Iron-copper-based single-walled carbon nanotube catalyst, method of making and use thereof

Abstract

The present application belongs to the field of carbon nanomaterial preparation and heterogeneous catalysis, and particularly relates to a layered double hydroxide derived iron-copper based single-walled carbon nanotube catalyst, a preparation method and application thereof. The catalyst comprises a magnesium-aluminum composite oxide carrier, iron-copper alloy nanoparticles and at least one additive selected from molybdenum oxide and cerium oxide; wherein iron source and copper source are introduced in the formation stage of a magnesium-aluminum layered double hydroxide precursor, and iron-copper alloy nanoparticles are formed in situ after calcination and temperature programmed reduction, and the iron-copper alloy nanoparticles are at least partially limited to be dispersed at defect sites of the magnesium-aluminum composite oxide carrier. When the catalyst is used for preparing single-walled carbon nanotubes by catalytic cracking of low-carbon hydrocarbons, the selectivity, graphitization degree and uniformity of the tube diameter distribution of the single-walled carbon nanotubes can be improved.

Description

Technical Field

[0001] The invention belongs to the field of advanced carbon material preparation and heterogeneous catalysis technology, specifically involving an iron-copper based single-walled carbon nanotube catalyst, its preparation method, and its application. Background Technology

[0002] Single-walled carbon nanotubes (SWCNTs) possess excellent electrical, optical, thermal, and mechanical properties, making them promising candidates for applications in nanoelectronic devices, transparent conductive films, sensors, high-strength composite materials, and energy storage materials. Catalytic chemical vapor deposition (CVD) has become a crucial technological route for SWCNT preparation due to its relatively simple equipment, continuous operation, and suitability for large-scale production. For this route, the composition, structure, and formation mechanism of the catalyst system are key factors determining the quality, selectivity, and reaction stability of the SWCNT product.

[0003] In existing technologies, catalysts used for carbon nanotube growth mostly employ transition metals such as iron, cobalt, and nickel, or bimetallic or multimetallic systems containing these metals. Among these, iron-based catalysts are widely used in carbon nanotube preparation due to their good activation ability for low-carbon hydrocarbons. However, conventional iron-based catalysts, especially those prepared by impregnation or deposition methods, are prone to metal particle migration, agglomeration, and sintering during high-temperature reactions. This leads to a reduction in active sites, a wider particle size distribution, and further increases in the outer diameter distribution of carbon nanotubes, as well as an increase in amorphous carbon and multi-walled carbon nanotube byproducts, making it difficult to stably achieve the selective growth of high-quality single-walled carbon nanotubes.

[0004] To improve the performance of iron-based catalysts, existing technologies employ bimetallic systems to regulate the iron-based active components. For example, a second metal can be introduced to alter the activation behavior of the carbon source, carbon dissolution / precipitation behavior, and the stability of the active particles. However, in existing bimetallic catalyst systems, if the introduction method, dispersion state, and interfacial structure between the second metal and the support are not effectively controlled, it remains difficult to fundamentally solve the problems of active particle growth, instability, and insufficient selectivity during the reaction process. Especially for the growth of single-walled carbon nanotubes, the size and stability of the active particles directly affect the tube formation behavior and the product tube diameter. Therefore, simply introducing bimetallic components through conventional loading methods often fails to simultaneously achieve high activity, narrow tube diameter distribution, and long-term stable operation.

[0005] On the other hand, the support structure has a significant impact on the formation, dispersion, and stability of active metal particles. Layered double hydroxides (LDHs) are widely regarded as promising catalyst precursors due to their uniform distribution of metal ions in the layers, tunable composition, and ability to transform into composite oxides after calcination. The composite oxides formed after LDH calcination typically possess high specific surface area and strong metal-support interactions, and may form defect sites that facilitate the anchoring of active components during structural transformation. However, current technologies lack mature solutions that balance structural controllability and single-walled carbon nanotube growth effects in order to utilize LDH precursors to introduce active metals during the precursor stage and construct stable bimetallic nano-active centers through subsequent calcination and reduction processes.

