SUPPORTED CATALYST FOR SYNTHESIZING CARBON NANOTUBES, METHOD FOR PREPARING THEREOF AND CARBON NANOTUBE USING THE SAME

[Technical Field]

<i> The present invention relates to supported catalysts for synthesizing carbon nanotubes, a method of preparing the same, and carbon nanotubes manufactured using the same. More particularly, the present invention relates to supported catalysts for synthesizing carbon nanotubes, which can synthesize a large amount of carbon nanotubes with high purities by increasing surface areas of metal catalysts using a spray-drying method, a method of preparing the same, and carbon nanotubes manufactured using the same.

<2>

[Background Art]

<3> Carbon nanotubes are graphite layers rolled into cylindrical forms, and they are largely applied to devices including electron emitting devices, electronic devices, sensors, and the like due to their excellent electrical properties. Additionally, the carbon nanotubes are diversely used in high strength composite materials and the like due to their excellent physical properties. The carbon nanotubes are classified into single-walled carbon nanotubes, double-walled carbon nanotubes and mult i-walled carbon nanotubes according to the number of rolled surfaces of the cylinder form, and have different properties according to the number of such walls.

<4> Recently, research and development of complexes using carbon nanotubes has been actively advanced. Particularly, engineering plastic complexes with electrical conductivities provided thereto have been applied to electric and electronic articles so that the engineering plastic complexes can be used as high-value added materials including electro-magnetic shielding materials and antistatic materials. <5> Since such carbon nanotubes are generally expensive, it is required to inexpensively synthesize the carbon nanotubes in large quantities in order to usefully apply them to various fields. However, in order to obtain desired electrical conductivities from small contents of carbon nanotubes, the electrical conductivities are greatly influenced by properties of carbon nanotubes themselves to be used as well as properties and processing conditions of resins. Therefore, high purity carbon nanotubes are required to be synthesized at a high productivity, and it is very important to develop catalyst for synthesizing such carbon nanotubes at the high productivity.

<6> In general, methods for synthesizing carbon nanotubes include electric discharge method, laser vaporization, high pressure chemical vapor deposition, atmospheric pressure thermal chemical vapor deposition, and the like. Although the electric discharge method and the laser vaporization among the methods have advantages in that they are easily applied due to their simple principles, these methods have disadvantages in that the carbon nanotubes comprise large amounts of impurities during the synthesis of carbon nanotubes and such methods are not suitable for mass production. In contrast, the thermal chemical vapor deposition is known to be the most suitable method for synthesizing a large amount of high purity carbon nanotubes at low cost.

<7> When synthesizing the carbon nanotubes through the thermal chemical vapor deposition, a catalyst plays a very important role since the growth of the carbon nanotubes varies according to, for example, types and composition ratios of transition metals and sizes of metal particles. Fe, Co, Ni and the like are used as the transition metals, and the transition metals are supported onto a supporting body to synthesize the carbon nanotubes. Methods of synthesizing the carbon nanotubes include a co-precipitation method for supporting pH-adjusted solution onto a supporting body by uniformly dissolving catalytic material into an aqueous solution and then adjusting pH of the dissolved solution, and an impregnation method for synthesizing carbon nanotubes by uniformly dissolving catalytic material into an aqueous solution, drying the dissolved solution through a drying process, polishing the dried material for uniformly supporting a metal catalyst onto the polished material, and then sintering the polished material at a high temperature of about 700 ^° C to about 900 ^° C for a long time of about 6 to about 10 hours. However, such methods are not suitable for mass production due to their long synthesis times and low yields.

<8> In order to improve catalytic efficiency, it is important to increase the surface area of the catalyst. Conventionally, mechanical and physical methods including grinding and ball milling have been used to increase the surface area of a catalyst. However, there is much difficulty in practically applying methods of increasing the surface area of the catalyst using grinding and ball milling, since additional equipment or processes are further required, costs and time are additionally consumed, and preparation of a very small catalyst is not technically easy.

