Method for producing single-walled carbon nanotubes by arc process

Abstract

The invention provides a method for producing a single-walled carbon nanotube by an electric arc method, and belongs to the technical field of preparation of carbon nanotubes. The method provided by the invention comprises the following steps: mixing a main catalyst, sulfur powder and selenium oxide powder, and calcining to obtain catalyst powder; mixing the catalyst powder with porous hard carbon to obtain a composite catalyst; a carbon source, the composite catalyst and reducing gas are subjected to a carbon atom recombination reaction in a reaction tube of a plasma electric arc furnace, and the single-walled carbon nanotube is obtained. According to the invention, the catalyst powder is firstly prepared and mixed with the porous hard carbon to form the composite catalyst, so that the catalyst can form a uniform catalyst core in a high-temperature area of a reaction tube, and the utilization rate of the catalyst and the quality of the single-walled carbon nanotube are improved; in addition, the porous hard carbon is easy to remove, impurities are not easy to introduce into the product, and the purity of the carbon nanotube is improved.

Description

Technical Field

[0001] The present invention relates to the technical field of carbon nanotube preparation, and particularly to a method for producing single-walled carbon nanotubes by an arc method. Background Art

[0002] Carbon nanotubes (Carbonnanotubes; CNT) have excellent mechanical and thermal properties and unique electrical properties, and thus have broad application prospects in many fields such as nanoelectronic devices, field emission technology, drug delivery in biology, and hydrogen storage technology. Carbon nanotubes can be divided into single-walled carbon nanotubes (SWNT), double-walled carbon nanotubes (DWNT), and multi-walled carbon nanotubes (MWNT). Among them, SWNT is an excellent quasi-one-dimensional nanomaterial and has become an ideal material for preparing high-performance and small-size electronic devices due to its high electrical conductivity and excellent electron transport properties.

[0003] At present, the main methods for preparing single-walled carbon nanotubes are chemical vapor deposition method, laser ablation method, and arc discharge method. Among them, the arc discharge method for preparing single-walled carbon nanotubes requires simple equipment, easily available raw materials, and low cost. At the same time, the prepared carbon nanotubes have high crystallinity, and it is a commonly used production method. However, the arc method uses transition metals and noble metals as catalysts. Although high-quality carbon nanotubes can be prepared, the cost of noble metals is high, and impurities are difficult to remove, resulting in low purity of the carbon nanotubes and difficulty in large-scale industrial production.

[0004] Therefore, there is an urgent need to provide a method for preparing single-walled carbon nanotubes with high purity. Summary of the Invention

[0005] The purpose of the present invention is to provide a method for producing single-walled carbon nanotubes by an arc method with high purity.

[0006] In order to achieve the above-mentioned invention purpose, the present invention provides the following technical solutions:

[0007] The present invention provides a method for producing single-walled carbon nanotubes by an arc method, comprising the following steps:

[0008] (1) Mixing a main catalyst, sulfur powder, and selenium oxide powder, and then calcining to obtain a catalyst powder; the main catalyst is a mixture of a transition metal and a rare earth oxide;

[0009] (2) Mixing the catalyst powder obtained in step (1) with porous hard carbon to obtain a composite catalyst;

[0010] (3) Performing a carbon atom recombination reaction on a carbon source, the composite catalyst obtained in step (2), and a reducing gas in a reaction tube of a plasma arc furnace to obtain single-walled carbon nanotubes.

[0011] Preferably, in the step (1), the mass ratio of the transition metal to the rare earth oxide in the main catalyst is 5-10:1.

[0012] Preferably, the transition metal in the main catalyst includes one or more of iron, cobalt, molybdenum, chromium, and tungsten; the rare earth oxide in the main catalyst is lanthanum oxide.

[0013] Preferably, in the step (1), the particle sizes of the main catalyst, sulfur powder, and selenium oxide powder are independently 1-20 nm.

[0014] Preferably, in the step (1), the mass ratio of the main catalyst, sulfur powder, selenium oxide powder, and porous hard carbon is (30-50):(10-20):(1-5):(10-30).

[0015] Preferably, in the step (1), the calcination temperature is 200-500 °C; the calcination time is 1-5 h.

[0016] Preferably, in the step (2), the pore diameter of the porous hard carbon is 50-100 nm; the porosity of the porous hard carbon is 10-30%.

[0017] Preferably, in the step (3), the carbon source includes one or more of ethanol, methane, ethylene, propane, propylene, and natural gas.

[0018] Preferably, in the step (3), the temperature of the carbon atom recombination reaction is 1200-2000 °C.

