Method for producing carbon and hydrogen, and carbon fiber

Abstract

Provided is a method for producing carbon and hydrogen, the method comprising: a step for thermally decomposing a hydrocarbon in a first reaction zone to generate carbon and hydrogen; and a step for thermally decomposing a hydrocarbon in a second reaction zone in the presence of the carbon generated in the first reaction zone by using the carbon as a catalyst to obtain carbon and hydrogen, wherein the second reaction zone has a temperature higher than that of the first reaction zone. In the method, the partial pressure of the hydrocarbon in the first reaction zone is 0.02 MPa or more, and the carbon obtained in the second reaction zone has a bulk density of 0.3 g / cm3 or more and an aspect ratio of 2 or more.

Description

Carbon and hydrogen production method, and carbon fiber

[0001] The present invention relates to a method for producing carbon and hydrogen, and to a carbon fiber.

[0002] BACKGROUND ART Conventionally, a process for obtaining hydrogen and carbon by thermal decomposition of hydrocarbons has been known, as disclosed in Patent Document 1.

[0003] WO2022 / 232942A1

[0004] However, in conventional methods, the bulk density of carbon obtained by pyrolysis of hydrocarbons is low, and it sometimes adheres to the wall of the reaction vessel.

[0005] The present invention has been made in view of the above-mentioned problems, and an object of the present invention is to provide a method for producing carbon and hydrogen that can produce carbon with high bulk density, and a novel carbon fiber obtained thereby.

[0006] [1] A process for producing carbon and hydrogen by thermally cracking hydrocarbons in a first reaction zone, and a process for producing carbon and hydrogen by thermally cracking hydrocarbons in a second reaction zone having a higher temperature than the first reaction zone in the presence of the carbon produced in the first reaction zone, using the carbon as a catalyst, wherein the partial pressure of hydrocarbons in the first reaction zone is 0.02 MPa or more, and the bulk density of carbon produced in the second reaction zone is 0.3 g / cm ３a method for producing carbon and hydrogen, wherein the carbon produced in the first reaction zone is a carbon fiber; [2] The method for producing carbon and hydrogen according to [1], wherein the carbon produced in the first reaction zone is a carbon fiber; [3] The method for producing carbon and hydrogen according to [1] or [2], wherein the temperature difference between the first reaction zone and the second reaction zone is 100 to 500°C; [4] The method for producing carbon and hydrogen according to any of [1] to [3], wherein the temperature of the first reaction zone is 500 to 900°C; [5] The method for producing carbon and hydrogen according to any of [1] to [4], wherein the temperature of the second reaction zone is 900 to 1200°C; [6] The method for producing carbon and hydrogen according to any of [1] to [5], wherein a gas containing hydrogen and hydrocarbons discharged from the first reaction zone is supplied to the second reaction zone; [7] The method for producing carbon and hydrogen according to any of [1] to [6], further comprising a heating device for heating at least one of the first reaction zone and the second reaction zone. [8] The method for producing carbon and hydrogen according to any one of [1] to [7], wherein at least one of carbon and hydrocarbons produced in the first reaction zone and supplied to the second reaction zone is heated before being supplied to the second reaction zone. [9] The method for producing carbon and hydrogen according to any one of [1] to [8], wherein the hydrocarbons are thermally decomposed in the presence of a transition metal oxide in the first reaction zone.

[10] The method for producing carbon and hydrogen according to any one of [1] to [9], wherein the hydrocarbons are thermally decomposed in the presence of iron (III) oxide in the first reaction zone.

[11] The method for producing carbon and hydrogen according to any one of [1] to

[10] , wherein the hydrocarbons include methane.

[12] A method for producing carbon and hydrogen according to any one of [1] to

[10] , wherein the bulk density is 0.3 g / cm ３

[13] The carbon fiber according to

[12] , wherein an exothermic peak appears at 500°C or higher in TG-DTA of the carbon fiber in an air atmosphere at 650°C or higher.

