CN114524466B

CN114524466B - Synthesis method of high-activity catalyst

Info

Publication number: CN114524466B
Application number: CN202210171373.XA
Authority: CN
Inventors: 沈宇栋; 万仁涛
Original assignee: Wuxi Dongheng New Energy Technology Co Ltd
Current assignee: Wuxi Dongheng New Energy Technology Co Ltd
Priority date: 2022-02-24
Filing date: 2022-02-24
Publication date: 2023-04-28
Anticipated expiration: 2042-02-24
Also published as: WO2023159698A1; CN114524466A

Abstract

The invention belongs to the field of catalyst preparation, and in particular relates to a preparation method of a catalyst synthesized by a carbon nano tube complexation method for olefin gas pyrolysis growth. The invention uses a two-step complexing method of Citric Acid (CA) and ethylenediamine tetraacetic acid (EDTA) ammonia solution, and prepares the high-activity metal oxide catalyst for synthesizing the carbon nano tube through carbonization, roasting, crushing and screening. The high-activity catalyst prepared by the invention has simple process, good stability, proper structure and better catalytic activity, the high-performance carbon nano tube is successfully obtained through experiments, and the yield of the carbon nano tube obtained by the catalyst with the weight of unit catalyst metal element is 169-246g/g, thus having better economic benefit.

Description

Synthesis method of high-activity catalyst

Technical Field

The invention relates to a synthesis method of a high-activity catalyst, and belongs to the technical field of catalysts.

Background

In recent years, carbon nanotubes have been widely used as an excellent conductive agent in the industries of lithium batteries for new energy automobiles and the like. This is due to the advantages of excellent heat conduction, electrical conductivity, good mechanical strength, etc. In addition, the one-dimensional structure of the carbon nano tube can enhance the bonding of the active material and improve the performance of the pole piece. Therefore, the battery has a great application prospect in the aspect of being used for new energy batteries. The current methods for synthesizing carbon nanotubes mainly include an arc discharge method, a thermal decomposition method, a chemical vapor deposition method and the like. Compared with other preparation methods, the chemical vapor deposition method has better superiority and has realized large-scale industrialized application. This is due to the advantages of low growth temperature, easy control and optimization of reaction conditions, etc. of the carbon nanotubes by the method.

The preparation of the carbon nano tube by the chemical vapor deposition method is completed under the action of a catalyst, and the catalyst currently used in the method mainly comprises an iron catalyst and other transition metal catalysts, but the catalyst reported in the prior art has the technical defects of lower yield of the synthesized carbon nano tube, higher metal residue in the prepared carbon nano tube, subsequent pickling and purification and the like, and high preparation cost.

Disclosure of Invention

[ technical problem ]

The existing reported catalyst has the technical defects of low yield of synthesized carbon nanotubes, high metal residue in the prepared carbon nanotubes, high preparation cost caused by the subsequent processes of acid washing, purification and the like.

In addition, the length of the synthesized carbon nanotubes is reported to be shorter and is mostly 5-30 micrometers; few reports of carbon nanotubes of hundreds of micrometers and even millimeters in length are available and synthesis is difficult.

Technical scheme

The first object of the present invention is to provide a method for preparing a carbon nanotube catalyst with high activity, using a citric acid-ethylenediamine tetraacetic acid complexation method, the method comprising the steps of:

(1) Mixing metal precursor salt, pure water and citric acid to prepare a mixed solution, adding ammonium heptamolybdate into the mixed solution, and stirring and dissolving the mixed solution to obtain a citrate mixed solution; stirring and concentrating the citrate mixed solution in a water bath, and cooling to obtain a solution A; wherein, the mole ratio of the citric acid and the metal precursor salt is (1-1.5): 1, a step of;

(2) EDTA is dissolved in ammonia water to obtain EDTA ammonia solution; mixing the solution A with an EDTA ammonia solution to obtain a mixed solution B; then, placing the mixed solution B into an open porcelain element dish, and carbonizing for 30min at 500 ℃ in a muffle furnace; then roasting in a muffle furnace at 450 ℃ for 240min, and sieving and crushing through a screen to form a powdery catalyst; wherein the molar ratio of EDTA to metal precursor salt is (0.35-0.75): 1.

As an embodiment of the present invention, the metal precursor salt in step (1) is Mg ²⁺ Salts, al ³⁺ Salts, fe ³ ⁺ Salts, co ²⁺ A mixture of salts.

