WO2007069267A2

WO2007069267A2 - Method and appartus for synthesis of carbon nanotubes

Info

Publication number: WO2007069267A2
Application number: PCT/IN2006/000403
Authority: WO
Inventors: Gaurav Namdhar
Original assignee: Gaurav Namdhar
Priority date: 2005-10-07
Filing date: 2006-10-06
Publication date: 2007-06-21
Also published as: WO2007069267A3

Abstract

The present invention describes a method and an apparatus for single-step, low cost and large scale synthesis of carbon nanotubes. The method describes synthesis of carbon nanotubes utilizing inorganic precursors for availability of required catalyst in-situ in a nanotube growth environment.

Description

METHOD AND APPARATUS FOR SYNTHESIS OF CARBON NANOTUBES

TECHNICAL FIELD

The present Invention relates generally to the large scale production of Carbon Nanotubes (CNT). Specifically it relates to a method and apparatus for single step continuous or semi-continuous synthesis of CNT, utilizing inorganic precursors for availability of required catalyst in situ in the nanotube growth environment.

BACKGROUND ART

CNT are long and thin hollow cylinders, made of carbon atoms. CNT are rolled tubes of graphite in appearance having a hexagonal network of carbon atoms forming seamless tubes which may or may not be capped with fullerene like structure having pentagons at ends. CNT are known for their remarkable mechanical, electronic, thermal and other properties. CNT are mainly of two types, Single Wall Carbon Nano-tube (SWCNT) and Multi Walled Carbon Nano-tube (MWCNT), depending on number of concentric tubes involved in forming the structure. CNT were first discovered and reported in 1991 by Sumio Ijima, in an arc discharge.

Various processes for producing CNT exist such as arc discharge, laser ablation, CVD, gas phase and plasma torch etc.

In arc discharge process, arc evaporates graphitic or composite anode (graphitic electrode doped with transition metals like Fe, Ni, Co etc.), which further yield CNT upon condensation on cathode and / or chamber walls. Arc discharge process can be performed in various environments including air, inert gases and even liquids. These processes have high power requirement and provide lower yield. Further, the product from the arc discharge process is difficult to purify.

In laser ablation process, a graphitic or composite (graphite doped with transition metals like Fe, Ni, Co etc.) target is evaporated using pulsed or continuous laser in various environments. CNT are obtained by condensation of evaporated graphitic or composite target on the walls of laser ablation chamber. These processes have high

i composite target on the walls of laser ablation chamber. These processes have high power requirement and provide lower yield. Further, the product from the arc discharge process is difficult to purify.

In plasma torch process, gaseous carbon precursors like hydrocarbon and organo-metallic catalyst precursors like metallocenes, metal carbonyls are subjected to plasma generated either by arc or microwave, which further yield CNT. These methods could be understood as improvement over arc discharge methodology which essentially works with solid state precursors. However, due to using expensive organo-metallic precursors and the product which contains various carbon species that are more stable than CNT, the product of the plasma torch process are difficult to purify and are not suitable for industrial scale production.

Chemical Vapor deposition (CVD) and gas phase processes are essentially dependent on catalytic growth of nanotubes from carbon feeds like hydrocarbons and CO upon nanometer size transition metals acting as catalyst.

CVD processes use prior fabricated solid state catalyst, which is further fed in to a

CNT growth reactor or furnace for CNT growth. Gas phase processes utilize organo- metallic precursors to generate required catalyst in-situ in the CNT growth reactor itself.

CVD methods are essentially multi-step process because of involvement of at least two process blocks; one for the fabrication of catalyst and other for the growth of CNT. Gas phase methods can be considered as single-step process where a single process block is responsible for generation of catalyst as well as nanotubes unlike CVD.

Presently inorganic precursors like transition metal compounds and salts are used to prepare solid state catalyst to feed in to CVD processes for large scale synthesis. The decomposition of inorganic precursors results in highly oxidative decomposition product which can be detrimental to CNT growth, especially in large scale production where significant amount of catalyst is needed for sizeable production. Therefore an inorganic precursor based system is essentially multi-step, where precursors are decomposed first out side CNT growth reactor and then solid state catalyst having very low or zero oxidation potential are fed into CNT growth reactor.

Further, single-step process like gas phase methods essentially utilizes organo metallic precursors which don't complicate CNT growth because of their oxidation potential.

