JP2003138431A - Carbon nanofiber and method for producing the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a new carbon nanofiber. SOLUTION: This carbon nanofiber has >=0.9 strength ratio of D band to G band in a Raman spectrum.

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention relates to a carbon nanofiber having a unique microstructure and a method for producing the same. The carbon nanofiber according to the present invention has a high degree of graphitization and is suitable for use as a conductor, a heat conductor, a catalyst carrier, a graphite lubricant, a host material of a graphite intercalation compound, and the like.

[0002]

2. Description of the Related Art Many reports have been made on the production of fibrous carbon, so-called carbon nanofiber, in the gas phase from hydrocarbons such as benzene and methane, or carbon monoxide. For example, carbon monoxide has a microstructure called a platelet in which graphite surfaces are laminated perpendicularly to the fiber axis (Patent No. 2890548).
(See Japanese Patent Laid-Open Publication No. JP-A-2003-13865), a structure having a herringbone structure laminated in a V shape, or a structure having a ribbon structure having a cylindrical shape parallel to the fiber axis (RT.
l. , Journal of Catalysis, 1
69, 212 (1997)) and the like. However, conventionally reported production of carbon nanofibers from carbon monoxide often produces a mixture of carbon nanofibers having various microstructures mixed with non-fibrous carbon. Further, carbon nanofibers from carbon monoxide that have been reported so far have a large crystallite size in the graphite layer inward direction and a small edge ratio.

The optimum temperature of the reaction for producing carbon nanofibers from carbon monoxide is about 600 ° C. Four
A mixture of amorphous carbon and fibrous carbon is likely to be formed at a low temperature of 00 ° C. or lower, while a plate-like substance is dominant at a high temperature, and it is difficult to preferentially form carbon nanofibers. ing. In addition, the carbon deposition reaction from carbon monoxide is an exothermic reaction and is an equilibrium reaction, so at high temperatures, the reaction equilibrium reduces the production rate of carbon nanofibers. On the contrary, the reaction at a low temperature is advantageous in terms of reaction equilibrium, but the reaction rate decreases. Therefore, it has been considered difficult to industrially produce carbon nanofibers using carbon monoxide as a raw material, from both aspects of the reaction and generation of the carbon nanofibers.

[0004]

Accordingly, the present invention is intended to provide a method for selectively producing carbon nanofibers from carbon monoxide. Further, the present invention aims to provide a carbon nanofiber having a novel microstructure, particularly a carbon nanofiber having a small crystallite size in a graphite layer and a large edge ratio, and a method for producing the same.

[0005]

The carbon nanofiber according to the present invention is characterized in that the intensity ratio of the D band to the G band in the Raman spectrum is 0.9 or more. And, this carbon nanofiber can be produced by causing a carbon deposition reaction in a mixed gas of carbon monoxide and hydrogen in the presence of preliminarily prepared metal particles of Groups 8 to 10 of the periodic table. it can.

[0006]

BEST MODE FOR CARRYING OUT THE INVENTION The carbon nanofiber according to the present invention has a Raman spectrum of D to G band
The band intensity ratio is 0.9 or more. The Raman spectrum of a carbon compound having a graphite structure is generally 15
It has an absorption in the vicinity of 80 cm ^-1 and near 1350 cm ^-1. The former is said to correspond to the E2g mode of the graphite surface, the G band, the latter is the edge mode,
It corresponds to the der mode and is called the D band. The larger the intensity ratio of the D band to the G band, the smaller the crystallite size in the graphite layer inward direction, and the larger the edge ratio of graphite, that is, the ratio of the outer surface of the end.

The carbon nanofiber according to the present invention has
Despite being formed at a low temperature of 000 ° C or less, the graphitization degree is high, and the graphitization degree calculated by the following formula based on the carbon structure model of Mering and Maire is usually 8
It is 0% or more, and in many cases 90% or more.

