Preparation method of multi-element metal spherical catalyst and application of multi-element metal spherical catalyst in synthesis of single / few-walled carbon nanotubes

Abstract

The invention discloses a preparation method of a multi-element metal spherical catalyst and application of the multi-element metal spherical catalyst in synthesis of single / few-walled carbon nanotubes, and belongs to the technical field of crossing of nano material preparation and catalytic chemistry. The method comprises the following steps: preparing a nano magnesium oxide carrier with a high specific surface area by adopting a polymer protection precipitation method, preparing a specific catalyst precursor solution, forming micron-sized liquid drops in combination with flame spray pyrolysis, and quickly evaporating, decomposing, reacting and sintering the liquid drops in high-temperature flame to form spherical nano catalyst particles. The obtained catalyst is spherical, has good fluidity, is suitable for a fluidized bed CVD reactor, and is convenient for continuous large-scale production; and when the carbon nano tube is grown by CVD (Chemical Vapor Deposition), high activity, high selectivity and excellent stability are shown, and the single-wall / few-wall carbon nano tube with narrow tube diameter distribution (1-4 nm) and high graphitization degree (ID / IG is less than 0.1) can be prepared.

Description

Technical Field

[0001] This invention relates to a method for preparing a multi-metal spherical catalyst and its application in the synthesis of single / oligowalled carbon nanotubes, belonging to the interdisciplinary field of nanomaterial preparation and catalytic chemistry. Background Technology

[0002] Carbon nanotubes (CNTs) have shown broad application prospects in energy storage, composite materials, electronic devices, and catalysis due to their unique structure and excellent physicochemical properties. In particular, single-walled carbon nanotubes (SWCNTs) and few-walled carbon nanotubes (FWCNTs, typically referring to those with 2-6 wall layers) have become a research and application hotspot due to their higher aspect ratio and superior performance. Chemical vapor deposition (CVD) is the mainstream method for preparing carbon nanotubes, and its core lies in the design and preparation of the catalyst. The active component of the catalyst, the properties of the support, and the preparation process directly affect the growth efficiency, diameter distribution, wall number control, and crystallinity of carbon nanotubes.

[0003] Currently, common methods for preparing carbon nanotube catalysts include impregnation, co-precipitation, and sol-gel methods. For example, CN202410530019A discloses a ternary metal catalyst and its sol-gel preparation method, which improves catalytic activity and stability through the synergistic effect of multiple metal components. However, its preparation process still involves multiple high-temperature calcination steps, making the process complex, and the metal dispersion is limited by the surface properties of the support. CN113292067A achieves wall number control by adjusting the hydrogen ratio, but it still requires the pre-preparation of a specific catalyst and lacks sufficient control over the support structure. Existing technologies generally suffer from the following problems: (1) Active metals are prone to agglomeration: Traditional methods are difficult to maintain the high dispersion of metal nanoparticles at high temperatures, resulting in a wide diameter distribution and uneven mass of carbon nanotubes; (2) Complex process flow: The multi-step preparation process (such as impregnation, drying and calcination) increases production costs and cycle time, and has poor repeatability; (3) Insufficient catalytic stability: The catalyst is prone to sintering and deactivation during high-temperature reactions, which affects the continuous growth of carbon nanotubes and the final yield.

[0004] Therefore, developing a catalyst preparation method that is simple in process, has high metal dispersion, excellent support structure, and is suitable for the efficient synthesis of high-quality single / oligowalled carbon nanotubes is of great scientific significance and industrial application value. Summary of the Invention

[0005] The purpose of this invention is to overcome the shortcomings of the prior art and provide a multi-component transition metal spherical nanocatalyst for the synthesis of high-quality single-walled / oligowalled carbon nanotubes by chemical vapor deposition and its preparation method. The catalyst has high specific surface area, high metal dispersion and excellent thermal stability, and is suitable for the synthesis of single-walled / oligowalled carbon nanotubes.