[14] The carbon fiber according to

[12] or

[13] , wherein a G / D ratio in Raman spectroscopic analysis of the carbon fiber is 1 or less.

[15] The carbon fiber according to

[12] or

[13] , wherein a specific surface area of ​​the carbon fiber measured by the BET method is 4 m or less. ２

[16] The carbon fiber according to any one of

[12] to

[15] , which is obtained by pyrolysis of hydrocarbons using carbon nanotubes as a catalyst.

[17] The carbon fiber according to any one of

[12] to

[16] , wherein the hydrocarbons include methane.

[0007] According to the present invention, there are provided a method for producing carbon and hydrogen that can produce carbon with high bulk density, and a novel carbon fiber obtained thereby.

[0008] Figure 1(a) is a low-magnification TEM image of carbon α produced in the first reaction zone in Example 1 and used as a catalyst in the second reaction zone, and Figure 1(b) is a high-magnification image of a different field of view from (a). Figure 2(a) is a low-magnification TEM image of carbon A produced in the second reaction zone in Example 1, and Figure 2(b) is a high-magnification image of a different field of view from (a). Figure 3(a) is a low-magnification TEM image of carbon B produced in the second reaction zone in Example 2, and Figure 3(b) is a low-magnification image of a different field of view from (a).

[0009] (Method for Producing Hydrogen and Carbon) The method for producing hydrogen and carbon according to this embodiment includes: (1) a step of pyrolyzing hydrocarbons in a first reaction zone to produce carbon and hydrogen; and (2) a step of pyrolyzing hydrocarbons in a second reaction zone having a higher temperature than the first reaction zone in the presence of the carbon produced in the first reaction zone to obtain carbon and hydrogen.

[0010] (1) Step In this step, hydrocarbons are thermally cracked in a first reaction zone to produce carbon and hydrogen.

[0011] There is no particular limitation on the hydrocarbon, and methane, ethane, etc. can be used, and a mixed gas can also be used. The hydrocarbon can be an aliphatic hydrocarbon or an aromatic hydrocarbon.

[0012] There are no particular limitations on the temperature of the first reaction zone, but from the viewpoint of producing carbon fibers with a large G / D ratio (for example, a G / D ratio of 1 or more), such as carbon nanotubes, a temperature of 500 to 900°C is preferable.

[0013] The partial pressure of the hydrocarbon in the first reaction zone is preferably high from the viewpoint of producing carbon with a high bulk density in the second reaction zone, and is preferably 0.02 MPa or more, may be 0.03 MPa or more, may be 0.05 MPa or more, or may be 0.08 MPa or more. There are no particular limitations on the pressure in the first reaction zone, but it is preferably 0.1 to 5 MPa.

[0014] The gas supplied to the first reaction zone may contain gases other than hydrocarbons. Examples of gases other than hydrocarbons include inert gases such as argon, nitrogen, hydrogen, CO, CO ２ The partial pressure of the gas other than the hydrocarbon is preferably lower than the partial pressure of the hydrocarbon, and may be 1 / 2 or less, 1 / 5 or less, or 1 / 10 or less of the partial pressure of the hydrocarbon.

[0015] In this step, it is preferable to use a catalyst.

[0016] Examples of the catalyst include transition metals and oxides of transition metals. Examples of transition metals include Fe, Ni, Co, Pd, and Pt. The catalyst may be iron (III) oxide. From the viewpoints of increasing the amount of carbon produced in the first reaction zone and increasing the bulk density of carbon produced in the second reaction zone, the catalyst is preferably an oxide of a transition metal, and particularly preferably iron (III) oxide.

[0017] The catalyst particles may be supported on a carrier. The carrier may be porous. Examples of the carrier include α-alumina and γ-alumina.

[0018] There is no particular limitation on the reaction vessel forming the first reaction zone, which may be a fixed bed or a fluidized bed of the catalyst or its support.

[0019] When a catalyst is used, the catalyst may be fed to the reaction vessel in batches, or the catalyst may be continuously discharged from the reaction vessel, regenerated, and then continuously fed to the reactor.