<9> Further, when synthesizing high purity carbon nanotubes to improve physical and electrical properties of carbon nanotubes, there are problems in that a purification process such as acid treatment or heat treatment is additionally required, and such a post-treatment process causes an increase in production cost and surface defects of the carbon nanotubes to thereby deteriorate inherent physical properties of the carbon nanotubes. Accordingly, a technique for synthesizing high purity carbon nanotubes in a large amount without a purification process is urgently needed.

<io> Therefore, in order to solve the aforementioned problems, the present inventors have developed a supported catalyst for synthesizing carbon nanotubes, which makes it possible to obtain high purity carbon nanotubes by forming spherical catalytic particles in a particle size range from some microns to some tens of microns through a spray-drying method and then splitting the spherical particles by high temperature sintering, thereby substantially increasing surface areas of the spherical particles. Also, there has been developed a method of preparing a supported catalyst for synthesizing carbon nanotubes, which makes it possible to reduce time and cost and is effective in mass production since a post-treatment process or an additional purification process is not necessary.

<11>

[Disclosure] [Technical Problem] <i2> An object of the present invention is to provide a supported catalyst with a remarkably increased surface area for synthesizing carbon nanotubes. <i3> Another object of the present invention is to provide a supported catalyst capable of mass-producing carbon nanotubes. <14> Another object of the present invention is to provide a supported catalyst that makes it possible to obtain high purity carbon nanotubes. <15> Another object of the present invention is to provide a supported catalyst that can be applied to a fixed bed reactor and a fluidized bed reactor when manufacturing carbon nanotubes. <16> Another object of the present invention is to provide a supported catalyst that enables carbon nanotubes to grow in both directions of front and rear sides of the supported catalyst. <17> Another object of the present invention is to provide a supported catalyst that is particularly suitable for a fixed bed reactor. <i8> Another object of the present invention is to provide a novel preparation method of a supported catalyst that makes it possible to reduce time and cost and is effective in mass production since a post-treatment process, such as a grinding or ball-milling process, or an additional purification process is not necessary. <19> Another object of the present invention is to provide a preparation method of a supported catalyst, which increases surface areas of spherical particles using a phenomenon that the spherical particles are split when sprayed and dried spherical catalytic particles are sintered at high temperatures. <20> Another object of the present invention is to provide carbon nanotubes having highly efficient productivity and selectivity and high purity using the supported catalysts. <2i> Hereinafter, the aforementioned and other objects of the present invention can be all accomplished by the present invention described in detail .

<22>

[Technical Solution]

<23> An aspect of the present invention relates to a supported catalyst for synthesizing carbon nanotubes. The supported catalyst is characterized in that a metal catalyst comprising one or more selected from the group consisting of Co, Ni, and Fe is supported onto an alumina, magnesium oxide or silica supporting body, and the supported catalyst has a surface area of

2 about 15 to about 100 m/g. According to an exemplary embodiment, the surface

2 area may be about 50 to about 100 m /g. <24> In one exemplary embodiment, the metal catalyst may be supported on both faces including front and back faces of the supported catalyst.

Therefore, carbon nanotubes can grow on both sides including front and back sides of the supported catalyst. <25> In one exemplary embodiment of the present invention, the supported catalyst may have the following molar ratio: <26> (Co, Ni)Fe : Mo : (Mg, Si)Al = x : y : z <27> where 1 < x < 10, 0 < y < 5, and 2 < z < 70. <28> In one exemplary embodiment, the supported catalyst may have the following molar ratio: <29> Fe : Mo : Al = x : y : z

<30> where 1 < x < 10, 0 < y < 5, and 2 < z < 70. <3i> According to another aspect of the present invention, a method of preparing a supported catalyst for synthesizing carbon nanotubes is provided.

The method comprises the steps of spray-drying an aqueous catalytic solution having a metal catalyst and a supporting body mixed, thereby preparing spherical catalytic particles; and crushing the spherical catalytic particles by sintering. <32> In one exemplary embodiment, the metal catalyst may comprise one or more selected from the group consisting of Fe(NO ₃ )S, Ni(N0 ₃ ) ₂ , Co(N0 ₃ ) ₂ ,

Fe(OAc) ₂ , Ni(OAc) ₂ , and Co(OAc) ₂ .