[0006] Specifically: This invention provides a method for preparing multi-component transition metal spherical nanocatalysts for synthesizing high-quality single / oligowalled carbon nanotubes, comprising the following steps: (1) Nano magnesium oxide powder was prepared by polymer protection uniform precipitation method: soluble magnesium salt, precipitant and polymer protection agent were dissolved in deionized water and reacted at 80-100℃ for a period of time. After the reaction was completed, the powder was aged, filtered, washed and dried, and then calcined in air at 400-600℃ to obtain a high specific surface area nano magnesium oxide carrier. (2) Preparation of catalyst precursor solution: The nano magnesium oxide powder obtained in step (1) is dispersed in a solvent to form a uniform suspension; then a soluble metal salt containing at least two active metal components and a soluble metal salt containing at least one auxiliary metal component are dissolved in the solvent to obtain a metal salt solution; the metal salt solution is mixed with the suspension, a complexing agent is added, and the mixture is stirred at room temperature to 80°C for 2-6 hours to form a uniform and stable precursor solution; (3) The precursor solution obtained in step (2) is fed to the nozzle of the flame spray pyrolysis device through the feeding system. The carrier gas is used to atomize and form micron-sized droplets. The droplets undergo evaporation, decomposition, reaction and sintering in the high-temperature flame to form spherical nanocatalyst particles.

[0007] In one specific embodiment of the present invention, in step (1), the soluble magnesium salt is a raw material including at least one of magnesium nitrate and magnesium chloride.

[0008] In one specific embodiment of the present invention, in step (1), the precipitant is urea or ammonium carbonate.

[0009] In one specific embodiment of the present invention, in step (1), the polymeric dispersant is selected from a polymeric material, including polyethylene glycol (PEG) and polyvinylpyrrolidone (PVP), with a molecular weight of 2000-10000.

[0010] In one specific embodiment of the present invention, in step (1), the reaction lasts for 2-6 hours.

[0011] In one specific embodiment of the present invention, in step (1), the roasting is carried out for 2-4 hours.

[0012] In one specific embodiment of the present invention, in step (1), the BET specific surface area of ​​the obtained high specific surface area nano-magnesium oxide powder is greater than 200 m² / g.

[0013] In one specific embodiment of the present invention, in step (2), the nano-magnesium oxide powder is dispersed in a solvent, wherein the solvent is water, ethanol or a mixture of both.

[0014] In one specific embodiment of the present invention, in step (2), the active metal is at least two of Fe, Co, and Ni.

[0015] In one specific embodiment of the present invention, in step (2), the auxiliary metal is at least one of Mo, V, and W. Mo and W are preferred.

[0016] In one specific embodiment of the present invention, in step (2), the metal salt includes, but is not limited to, nitrates, chlorides, and acetates.

[0017] In one specific embodiment of the present invention, in step (2), the complexing agent is one or more of citric acid, tartaric acid, oxalic acid, and glucose.

[0018] In one specific embodiment of the present invention, in step (2), the molar ratio of the complexing agent to the metal ions is 1:1 to 3:1.

[0019] In one specific embodiment of the present invention, in step (2), the total active metal loading is 1%-10% of the mass of the carrier nano-magnesium oxide powder, calculated as metal elements.

[0020] In one specific embodiment of the present invention, in step (3), the carrier gas is nitrogen, air or argon, and the flow rate is 10-50 L / min.

[0021] In one specific embodiment of the present invention, in step (3), the feed rate of the precursor solution is 1-10 mL / min.

[0022] In one specific embodiment of the present invention, in step (3), the flame is generated by the combustion of combustible gases such as methane and hydrogen with oxygen or air, and the flame temperature is controlled at 1500-2500℃.

[0023] In one specific embodiment of the present invention, in step (3), the obtained catalyst particles are recovered by a downstream filtration and collection device.

[0024] In one specific embodiment of the present invention, the active metal component of the obtained catalyst is highly dispersed on the surface of a spherical nano-magnesium oxide support, forming a core-shell structure. The particle size of the metal nanoparticles is less than 5 nm, and the overall particle size distribution of the catalyst is 50-500 nm.

[0025] This invention provides a multi-element transition metal spherical nanocatalyst prepared based on the above method.

