A defective double-metal center composite oxynitride catalyst for non-thermal plasma synthesis of ammonia and a preparation method and application thereof

Abstract

This invention discloses a defect-type bimetallic-centered composite oxynitride catalyst for ammonia synthesis via nonthermal plasma, its preparation method, and its application, belonging to the field of plasma catalytic nitrogen fixation technology. The catalyst comprises a stable oxide framework phase and a surface oxynitride active phase. The catalyst simultaneously contains BI–BII bimetallic synergistic active sites, lattice nitrogen active sites, oxygen vacancies and / or nitrogen vacancies, and low-valence transition metal sites, where BI is Co, Mo, Fe, or Ru, and BII is Zr, Ni, Pt, or Pd. Through precursor mixing, calcination, pre-activation, nitriding, and optional post-treatment, a lattice-rich nitrogen defect layer is formed on the catalyst surface, while the stable framework structure is retained internally, achieving synergistic promotion of nitrogen molecule activation, active hydrogen coupling, and lattice nitrogen recycling. The catalyst can be applied to nonthermal plasma reactors for efficient ammonia synthesis under atmospheric or low-pressure, low-temperature conditions, exhibiting advantages such as high activity, excellent stability, good low-power response, and compatibility with various hydrogen-containing feed gases.

Description

Technical Field

[0001] This invention belongs to the field of synergistic nitrogen fixation technology of catalytic materials and nonthermal plasma, specifically relating to a defective bimetallic center composite oxynitride catalyst, its preparation method, and its application in nonthermal plasma ammonia synthesis. Background Technology

[0002] Ammonia is an important basic chemical in the fertilizer industry and hydrogen energy storage. The traditional Haber-Bosch process requires high temperature and high pressure conditions, resulting in high energy consumption, high equipment costs, high carbon emissions, and poor equipment flexibility, making it unsuitable for distributed production. Non-thermal plasma technology can excite nitrogen and hydrogen to generate active particles, such as vibrational excited states, electronic excited states, and free radicals, under normal pressure and relatively low temperature conditions, providing a new reaction pathway for ammonia synthesis. This technology places higher demands on catalysts: catalysts need to have efficient nitrogen molecule adsorption capacity, electron transfer capacity, recyclable active sites, and long-term structural stability. At the same time, the defect structure, lattice sites, and micropore / mesopore structure on the catalyst surface have a significant impact on the non-thermal plasma reaction activity and stability. Existing catalyst systems usually have the following problems: (1) insufficient activity, resulting in low ammonia production efficiency under low-power plasma; (2) lack of recyclable active sites, leading to rapid performance degradation of the catalyst during long-term operation; (3) easy structural degradation, with unstable surface defect sites and pore structures, affecting continuous reaction; (4) limited adaptability to different hydrogen-containing feed gases, making it difficult to achieve stable operation under diverse working conditions. Therefore, there is an urgent need to develop a novel defect-type bimetallic center composite oxynitride catalyst that can balance high activity, low power response, long-term stability and adaptability to various operating conditions. Summary of the Invention

[0003] This invention provides a defect-type bimetallic-center composite oxynitride catalyst, its preparation method, and its applications, to address the problems of insufficient activity, short lifetime, limited low-power response, and poor compatibility with single active centers in existing catalysts. The catalyst described in this invention has the general formula: A 1-a A' a (BI 1-b BII b )O x N y in: • A can be one or more of La, Ce, Y, Sr, and Ba; • A' represents the modifying element (Ce, Zr, Y, Sr, etc.); • BI is one or more of Co, Mo, Fe, and W; • BII is one or more of Zr, Ni, Pt, and Pd; • 0 <a≤0.5,0<b≤0.5; • c and d satisfy the non-stoichiometric ratio of oxygen and nitrogen, and contain oxygen vacancies and / or nitrogen vacancies.