[0008]

[Formula 2] Graphitization degree = [{3.44−interlayer distance of graphite (Å)} / 0.086] × 100 (%)

Here, the interlayer distances are Mering and Mai.
It is the inter-plane distance (d ₀₀₂ ) measured by XRD based on the carbon structure model of re (Shin Carbon Industry, page 65, Modern Editing Company, published in 1980). Carbon nanofibers having a graphitization degree of 80% or more are difficult to obtain by the conventionally reported vapor phase growth method. For example, Japanese Patent No. 2890584
According to the method described in the publication, the degree of graphitization of carbon nanofibers obtained by vapor-phase growth from a mixed gas of carbon monoxide, hydrogen and iron pentacarbonyl (Fe (Co) ₅ ) is about 72%. A graphitization degree of 80% or more, particularly 90% or more is the highest graphitization degree among those obtained by the vapor phase growth method. A cylindrical carbon nanofiber having a ring-shaped cross section and a hollow portion is used. It is a level that is finally achieved by treating at 2400 ° C. Also, if it is an industrially produced carbon fiber, the mesophase pitch type carbon fiber, which is said to have a high degree of graphitization, is used at
This is the level achieved by heat treatment in.

The carbon nanofiber according to the present invention is
Structurally usually has a platelet structure in which graphite planes are laminated substantially perpendicular to the fiber axis,
In addition, it does not have a hollow portion as seen in ordinary carbon nanofibers. The cross section perpendicular to the fiber axis is not a circle but a rectangle or a flat ellipse, and the major axis of the cross section is 10 to 800 nm. The ratio of the length of the major axis to the minor axis of the cross section is usually 2 or more, and in many cases 5 to 15. The length of the fiber is 1-1000 μm,
In particular, the thickness is 5 to 100 μm, and the shape thereof is various, such as a relatively straight one, a curved one, and a bent one.

The carbon nanofiber according to the present invention is
It can be produced by using particles of metals of Groups 8 to 10 prepared in advance as a catalyst and bringing them into contact with a mixed gas of carbon monoxide and hydrogen. Since carbon nanofibers themselves are produced from carbon monoxide, it is possible to use a raw material gas that does not contain hydrogen, but the coexistence of hydrogen enhances the catalytic activity and reacts with carbon dioxide produced as a by-product to produce water. Therefore, the coexistence of hydrogen promotes the carbon deposition reaction from carbon monoxide by decreasing the carbon dioxide in the reaction system and increases the production amount of carbon nanofibers. However, when the ratio of hydrogen in the raw material gas is increased, the partial pressure of carbon monoxide is lowered and the reaction rate is lowered. Therefore, hydrogen is contained in an amount of 0.5 to 10 times mol, particularly 0.5 to 3 times, relative to carbon monoxide. It is preferable to use it so as to have a double mole.
A source gas consisting essentially of carbon monoxide and hydrogen is usually used, but other gases may coexist. Examples of such a gas include rare gases such as helium and argon of Group 18 of the periodic table, nitrogen, oxygen, and water vapor. Further, hydrocarbons and hydrocarbons containing heteroatoms such as oxygen, nitrogen, sulfur and phosphorus are also included.

In the present invention, a transition metal such as titanium, vanadium, manganese, iron, cobalt, nickel, yttrium, niobium, molybdenum, ruthenium, rhodium, palladium, tantalum, dangsten, rhenium, iridium or platinum is used as the catalyst. These may be used alone or as an alloy. Among these, it is preferable to use a metal of Group 8 to 10 of the periodic table as a simple substance or an alloy. Of these, iron, cobalt and nickel are preferable, and iron is most preferable. It is necessary to use the catalyst as preliminarily prepared fine particles. For example, Japanese Patent No. 2890
In the method for producing carbon nanofibers while forming fine metal particles in the reaction system by using the source gas containing iron pentacarbonyl as described in the examples of Japanese Patent No. 548, the carbon nanofibers according to the present invention are used. Can't get Further, the carbon nanofiber according to the present invention cannot be obtained even by using a catalyst in which the above-mentioned metal is supported on a carrier such as silica or alumina as in a usual catalytic reaction.

