbon paper to make a working electrode, and the electrochemical synthesis of ammonia reaction is carried out by using a three-electrode system.

[0023] The beneficial effects of the present invention compared with the prior art are as follows:

[0024] (1) The preparation process of the PdCu@UiO-S@PDMS core-shell structure composite catalyst provided by the present invention is simple and easy to control, the reaction conditions are mild, the metal loading is only about 2%, and the recycling rate is high, which has high practical value and application prospects. In the preparation process of the catalyst of the present invention, the reaction principles of each step are as follows:

[0025] S1. Prepare a mixed precursor solution. The role of acetic acid is a regulator, which is used to control the morphology and size of UiO-S nanoparticles. The more acetic acid is added, the smaller the particle size of the zirconium-based MOF catalyst UiO-66(SO 3 H) will be, and its size and morphology have a great influence on the catalytic performance.

[0026] S2. Add the synthesized product UiO-S to sulfuric acid for soaking to completely acidify the sulfonate groups; vacuum overnight for drying and activating UiO-S to remove small molecules such as H 2 O in its pores.

[0027] S3. Disperse UiO-S in hydrophobic n-hexane. Since the interior of the UiO-S pores is hydrophilic, when an aqueous solution of a metal precursor is dropped, due to capillary action, the hydrophilic metal precursor will enter the hydrophilic UiO-S pores, which is beneficial to the formation of the subsequent core-shell structure.

[0028] S4. Use nitrogen or inert gas or H2 The low-temperature reaction of the Ar mixed gas is mainly used to reduce metal precursors.

[0029] S5. Since the boiling point of PDMS is 155 - 220 °C, heat treatment at a temperature higher than its boiling point can cause partial vaporization of PDMS, which deposits on the surface of the PdCu@UiO-S precursor to hydrophobically modify its surface.

[0030] (2) The PdCu@UiO-S@PDMS catalyst of the present invention has a core-shell structure, where Pd serves as the active site for the NRR reaction. The introduction of Cu regulates the electronic state of Pd to avoid poisoning of molecular hydrogen on the Pd surface. The hydrophobic microenvironment created by the PDMS layer around the active site can effectively inhibit the competitive HER reaction. In addition, the introduction of the protonated functional group sulfonic acid (-SO 3 H) will provide protons for the NRR process, and the deprotonated -SO 3 - may be able to capture protons through the hydrophobic microenvironment. The synergistic effect of the above parts enables the PdCu@UiO-S@PDMS catalyst to have excellent electrochemical ammonia synthesis performance.

[0031] (3) The PdCu@UiO-S@PDMS core-shell structured composite catalyst of the present invention has excellent ammonia production rate and selectivity in electrochemical ammonia synthesis. In a 0.1 molar hydrochloric acid solution, the ammonia production rate reaches 20.24 μg h -1 mg cat. -1 , and the Faraday efficiency can reach 13.16%. Description of the Drawings

[0032] To more clearly illustrate the technical solutions of the embodiments of the present invention, the following will briefly introduce the drawings required for the description of the embodiments. Obviously, the drawings in the following description are only some embodiments of the present invention. For those of ordinary skill in the art, without creative efforts, other drawings can also be obtained based on these drawings.

[0033] Figure 1 This is the morphological characterization diagram of the finally prepared PdCu@UiO-S@PDMS core-shell structured composite catalyst in Example 1 of the present invention, where Figure 1 a - d are respectively the field emission scanning electron microscope (FESEM) photo, low-magnification TEM photo, high-magnification TEM photo, and different element distribution maps of the catalyst.

