U.S. patent application number 14/432027 was filed with the patent office on 2015-08-27 for supported catalyst, carbon nanotube assembly, and preparation method therefor.
The applicant listed for this patent is LG CHEM. LTD.. Invention is credited to Jinmyung Cha, KyungYeon Kang, SungJin Kim, Dongchul Lee, Seungyong Lee, JaeKeun Yoon.
Application Number | 20150238937 14/432027 |
Document ID | / |
Family ID | 52570272 |
Filed Date | 2015-08-27 |
United States Patent
Application |
20150238937 |
Kind Code |
A1 |
Kang; KyungYeon ; et
al. |
August 27, 2015 |
SUPPORTED CATALYST, CARBON NANOTUBE ASSEMBLY, AND PREPARATION
METHOD THEREFOR
Abstract
The present invention relates to an impregnated supported
catalyst, a carbon nanotube aggregate, and a method for producing
the carbon nanotube aggregate. The carbon nanotube aggregate
includes a four-component catalyst in which catalytic components
and active components are supported on a granular support, and
bundle type carbon nanotubes grown on the catalyst. The carbon
nanotube aggregate has an average particle diameter of 100 to 800
.mu.m, a bulk density of 80 to 250 kg/m.sup.3, and a spherical or
potato-like shape.
Inventors: |
Kang; KyungYeon; (Daejeon,
KR) ; Lee; Dongchul; (Daejeon, KR) ; Kim;
SungJin; (Daejeon, KR) ; Yoon; JaeKeun;
(Daejeon, KR) ; Lee; Seungyong; (Daejeon, KR)
; Cha; Jinmyung; (Daejeon, KR) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
LG CHEM. LTD. |
Yeongdeungpo-gu Seoul |
|
KR |
|
|
Family ID: |
52570272 |
Appl. No.: |
14/432027 |
Filed: |
July 10, 2014 |
PCT Filed: |
July 10, 2014 |
PCT NO: |
PCT/KR2014/006230 |
371 Date: |
March 27, 2015 |
Current U.S.
Class: |
428/403 ;
427/249.1; 502/312 |
Current CPC
Class: |
B01J 35/1042 20130101;
C01P 2004/60 20130101; C23C 16/26 20130101; Y10T 428/2991 20150115;
B01J 37/0203 20130101; B01J 2523/00 20130101; C01B 2202/08
20130101; B01J 35/0026 20130101; B01J 37/086 20130101; B01J 2523/00
20130101; B01J 37/0205 20130101; B01J 37/0213 20130101; C01B
2202/36 20130101; B01J 23/8877 20130101; B01J 35/023 20130101; B01J
35/1019 20130101; C01B 32/162 20170801; B01J 2523/845 20130101;
B01J 2523/842 20130101; B01J 2523/55 20130101; B01J 2523/68
20130101 |
International
Class: |
B01J 23/887 20060101
B01J023/887; B01J 37/02 20060101 B01J037/02; C23C 16/26 20060101
C23C016/26; B01J 21/04 20060101 B01J021/04; C01B 31/02 20060101
C01B031/02; B01J 35/00 20060101 B01J035/00; B01J 37/08 20060101
B01J037/08 |
Foreign Application Data
Date |
Code |
Application Number |
Jul 10, 2013 |
KR |
10-2013-0080813 |
Jul 10, 2014 |
KR |
10-2014-0087041 |
Claims
1. An impregnated supported catalyst prepared by sequentially
adding a multi-carboxylic acid and precursors of first and second
catalytic components to precursors of first and second active
components to obtain a transparent aqueous metal solution,
impregnating an aluminum-based granular support with the
transparent aqueous metal solution, followed by drying and
calcination, wherein the supported catalyst has a bulk density of
0.8 to 1.5 g/cm.sup.3.
2. The impregnated supported catalyst according to claim 1, wherein
the catalyst comprises first and second catalytic components and
first and second active components, and the number of moles (x) of
the first catalytic component, the number of moles (y) of the
second catalytic component, the number of moles (p) of the first
active component, and the number of moles (q) of the second active
component with respect to 100 moles of the support satisfy the
following relationships: 10.ltoreq.x.ltoreq.40;
1.ltoreq.y.ltoreq.20; 0.1.ltoreq.y/[x+y].ltoreq.0.5;
1.ltoreq.p+q.ltoreq.20; and 0.1.ltoreq.[p+q]/[x+y].ltoreq.0.5.
3. The impregnated supported catalyst according to claim 1, wherein
the granular support has a bulk density of 0.6 to 1.2 g/cm.sup.3,
and the catalyst in which the catalytic components and the active
components are supported has a bulk density of 0.8 to 1.5
g/cm.sup.3.
4. The impregnated supported catalyst according to claim 1, wherein
the granular support has an aspect ratio of 1.2 or less, and the
average aspect ratio (As) of the support before the catalytic
components and the active components are supported on the support
and the average aspect ratio (A.sub.CAT) of the catalyst after the
catalytic components and the active components are supported on the
support satisfy 0.8.ltoreq.A.sub.CAT/As.ltoreq.1.2.
5. The impregnated supported catalyst according to claim 1, wherein
the multi-carboxylic acid is used in an amount of 0.2 to 2.0 moles,
assuming that the sum of the moles (p+q) of the first and second
active components equals to 1.
6. The impregnated supported catalyst according to claim 1, wherein
the multi-carboxylic acid is selected from dicarboxylic acids,
tricarboxylic acids, tetracarboxylic acids, and mixtures
thereof.
7. The impregnated supported catalyst according to claim 1, wherein
the calcination is performed at 650 to 800.degree. C.
8. The impregnated supported catalyst according to claim 1, wherein
the first catalytic component is cobalt (Co), the second catalytic
component is selected from iron (Fe), nickel (Ni), and a mixture
thereof, the first active component is molybdenum (Mo), and the
second active component is vanadium (V).
9. The impregnated supported catalyst according to claim 1, wherein
the first and second active components are in a weight ratio of
6-0.1:0.1-6.
10. The impregnated supported catalyst according to claim 1,
wherein the catalyst has a structure in which the surface and pores
of the aluminum-based support are coated with a monolayer or
multilayer of the catalytic components and the active components,
and the amount of a fine powder having a number average particle
diameter not larger than 32 .mu.m, as measured after ultrasonic
shaking at 40 watts for 1 minute, is 10% or less of the amount of
the catalyst.
11. The impregnated supported catalyst according to claim 6,
wherein the transparent aqueous metal solution has a concentration
of 0.01 to 0.4 g/ml.
