A catalyst doped with transition metals in a molecular sieve, its preparation method and electrocatalytic application.

Abstract

This invention discloses a catalyst doped with a transition metal in a molecular sieve, its preparation method, and its electrocatalytic application. The catalyst uses NaX molecular sieve as a precursor and introduces Fe... 3+ The catalyst, named FeX, was obtained through ion exchange of ions; X-ray diffraction characterization showed that Fe... 3+ The embedding disrupts the ordered lattice of NaX, initiating a phase transition and framework collapse, resulting in an amorphous FeX. The FeX catalyst prepared in this invention, when applied to the electrocatalytic reduction of nitrate wastewater to ammonia, can [achieve a certain effect] in NO3 [reduction / depletion]. ‑ NO3 is preferentially and efficiently reduced in the H2O system. ‑ It is ammonia nitrogen, with high Faraday efficiency and ammonia yield.

Description

Technical Field

[0001] This invention belongs to the field of electrocatalysis technology, specifically relating to a catalyst doped with transition metals in a molecular sieve, its preparation method, and its electrocatalytic application. Background Technology

[0002] Currently, industrial NOx sources x While SCR technology has been successfully used to treat flue gas, its application has a fixed temperature window, making it difficult to handle low-temperature flue gas below 180℃. With the development of new energy sources and adjustments to the energy structure, low-load operation of similar natural gas units has become the norm, generating a large amount of low-temperature flue gas that is difficult for SCR technology to handle. In addition, the exhaust temperatures of industries such as cement kilns, lime kilns, and glass kilns are generally below 180℃. Therefore, wet denitrification technology, due to its strong temperature adaptability and higher operability, can become an important way to fill the gap in SCR technology. However, wet denitrification technology has a significant drawback—the generation of secondary nitrate wastewater. If the nitrate wastewater generated by wet denitrification technology is converted into ammonia through denitrification, not only can the goals of wastewater and exhaust gas emission reduction and ultra-low emission be achieved, but ammonia (NH3), a core raw material for modern agriculture and the energy industry, can also be produced, thereby maximizing economic benefits and energy utilization efficiency.

[0003] Currently, photocatalytic reduction, biological reduction, and electrocatalysis are the three mainstream processes for the removal and conversion of nitrates. Photocatalytic reduction of nitrates to ammonia is a green technology that uses light energy to drive semiconductor materials (such as titanium dioxide or tungsten-based compounds) to convert nitrates into ammonia. It has advantages such as mild reaction conditions (room temperature and pressure), environmental friendliness (using water as a medium), and the ability to utilize solar energy. However, it also has disadvantages such as generally low reaction efficiency, insufficient catalyst stability, complex reaction pathways with numerous byproducts, and strong dependence on light sources. Biological reduction of nitrates to ammonia utilizes microorganisms or enzyme catalysis systems to convert nitrates (NO3-) into ammonia. - The low-carbon technology of converting nitrates (NH3) to ammonia has the advantages of low energy consumption and the ability to utilize nitrate resources from biomass or wastewater, exhibiting a certain degree of environmental compatibility. Although biological methods have potential in terms of sustainability, their low efficiency, poor controllability, and complex processes still require breakthroughs through synthetic biology techniques such as modifying strains, optimizing enzyme immobilization technology, or coupling with electrochemical systems. Overall, electrocatalytic reduction of ammonia demonstrates unique advantages in efficiency and controllability, but its large-scale application still requires continuous breakthroughs in areas such as low-cost catalyst development, reactor optimization, and integration with green energy systems.

[0004] Pore ​​structure, as a key structural characteristic of molecular sieves, can be used for targeted screening of molecular sieve precursors suitable for electrocatalytic reduction experiments. Among them, molecular sieves with the FAU topology exhibit the best electrocatalytic performance due to their combination of the highest Faradaic efficiency and high ammonia yield. This is due to the synergistic effect of the unique crystal structure and chemical properties of FAU-configured molecular sieves. The three-dimensional interconnected supercage structure and abundant microporous network of FAU-configured molecular sieves provide efficient migration pathways for cations, enabling them to rapidly diffuse to internal active sites. Furthermore, the hydroxyl groups (Si-OH-Al) in their pore structure form acidic sites that can specifically adsorb and activate cations. It is precisely for these reasons that FAU molecular sieves exhibit excellent catalytic potential. Summary of the Invention

[0005] To address the aforementioned technical problems in the existing technology, the present invention aims to provide a catalyst doped with transition metals in a molecular sieve, its preparation method, and its electrocatalytic application. The high-performance transition metal-doped FAU-type molecular sieve catalyst of the present invention can efficiently reduce nitrate ions in water to ammonia.