As one embodiment of the present invention, the mass fraction of ammonia is 25wt%; the mass ratio of the ammonia water to the EDTA is (4-7): 1.

As an embodiment of the present invention, the step (1) specifically includes: 102.6g of magnesium nitrate hexahydrate, 30g of aluminum nitrate nonahydrate, 8.9g of ferric nitrate nonahydrate, 5.2g of cobalt nitrate hexahydrate, 585g of pure water and 100g of citric acid are mixed, stirred and dissolved in a constant-temperature water bath at 92 ℃ to obtain a mixed solution; then 0.71g of ammonium heptamolybdate is added into the mixture, stirred and dissolved, stirred and heated continuously until the volume of the solution is 500mL, cooled to normal temperature, and then solution A is obtained.

As an embodiment of the present invention, the step (2) specifically includes: 83.6g of EDTA was dissolved in 493.1g of 25wt% aqueous ammonia to obtain an EDTA aqueous ammonia solution; mixing the solution A with EDTA ammonia solution to obtain a mixed solution B.

The second object of the present invention is to provide a carbon nanotube catalyst with high activity prepared by the aforementioned method.

The third object of the present invention is to provide an application of the carbon nanotube catalyst with high activity in preparing carbon nanotubes by cracking olefin gases.

A fourth object of the present invention is to provide a method for improving productivity of carbon nanotubes by cracking an olefin gas, which uses the aforementioned carbon nanotube catalyst having high activity, the method comprising the steps of:

placing the carbon nano tube catalyst with high activity in a tube furnace, and heating to 300 ℃ under the protection of inert gas; after the temperature reaches 300 ℃, continuously introducing 1000sccm of hydrogen, and heating to 700 ℃ at 10 ℃/min; and continuously introducing a mixed gas of 300sccm ethylene, 300sccm nitrogen and 100sccm hydrogen after the temperature reaches 700 ℃, reacting for 60 minutes at constant temperature, and then cooling to obtain the carbon nanotube.

A fifth object of the present invention is to provide a carbon nanotube, wherein the metal element residue is not higher than 0.6%, and the structure is an ultra-long bundle structure.

The beneficial effects are that:

(1) The invention uses a two-step complexing method of Citric Acid (CA) and ethylenediamine tetraacetic acid (EDTA) ammonia solution to synthesize the high-activity metal oxide catalyst, which is used for synthesizing the carbon nano tube. The yield of the carbon nano tube obtained by the high-efficiency catalyst per the weight of the metal element of the catalyst is not lower than 169g/g, and can reach 246g/g at most, namely, the metal residue in the carbon nano tube is about 0.4 percent, so that the purification treatment of the carbon nano tube can be reduced or even avoided, and the production cost of the carbon nano tube is reduced.

(2) The catalyst prepared based on the CA-EDTA ammonia solution two-step complexation method presents a loose flaky stacking mode, so that the grown carbon nanotubes are in an ultra-long bundle shape, and the utilization efficiency of the catalyst is greatly improved; the more uniform active center distribution can be fully utilized, and the carbon nano tube with higher yield is obtained.

(3) The catalyst prepared by the two-step complexation method based on the CA-EDTA ammonia solution has simple process, greatly reduces the production cost and improves the production profit of enterprises.

(4) The carbon nano tube prepared by the invention has the characteristic structure of ultra-long beam shape, the length of the carbon nano tube can reach more than 50 microns, the length-diameter ratio is large, the electric conduction and the heat conduction are good, and the carbon nano tube has good mechanical properties and can be used as carbon fiber, lithium battery cathode material and conductive agent.

Drawings

FIG. 1 is a scanning electron microscope image of the catalyst prepared in example 2 of the present invention, with a scale of 1. Mu.m.

FIG. 2 is a scanning electron microscope image of the catalyst prepared in example 2 of the present invention, with a scale of 5. Mu.m.

FIG. 3 is a scanning electron microscope image of a carbon nanotube synthesized by using the catalyst prepared in example 2 of the present invention, with a scale of 2. Mu.m.

FIG. 4 is a scanning electron microscope image of a carbon nanotube synthesized using the catalyst prepared in example 2 of the present invention, with a scale of 10. Mu.m.

FIG. 5 is a scanning electron microscope image of a carbon nanotube synthesized by using the catalyst prepared in example 2 of the present invention, with a scale of 20. Mu.m.

FIG. 6 is a scanning electron microscope image of a carbon nanotube synthesized using the catalyst prepared in example 2 of the present invention, with a scale of 50. Mu.m.