[0026] This invention also provides the application of the above-mentioned multi-component transition metal spherical nanocatalysts in the synthesis of single / oligowalled carbon nanotubes.

[0027] In one specific embodiment of the present invention, a method for synthesizing single / oligowalled carbon nanotubes includes: (1) The above-mentioned multi-component transition metal spherical nanocatalyst was reduced in pure hydrogen for a period of time. (2) Then, CH4, H2 and Ar were used as reaction gases, and the reaction was carried out for a period of time under the action of the reduced multi-element transition metal spherical nanocatalyst.

[0028] In one specific embodiment of the present invention, the volume ratio of CH4, H2 and Ar in the reaction gas is 1:2:1.

[0029] In one specific embodiment of the present invention, a method for synthesizing single / oligowalled carbon nanotubes specifically includes: A multi-element transition metal spherical nanocatalyst was evenly spread in a quartz boat and placed in a tube furnace. The furnace was then purged with Ar at a flow rate of 150-200 mL / min for 20-60 min to displace the gas in the reaction tube. The temperature was then increased to 700-800℃ at a rate of 5-20℃ / min, during which the catalyst was reduced with pure hydrogen at a flow rate of 200-300 mL / min for 1-3 h. At 700-800℃, a reaction gas (CH4: 200 mL / min, H2: 400 mL / min, Ar: 200 mL / min) was introduced, and the reaction was allowed to proceed for 30-60 minutes.

[0030] Advantages and effects of the present invention: 1. A high specific surface area nano-magnesium oxide carrier was prepared by polymer protective precipitation method. It has a large specific surface area (>200 m² / g) and abundant surface hydroxyl groups, which is conducive to metal anchoring and dispersion. 2. The flame spray pyrolysis process is extremely fast (millisecond level), which effectively inhibits metal agglomeration and achieves nanoscale uniform dispersion of active components on the carrier surface; 3. The catalyst is spherical with good fluidity, making it suitable for fluidized bed CVD reactors and facilitating continuous large-scale production; 4. The process is simple and efficient, integrating support synthesis and catalyst loading into one step, avoiding the cumbersome and uncertainties of traditional multi-step processes; 5. The catalyst exhibits high activity, high selectivity and excellent stability during CVD growth of carbon nanotubes, and can produce single-walled / oligowalled carbon nanotubes with narrow diameter distribution (1-4 nm) and high degree of graphitization (ID / IG < 0.1). Attached Figure Description

[0031] Figure 1 This is a schematic diagram of the catalyst preparation process of the present invention; Figure 2 The image shows a SEM image of the catalyst prepared in Example 1. Figure 3 Here is a SEM image of the catalyst prepared in Comparative Example 2; Figure 4 This is a TEM image of carbon nanotubes grown using the catalyst in Example 1. Figure 5 This is a SEM image of carbon nanotubes grown using catalyst 3 in Comparative Example 3. Detailed Implementation

[0032] The technical solution of the invention will be described in detail below with reference to the accompanying drawings: The present invention will be described in detail below through examples, but the scope of protection of the present invention is not limited thereto. Unless otherwise specified, the reagents and instruments described in the examples are all available through conventional commercial channels.

[0033] The technical terms involved in this invention are explained as follows: SWCNT refers to single-walled carbon nanotubes, DWCNT refers to double-walled carbon nanotubes, FWCNT refers to oligo-walled carbon nanotubes, and MWCNT refers to multi-walled carbon nanotubes.

[0034] Example 1 (1) Preparation of nano-magnesium oxide carrier: 25.6 g of Mg(NO3)2·6H2O and 10 g of PEG-4000 were weighed and dissolved in 500 mL of deionized water, and stirred until dissolved. 15 g of urea was added, and the mixture was stirred in an oil bath at 95 °C for 4 hours. After the reaction was complete, the mixture was cooled to room temperature, filtered, and washed three times each with deionized water and anhydrous ethanol. The resulting precipitate was dried in an oven at 110 °C for 12 hours, and then calcined in a muffle furnace at a temperature increased to 500 °C at 2 °C / min for 3 hours to obtain nano-magnesium oxide powder. BET analysis showed its specific surface area to be 235 m². 2 / g.