[0006] The technical solution adopted in this invention is as follows: A catalyst doped with a transition metal in a molecular sieve, wherein the catalyst uses NaX molecular sieve as a precursor and is obtained by introducing Fe... 3+ The catalyst, named FeX, was obtained through ion exchange of ions; X-ray diffraction characterization showed that Fe... 3+ The embedding disrupts the ordered lattice of NaX, triggering a phase transition and framework collapse, causing FeX to exhibit an amorphous state. The Si / Al molar ratio in the FeX catalyst is 1.5-4:1, preferably 2-2.5:1; the Fe / Al molar ratio in the FeX catalyst is 1.8-2.5:1.

[0007] The method for preparing a catalyst doped with a transition metal in a molecular sieve includes the following steps: Step 1: Dissolve the trivalent Fe salt in distilled water to obtain a homogeneous Fe salt solution. Then add NaX molecular sieve and stir to allow the NaX molecular sieve to undergo a full ion exchange reaction with the trivalent Fe salt. Step 2: After the reaction in Step 1 is completed, the catalyst is obtained by centrifugation, washing, drying overnight, and grinding.

[0008] Furthermore, in step 1, the feeding ratio of trivalent Fe salt to NaX molecular sieve is 2-8 mmol:1g, preferably 4-5 mmol:1g.

[0009] Furthermore, the reaction temperature in step 1 is room temperature, and the reaction time is 20-30 hours, preferably 24-26 hours.

[0010] Furthermore, the Si / Al molar ratio of the NaX molecular sieve is 0.8-2:1, preferably 1:1.

[0011] This invention also discloses the application of a catalyst doped with transition metals in a molecular sieve in the electrocatalytic reduction of nitrate wastewater to ammonia. The catalyst is used to make an electrocatalytic cathode material. The preparation process of the cathode material is as follows: the catalyst is mixed with ultrapure water, ethanol and Nafion solution, and ultrasonically homogenized to obtain a catalyst coating. The catalyst coating is uniformly dropped onto the surface of carbon paper and dried to obtain the cathode material.

[0012] Compared with the prior art, the beneficial effects achieved by the present invention are: 1. The NaX molecular sieve framework does not merely function as an inert support; its unique three-dimensional microporous network and surface hydroxyl groups can synergistically interact with transition metal active sites through a structure-activity synergy mechanism. Specifically, the NaX silica-alumina framework can inhibit iron species aggregation, achieving atomic-level dispersion of active sites; surface hydroxyl groups accelerate charge transfer through the inner sphere electron transfer (ISET) mechanism; and the electrostatic environment of the pores forms a synergistic adsorption field with the metal sites, optimizing the activation pathway of nitrate ions.

[0013] 2. The introduction of transition metal Fe not only provides highly efficient reactive centers, but its interaction with hydroxyl groups on the surface of molecular sieves can also optimize electron transport pathways. Ultimately, through the dual mechanisms of active site construction and conductive network enhancement, comprehensive optimization of catalytic performance is achieved.

[0014] 3. The "molecular sieve framework-transition metal catalyst" mechanism constructed in this invention provides new possibilities for designing low-cost, high-stability electrocatalytic reduction catalysts for ammonia production and for treating secondary nitrate wastewater generated from wet denitrification.

[0015] 4. The FeX catalyst prepared in this invention, when applied to the electrocatalytic reduction of nitrate wastewater to ammonia, can reduce NO3... - In the H2O system, NO3 is preferentially and efficiently reduced. - It is ammonia nitrogen, with high Faraday efficiency and ammonia yield. Attached Figure Description

[0016] Figure 1 This is a schematic diagram of the XRD test results of FeX in Example 1; Figure 2 This is a schematic diagram of the ICP test results of the supernatant sample and the FeX catalyst sample in Example 1; Figure 3 This is a schematic diagram of the XPS full spectrum test results of the FeX catalyst in Example 1; Figure 4This is a schematic diagram of the XPS spectra of the Fe 2p orbitals of the FeX catalyst in Example 1; Figure 5 This is a schematic diagram of the electrocatalytic performance test results of FAU molecular sieves with different silicon-to-aluminum ratios in Example 2; Figure 6 This is a schematic diagram showing the test results of the molecular sieve electrocatalytic performance of different modified ions in Comparative Example 1; Figure 7 This is a schematic diagram showing the electrocatalytic performance test results of the ball-milled sample in Comparative Example 2 and the FeX molecular sieve in Example 1. Figure 8 This is a schematic diagram of the XPS full spectrum test results of the ball-milled sample in Comparative Example 2; Figure 9 This is a schematic diagram of the XPS spectra of the Fe 2p orbitals of the ball-milled sample in Comparative Example 2. Detailed Implementation

[0017] The present invention will be further described below with reference to specific embodiments, but the scope of protection of the present invention is not limited thereto.