<33> The supporting body may comprise one or more selected from the group consisting of aluminum nitrate, magnesium nitrate, and silica.

<34> Preferably, the metal catalyst and the supporting body may be formed in the phase of an aqueous solution.

<35> The spray-drying may be performed at about 200 to about 350 ^° C . In another exemplary embodiment, the spray-drying may be performed at about 250 to about 300 ^° C . Also, the spray-drying may be performed at a disc rotating speed of about 5,000 to about 20,000 rpm and a solution injection rate of about 10 to about 100 ml/min.

<36> The sintering may be carried out at about 350 to about 1,100 "C .

<37> A supported catalyst prepared by the method has an irregular shape in which hollow spherical particles are crushed.

<38> According to further aspects of the present invention, a supported catalyst for synthesizing carbon nanotubes is provided, and carbon nanotubes prepared using such a supported catalyst are provided. The carbon nanotubes grow in directions of both sides including front and back sides of the supported catalyst.

<39> The carbon nanotubes may be prepared in a fixed bed reactor or a fluidized bed reactor. Preferably, the carbon nanotubes may be prepared in the fixed bed reactor. In an exemplary embodiment, the carbon nanotube may be prepared by injecting hydrocarbon gases at a temperature of about 600 to about 1,100 ^° C in the presence of the supported catalyst.

<40>

[Description of Drawings] <4i> Fig. 1 is a schematic view of a supported catalyst for synthesizing carbon nanotubes according to the present invention. <42> Fig. 2 is a schematic view showing a shape in which carbon nanotubes are grown in one direction of a supported catalyst. <43> Fig. 3 is a schematic view showing a shape in which the carbon nanotubes are grown in both direction of the supported catalyst according to the present invention. <44> Fig. 4(a) is a Scanning Electron Microscopic (SEM) image of particles spray-dried in Example 1, and Fig. 4(b) is an SEM image of a supported catalyst prepared according to Example 1. <45> Fig. 5 is an SEM image showing a shape of carbon nanotubes prepared according to Example 1. <46> Fig. 6(a) is an SEM image of a supported catalyst prepared according to

Comparative Example 1, and Fig. 6(b) is an SEM image of a supported catalyst prepared according to Comparative Example 2. <47> Fig. 7 is a graph showing a relationship between surface areas of catalytic particles and productivities of carbon nanotubes.

<48>

[Best Mode]

<49> Supported catalyst

<50> The present invention provides a supported catalyst for synthesizing carbon nanotubes. Fig. 1 is a schematic view of a supported catalyst for synthesizing carbon nanotubes according to the present invention. The supported catalyst has a metal catalyst (2) supported onto a supporting body(l) and has an irregular shape in which hollow spherical particles are crushed. For example, examples of the shape of the supported catalyst include, but are not limited to, semicircular, sectorial, fragmental, planar, and crescent shapes. As illustrated in Fig. 1, pores may be formed on the surface of the supporting body(l). Further, the surface of a supported catalyst of the present invention may be curved or have protrusions formed thereon. Also, the metal catalyst(2) is distributed on both sides including front and back sides of the supported catalyst. Here, the front and back sides, as one face with the metal catalyst present and another face opposite to the one face, correspond to an outer surface and an inner surface (or an inner surface and an outer surface) of hollow spherical particles before they crushed. Since metal particles are present on the front and back sides of the supported catalyst of the present invention, carbon nanotubes may be grown on both sides including the front and back sides of the supported catalyst. Therefore, carbon nanotubes with excellent purity and productivity can be obtained when synthesizing the carbon nanotubes afterward. <5i> A surface area of the supported catalyst measured using BET

2

(Brunauer-Emmett-Teller) is about 15 to about 100 m /g, preferably about 40 to

2 about 100 m /g. In an exemplary embodiment, the surface area of the supported

2 catalyst is about 50 to about 100 m/g, in another exemplary embodiment about

2

60 to about 100 m /g, in another exemplary embodiment about 70 to about 100

2 2 m/g, in another exemplary embodiment about 80 to about 100 m/g. Most

2 preferably, the surface area is about 90 to about 100 m/g.