[0006] Furthermore, existing technologies for carbon nanotube growth catalysts often employ the introduction of promoters to modulate the catalyst surface electronic properties, coking behavior, and reaction stability. For example, certain metal oxide promoters can improve the interfacial state between the active component and the support, suppressing side reactions and deactivation due to coking. However, for the preparation of single-walled carbon nanotubes, promoters cannot simply be added to achieve ideal results; their effects are usually closely related to the formation mode of the basic active framework. If the basic catalytic structure itself is unstable, or if the interface between the active particles and the support is not effectively regulated, even the introduction of promoters will not significantly improve the selectivity and catalyst lifetime of single-walled carbon nanotubes.

[0007] Therefore, the existing technology has at least the following shortcomings: First, it lacks a catalyst system that can introduce iron and copper components during the formation stage of the magnesium-aluminum layered double hydroxide precursor and form confined dispersed iron-copper alloy nanoparticles during the subsequent calcination and reduction process; second, it lacks a single-walled carbon nanotube growth catalyst that can introduce molybdenum oxide or cerium oxide additives on the basis of the above-mentioned confined dispersed iron-copper active particles, thereby taking into account activity, selectivity and anti-coking stability; third, it lacks a preparation method that matches the structure of the catalyst to achieve a synergistic improvement in in-situ formation of active particles, size control, interface regulation and reaction stability.

[0008] Therefore, it is necessary to provide a novel layered double hydroxide-derived iron-copper based single-walled carbon nanotube catalyst. By introducing iron and copper components into the magnesium-aluminum layered double hydroxide precursor stage, and through calcination, additive impregnation, and programmed temperature reduction processes, confined and dispersed iron-copper alloy nanoparticles and their interfacial structure with additives and magnesium-aluminum composite oxide supports are constructed, thereby improving the selectivity, product quality, and reaction stability of single-walled carbon nanotube growth. Summary of the Invention

[0009] To address the problems of existing transition metal catalysts used for carbon nanotube growth, which are prone to metal particle migration, agglomeration, and sintering under high-temperature reaction conditions, leading to activity decay, broadened product diameter distribution, and increased byproducts such as amorphous carbon and multi-walled carbon nanotubes, this invention provides a layered double hydroxide-derived iron-copper based single-walled carbon nanotube catalyst, its preparation method, and its applications. This method utilizes the highly dispersed metal ions in the magnesium-aluminum layered double hydroxide precursor. Iron and copper active components are introduced during the precursor formation stage. After calcination to form a magnesium-aluminum composite oxide, a programmed temperature reduction process is performed to allow iron-copper alloy nanoparticles to precipitate in situ and disperse within the defect sites of the support. Furthermore, molybdenum or cerium additives are introduced to improve the interfacial electronic structure and anti-carbon deposition properties, thereby achieving high-quality growth of single-walled carbon nanotubes.

[0010] To achieve the above objectives, the present invention adopts the following technical solution: A layered double hydroxide-derived iron-copper based single-walled carbon nanotube catalyst, characterized in that the catalyst comprises a magnesium-aluminum composite oxide support, iron-copper alloy nanoparticles dispersed in the magnesium-aluminum composite oxide support, and at least one auxiliary agent selected from molybdenum oxide and cerium oxide supported on the magnesium-aluminum composite oxide support; the iron-copper alloy nanoparticles are formed in situ from an iron- and copper-containing magnesium-aluminum layered double hydroxide precursor by calcination and programmed temperature reduction, and are at least partially dispersed in the defect sites of the magnesium-aluminum composite oxide support.

[0011] Preferably, the magnesium-aluminum composite oxide carrier is derived from the topological transformation product of a magnesium-aluminum layered double hydroxide precursor after calcination.

[0012] Preferably, the iron-copper alloy nanoparticles are uniformly dispersed nanoparticles and form a strong metal-carrier interaction with the magnesium-aluminum composite oxide carrier.

[0013] Preferably, the additive is dispersed in the form of an oxide on the surface of the carrier and / or located at the interface between the iron-copper alloy nanoparticles and the carrier.

