CN1833055A - Improved catalyst and process to produce nanocarbon materials in high yield and at high selectivity at reduced reaction temperatures - Google Patents

Abstract

A carbon nanofiber system is synthesized with very high purity (above 95%), selectivity of the carbon morphology, and exceptionally high yield. A custom made catalyst with a particle size of <=10 nm and a high surface area (>50 m2/g), provides a higher morphological selectivity and higher yield. The reactivity of these catalyst particles is maintained even after 24 hours reaction such that yield exceeds 200 g carbon per gram of catalyst. The catalysts which are key to the products and yields achieved are prepared to specific parameters (size distribution, composition and crystallinity) specified and via a flame synthesis process as taught in U.S. Pat. No. 6,132,653.

Description

Improved catalyst and process for producing nanocarbon materials at reduced reaction temperatures in high yield and high selectivity

The inventor: PRADHAN, Bhabendra, 360 Bloombridge Way n.w.,Marietta, GA 30066, India citizen.

The assignee: COLUMBIAN CHEMICALS COMPANY (Delaware Corporation), 1800 West Oak Commons Coirt, Marietta, Georgia 30062

Cross Reference to Related Applications

In the united states, this application is a continuation-in-part application of united states application 10/628,842 filed on 28/7/2003.

Thereby claiming priority from U.S. application 10/628,842 filed on 28/7/2003.

U.S. application 10/628,842 filed on 28/7/2003 is hereby incorporated by reference.

Statement regarding federally sponsored research or development

Not applicable to

Reference to "Microfilm appendix"

Not applicable to

Background

1. Field of the invention

The present invention relates to the preparation of nanocarbon materials, and more particularly, to an improved catalyst and method for preparing nanocarbon materials at reduced reaction temperatures with high yield and high selectivity.

2. General background

Nanostructured materials, and more particularly, carbon nanostructured materials, are increasingly important for various commercial applications. Such applications include their use for storing molecular hydrogen, acting as catalyst supports, as reinforcing components for polymer composites, for electromagnetic shielding, and in various types of batteries and other energy storage devices. Carbon nanostructured materials are prepared by decomposing a carbon-containing gas on the surface of a selected catalytic metal, typically at a temperature of about 500 ℃ to about 1200 ℃.

For example, carbon nanofibers can be used in lithium ion batteries, where the anode will consist of graphite nanofibers. The graphite platelets are substantially perpendicular or parallel to the longitudinal axis of the carbon nanofibers. An example of such an application is found in us patent 6,503,660. Furthermore, us patent 5,879,836 teaches the use of fibrils as raw material for lithium ion batteries. A fibril is described as being composed of parallel carbon layers in the form of a series of concentric tubes arranged about a longitudinal axis, rather than a multi-layered planar graphite sheet.

Further, in us patent 6,485,858, graphite nanofibers have a structure in which graphite flakes are aligned in a direction substantially perpendicular or substantially parallel to the fiber axis and are defined as a sheet and a ribbon, respectively. Furthermore, the exposed faces of the nanofibers are composed of at least 95% edge regions (edge regions), as opposed to conventional graphite which is composed almost entirely of primary face regions and a very small number of edge sites.

Other references include "Catalytic Growth of Carbon fibers," from the 1989 Chemical Engineering Department of Auburn University article, in which the formation of fibrous Carbon is discussed. Another source of information is the product of the article entitled "A Review of catalytic growth Carbon fibers," published by Material Research Society in 1993. In this paper, the carbon nanofibers discussed are produced on a relatively large scale by catalytic decomposition of specific hydrocarbons on small metal particles.

In all of the above cases, synthesizing pure carbon nanomaterials is challenging. Most applications of these materials require pure carbon nanomaterial systems. It would therefore be advantageous to provide a system for producing pure carbon nanomaterials in which the carbon system can be synthesized with very high purity (greater than 95%), high crystallinity, carbon morphology selectivity, and exceptionally high yield. In addition, custom made catalysts (custom made catalysts) with specific particle size and high surface area will yield higher selectivity and higher activity.

