Chemistry of inorganic compounds – Carbon or compound thereof – Elemental carbon
Reexamination Certificate
2000-09-08
2003-03-25
Hendrickson, Stuart L. (Department: 1754)
Chemistry of inorganic compounds
Carbon or compound thereof
Elemental carbon
Reexamination Certificate
active
06537515
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a process for producing substantially crystalline graphitic carbon nanofibers comprised of graphite sheets. The graphite sheets are substantially perpendicular to the longitudinal axis of the carbon nanofiber. These carbon nanofibers are produced by contacting an iron:copper bimetallic bulk catalyst with a mixture of carbon monoxide and hydrogen at temperatures from about 550° C. to about 670° C. for an effective amount of time.
2. Description of Related Art
Nanostructure materials, particularly carbon nanostructure materials, are quickly gaining importance for various potential commercial applications. Such applications include their use to store molecular hydrogen, serve as catalyst supports, as reinforcing components for polymeric composites and to be useful in various batteries. Carbon nanostructure materials are typically prepared from the decomposition of carbon-containing gases over selected catalytic metal surfaces at temperatures ranging from about 500° to about 1,200° C.
For example, U.S. Pat. Nos. 5,149,584 and 5,618,875, to Baker et al., teach carbon nanofibers as reinforcing components in polymer reinforced composites. The carbon nanofibers can either be used as is or as part of a structure comprised of carbon fibers having carbon nanofibers grown therefrom. The examples of these patents show the preparation of various carbon nanostructures by the decomposition of a mixture of ethylene and hydrogen in the presence of metal catalysts such as iron, nickel, a nickel:copper alloy, an iron:copper alloy, etc. Also,U.S. Pat. No. 5,413,866, to Baker et al., teaches carbon nanostructures characterized as having: (i) a surface area from about 50 m
2
/g to 800 m
2
/g; (ii) an electrical resistivity from about 0.3 &mgr;ohm·m to 0.8 &mgr;ohm·m; (iii) a crystallinity from about 5% to about 100%; (iv) a length from about 1 &mgr;m to about 100 &mgr;m; and (v) a shape that is selected from the group consisting of branched, spiral, and helical. These carbon nanostructures are taught as being prepared by depositing a catalyst containing at least one Group IB metal, and at least one other metal, on a suitable refractory support and then subjecting the catalyst-treated support to a carbon-containing gas at a temperature from the decomposition temperature of the carbon-containing gas to the deactivation temperature of the catalyst.
U.S. Pat. No. 5,458,784, also to Baker et al., teaches the use of the carbon nanostructures of U.S. Pat. No. 5,413,866 for removing contaminants from aqueous and gaseous steams; and U.S. Pat. No. 5,653,951, to Rodriguez et al., discloses and claims that molecular hydrogen can be stored in layered nanostructure materials having specific distances between layers. The examples of these patents teach the aforementioned preparation methods as well as the decomposition of a mixture of carbon monoxide and hydrogen in the presence of an iron powder catalyst at 600° C. All of the above referenced U.S. patents are incorporated herein by reference.
While various carbon nanostructures and their uses are taught in the art, there is still a need for improvements before such nanostructure materials can reach their full commercial and technical potential. For example, while the art broadly discloses carbon nanostructures having crystallinities from about 5 to 95%, it has heretofore not been possible to produce carbon nanostructures with crystallinities greater than about 95%.
SUMMARY OF THE INVENTION
In accordance with the present invention, there is provided a substantially crystalline graphitic carbon nanofibers comprised of graphite sheets that are substantially perpendicular to its longitudinal axis of the nanofibers, wherein the distance between graphite sheets is from about 0.335 nm to about 0.67 nm, and having a crystallinity greater than about 95%.
In a preferred embodiment, the distance between the graphite sheets is from about 0.335 and 0.40 nm.