[0019] Preferably, in the step (3), the reactor power of the plasma arc furnace is 15-150 kW, and the arc length at the bottom of the furnace of the plasma arc furnace is 3-20 cm.

[0020] The present invention provides a method for producing single-walled carbon nanotubes by an arc method, which includes the following steps: mixing a main catalyst, sulfur powder and selenium oxide powder and then calcining to obtain a catalyst powder; the main catalyst is a mixture of a transition metal and a rare earth oxide; mixing the catalyst powder with porous hard carbon to obtain a composite catalyst; carrying out a carbon atom recombination reaction on a carbon source, the composite catalyst and a reducing gas in a reaction tube of a plasma arc furnace to obtain single-walled carbon nanotubes. The present invention uses a mixture of a transition metal and a rare earth oxide as the main catalyst. Adding a rare earth element can form a rare earth carbide during the reaction, reduce the activation energy of the reaction, and interact with the transition metal to improve the purity of single-walled carbon nanotubes; the present invention uses sulfur powder and selenium oxide powder as co-catalysts, mixing them and then calcining to make the main catalyst and the co-catalyst in the obtained catalyst powder evenly mixed; mixing the catalyst powder with porous hard carbon can make the catalyst powder evenly distributed in the pore structure and on the surface of the porous hard carbon, which can enable the catalyst to evaporate at high temperature after the composite catalyst enters the reaction tube of the plasma arc furnace, and then form uniform catalyst particles, preventing the problem that uniform catalyst nuclei cannot be formed due to different densities or melting points of different catalyst components, thereby being beneficial to improving the utilization rate of the catalyst and the quality of single-walled carbon nanotubes; moreover, sulfur powder and selenium oxide powder can form a unique microstructure on the surface of the main catalyst at high temperature, and different contact angles with the main catalyst are beneficial to the formation of the crystal structure of carbon nanotubes; furthermore, the co-catalyst has good wettability to carbon nanotubes, and a large amount of high-quality single-walled carbon nanotubes can be generated under a plasma arc furnace; in addition, porous hard carbon is easy to remove and is not likely to introduce impurities into the product, improving the purity of carbon nanotubes; in the present invention, a carbon atom recombination reaction is carried out on a carbon source, a composite catalyst and a reducing gas in a reaction tube of a plasma arc furnace to obtain single-walled carbon nanotubes. The results of the examples show that the product prepared by the present invention is single-walled carbon nanotubes, and the purity is higher than 90%. Description of the Drawings

[0021] Figure 1 TEM image of the single-walled carbon nanotubes prepared in Example 1 of the present invention;

[0022] Figure 2 TEM image of the single-walled carbon nanotubes prepared in Comparative Example 2 of the present invention;

[0023] Figure 3 Raman spectrum of the single-walled carbon nanotubes prepared in Example 1 of the present invention;

[0024] Figure 4 Raman spectrum of the single-walled carbon nanotubes prepared in Comparative Example 2 of the present invention. Detailed Embodiments

[0025] The present invention provides a method for producing single-walled carbon nanotubes by an arc method, comprising the following steps:

[0026] (1) Mixing a main catalyst, sulfur powder and selenium oxide powder, and then calcining to obtain a catalyst powder; the main catalyst is a mixture of a transition metal and a rare earth oxide;

[0027] (2) Mixing the catalyst powder obtained in step (1) with porous hard carbon to obtain a composite catalyst;

[0028] (3) Performing a carbon atom recombination reaction on a carbon source, the composite catalyst obtained in step (2), and a reducing gas in a reaction tube of a plasma arc furnace to obtain single-walled carbon nanotubes.

[0029] In the present invention, a main catalyst, sulfur powder and selenium oxide powder are mixed and then calcined to obtain a catalyst powder.

[0030] In the present invention, the main catalyst is a mixture of a transition metal and a rare earth oxide. In the present invention, the mass ratio of the transition metal to the rare earth oxide is preferably 5-10:1, more preferably 6-8:1. By using a mixture of a transition metal and a rare earth oxide as the main catalyst in the present invention, the rare earth element can form rare earth carbides during the reaction, reducing the activation energy of the reaction, and interacting with the transition metal can improve the quality of single-walled carbon nanotubes.

[0031] In the present invention, the transition metal in the main catalyst preferably includes one or more of iron, cobalt, molybdenum, chromium and tungsten, more preferably one or more of ferrocene, cobalt oxide, molybdenum sulfide, chromium sulfide and tungsten sulfide, and further preferably one or two of ferrocene, cobalt oxide, molybdenum sulfide, chromium sulfide and tungsten sulfide. As an embodiment of the present invention, the main catalyst may be a mixture of ferrocene and cobalt oxide, a mixture of ferrocene and molybdenum sulfide, or a mixture of ferrocene and tungsten sulfide; the mass ratio of the mixture of ferrocene and cobalt oxide may be 1:1; the mass ratio of ferrocene to molybdenum sulfide in the mixture of ferrocene and molybdenum sulfide may be 1:1; the mass ratio of ferrocene to tungsten sulfide in the mixture of ferrocene and tungsten sulfide may be 1:1.