12. A carbon nanotube aggregate comprising the impregnated
supported catalyst according to claim 1 and bundle type carbon
nanotubes grown on the catalyst wherein the carbon nanotube
aggregate has an average particle diameter of 100 to 800 .mu.m, a
bulk density of 80 to 250 kg/m.sup.3, and a spherical or
potato-like shape.
13. The carbon nanotube aggregate according to claim 12, wherein
the carbon nanotubes have an aspect ratio of 0.9 to 1 and a strand
diameter of 10 to 50 nm.
14. The carbon nanotube aggregate according to claim 12, wherein
the catalyst has an average aspect ratio (A.sub.CAT) of 1.2 or less
and the carbon nanotube aggregate has an average aspect ratio
(A.sub.CNT) of 1.2 or less.
15. The carbon nanotube aggregate according to claim 12, wherein
the bundle type carbon nanotubes have a particle size distribution
(Dcnt) of 0.5 to 1.0.
16. A method for producing a carbon nanotube aggregate, comprising:
1) sequentially blending a multi-carboxylic acid component and an
aqueous solution of precursors of first and second catalytic
components with an aqueous solution of precursors of first and
second active components to obtain a transparent aqueous metal
solution, and mixing an aluminum-based granular support with the
transparent aqueous metal solution; 2) drying the mixture under
vacuum at 40 to 80.degree. C. and calcining the dried mixture at
650 to 800.degree. C. to obtain a catalyst for carbon nanotube
production in which the surface and pores of the aluminum-based
support are impregnated and coated with the catalytic components
and the active components; 3) feeding the catalyst for carbon
nanotube production into a fluidized bed reactor and introducing at
least one carbon source selected from C.sub.1-C.sub.4 saturated or
unsaturated hydrocarbons, and optionally together with a mixed gas
hydrogen and nitrogen, into the reactor at 500 to 900.degree. C.;
and 4) decomposing the carbon source and growing carbon nanotubes
on the catalyst surface by chemical vapor synthesis.
17. The method according to claim 16, further comprising aging at
45 to 80.degree. C. before the drying under vacuum in step 2).
18. The method according to claim 16, further comprising
preliminarily calcining at 250 to 400.degree. C. before the
calcination in step 2).
19. The method according to claim 18, further comprising
impregnating a portion of the total amount of the aqueous metal
solution into the aluminum-based granular support just before the
preliminary calcination and impregnating the remainder of the
aqueous metal solution into the aluminum-based granular support
just before the calcination.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention relates to a supported catalyst, a
carbon nanotube aggregate, and a method for producing the carbon
nanotube aggregate.
[0003] 2. Description of the Related Art
[0004] Carbon nanostructures (CNSs) refer collectively to
nano-sized carbon structures having various shapes, such as
nanotubes, nanohairs, fullerenes, nanocones, nanohorns, and
nanorods. Carbon nanostructures can be widely utilized in a variety
of technological applications because they possess excellent
characteristics.
[0005] Carbon nanotubes (CNTs) are tubular materials consisting of
carbon atoms arranged in a hexagonal pattern and have a diameter of
approximately 1 to 100 nm. Carbon nanotubes exhibit insulating,
conducting or semiconducting properties depending on their inherent
chirality. Carbon nanotubes have a structure in which carbon atoms
are strongly covalently bonded to each other. Due to this
structure, carbon nanotubes have a tensile strength approximately
100 times that of steel, are highly flexible and elastic, and are
chemically stable.
[0006] Carbon nanotubes are divided into three types: single-walled
carbon nanotubes (SWCNTs) consisting of a single sheet and having a
diameter of about 1 nm; double-walled carbon nanotubes (DWCNTs)
consisting of two sheets and having a diameter of about 1.4 to
about 3 nm; and multi-walled carbon nanotubes (MWCNTs) consisting
of three or more sheets and having a diameter of about 5 to about
100 nm.
[0007] Carbon nanotubes are being investigated for their
commercialization and application in various industrial fields, for
example, aerospace, fuel cell, composite material, biotechnology,
pharmaceutical, electrical/electronic, and semiconductor
industries, due to their high chemical stability, flexibility and
elasticity. However, carbon nanotubes have a limitation in directly
controlling the diameter and length to industrially applicable
dimensions for practical use owing to their primary structure.
Accordingly, the industrial application and use of carbon nanotubes
are limited despite their excellent physical properties.
[0008] Carbon nanotubes are generally produced by various
techniques, such as arc discharge, laser ablation, and chemical
vapor deposition. However, arc discharge and laser ablation are not
appropriate for mass production of carbon nanotubes and require
high arc production costs or expensive laser equipment. Chemical
vapor deposition using a vapor dispersion catalyst has the problems
of a very low synthesis rate and too small a size of final CNT
particles. Chemical vapor deposition using a substrate-supported
catalyst suffers from very low efficiency in the utilization of a
reactor space, thus being inappropriate for mass production of
CNTs. Thus, studies on catalysts and reaction conditions for
chemical vapor deposition are currently underway to increase the
yield of carbon nanotubes.
[0009] Catalytically active components of the catalysts usually
take the form of oxides, partially or completely reduced products,
or hydroxides. The catalysts may be, for example, supported
catalysts or coprecipitated catalysts, which can be commonly used
for CNT production. Supported catalysts are preferably used for the
following reasons: supported catalysts have a higher inherent bulk
density than coprecipitated catalysts; unlike coprecipitated
catalysts, supported catalysts produce a small amount of a fine
powder with a size of 10 microns or less, which reduces the
possibility of occurrence of a fine powder due to attrition during
fluidization; and high mechanical strength of supported catalysts
effectively stabilizes the operation of reactors.
[0010] Many methods have been proposed to prepare supported
catalysts. For example, a supported catalyst is prepared by an
impregnation method including mixing an aqueous metal solution with
a support, followed by coating-drying. This method has the
disadvantage that the amount of the catalyst loaded is limited.
Non-uniform distribution of active components and catalytic
components greatly affects CNT growth yield and CNT diameter
distribution, but no technique has been proposed to control the
non-uniform distribution.
[0011] Particularly, a supported catalyst prepared by the
conventional impregnation method can be used for the synthesis of
carbon nanotubes. In this case, however, the yield of carbon
nanotubes is less than 1000% and the amount of the catalyst loaded
is large, showing a limited influence on yield. The catalyst is a
bundle type with low bulk density and the feeding rate of reaction
gas is thus lowered, resulting in low CNT productivity.
[0012] Under these circumstances, more research needs to be done to
synthesize carbon nanotubes with high bulk density in high yield
despite the use of supported catalysts.