[0020] The heating means for the first reaction zone is not particularly limited, and may be, for example, an electric heater, an infrared heater, a heat transfer tube through which a heat medium such as combustion exhaust gas flows, a microwave heating device, or a gas and / or catalyst preheating device that heats the hydrocarbon gas and / or catalyst before supplying them to the reaction vessel.

[0021] The carbon obtained in the first reaction zone may be carbon fibers such as carbon nanotubes. The carbon fibers may have a diameter of 5 to 200 nm. The carbon fibers may have a G / D ratio of 1 or greater. The carbon nanotubes may be single-walled or multi-walled nanotubes.

[0022] (2) Step In this step, hydrocarbons are thermally decomposed in the presence of carbon produced in the first reaction zone in a second reaction zone having a temperature higher than that of the first reaction zone to obtain carbon and hydrogen.

[0023] The partial pressure of the hydrocarbon in the second reaction zone is preferably 0.02 MPa or more in order to produce carbon with a high bulk density, and may be 0.03 MPa or more, 0.05 MPa or more, or 0.08 MPa or more. The hydrocarbon supplied to the second reaction zone may be the same as or different from the hydrocarbon supplied to the first reaction zone. The partial pressure of the hydrocarbon in the first reaction zone and the partial pressure of the hydrocarbon in the second reaction zone may be the same as or different from each other.

[0024] The temperature difference between the first and second reaction zones may be from 100 to 500°C.

[0025] There are no particular limitations on the temperature of the second reaction zone, but it is preferably 900° C. to 1200° C. There are no particular limitations on the reaction vessel and heating method of the second reaction zone, and examples are as shown for the first reaction zone.

[0026] The pressure in the second reaction zone is not particularly limited, but is preferably 0.1 to 5 MPa. The pressures in the first reaction zone and the second reaction zone may be the same or different.

[0027] A gas containing hydrogen and hydrocarbons discharged from the first reaction zone may be supplied to the second reaction zone, or hydrocarbons supplied from a source other than the first reaction zone may be supplied to the second reaction zone, or these may be mixed and supplied to the second reaction zone.

[0028] When the carbon produced in the first reaction zone is supplied to the second reaction zone, the carbon produced may be supplied to the second reaction zone together with the catalyst, or the carbon produced may be supplied to the second reaction zone after at least a portion of the catalyst has been removed.

[0029] The heating means for the second reaction zone is not particularly limited and may be, for example, an electric heater, an infrared heater, a heat transfer tube through which a heat medium such as combustion exhaust gas flows, or a microwave heating device. The heating means may also be a preheating device that heats at least one of the hydrocarbon-containing gas, catalyst, and carbon discharged from the first reaction zone, or a preheating device that heats hydrocarbons to be supplied to the second reaction zone separately from the first reaction zone.

[0030] The gas supplied to the second reaction zone may contain gases other than hydrocarbons. Examples of gases other than hydrocarbons include inert gases such as argon, nitrogen, hydrogen, CO, CO ２ The partial pressure of the gas other than the hydrocarbon is preferably lower than the partial pressure of the hydrocarbon, and may be 1 / 2 or less, 1 / 5 or less, or 1 / 10 or less of the partial pressure of the hydrocarbon. The gas other than the hydrocarbon supplied to the second reaction zone may be the same as or different from the gas other than the hydrocarbon supplied to the first reaction zone. From the viewpoint of hydrogen utilization, it is also preferable that the partial pressure of the gas other than the hydrocarbon is low.

[0031] Furthermore, there are no particular limitations on the hydrocarbons supplied to the second reaction zone, and methane, ethane, or a mixed gas thereof can be used, and the hydrocarbons may be the same as or different from the hydrocarbons supplied to the first reaction zone.

[0032] The hydrogen produced by this method can be used in a variety of ways, such as for power generation, after being separated from hydrocarbons and other components.

[0033] According to this step, carbon fibers with high bulk density can be obtained.

[0034] The reason for this is not clear, but the following is thought to be the reason.