<52> The supported catalyst of the present invention has a wide particle-size distribution since the supported catalyst is naturally crushed by sintering (baking). In an exemplary embodiment, the supported catalyst may have a longest length of about 0.01 to about 200 μm, in another exemplary embodiment about 0.1 to about 100 μm.

<53> Co, Ni, Fe, or alloys or combinations thereof may be used as the metal catalyst. Alumina, magnesium oxide, silica, or combinations thereof may be used as the support.

<54> In one exemplary embodiment of the present invention, the supported catalyst may have the following molar ratio:

<55> (Co, Ni)Fe : Mo : (Mg, Si)Al = x : y : z

<56> where 1 < x < 10, 0 < y < 5, and 2 < z < 70.

<57> In a preferred exemplary embodiment, the supported catalyst may have the following molar ratio:

<58> Fe : Mo : Al = x : y : z

<59> where 1 < x < 10 , 0 < y < 5 , and 2 < z < 70. <60>

<6i> Preparation method of supported catalyst

<62> Another aspect of the present invention relates to a method of preparing the supported catalyst. The method comprises the steps of spray-drying an aqueous catalytic solution, in which a metal catalyst and a supporting body are mixed, to prepare spherical catalytic particles, and crushing the spherical catalytic particles by sintering.

<63> In an exemplary embodiment, the metal catalyst may be Fe(N0s)3,

Ni(N0 ₃ ) ₂ , Co(NOa) ₂ , Fe(OAc) ₂ , Ni(OAc) ₂ , Co(OAc) ₂ , or mixtures of two or more thereof. In an exemplary embodiment, the metal catalyst may be in the form of hydrates. For example, the metal catalyst may be used in the form of iron(III) nitrate nonahydrate, nickel(II) nitrate hexahydrate, and cobalt nitrate hexahydrate.

<64> Examples of the supporting body may include, but are not limited to, aluminum nitrate, magnesium nitrate, and the like, or mixtures of two or more thereof. Preferably, aluminum nitrate nonahydrate may be used as the supporting body.

<65> The metal catalyst and supporting body are dissolved into water and mixed into an aqueous solution phase.

<66> In another exemplary embodiment of the present invention, a molybdenum (Mo) based activator such as ammonium molybdate tetrahydrate is injected into water to prevent agglomeration of nano-sized metal catalysts during a sintering process at high temperatures. In another exemplary embodiment, an activator such as citric acid may also be used.

<67> The metal catalyst and supporting body are mixed, and optionally the molybdenum (Mo) based activator, are stirred and completely dissociated in the aqueous catalytic solution.

<68> The aqueous catalytic solution in which the metal catalyst and supporting body are mixed is prepared in the form of spherical particles by a spray-drying method afterward. Among methods of synthesizing a metal supporting body with uniform spherical shape and size, a method capable of producing the largest amount of the metal supporting body in the easiest manner is a spray-drying method. The spray-drying method allows the supplied material to be dried almost instantaneously by spraying a supplied material in a fluid state into dry gas. The supplied material is dried very fast since the supplied material is atomized by an atomizer to result in a considerable increase of the surface area of the supplied material. Spray-drying equipment has an effect on the size of a catalytic powder formed according to density and spray amount of a solution and a rotating speed of an atomizer disc. In an exemplary embodiment, the spray-drying method may be performed at about 200 to about 350 °C , preferably about 250 to about 300 ^° C . The spray-drying method includes a method using a nozzle and a method of spraying drops of water after forming drops of water according to the rotation of the disc. In a preferred exemplary embodiment, a disc type spray-drying method is applied to prepare a supported catalytic powder with a more uniform size. The disc type spray-drying method includes a vane or pin type spray-drying method. Particle size and distribution may be controlled according to a rotating speed of the disc and injection quantity and density of the solution. In an exemplary embodiment of the present invention, the spray-drying method may be carried out at a disc rotating speed of about 5,000 to about 20,000 rpm and a solution injection rate of about 10 to about 100 ml/min. In another exemplary embodiment, the disc rotating speed may be about 10,000 to about 18,000 rpm, about 12,000 to about 19,000 rpm, or about 5,000 to about 9,000 rpm. Further, the spray-drying method may be performed at a solution injection rate of about 15 to about 60 ml/min, about 50 to about 75 ml/min, or about 80 to about 100 ml/min.