In catalytic growth of CNT, it is apparent that a single step process based on inorganic precursors can provide a way for very cost effective large scale production. As catalytic growth methods, especially single-step methods have already shown their potential for large scale production; however the use of organo-metallic compounds comes on the expensive side. Further, inorganic precursor based single-step method would also be cost effective in comparison to inorganic precursor based multi-step methods due to lower handling and processing cost.

DISCLOSURE OF INVENTION

An object of the present invention is synthesis of carbon nanotubes.

Another object of the present invention is to provide a method and an apparatus for single-step, low cost and large scale synthesis of carbon nanotubes.

Yet another object of the present invention is to provide a solid-gas phase synthesis of carbon nanotubes, where catalyst is generated in-situ utilizing inorganic precursor.

Yet another object of the present invention is synthesis of carbon nanotubes which is easy in purification and its use in various applications.

BRIEF DESCRIPTION OF THE DRAWINGS

The preferred embodiments of the invention will hereinafter be described in conjunction with the appended drawings provided to illustrate and not to limit the invention, wherein like designations denote like elements, and in which:

FIG. 1 is a block diagram illustrating a system for the production of carbon nanotubes in accordance with an embodiment of the present invention. FlG. 2 is a flow chart illustrating a method for production of carbon nanotubes in accordance with an embodiment of the present invention.

FIG. 3 is a flow chart illustrating a method for production of carbon nanotubes in accordance with another embodiment of the present invention.

FIG. 4 is a Scanning Electron Microscopy (SEM) image of carbon nanotube material produced in accordance to an embodiment of the present invention.

FIG. 5 is a Raman spectra plot of carbon nanotube material produced in accordance to an embodiment of the present invention.

DESCRIPTION OF PREFERRED EMBODIMENTS While the preferred embodiments of the invention have been illustrated and described, it will be clear that the invention is not limited to these embodiments only. Numerous modifications, changes, variations, substitutions and equivalents will be apparent to those skilled in the art without departing from the spirit and scope of the invention as described in the claims.

FIG. 1 is a block diagram illustrating an apparatus 100 for the production of carbon nanotubes in accordance with an embodiment of the present invention. Apparatus 100 is a growth reactor capable of growing carbon nanotubes. Apparatus 100 is attached with an inorganic precursor input module 101 and a support material input module 103. Inorganic precursor input module 101 helps in introducing inorganic precursor. Examples of inorganic precursor are iron nitrate, iron acetate and the like in accordance with an embodiment of the present invention. In accordance with another embodiment of the present invention the inorganic precursor may be a mixture of two or more inorganic precursors along with water. Support material input module 103 is used to introduce the support material into the growth reactor. The support material is selected from a group comprising TiO₂, MgO, AI₂O₃, SiO₂ and the like. Support material provides required surface area for heterogeneous stabilization of liquid phase precursor and it further provides nucleation sites for decomposing inorganic precursor material. Use of support is not a necessary condition for nanotube growth but helps in achieving higher production by significantly improving available surface area for heterogeneous catalyst nucleation. Apparatus 100 is further attached with a reactant gas inlet module 105. Reactant gas input module 105 is utilized to introduce reactant gas flowing in counter to inorganic precursor and support. The reactant gas is selected from the group comprising methane, ethane, propane, ethylene, propylene, butane, liquefied petroleum gas, natural gas and the like.

Apparatus 100 is a concentric tubular in shape, comprising an inner tube and an outer tube. Inner tube allows decomposition and partial reduction of inorganic precursor material in a counter flow of gases. Nanotube growth takes place in a reducing atmosphere provided by the reactant gas in annular space. The decomposition products of inorganic precursor material have significant oxidation potential which can prove to be detrimental for nanotubes growth. The annular nature of apparatus 100 allows the decomposition and partial reduction of inorganic precursor material to take place without mixing with the reducing atmosphere required for nanotube growth. G1 in apparatus 100 is exhaust containing highly oxidative gaseous product resulting from the decomposition of inorganic precursor material. G2 in apparatus 100 is exhaust containing unused hydrocarbon and gaseous products resulting due to dehydrogenation of hydrocarbons. Annular space could support growth of nanotubes on catalyst clusters generated in inner tube in plug flow, fluidized bed or moving bed designs.