[0004] The specific preparation method includes the following steps: (1) Select the corresponding metal source as raw material according to the stoichiometric ratio of the A-site element, A'-site element, BI-site element and BII-site element in the target product. The A-site and A'-site metal sources can be selected from one or more of the corresponding metal oxides, carbonates, nitrates and acetates, preferably oxides or carbonates; the BI-site and BII-site metal sources can be selected from one or more of the corresponding metal oxides, hydroxides, carbonates, nitrates and ammonium salts, preferably oxides. After mixing the above metal sources according to the set molar ratio, add molten salt medium. The molten salt medium is selected from one or more of KCl, NaCl, KNO3 and Na2SO4, preferably KCl. The molar ratio of the total number of metal sources to the molten salt medium is preferably 1:(2~10), more preferably 1:(3~6).

[0005] (2) The mixture obtained in step (1) is placed in an agate mortar and ground thoroughly to ensure uniform mixing of all components. The grinding time is preferably 20-60 min, preferably under infrared lamp-assisted heating conditions, to promote uniform mixing and remove adsorbed water. The uniformly ground mixture is transferred to a crucible, covered but not completely sealed, and placed in a muffle furnace for high-temperature calcination. The heating rate is preferably 2-5 ℃ / min; the calcination temperature is preferably 900-1250 ℃, more preferably 1000-1200 ℃; the holding time is preferably 5-15 h, more preferably 8-12 h. After calcination, it is naturally cooled to room temperature to obtain the composite oxide precursor. In this step, the molten salt medium can promote the diffusion and rearrangement of the reactants at high temperature, which is beneficial to the formation of a polymetallic composite oxide precursor with uniform crystal phase and controllable composition, and improves the nitrogen incorporation efficiency in the subsequent ammoniation process.

[0006] (3) The product obtained in step (2) is washed with hot deionized water to remove residual molten salt; then washed with acid to remove unreacted alkaline earth metal oxides, rare earth oxides or other impurities. The acid is preferably a 0.1~2 mol / L hydrochloric acid solution, more preferably a 1 mol / L hydrochloric acid solution. After washing, it is repeatedly washed with deionized water until neutral. If necessary, it can be further rinsed with ethanol once to promote drying. Then it is dried at 60~100 ℃ for 6~24 h to obtain composite oxide precursor powder.

[0007] (4) The composite oxide precursor obtained in step (3) is placed in an alumina boat or a high-temperature resistant ceramic boat and transferred to a horizontal tube furnace for heat treatment under an ammonia atmosphere. The ammonia flow rate is preferably 30~200 mL / min, more preferably 50~100 mL / min; the treatment temperature is preferably 700~1100 ℃, more preferably 850~1000 ℃; and the treatment time is preferably 2~20 h, more preferably 5~12 h. During the ammonia heat treatment, some oxygen in the precursor is replaced by nitrogen to form a composite oxynitride structure; at the same time, due to the high-temperature reduction-nitridation coupling effect, oxygen vacancies and / or nitrogen vacancies are introduced into the lattice, thereby obtaining a defect-type bimetallic center composite oxynitride catalyst. Beneficial effects

[0008] Compared with existing technologies, this invention constructs a composite oxynitride catalyst that combines the synergistic effect of bimetallic centers with the characteristics of lattice defect regulation. By combining molten salt-assisted calcination with ammonia heat treatment, it achieves uniform phase formation of multi-metal components, oxygen-nitrogen exchange, and simultaneous construction of oxygen and / or nitrogen vacancies, thereby effectively optimizing the electronic structure, local coordination environment, and surface active site distribution of the catalyst. The BI and BII sites in the catalyst can respectively or synergistically undertake functions such as nitrogen molecule adsorption and activation, electron transfer, and hydrogen-containing intermediate migration and transformation, overcoming the shortcomings of a single active center in handling multi-step reaction processes. At the same time, the coupling of defect structure and stable framework significantly improves the material's response capability, sustained reaction activity, and long-term operational stability under low-power non-thermal plasma conditions. In addition, the preparation method adopted in this invention is simple, has a wide range of applicable raw materials, and strong compositional controllability. The resulting catalyst is applicable to non-thermal plasma ammonia synthesis processes under N2 / H2, N2 / water vapor, and other hydrogen-containing systems, and has good application prospects. Attached Figure Description

[0009] Figure 1 This is a schematic diagram of the preparation process of the catalyst of the present invention (using LaW). 0.9 Zr 0.1 O x N y (For example).