<69> A catalytic powder prepared through a spray-drying method is heat-treated through a sintering process. The catalytic powder is crystallized into a supported catalyst, and disintegration of the catalytic powder into spherical particles occurs through such a sintering process. Therefore, the catalytic powder is split into random particles with small sizes to increase the surface area of the catalytic powder. Diameters and properties of carbon nanotubes vary according to sintering temperature and time of the catalytic powder. In an exemplary embodiment, the sintering process may be performed at about 350 to about 1,100 ^° C , preferably at about 450 to about 900 ^° C , and more preferably at about 500 to about 800 ^° C . Also, the sintering process may be performed at about 350 to about 500 ^" C , at about 550 to about 700 V, at about 650 to about 900 1C, or at about 750 to about 1,100 TC . The sintering process may be carried out for about 15 minutes to about 3 hours, preferably for about 30 minutes to 1 hour. When the catalytic powder is subjected to such a sintering process, hollow spherical particles prepared by spray-drying are broken. Therefore, a supported catalyst prepared by the aforementioned method has an irregular shape in which the hollow spherical particles are crushed. Further, particles after the spray-drying process and before the sintering process are hollow spherical particles having a metal catalyst being present on outer and inner surfaces thereof. The hollow spherical particles are crushed by the sintering process to thereby prepare a supported catalyst having the metal catalyst(2) distributed both the front and rear sides thereof.

<70>

<7i> Carbon nanotubes

<72> According to a further aspect of the present invention, carbon nanotubes prepared using the aforementioned supported catalyst are provided. The supported catalyst of the present invention may be applied to a fixed bed reactor or a fluidized bed reactor, preferably the fixed bed reactor.

<73> In an exemplary embodiment, the carbon nanotubes may be prepared by injecting a hydrocarbon gas in the presence of the supported catalyst at a temperature of about 600 to about 1,100 ^° C , preferably at about 650 to about 950 ^° C . In one exemplary embodiment, carbon nanotubes may be prepared at about 650 to about 800 ^° C . In another exemplary embodiment, carbon nanotubes may be prepared at about 800 to about 990 ^° C . In another exemplary embodiment, carbon nanotubes may be prepared at about 980 to about 1,100 ^° C . Examples of the hydrocarbon gas include, but are not limited to, methane, ethylene, acetylene, LPG (Liquefied Petroleum Gas), or mixed gases thereof. The hydrocarbon gas is supplied for about 15 minutes to about 2 hours, preferably for about 30 to about 60 minutes.

<74> Carbon nanotubes prepared using a supported catalyst of the present invention are grown in both directions including front and rear sides of the supported catalyst. Fig. 2 is a schematic view showing a shape in which carbon nanotubes(3) are grown in one direction of a supported catalyst. Since a metal catalyst(2) is generally present only on one side of a supported catalyst prepared by a conventional combustion method, the carbon nanotubes are grown only in one direction when preparing carbon nanotubes using the supported catalyst. However, according to a supported catalyst according to the present invention, since a metal catalyst is present on both sides including front and rear sides of the supported catalyst, the carbon nanotubes are capable of being grown in both directions when preparing carbon nanotubes using such a supported catalyst. Fig. 3 is a schematic view showing a shape in which the carbon nanotubes(3) are grown in both direction of the supported catalyst according to the present invention. As illustrated in Fig. 3, it can be seen that the carbon nanotubes(3) are grown in both directions since the metal catalyst(2) is present on both sides including front and rear sides of the supported catalyst. The productivity of carbon nanotubes prepared using a supported catalyst of the present invention [(weight of synthesized carbon nanotubes - catalyst weight )/catalyst weight x 100] is about 5,000 % or more, preferably about 7,000 % or more, more preferably about 9,000 % or more. In an exemplary embodiment, the carbon nanotubes according to the present invention have a productivity of about 9,010 to about 15,000 %.