The metal particles of the catalyst have a major axis of 1 μm when observed under a microscope.
It is preferable that substantially less than m is not included. Major axis 1
If the metal particles containing a large amount of particles less than μm are used as the catalyst, the production selectivity of the carbon nanofibers according to the present invention is poor. The method for producing the metal particles used as the catalyst is arbitrary, and for example, those produced by pulverizing a metal block and then removing fine powder can be used. Further, reduction of a precursor metal salt or metal compound, for example, a precipitate formed by adding an aqueous alkali solution to an aqueous solution of metal nitrate is dried and calcined to form a metal oxide, which is crushed and then reduced to produce metal particles. It can also be used.

The carbon nanofibers can be produced by accommodating the metal particles obtained as described above in a reactor and supplying a raw material gas containing carbon monoxide thereto. The amount of raw material gas supplied is usually 1 ml per 1 g of metal particles.
/ Min to 10 ⁴ ml / min, preferably 10 ml / min to 2 ×
It is in the range of 10 ³ ml / min. In addition to the method of preliminarily accommodating the catalyst in the reactor, metal particles of the catalyst are continuously supplied to the reactor together with the raw material gas using a kiln, that is, an inclined rotating cylindrical reactor. It is also possible to use a method of continuously discharging the carbon nanofibers simultaneously produced from the reactor. In this case, 1 ml to 2 × 10 ⁴ ml of raw material gas may be supplied per 1 g of metal particles. The reaction temperature is usually 400 to 1000 ° C., but 4
50-1000 degreeC, especially 550-850 degreeC are preferable.
The reaction pressure is usually normal pressure.

[0015]

EXAMPLES The present invention will be described in more detail with reference to the following examples. Example 1 Preparation of catalyst; iron nitrate _{(Fe (NO 3) 3 ·} 9H 2 O) 10
g was dissolved in 200 ml of water. A saturated aqueous solution of ammonium carbonate was added dropwise to this aqueous solution with stirring to adjust the pH to 9-1.
The dropping was stopped when 0 was reached. After standing at room temperature for 1 hour, the precipitate was collected by filtration and washed with 500 ml of water. This was air-dried and then fired in air at 400 ° C. for 2 hours. After cooling, reduction treatment was performed with hydrogen gas at 400 ° C. for 2 hours. After cooling to room temperature under hydrogen atmosphere, 2
Treated with nitrogen gas containing% oxygen. This was crushed and sieved to obtain metallic iron particles having a particle size of 90 to 250 μm.

Production of carbon nanofibers: 0.3 g of the iron particles prepared above was dispersed and placed in a quartz reaction tube (40 mmφ × 480 mmL). A mixed gas of carbon monoxide and hydrogen (volume ratio 1: 1, 1 atm) was added to this for 64 m.
The temperature was raised to 650 ° C. while supplying at a flow rate of 1 / min, and the temperature was maintained at this temperature for 129 minutes and then cooled. 1.297 g of carbon nanofibers were recovered. The degree of graphitization of this product was 90%. The intensity ratio of the D band to the G band was 1.04. The Raman spectrum was measured and the intensity ratio was calculated as follows.

Measurement of Raman spectrum: NR180 manufactured by JASCO Corporation equipped with Ar ⁺ laser and OCD detector
Type 0 was used. Excitation wavelength; Ar ⁺ 514.5 nm Irradiation diameter; 100 μmφ Excitation output; 5 mW (a) Material position exposure time; 180 seconds × 2 times slit; S1, S5 = 400 μm, S2 = 24000
μm

Calculation of intensity ratio; G band is 1580 cm ^-1
Absorbs in the vicinity, and the D band absorbs in the vicinity of 1350 cm ^-1 . There is also unspecified absorption near 1620 cm ^-1 . Therefore, in the range of 1650 to 1500 cm ^-1 including the band near 1620 cm ^-1 and the G band, the baseline of the peak is drawn and the intensity (area) of the G band is calculated after dividing into two peaks (curve fitting). It was Next is 1420 cm for the D band
^The baseline of the peak was drawn in the range of ^{−1 to} 1270 cm ⁻¹ , and curve fitting was performed with one band, and then the intensity (area) of the D band was obtained. Finally G from the strength of both
The intensity ratio of the D band to the band was calculated.