[0032] In the present invention, the rare earth oxide in the main catalyst is preferably lanthanum oxide.

[0033] In the present invention, the particle size of the main catalyst is preferably 1-20 nm, more preferably 5-10 nm. Using a nanoscale main catalyst in the present invention is more conducive to dispersion in porous hard carbon.

[0034] In the present invention, the particle size of the sulfur powder is preferably 1-20 nm, more preferably 5-10 nm. Using a nanoscale sulfur powder in the present invention is more conducive to dispersion in porous hard carbon.

[0035] In the present invention, the particle size of the selenium oxide powder is preferably 1 to 20 nm, more preferably 5 to 10 nm. Using nano-sized selenium oxide powder in the present invention is more conducive to dispersion in the porous hard carbon. As an example of the present invention, the mass ratio of the selenium powder to the selenium oxide powder can be 1:1.

[0036] In the present invention, the method of mixing the main catalyst, sulfur powder, and selenium oxide powder is preferably high-energy ball milling. The rotation speed of the high-energy ball milling is preferably 1000 to 2000 r / min, more preferably 1000 to 1500 r / min; the time of the high-energy ball milling is preferably 0.5 to 2 h, more preferably 1 to 1.5 h. High-energy ball milling in the present invention is more conducive to the main catalyst, sulfur powder, and selenium oxide powder being fully and evenly mixed.

[0037] In the present invention, the calcination temperature is preferably 200 to 500 °C, more preferably 300 to 400 °C; the calcination time is preferably 1 to 5 h, more preferably 3 to 4 h. In the present invention, the calcination atmosphere is preferably argon. Calcination in the present invention can make the main catalyst, sulfur powder, and selenium oxide powder mix more evenly.

[0038] After obtaining the catalyst powder, in the present invention, the catalyst powder is mixed with the porous hard carbon to obtain a composite catalyst.

[0039] In the present invention, the pore diameter of the porous hard carbon is preferably 50 to 100 nm, more preferably 60 to 80 nm; the porosity of the porous hard carbon is preferably 10 to 30%, more preferably 15 to 20%. The present invention has no special limitation on the source of the porous hard carbon. Conventional porous hard carbon with a pore diameter and porosity within the above ranges can be used. In the present invention, the porous hard carbon is preferably carbon black.

[0040] In the present invention, the particle size of the porous hard carbon is preferably 300 to 800 nm, more preferably 500 to 600 nm.

[0041] In the present invention, the mass ratio of the main catalyst, sulfur powder, selenium oxide powder, and porous hard carbon is preferably (30 to 50):(10 to 20):(1 to 5):(10 to 30), more preferably (40 to 50):(15 to 20):(2 to 4):(15 to 20).

[0042] In the present invention, the method of mixing the catalyst powder and the porous hard carbon is preferably high-energy ball milling. The rotation speed of the high-energy ball milling is preferably 1000 to 2000 r / min, more preferably 1000 to 1500 r / min; the time of the high-energy ball milling is preferably 1 to 2 h, more preferably 1.5 to 2 h. High-energy ball milling in the present invention is more conducive to the catalyst powder being evenly distributed in the pores of the porous hard carbon.

[0043] After obtaining the composite catalyst, the present invention conducts a carbon atom recombination reaction on a carbon source, the composite catalyst, and a reducing gas in the reaction tube of a plasma arc furnace to obtain single-walled carbon nanotubes.

[0044] In the present invention, the carbon source preferably includes one or more of ethanol, methane, ethylene, propane, propylene, and natural gas, and more preferably ethanol, methane, ethylene, or propane. The present invention uses the above carbon source to provide carbon atoms.

[0045] In the present invention, the reducing gas is preferably a mixed gas of hydrogen and argon, and the volume ratio of hydrogen to argon is preferably 2-3:1, and more preferably 2-2.5:1.

[0046] In the present invention, the mass ratio of the carbon source to the composite catalyst is preferably 10:(0.1-1), and more preferably 10:(0.5-0.8).

[0047] In the present invention, the carbon source is preferably transported by a carrier gas, and the flow rate of the carbon source is preferably 3-100 L / min, and more preferably 5-50 L / min. In the present invention, the flow rate of the carbon source is preferably the same as that of the carrier gas. In the present invention, the flow rate of the reducing gas is preferably 3-20 L / min, and more preferably 5-10 L / min. The present invention can improve the product purity by controlling the flow rates of the reducing gas and the carbon source.