[0035] By setting the temperature of the first reaction zone to a relatively low temperature, it is possible to produce carbon fibers such as carbon nanotubes having a relatively high G / D ratio (e.g., 1 or more or greater than 1), and it is further thought that by supplying the obtained carbon fibers to a second reaction zone having a higher temperature, it is possible to obtain carbon fibers with a high bulk density using the carbon fibers as a catalyst.

[0036] Even if the temperature is raised only in the first reaction zone without the two-stage reaction, carbon fibers having a low G / D ratio (for example, less than 1) tend to be produced, making it difficult to produce carbon fibers having a high bulk density. Furthermore, in the first reaction zone, the partial pressure of the hydrocarbons in the gas supplied is high, and therefore, the bulk density in the second reaction zone is likely to be less than 0.3 g / cm. ３ The reason for this is also unclear, but one possible reason is that the high partial pressure of the hydrocarbons fed to the first reaction zone makes the structure of the carbon produced in the first reaction zone denser.

[0037] (Carbon fiber) The carbon fiber obtained by the above method has a bulk density of 0.1 g / cm ３ or more, the aspect ratio is 2 or more, and the diameter is less than 1000 nm.

[0038] The bulk density of the carbon fiber is 0.2 g / cm ３ or more, and ３ The bulk density may be 1.0 g / cm or more. ３ It may be the following:

[0039] The aspect ratio of the carbon fibers may typically be 5 or more, and may be 10 or more.

[0040] The diameter of the carbon fiber is less than 1000 nm, and may be 500 nm or less, 300 nm or less, or 200 nm or less. The diameter of the carbon fiber may be 10 nm or more, or 20 nm or more. The diameter and aspect ratio may be the average of measurements at 10 points on a TEM photograph.

[0041] In Raman spectroscopic analysis of the carbon fiber, the G / D ratio may be 1 or less, 0.9 or less, 0.8 or less, or 0.7 or less. This condition indicates that the carbon fiber has many portions that do not exhibit a graphite structure.

[0042] In TG-DTA of the carbon fiber in an air atmosphere, it is preferable that the exothermic peak appearing at 500° C. or higher appears at 650° C. or higher, preferably 700° C. or higher. This means that the carbon fiber is less susceptible to oxidation than usual, and has the advantage of being easy to store and transport.

[0043] The specific surface area of ​​carbon fiber measured by the BET method is 4m ２ / g or more.

[0044] Such carbon fibers have high bulk density, making them easy to handle and easy to add to resins, making them suitable for use as thermal conductivity improvers, conductive additives in battery electrodes, and as materials for asphalt pavement.

[0045] Example 1 (Catalyst A → Carbon α → Carbon A) The iron-supported alumina catalyst used in Example 1 was synthesized according to the following procedure. A γ-alumina support (manufactured by Sumitomo Chemical Co., Ltd. (alumina content 96 wt %, silica content 4 wt %) was crushed in a mortar, and the crushed alumina was classified using metal sieves with 45 μm and 90 μm mesh sizes to obtain a support with a particle size range of 45 to 90 μm. 36.2 g of iron (III) nitrate nonahydrate (manufactured by Kanto Chemical Co., Inc.) was dissolved in 14.0 g of pure water, and 5.0 g of the classified alumina support was added. The mixture was stirred with a stirrer at 80°C for 3 hours. The solvent was evaporated using a rotary evaporator, and the resulting powder was placed in a polytetrafluoroethylene beaker and dried in an oil bath at 120°C for 3 hours. 24.2 g of the dried sample was placed on a porcelain dish and placed in a muffle furnace, and the temperature was raised from room temperature to 900°C at a heating rate of 10°C / min without introducing air from the outside, and then maintained at 900°C for 5 hours. After cooling to room temperature and recovering the solid matter, 9.0 g of Catalyst A in which iron (III) oxide was supported on α-alumina was obtained.

[0046] The synthesis reactions of hydrogen and carbon were carried out according to the following procedure.