FIG. 2 is a flow chart illustrating a method for production of carbon nanotubes in accordance with an embodiment of the present invention. At step 201, support material is introduced into apparatus 100. Examples of support material is selected from the group comprising Magnesium Oxide (MgO), Aluminum Oxide (AI₂O₃), Titanium-di-oxide (TiO₂), Silicon-di-oxide (SiO₂) and the like. At step 203, inorganic precursor material is introduced into apparatus 100. Examples of inorganic precursor material comprises one or more compound selected from a group comprising transition metal nitrate, transition metal acetate transition metal sulphates, transition metal chlorides, and the like in accordance with an embodiment of the present invention. The inorganic precursor material further comprises one or more compounds selected from a group comprising nitrates, sulphates, chlorides and acetates of magnesium, titanium and aluminum. The inorganic precursor material may further comprise additives selected from a group comprising urea, citric acid, glucose, glycerol, ammonia, ammonium nitrate, toluene, benzene and xylene. In accordance with an embodiment of the present invention, the inorganic precursor material is selected from the group comprising ferric nitrate (Fe(NO₃)₃), ferric acetate (Fe(CH₃COO)₃), and the like. In accordance with another embodiment of the present invention the inorganic precursor may be a mixture of two or more inorganic precursors along with water.

At step 205, reactant gas is introduced into growth reactor 100. In accordance with an embodiment of the present invention the reactant gas comprises one or more gases selected from the group comprising methane, ethane, propane, butane, ethylene, propylene, acetylene, carbon monoxide and the like. In accordance with another embodiment reactant gas comprises commercial fuel gases such as liquefied petroleum gas, natural gas and the like. In accordance with another embodiment of the present invention reactant gas may additionally comprise gases such as argon, neon, nitrogen and hydrogen. At step 207, the inorganic precursor material is decomposed to metal oxide in fine clusters. The particle size of the fine clusters ranges from few nanometers to few microns. At step 209, the decomposed inorganic precursor material is partially reduced to metallic clusters. The metallic clusters help in initiating the growth of carbon nanotubes. The catalytic reaction of reactant gas over metallic clusters leads to growth of carbon nanotubes at step 211.

FIG. 3 is a flow chart illustrating a method for production of carbon nanotubes in accordance with an embodiment of the present invention. At step 301 , inorganic precursor material is introduced into apparatus 100. Examples of inorganic precursor material comprises one or more compound selected from a group comprising transition metal nitrate, transition metal acetate transition metal sulphates, transition metal chlorides, and the like in accordance with an embodiment of the present invention. The inorganic precursor material further comprises one or more compounds selected from a group comprising nitrates, sulphates, chlorides and acetates of magnesium, titanium and aluminum. The inorganic precursor material may further comprise additives selected from a group comprising urea, citric acid, glucose, glycerol, ammonia, ammonium nitrate, toluene, benzene and xylene. In accordance with an embodiment of the present invention, the inorganic precursor material is selected from the group comprising ferric nitrate (Fe(NO₃)₃), ferric acetate (Fe(CH₃COO)₃), and the like. In accordance with another embodiment of the present invention the inorganic precursor may be a mixture of two or more inorganic precursors along with water.

At step 303, reactant gas is introduced into growth reactor 100. In accordance with an embodiment of the present invention the reactant gas comprises one or more gases selected from the group comprising methane, ethane, propane, butane, ethylene, propylene, acetylene, carbon monoxide and the like. In accordance with another embodiment reactant gas comprises commercial fuel gases such as liquefied petroleum gas, natural gas and the like. In accordance with another embodiment of the present invention reactant gas may additionally comprise gases such as argon, neon, nitrogen and hydrogen. At step 305, the inorganic precursor material is decomposed to metal oxide in fine clusters. The particle size of the fine clusters ranges from few nanometers to few microns. At step 307, the decomposed inorganic precursor material is partially reduced to metallic clusters. The metallic clusters help in initiating the growth of carbon nanotubes. The catalytic reaction of reactant gas over metallic clusters leads to growth of carbon nanotubes at step 309.

A sample of carbon nanotubes produced by the process in accordance with an embodiment of the present invention was collected and was investigated by a scanning electron microscope (SEM). Fig. 4 is an SEM image showing bundle of carbon nanotubes.

FIG. 5 is a Raman spectra plot of carbon nanotube material produced in accordance to an embodiment of the present invention. The peaks at 1584 cm^"1 and 2707 cm^"1 in FIG. 6 clearly confirms the presence of carbon nanotubes. Further, the side peak at 1349 cm^"1 indicates the existence defects and impurities.

While the preferred embodiments of the invention have been illustrated and described, it will be clear that the invention is not limited to these embodiments only. Numerous modifications, changes, variations, substitutions and equivalents will be apparent to those skilled in the art without departing from the spirit and scope of the invention as described in the claims.