Brief description of the invention

In the present invention, carbon nanofiber systems are synthesized with very high purity (greater than 95%), high crystallinity, carbon morphology selectivity, and exceptionally high yield. Compared with the catalysts available hitherto, an average single crystal particle size of 10nm or less and a high surface area (>50 m)²The customized catalysts of/g) provide higher morphological selectivity and higher activity. The activity of these catalyst particles was maintained even after 24 hours of reaction, resulting in a yield of over 200g carbon/g catalyst. Flame synthesis as taught by U.S. Pat. No. 6,132,653 would beThe key catalyst to achieve product and yield is prepared to the specified specific parameters (particle size distribution, composition and crystallinity). The contents of U.S. Pat. No. 6,132,653 are incorporated herein by reference.

For purposes of this application, the terms used herein will have the following definitions:

"purity" is defined as the carbon content with impurities considered to comprise the catalyst.

"selectivity" is defined as the fraction of carbonaceous products having the expected morphology (orientation of the graphitic layer); and

"yield" is defined as the weight of carbon produced divided by the weight of catalyst; in this catalytic process, the utilization rate (turn over) may be expressed.

Therefore, the main object of the present invention is to synthesize carbon nanomaterials with extremely high purity, high selectivity of carbon morphology and exceptionally high yield.

It is another object of the present invention to synthesize carbon nanomaterials in the presence of tailored catalysts having specific particle size, surface area and chemical composition to achieve high morphological selectivity, yield and purity.

It is yet another object of the present invention to prepare carbon nanomaterials in the presence of tailored catalysts so that the yields exceed 200g of carbon per g of catalyst in a given amount of time.

Brief Description of Drawings

For a further understanding of the nature, objects, and advantages of the present invention, reference should be made to the following detailed description, taken in conjunction with the accompanying drawings, in which like reference characters identify like elements, and wherein:

FIG. 1 is a graph of the effect of time on carbon nanofiber growth in the presence of an iron oxide catalyst over a 24 hour period;

FIG. 2 shows the results of the reaction between iron: a plot of the effect of time on carbon nanofiber growth in the presence of a nickel catalyst over a 24 hour period;

FIG. 3 is a particular morphology of a carbon nanofiber carbon microstructure prepared in the presence of an iron oxide catalyst as described in FIG. 1;

figure 4 is a high resolution image of a specific morphology of a carbon nanofiber carbon microstructure prepared in the presence of an iron oxide catalyst as described in figure 1.

Fig. 5 is a graph showing the relationship between iron: the specific morphology of the carbon microstructure of the carbon nanofibers prepared in the presence of a nickel catalyst;

fig. 6 is a graph showing the relationship between iron: a high resolution map of the specific morphology of the carbon nanofiber carbon microstructure prepared in the presence of a nickel catalyst;

FIG. 7 is a graph of the preparation of carbon nanofibers with platelet morphology (platelet morphology) prepared with an iron oxide catalyst compared to a conventional catalyst; and

fig. 8 is a graph of the reaction rate of a catalyst having a molar ratio of iron: preparation of tubular morphology (tubulartorphism) carbon nanofibers prepared with nickel catalyst.

PREFERRED EMBODIMENTS FOR CARRYING OUT THE INVENTION

Method for producing catalyst

The preparation of the catalyst used in the production of the nanofibers disclosed herein is similar to that disclosed in U.S. Pat. No. 6,132,653, previously referenced and incorporated herein.

Metals that may be part of the catalyst are listed below:

iron (Fe), nickel (Ni), cobalt (Co), molybdenum (Mo), copper (Cu), lanthanum (La), silver (Ag), gold (Au), and alloys.

Nanomaterials prepared with said catalysts

Reference is now made to the following table and information which discusses the properties of materials prepared with the novel catalysts described above (flame synthesis) and conventional catalysts (co-precipitation).

TABLE 1

Properties of	New pattern catalyst (flame synthesis)	Conventional or commercial catalysts (coprecipitation)
			Chemical form	Metal oxides	Pre-reduced metal with thin oxide layer
Size (nm)	～10	500-2000
			Form of the composition	Single crystal	Polycrystalline
Surface area (m)2/g)	～130	＜20
			Density of compaction	Less than bulk density	Same as bulk density