[0048] The present invention has no special limitation on the method of transporting the composite catalyst to the reaction tube of the plasma arc furnace. By using a conventional method, the composite catalyst can be placed in the reaction tube of the plasma arc furnace. In the present invention, the composite catalyst is preferably transported to the reaction tube of the plasma arc furnace by powder injection or at a uniform speed with a carrier gas. In the embodiments of the present invention, the carrier gas can be argon, and the flow rate of the carrier gas can be 5 L / min.

[0049] In the present invention, the temperature of the carbon atom recombination reaction is 1200-2000 °C, preferably 1500-1800 °C; the catalyst for the carbon atom recombination reaction is preferably the composite catalyst described in the above technical solution.

[0050] In the present invention, the carbon atom recombination reaction is carried out in the reaction tube of the plasma arc furnace. The present invention has no special limitation on the device model of the plasma arc furnace, and a conventional plasma arc furnace can be used.

[0051] In the present invention, the reactor power of the plasma arc furnace is preferably 50 - 100 KW, more preferably 50 - 80 KW; the arc length at the bottom of the hearth of the plasma arc furnace is preferably 3 - 20 cm, more preferably 10 - 15 cm, and further preferably 12 - 15 cm.

[0052] The present invention has no special limitation on the operation method of the carbon atom recombination reaction, and the conventional operation method of the plasma arc furnace can be adopted. In the examples of the present invention, the operation method of the carbon atom recombination reaction can be as follows: an inert gas is introduced through the electrode gun of the plasma arc furnace, the arc furnace is started, the arc length is controlled to preheat the arc furnace. After the temperature of the carbon atom recombination reaction is detected by the thermocouple at 5 cm from the bottom anode inside the hearth wall, the composite catalyst is sprayed into the high-temperature area at the bottom of the hearth of the plasma arc furnace together with the carrier gas by means of powder spraying under the carrier gas, and at the same time, the flow rate of the reducing gas and the flow rate of the carbon source are adjusted to grow carbon nanotubes. After the reaction ends, the product is collected and weighed.

[0053] The present invention preferably purifies the product obtained from the carbon atom recombination reaction to obtain single-walled carbon nanotubes.

[0054] In the present invention, the purification method is preferably: the product obtained from the carbon atom recombination reaction is subjected to heat treatment, pickling and post-treatment in sequence to obtain single-walled carbon nanotubes.

[0055] In the present invention, the temperature of the heat treatment is preferably 400 - 500 °C, more preferably 420 - 450 °C; the time of the heat treatment is preferably 1 - 2 h, more preferably 1.5 - 2 h. In the present invention, the atmosphere of the heat treatment is preferably air. Heat treatment at the above temperature in the present invention can fully remove the residual porous hard carbon in the carbon nanotubes and can also reduce the damage to the carbon nanotubes.

[0056] In the present invention, the pickling acid solution is preferably a hydrochloric acid solution, and the concentration of the hydrochloric acid solution is preferably 1 - 5 mol / L, more preferably 3 - 5 mol / L. By pickling in the present invention, the residual metal impurities in the carbon nanotubes can be removed. The present invention has no special limitation on the operation and number of times of the pickling, as long as the metal impurities therein are fully removed.

[0057] In the present invention, the post-treatment method is preferably: the carbon nanotubes obtained after pickling are uniformly dispersed in an aqueous surfactant solution to form a uniform mixed solution; the impurities in the mixed solution are separated by high-speed centrifugation, acetone is added to the supernatant to dissolve the surfactant on the surface of the single-walled carbon nanotubes and in the water, so that the single-walled carbon nanotubes flocculate; the solution is removed by membrane filtration and naturally dried at room temperature to obtain high-purity single-walled carbon nanotubes.

[0058] In the present invention, the surfactant aqueous solution is preferably an aqueous solution of sodium dodecyl sulfate, and the mass concentration of the aqueous solution of sodium dodecyl sulfate is preferably 1-10%, more preferably 5-8%.

[0059] In the present invention, the ratio of the mass of the product obtained from the carbon nanotubes after pickling to the volume of the surfactant aqueous solution is preferably 1-2 g:1 mL, more preferably 1.5-2 g:1 mL.

[0060] By adopting the above purification method, the present invention can remove impurities in single-walled carbon nanotubes, improve their purity, and do not damage the structure of single-walled carbon nanotubes.