SUMMARY OF THE INVENTION
[0013] The present invention has been made in an effort to overcome
the disadvantage of low CNT yield encountered in the use of
conventional supported catalysts, and it is an object of the
present invention to provide carbon nanotubes whose bulk density
and yield are improved by simultaneously controlling the activity
of a catalyst and the amount of a fine powder, and a method for
producing the carbon nanotubes.
[0014] One aspect of the present invention provides an impregnated
supported catalyst prepared by sequentially adding a
multi-carboxylic acid and precursors of first and second catalytic
components to precursors of first and second active components to
obtain a transparent aqueous metal solution, impregnating an
aluminum-based granular support with the transparent aqueous metal
solution, followed by drying and calcination, wherein the supported
catalyst has a bulk density of 0.8 to 1.5 g/cm.sup.3.
[0015] Another aspect of the present invention provides a carbon
nanotube aggregate including a four-component catalyst in which
catalytic components and active components are supported on a
granular support, and bundle type carbon nanotubes grown on the
catalyst wherein the carbon nanotube aggregate has an average
particle diameter of 100 to 800 .mu.m, a bulk density of 80 to 250
kg/m.sup.3, and a spherical or potato-like shape.
[0016] The carbon nanotubes may have an aspect ratio of 0.9 to 1
and a strand diameter of 10 to 50 nm.
[0017] The catalyst may have an average aspect ratio (A.sub.CAT) of
1.2 or less and the carbon nanotube aggregate may have an average
aspect ratio (A.sub.CNT) of 1.2 or less.
[0018] The bundle type carbon nanotubes grown on the aluminum-based
granular support may have a particle size distribution (Dcnt) of
0.5 to 1.0.
[0019] The four-component catalyst includes first and second
catalytic components and first and second active components, and
the number of moles (x) of the first catalytic component, the
number of moles (y) of the second catalytic component, the number
of moles (p) of the first active component, and the number of moles
(q) of the second active component with respect to 100 moles of the
support may satisfy the following relationships:
10.ltoreq.x.ltoreq.40;
1.ltoreq.y.ltoreq.20;
0.1.ltoreq.y/[x+y].ltoreq.0.5;
1.ltoreq.p+q.ltoreq.20; and
0.1.ltoreq.[p+q]/[x+y].ltoreq.0.5.
[0020] The granular support may have a bulk density of 0.6 to 1.2
g/cm.sup.3, and the catalyst in which the catalytic components and
the active components are supported may have a bulk density of 0.8
to 1.5 g/cm.sup.3.
[0021] The granular support may have an aspect ratio of 1.2 or
less, and the average aspect ratio (As) of the support before the
catalytic components and the active components are supported on the
support and the average aspect ratio (A.sub.CAT) of the catalyst
after the catalytic components and the active components are
supported on the support may satisfy
0.8.ltoreq.A.sub.CAT/As.ltoreq.1.2.
[0022] The multi-carboxylic acid may be used in an amount of 0.2 to
2.0 moles, assuming that the sum of the moles (p+q) of the first
and second active components equals to 1.
[0023] The multi-carboxylic acid may be selected from dicarboxylic
acids, tricarboxylic acids, tetracarboxylic acids, and mixtures
thereof.
[0024] The calcination may be performed at 650 to 800.degree.
C.
[0025] The transparent aqueous metal solution may have a
concentration of 0.01 to 0.4 g/ml.
[0026] According to one embodiment, the first catalytic component
may be cobalt (Co), the second catalytic component may be selected
from iron (Fe), nickel (Ni), and a mixture thereof, the first
active component may be molybdenum (Mo), and the second active
component may be vanadium (V).
[0027] The first and second active components may be in a weight
ratio of 6-0.1:0.1-6.
[0028] According to one embodiment, the catalyst may have a
structure in which the surface and pores of the aluminum-based
support are coated with a monolayer or multilayer of the catalytic
components and the active components, and the amount of a fine
powder having a number average particle diameter not larger than 32
.mu.m, as measured after ultrasonic shaking at 40 watts for 1
minute, may be 10% or less of the amount of the catalyst.
[0029] Yet another aspect of the present invention provides a
method for producing a carbon nanotube aggregate, including 1)
sequentially blending a multi-carboxylic acid component and an
aqueous solution of precursors of first and second catalytic
components with an aqueous solution of precursors of first and
second active components to obtain a transparent aqueous metal
solution, and mixing an aluminum-based granular support with the
transparent aqueous metal solution, 2) drying the mixture under
vacuum at 40 to 80.degree. C. and calcining the dried mixture at
650 to 800.degree. C. to obtain a catalyst for carbon nanotube
production in which the surface and pores of the aluminum-based
support are impregnated and coated with the catalytic components
and the active components, 3) feeding the catalyst for carbon
nanotube production into a fluidized bed reactor and introducing at
least one carbon source selected from C.sub.1-C.sub.4 saturated or
unsaturated hydrocarbons, and optionally together with a mixed gas
hydrogen and nitrogen, into the reactor at 500 to 900.degree. C.,
and 4) decomposing the carbon source and growing carbon nanotubes
on the catalyst surface by chemical vapor synthesis.
[0030] According to one embodiment, the method may further include
aging at 45 to 80.degree. C. before the drying under vacuum in step
2).
[0031] The method may further include preliminarily calcining at
250 to 400.degree. C. before the calcination in step 2).
[0032] The method may further include impregnating a portion of the
total amount of the aqueous metal solution into the aluminum-based
granular support just before the preliminary calcination and
impregnating the remainder of the aqueous metal solution into the
aluminum-based granular support just before the calcination.
[0033] Details of other embodiments of the present invention are
included in the detailed description that follows.
[0034] The present invention can overcome the disadvantage of low
CNT yield encountered in the use of conventional impregnated
catalysts for carbon nanotube production. The supported catalyst of
the present invention has controlled activity and can produce a
controlled amount of a fine powder, enabling the synthesis of
bundle type carbon nanotubes in high yield. Therefore, the
supported catalyst of the present invention can find application in
various fields, such as energy materials, functional composites,
pharmaceuticals, batteries, and semiconductors, where carbon
nanotubes are used.
BRIEF DESCRIPTION OF THE DRAWINGS
[0035] FIG. 1 is a SEM image showing a bulk shape of CNTs produced
in Example 1-1;
[0036] FIGS. 2a and 2b are SEM images of CNT aggregates produced in
Reference Example 1-2 and Example 1-1, respectively;
[0037] FIG. 3 is a graph showing changes in the yield of CNTs
depending on Mo:V ratio and calcination temperature in Reference
Examples 1-1 and 1-2 and Examples 1-1, 1-2, and 1-3; and
[0038] FIG. 4 is a graph showing changes in the yield of CNTs as a
function of reaction time in Example 3 and Reference Example 2.