[0018] The sources of the different molecular sieves used in the embodiments of the present invention are shown in Table 1.

[0019] Table 1 .

[0020] Example 1: Clean conical flasks were used as exchange containers. 100 mL of distilled water was added to each flask, followed by 2 g (approximately 5 mmol) of Fe(NO3)3·9H2O. The flasks were placed on a magnetic stirrer and stirred until the ferric nitrate was completely dissolved, resulting in a homogeneous ferric nitrate solution. Subsequently, 1 g of NaX molecular sieve was weighed and added sequentially to the flasks, and stirring continued for 24 h to allow for sufficient ion exchange. After 24 h, the molecular sieve mixture was centrifuged and washed four times, dried overnight at 60 °C, and ground to obtain the FeX catalyst.

[0021] The FeX was characterized by XRD, and the test results were as follows: Figure 1 ICP characterization was performed on the supernatant sample and the FeX catalyst sample, and the test results were as follows: Figure 2 Parts a and b in the figure were used to characterize FeX using XPS, and the test results were as follows: Figure 3 and Figure 4 The mass ratio of Si:Al:Fe in the FeX catalyst is 11:5:21, which comprehensively indicates that during the preparation process of the synthesized FeX catalyst, a Fe... 3+ Exchange Al in 1.6 NaX molecular sieves 3+ Fe 3+The insertion of O atoms disrupts the ordered lattice of NaX, causing it to exhibit an amorphous state. The highly electronegative O atoms reduce the electron cloud density of Fe atoms through the inductive effect, optimize the hybridization efficiency of d orbitals and nitrate π orbitals, and enhance electron transfer capability.

[0022] according to Figures 3-4 XPS characterization showed that Fe coexists in both +2 and +3 oxidation states within the molecular sieve framework, with Fe... 3+ The FeOOH generated by hydrolysis is anchored in the molecular sieve framework through Fe-O-Si / Al bonds.

[0023] Example 2: Clean conical flasks were used as exchange containers. 100 mL of distilled water was added to each flask, followed by 2 g of Fe(NO3)3·9H2O. The flasks were placed on a magnetic stirrer and stirred until the ferric nitrate was completely dissolved, resulting in a homogeneous ferric nitrate solution. Subsequently, 1 g of USY-15 molecular sieve, USY-5.5 molecular sieve, or NaX molecular sieve were weighed and added to the flasks respectively. Stirring was continued for 24 hours to allow for sufficient ion exchange. After 24 hours, the molecular sieve mixture was centrifuged and washed four times, dried overnight at 60°C, and ground to obtain the target catalyst.

[0024] The target catalyst was used to make an electrode, and it was electroreduced to nitrate to obtain ammonia. The electrode was made by dispersing 5 mg of catalyst in a mixed solution of 495 μL ultrapure water, 495 μL ethanol and 10 μL Nafion (5 wt%), sonicating for 1 h, taking 100 μL with a pipette and coating it on both sides of a 1 cm × 1 cm carbon paper, drying it under an infrared lamp, and repeating the process twice on the same side of the carbon paper.

[0025] Catalytic application experiment: An H-type electrolytic cell was used, which was divided into two chambers by a proton exchange membrane. The catholyte in the cathode chamber was a mixed aqueous solution of 1M KOH and 0.1M potassium nitrate, and the anolyte in the anode chamber was a mixed aqueous solution of 1M KOH. The electrode prepared above was inserted into the catholyte as the cathode, and a platinum sheet electrode was inserted into the anolyte as the counter electrode. A saturated calomel electrode was also set in the cathode chamber as a reference electrode.

[0026] Following the above catalytic application experimental procedure, the electrocatalytic performance of the prepared catalyst and the ammonia yield were tested. Test conditions: temperature 25℃, initial voltage -1.6 V, electrolysis time 2 h, initial NO3... - The concentration is 0.1 mol·L⁻¹ -1 The results are as follows Figure 5 .

[0027] Comparative Example 1: Using clean conical flasks as exchange containers, add 100 mL of distilled water to each flask, and then weigh out 5 mmol of Cu(NO3)2·3H2O, Ni(NO3)2·6H2O, Co(NO3)2·6H2O, and C5H2O respectively. 14 ClNO (ChCl), C4H 12 NCl(TMACl) or Mn(NO3)2 was added to separate conical flasks. The flasks were placed on a magnetic stirrer and stirred until completely dissolved, resulting in a homogeneous solution. Then, 1 g of NaX molecular sieve was weighed into each flask, and stirring continued for 24 hours to allow for sufficient ion exchange. After 24 hours, the molecular sieve mixture was centrifuged and washed four times, dried overnight at 60°C, and ground to obtain the target catalyst.