[0034] Figure 2 This is the performance comparison diagram of different composite catalysts, where Figure 2a is the performance comparison diagram of the PdCu@UiO-S@PDMS core-shell structure composite catalyst prepared in Example 1 of the present invention; Figure 2 b is the comparison of the electrochemical ammonia synthesis performance of carbon paper, the catalyst prepared in Comparative Example 1, the catalyst prepared in Example 1, the catalyst prepared in Comparative Example 2, and the catalyst prepared in Comparative Example 3 at a voltage of -0.25 V; Figure 2 c is the nuclear magnetic resonance performance characterization of the PdCu@UiO-S@PDMS core-shell structure composite catalyst prepared in Example 1; Figure 2 d is the 5-cycle performance comparison diagram of the PdCu@UiO-S@PDMS core-shell structure composite catalyst prepared in Example 1.

[0035] Figure 3 In a-d are the X-ray photoelectron spectroscopy (XPS) comparison, diffuse reflectance infrared Fourier transform (DRIFT) spectroscopy comparison of CO adsorption, and in-situ Fourier transform infrared (FT-IR) spectroscopy comparison of the finally prepared PdCu@UiO-S@PDMS core-shell structure composite catalyst in Example 1 of the present invention and the Pd@UiO-S@PDMS core-shell structure composite catalyst prepared in Comparative Example 4.

[0036] Figure 4 In a-c are the comparison of Gibbs free energy, d-band center, and electron localization function (ELF) of different reaction steps of the finally prepared PdCu@UiO-S@PDMS core-shell structure composite catalyst in Example 1 of the present invention and the Pd@UiO-S@PDMS core-shell structure composite catalyst prepared in Comparative Example 4. Detailed implementation mode

[0037] In order to make the purpose, technical solution and advantages of the present invention clearer, the present invention will be further described in detail below with reference to the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative efforts belong to the protection scope of the present invention.

[0038] Example 1

[0039] (1) Dissolve 40.8 mg of zirconium chloride in 5 ml of N,N-dimethylformamide (DMF), stir at room temperature and use it as solution a; dissolve 26.6 mg of terephthalic acid and 9.27 mg of sodium 2-sulfoterephthalate monohydrate in 5 ml of DMF, stir at room temperature and use it as solution b; mix solution a and solution b, add 1.2 ml of acetic acid with a concentration of 17.5 mol / L to adjust, and the molar ratio of zirconium chloride, terephthalic acid, sodium 2-sulfoterephthalate monohydrate and acetic acid in the mixed precursor solution is 1:1:2:130, and continuously stir at a rotation speed of 500 r / min at room temperature for 60 min to obtain a precursor mixed solution;

[0040] (2) Take 20 ml of the mixed precursor solution in step S1 and react it at 120 °C for 24 h. After cooling to room temperature, centrifuge the obtained product. Add the precipitate to 15 ml of an H 2 SO 4 solution and disperse and soak it for 40 min to completely acidify the sulfonate groups. Then wash it with DMF and methanol and dry it overnight under vacuum at 60 °C to obtain UiO-66(SO 3 H), abbreviated as UiO-S; after testing, the size of UiO-S is 120 - 460 nanometers;

[0041] (3) Add 50 mg of UiO-S prepared in step (2) to 10 ml of n-hexane, ultrasonically disperse it for 1 h to obtain a dispersion. Then, sequentially add dropwise the potassium chloropalladate / dichloride copper dihydrate precursor mixture to the above dispersion. The potassium chloropalladate / dichloride copper dihydrate precursor mixture is composed of 18 μl of a 0.6 mol / L potassium chloropalladate solution and 2 μl of a 0.6 mol / L dichloride copper dihydrate solution. Stir continuously during the dropping process until the total amount of palladium + copper in the dispersion is 0.012 mmol. Then continue to ultrasonically disperse for 3 h, and then centrifuge and dry to obtain a precursor precipitate;

[0042] (4) Place the precursor precipitate in a quartz tube, and introduce H 2 an H 2 / Ar mixed gas with a volume ratio of 20%, a gas flow rate of 40 ml / min, and react at 200 °C for 4 h to prepare the PdCu@UiO-S precursor;