[0061] The method provided by the present invention is simple to operate. By using a composite catalyst, the main catalyst, sulfur powder and selenium oxide powder can enter the reaction tube of the plasma arc furnace under the condition of uniform mixing, which can solve the problem that it is difficult to form uniform catalyst particles due to the different masses of different components when the conventional catalyst is introduced into the high-temperature zone; by forming a uniform catalyst core, the present invention improves the utilization rate and efficiency of the catalyst and improves the purity of carbon nanotubes.

[0062] Next, the technical solutions in the present invention will be clearly and completely described in conjunction with the embodiments in the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all the embodiments. Based on the embodiments in the present invention, all other embodiments obtained by those of ordinary skill in the art without making creative efforts shall fall within the protection scope of the present invention.

[0063] In the embodiments of the present invention, the particle sizes of the transition metal, rare earth oxide, sulfur powder and selenium oxide powder used are all 5-10 nm;

[0064] In the embodiments of the present invention, the pore diameter of the carbon black used is 60-80 nm, the particle size is 500-600 nm, and the porosity of the porous carbon black is 10-15%.

[0065] Example 1

[0066] A method for producing single-walled carbon nanotubes by arc method, the steps are as follows:

[0067] (1) 20 g of ferrocene, 5 g of lanthanum oxide, 20 g of cobalt oxide, 10 g of sulfur powder and 3 g of selenium oxide powder are subjected to high-energy ball milling at a rotation speed of 1500 r / min for 0.5 h, and then calcined at 400 °C for 3 h in an argon atmosphere to obtain a catalyst powder;

[0068] (2) The catalyst powder obtained in step (1) and 15 g of carbon black are subjected to high-energy ball milling at a rotation speed of 1500 r / min for 1.5 h to obtain a composite catalyst;

[0069] (3) Use a plasma arc furnace with a power of 50 KW. Pass argon (60 L / min) through the electrode gun of the plasma arc furnace, start the arc furnace, control the arc length to be 12 cm, and preheat the arc furnace. After the thermocouple detects a temperature of 1500 °C at a position 5 cm from the bottom anode inside the furnace wall, spray the composite catalyst into the high-temperature area at the bottom of the furnace together with argon by means of powder injection under argon with a flow rate of 5 L / min. At the same time, adjust the flow rate of the reducing gas (the volume ratio of hydrogen to argon is 2:1) to be 10 L / min and the flow rate of the carbon source methane to be 5 L / min (the mass ratio of the carbon source to the composite catalyst is preferably 10:0.5), and carry out the growth of carbon nanotubes. After the reaction ends, collect the products; then heat-treat the products in air at 400 °C for 1.8 h, then wash them 3 times in a hydrochloric acid solution with a concentration of 3 mol / L, and then uniformly disperse the carbon nanotubes obtained after pickling in an aqueous solution of sodium dodecyl sulfate with a mass concentration of 8% to form a uniform mixed solution; separate the impurities in the mixed solution by high-speed centrifugation, add acetone to the supernatant to dissolve the surfactant on the surface of the single-walled carbon nanotubes and in water, so that the single-walled carbon nanotubes flocculate; remove the solution by filtration through a filter membrane, and obtain high-purity single-walled carbon nanotubes after natural drying at room temperature. After testing, its purity is 94%.

[0070] Example 2

[0071] A method for producing single-walled carbon nanotubes by an arc method, the steps are as follows:

[0072] (1) High-energy ball-mill 25 g of ferrocene, 5 g of lanthanum oxide, 25 g of molybdenum sulfide, 15 g of sulfur powder and 4 g of selenium oxide powder at a rotation speed of 1500 r / min for 0.5 h, and then calcine at 400 °C in an argon atmosphere for 3 h to obtain a catalyst powder;

[0073] (2) High-energy ball-mill the catalyst powder obtained in step (1) and 17 g of carbon black at a rotation speed of 1500 r / min for 1.5 h to obtain a composite catalyst;

[0074] (3) Use a plasma arc furnace with a power of 50 KW. Pass argon (60 L / min) through the electrode gun of the plasma arc furnace, start the arc furnace, control the arc length to be 12 cm, and preheat the arc furnace. After the thermocouple detects a temperature of 1500 °C at a position 5 cm from the bottom anode inside the furnace wall, spray the composite catalyst into the high-temperature area at the bottom of the furnace together with argon at a flow rate of 5 L / min by means of powder injection. At the same time, adjust the flow rate of the reducing gas (the volume ratio of hydrogen to argon is 2.2:1) to be 10 L / min, and the flow rate of the carbon source methane to be 5 L / min. (The mass ratio of the carbon source to the composite catalyst is preferably 10:0.6), and carry out the growth of carbon nanotubes. After the reaction ends, collect the product; then heat-treat the product in air at 400 °C for 1.8 h, then wash it 3 times in a hydrochloric acid solution with a concentration of 3 mol / L, and then uniformly disperse the carbon nanotubes obtained after pickling in an aqueous solution of sodium dodecyl sulfate with a mass concentration of 8% to form a uniform mixed solution; separate the impurity particles in the mixed solution by high-speed centrifugation, add acetone to its supernatant to dissolve the surfactant on the surface of the single-walled carbon nanotubes and in water, so that the single-walled carbon nanotubes flocculate; remove the solution by filtration through a filter membrane, and obtain high-purity single-walled carbon nanotubes after natural drying at room temperature. After testing, its purity is 92%.