[0035] (2) Preparation of precursor solution: Weigh 1.0 g of the above magnesium oxide powder, disperse it in 100 mL of anhydrous ethanol, and sonicate for 30 minutes. Separately weigh 0.18 g of Fe(NO3)3·9H2O and 0.044 g of (NH4)6Mo7O. 24 ·4H2O and 0.124 g of Co(NO3)2·6H2O were dissolved in 20 mL of deionized water. The metal salt solution was slowly added dropwise to the magnesium oxide suspension, followed by the addition of 0.5 g of citric acid. The mixture was stirred at 60 °C for 4 hours to obtain a homogeneous precursor solution.

[0036] (3) Flame spray pyrolysis: The precursor solution was pumped into a flame spray pyrolysis apparatus at a rate of 5 mL / min, using compressed air as the atomizing gas at a flow rate of 30 L / min, and methane and oxygen as the combustion gases at flow rates of 30 L / min and 10 L / min, respectively. The product was collected using a glass fiber filter to obtain Fe-Co-Mo / MgO spherical nanocatalysts. SEM ( Figure 2 The results show that the catalyst is ideally spherical with a particle size of 50-500 nm.

[0037] (4) Carbon nanotube growth test: 300 mg of catalyst was evenly spread in a quartz boat and placed in a tube furnace with an inner diameter of 60 mm. The furnace was purged with Ar at a flow rate of 150 mL / min for 30 min to displace the gas in the reaction tube. The temperature was then increased to 750 °C at a rate of 10 °C / min, during which the catalyst was reduced with pure hydrogen at a flow rate of 300 mL / min for 1 h. At 750 °C, a reaction gas mixture (CH4: 200 mL / min, H2: 400 mL / min, Ar: 200 mL / min) was introduced, and the reaction was allowed to proceed for 30 min. After cooling, the carbon nanotubes were weighed, and the yield was 6.2 g / gcat. TEM (Precision Transformation) Figure 4 The product is predominantly single-walled / double-walled, with a diameter of 1.5-2.5 nm and smooth tube walls. Raman spectroscopy (ID / IG=0.045) indicates good crystallinity.

[0038] Example 2 (1) The preparation of magnesium oxide support is the same as in Example 1.

[0039] (2) Precursor solution: Weigh 1.0 g of magnesium oxide and disperse it in 100 mL of ethanol. Separately weigh 0.130 g of Fe(NO3)3·9H2O and 0.044 g of (NH4)6Mo7O. 24 ·4H2O and 0.066 g Co(NO3)2·6H2O were dissolved in 20 mL of a water-ethanol mixture. Subsequent steps were the same as in Example 1.

[0040] (3) The flame spray pyrolysis process is the same as in Example 1, and Fe-Co-Mo / MgO spherical nanocatalysts are obtained.

[0041] The carbon nanotube growth conditions are the same as in Example 1.

[0042] The yield of carbon nanotubes obtained is related to I D / I G See Table 1.

[0043] Comparative Example 1 (Carrier-free spray pyrolysis method) (1) Preparation of precursor solution: Weigh out 6.16 g of Mg(NO3)2·6H2O, 0.18 g of Fe(NO3)3·9H2O, and 0.044 g of (NH4)6Mo7O. 24 Dissolve 4H2O and 0.124 g of Co(NO3)2·6H2O in 20 mL of deionized water, add 0.5 g of citric acid, and stir at 60 °C for 4 hours to obtain a homogeneous precursor solution.

[0044] (2) Flame spray pyrolysis: The precursor solution was pumped into a flame spray pyrolysis apparatus at a rate of 5 mL / min, using compressed air as the atomizing gas at a flow rate of 30 L / min, and methane and oxygen as the combustion gases. The product was collected through a glass fiber filter to obtain the Mg-Fe-Co-Mo catalyst.