[0043] (5) Place 5 g of solidified PDMS and the above 50 mg of PdCu@UiO-S precursor in a porcelain boat, and then put them together into a tube furnace. Among them, introduce a protective atmosphere of N 2 . The solidified PDMS is placed upstream of the inlet of the tube furnace, and the PdCu@UiO-S precursor is placed downstream of the inlet of the tube furnace. And cover the upper part of the porcelain boat with a glass sheet, leaving only a small gap for gas to enter. Then perform heat treatment on the tube furnace. The heat treatment temperature is 230 °C and the heat treatment time is 1 h. After cooling, prepare the PdCu@UiO-S@PDMS core-shell structure composite catalyst.

[0044] Example 2

[0045] It is recommended that the inventor add another example, which is the same as the steps of Example 1, except for the raw material ratio and process parameters, etc.

[0046] This embodiment provides a method for preparing a core-shell structured composite catalyst. The specific steps refer to Embodiment 1, with the difference being that in step (3), the potassium palladium(II) chloride / dichloride copper dihydrate precursor mixture is modified to "composed of 14 μL of a 0.6 mol / L potassium palladium(II) chloride solution and 6 μL of a 0.6 mol / L dichloride copper dihydrate solution", thereby preparing a PdCu@UiO-S@PDMS core-shell structured composite catalyst.

[0047] Comparative Example 1

[0048] This comparative example provides a method for preparing a core-shell structured composite catalyst. The specific steps refer to Embodiment 1, with the difference being that in step (5), "cured PDMS" is removed, and the PdCu@UiO-S precursor is placed alone in a porcelain boat, thereby preparing a PdCu@UiO-S core-shell structured composite catalyst.

[0049] Comparative Example 2

[0050] This comparative example provides a method for preparing a core-shell structured composite catalyst. The specific steps refer to Embodiment 1, with the difference being that in step (1), sodium 2-sulfoterephthalate monosodium salt is not added, and the addition amount of acetic acid is 0.6 mL, thereby preparing a PdCu@UiO@PDMS core-shell structured composite catalyst.

[0051] Comparative Example 3

[0052] This comparative example provides a method for preparing a core-shell structured composite catalyst. The specific steps refer to Embodiment 1, with the difference being that the process of adding the potassium palladium(II) chloride / dichloride copper dihydrate precursor mixture in step (3) is removed, thereby preparing a UiO-S@PDMS core-shell structured composite catalyst.

[0053] Comparative Example 4

[0054] This comparative example provides a method for preparing a core-shell structured composite catalyst. The specific steps refer to Embodiment 1, with the difference being that in step (3), "composed of 18 μL of a 0.6 mol / L potassium palladium(II) chloride solution and 2 μL of a 0.6 mol / L dichloride copper dihydrate solution" is changed to "20 μL of a 0.6 mol / L potassium palladium(II) chloride solution", and the total amount of palladium added to the dispersion is 0.012 mmol, thereby preparing a Pd@UiO-S@PDMS core-shell structured composite catalyst.

[0055] Specifically, during the implementation of Embodiment 1 of the present invention above, morphology, composition, and performance were detected:

[0056] (1) The morphology of the finally prepared PdCu@UiO@PDMS core-shell structured composite catalyst in Example 1 of the present invention was characterized by electron microscopy. Figure 1 In a, it is the FESEM photograph of the PdCu@UiO@PDMS core-shell structured composite catalyst finally prepared in Example 1 of the present invention. Figure 1 In b, it is the low-magnification TEM photograph of the PdCu@UiO@PDMS core-shell structured composite catalyst finally prepared in Example 1 of the present invention. Figure 1 In c, it is the high-magnification TEM photograph of the PdCu@UiO@PDMS core-shell structured composite catalyst finally prepared in Example 1 of the present invention. Figure 1 In d, it is the elemental energy spectrum diagram of the PdCu@UiO@PDMS core-shell structured composite catalyst finally prepared in Example 1 of the present invention. Figure 1 It can be seen that: the particles of the PdCu@UiO@PDMS core-shell structured composite catalyst finally prepared in Example 1 of the present invention are relatively uniform, with a size of about ~120 nanometers, and the outermost PDMS layer is about ~1 nanometer. The three elements of Pd, Cu, and Zr are evenly distributed, indicating that the catalyst has been successfully prepared.