[0075] Example 3

[0076] A method for producing single-walled carbon nanotubes by an arc method, the steps are as follows:

[0077] (1) High-energy ball mill 25 g of ferrocene, 5 g of lanthanum oxide, 25 g of tungsten sulfide, 15 g of sulfur powder and 4 g of selenium oxide at a rotation speed of 1500 r / min for 0.5 h, and then calcine at 400 °C for 3 h in an argon atmosphere to obtain a catalyst powder;

[0078] (2) High-energy ball mill the catalyst powder obtained in the step (1) and 17 g of carbon black at a rotation speed of 1500 r / min for 1.5 h to obtain a composite catalyst;

[0079] (3) Use a plasma arc furnace with a power of 50 KW. Pass argon (60 L / min) through the electrode gun of the plasma arc furnace, start the arc furnace, control the arc length to be 12 cm, and preheat the arc furnace. After the thermocouple detects a temperature of 1500 °C at a position 5 cm from the bottom anode inside the furnace wall, spray the composite catalyst into the high-temperature area at the bottom of the furnace together with argon by means of powder injection under argon with a flow rate of 5 L / min. At the same time, adjust the flow rate of the reducing gas (the volume ratio of hydrogen to argon is 2.3:1) to be 10 L / min, and the flow rate of the carbon source methane to be 5 L / min. (The mass ratio of the carbon source to the composite catalyst is preferably 10:0.6), and carry out the growth of carbon nanotubes. After the reaction is completed, collect the product; then heat-treat the product in air at 400 °C for 1.8 h, then wash it 3 times in a hydrochloric acid solution with a concentration of 3 mol / L, and then uniformly disperse the carbon nanotubes obtained after pickling in an aqueous solution of sodium dodecyl sulfate with a mass concentration of 8% to form a uniform mixed solution; separate the impurity particles in the mixed solution by high-speed centrifugation, add acetone to its supernatant to dissolve the surfactant on the surface of the single-walled carbon nanotubes and in water, so that the single-walled carbon nanotubes flocculate; remove the solution by filtration through a filter membrane, and obtain high-purity single-walled carbon nanotubes after natural drying at room temperature. After testing, its purity is 93%.

[0080] Example 4

[0081] A method for producing single-walled carbon nanotubes by an arc method, the steps are as follows:

[0082] (1) High-energy ball mill 25 g of ferrocene, 5 g of lanthanum oxide, 25 g of tungsten sulfide, 15 g of sulfur powder and 4 g of selenium oxide at a rotation speed of 1500 r / min for 0.5 h, and then calcine at 400 °C for 3 h in an argon atmosphere to obtain a catalyst powder;

[0083] (2) High-energy ball mill the catalyst powder obtained in step (1) and 17 g of carbon black at a rotation speed of 1500 r / min for 1.5 h to obtain a composite catalyst;

[0084] (3) Use a plasma arc furnace with a power of 50 KW. Pass argon (60 L / min) through the electrode gun of the plasma arc furnace, start the arc furnace, control the arc length to be 15 cm, and preheat the arc furnace. After the thermocouple detects a temperature of 1700 °C at a position 5 cm from the bottom anode inside the furnace wall, spray the composite catalyst into the high-temperature area at the bottom of the furnace together with argon by means of powder injection under argon with a flow rate of 5 L / min. At the same time, adjust the flow rate of the reducing gas (the volume ratio of hydrogen to argon is 2.5:1) to be 10 L / min, and the flow rate of the carbon source methane to be 5 L / min. (The mass ratio of the carbon source to the composite catalyst is preferably 10:0.5), and carry out the growth of carbon nanotubes. After the reaction ends, collect the products; then heat-treat the products in air at 400 °C for 2 h, then wash them 3 times in a hydrochloric acid solution with a concentration of 3 mol / L, and then uniformly disperse the carbon nanotubes obtained after pickling in an aqueous solution of sodium dodecyl sulfate with a mass concentration of 8% to form a uniform mixed solution; separate the impurity particles in the mixed solution by high-speed centrifugation, add acetone to its supernatant to dissolve the surfactant on the surface of the single-walled carbon nanotubes and in water, so that the single-walled carbon nanotubes flocculate; remove the solution by filtration through a filter membrane, and obtain high-purity single-walled carbon nanotubes after natural drying at room temperature. After testing, its purity is 95%.