Experimental details to obtain the above results:

a: conventional or commercial catalysts

A known amount of pre-reduced catalyst (0.1g) was placed in a ceramic dish or quartz graduated cylinder. The dish was then transferred to a quartz reactor (. about.47 mm). The reactor was purged with nitrogen at a rate of 200sccm for 30 minutes. Heating at 10-20% H at 5 deg.C/min₂(the remainder being N)₂) The reactor was heated to 450 ℃. The temperature was maintained for 1 hour. Then N within 30 minutes₂The stream raises the temperature to the reaction temperature of 600 ℃ (iron) or 650 ℃ (iron-nickel oxide catalyst). After the set temperature stabilized, the reaction gas (CO/H) was allowed to react for different periods of time (1, 2, 4, 6, 8 and 24 hours)₂Or C₂H₄/H₂) Is introduced into the reactor.

b: novel catalyst

A known amount of oxide catalyst (0.1g) was placed in a ceramic dish or quartz graduated cylinder. The dish was then transferred to a quartz reactor (. about.47 mm). The reactor was purged with nitrogen at a flow rate of 200sccm for 30 minutes. Heating at 10-20% H at 5 deg.C/min₂(the remainder being N)₂) The reactor was heated to 450 ℃. The temperature was maintained for 1 hour. Then N within 30 minutes₂Flow down the temperatureThe reaction temperature was raised to 550 ℃ (iron oxide and iron-nickel oxide catalyst). After the set temperature stabilized, the reaction gas (CO/H) was used for different periods of time (1, 2, 4, 6, 8 and 24 hours)₂Or C₂H₄/H₂) Is introduced into the reactor.

At 550 ℃ in CO: H₂The ratio of: 4: 1 the use of the iron oxide catalyst produces a carbon microstructure of a specific morphology in which the graphite planes areperpendicular to the carbon growth axis, as shown in fig. 3 and 4. Compared with the conventional catalystThis experiment shows better carbon yield (2 to 3 times higher) and synthesis temperature 50 ℃ lower (550 ℃ versus 600 ℃). A carbon product of greater than 99.6% purity was obtained in the system. The morphological selectivity was 100%.

In a second example, an iron to nickel catalyst was used as C₂H₂∶H₂The ratio of: 1: 4 produces a carbon microstructure of a specific morphology at 550 deg.C, i.e., wherein the graphite planes are parallel and/or at an angle to the carbon growth axis, as shown in FIGS. 5 and 6. This experiment shows better carbon yields (2 to 3 times higher) and synthesis temperatures as low as 100 ℃ (550 ℃ versus 650 ℃) compared to other conventional or commercially available catalysts. Carbon products of greater than 99.2% purity can be obtained in this system. The morphology selectivity was greater than 95%. In both of the above-described embodiments, the catalyst may be a metal oxide catalyst selected from metals including iron, nickel, cobalt, lanthanum, gold, silver, molybdenum, iron-nickel, iron-copper, and alloys thereof.

c. The fluidized bed process scheme is as follows:

a known amount of an oxide catalyst (0.1-1.2g) was placed with Al₂O₃(14.9-13.8g) in an ebullated fluidized bed reactor. The reactor was purged with nitrogen at a flow rate of 1000sccm for 30 minutes. Heating at 10-20% H at 5 deg.C/min₂(the remainder being N)₂) The reactor was heated to 450 ℃. The temperature was maintained for 1 hour. Then N within 30 minutes₂The stream was then allowed to increase in temperature to a reaction temperature of 550 ℃ (iron-nickel oxide catalyst). After the set temperature is stabilized, the reaction gas (C) is introduced₂H₄/H₂) Introduced into the reactor over a known period of time (2 hours). The yield can reach 140g carbon/g catalyst.

Referring now to FIG. 1, FIG. 1 shows time versus CO: H using an iron oxide catalyst₂Effect of carbon nanofibers grown at 550 ℃ in the ratio of: 4: 1. In this figure, the carbon nanofibers produced comprise a carbon platelet morphology, as shown in fig. 3 and 4. Curve 10 records (track) g carbon/g catalyst. Curve 20 records the metal content (wt%). Referring to FIG. 1, when the process lasts about 24 hours, the metal content is based on the weight of the productThe% by weight drops to 0.3% and the yield (carbon/g catalyst) is>300 g/g. It also shows that the catalytic particles are active even after 24 hours of reaction. In this embodiment, the iron oxide catalyst is present in the form of CO: H₂The ratio of: 4: 1 produced a carbon microstructure of a specific morphology at 550 ℃, i.e., where the graphite planes were perpendicular to the carbon growth axis, again as shown in FIGS. 3 and 4. In addition, this experiment showed better carbon yields (2 to 3 higher) compared to the commercial catalyst, as described previouslyDouble) and a synthesis temperature 50 ℃ lower. A pure carbon product of 99.7% was obtained with a morphological selectivity of 100%. As shown in fig. 3 and 4, the carbon microstructure of the particular morphology shows graphitic planes perpendicular to the carbon growth axis.