A scanning electron micrograph of the obtained carbon nanofibers is shown in FIG. 1, and a transmission electron micrograph is shown in FIG. From FIG. 1, it was found that flat carbon nanofibers having a long axis of the fiber cross section of about 200 nm were produced. Although the length of the minor axis of the cross section is not clear, it is estimated to be about 20 nm. It was also found from FIG. 2 that the fiber has no hollow portion and has a platelet structure having a graphene structure perpendicular to the fiber axis.

Example 2 Preparation of catalyst: In the preparation of the catalyst of Example 1, 10 g of iron nitrate and 1.8 g of nickel nitrate (Ni (NO ₃ ) ₂ .6H ₂ O) were used instead of 10 g of iron nitrate. An iron-nickel catalyst was prepared in exactly the same manner as in Example 1 except that the above was used.

Production of carbon nanofibers; Example 1
In, the reaction was carried out in exactly the same manner as in Example 1 except that 0.3 g of the iron-nickel catalyst obtained above was used instead of the iron catalyst. The recovery amount of carbon nanofibers is 1.
The degree of graphitization was 100%, and the intensity ratio of the D band to the G band was 1.14. Further, according to the scanning electron microscope photograph, the length of the major axis of the fiber cross section was about 200 nm. Although the length of the short axis is not clear, it is estimated to be about 20 nm.

Comparative Example 1 Mixed gas of carbon monoxide and hydrogen (volume ratio 1: 1, atmospheric pressure)
Was bubbled through a pentacarbonyliron (Fe (CO) ₅ ) solution cooled to 0 ° C. at a flow rate of 64 ml / min, and then the solution was supplied to the same quartz reaction tube as that used in Example 1, and 850
The reaction was allowed to proceed at 129 ° C. for 129 minutes. 500 mg of carbon nanofibers were collected. The graphitization degree of this product is 72
%, And the intensity ratio of the D band to the G band was 0.53. According to a scanning electron micrograph, this product has a length of the major axis of the fiber cross section of 20 as in the first embodiment.
It was about 0 nm.

[Brief description of drawings]

FIG. 1 is a scanning electron micrograph of carbon nanofibers. One scale is 150 nm.

FIG. 2 is a transmission electron micrograph of carbon nanofibers.

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Claims (7)

[Claims] 1. A carbon nanofiber characterized in that the intensity ratio of the D band to the G band in the Raman spectrum is 0.9 or more. 2. The graphitization degree defined by the formula (1) is 80%.
It is above, The carbon nanofiber of Claim 1 characterized by the above-mentioned. [Formula 1] Graphitization degree = [{3.44−interlayer distance of graphite (Å)} / 0.086] × 100 (%) (1) 3. The cross section perpendicular to the fiber axis is rectangular or flat elliptical, and the major axis length of the cross section is 10 nm to 800.
2. The vapor-grown carbonaceous fiber having a thickness of nm and a ratio of the length of the long axis to the length of the short axis of 2 or more.
Alternatively, the carbon nanofiber according to item 2. 4. A metal particle prepared from a metal of Groups 8 to 10 of the Periodic Table is housed in a reactor, the reactor is kept at a high temperature, and a mixed gas of carbon monoxide and hydrogen is supplied to the reactor. 4. The method for producing carbon nanofiber according to claim 1, wherein 5. Particles of a metal of Groups 8 to 10 of the Periodic Table,
The method for producing carbon nanofibers according to claim 1, wherein the mixed gas of carbon monoxide and hydrogen is supplied to a reactor maintained at a high temperature. 6. The carbon nanofiber according to claim 4, wherein the mixed gas of carbon monoxide and hydrogen contains 0.5 to 10 times mol of hydrogen with respect to carbon monoxide. Production method. 7. The method for producing carbon nanofiber according to claim 4, wherein the reactor is maintained at 450 to 1000 ° C.