[0085] Example 5

[0086] A method for producing single-walled carbon nanotubes by an arc method, the steps are as follows:

[0087] (1) High-energy ball-mill 30 g of ferrocene, 5 g of lanthanum oxide, 10 g of sulfur powder and 3 g of selenium oxide powder at a rotation speed of 1500 r / min for 0.5 h, and then calcine at 400 °C for 3 h in an argon atmosphere to obtain catalyst powder;

[0088] (2) High-energy ball-mill the catalyst powder obtained in step (1) and 15 g of carbon black at a rotation speed of 1500 r / min for 1.5 h to obtain a composite catalyst;

[0089] (3) Use a plasma arc furnace with a power of 50 KW. Introduce argon (60 L / min) through the electrode gun of the plasma arc furnace, start the arc furnace, control the arc length to 12 cm, and preheat the arc furnace. After the thermocouple detects a temperature of 1500 °C at a position 5 cm from the bottom anode inside the furnace wall, spray the composite catalyst into the high-temperature area at the bottom of the furnace together with argon by means of powder injection under argon with a flow rate of 5 L / min. At the same time, adjust the flow rate of the reducing gas (the volume ratio of hydrogen to argon is 2:1) to 10 L / min and the flow rate of the carbon source methane to 5 L / min. (The mass ratio of the carbon source to the composite catalyst is preferably 10:0.5), and carry out the growth of carbon nanotubes. After the reaction ends, collect the product; then heat-treat the product in air at 400 °C for 1.8 h, then wash it 3 times in a hydrochloric acid solution with a concentration of 3 mol / L, and then uniformly disperse the carbon nanotubes obtained after pickling in an aqueous solution of sodium dodecyl sulfate with a mass concentration of 8% to form a uniform mixed solution; separate the impurity particles in the mixed solution by high-speed centrifugation, add acetone to the supernatant to dissolve the surfactant on the surface of the single-walled carbon nanotubes and in water, so that the single-walled carbon nanotubes flocculate; remove the solution by filtration through a filter membrane, and obtain high-purity single-walled carbon nanotubes after natural drying at room temperature. After testing, its purity is 91%.

[0090] Comparative Example 1

[0091] A method for producing single-walled carbon nanotubes by an arc method, which is different from Example 1 in that lanthanum oxide is not added. After testing, the purity of the obtained single-walled carbon nanotubes is 81%.

[0092] It can be seen from Comparative Example 1 that when lanthanum oxide is not added, the quality of single-walled carbon nanotubes will be less. This is because lanthanum oxide can reduce the activation energy of the reaction and interact with transition metals to improve the purity of single-walled carbon nanotubes.

[0093] Comparative Example 2

[0094] A method for producing single-walled carbon nanotubes by an arc method, which is different from Example 1 in that selenium oxide powder is not added. After testing, the purity of the obtained single-walled carbon nanotubes is 73%.

[0095] It can be seen from Comparative Example 2 that when selenium oxide powder is not added, the purity of single-walled carbon nanotubes decreases.

[0096] Comparative Example 3

[0097] A method for producing single-walled carbon nanotubes by an arc method, which is different from Example 1 in that carbon black is not added. After testing, the purity of the obtained single-walled carbon nanotubes is 65%.

[0098] As can be seen from Comparative Example 3, when carbon black is not added, the purity of single-walled carbon nanotubes decreases. This is because when carbon black is absent, there is a problem that the components of the catalyst powder cannot form a uniform catalyst core due to different densities or melting points, which leads to a low utilization rate of the catalyst and a decrease in purity.

[0099] Test Example 1

[0100] The TEM image of the single-walled carbon nanotubes prepared in Example 1 is as Figure 1 shown. From Figure 1 it can be seen that the diameter of the single-walled carbon nanotube bundles prepared in Example 1 is about 10 - 20 nm, the diameter of a single tube is about 2 nm, and it has high crystallinity, excellent uniformity and continuity.

[0101] The TEM image of the single-walled carbon nanotubes prepared in Comparative Example 2 is as Figure 2 shown. From Figure 2 it can be seen that the diameter of the single-walled carbon nanotube bundles prepared in Comparative Example 2 is relatively thick, about 30 nm, and it has obvious tumor-like structures. These tumor-like structures are impurities such as carbon, fine carbon nanotubes or graphene that are difficult to remove during the preparation of single-walled carbon nanotubes, and these impurities result in poor quality of the single-walled carbon nanotubes. This is because selenium is not added to the catalyst used in Comparative Example 2, which is not conducive to the orderly growth of single-walled carbon nanotubes.