DETAILED DESCRIPTION OF THE INVENTION
[0039] The present invention will now be described in more
detail.
[0040] The present invention provides an impregnated supported
catalyst prepared by sequentially adding a multi-carboxylic acid
and precursors of first and second catalytic components to
precursors of first and second active components to obtain a
transparent aqueous metal solution, impregnating an aluminum-based
granular support with the transparent aqueous metal solution,
followed by drying and calcination, wherein the supported catalyst
has a bulk density of 0.8 to 1.5 g/cm.sup.3.
[0041] The present invention also provides a carbon nanotube
aggregate including a four-component catalyst in which catalytic
components and active components are supported on a granular
support, and bundle type carbon nanotubes grown on the catalyst
wherein the carbon nanotube aggregate has an average particle
diameter of 100 to 800 .mu.m, a bulk density of 80 to 250
kg/m.sup.3, and a spherical or potato-like shape.
[0042] The bulk density can be defined by Expression 1:
Bulk density=CNT weight (kg)/CNT volume (m.sup.3) (1)
[0043] where CNT refers to the carbon nanotube aggregate.
[0044] According to the present invention, the use of the
four-component catalyst with little fine powder allows the carbon
nanotubes grown on the catalyst to have a density distribution and
an average particle diameter in specific ranges.
[0045] The carbon nanotubes of the carbon nanotube aggregate
according to the present invention may have a strand diameter of 10
to 50 nm.
[0046] The bundle type carbon nanotubes grown on the aluminum-based
granular catalyst have a particle size distribution (Dcnt) of 0.5
to 1.0.
[0047] Unless otherwise mentioned, the term "bundle type carbon
nanotubes" used herein refers to a type of carbon nanotubes in
which the carbon nanotubes are arranged in parallel or get
entangled to form bundles or ropes.
[0048] The particle size distribution (Dcnt) can be defined by
Expression 2:
Dcnt=[Dn90-Dn10]/Dn50 (2)
[0049] where Dn90, Dn10, and Dn50 are the number average particle
diameters of the CNTs after standing in distilled water for 3
hours, as measured under 90%, 10%, and 50% in the absorption mode
using a particle size analyzer (Microtrac), respectively.
[0050] The particle size distribution may be, for example, from
0.55 to 0.95 or from 0.55 to 0.9.
[0051] The bundle type carbon nanotubes may have an aspect ratio of
0.9 to 1. The bundle type carbon nanotubes may have a diameter of 1
to 50 .mu.m. The aspect ratio range and the type of the carbon
nanotubes can be achieved by a specific process of the
four-component catalyst, which is presented in the present
invention. Specifically, the aspect ratio is defined by Expression
3:
Aspect ratio=the shortest diameter passing through the center of
CNT/the longest diameter passing through the center of CNT (3)
[0052] where CNT refers to the carbon nanotube aggregate.
[0053] In the catalyst used for the production of the carbon
nanotube assembly, catalytic components and active components are
supported on a granular support. The catalytic components include
first and second catalytic components. The active components
include first and second active components. The four-component
catalyst is prepared by calcination of the catalytic components and
the active components. The number of moles (x) of the first
catalytic component, the number of moles (y) of the second
catalytic component, the number of moles (p) of the first active
component, and the number of moles (q) of the second active
component with respect to 100 moles of the support may satisfy the
following relationships:
1.ltoreq.y.ltoreq.20;
0.1.ltoreq.y/[x+y].ltoreq.0.5;
1.ltoreq.p+q.ltoreq.20; and
0.1.ltoreq.[p+q]/[x+y].ltoreq.0.5.
[0054] In a preferred embodiment of the present invention, based on
100 moles of the aluminum-based support, the number of moles (x) of
the first catalytic component and the number of moles (y) of the
second catalytic component satisfy 30.ltoreq.x+y.ltoreq.53, and the
number of moles (p) of the first active component and the number of
moles (q) of the second active component satisfy
3.ltoreq.p+q.ltoreq.13. More preferably, based on 100 moles of the
aluminum-based support, x and y satisfy 30.ltoreq.x+y.ltoreq.44 or
35.ltoreq.x+y.ltoreq.44, and p and q satisfy 3.ltoreq.z.ltoreq.9.5
or 5.ltoreq.z.ltoreq.9.5.
[0055] In a preferred embodiment of the present invention, the
supported catalyst is a calcined catalyst in which the catalytic
components and the active components are supported on the granular
support. The granular support may have an aspect ratio of 1.2 or
less, and the average aspect ratio (As) of the support before the
catalytic components and the active components are supported on the
support and the average aspect ratio (A.sub.CAT) of the catalyst
after the catalytic components and the active components are
supported on the support may satisfy
0.8.ltoreq.A.sub.CAT/As.ltoreq.1.2.
[0056] According to a preferred embodiment of the present
invention, the granular support may have a bulk density of 0.6 to
1.2 g/cm.sup.3, and the supported catalyst prepared by impregnation
of the catalytic components and the active components into the
granular support may have a bulk density of 0.8 to 1.5
g/cm.sup.3.
[0057] The supported catalyst of the present invention has a
structure in which the catalytic components and the active
components are uniformly impregnated into and coated on the surface
and pores of the support. Due to this structure, the amount of a
fine powder as an aggregate of the catalytic metals remaining
uncoated can be reduced to less than 5% and the spherical or
potato-like shape of the support is maintained unchanged even after
completion of the catalyst production. The spherical or potato-like
shape refers to a three-dimensional shape having an aspect ratio of
1.2 or less such as a sphere or ellipse.
[0058] The catalyst of the present invention may have a particle
diameter (or average particle diameter) of 30 to 150 .mu.m, as
measured before the calcination, and each of the support and
catalyst may have a spherical or potato-like shape with a primary
particle diameter of 10 to 50 nm.
[0059] According to one embodiment, the first catalytic component
may be selected from Fe, Ni, and a mixture thereof, the second
catalytic component may be Co, the first active component may be
Mo, and the second active component may be V. Particularly, the
addition of vanadium (V) as the second active component leads to a
high yield of carbon nanotubes and a significant increase in the
density (or bulk density) of carbon nanotubes.