<75> The present invention will be well understood by the following examples. The following examples of the present invention are only for illustrative purposes and are not to be construed as limiting the scope of the present invention defined by the appended claims. <76>

[Mode for Invention] <77> Example 1

<78> Spherical catalytic particles were prepared by injecting an aqueous catalytic solution comprising Fe, Co, Mo and AI2O3 (a molar ratio of

TH

Fe:Co:Mo:Al=2:3:l:12) into a spray-dryer (Niro Spray-dryer Mobile Minor ) and simultaneously spraying and drying the aqueous catalytic solution using hot air of about 290 ^° C . An SEM image of one hundred magnifications showing catalytic particles prepared at a disc rotating speed of 5,000 to 20,000 rpm and a solution injection rate of 10 to 100 ml/min is illustrated in Fig. 4(a). A supported catalyst was synthesized by sintering a prepared catalytic powder at about 550 "C for 30 minutes under normal pressure and an air atmosphere. An SEM image of the prepared supported catalyst is illustrated in Fig. 4(b). As illustrated in Fig. 4(b), it can be confirmed that the spherical catalytic particles are randomly broken into small-sized particles after the sintering process. A surface area of the prepared catalyst was measured using BET.

<79> Carbon nanotubes were synthesized for 45 minutes while 0.01 g of the supported catalyst synthesized by the aforementioned method flowed in a fixed-bed thermal chemical vapor deposition system at about 700 °C at 100/100 seem of ethylene and hydrogen at a ratio of 1:1. An SEM image of the synthesized carbon nanotubes was photographed at 100,000 magnifications and illustrated in Fig. 5. The carbon purity of the synthesized carbon nanotubes was measured using TGA and the productivity was measured as an increased weight of the carbon nanotubes after the synthesis, which are shown in Table

2

1. The surface area of the catalyst was about 57 m /g, the carbon nanotubes could be produced up to about 90 g from about 1 g of the catalyst, and the carbon purity was 98.8 %.

<80>

<8i> Comparative Example 1 <82> A supported catalyst was prepared in the same manner as in Example 1 except that a water-soluble polyvinylpyrrolidone (PVP) polymer was added to an aqueous catalytic solution at a ratio of 20 % by weight with respect to the solid content. An SEM image of the prepared supported catalyst is illustrated in Fig. 6(a). As illustrated in Fig. 6(a), it can be seen that spherical particles in the polymer-mixed aqueous catalytic solution are not broken, but are maintained in the spherical shape even after the sintering process. A surface area of the prepared catalyst was measured using BET, carbon nanotubes were synthesized under the same conditions as Example 1, and the purity and productivity of the carbon nanotubes are illustrated in Table 1.

<83>

<84> Comparative Example 2

<85> A supported catalyst was prepared in the same manner as in Example 1 except that an aqueous catalytic solution was directly subjected to the sintering process without performing the spray-drying process. An SEM image of the prepared supported catalyst is illustrated in Fig. 6(b). As illustrated in Fig. 6(b), it can be seen that the prepared supported catalyst was formed in a random shape without having a specific shape, or a metal catalyst was formed only on one side of the supported catalyst. A surface area of the prepared catalyst was measured using BET, the carbon nanotubes were synthesized under the same conditions as Example 1, and the purity and productivity of the carbon nanotubes are illustrated in Table 1.

<86> <87> [Table 1]

<88> <89> * Productivity of carbon nanotubes = (weight of synthesized carbon nanotubes - catalyst weight )/catalyst weight x 100

<90> <91> Fig. 7 is a graph showing a relationship between the surface areas of the catalytic particles prepared according to Example 1 and Comparative Examples 1 and 2 and the productivities of the carbon nanotubes prepared according thereto. It can be seen that the production efficiencies increase as the surface areas are enlarged. This shows that the surface areas of the catalysts are closely related to the productivities of the carbon nanotubes. Further, it can be seen that it is important to increase the surface area of a catalytic metal in order to mass-produce high purity carbon nanotubes at low costs since the productivities of the carbon nanotubes are also related to the purities of the carbon nanotubes.

<92> <93> It will be apparent that those skilled in the art can make various modifications and changes thereto within the scope of the invention defined by the claims, and the modifications and changes will be included in the scope of the present invention.