Referring now to FIG. 2, the graph depicts time versus C for the use of an iron-nickel catalyst₂H₂∶H₂Effect of carbon nanofibers grown at 550 ℃ in the ratio of: 1: 4. Curve 30 records g carbon/g catalyst. Curve 40 records the metal content (wt%). Carbon nanofibers fabricated as shown in this figure produce a carbon microstructure of a particular morphology, i.e., graphite planes parallel to the growth axis or at an angle, as shown in fig. 5 and 6. This shows better carbon yield anda synthesis temperature of 100 ℃ lower compared to conventional catalysts. Carbon products with 99.6% purity are also obtained with a morphology selectivity of greater than 95%. At the end of the 24-hour reaction period, the metal content of the product was 0.4%, while the carbon yield was 200-250g/g catalyst.

In both systems, 99% carbon was obtained in 8 hours of reaction time, as shown in figures 1 and 2. These results are shown in tables 2 and 3.

In each of these tables and as depicted in fig. 7 and 8, respectively, the iron catalyst and the iron: nickel catalyst, respectively, gave carbon nanomaterial platelet or tubular morphology at lower temperatures with greater than 95% morphology selectivity, higher yields, and lower metal impurities than the commercially available or conventional catalysts. Curve 50 records the g carbon/gMCT catalyst at 550 ℃. Curve 60 records the metal content (wt%). Curve 70 records the g carbon/gJT Baker catalyst at 600 ℃. Curve 80 records the metal content (wt%). Curve 90 records the g carbon/gMCT catalyst at 550 ℃. Curve 100 records the metal content (wt%). Curve 110 records g carbon/g CCC at 600 ℃. Curve 120 records the metal content (wt%).

For platelet morphology, the catalyst iron, CO: H₂∷4∶1。

TABLE 2

Catalyst and process for preparing same	Temperature (. degree.C.)	Selectivity (visual inspection)	Yield (g/6h)	Impurities (Metal)
					Flame(s)	550	100	77	1.3
Market selling (J.T.Baker)	600	90	50	2

For the tubular morphology, the catalysts were iron: nickel: 8: 2, C₂H₂∶H₂∷1∶4

TABLE 3

Catalyst and process for preparing same	Temperature (. degree.C.)	Selectivity (visual inspection)	Yield (g/6h)	Impurities (Metal)
					Flame(s)	550	＞95	81	1.25
Conventionally prepared CCC	650	60	26.33	3.8

The "conventionally prepared CCC" catalyst was prepared using a liquid precipitation method. Iron, nickel and copper metal nitrates are used. The metal nitrate and water were mixed stoichiometrically and stirred rapidly at room temperature. Aluminum bicarbonate was added to give a pH of about 9 and stirred for about 5 minutes. After one night a precipitate formed, which was washed and dried. The metal carbonate was dried at 110 ℃ for 24 hours and then calcined in air at 400 ℃ for 4 hours. The metal oxide was ball milled for 6 hoursand reduced at 500 c in 10% hydrogen in 200sscm nitrogen for 20 hours. The metal powder was passivated in 2% oxygen in nitrogen for 1 hour. The above-described methods and reactions are carried out as described below with reference to r.j.best and w.w.russel, j.am.chem.soc.76, 8383 (1954).

Synthesis of powdered catalyst by flame/plasma method:

an ethanolic solution of a nitrate/sulphate mixture of metals (iron, nickel and copper) is prepared and evaporated/atomized into a flame or plasma plume (torch) and by this process a powder of pure or mixed metal oxides is obtained using the method described in us patent 6,123,653.

Generally, the method of making the nanocarbon material is by providing an averageParticle size of 10nm or less, surface area of more than 50m²Per g of catalyst, but may vary. And then, reacting the carbon-containing reactant in the presence of a catalyst within a given time to obtain the carbon nanofiber with the purity of more than 99 percent and the morphological selectivity of 100 percent and higher activity.