[0057] (2) The PdCu@UiO@PDMS core-shell structured composite catalyst prepared in Example 1 of the present invention, the PdCu@UiO-S core-shell structured composite catalyst prepared in Comparative Example 1, the PdCu@UiO@PDMS core-shell structured composite catalyst prepared in Comparative Example 2, the UiO-S@PDMS core-shell structured composite catalyst prepared in Comparative Example 3, the Pd@UiO-S@PDMS core-shell structured composite catalyst prepared in Comparative Example 4, and the carbon paper substrate were respectively used as the working electrodes for the reaction. Silver / silver chloride was used as the reference electrode and a carbon rod was used as the counter electrode. Electrochemical ammonia synthesis tests were respectively carried out in a HCl solution with a concentration of 0.1 mol / L, so as to obtain the Figure 2 performance diagrams as shown.

[0058] Figure 2 In a, it is the performance comparison diagram of the PdCu@UiO@PDMS core-shell structured composite catalyst tested by the indophenol blue method at different voltages. When the voltage is -0.25 V, this catalyst has the optimal electrochemical and ammonia synthesis activities. Figure 2 In b, it is the performance comparison of different catalysts at -0.25 V. It is found that the performance of the PdCu@UiO@PDMS core-shell structured composite catalyst prepared in Example 1 is the best, and the ammonia production rate is 20.24 μg h -1 mg cat. -1 , and the Faraday efficiency reaches 13.16%. Figure 2 In c, it is when 15 N 2 is introduced as the reaction gas to obtain the product 15 NH4 + The yield was characterized by nuclear magnetic resonance spectroscopy (NMR). The results showed that the ammonia production rate of the PdCu@UiO@PDMS core-shell structured composite catalyst was 20.92 μg h -1 mg cat. -1 , which was basically consistent with the indophenol blue method. This not only confirmed the reliability of the ammonia yield but also indicated that the product NH 4 + was derived from N 2 ; Figure 2 In d of Figure 2 , it is a comparison chart of ammonia production yield and Faraday efficiency of the PdCu@UiO@PDMS core-shell structured composite catalyst obtained after 5 cycles of testing. The results showed that the catalyst had good cycling performance after 5 cycles of testing.

[0059] The PdCu@UiO-S@PDMS core-shell structured composite catalyst prepared in Example 2 was tested for the above-mentioned electrochemical synthesis of ammonia. In a 0.1 mol hydrochloric acid solution, the ammonia production rate was 16.51 μg h -1 mg cat. -1 , and the Faraday efficiency could reach 8.42%, also showing excellent ammonia production rate and selectivity.

[0060] (3) X-ray photoelectron spectroscopy (XPS) was used to characterize the PdCu@UiO@PDMS core-shell structured composite catalyst prepared in Example 1 and the Pd@UiO-S@PDMS core-shell structured composite catalyst prepared in Comparative Example 4 ( Figure 3 in a)). After introducing the second element Cu, the XPS binding energy of the PdCu@UiO@PDMS core-shell structured composite catalyst prepared in Example 1 shifted towards lower energy, indicating that it gained electrons on the surface, which would be beneficial to the adsorption of N 2 molecules on the catalyst surface. Figure 3 In b of Figure 3 , diffuse reflectance infrared Fourier transform (DRIFT) spectroscopy using CO adsorption was used to characterize the catalyst. The results showed that the introduction of Cu caused the PdCu@UiO@PDMS core-shell structured composite catalyst to shift towards lower wavenumbers, indicating that the Pd surface was electron-rich, which would be beneficial to the progress of the reaction; Figure 3 In c of Figure 3 , in-situ electrochemical Fourier transform infrared (FT-IR) spectroscopy was used to characterize the two catalysts respectively. The results showed that as the reaction time extended, the peak at 1230 wavenumbers of the PdCu@UiO@PDMS core-shell structured composite catalyst prepared in Example 1 gradually increased, while the change of the Pd@UiO-S@PDMS core-shell structured composite catalyst prepared in Comparative Example 4 was relatively small, indicating that the introduction of the second element could greatly enhance the activity of electrochemical ammonia synthesis.