[0060] The catalytic components used in the present invention may
include at least one metal selected from Fe and Ni as the first
catalytic component and Co as the second catalytic component. For
example, the first catalytic component may be selected from the
group consisting of Fe salts, Fe oxides, Fe compounds, Ni salts, Ni
oxides, Ni compounds, and mixtures thereof, and the second
catalytic component may be selected from the group consisting of Co
salts, Co oxides, Co compounds, and mixtures thereof. As another
example, the first catalytic component may be selected from the
group consisting of Fe(NO.sub.3).sub.2.6H.sub.2O,
Fe(NO.sub.3).sub.2.9H.sub.2O, Fe(NO.sub.3).sub.3, Fe(OAc).sub.2,
Ni(NO.sub.3).sub.2.6H.sub.2O, and mixtures thereof, and the second
catalytic component may be selected from the group consisting of
Co(NO.sub.3).sub.2.6H.sub.2O, Co.sub.2(CO).sub.8,
[Co.sub.2(CO).sub.6(t-BuC.dbd.CH)], Co(OAc).sub.2, and mixtures
thereof.
[0061] The first active component may be Mo, for example, a Mo
salt, a Mo oxide, or a Mo compound. As another example, the first
active component may be (NH.sub.4).sub.6Mo.sub.7O.sub.24.4H.sub.2O,
Mo(CO).sub.6, or (NH.sub.4)MoS.sub.4, which may be dissolved in
distilled water before use.
[0062] The second active component may be V, for example, a V salt,
a V oxide, or a V compound. As another example, the second active
component may be NH.sub.4VO.sub.3, which may be dissolved in
distilled water before use.
[0063] The total amount of the first and second active components
may be from 0.2 to 4% by weight, based on the weight of the aqueous
metal solution.
[0064] The first active component (Mo) and the second active
component (V) may be in a weight ratio of 6-0.1:0.1-6, more
preferably 5-1:2-4. The catalyst essentially includes molybdenum
(Mo) and vanadium (V) as the active components. The ratio of the
metal components may be controlled to obtain carbon nanotubes in a
high yield, for example, 5000% or more, 6000% or more, or 7500% or
more.
[0065] The four-component catalyst of the present invention has a
structure in which the surface and pores of the aluminum-based
support are coated with a monolayer or multilayer of the catalytic
components and the active components, and the amount of a fine
powder after ultrasonication is 10% or less of the amount of the
catalyst. Accordingly, the density distribution of the carbon
nanotubes grown on the catalyst is much more uniform than that of
carbon nanotubes grown on a conventional catalyst. In one
embodiment of the present invention, the amount of a fine powder
having a number average particle diameter not larger than 32 .mu.m,
as measured after ultrasonic shaking at 40 watts for 1 minute, may
be 10% or less, preferably 5% or less, of the amount of the
catalyst taking into consideration the particle diameter (or
average particle diameter) range (32-95 .mu.m) of the
aluminum-based support.
[0066] For reference, the fine powder is defined as an aggregate of
the catalytic materials and the active materials attached to the
catalyst after ultrasonication. The fine powder passes through a
sieve but is fundamentally different from the catalytically active
materials well-coated on the support in terms of particle size and
catalytic activity. The fine powder is an island-like aggregate
attached to the catalyst and is a cause of low CNT yield. Portions
of the catalytic materials and the active materials are somewhat
weakly attached to the catalyst and are thus separated from the
catalyst during ultrasonication to form the fine powder.
[0067] The supported catalyst of the present invention is
preferably prepared by an impregnation method for the following
reasons: the supported catalyst has a higher inherent bulk density
than coprecipitated catalysts; unlike coprecipitated catalysts, the
supported catalyst produces a small amount of a fine powder with a
size of 10 microns or less, which reduces the possibility of
occurrence of a fine powder due to attrition during fluidization;
and high mechanical strength of the supported catalyst effectively
stabilizes the operation of a fluidized bed reactor.
[0068] The aluminum-based granular support used in the catalyst of
the present invention may be made of an aluminum compound selected
from the group consisting of Al.sub.2O.sub.3, AlO(OH),
Al(OH).sub.3, and mixtures thereof. The aluminum-based granular
support is preferably made of alumina (Al.sub.2O.sub.3). The
support may be in the form of a powder instead of a water-soluble
salt such as aluminum nitrate. The use of the support in the form
of a powder allows the impregnated supported catalyst to have a
very high bulk density of 0.5 to 1.5 g/cm.sup.3. This is considered
a significant difference between the impregnated catalyst and
coprecipitated catalysts. The high bulk density of the catalyst
enables the operation of a reactor at a high linear velocity in the
production of carbon nanotubes and serves to markedly increase the
hourly productivity of carbon nanotubes.
[0069] The aluminum (Al)-based support may further include at least
one oxide selected from the group consisting of ZrO.sub.2, MgO, and
SiO.sub.2. The aluminum (Al)-based granular support has a spherical
or potato-like shape. The material for the aluminum (Al)-based
granular support has a structure suitable to provide a relatively
high surface area per unit weight or volume, such as a porous
structure, a molecular sieve structure, or a honeycomb
structure.
[0070] According to a preferred embodiment of the present
invention, the granular support may have a primary particle
diameter of 10 to 50 nm, a porosity of 0.1 to 1.0 cm.sup.3/g, and a
specific surface area of 100 to 300 m.sup.2/g.
[0071] As described above, the supported catalyst may be prepared
by sequentially adding a multi-carboxylic acid and the precursors
of first and second catalytic components to the precursors of first
and second active components to obtain a transparent aqueous metal
solution, providing the transparent aqueous metal solution to the
aluminum-based support, followed by drying under vacuum and
calcination.
[0072] The transparent aqueous metal solution means an aqueous
solution free of precipitates. The term "precipitates" means, for
example, deep yellow precipitates such as Fe(MoO).sub.3.dwnarw. or
dark red precipitates such as Fe(VO.sub.3).sub.3.dwnarw. formed as
a result of the reaction of Fe.sup.3+ with 3MoO.sup.- or
3VO.sub.3.sup.- at room temperature when an iron (Fe) precursor
(such as iron nitrate) as the catalytic component is added to water
and a Mo precursor (such as ammonium molybdate) and a V precursor
(such as ammonium vanadate) as the active components are added
thereto.
[0073] The multi-carboxylic acid used in the present invention is a
compound having one or more carboxyl groups. The multi-carboxylic
acid is highly soluble and serves as a complexing agent to suppress
the formation of precipitates and to facilitate the synthesis of
the catalyst. The multi-carboxylic acid also serves as an activator
to promote the synthesis of carbon nanotubes.