[0014] Preferably, the molar ratio of magnesium to aluminum in the iron-copper based single-walled carbon nanotube catalyst is 2:1 to 4:1, and the molar ratio of iron to copper is 2:1 to 5:1.

[0015] Preferably, based on the total mass of the iron-copper based single-walled carbon nanotube catalyst, the mass fraction of the iron-copper alloy is 2% to 8%, of which copper accounts for 15% to 30% of the mass of the iron-copper alloy.

[0016] Preferably, the mass fraction of the additive, calculated as MoO3 or CeO2, is 0.2% to 1.5%.

[0017] Preferably, the average particle size of the iron-copper alloy nanoparticles is 4–10 nm.

[0018] Preferably, the additive is dispersed in oxide form on the surface of the magnesium-aluminum composite oxide carrier and / or located in the interface region between the iron-copper alloy nanoparticles and the magnesium-aluminum composite oxide carrier.

[0019] This invention also provides a method for preparing a layered double hydroxide-derived iron-copper based single-walled carbon nanotube catalyst, characterized by comprising the following steps: S1. Dissolve magnesium source, aluminum source, iron source and copper source in deionized water to form a mixed salt solution, and dissolve sodium hydroxide and sodium carbonate in deionized water to form a mixed alkaline solution. Under stirring conditions, a co-precipitation reaction is carried out to obtain an iron and copper doped magnesium-aluminum layered double hydroxide precursor. S2. The precursor obtained in step S1 is calcined to obtain iron-copper doped magnesium-aluminum composite oxide. S3. Using an impregnation method, ammonium molybdate solution and / or cerium nitrate solution are introduced into the material obtained in step S2 to obtain a precursor loaded with additives; S4. The material obtained in step S3 is subjected to programmed temperature reduction in a hydrogen atmosphere to obtain the layered double hydroxide-derived iron-copper based single-walled carbon nanotube catalyst.

[0020] Preferably, in the above preparation method, the pH of the coprecipitation reaction in step S1 is controlled to be 9.5-10.5, and the molar ratio of sodium hydroxide to sodium carbonate is 2:1.

[0021] Preferably, in the above preparation method, after co-precipitation in step S1, the mixture is stirred and aged for 4-8 hours, and the resulting precipitate is washed until neutral and then dried at 60-80°C for 12-24 hours.

[0022] Preferably, in the above preparation method, in step S2, the temperature is increased to 450-600℃ at a rate of 1-3℃ / min and maintained for 4-6 hours.

[0023] Preferably, in the above preparation method, the programmed temperature reduction in step S4 includes: first, heating to 300-350°C at 2-5°C / min and reducing at a constant temperature for 1-2 hours, then heating to 500-600°C at 1-2°C / min and reducing for 2-3 hours.

[0024] Preferably, in the above preparation method, the temperature-programmed reduction in step S4 uses a mixture of hydrogen and inert gas, with the hydrogen integral gradually increasing from 5% to 50%, and the space velocity being 500–2000 h⁻¹. -1 After reduction, the sample is cooled in an inert atmosphere and then passivated for 1-2 hours by introducing an inert gas containing 50-200 ppm oxygen.

[0025] Preferably, in step S3, an ammonium molybdate solution and / or cerium nitrate solution are introduced into the material obtained in step S2 using an equal-volume impregnation method. After impregnation, the material is left to stand at room temperature for 6–12 hours and then dried at 80–100°C for 6–10 hours.

[0026] The present invention also provides the application of the above-mentioned layered double hydroxide-derived iron-copper based single-walled carbon nanotube catalyst in the preparation of single-walled carbon nanotubes.

[0027] Preferably, the specific application involves placing the catalyst in a reactor and using methane or ethylene as a carbon source to carry out a catalytic cracking reaction to prepare single-walled carbon nanotubes.

[0028] Preferably, the catalytic cracking reaction is carried out in a vertical fixed-bed reactor.

[0029] Preferably, the reaction gas includes methane, hydrogen, and argon, with a volume ratio of 50–70:20–40:5–15.