[0061] (4) Figure 4In a, the Gibbs free energy of different reaction steps of the PdCu@UiO@PDMS core-shell structure composite catalyst prepared in Example 1 and the Pd@UiO-S@PDMS core-shell structure composite catalyst prepared in Comparative Example 4 was calculated by density functional theory (DFT). The results showed that N 2 The formation of H* is the rate-determining step (RDS) of both the alternating and distal pathways, where the energy barrier for RDS of PdCu@UiO-S@PDMS (0.40 eV) is much lower than that of Pd@UiO-S@PDMS (1.16 eV); in order to rationalize the lower RDS barrier of the PdCu counterpart, Figure 4 Figure b shows the d-band center of Pd, which reflects the 2 The binding strength between H* and the active site. When the d-band center of the catalytic site moves up to the Fermi level, the bond strength between the intermediate and the catalytic site will be stronger and the electron transfer will be accelerated. Compared with PdNPs, it can be seen that the introduction of Cu will cause the d-band center of the Pd site to move up, indicating that the d-band electrons on the PdCu counterpart are more occupied. Figure 4 The electron localization function (ELF) in the c-section shows that the electron overlap in PdCu@UiO-S@PDMS is higher, which indicates that N 2 The Pd-N interaction between H* and Pd sites is stronger, and the above results reveal the mechanism of the catalytic reaction.

[0062] In summary, the embodiments of the present invention not only have very good electrochemical ammonia synthesis activity, but also have a simple and easy-to-control preparation process, mild reaction conditions, a metal loading of only about 2%, a high recycling rate, and high practical value and application prospects.

[0063] The above is only a preferred specific embodiment of the present invention, but the protection scope of the present invention is not limited thereto. Any changes or substitutions that can be easily thought of by a person skilled in the art within the technical scope disclosed in the present invention should be included in the protection scope of the present invention. Therefore, the protection scope of the present invention should be based on the protection scope of the claims.