[0074] The multi-carboxylic acid may be selected from dicarboxylic
acids, tricarboxylic acids, tetracarboxylic acids, and mixtures
thereof. Examples of such multi-carboxylic acids include citric
acid, oxalic acid, succinic acid, and tartaric acid. The
multi-carboxylic acid may be used in an amount of 0.1 to 1.5% by
weight, based on the weight of the aqueous metal solution. The
ratio of the moles of the multi-carboxylic acid to the sum of the
moles of the first and second active components is 0.2-2.0:1, more
preferably 0.2-1.0:1, most preferably 0.2-0.5:1. Within this range,
no precipitation may take place in the aqueous metal solution and
no cracks may be caused during calcination.
[0075] The order of addition of the multi-carboxylic acid and the
catalytic components may be changed. For example, the
multi-carboxylic acid may be added to a Mo component and/or a V
component before the addition of a Fe component or a Co component
is added. In this case, the formation of precipitates is
suppressed, and as a result, the area of the support surface
covered by precipitates is reduced, resulting in an improvement in
the activity of the catalyst.
[0076] Specifically, the supported catalyst of the present
invention may be prepared by: sequentially blending the
multi-carboxylic acid component and an aqueous solution of the
precursors of first and second catalytic components with an aqueous
solution of the precursors of first and second active components to
obtain a transparent aqueous metal solution, and mixing the
aluminum-based support with the transparent aqueous metal solution;
and drying the mixture under vacuum at 40 to 80.degree. C. and
calcining the dried mixture at 650 to 800.degree. C. In the
supported catalyst for carbon nanotube production, the surface and
pores of the aluminum-based support are impregnated and coated with
the catalytic components and the active components.
[0077] The drying under vacuum may be performed by rotary
evaporation at a temperature of 40 to 80.degree. C. for 30 minutes
to 3 hours.
[0078] Thereafter, the calcination may be performed at a
temperature of 650 to 800.degree. C., preferably 700 to 750.degree.
C. The calcination time is not limited and may be, for example, in
the range of 30 minutes to 15 hours. Within these ranges, a large
amount of the catalyst can be synthesized in a short time and the
catalytic components and the active components can be uniformly
dispersed on the surface of the aluminum-based support.
[0079] It is preferred to add the multi-carboxylic acid to the
aqueous solution of the precursors of first and second active
components before addition of the aqueous solution of the
precursors of first and second catalytic components. In the case
where the order of addition of the multi-carboxylic acid and the
catalytic components is controlled such that the multi-carboxylic
acid is added to the aqueous solution of the precursors of active
components before addition of the aqueous solution of the
precursors of catalytic components, the formation of precipitates
is suppressed, and as a result, the area of the support surface
covered by precipitates is reduced, resulting in an improvement in
the activity of the catalyst. The transparent aqueous metal
solution thus obtained has a concentration of 0.01 to 0.4 g/ml,
specifically 0.01 to 0.3 g/ml, which is efficient in terms of
reactivity.
[0080] According to one embodiment, the method may further include
aging with rotation or stirring at 45 to 80.degree. C. before the
drying under vacuum. The aging may be performed for a maximum of 5
hours, for example, 20 minutes to 5 hours or 1 to 4 hours.
[0081] The method may further include preliminarily calcining the
vacuum-dried mixture at 250 to 400.degree. C. before the
calcination.
[0082] A portion of the total amount of the aqueous metal solution
may be impregnated into the aluminum-based support just before the
preliminary calcination and the remainder of the aqueous metal
solution may be impregnated into the aluminum-based support just
before the calcination. Specifically, it is preferred in terms of
reaction efficiency that a maximum of 50 vol %, for example, 1 to
45 vol % or 5 to 40 vol %, of the aqueous metal solution is
impregnated into the aluminum-based support just before the
preliminary calcination and the remainder of the aqueous metal
solution is impregnated into the aluminum-based support just before
the calcination.
[0083] The carbon nanotube aggregate of the present invention may
be produced by a method including: feeding the supported catalyst
for carbon nanotube production into a fluidized bed reactor and
introducing at least one carbon source selected from
C.sub.1-C.sub.4 saturated or unsaturated hydrocarbons, and
optionally together with a mixed gas of hydrogen and nitrogen, into
the reactor at 500 to 900.degree. C.; and decomposing the carbon
source and growing carbon nanotubes on the catalyst surface by
chemical vapor synthesis.
[0084] According to the chemical vapor synthesis, the catalyst for
carbon nanotube production is charged into the reactor and the
carbon source is then supplied to the reactor at ambient pressure
and high temperature to produce carbon nanotubes. The hydrocarbon
is thermally decomposed and is infiltrated into and saturated in
the catalyst particles. Carbon is deposited from the saturated
catalyst particles and grows into carbon nanotubes.
[0085] According to one embodiment, carbon nanotubes may be grown
for 30 minutes to 8 hours after the carbon source is introduced
into the catalyst for carbon nanotube production. According to the
present invention, the yield of bundles of carbon nanotubes
increases and the rate of increase in yield decreases moderately
with increasing reaction time, enabling control over yield
depending on processing time and achieving a yield of 5000% or
more, 6000% or more, 7500% or more, presumably 10,000% or more,
without being bound by processing time.
[0086] The carbon source may be a C.sub.1-C.sub.4 saturated or
unsaturated hydrocarbon. Examples of such hydrocarbons include, but
are not limited to, ethylene (C.sub.2H.sub.4), acetylene
(C.sub.2H.sub.2), methane (C.sub.2H.sub.4), and propane
(C.sub.3H.sub.8). The mixed gas of hydrogen and nitrogen transports
the carbon source, prevents carbon nanotubes from burning at high
temperature, and assists in the decomposition of the carbon
source.
[0087] The use of the supported catalyst according to the present
invention for the synthesis of carbon nanotubes enables the
formation of an aggregate of carbon nanotubes grown on the
spherical or potato-like catalyst without changing the shape of the
catalyst. As a result, the carbon nanotubes have a high bulk
density while maintaining a normal distribution in particle size.
That is, the carbon nanotubes increase in size without substantial
change in the catalyst shape. Therefore, the average aspect ratio
(A.sub.CAT) of the catalyst may be 1.2 or less and the average
aspect ratio (A.sub.CNT) of the carbon nanotube aggregate may also
be 1.2 or less.
[0088] The following examples are provided to assist in
understanding the invention. However, it will be obvious to those
skilled in the art that these examples are merely illustrative and
various modifications and changes are possible without departing
from the scope and spirit of the invention. Accordingly, it should
be understood that such modifications and changes are encompassed
within the scope of the appended claims.