Detailed Description

The mass fraction of aqueous ammonia in each of the examples and comparative examples was 25wt%, and the aqueous ammonia solution was reagent grade.

Yield of carbon nanotubes = weight of carbon nanotubes/weight of catalyst metal element;

catalyst metal element weight = catalyst weight iron cobalt element/all elements = 0.13 catalyst weight.

Example 1

A preparation method of a synthetic carbon nano tube catalyst comprises the following steps:

(1) 102.6g of magnesium nitrate hexahydrate, 30g of aluminum nitrate nonahydrate, 8.9g of ferric nitrate nonahydrate, 5.2g of cobalt nitrate hexahydrate, 585g of pure water and 100g of citric acid are sequentially added into a 1L beaker, the mixture is placed into a 92 ℃ constant temperature water bath kettle, a tetrafluoro paddle is used for mechanically stirring and dissolving raw materials at 400rpm, then 0.71g of ammonium heptamolybdate is added, stirring and dissolving are carried out, stirring and heating are continued, and the solution liquid level is 500mL. Taking out the beaker, and cooling to normal temperature to obtain a solution A.

(2) 60.8g of EDTA and 359g of ammonia water are added into a 1L beaker in sequence, and the mixture is stirred and dissolved at normal temperature to form an EDTA ammonia solution.

(3) Pouring the solution A into EDTA ammonia solution, continuously stirring for 30min, and then weighing 30g of the solution and filling into an open porcelain dish. And heating the muffle furnace to a constant temperature of 500 ℃, placing the ceramic element dish filled with the solution into the muffle furnace, carbonizing for 30min to form a loose porous catalyst precursor, setting the temperature of the muffle furnace to 450 ℃ after carbonization, and roasting for 240min at the constant temperature. The product was crushed by sieving with a 80 mesh sieve to form a powdery catalyst.

The method for preparing the carbon nanotubes by using the catalyst prepared in example 1 is as follows:

weighing 0.3g of the prepared powdery catalyst, placing the powdery catalyst in the middle area of a phi 60mm tubular furnace, heating to 300 ℃ at a heating rate of 10 ℃/min under the protection of 1000sccm nitrogen, continuously introducing 1000sccm hydrogen, continuously heating to 700 ℃ at the heating rate of 10 ℃/min, introducing a mixed gas of 300sccm ethylene, 300sccm nitrogen and 100sccm hydrogen after the temperature reaches 700 ℃, and reacting at constant temperature for 60 minutes. Cooling and then taking out the product, namely the carbon nano tube.

The results show that the catalyst of example 1 was able to successfully produce carbon nanotubes as a product, the weight of the resulting carbon nanotube product was 6.6g and the bulk density was 0.006g/ml. The yield of carbon nanotubes obtained from the catalyst of example 1 per weight of catalyst metal element was 169g/g. The residual iron and cobalt metal elements in the carbon nano tube is about 0.6 percent.

Example 2

A method for preparing a synthetic carbon nanotube catalyst, referring to example 1, differs only in adjusting the amounts of EDTA and ammonia, specifically:

(1) 102.6g of magnesium nitrate hexahydrate, 30g of aluminum nitrate nonahydrate, 8.9g of ferric nitrate nonahydrate, 5.2g of cobalt nitrate hexahydrate, 585g of pure water and 100g of citric acid are sequentially added into a 1L beaker, the mixture is placed into a 92 ℃ constant temperature water bath kettle, a tetrafluoro blade is mechanically stirred at 400rpm to dissolve raw materials, then 0.71g of ammonium heptamolybdate is added, stirring and dissolution are carried out, stirring and heating are continued, and the solution liquid level is 500mL. Taking out the beaker, and cooling to normal temperature to obtain a solution A.

(2) 83.6g of EDTA and 493.1g of ammonia water are added into a 1L beaker in sequence, and the mixture is stirred and dissolved at normal temperature to form an EDTA ammonia solution.

(3) Pouring the solution A into EDTA ammonia solution, continuously stirring for 30min, and then weighing 30g of the solution and filling into an open porcelain dish. After the muffle furnace is heated to 500 ℃ and kept at a constant temperature, the ceramic element dish filled with the solution is placed into the muffle furnace to be carbonized to form loose porous catalyst precursor, the temperature of the muffle furnace is set to 450 ℃ after carbonization for 30min at the temperature of 500 ℃, and the ceramic element dish is baked for 240min at the constant temperature. The product was crushed by sieving with a 80 mesh sieve to form a powdery catalyst.