Claims

1. Preparation method of PdCu@UiO-S@PDMS core-shell structure composite catalyst, characterized in that, it includes the following steps: Step S1: Add the zirconium-based MOF catalyst UiO-66(SO 3 H), abbreviated as UiO-S, with a size of 120 - 460 nm into n-hexane, and ultrasonically disperse it for 1 - 2 h to obtain a dispersion. Subsequently, dropwise add the potassium chloropalladate / dichloride copper dihydrate precursor mixture into the above dispersion in sequence. The molar concentration ratio of potassium chloropalladate to dichloride copper dihydrate in the potassium chloropalladate / dichloride copper dihydrate precursor mixture is 9:1, and the molar concentration of potassium chloropalladate is 0.4 - 0.6 mol / L. Stir continuously during the dropping process until the total molar amount of palladium and copper in the dispersion reaches 0.01 - 0.02 mmol / 50 mg UiO-S. After continuing to ultrasonicate for a period of time, centrifuge and dry to obtain a precursor precipitate; Step S2: Place the precursor precipitate in a quartz tube, and introduce nitrogen or an inert gas or a mixed gas of H 2 , Ar. The gas flow rate is 30 - 50 mL / min, and react at 200 - 300 °C for 3 - 5 h to prepare a precursor in which Pd and Cu alloy nanoparticles are coated in UiO-66(SO 3 H), which is abbreviated as PdCu@UiO-S precursor; Step S3: Place the solidified polydimethylsiloxane, i.e., PDMS, and the above PdCu@UiO-S precursor in a porcelain boat, and then put them together into a tube furnace. Introduce nitrogen or inert gas or a mixed gas of H 2 , Ar, and conduct heat treatment in the tube furnace. After cooling, a core-shell structured composite catalyst with Pd and Cu alloy nanoparticles coated in UiO-66(SO 3 H) and the surface of UiO-66(SO 3 H) coated with PDMS is prepared, which is simply referred to as the PdCu@UiO-S@PDMS core-shell structured composite catalyst; Among them, the preparation method of UiO-S in step S1 is as follows: Step S11: Dissolve zirconium chloride in N,N-dimethylformamide (i.e., DMF), stir at room temperature and use it as solution a; dissolve terephthalic acid and sodium 2-sulfoterephthalate in DMF, stir at room temperature and use it as solution b; mix solution a and solution b and continue to stir at room temperature, add acetic acid for adjustment to obtain a mixed precursor solution; Step S12: The mixed precursor solution obtained in Step S11 is reacted at 110 - 130 °C for 18 - 36 h, cooled to room temperature, and the resulting product is centrifuged. The precipitate is added to the H 2 SO 4 solution for dispersion and immersion to fully acidify the sulfonate groups. Subsequently, it is washed with DMF and methanol and then dried under vacuum at 60 - 80 °C overnight to obtain the zirconium-based MOF catalyst UiO-66(SO 3 H), abbreviated as UiO-S.

2. The preparation method of PdCu@UiO-S@PDMS core-shell structure composite catalyst according to claim 1, characterized in that, in the mixed precursor solution in step S11, the molar ratio of zirconium chloride, terephthalic acid, sodium 2-sulfoterephthalate and acetic acid is 1:1:2:(30-130), and the concentration of acetic acid is 0.525-2.1 mol / L.

3. The preparation method of PdCu@UiO-S@PDMS core-shell structure composite catalyst according to claim 1 or 2, characterized in that, when preparing solution a, preparing solution b and mixing solution a and solution b in step S11, the stirring speed at room temperature is 500 revolutions / min, and the stirring time is 30-60 min.

4. The preparation method of PdCu@UiO-S@PDMS core-shell structure composite catalyst according to claim 1, characterized in that, In the step S12, H 2 SO 4 The concentration of the solution is 0.01 - 0.05 M, and the time for the precipitate to be dispersed and soaked in the H 2 SO 4 solution is 30 - 60 min.

5. The preparation method of PdCu@UiO-S@PDMS core-shell structure composite catalyst according to claim 1, characterized in that, the temperature of heat treatment in step S3 is 200-300 °C, and the heat treatment time is 0.5-2 h.

6. The preparation method of PdCu@UiO-S@PDMS core-shell structure composite catalyst according to claim 1, characterized in that, the solidified PDMS in step S3 is placed upstream of the inlet of the tubular furnace in the porcelain boat, and the PdCu@UiO-S precursor is placed downstream of the inlet of the tubular furnace in the porcelain boat, and a glass sheet is covered on the upper part of the porcelain boat, leaving only small gaps.

7. A PdCu@UiO-S@PDMS core-shell structure composite catalyst prepared by the preparation method according to any one of claims 1 to 6.

8. Application of the core-shell structure composite catalyst according to claim 7 in the electrochemical synthesis of ammonia reaction.

9. The application of the core-shell structure composite catalyst according to claim 8, characterized in that, disperse the PdCu@UiO-S@PDMS core-shell structure composite catalyst in water, ethanol or Nafion solution, after ultrasonic treatment, drop it on the hydrophobic carbon paper to make a working electrode, and use a three-electrode system to carry out the chemical synthesis of ammonia reaction.

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