Example 1
Production of CNTs Depending on the Weight Ratio of Mo:V
Example 1-1
Metal Catalyst (Mo:V=3:3)
[0089] A. Preparation of Aqueous Metal Solution
[0090] A four-component metal catalyst having a combination of Co
and Fe as catalytic components and Mo and V as active components
was prepared by the following procedure. 0.055 g of
(NH.sub.4).sub.6Mo.sub.7O.sub.24.H.sub.2O as a Mo precursor and
0.069 g of NH.sub.4VO.sub.3 as a V precursor were dissolved in 20
ml of water in flask A, and then 0.037 g of citric acid as a
multi-carboxylic acid, 2.175 g of Co(NO.sub.3).sub.2.H.sub.2O as a
Co precursor, and 0.318 g of Fe(NO.sub.3).sub.2.H.sub.2O as a Fe
precursor were added thereto to prepare an aqueous metal
solution.
[0091] The aqueous metal solution was observed to be clear and free
of precipitates. Since 7 moles of Mo was present in one mole of
(NH.sub.4).sub.6Mo.sub.7O.sub.24, the number of moles of the active
components Mo and V and the multi-carboxylic acid were 0.3127,
0.5889, and 0.1926 moles, respectively, indicating that the molar
ratio of the multi-carboxylic acid to the active components was
0.21:1.
[0092] The molar ratio of Co:Fe was fixed to 30:8 and the weight
ratio of Mo:V was adjusted to 3:3.
[0093] B. Preparation of Support
[0094] 2.5 g of Al.sub.2O.sub.3 (D50v=76 microns, pore volume: 0.64
cm.sup.3/g, surface area: 237 m.sup.2/g, Saint Gobain) as an
aluminum-based support was placed in flask B.
[0095] C. Preparation of Supported Catalyst Having First Catalyst
Layer
[0096] One half (10.6 g) of the total amount (21.3 g) of the
solution in flask A was added to flask B. The catalytically active
metal precursors were sufficiently supported on the
Al.sub.2O.sub.3, followed by aging with stirring in a thermostatic
bath at 60.degree. C. for 5 min. The mixture was dried with
rotation at 150 rpm under vacuum for 30 min while maintaining the
temperature. The dried mixture was calcined at 350.degree. C. for 1
h to prepare a homogeneous supported catalyst.
[0097] When the number of moles of the Al.sub.2O.sub.3 (2.5 g) was
assumed to be 100 moles, the numbers of moles of Fe, Co, Mo, and V
were 8, 30, 3, and 6 moles, respectively.
[0098] D. Preparation of Supported Catalyst Having Second Metal
Catalyst Layer
[0099] The catalyst having the first metal catalyst layer was
placed in flask C, and the other half (10.6 g) of the metal
solution in flask A was added thereto. The catalytically active
metal precursors were sufficiently supported on Al.sub.2O.sub.3,
followed by aging with stirring in a thermostatic bath at
60.degree. C. for 5 min.
[0100] The mixture was dried with rotation at 150 rpm under vacuum
for 30 min while maintaining the temperature. The dried mixture was
calcined at 725.degree. C. for 3 h to prepare a homogeneous
catalyst.
[0101] The catalyst was passed through a 32-micron sieve and the
passed particles were weighed to calculate the content of a fine
powder, which was defined as the aggregate of the particles. The
calculated content of the fine powder was 0 wt %. After dispersion
in water and ultrasonic shaking at 40 watts for 1 min, the
proportion of particles having a size of 32 .mu.m or less was
measured using a particle size analyzer (Microtrac, bluewave). As a
result, the amount of the fine powder after ultrasonication
corresponded to 0% on the basis of number average particle
diameter.
[0102] E. CNT Synthesis
[0103] A test for the synthesis of carbon nanotubes using the
catalyst prepared in D was conducted in a fluidized bed reactor on
a laboratory scale. Specifically, the catalyst was mounted at the
center of a quartz tube having an inner diameter of 55 mm and
heated to 700.degree. C. under a nitrogen atmosphere. A mixed gas
of nitrogen, hydrogen and ethylene gas in the same volume ratio was
allowed to flow at a rate of 180 ml/min for a total of 1 h while
maintaining the same temperature, affording a carbon nanotube
aggregate.
Example 1-2
Metal Catalyst (Mo:V=4.5:1.5)
[0104] The procedure of Example 1-1 was repeated except that the
weight ratio of Mo:V was adjusted to 4.5:1.5.
Example 1-3
Metal Catalyst (Mo:V=4:2)
[0105] The procedure of Example 1-1 was repeated except that the
weight ratio of Mo:V was adjusted to 4:2.
Reference Example 1
Production of CNTs Using Trimetallic Catalyst (Co--Fe--Mo or V)
Reference Example 1-1
Trimetallic Catalyst (Co--Fe--Mo)
[0106] The procedure of Example 1-1 was repeated except that the
weight ratio of Mo:V was adjusted to 6:0.
Reference Example 1-2
Trimetallic Catalyst (Co--Fe--V)
[0107] The procedure of Example 1-1 was repeated except that the
weight ratio of Mo:V was adjusted to 0:6.
[0108] SEM Images
[0109] The CNT aggregates of Example 1-1 and Reference Example 1-1
were observed using FE-SEM (HITACHI S-4800, Cold cathode field
emission gun, 3-stage electromagnetic lens system, SE detector) at
an accelerated voltage of 5 kV, an emission current of 10 .mu.A,
and a working distance of 8 mm, and the SEM images are shown in
FIGS. 1, 2a and 2b, respectively.
[0110] FIG. 1 shows a bulk shape of the CNTs produced in Example
1-1. The CNTs were observed to be potato-like or spherical in bulk
shape.
[0111] The support had an average aspect ratio (As) of 1.2 or less
before the catalytic components and the active components were
supported thereon. The average aspect ratio (A.sub.CAT) of the
catalyst after the catalytic components and the active components
were supported thereon and the average aspect ratio (As) of the
support were shown to satisfy
0.8.ltoreq.A.sub.CAT/As.ltoreq.1.2.
[0112] FIG. 2a shows that the CNT aggregate produced using the
trimetallic catalyst (Co--Fe--Mo) in Reference Example 1-1
consisted of a number of CNTs, which simply got entangled and were
random in shape. In contrast, FIG. 2b shows that the CNT aggregate
produced using the tetrametallic catalyst (Co--Fe--Mo-V) in Example
1-1 consisted of a number of CNTs, which were grown with high
density, had a spherical or potato-like bulk shape, and aggregated
regularly to form bundles or ropes.
[0113] Bulk Densities of the Catalysts
[0114] Each of the catalysts was filled in a measuring cylinder and
weighed.
[0115] The weight was divided by the volume of the measuring
cylinder. As a result, the catalysts of Example 1-1, Example 1-2,
and Example 1-3 were confirmed to have bulk densities of 1.0
g/cm.sup.3, 1.2 g/cm.sup.3, and 1.1 g/cm.sup.3, respectively.