Claims

What is claimed is:

1. A method for producing Carbon Nanotubes (CNT) in a single step growth reactor, the single step growth reactor being used to carry out various process for the growth of CNT, the method comprising the steps of: a. introducing a support material in the growth reactor; b. introducing an inorganic precursor material in the growth reactor; c. introducing a reactant gas counter flowing to the inorganic precursor and support in the growth reactor; d. heating the support material, precursor material in the presence of the reactant gas in the growth reactor; and e. growing carbon nanotubes in the growth reactor.

2. The method of claim 1 , wherein the support material is selected from the group comprising Magnesium Oxide (MgO), Aluminum Oxide (AI2O3), Titanium-di-oxide (TiO₂), Silicon-di-oxide (SiO₂) and the like.

3. The method of claim 1 , wherein the inorganic precursor material comprises a compound selected from a group comprising transition metal nitrate, transition metal acetate transition metal sulphates, transition metal chlorides, and the like.

4. The method of claim 1 , wherein the inorganic precursor material is a liquid solution, the liquid solvent selected from a group comprising water, methanol, ethanol, propanol, isopropanol and the like.

5. The method of claim 1 , wherein the inorganic precursor material further comprises a compound selected from a group comprising nitrates, sulphates, chlorides and acetates of magnesium, titanium and aluminum.

6. The method of claim 1 , wherein the inorganic precursor material further comprises additives selected from a group comprising urea, citric acid, glucose, glycerol, ammonia, ammonium nitrate, toluene, benzene and xylene.

7. The method of claim 1 , wherein the precursor material is selected from the group comprising ferric nitrate (Fe(NO₃)3), ferric acetate (Fe(CH₃COO)₃), and the like.

8. The method of claim 1 , wherein the reactant gas is selected from the group comprising methane, ethane, propane, ethylene, propylene, butane, liquefied petroleum gas, natural gas and the like.

9. The method of claim 1 , wherein the growth reactor is preheated to a temperature in the range of 500-1200⁰C.

10. The method of claim 1, wherein in the precursor material is introduced in the growth reactor in the form of liquid, spray or aerosol.

11. A method for producing Carbon Nanotubes (CNT) in gas phase in a growth reactor, the growth reactor being used to carry out various process for the growth of CNT, the method comprising the steps of: a. introducing an inorganic precursor material in the growth reactor; b. introducing a reactant gas in the growth reactor; c. heating the inorganic precursor material in the presence of the reactant gas in the growth reactor; and d. growing carbon nanotubes in the growth reactor.

12. The method of claim 11, wherein the inorganic precursor material comprises a compound selected from a group comprising transition metal nitrate, transition metal acetate transition metal sulphates, transition metal chlorides, and the like.

13. The method of claim 11, wherein the inorganic precursor material further comprises a compound selected from a group comprising nitrates, sulphates, chlorides and acetates of magnesium, titanium and aluminum.

14. The method of claim 11 , wherein the inorganic precursor material further comprises additives selected from a group comprising urea, citric acid, glucose, glycerol, ammonia, ammonium nitrate, toluene, benzene and xylene.

15. The method of claim 11 , wherein the precursor material is selected from the group comprising ferric nitrate (Fe(NO₃)₃), ferric acetate (Fe(CHsCOO)₃), and the like.

16. The method of claim 11 , wherein the reactant gas is selected from the group comprising methane, ethane, propane, ethylene, propylene, butane, liquefied petroleum gas, natural gas and the like.

17. The method of claim 11 , wherein the growth reactor is preheated to a temperature in the range of 500-1200⁰C.

18. The method of claim 11 , wherein in the precursor material is introduced in the growth reactor in the form of liquid, spray or aerosol.

19. An apparatus for producing carbon nanotubes (CNT), the apparatus being a single step carbon nanotubes growth reactor, the apparatus comprising: a. a concentric tubular reactor with an inner and an outer tube; b. an inorganic precursor material input module; c. a support input module; d. a reactant gas input module; and e. an exhaust module, the exhaust module being used to exit gases, the gases comprising oxygen and hydrogen.

20. The apparatus of claim 19, wherein the inner tube being used for inorganic precursor decomposition and annular space being used for the CNT growth;

21. The apparatus of claim 19, wherein the growth reactor is preheated at a temperature ranging from 500 to 1200⁰C.