[0047] (Production of Carbon α Using Catalyst A in the First Reaction Zone) 0.05 g of catalyst A was dispersed in a 2.0 cm x 7.7 cm quartz sample dish and introduced into a 36.0 cm diameter quartz tube used as the first reaction zone. While flowing methane at a rate of 15 mL / min (i.e., methane partial pressure 0.1 MPa), the temperature was raised to 700°C from the outside using an electric furnace. After the internal temperature reached 700°C, the temperature was maintained for 6 hours. After the reaction, the solid matter (carbon α) on the sample dish in the reactor was collected and weighed. The weight increase calculated by subtracting the weight of the catalyst from the weight of the recovered solid matter was calculated as the carbon produced. The weight ratio of the catalyst A introduced into the first reaction zone was calculated to be 14.7 (g of carbon α / g of catalyst A).

[0048] A TEM photograph of the carbon α obtained in the first reaction zone is shown in Figure 1. It was confirmed that the carbon α obtained in the first reaction zone had a carbon nanotube structure.

[0049] (Production of Carbon A Using Carbon α as a Catalyst in the Second Reaction Zone) The same apparatus as the first reaction zone was used for the second reaction zone. 0.1 g of the carbon α obtained above was dispersed and introduced into a sample dish. While flowing methane at a rate of 15 mL / min, the temperature was raised to 1000°C from an external electric furnace. After the internal temperature reached 1000°C, the temperature was maintained for 3 hours. After the reaction, the solid matter on the sample dish in the reactor was collected, yielding 0.37 g of carbon A. The ratio of the weight gain of the sample dish (carbon A) to the weight of the carbon α catalyst was 3.7. Almost no soot fouling was observed on the inner wall of the reaction tube, and the amount was below the detection limit. The weight gain (carbon α + carbon A) in the two stages and the catalyst A charge ratio was 54.4.

[0050] Carbon α obtained in the first reaction zone and carbon A obtained in the second reaction zone were analyzed for bulk density, thermal analysis by TG-DTA, BET specific surface area, graphite / diamond ratio by microscopic laser Raman analysis, amount of contained elements, etc. The measurement results are summarized in Tables 1 and 2. A TEM photograph is shown in Figure 2. As shown in Figure 2, the aspect ratio of carbon A was 2 or more.

[0051] The various analytical methods are as follows:

[0052] Bulk density: 1 mL or more of sample was added to a 5 mL graduated cylinder equipped with a resin funnel using the funnel, and the volume was measured by reading the volume without tapping. The bulk density of each sample was obtained by dividing the weight of the added sample by the volume.

[0053] TG-DTA thermal analysis: Measurements were performed under the following conditions using a Thermo Plus Evo2 TG-DTA8122 simultaneous differential thermal and thermogravimetric analyzer manufactured by Rigaku Corporation. Sample pan: made of platinum Reference sample: alumina standard sample Measurement conditions: temperature increased from room temperature to 1000°C at a heating rate of 10°C / min Measurement atmosphere: air flow at 500 mL / min Exothermic peak temperature: the temperature at which the endothermic heat in the differential thermal analysis was maximum was used.

[0054] BET specific surface area: Measured using a BELSORP-mini manufactured by Japan BEL Co., Ltd. under the following treatment conditions. Pretreatment device: BELPREP-vac2 manufactured by Japan BEL Co., Ltd. Pretreatment conditions: 2 hours at 120°C, vacuum degassing Measurement conditions: Measurement of adsorption / desorption isotherm with nitrogen using a constant volume method. Adsorption temperature: 77K Adsorbate: Nitrogen Saturated vapor pressure: Actual measurement Adsorbate cross-sectional area: 0.162 nm ２ Sample amount: 50 mg

[0055] Graphite / Diamond Ratio (G / D Ratio): The G / D ratio was measured by microscopic laser Raman spectroscopy of carbon using a RAMAN-11 manufactured by Nanophoton under the following conditions. The G / D ratio was calculated as the peak intensity ratio by measuring the maximum peak intensity in each of the following bands: Graphite band: 1578 cm －１ ~1590cm －１ Diamond - Band: 1339cm －１ ~1357cm －１ Measurement location: Surface Excitation wavelength: 532 nm Grating: 600 grooves / mm Objective lens: ×20, N.A. 0.45 Wavenumber range: 110 cm －１ ~2000cm －１