The method for preparing carbon nanotubes using the catalyst prepared in example 2 was the same as in example 1.

The scanning electron microscope images of the powdery catalyst of example 2 and the carbon nanotubes produced therefrom are shown in FIGS. 1-2 and 3-4, respectively. As shown in fig. 1-2, the catalyst of the present invention exhibits a loose sheet stacking mode, a relatively uniform distribution of active centers, and the structure has the effect of: the sheet structure makes the grown carbon nano tube into ultra-long beam shape; the uniform active center distribution improves the activity of the catalyst, obtains the carbon nano tube with higher yield, and greatly improves the utilization efficiency of the catalyst. The carbon nano tube prepared by the invention has good electric conduction, heat conduction and mechanical properties, and can be used as a lithium battery cathode material and a conductive agent.

The results show that the catalyst of example 2 was able to successfully produce carbon nanotubes as a product, the weight of the carbon nanotube product obtained in example 2 was 9.6g and the bulk density was 0.006g/ml. The yield of carbon nanotubes obtained from the catalyst of example 2 per weight of catalyst metal element was 246g/g. The residual iron and cobalt metal elements in the carbon nano tube is about 0.4 percent.

Example 3

(2) 98.78g of EDTA and 582.8g of ammonia water are added into a 1L beaker in sequence, and the mixture is stirred and dissolved at normal temperature to form an EDTA ammonia solution.

The method for preparing carbon nanotubes using the catalyst prepared in example 3 was the same as in example 1.

The results show that the catalyst of example 3 was able to successfully produce carbon nanotubes as a product, the weight of the carbon nanotube product obtained in example 3 was 7.8g and the bulk density was 0.006g/ml. The yield of carbon nanotubes obtained from the catalyst of example 3 per weight of catalyst metal element was 200g/g. The residual iron and cobalt metal elements in the carbon nano tube is about 0.5 percent.

From examples 1-3, it can be analyzed that the yield of the carbon nanotubes prepared in example 2 is highest, up to 246g/g, which indicates that the EDTA and ammonia water content in example 2 are optimal, and too high or too low may cause structural changes of the catalyst, thereby affecting the activity of the catalyst for synthesizing carbon nanotubes.

Comparative example 1

A method for preparing a synthetic carbon nanotube catalyst, referring to example 2, except that only the amount of EDTA is adjusted such that the molar ratio of EDTA to metal precursor salt is less than 0.35:1. For example, the molar ratio of EDTA to metal precursor salt is 0.3:1.

Carbon nanotubes were prepared using the catalyst of comparative example 1 in the same manner as in example 2, and the result showed that the weight of the carbon nanotubes obtained in comparative example 1 was 3.3g. The yield of carbon nanotubes obtained from the catalyst of comparative example 1 per weight of catalyst metal element was 85g/g.

Comparative example 2

A method for preparing a synthetic carbon nanotube catalyst, referring to example 2, except that only the amount of EDTA is adjusted such that the molar ratio of EDTA to metal precursor salt is higher than 0.75:1. For example, the molar ratio of EDTA to metal precursor salt is 0.8:1.

Carbon nanotubes were prepared using the catalyst of comparative example 2 in the same manner as in example 2, and the result showed that the weight of the carbon nanotubes obtained in comparative example 2 was 5.1g. The yield of carbon nanotubes obtained from the catalyst of comparative example 2 per weight of catalyst metal element was 131g/g.

As can be seen from comparative analysis of examples 1-3 and comparative examples 1-2, the optimal range of molar ratio of EDTA to metal precursor salt is 0.35-0.75:1, and within this range, 6.6-9.6 g of product carbon nanotubes can be successfully prepared, and the yield of carbon nanotubes obtained by the high-efficiency catalyst per weight of metal element of the catalyst is not lower than 169g/g; outside this range, the aforementioned effects cannot be achieved.

Comparative example 3

A method for preparing a synthetic carbon nanotube catalyst, referring to example 2, only differs in that the amount of ammonia is adjusted so that the mass ratio of ammonia to EDTA is lower than 4:1, for example, so that the mass ratio of ammonia to EDTA is 3:1.

Carbon nanotubes were prepared using the catalyst of comparative example 3 in the same manner as in example 2, and the result showed that the weight of the carbon nanotubes obtained in comparative example 3 was 5.4g. The yield of carbon nanotubes obtained from the catalyst of comparative example 3 per weight of catalyst metal element was only 138g/g.