[0028] Following the catalytic application experiment procedure in Example 2, the electrocatalytic performance and ammonia yield of the catalyst prepared in Comparative Example 1 were tested. Test conditions: temperature 25℃, initial voltage -1.6 V, electrolysis time 2 h, initial NO3... - The concentration is 0.1 mol·L⁻¹ -1 The results are as follows Figure 6 .

[0029] Comparative Example 2: Following the ratio of the main elements (Si:Al:Fe mass ratio of 11:5:21) of the FeX molecular sieve catalyst obtained by ICP testing in Example 1, equal proportions of SiO2, Al2O3, and Fe2O3 were prepared. This mixture was then ball-milled, and the resulting mixture was the ball-milled sample. XPS characterization of the ball-milled sample yielded the following results: Figure 8 and Figure 9 .

[0030] Following the catalytic application experiment procedure in Example 2, the electrocatalytic performance and ammonia yield of the FeX catalyst obtained in Example 1 and the ball-milled sample obtained in Comparative Example 2 were tested. The test conditions were: temperature 25℃, initial voltage -1.6 V, electrolysis time 2 h, and initial NO3... - The concentration is 0.1 mol·L⁻¹ -1 The results are as follows Figure 7 .

[0031] Ball milling control experiments showed that the ammonia yield and Faraday efficiency of the physically mixed sample were lower than those of the FeX catalyst. This result reveals that the NaX molecular sieve framework does not merely exist as an inert support; its unique three-dimensional microporous network and surface hydroxyl groups can synergistically interact with transition metal active sites through a structure-activity synergy mechanism. Specifically, the NaX silica-alumina framework can inhibit iron species aggregation, achieving atomic-level dispersion of active sites; surface hydroxyl groups accelerate charge transfer through the inner sphere electron transfer (ISET) mechanism; and the electrostatic environment of the pores forms a synergistic adsorption field with the metal sites, optimizing the activation pathway of nitrate ions. These multi-dimensional synergistic effects (enhanced electron transfer, regulated site dispersion, and optimized adsorption configuration) collectively endow the FeX catalyst with highly efficient electrocatalytic performance.

[0032] The contents described in this specification are merely an enumeration of the implementation forms of the inventive concept, and the scope of protection of this invention should not be regarded as limited to the specific forms described in the embodiments.

Claims

1. A catalyst doped with a transition metal in a molecular sieve, characterized in that, The catalyst uses NaX molecular sieve as a precursor and introduces Fe... 3+ The catalyst, named FeX, was obtained through ion exchange of ions; X-ray diffraction characterization showed that Fe... 3+ The embedding disrupts the ordered lattice of NaX, triggering a phase transition and framework collapse, causing FeX to exhibit an amorphous state. The Si / Al molar ratio in the FeX catalyst is 1.5-4:1, preferably 2-2.5:1; the Fe / Al molar ratio in the FeX catalyst is 1.8-2.5:

1.

2. The method for preparing a catalyst doped with a transition metal in a molecular sieve as described in claim 1, characterized in that, Includes the following steps: Step 1: Dissolve the trivalent Fe salt in distilled water to obtain a homogeneous Fe salt solution. Then add NaX molecular sieve and stir to allow the NaX molecular sieve to undergo a full ion exchange reaction with the trivalent Fe salt. Step 2: After the reaction in Step 1 is completed, the catalyst is obtained by centrifugation, washing, drying overnight, and grinding.

3. The method for preparing a catalyst doped with a transition metal in a molecular sieve as described in claim 2, characterized in that, In step 1, the feeding ratio of trivalent Fe salt to NaX molecular sieve is 2-8 mmol:1g, preferably 4-5 mmol:1g.

4. The method for preparing a catalyst doped with a transition metal in a molecular sieve as described in claim 2, characterized in that, The reaction temperature in step 1 is room temperature, and the reaction time is 20-30 hours, preferably 24-26 hours.

5. The method for preparing a catalyst doped with a transition metal in a molecular sieve as described in claim 2, characterized in that, The Si / Al molar ratio of the NaX molecular sieve is 0.8-2:1, preferably 1:

1.

6. The application of a transition metal-doped molecular sieve catalyst as described in claim 1 in the electrocatalytic reduction of nitrate wastewater to ammonia.

7. The application as described in claim 6, characterized in that... The catalyst is used to make an electrocatalytic cathode material. The preparation process of the cathode material is as follows: the catalyst is mixed with ultrapure water, ethanol and Nafion solution, and ultrasonically homogenized to obtain a catalyst coating. The catalyst coating is uniformly dropped onto the surface of carbon paper and dried to obtain the cathode material.

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