[0102] The Raman spectrum of the single-walled carbon nanotubes prepared in Example 1 is as Figure 3 shown. The Raman spectrum of the single-walled carbon nanotubes prepared in Comparative Example 2 is as Figure 4 shown. From Figure 3 and Figure 4 it can be seen that the I G / I D value of the single-walled carbon nanotubes prepared in Example 1 is about 66.5, while the I G / I D value of the single-walled carbon nanotubes prepared in Comparative Example 2 is about 16.5. From Figure 3 and Figure 4 it can be seen that the single-walled carbon nanotubes prepared by the present invention have high crystallinity, high graphitization degree and high quality.

[0103] As can be seen from the above results, the product obtained by the method provided by the present invention is single-walled carbon nanotubes, with a purity higher than 90%, and a large amount of single-walled carbon nanotubes can be obtained. This is because the present invention uses a mixture of transition metals and rare earth oxides as the main catalyst, and adds rare earth elements to lower the activation energy of the reaction. The interaction with transition metals can improve the purity and quality of single-walled carbon nanotubes. The present invention mixes the catalyst powder with porous hard carbon, which can make the catalyst powder evenly distributed in the pore structure and surface of the porous hard carbon, improving the utilization rate of the catalyst and the quality of single-walled carbon nanotubes. Moreover, sulfur powder and selenium oxide powder can form a unique microstructure on the surface of the main catalyst at high temperature, and different contact angles with the main catalyst are beneficial to the formation of the crystal structure of carbon nanotubes. In addition, the promoter has good wettability to carbon nanotubes, and a large amount of high-quality single-walled carbon nanotubes can be generated under a plasma arc furnace. Furthermore, the porous hard carbon is easy to remove and is not likely to introduce impurities into the product, improving the purity of carbon nanotubes. In the present invention, the carbon source, composite catalyst, and reducing gas undergo a carbon atom recombination reaction in the reaction tube of the plasma arc furnace to obtain single-walled carbon nanotubes with a high degree of graphitization.

[0104] The above are only the preferred embodiments of the present invention. It should be noted that for those of ordinary skill in the art, without departing from the principle of the present invention, several improvements and refinements can be made, and these improvements and refinements should also be regarded as the protection scope of the present invention.

Claims

1. A method for producing single-walled carbon nanotubes by an arc process, comprising the following steps: (1) mixing a main catalyst, sulfur powder and selenium oxide powder and calcining the mixture to obtain catalyst powder; the main catalyst is a mixture of transition metal and rare earth oxide; (2) mixing the catalyst powder obtained in step (1) with porous hard carbon to obtain a composite catalyst; (3) subjecting the carbon source, the composite catalyst obtained in step (2) and a reducing gas to a carbon atom recombination reaction in a reaction tube of a plasma arc furnace to obtain single-walled carbon nanotubes.

2. The preparation method according to claim 1, characterized in that: In the step (1), the mass ratio of transition metal to rare earth oxide in the main catalyst is 5 to 10:

1.

3. The preparation method according to claim 1 or 2, characterized in that: The transition metal in the main catalyst includes one or more of iron, cobalt, molybdenum, chromium and tungsten; the rare earth oxide in the main catalyst is lanthanum oxide.

4. The preparation method according to claim 1, characterized in that: In the step (1), the particle sizes of the main catalyst, sulfur powder and selenium oxide powder are independently 1 to 20 nm.

5. The preparation method according to claim 1, characterized in that: In the step (1), the mass ratio of the main catalyst, sulfur powder, selenium oxide powder and porous hard carbon is (30-50): (10-20): (1-5): (10-30).

6. The preparation method according to claim 1, characterized in that: The calcination temperature in the step (1) is 200 to 500° C. and the calcination time is 1 to 5 hours.

7. The preparation method according to claim 1, characterized in that: The pore size of the porous hard carbon in the step (2) is 50-100 nm; the porosity of the porous hard carbon is 10-30%.

8. The preparation method according to claim 1, characterized in that: The carbon source in step (3) includes one or more of ethanol, methane, ethylene, propane, propylene and natural gas.

9. The preparation method according to claim 1, characterized in that: The temperature of the carbon atom recombination reaction in step (3) is 1200-2000°C.

10. The preparation method according to claim 1, characterized in that: In the step (3), the reactor power of the plasma arc furnace is 15-150 kW, and the arc length at the bottom of the furnace of the plasma arc furnace is 3-20 cm.

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