[0045] (3) Catalyst used for carbon nanotube growth testing: 300 mg of catalyst was evenly spread in a quartz boat and placed in a tube furnace with an inner diameter of 60 mm. The furnace was purged with Ar at a flow rate of 150 mL / min for 30 min to displace the gas in the reaction tube. The temperature was then increased to 750 °C at a rate of 10 °C / min, during which the catalyst was reduced with pure hydrogen at a flow rate of 300 mL / min for 1 h. At 750 °C, a reaction gas mixture (CH4: 200 mL / min, H2: 400 mL / min, Ar: 200 mL / min) was introduced, and the reaction was allowed to proceed for 30 min. After cooling, the carbon nanotubes were weighed, and the yield was 2.7 g / gcat. The yield of the obtained carbon nanotubes was comparable to that of I... D / I G See Table 1.

[0046] Comparative Example 2 (Equal Volume Impregnation Method) (1) Preparation of nano-magnesium oxide carrier To ensure consistent carrier properties and fair comparison, the comparative sample used the same nano-magnesium oxide carrier prepared by the same method described in Example 1. Specifically: 25.6 g of Mg(NO3)2·6H2O and 10 g of PEG-4000 were weighed and dissolved in 500 mL of deionized water, and stirred until dissolved. 15 g of urea was added, and the mixture was stirred and reacted in a 95°C oil bath for 4 hours. After the reaction, the mixture was cooled to room temperature, filtered, and washed three times each with deionized water and anhydrous ethanol. The resulting precipitate was dried in an oven at 110°C for 12 hours, and then calcined in a muffle furnace at a temperature increased to 500°C at 2°C / min for 3 hours to obtain nano-magnesium oxide powder. BET analysis showed its specific surface area to be 235 m² / g.

[0047] (2) Preparation of impregnation solution: Accurately weigh 0.18 g of Fe(NO3)3·9H2O and 0.044 g of (NH4)6Mo7O. 24 • 4H2O and 0.124 g of Co(NO3)2·6H2O were dissolved together in an appropriate amount of deionized water. The pore volume of the magnesium oxide support obtained in step (1) above was measured in advance. To ensure equal volume impregnation, the above metal salt was dissolved in deionized water corresponding to the pore volume of 1.0 g of nano-magnesium oxide to prepare a saturated impregnation solution. To avoid incomplete dissolution, slight heating or ultrasonication can be used to aid dissolution.

[0048] (3) Impregnation process: Place 1.0 g of the nano-magnesium oxide support prepared in step (1) in an evaporating dish. While stirring continuously, slowly and dropwise add the prepared metal salt impregnation solution onto the support powder using a pipette. During this process, ensure that the liquid is evenly dispersed and avoid local over-wetting. After the addition is complete, seal the container and allow it to stand at room temperature for 12 hours to allow the active component precursor to fully diffuse within the pores of the support.

[0049] (4) Drying and roasting: The aged wet solids were transferred to a 110°C oven and dried for 12 hours to completely remove moisture.

[0050] The dried sample was placed in a muffle furnace and heated to 500°C at a rate of 2°C / min. It was then calcined at this temperature for 3 hours to decompose the metal nitrate and ammonium molybdate into the corresponding metal oxides, thus obtaining the Fe-Mo / MgO catalyst of the control sample 4.

[0051] (5) The carbon nanotube growth conditions were the same as in Example 1. The yield of the obtained carbon nanotubes was the same as that of I. D / I G See Table 1.

[0052] Comparative Example 3 (Sol-Gel Method) (1) Sol preparation: Weigh out 25.6 g of Mg(NO3)2·6H2O, 0.18 g of Fe(NO3)3·9H2O, and 0.044 g of (NH4)6Mo7O. 24 Dissolve 4H2O and 0.124 g of Co(NO3)2·6H2O in 200 mL of deionized water and stir until clear.

[0053] Add 19.35 g of citric acid as a complexing agent, with the amount of citric acid calculated at a 1:1 molar ratio to the total number of metal ions. After stirring to dissolve, adjust the pH of the solution to 7.0 with ammonia.

[0054] (2) Gel formation and drying: The solution was placed in an oil bath at 80°C and stirred for 4 hours. The solution gradually formed a transparent sol and further transformed into a gel. The gel was transferred to a petri dish and dried in an oven at 110°C for 12 hours to obtain a dry gel.