[0010] Figure 2 This is a schematic diagram of the reaction apparatus for non-thermal plasma ammonia synthesis according to the present invention.

[0011] Figure 3 This is an X-ray diffraction characterization pattern of the catalyst of this invention.

[0012] Figure 4 This is a catalytic rate diagram of the catalyst of the present invention in the plasma ammonia synthesis reaction.

[0013] Figure 5The accompanying drawings are for the abstract of this invention. Detailed Implementation

[0014] Example 1: LaW 0.9 Zr 0.1 O x N y Preparation of defective bimetallic center complex oxynitride catalysts.

[0015] (1) Weigh 1.629 g of La2O3, 2.087 g of WO3 and 0.123 g of ZrO2, wherein the molar ratio of La, W and Zr is 1:0.9:0.1. Weigh 4.47 g of KCl as the molten salt medium.

[0016] (2) Add the above raw materials to an agate mortar and grind for 30 min under the assistance of an infrared lamp to ensure that the components are fully mixed. Then transfer the mixture to a 100 mL covered alumina crucible, cover it but not completely seal it, place it in a muffle furnace, heat it to 1100 ℃ at a heating rate of 150 ℃ / h, hold it at that temperature for 10 h, and then cool it naturally to room temperature to obtain the composite oxide precursor.

[0017] (3) The precursor obtained above was washed three times with hot deionized water at 80 °C to remove residual KCl; then washed three times with 1 mol / L hydrochloric acid solution to remove unreacted impurities; then washed with deionized water until the pH of the filtrate was close to neutral, and finally dried at 80 °C for 12 h to obtain precursor powder.

[0018] (4) The precursor powder was evenly spread in a 15 mL alumina boat and placed in a horizontal tube furnace. The furnace was then heat-treated at 900 °C for 8 h under ammonia flow rate of 50 mL / min. After the reaction, the mixture was allowed to cool naturally to room temperature. The sample was then removed, rapidly washed once with deionized water, and dried at 80 °C for 12 h to obtain LaW. 0.9 Zr 0.1 O x N y Defective bimetallic center complex oxynitride catalyst.

[0019] Example 2: LaW 0.9 Ni 0.1 O x N y Preparation of defective bimetallic center complex oxynitride catalysts (1) Weigh 1.629 g of La2O3, 2.087 g of WO3 and 0.075 g of NiO, wherein the molar ratio of La, W and Ni is 1:0.9:0.1; weigh 4.47 g of KCl as molten salt medium.

[0020] (2) Add the above raw materials to an agate mortar and grind for 30 min under the assistance of an infrared lamp to ensure that the components are fully mixed. Then transfer the mixture to a 100 mL covered alumina crucible, cover it but not completely seal it, place it in a muffle furnace, heat it to 1100 ℃ at a heating rate of 150 ℃ / h, hold it at that temperature for 10 h, and then cool it naturally to room temperature to obtain the composite oxide precursor.

[0021] (3) The precursor obtained above was washed three times with hot deionized water at 80 °C to remove residual KCl; then washed three times with 1 mol / L hydrochloric acid solution to remove unreacted impurities; then washed with deionized water until the pH of the filtrate was close to neutral, and finally dried at 80 °C for 12 h to obtain precursor powder.

[0022] (4) The precursor powder was evenly spread in a 15 mL alumina boat and placed in a horizontal tube furnace. The furnace was then heat-treated at 900 °C for 8 h under ammonia flow rate of 50 mL / min. After the reaction, the mixture was allowed to cool naturally to room temperature. The sample was then removed, rapidly washed once with deionized water, and dried at 80 °C for 12 h to obtain LaW. 0.9 Ni 0.1 O x N y Defective bimetallic center complex oxynitride catalyst.

[0023] Example 3: La 0.9 Ce 0.1 W 0.9 Zr 0.1 O x N y Preparation of defective bimetallic center complex oxynitride catalysts (1) Weigh 1.466 g of La2O3, 0.172 g of CeO2, 2.087 g of WO3 and 0.123 g of ZrO2, wherein the molar ratio of La, Ce, W and Zr is 0.9:0.1:0.9:0.1; weigh 4.77 g of KCl as molten salt medium.

[0024] (2) Add the above raw materials to an agate mortar and grind for 30 min under the assistance of an infrared lamp to ensure that the components are fully mixed. Then transfer the mixture to a 100 mL covered alumina crucible, cover it but not completely seal it, place it in a muffle furnace, heat it to 1100 ℃ at a heating rate of 150 ℃ / h, hold it at that temperature for 10 h, and then cool it naturally to room temperature to obtain the composite oxide precursor.

[0025] (3) The precursor obtained above was washed three times with hot deionized water at 80 °C to remove residual KCl; then washed three times with 1 mol / L hydrochloric acid solution to remove unreacted impurities; then washed with deionized water until the pH of the filtrate was close to neutral, and finally dried at 80 °C for 12 h to obtain precursor powder.

[0026] (4) The precursor powder was evenly spread in a 15 mL alumina boat and placed in a horizontal tube furnace. The furnace was then heat-treated at 900 °C for 8 h under ammonia flow rate of 50 mL / min. After the reaction, the mixture was allowed to cool naturally to room temperature. The sample was then removed, rapidly washed once with deionized water, and dried at 80 °C for 12 h to obtain La. 0.9 Ce 0.1 W 0.9 Zr 0.1 O x N y Defective bimetallic center complex oxynitride catalyst.

[0027] Example 4: SrW 0.9 Zr 0.1 O x N y Preparation of defective bimetallic center complex oxynitride catalysts (1) Weigh out 1.476 g of SrCO3, 2.087 g of WO3 and 0.123 g of ZrO2, wherein the molar ratio of Sr, W and Zr is 1:0.9:0.1; weigh out 4.47 g of KCl as molten salt medium.

[0028] (2) Add the above raw materials to an agate mortar and grind for 30 min under the assistance of an infrared lamp to ensure that the components are fully mixed. Then transfer the mixture to a 100 mL covered alumina crucible, cover it but not completely seal it, place it in a muffle furnace, heat it to 1100 ℃ at a heating rate of 150 ℃ / h, hold it at that temperature for 10 h, and then cool it naturally to room temperature to obtain the composite oxide precursor.

[0029] (3) The precursor obtained above was washed three times with hot deionized water at 80 °C to remove residual KCl; then washed three times with 1 mol / L hydrochloric acid solution to remove unreacted impurities; then washed with deionized water until the pH of the filtrate was close to neutral, and finally dried at 80 °C for 12 h to obtain precursor powder.

[0030] (4) The precursor powder was evenly spread in a 15 mL alumina boat and placed in a horizontal tube furnace. The furnace was then heat-treated at 900 °C for 8 h under ammonia flow rate of 50 mL / min. After the reaction, the mixture was allowed to cool naturally to room temperature. The sample was then removed, rapidly washed once with deionized water, and dried at 80 °C for 12 h to obtain SrW. 0.9 Zr0.1 O x N y Defective bimetallic center complex oxynitride catalyst.

[0031] Example 5: Application of the catalyst of the present invention in DBD nonthermal plasma ammonia synthesis Take the LaW prepared in Example 1 0.9 Zr 0.1 O x N y 0.50 g of a defect-type bimetallic central composite oxynitride catalyst was packed into the discharge region of a dielectric barrier discharge (DBD) reactor. The reactor used a quartz tube as the dielectric layer and was powered by a high-voltage AC power supply. Before the reaction, the reactor was purged with N2 for 30 min to remove air from the system. The feed gas was then switched to an N2 / H2 mixture, with an N2 flow rate of 10 mL / min and an H2 flow rate of 30 mL / min, for a total flow rate of 40 mL / min. The discharge power was controlled at 30 W, and the athermal plasma ammonia synthesis reaction was carried out at atmospheric pressure.

[0032] After the reaction stabilizes, the tail gas is passed into the absorption liquid to collect the generated ammonia. The ammonia content in the absorption liquid is then determined by indophenol blue spectrophotometry to evaluate the ammonia synthesis performance of the catalyst.

[0033] The above examples are further detailed descriptions of the present invention and should not be construed as limiting the specific embodiments of the present invention to these examples. The above examples illustrate that the supported metal catalyst coupled with porous material strategy we provide has good catalytic activity and good cycle stability in plasma ammonia synthesis reaction, and is an efficient and practical plasma catalytic ammonia synthesis strategy.

Claims

1. A defect-type bimetallic center composite oxynitride catalyst, characterized in that, The chemical composition of the catalyst is: A 1-a A' a (BI 1-b BII b )O x N y , wherein A is one or more selected from La, Ce, Y, Sr and Ba, A' is one or more selected from Ce, Zr, Y and Sr, BI is one or more selected from Co, Mo, Fe and W, BII is one or more selected from Zr, Ni, Pt and Pd, 0<a≤0.5, 0<b≤0.5, x and y respectively represent the atomic stoichiometric numbers of oxygen and nitrogen, and satisfy a non-stoichiometric relationship, so that the catalyst contains oxygen vacancies and / or nitrogen vacancies.

2. The catalyst according to claim 1, characterized in that, The A is preferably one or two of La, Ce, or Sr.

3. The catalyst according to claim 1, characterized in that, The BI is preferably one of W, Mo, Fe or Co, and the BII is preferably one of Zr, Ni, Pt or Pd.

4. The catalyst according to claim 1, characterized in that, The catalyst surface contains an oxynitridation active layer, and the concentration of oxygen vacancies and / or nitrogen vacancies in the surface region is higher than that in the internal region.

5. The catalyst according to any one of claims 1 to 4, characterized in that, In the catalyst, A' is used to regulate the lattice environment at the A site, and BII is used to regulate the electronic structure and local coordination environment at the B1 site.

6. The catalyst according to any one of claims 1 to 5, characterized in that, The catalyst is a powder, granules, tablet, extruded material, supported coating, or a composite material with dielectric filler.

7. A method for preparing the catalyst according to any one of claims 1 to 6, characterized in that, Includes the following steps: (1) Weigh out the A-site metal source, A'-site metal source, BI-site metal source and BII-site metal source according to the target chemical composition, and mix them with the molten salt medium; (2) The mixture obtained in step (1) is calcined to obtain a composite oxide precursor; (3) The composite oxide precursor is washed and dried; (4) The dried composite oxide precursor is nitrided under ammonia or an ammonia-containing atmosphere to obtain the defective bimetallic center composite oxynitride catalyst.

8. The method according to claim 7, characterized in that, The A-site metal source and A'-site metal source are selected from one or more of the corresponding metal oxides, carbonates, nitrates, and acetates, and the BI-site metal source and BII-site metal source are selected from one or more of the corresponding metal oxides, hydroxides, carbonates, nitrates, or ammonium salts.

9. The method according to claim 7, characterized in that, The molten salt medium is one or more of KCl, NaCl, KNO3, and Na2SO4, preferably KCl; the molar ratio of the total number of metal sources to the molten salt medium is 1:(2~10), preferably 1:(3~6).

10. The method according to claim 7, characterized in that, The roasting temperature in step (2) is 900~1250 ℃, preferably 1000~1200 ℃; the roasting time is 5~15 h, preferably 8~12 h.

11. The method according to claim 7, characterized in that, The nitriding temperature in step (4) is 700~1100℃, preferably 850~1000℃; the nitriding time is 2~20 h, preferably 5~12 h; and the ammonia flow rate is 30~200 mL / min, preferably 50~100 mL / min.

12. The method according to any one of claims 7 to 11, characterized in that, The washing in step (3) includes washing with hot deionized water and washing with acid solution, wherein the acid solution is a 0.1~2 mol / L hydrochloric acid solution, preferably a 1 mol / L hydrochloric acid solution.

13. The use of the catalyst according to any one of claims 1 to 6 in nonthermal plasma ammonia synthesis.

14. The application according to claim 13, characterized in that, The non-thermal plasma reactor is a dielectric barrier discharge reactor, a packed bed dielectric barrier discharge reactor, a microwave plasma reactor, a radio frequency plasma reactor, or a pulsed discharge plasma reactor.

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