[0056] (Elemental Analysis) The amounts of elements contained in carbon α and carbon A were calculated as follows. Catalyst A contained 50 wt % Fe atoms, 26 wt % Al atoms, and 24 wt % other components. The amounts of elements contained in the produced carbon α and carbon A were calculated from the total weight of produced carbon and the initial weight of the catalyst, assuming that the catalyst was homogeneously dispersed in the produced carbon material. The contents of elements contained in carbon α and carbon A can also be measured by elemental analysis techniques such as high-frequency inductively coupled plasma atomic emission spectroscopy or X-ray fluorescence analysis.

[0057] Example 2 (Catalyst A → Carbon α → Carbon B) Carbon B was synthesized using carbon α as a catalyst in the same manner as in Example 1, except that the reaction time in the second reaction zone was changed to 6 hours. The ratio of the weight gain of the catalyst boat after storage (carbon B) to the amount of carbon α charged as catalyst was 6.6. Almost no soot fouling was observed on the inner wall of the reaction tube, and the amount was below the detection limit. The ratio of the weight gain in the two stages (carbon α + carbon B) to the catalyst A charge was 97.0.

[0058] The obtained carbon B was analyzed for bulk density, thermal analysis by TG-DTA, and BET specific surface area, and the bulk density and the amount of graphite and diamond specific elements were analyzed by microscopic laser Raman analysis. The measurement results are summarized in Tables 1 and 2. A TEM photograph is shown in Figure 3. As shown in Figure 3, the aspect ratio of carbon B was 2 or more.

[0059] Comparative Example 1 (Carbon γ → Carbon δ) Carbon δ was synthesized by the thermal decomposition of methane in the same manner as in Example 1, using 0.1 g of granular Shirasagi KL (carbon γ), a carbon without a carbon nanotube structure, in the second reaction zone. The ratio of the weight gain of the catalyst boat (carbon δ) to the amount of carbon γ charged as catalyst after the reaction was 1.6. In this reaction, carbon δ' was observed not only on the catalyst boat but also on the inner wall of the reaction tube. When its weight was measured, the ratio of the weight gain (carbon δ') to the amount of carbon γ charged as catalyst was 1.4. (In other words, the ratio of the total carbon weight gain (carbon δ + δ') to the amount of catalyst charged was 3.0.) The carbon γ used as the raw material and the carbon δ obtained on the catalyst board were analyzed for bulk density, thermal analysis by TG-DTA, and BET specific surface area. The bulk density and graphite / diamond ratios were measured by microscopic laser Raman analysis. The measurement results are shown in Tables 1 and 2.

[0060] As is clear from the examples and comparative examples, by introducing carbon obtained by decomposing hydrocarbons in the first reaction zone into the second reaction zone, a carbon material with high bulk density can be produced without adhesion to the reaction vessel.

[0061] The carbon material obtained in this application has a higher bulk density than commercially available carbon nanotubes, yet is characterized by a higher thermal decomposition temperature.

[0062]

[0063]