[0030] Preferably, the catalyst is heated to the reaction temperature under an inert atmosphere before the reaction.

[0031] Preferably, in the preferred embodiment, the proportion of single-walled carbon nanotube bundles obtained is not less than 92%.

[0032] Preferably, the average outer diameter of the single-walled carbon nanotubes obtained is 1.3 ± 0.2 nm.

[0033] The present invention has at least the following beneficial effects: This invention introduces iron and copper components during the formation stage of a magnesium-aluminum layered double hydroxide precursor, enabling more uniform dispersion of iron and copper species within the precursor. After calcination to form a magnesium-aluminum composite oxide support, the iron and copper species form iron-copper alloy nanoparticles in situ during subsequent programmed temperature reduction. Compared to introducing iron and copper components by post-impregnation onto the calcined magnesium-aluminum composite oxide support, the iron-copper alloy nanoparticles obtained by this invention are smaller and more concentrated in size. This indicates that introducing iron and copper components during the precursor stage facilitates the formation of a spatially confined relationship between them and the defect sites of the magnesium-aluminum composite oxide support, thereby inhibiting particle migration, aggregation, and sintering during high-temperature reduction and catalytic cracking.

[0034] Meanwhile, by introducing molybdenum oxide or cerium oxide additives, this invention can further adjust the interfacial state between the iron-copper alloy nanoparticles, the additives, and the magnesium-aluminum composite oxide support, thereby improving the catalyst's resistance to carbon deposition and reaction stability. The results of the examples show that when the catalyst of this invention is used for the catalytic cracking of methane to prepare single-walled carbon nanotubes, a higher proportion of single-walled carbon nanotube bundles, a higher Raman G / D peak intensity ratio, and a narrower diameter distribution can be obtained. Attached Figure Description

[0035] The accompanying drawings are provided to further illustrate the invention and form part of the specification. They are used together with the embodiments of the invention to explain the invention and do not constitute a limitation thereof.

[0036] Figure 1 TEM image of the single-walled carbon nanotubes prepared in Example 1.

[0037] Figure 2 TEM image of the single-walled carbon nanotubes prepared in Example 2.

[0038] Figure 3 TEM image of the single-walled carbon nanotubes prepared in Example 3.

[0039] Figure 4 TEM image of the single-walled carbon nanotubes prepared for Comparative Example 1.

[0040] Figure 5 Raman spectra of single-walled carbon nanotubes prepared in Examples 1-3 and Comparative Examples 1-3. Detailed Implementation

[0041] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below in conjunction with the embodiments of the present invention. All other embodiments obtained by those skilled in the art based on the embodiments of the present invention without creative effort are within the scope of protection of the present invention.