[0055] (3) Roasting: The dry gel was ground into powder, placed in a muffle furnace, heated to 500°C at 2°C / min, and calcined for 3 hours to obtain Fe-Mo / MgO catalyst powder.

[0056] (4) The carbon nanotube growth conditions were the same as in Example 1. The yield of the obtained carbon nanotubes was the same as that of I. D / I G See Table 1.

[0057] Comparative Example 4 (Glycine Nitrate Solution Combustion Method) (1) Preparation of precursor solution: Weigh out 25.6 g of Mg(NO3)2·6H2O, 0.18 g of Fe(NO3)3·9H2O, and 0.044 g of (NH4)6Mo7O. 24 Dissolve 0.124 g of 4H₂O and Co(NO₃)₂·6H₂O in 200 mL of deionized water and stir until clear. Weigh 7.56 g of glycine as fuel and add it to the above solution, stirring until dissolved evenly. The amount of glycine used is calculated based on a 1:1 molar ratio of total metal ions.

[0058] (2) Combustion reaction: The precursor solution is transferred to a refractory crucible and placed on a hot plate inside a fume hood. Heating to 80°C and maintaining this temperature allows the moisture to evaporate slowly, gradually concentrating the solution into a viscous slurry. Heating is then continued to approximately 200°C, at which point the solution undergoes self-propagating combustion, producing a large amount of gas and forming a fluffy solid product. The combustion process lasts approximately 1-2 minutes.

[0059] (3) Post-processing: The solid product obtained from combustion was cooled to room temperature and then ground into a fine powder. The powder was placed in a muffle furnace and heated to 500°C at a rate of 2°C / min for 3 hours to remove residual carbonaceous material, thus obtaining Fe-Mo / MgO catalyst powder.

[0060] (4) The carbon nanotube growth conditions were the same as in Example 1. The yield of the obtained carbon nanotubes was the same as that of I. D / I G See Table 1.

[0061] Comparative Example 5 (Spray Drying + Sintering) Following steps (1) and (2) in Example 1, nano-magnesium oxide carrier and precursor solution were prepared.

[0062] (3) Spray drying: The obtained precursor solution was injected into a spray dryer, and spherical catalyst particles were prepared by simultaneously spraying and drying the precursor solution with hot air at approximately 290°C using a disc-type spray drying method. The disc speed was 5000-20000 rpm, and the solution injection rate was 10-100 mL / min.

[0063] (4) Roasting: The catalyst product was obtained by sintering at approximately 55°C for 30 minutes under normal pressure and air atmosphere.

[0064] (5) The carbon nanotube growth conditions were the same as in Example 1. The yield of the obtained carbon nanotubes was the same as that of I. D / I G See Table 1.

[0065] Comparative Example 6 (Immersion Method) Following steps (1) and (2) in Example 1, nano-magnesium oxide carrier and precursor solution were prepared.

[0066] The obtained precursor solution was continuously stirred and heated to a reaction temperature of 110°C for 3 hours. The solid obtained by calcination at 500°C for 4 hours was then ground and passed through a 100-mesh sieve to obtain the catalyst.

[0067] The carbon nanotube growth conditions were the same as in Example 1. The yield of the obtained carbon nanotubes was the same as that of I. D / I G See Table 1.

[0068] Table 1. Comparison of catalyst performance between the examples and the comparative examples

[0069] As can be seen from Table 1, the catalyst synthesized by flame spray combustion method using nano-magnesium oxide prepared by polymer-protected uniform precipitation as a carrier grows high-quality and high-yield carbon nanotubes and has excellent high-temperature stability. Example 1 and Comparative Example 1 confirm the importance of the catalyst active components being uniformly dispersed on the surface of the catalyst carrier. The catalyst prepared by the former has the best yield and quality of carbon nanotubes.

[0070] Comparative Example 7 Referring to Example 1, in step (2), Co(NO3)2·6H2O was replaced with an equal mass of (NH4). 10 H2(W2O7)6, with other parameters remaining unchanged, yielded the corresponding catalyst.