[0116] Bulk Densities of CNTs
[0117] The CNTs were filled in a measuring cylinder and weighed.
The weight was divided by the volume of the measuring cylinder. As
a result, the CNTs of Example 1-1, Example 1-2, and Example 1-3
were confirmed to have bulk densities of 210 kg/m.sup.3, 183
kg/m.sup.3, and 170 kg/m.sup.3, respectively.
[0118] Aspect Ratios
[0119] The longest diameter and the shortest diameter passing
through the center of CNT were measured from the corresponding SEM
image. The aspect ratio of the CNT was determined by dividing the
longest diameter by the shortest diameter. The CNTs of Example 1-1,
Example 1-2, and Example 1-3 were confirmed to have aspect ratios
of 0.90, 0.95, and 1.0, respectively.
[0120] Particle Diameter Distributions (Dcnt)
[0121] Each of the CNT aggregates was allowed to stand in distilled
water for 3 h. The number average particle diameter of the CNT
aggregate was measured in the absorption mode using a particle size
analyzer (Microtrac) and was substituted into Formula 1 to
calculate the particle diameter distribution (Dcnt) of the CNTs.
The CNTs of Example 1-1, Example 1-2, and Example 1-3 were
confirmed to have particle diameter distributions (Dcnt) of 0.88,
0.92, and 0.95, respectively.
[0122] The experimental results are summarized in Table 1.
TABLE-US-00001 TABLE 1 Bulk Bulk Particle Weight density of density
of diameter x y p q ratio of catalyst CNTs Aspect distribution
(mole) (mole) (mole) (mole) Mo:V (g/cm.sup.3) (kg/m.sup.3) ratio
(Dcnt) Example 1-1 30 8 3 6 3:3 1.0 210 0.90 0.88 Example 1-2 30 8
5.5 3.5 4.5:1.5 1.2 183 0.95 0.92 Example 1-3 30 8 6 3 4:2 1.1 170
1.0 0.95
[0123] Reaction Yield Measurement
[0124] The contents of the carbon nanotubes obtained at room
temperature were measured using an electronic scale. The reaction
yield was calculated by substituting the weight of the catalyst
used for CNT synthesis and the total weight after the reaction into
the following expression:
CNT yield (%)=(the total weight after the reaction (g)-the weight
of the catalyst used (g))/the weight of the catalyst used
(g).times.100
[0125] Referring to FIG. 3, the yield of the CNTs produced using
the metal catalyst consisting of Co, Fe, and Mo in Reference
Example 1-1 was about 3500%, and the yield of the CNTs produced
using the metal catalyst consisting of Co, Fe, and V in Reference
Example 1-2 was lower than 5000%. In contrast, the yield of the
CNTs produced using the metal catalyst including Mo and V in a
ratio of 4.5:1.5 in Example 1-2 was 6000% or more, the yield of the
CNTs produced using the metal catalyst including Mo and V in a
ratio of 4:2 in Example 1-3 was 6500% or more, and the yield of the
CNTs produced using the metal catalyst including Mo and V in a
ratio of 3:3 in Example 1-1 was close to 5000%.
Example 2
CNT Yield Depending on Calcination Temperature
Example 2-1
Mo:V=3:3
[0126] The procedure of Example 1-1 was repeated except that the
metal catalyst was calcined at 710.degree. C.
Example 2-2
Mo:V=4.5:1.5
[0127] The procedure of Example 1-2 was repeated except that the
metal catalyst was calcined at 710.degree. C.
Example 2-3
Mo:V=4:2
[0128] The procedure of Example 1-3 was repeated except that the
metal catalyst was calcined at 710.degree. C.
[0129] Referring to FIG. 3, the yield of the CNTs produced in
Example 2-2 was comparable to the yield of the CNTs produced in
Example 1-2. The CNTs were produced using the catalysts including
Mo and V in the same weight ratio (4.5:1.5), except that the
calcination temperatures were different (710.degree. C. in Example
2-2 and 725.degree. C. in Example 1-2). In contrast, the yield
(.gtoreq.7500%) of the CNTs produced in Example 2-3 was higher by
.gtoreq.+1000% than that of the CNTs produced in Example 1-3. The
CNTs were produced using the catalysts including Mo and V in the
same weight ratio (4:2), except that the calcination temperatures
were different (710.degree. C. in Example 2-3 and 725.degree. C. in
Example 1-3). The yield (.about.6900%) of the CNTs produced in
Example 2-1 was higher by .gtoreq.+2000% than that of the CNTs
produced in Example 1-1. The CNTs were produced using the catalysts
including Mo and V in the same weight ratio (3:3), except that the
calcination temperatures were different (710.degree. C. in Example
2-1 and 725.degree. C. in Example 1-1).
Example 3
CNT Yield Depending on Reaction Time
[0130] CNTs were synthesized in the same manner as in Example 1-1,
except that the reaction time was changed. Changes in the CNT yield
as a function of reaction time from 1 to 8 h were observed.
Reference Example 2
CNT Yield Depending on Reaction Time
[0131] CNTs were synthesized in the same manner as in Reference
Example 1-1, except that the reaction time was changed. Changes in
the CNT yield as a function of reaction time from 1 to 8 h were
observed.
[0132] Referring to FIG. 4, when the reaction time was 1 h, the use
of the four-component catalyst (cobalt (Co)-iron (Fe)-molybdenum
(Mo)-vanadium (V)) led to an at least 30% increase in CNT yield
compared to the use of the three-component catalyst (Co--Fe--Mo).
In addition, when the three-component catalyst (Co--Fe--Mo) was
used, the rate of increase in yield steeply decreased with
increasing reaction time. It could be thus expected that the yield
would not be increased any more when the reaction time reaches 8 h.
In contrast, the use of the four-component catalyst (cobalt
(Co)-iron (Fe)-molybdenum (Mo)-vanadium (V)) led to a high yield
and moderately decreased the rate of increase in yield with
increasing reaction time, allowing the reaction to proceed for a
longer time, for example, 8 h or more or 10 h or more, and enabling
the production of CNT bundles in higher yield.
[0133] The present invention can overcome the disadvantage of low
CNT yield encountered in the use of conventional impregnated
catalysts for carbon nanotube production. The supported catalyst of
the present invention has controlled activity and can produce a
controlled amount of a fine powder, enabling the synthesis of
bundle type carbon nanotubes in high yield. Therefore, the
supported catalyst of the present invention can find application in
various fields, such as energy materials, functional composites,
pharmaceuticals, batteries, and semiconductors, where carbon
nanotubes are used.
* * * * *