[0042] Example 1: This Example 1 describes a layered double hydroxide-derived iron-copper based single-walled carbon nanotube catalyst, prepared from the following raw materials in parts by weight: Magnesium nitrate hexahydrate 102.6 parts, aluminum nitrate nonahydrate 56.3 parts, ferric nitrate nonahydrate 20.2 parts, copper nitrate trihydrate 7.2 parts, appropriate amount of mixed solution of sodium hydroxide and sodium carbonate (NaOH concentration 2 mol / L, Na2CO3 concentration 1 mol / L), ammonium molybdate 1.2 parts; The preparation method of the iron-copper doped magnesium-aluminum layered double hydroxide precursor is as follows: S1. Dissolve magnesium nitrate hexahydrate, aluminum nitrate nonahydrate, ferric nitrate nonahydrate, and copper nitrate trihydrate in 500 parts of deionized water to prepare mixed salt solution A (total metal ion concentration 1.5 mol / L); dissolve sodium hydroxide and sodium carbonate in deionized water at a molar ratio of NaOH to Na2CO3 of 2:1 to prepare mixed alkaline solution B ([OH-] - [Concentration 3 mol / L] S2. At room temperature, solutions A and B were simultaneously added dropwise to a reactor containing 200 parts of deionized water. The mixture was stirred vigorously, and the pH was controlled at 10.0±0.2. After the addition was completed, the mixture was stirred and aged for 6 hours. Then, it was centrifuged and washed with deionized water until the pH of the filtrate was 7. The filtrate was dried at 80°C for 18 hours to obtain the iron-copper doped magnesium-aluminum layered double hydroxide precursor. S3. The precursor obtained in S2 is placed in a muffle furnace and calcined at 500℃ for 5 hours with a heating rate of 2℃ / min to obtain an iron-copper doped magnesium-aluminum composite oxide carrier. S4. Weigh ammonium molybdate and dissolve it in deionized water to prepare an impregnation solution (concentration 0.1 mol / L). Use the equal volume impregnation method to uniformly add the impregnation solution to the support obtained in S3. Let it stand at room temperature for 12 hours, and then dry it in an oven at 90℃ for 8 hours to obtain the molybdenum-loaded precursor. S5. The material obtained in S4 is loaded into a fixed-bed quartz reactor (inner diameter 25 mm), and a mixture of hydrogen and argon is introduced. The initial volume fraction of hydrogen is 10%, and the total space velocity is 1200 h⁻¹. -1 The temperature was increased to 320℃ at 3℃ / min and reduced at a constant temperature for 1.5h. During this stage, the hydrogen gas fraction increased linearly to 30%. The temperature was then increased to 550℃ at 1.5℃ / min and the reduction was continued for 2.5h, with the hydrogen gas fraction maintained at 50%. After the reduction was completed, the temperature was switched to pure argon gas for 30min, cooled to 80℃, and then passivated with argon gas containing 100 ppm oxygen for 1.5h to obtain the final catalyst.

[0043] Based on the total mass of the catalyst, the mass fraction of the iron-copper alloy is 5.3% (of which copper accounts for 23% of the alloy mass fraction), and the mass fraction of molybdenum (calculated as MoO3) is 0.8%; the iron-copper alloy nanoclusters (nanoclusters are a type of nanoparticle morphology) have an average size of 6.8 nm and are uniformly embedded in the magnesium-aluminum composite oxide support.

[0044] The preparation method of single-walled carbon nanotubes is as follows: S1. Take 0.5g of the above catalyst and load it into the middle isothermal zone of a vertical quartz fixed bed reactor (inner diameter 20 mm). Heat the reactor from room temperature to 700℃ under argon protection (200 mL / min). S2. After reaching the temperature, switch the gas to the reaction gas (methane:hydrogen:argon = 60:30:10, total flow rate 400mL / min), and maintain the temperature at 700℃ for catalytic cracking reaction for 60 min. S3. After the reaction is complete, stop the flow of methane and hydrogen, cool the reactor to room temperature under argon protection, and collect the product in the reactor to obtain single-walled carbon nanotubes.

[0045] More than 93% of the obtained single-walled carbon nanotube products were single-walled carbon nanotube bundles. Figure 1To prepare TEM images of single-walled carbon nanotubes with an average outer diameter of 1.4 nm and a Raman spectrum G / D peak intensity ratio of 65, Figure 5 The length can reach 80 μm, with low defect density and high degree of graphitization.

[0046] Example 2: This Example 2 describes a layered double hydroxide-derived iron-copper based single-walled carbon nanotube catalyst, prepared from the following raw materials in parts by weight: Magnesium nitrate hexahydrate 102.6 parts, aluminum nitrate nonahydrate 56.3 parts, ferric nitrate nonahydrate 20.2 parts, copper nitrate trihydrate 7.2 parts, appropriate amount of mixed solution of sodium hydroxide and sodium carbonate, cerium nitrate 2.5 parts; The preparation method of the iron-copper doped magnesium-aluminum layered double hydroxide precursor is the same as in Example 1; in the catalyst preparation step, the auxiliary agent is replaced with cerium nitrate (dissolved in water to prepare an impregnation solution), and the rest is the same as in Example 1.