The catalyst prepared by the process described in us patent 6123653 is a metal oxide catalyst selected from the group of metals consisting of: iron, nickel, cobalt, lanthanum, gold, silver, molybdenum, iron-nickel, iron-copper, and alloys thereof. Other suitable metal oxides may be found as the test proceeds further. The catalyst itself is prepared to the specified specific parameters (particle size, composition and crystallinity) by flame synthesis; it has a single crystal morphology. By using a catalyst selected from the above, the carbon nanomaterial obtained has a yield of 140g carbon/g catalyst or more, but the yield can be greater, while the morphology of the carbon microstructure comprises graphite planes with controlled orientationperpendicular or parallel to the carbon growth axis (depending on the catalyst composition and the carbonaceous feedstock), resulting in a carbon product of 99.6% purity.

The foregoing embodiments are given by way of example only, and the scope of the invention is limited only by the claims.

Claims (19)

1. A method of making a nanocarbon material, comprising the steps of:

a. providing a particle size of 10nm or less and a surface area of more than 50m²A catalyst per gram;

b. reacting a carbon-containing feedstock in the presence of the catalyst for a given time to produce carbon nanofibers having a purity greater than 99% and a morphology selectivity approaching 100%, a yield greater than or equal to 140g carbon/g catalyst, and higher activity.

2. The process of claim 1 wherein the catalyst is a metal oxide catalyst selected from the group consisting of iron, nickel, cobalt, lanthanum, gold, silver, molybdenum, iron-nickel, iron-copper, and alloys thereof.

3. The process of claim 1, wherein the catalyst is made to specified specific parameters (particle size distribution, composition and crystallinity) by flame synthesis.

4. The process of claim 1 wherein the catalyst has a single crystal morphology.

5. The method of claim 1, wherein the carbon nanomaterial obtained has a yield of 140g or more carbon/g catalyst.

6. The method of claim 1, wherein the morphology of the carbon microstructure can be selectively controlled to obtain various desired orientations with a selectivity of ≥ 90%.

7. A method of making a nanocarbon material, comprising the steps of:

a. providing a particle size of about 10nm or less and a surface area of greater than 50m²A metal oxide catalyst per gram;

b. reacting a carbon-containing feedstock in the presence of the catalyst for a given time to produce carbon nanofibers having a purity greater than 99% and a morphology selectivity approaching 100% with a yield of 140g or greater carbon/g catalyst.

8. The process of claim 7 wherein the reaction is carried out at a temperature not exceeding 550 ℃.

9. The method of claim 7, wherein the purity of the carbon nanofibers after 8 hours of reaction is 99% or more.

10. The method of claim 7, wherein the metal oxide catalyst is selected from the group consisting of metals including iron, nickel, cobalt, lanthanum, gold, silver, molybdenum, iron-nickel, iron-copper, and alloys thereof.

11. A high purity and high activity carbon nanofiber produced by the steps of:

a. providing a particle size of 10nm or less and a surface area of more than 50m²A metal oxide catalyst per gram;

b. reacting a carbonaceous feedstock in the presence of the catalyst for a given time to produce carbon nanofibers having a purity greater than 99% and a selectivity approaching 100%, and having a higher activity.

12. The carbon nanofiber produced by the method of claim 11, wherein the metal oxide catalyst is selected from the group consisting of metals including iron, nickel, cobalt, lanthanum, gold, silver, molybdenum, iron-nickel, iron-copper, and alloys thereof.

13. The carbon nanofiber produced by the method of claim 11, wherein the purity of the carbon nanofiber is 99% or more after 8 hours of reaction.

14. A carbon nanofiber prepared in the presence of a metal oxide catalyst, the carbon nanofiber comprising at least 99% pure carbon and being produced in high yield and with>90% morphological selectivity.

15. The carbon nanofiber of claim 14, wherein the metal oxide catalyst is selected from the group consisting of metals including iron, nickel, cobalt, lanthanum, gold, silver, molybdenum, iron-nickel, iron-copper, and alloys thereof.

16. A carbon nanofiber composition exhibiting 90% selectivity to a single morphology.

17. The composition of claim 16, wherein the morphology comprises graphite layers oriented parallel to the fiber axis.

18. The composition of claim 16, wherein the morphology comprises graphite layers oriented perpendicular to the fiber axis.

19. The composition of claim 16, wherein the morphology comprises graphitic layers oriented at specific and identical (± 10 °) angles to the fiber axis.

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