[0071] Comparative Example 8 Referring to Example 1, in step (2), Co(NO3)2·6H2O was replaced with an equal mass of Ni(NO3)2·6H2O, while other steps remained unchanged. The corresponding catalyst was obtained.

[0072] Using the same carbon nanotube growth conditions as in Example 1, but replacing them with the catalysts obtained in Comparative Examples 7 and 8. The resulting carbon nanotube synthesis results are shown in Table 2.

[0073] Table 2

[0074] Although the present invention has been disclosed above with reference to preferred embodiments, it is not intended to limit the present invention. Anyone skilled in the art can make various modifications and alterations without departing from the spirit and scope of the present invention. Therefore, the scope of protection of the present invention should be determined by the claims.

Claims

1. A method for preparing a multi-component transition metal spherical nanocatalyst, characterized in that, Includes the following steps: (1) Nano magnesium oxide powder was prepared by polymer protection uniform precipitation method: soluble magnesium salt, precipitant and polymer protection agent were dissolved in deionized water and reacted at 80-100℃ for a period of time. After the reaction was completed, the powder was aged, filtered, washed and dried, and then calcined in air at 400-600℃ to obtain a high specific surface area nano magnesium oxide carrier. (2) Preparation of catalyst precursor solution: Disperse the nano magnesium oxide powder obtained in step (1) in a solvent to form a uniform suspension; Subsequently, a soluble metal salt containing at least two active metal components and a soluble metal salt containing at least one auxiliary metal component are dissolved in a solvent to obtain a metal salt solution; the metal salt solution is mixed with a suspension, a complexing agent is added, and the mixture is stirred at room temperature to 80°C for 2-6 hours to form a uniform and stable precursor solution. (3) The precursor solution obtained in step (2) is fed to the nozzle of the flame spray pyrolysis device through the feeding system. The carrier gas is used to atomize and form micron-sized droplets. The droplets undergo evaporation, decomposition, reaction and sintering in the high-temperature flame to form spherical nanocatalyst particles.

2. The preparation method according to claim 1, characterized in that, In step (1), the soluble magnesium salt is a raw material including at least one of magnesium nitrate and magnesium chloride; the precipitant is urea or ammonium carbonate; the polymeric dispersant is a polymeric material including polyethylene glycol and polyvinylpyrrolidone, with a molecular weight of 2000-10000.

3. The preparation method according to claim 1, characterized in that, In step (2), the active metal is at least two of Fe, Co, and Ni; and the total loading of the active metal is 1%-10% of the mass of the carrier nano-magnesium oxide powder, calculated as metal elements.

4. The preparation method according to claim 1, characterized in that, In step (2), the auxiliary metal is at least one of Mo, V, and W.

5. The preparation method according to claim 1, characterized in that, In step (2), the complexing agent is one or more of citric acid, tartaric acid, oxalic acid, and glucose; the molar ratio of the complexing agent to the metal ion is 1:1 to 3:

1.

6. The preparation method according to claim 1, characterized in that, In step (3), the carrier gas is nitrogen, air or argon, with a flow rate of 10-50 L / min; the precursor solution feed rate is 1-10 mL / min; the flame is generated by the combustion of combustible gases such as methane and hydrogen with oxygen or air, and the flame temperature is controlled at 1500-2500℃.

7. A multi-element transition metal spherical nanocatalyst prepared by the preparation method according to any one of claims 1-6.

8. The application of the multi-component transition metal spherical nanocatalyst according to claim 7 in the synthesis of single / oligowalled carbon nanotubes.

9. A method for synthesizing single / oligowalled carbon nanotubes, characterized in that, Includes the following steps: (a) The above-mentioned multi-component transition metal spherical nanocatalyst was reduced in pure hydrogen for a period of time. (b) Subsequently, CH4, H2 and Ar were used as reactants, and the reaction was carried out for a period of time under the action of the reduced multi-element transition metal spherical nanocatalyst.

10. The method according to claim 9, characterized in that, The volume ratio of CH4, H2 and Ar in the reaction gas is 1:2:1.

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