Also in accordance with the present invention, there is provided a process of producing substantially crystalline graphitic carbon nanofibers which process comprises reacting a mixture of CO/H
2
in the presence of a powder Fe:Cu bimetallic catalyst for an effective amount of time at a temperature from about 550° C. to about 670° C.
In a preferred embodiment, the ratio of Fe to Cu is from about 5:95 to about 95:5 and the ratio of CO to H
2
is from about 95:5 to about 5:95, preferably from about 80:20 to about 20:80.
DETAILED DESCRIPTION OF THE INVENTION
The carbon nanofibers of the present invention possess a novel structure in which graphite sheets, constituting the material, are aligned in a direction that is substantially perpendicular to the growth axis (longitudinal axis) of the nanofiber. The carbon nanfibers are sometimes referred to herein as “platelet” nanofibers. In addition, the nanofibers have a unique set of properties, which include: (i) a nitrogen surface area from about 40 to 120 m
2
/g; (ii) an electrical resistivity of 0.4 ohm·cm to 0.1 ohm·cm; (iii) a crystallinity from about 95% to 100%; and (iv) a spacing between adjacent graphite sheets of 0.335 nm to about 1.1 nm, preferably from about 0.335 nm to about 0.67 nm, and more preferably from about 0.335 to about 0.40 nm.
The catalysts used to prepare the carbon nanofibers of the present invention are iron:copper bulk bimetallic catalysts in powder form. It is well established that the ferromagnetic metals, iron, cobalt and nickel, are active catalysts for the growth of carbon nanofibers during decomposition of certain hydrocarbons or carbon monoxide. Efforts are now being directed at modifying the catalytic behavior of these metals, with respect to nanofiber growth, by introducing other metals and non-metals into the system.
In this respect, copper is an enigma, appearing to be relatively inert towards carbon deposition during the CO/H
2
reaction. Thus, it is unexpected that the combination of Cu with Fe has such a dramatic effect on carbon nanofiber growth in the CO/H
2
system.
The average powder particle size of the metal catalyst will range from about 0.5 nanometer to about 5 micrometer, preferably from about 2.5 nanometer to about 1 micrometer. The ratio of the two metals can be any effective ratio that will produce substantially crystalline carbon nanofibers in which the graphite sheets are substantially perpendicular to the longitudinal axis of the nanofiber, at temperatures from about 550° C. to about 670° C. in the presence of a mixture of CO/H
2
. The ratio of iron:copper will, typically, be from about 5:95 to about 95:5, preferably from about 3:7 to about 7:3; and more preferably from about 6:4 to about 7:3. The bimetallic catalyst can be prepared by any suitable technique. One preferred technique is by co-precipitation of aqueous solutions containing soluble salts of the two metals. Preferred salts include the nitrates, sulfates, and chlorides of iron and copper, particularly iron nitrate and copper nitrate. The resulting precipitates are dried and calcined to convert the salts to the mixed metal oxides. The calcined metal powders are then reduced at an effective temperature and for an effective time.
The iron:copper catalyst powders used in the present invention are prepared by the co-precipitation of aqueous solutions containing appropriate amounts of nickel and copper nitrate using ammonium bicarbonate. The precipitates were dried overnight at the 110° C. before being calcined in air at 400° C. to convert the carbonates into mixed metal oxides. The calcined powders were then reduced in hydrogen for 20 hours at 400° C. Following this treatment, the reduced catalyst was cooled to room temperature in a helium environment before being passivated in a 2% oxygen/helium mixture for 1 hour at about room temperature (24° C.).
Gas flow reactor experiments were carried out in a horizontal quartz tube (40 mm i.d. and 90 cm ong) contained in a Linberg tube furnace, at temperatures over the range of about 450° C. to 700° C. Gas flow rates to the reactor were regulated by M
Baker R. Terry K.
Rodriguez Nelly M.
Catalytic Materials LLC
Hendrickson Stuart L.
Naylor Henry E.
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