[0064] Comparative Example 2 (Catalyst A → Carbon ε → Carbon C) (Production of Carbon ε Using Catalyst A in the First Reaction Zone) 0.05 g of catalyst A was dispersed in a 2.0 cm x 7.7 cm quartz sample dish and introduced into a 36.0 cm diameter quartz tube used as the first reaction zone. Methane was introduced at a rate of 15 mL / min and nitrogen at a rate of 285 mL / min (i.e., a methane partial pressure of 0.005 MPa). The temperature was raised to 700°C using an external electric furnace. After the internal temperature reached 700°C, the temperature was maintained for 6 hours. After the reaction, the solid matter (carbon ε) on the sample dish in the reactor was collected and weighed. The weight increase calculated by subtracting the weight of the catalyst from the weight of the recovered solid matter was calculated as the carbon produced. The weight ratio of the carbon produced to the introduced catalyst A was calculated to be 1.1 (carbon ε-g / catalyst A-g). (Production of Carbon C Using Carbon ε as a Catalyst in the Second Reaction Zone) The same apparatus as the first reaction zone was used for the second reaction zone. 0.1 g of the carbon ε obtained above was dispersed and introduced into a sample dish. While flowing methane at a rate of 15 mL / min, the temperature was raised to 1000°C from an external electric furnace. After the internal temperature reached 1000°C, the temperature was maintained for 3 hours. After the reaction, the solid matter on the sample dish in the reactor was collected, yielding 0.403 g of carbon C. The ratio of the weight gain of the sample dish (carbon C) to the weight of carbon ε as a catalyst was 4.1. The weight gain in the two stages (carbon ε + carbon C) and the catalyst A charging ratio were 9.1.

[0065]

Claims

1. A process for producing carbon and hydrogen by thermally decomposing hydrocarbons in a first reaction zone; and a process for thermally decomposing hydrocarbons in a second reaction zone having a higher temperature than the first reaction zone using the carbon as a catalyst in the presence of the carbon produced in the first reaction zone to obtain carbon and hydrogen, wherein the partial pressure of the hydrocarbons in the first reaction zone is 0.02 MPa or more, and the bulk density of the carbon produced in the second reaction zone is 0.3 g / cm. ３ The above method for producing carbon and hydrogen, wherein the aspect ratio is 2 or more.

2. The method for producing carbon and hydrogen according to claim 1, wherein the carbon produced in the first reaction zone is carbon fiber.

3. The method for producing carbon and hydrogen according to claim 1 or 2, wherein the temperature difference between the first reaction zone and the second reaction zone is 100-500°C.

4. The method for producing carbon and hydrogen according to claim 1 or 2, wherein the temperature of the first reaction zone is between 500°C and 900°C.

5. The method for producing carbon and hydrogen according to claim 1 or 2, wherein the temperature of the second reaction zone is between 900°C and 1200°C.

6. The method for producing carbon and hydrogen according to claim 1 or 2, wherein a gas containing hydrogen and a hydrocarbon discharged from the first reaction zone is supplied to the second reaction zone.

7. The method for producing carbon and hydrogen according to claim 1 or 2, further comprising a heating device for heating at least one of the first reaction zone and the second reaction zone.

8. The method for producing carbon and hydrogen according to claim 1 or 2, wherein at least one of the carbon and the hydrocarbons produced in the first reaction zone and supplied to the second reaction zone is heated before being supplied to the second reaction zone.

9. The method for producing carbon and hydrogen according to claim 1 or 2, wherein the hydrocarbon is thermally decomposed in the presence of an oxide of a transition metal in the first reaction zone.

10. The method for producing carbon and hydrogen according to claim 1 or 2, wherein the hydrocarbons are thermally cracked in the presence of iron(III) oxide in the first reaction zone.

11. The method for producing carbon and hydrogen according to claim 1 or 2, wherein the hydrocarbon comprises methane.

12. Bulk density is 0.3 g / cm ３ or more, having an aspect ratio of 2 or more and a diameter of less than 1000 nm.

13. The carbon fiber according to claim 12, wherein in TG-DTA of the carbon fiber in an air atmosphere, an exothermic peak appears at 500° C. or higher and 650° C. or higher.

14. The carbon fiber according to claim 12 or 13, wherein the G / D ratio of the carbon fiber is 1 or less in Raman spectroscopic analysis.

15. The specific surface area of ​​the carbon fiber measured by the BET method is 4 m ２ The carbon fiber according to claim 12 or 13, wherein the carbon fiber has a viscosity of 1 / g or more.

16. Carbon fiber according to claim 12 or 13, obtained by pyrolysis of hydrocarbons using carbon nanotubes as a catalyst.

17. The carbon fiber of claim 16, wherein the hydrocarbon comprises methane.

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