Comparative example 4

A method for preparing a synthetic carbon nanotube catalyst, referring to example 2, only differs in that the amount of ammonia is adjusted so that the mass ratio of ammonia to EDTA is higher than 7:1, for example, so that the mass ratio of ammonia to EDTA is 8:1.

Carbon nanotubes were prepared using the catalyst of comparative example 4 in the same manner as in example 2, and the result showed that the weight of the carbon nanotubes obtained in comparative example 4 was 6.0g. The yield of carbon nanotubes obtained from the catalyst of comparative example 4 per weight of catalyst metal element was 153.8g/g.

As can be seen from comparative analysis of examples 1-3 and comparative examples 3-4, the optimal mass ratio of ammonia water to EDTA is 4-7:1, 6.6-9.6 g of carbon nanotubes can be successfully prepared, and the yield of the carbon nanotubes obtained by the catalyst per the weight of the metal element of the catalyst is not lower than 169g/g; outside this range, the aforementioned effects cannot be achieved.

Comparative example 5

A method for preparing a synthetic carbon nanotube catalyst, referring to example 2, only differs in that the amount of citric acid is adjusted so that the molar ratio of citric acid to metal precursor salt is lower than 1:1, for example, the molar ratio of citric acid to metal precursor salt is 0.7:1.

Carbon nanotubes were prepared using the catalyst of comparative example 5 in the same manner as in example 2, and the result showed that the weight of the carbon nanotubes obtained in comparative example 5 was 2.6g. The yield of carbon nanotubes obtained from the catalyst of comparative example 5 per weight of catalyst metal element was only 66.7g/g.

Comparative example 6

A method for preparing a synthetic carbon nanotube catalyst, referring to example 2, differs only in that the amount of citric acid is adjusted such that the molar ratio of citric acid to metal precursor salt is higher than 1.5:1, for example, such that the molar ratio of citric acid to metal precursor salt is 2:1.

Carbon nanotubes were prepared using the catalyst of comparative example 6 in the same manner as in example 2, and the result showed that the weight of the carbon nanotubes obtained in comparative example 6 was 4.6g. The yield of carbon nanotubes obtained from the catalyst of comparative example 6 per weight of catalyst metal element was only 118g/g.

Comparative example 7 citric acid complexation alone

A method for preparing a catalyst for synthesizing carbon nanotubes, with reference to example 2, differs only in that EDTA addition is omitted, and the amount of citric acid to be used is adjusted to be the optimum amount of 149.9g (determined according to the highest yield of carbon nanotubes obtained per weight of catalyst metal element).

Carbon nanotubes were prepared using the catalyst of comparative example 7 in the same manner as in example 2, and the result showed that the weight of the carbon nanotubes obtained in comparative example 7 was 1.8g. The yield of carbon nanotubes obtained from the catalyst of comparative example 7 per weight of catalyst metal element was only 46g/g.

Analysis of example 2 and comparative example 7 revealed that the catalytic activity of the prepared catalyst (the yield of carbon nanotubes obtained per weight of catalyst of metal element of the catalyst) was significantly reduced after omitting the EDTA complexing agent, indicating that the addition of EDTA had an unexpected effect on the improvement of the catalyst activity of the present invention.

Comparative example 8 EDTA complexation alone

A method for preparing a catalyst for synthesizing carbon nanotubes, which is described in example 2, differs from the method in that the addition of citric acid is omitted and the amount of EDTA is adjusted to be 98.78g.

Carbon nanotubes were prepared using the catalyst of comparative example 8 in the same manner as in example 2, and the result showed that the weight of the carbon nanotubes obtained in comparative example 8 was 0.90g. The yield of carbon nanotubes obtained from the catalyst of comparative example 8 per weight of catalyst metal element was only 23g/g.

Analysis of example 2 and comparative example 8 revealed that the catalytic activity (yield of carbon nanotubes obtained per weight of catalyst per unit weight of catalyst metal element) of the prepared catalyst was significantly reduced after omitting citric acid, indicating that the addition of citric acid had an unexpected effect on the improvement of the catalyst activity of the present invention.