[0047] Based on the total mass of the catalyst, the iron-copper alloy had a mass fraction of 5.1% (of which copper accounted for 23% of the alloy mass fraction) and a cerium (calculated as CeO2) mass fraction of 0.7%; the average size of the iron-copper alloy nanoclusters was 7.2 nm; more than 92% of the obtained single-walled carbon nanotube products were single-walled carbon nanotube bundles. Figure 2 To prepare TEM images of single-walled carbon nanotubes with an average outer diameter of 1.5 nm and a Raman spectrum G / D peak intensity ratio of 62 (…), Figure 5 ).

[0048] Example 3: This Example 3 describes a layered double hydroxide-derived iron-copper based single-walled carbon nanotube catalyst, prepared from the following raw materials in parts by weight: Magnesium nitrate hexahydrate 102.6 parts, aluminum nitrate nonahydrate 56.3 parts, ferric nitrate nonahydrate 24.2 parts, copper nitrate trihydrate 4.8 parts (iron-copper molar ratio adjusted to 4:1), appropriate amount of mixed solution of sodium hydroxide and sodium carbonate, ammonium molybdate 1.2 parts; The preparation method of the iron-copper doped magnesium-aluminum layered double hydroxide precursor is the same as that in Example 1, except that the amount of iron salt and copper salt is adjusted; the catalyst preparation steps are the same as in Example 1.

[0049] Based on the total mass of the catalyst, the iron-copper alloy had a mass fraction of 5.5% (of which copper accounted for 15% of the alloy mass fraction), and the molybdenum (calculated as MoO3) mass fraction was 0.8%; the average size of the iron-copper alloy nanoclusters was 6.2 nm; more than 94% of the obtained single-walled carbon nanotube products were single-walled carbon nanotube bundles. Figure 3 To prepare TEM images of single-walled carbon nanotubes with an average outer diameter of 1.3 nm and a Raman spectrum G / D peak intensity ratio of 68 (…), Figure 5 ).

[0050] Comparative Example 1: The catalyst of Comparative Example 1 was prepared from the following raw materials in parts by weight: Magnesium nitrate hexahydrate 102.6 parts, aluminum nitrate nonahydrate 56.3 parts, ferric nitrate nonahydrate 20.2 parts, copper nitrate trihydrate 7.2 parts, and an appropriate amount of mixed solution of sodium hydroxide and sodium carbonate; The preparation method of the iron-copper doped magnesium-aluminum layered double hydroxide precursor is the same as in Example 1; however, in the catalyst preparation step, the impregnation of the auxiliary agent (molybdenum or cerium) is not carried out, and the rest is the same as in Example 1.

[0051] Based on the total mass of the catalyst, the iron-copper alloy had a mass fraction of 5.4% (of which copper accounted for 23% of the alloy mass fraction), and the average size of the iron-copper alloy nanoclusters was 7.5 nm. More than 85% of the obtained single-walled carbon nanotube products were single-walled carbon nanotube bundles. Figure 4 To prepare TEM images of single-walled carbon nanotubes with an average outer diameter of 1.7 nm and a Raman spectrum G / D peak intensity ratio of 41 (…), Figure 5 A small amount of amorphous carbon byproducts are produced during the reaction.

[0052] Comparative Example 2: The catalyst of Comparative Example 2 was prepared from the following raw materials in parts by weight: Magnesium nitrate hexahydrate 102.6 parts, aluminum nitrate nonahydrate 56.3 parts, ferric nitrate nonahydrate 20.2 parts, copper nitrate trihydrate 7.2 parts, ammonium molybdate 1.2 parts, and an appropriate amount of mixed solution of sodium hydroxide and sodium carbonate; First, a magnesium-aluminum layered double hydroxide precursor was prepared without adding ferric nitrate nonahydrate or copper nitrate trihydrate, using the same method as in Example 1. Then, iron and copper were loaded by an equal-volume impregnation method, with the iron and copper loading amounts being comparable to those in Example 1. Ammonium molybdate auxiliaries were then impregnated, and the mixture was treated according to the reduction conditions of Example 1.

[0053] Based on the total mass of the catalyst, the mass fraction of the iron-copper alloy was 5.3%, the mass fraction of molybdenum (MoO3) was 0.8%, and the particle size of the iron-copper alloy was 5-15 nm, exhibiting agglomeration. More than 65% of the obtained single-walled carbon nanotube product consisted of single-walled carbon nanotube bundles, and the Raman spectrum G / D peak intensity ratio was 38 (…). Figure 5 During the reaction, a large amount of amorphous carbon byproducts were produced, and the activity decreased significantly after 1 hour.

[0054] Comparing Example 1 and Comparative Example 2, it can be seen that, under the same or similar conditions of iron-copper alloy mass fraction, molybdenum additive content, and reduction conditions, the introduction method of iron and copper components has a significant impact on the particle size of the iron-copper alloy and the growth results of single-walled carbon nanotubes. In Example 1, the iron and copper sources were introduced during the formation stage of the magnesium-aluminum layered double hydroxide precursor. The average size of the resulting iron-copper alloy nanoclusters was 6.8 nm, and more than 93% of the obtained single-walled carbon nanotube products were single-walled carbon nanotube bundles, with a Raman spectrum G / D peak intensity ratio of 65. In Comparative Example 2, the iron and copper components were introduced by impregnation after the formation of the magnesium-aluminum composite oxide support. The resulting iron-copper alloy particles had a size of 5–15 nm and exhibited agglomeration. The proportion of single-walled carbon nanotube bundles in the obtained product decreased to more than 65%, the Raman spectrum G / D peak intensity ratio decreased to 38, and a large number of amorphous carbon byproducts appeared during the reaction.

[0055] The above results indicate that, under similar or identical magnesium-aluminum composite oxide support systems, introducing iron and copper components in the precursor stage is more beneficial than subsequent impregnation methods for obtaining smaller, more concentrated iron-copper alloy nanoparticles. Considering the structural characteristics of magnesium-aluminum layered double hydroxides forming composite oxides and generating defect sites after calcination, it can be argued that the uniform introduction of iron and copper components in the precursor stage facilitates the formation of spatially confined relationships with the defect sites of the magnesium-aluminum composite oxide support during calcination and temperature-programmed reduction, thereby inhibiting the migration, aggregation, and growth of iron and copper species during high-temperature reduction and catalytic cracking. This confined dispersion structure helps maintain an active particle size suitable for single-walled carbon nanotube growth, thereby increasing the proportion of single-walled carbon nanotube bundles, the degree of graphitization, and reaction stability.

[0056] Comparative Example 3: The catalyst of Comparative Example 3 was prepared from the following raw materials in parts by weight: Commercial γ-Al₂O₃ support (specific surface area 200 m²) was used. 2 ( / g) instead of LDH-derived carrier, iron and copper were loaded by equal volume impregnation, with the iron and copper loading amounts being equivalent to those in Example 1, and then impregnated with ammonium molybdate additive, and then treated according to the reduction conditions of Example 1.

[0057] Based on the total mass of the catalyst, the iron-copper alloy had a mass fraction of 5.2%, and the molybdenum (MoO3) had a mass fraction of 0.8%. The iron-copper alloy particles had a wide size distribution (10-25 nm) and exhibited significant agglomeration. The obtained product contained approximately 60% single-walled carbon nanotubes, with a large amount of multi-walled carbon nanotubes and carbon fibers. The Raman spectrum G / D peak intensity ratio was 31 (…). Figure 5 The activity decreased rapidly after 30 minutes of reaction.

[0058] The above embodiments are only used to illustrate the technical solutions of the present invention, and are not intended to limit it. Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some of the technical features. Such modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the spirit and scope of the technical solutions of the embodiments of the present invention.

Claims

1. A layered double hydroxide-derived iron-copper based single-walled carbon nanotube catalyst, characterized in that, The catalyst comprises a magnesium-aluminum composite oxide support, iron-copper alloy nanoparticles dispersed in the magnesium-aluminum composite oxide support, and at least one auxiliary agent selected from molybdenum oxide and cerium oxide loaded on the magnesium-aluminum composite oxide support; the iron-copper alloy nanoparticles are formed in situ by calcination and programmed temperature reduction of an iron- and copper-containing magnesium-aluminum layered double hydroxide precursor, and are at least partially dispersed at defect sites of the magnesium-aluminum composite oxide support.

2. The layered double hydroxide-derived iron-copper based single-walled carbon nanotube catalyst according to claim 1, characterized in that, The catalyst has a magnesium to aluminum molar ratio of 2:1 to 4:1 and an iron to copper molar ratio of 2:1 to 5:

1. Based on the total mass of the catalyst, the iron-copper alloy has a mass fraction of 2% to 8%, of which copper accounts for 15% to 30% of the iron-copper alloy mass. The additive has a mass fraction of 0.2% to 1.5% based on MoO3 or CeO2.

3. The layered double hydroxide-derived iron-copper based single-walled carbon nanotube catalyst according to claim 1, characterized in that, The additive is dispersed in oxide form on the surface of the magnesium-aluminum composite oxide carrier and / or located in the interface region between the iron-copper alloy nanoparticles and the magnesium-aluminum composite oxide carrier.

4. A method for preparing a layered double hydroxide-derived iron-copper based single-walled carbon nanotube catalyst, characterized in that, Includes the following steps: S1. Dissolve magnesium source, aluminum source, iron source and copper source in deionized water to form a mixed salt solution, and dissolve sodium hydroxide and sodium carbonate in deionized water to form a mixed alkaline solution. Under stirring conditions, a co-precipitation reaction is carried out to obtain an iron and copper doped magnesium-aluminum layered double hydroxide precursor. S2. The precursor obtained in step S1 is calcined to obtain iron-copper doped magnesium-aluminum composite oxide. S3. Ammonium molybdate solution and / or cerium nitrate solution are introduced into the material obtained in step S2 by impregnation to obtain a precursor loaded with additives; S4. The material obtained in step S3 is subjected to programmed temperature reduction under a hydrogen atmosphere to obtain the layered double hydroxide-derived iron-copper based single-walled carbon nanotube catalyst.

5. The method for preparing the layered double hydroxide-derived iron-copper based single-walled carbon nanotube catalyst according to claim 4, characterized in that, In step S1, the pH of the coprecipitation reaction is controlled at 9.5–10.5, and the molar ratio of sodium hydroxide to sodium carbonate is 2:

1. After the coprecipitation is completed in step S1, the mixture is stirred and aged for 4–8 hours. The resulting precipitate is washed until neutral and then dried at 60–80°C for 12–24 hours.

6. The method for preparing the layered double hydroxide-derived iron-copper based single-walled carbon nanotube catalyst according to claim 4, characterized in that, In step S2, the temperature is increased to 450-600℃ at a rate of 1-3℃ / min and maintained for 4-6 hours.

7. The method for preparing the layered double hydroxide-derived iron-copper based single-walled carbon nanotube catalyst according to claim 4, characterized in that, The programmed temperature reduction in step S4 includes: first, increasing the temperature to 300-350℃ at 2-5℃ / min and reducing it at a constant temperature for 1-2 hours, then increasing the temperature to 500-600℃ at 1-2℃ / min and reducing it for 2-3 hours.

8. The method for preparing the layered double hydroxide-derived iron-copper based single-walled carbon nanotube catalyst according to claim 4, characterized in that, The programmed temperature reduction in step S4 uses a mixture of hydrogen and inert gas, with the hydrogen integral gradually increasing from 5% to 50%, and the space velocity being 500–2000 h⁻¹. -1 After reduction, the sample is cooled in an inert atmosphere and then passivated for 1-2 hours by introducing an inert gas containing 50-200 ppm oxygen.

9. The method for preparing the layered double hydroxide-derived iron-copper based single-walled carbon nanotube catalyst according to claim 4, characterized in that, In step S3, ammonium molybdate solution and / or cerium nitrate solution are introduced into the material obtained in step S2 using an equal-volume impregnation method. After impregnation, the material is left to stand at room temperature for 6–12 hours and then dried at 80–100°C for 6–10 hours.

10. The use of the layered double hydroxide-derived iron-copper based single-walled carbon nanotube catalyst according to any one of claims 1 to 3 in the preparation of single-walled carbon nanotubes.

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