Comparative example 9

A method for preparing a synthetic carbon nanotube catalyst, referring to example 2, which is different only in that EDTA ammonia solution is used for complexation and then citric acid is added for complexation, specifically:

(1) 102.6g of magnesium nitrate hexahydrate, 30g of aluminum nitrate nonahydrate, 8.9g of ferric nitrate nonahydrate, 5.2g of cobalt nitrate hexahydrate and 585g of pure water are sequentially added into a 1L beaker, 83.6g of EDTA and 493.1g of ammonia water are added, stirring is carried out for dissolution, 0.71g of ammonium heptamolybdate is added, and stirring is carried out for 30min at normal temperature to fully dissolve, so as to obtain a solution B.

(2) Weighing 100g of citric acid to prepare an aqueous solution;

(3) Pouring the solution B obtained in the step (1) into a citric acid aqueous solution, putting the solution B into a constant-temperature water bath kettle with the temperature of 92 ℃, mechanically stirring the solution B by using a tetrafluoro blade at 400rpm, and heating and concentrating the solution B until the volume of the solution B is equal to the volume of the mixed solution obtained in the step (3) in the embodiment 1. Taking out the beaker, cooling to normal temperature, weighing 30g of solution, and filling into an open ceramic element dish. And heating the muffle furnace to a constant temperature of 500 ℃, placing the ceramic element dish filled with the solution into the muffle furnace, carbonizing for 30min to form a loose porous catalyst precursor, setting the temperature of the muffle furnace to 450 ℃ after carbonization, and roasting for 240min at the constant temperature. The product was crushed by sieving with a 80 mesh sieve to form a powdery catalyst.

Carbon nanotubes were prepared using the catalyst of comparative example 9 in the same manner as in example 2, and the result showed that the weight of the carbon nanotubes obtained in comparative example 9 was only 0.3g, and that almost no carbon nanotube product was produced.

Claims

1. The preparation method of the carbon nano tube catalyst with high activity is characterized by adopting a citric acid-ethylenediamine tetraacetic acid two-step complexation method, and comprises the following steps:

(1) Mixing metal precursor salt, pure water and citric acid to prepare a mixed solution, adding ammonium heptamolybdate into the mixed solution, and stirring and dissolving the mixed solution to obtain a citrate mixed solution; stirring and concentrating the citrate mixed solution in a water bath, and cooling to obtain a solution A; wherein, the mole ratio of the citric acid to the metal precursor salt is (1-1.5): 1, a step of;

(2) EDTA is dissolved in ammonia water to obtain EDTA ammonia solution; mixing the solution A with an EDTA ammonia solution to obtain a mixed solution B; then, placing the mixed solution B into an open porcelain element dish, and carbonizing for 30min at 500 ℃ in a muffle furnace; then roasting in a muffle furnace at 450 ℃ for 240min, and sieving and crushing through a screen to form a powdery catalyst; wherein the molar ratio of EDTA to metal precursor salt is (0.35-0.75): 1;

the mass fraction of the ammonia water is 25wt%; the mass ratio of the ammonia water to the EDTA is (4-7) 1.

2. The method of claim 1, wherein the metal precursor salt in step (1) is Mg ²⁺ Salts, al ³⁺ Salts, fe ³⁺ Salts, co ²⁺ A mixture of salts.

3. The method according to claim 2, wherein step (1) is specifically: mixing 102.6g magnesium nitrate hexahydrate, 30g aluminum nitrate hexahydrate, 8.9g ferric nitrate hexahydrate, 5.2g cobalt nitrate hexahydrate, 585g pure water and 100g citric acid, and stirring and dissolving in a constant-temperature water bath at 92 ℃ to obtain a mixed solution; then 0.71g of ammonium heptamolybdate is added into the mixture, stirred and dissolved, stirred and heated continuously until the volume of the solution is 500mL, cooled to normal temperature, and then solution A is obtained.

4. A method according to claim 3, wherein step (2) is specifically: 83.6g of EDTA was dissolved in 493.1g of 25wt% aqueous ammonia to obtain an EDTA aqueous ammonia solution; mixing the solution A with EDTA ammonia solution to obtain a mixed solution B.

5. A carbon nanotube catalyst having high activity produced by the method of any one of claims 1 to 4.

6. The method for preparing carbon nanotubes by cracking olefin gas, which comprises the steps of preparing the carbon nanotubes by using the carbon nanotube catalyst with high activity as claimed in claim 5.

7. A method for improving the productivity of carbon nanotubes produced by cracking an olefin gas, characterized in that the carbon nanotube catalyst having high activity according to claim 5 is used, comprising the steps of: