Classifications

• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B82—NANOTECHNOLOGY
  • B82Y—SPECIFIC USES OR APPLICATIONS OF NANOSTRUCTURES; MEASUREMENT OR ANALYSIS OF NANOSTRUCTURES; MANUFACTURE OR TREATMENT OF NANOSTRUCTURES
  • B82Y30/00—Nanotechnology for materials or surface science, e.g. nanocomposites
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B82—NANOTECHNOLOGY
  • B82Y—SPECIFIC USES OR APPLICATIONS OF NANOSTRUCTURES; MEASUREMENT OR ANALYSIS OF NANOSTRUCTURES; MANUFACTURE OR TREATMENT OF NANOSTRUCTURES
  • B82Y40/00—Manufacture or treatment of nanostructures
• • C—CHEMISTRY; METALLURGY
  • C01—INORGANIC CHEMISTRY
  • C01B—NON-METALLIC ELEMENTS; COMPOUNDS THEREOF; METALLOIDS OR COMPOUNDS THEREOF NOT COVERED BY SUBCLASS C01C
  • C01B32/00—Carbon; Compounds thereof
  • C01B32/05—Preparation or purification of carbon not covered by groups C01B32/15, C01B32/20, C01B32/25, C01B32/30
• • C—CHEMISTRY; METALLURGY
  • C01—INORGANIC CHEMISTRY
  • C01B—NON-METALLIC ELEMENTS; COMPOUNDS THEREOF; METALLOIDS OR COMPOUNDS THEREOF NOT COVERED BY SUBCLASS C01C
  • C01B32/00—Carbon; Compounds thereof
  • C01B32/15—Nano-sized carbon materials
  • C01B32/158—Carbon nanotubes
  • C01B32/16—Preparation
  • C01B32/162—Preparation characterised by catalysts
• • C—CHEMISTRY; METALLURGY
  • C01—INORGANIC CHEMISTRY
  • C01B—NON-METALLIC ELEMENTS; COMPOUNDS THEREOF; METALLOIDS OR COMPOUNDS THEREOF NOT COVERED BY SUBCLASS C01C
  • C01B32/00—Carbon; Compounds thereof
  • C01B32/15—Nano-sized carbon materials
  • C01B32/18—Nanoonions; Nanoscrolls; Nanohorns; Nanocones; Nanowalls
• • D—TEXTILES; PAPER
  • D01—NATURAL OR MAN-MADE THREADS OR FIBRES; SPINNING
  • D01F—CHEMICAL FEATURES IN THE MANUFACTURE OF ARTIFICIAL FILAMENTS, THREADS, FIBRES, BRISTLES OR RIBBONS; APPARATUS SPECIALLY ADAPTED FOR THE MANUFACTURE OF CARBON FILAMENTS
  • D01F9/00—Artificial filaments or the like of other substances; Manufacture thereof; Apparatus specially adapted for the manufacture of carbon filaments
  • D01F9/08—Artificial filaments or the like of other substances; Manufacture thereof; Apparatus specially adapted for the manufacture of carbon filaments of inorganic material
  • D01F9/12—Carbon filaments; Apparatus specially adapted for the manufacture thereof
  • D01F9/127—Carbon filaments; Apparatus specially adapted for the manufacture thereof by thermal decomposition of hydrocarbon gases or vapours or other carbon-containing compounds in the form of gas or vapour, e.g. carbon monoxide, alcohols
  • D01F9/1278—Carbon monoxide

Definitions

• This invention relates to a process for producing substantially crystalline graphitic carbon nanofibers comprised of graphite sheets.
• the graphite sheets are substantially parallel to the longitudinal axis of the carbon nanofiber.
• These carbon nanofibers are produced by contacting a bulk iron, or an iron opper bimetallic, or an iron:nickel bimetallic catalyst with a mixture of carbon monoxide and hydrogen at temperatures from about 625°C to about 725°C for an effective amount of time.
• Nanostructure materials are quickly gaining importance for various potential commercial applications. Such applications include their use to store molecular hydrogen, to serve as catalyst supports, as reinforcing components for polymeric composites, and for use in various types of batteries.
• Carbon nanostructure materials are generally prepared from the decomposition of carbon-containing gases over selected catalytic metal surfaces at temperatures ranging from about 500°C to about 1,200°C.
• U.S. Patent 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 carbon-carbon structure comprised of carbon fibers having carbon nanofibers grown therefrom.
• the examples in these patents show the preparation of various carbon nanostructui'es 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.
• U.S. Patent No. 5,413,866 to Baker et al. teaches carbon nanostrucrures characterized as having a shape that is selected from the group consisting of branched, spiral, and helical. These carbon nanostructui'es 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, 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. Patent No. 5,458,784 also to Baker et al. teaches the use of the carbon nanosrructures of U.S. Patent No. 5,413,866 for removing contaminants from aqueous and gaseous steams; and U.S. Patent No. 5,653,951 to Rodriguez et al. discloses and claims that molecular hydrogen can be stored in layered carbon nanostructure materials having specific distances between layers.
• the examples in 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 US patents are incorporated herein by reference.
• substantially crystalline graphitic carbon nanofibers comprised of graphite sheets that are substantially parallel to the longitiidinal axis of the nanofibers, wherein the distance between the graphite sheets is from about 0.335 nm to about 0.67 nm, and having a crystallinity greater than about 95%.
• the distance between the graphite sheets is from about 0.335 and 0.40 nm.
• a process for producing substantially crystalline graphitic carbon nanofibers which process comprises reacting a mixture of C07H 2 in the presence of a bulk powder catalyst comprised of iron, ironxopper bimetallic, or iron:nickel bimetallic for an effective amount of time at a temperature from about 625°C to about 725°C.
• the catalyst is an ironxopper bimetallic catalyst wherein the ratio of iron to copper is from about 1:99 to about 99: 1 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.
• Figure la is a representation of a platelet carbon nanofiber, which is comprised of substantially graphite sheets that are substantially perpendicular to the longitudinal axis, or growth axis, of the nanofiber.
• Figure lb is a representation of a cylindrical carbon nanostructure that is comprised of continuous carbon sheets and is in the form of tube within a tube within a tube and having a substantially hollow center.
• Figure lc is a representation of a ribbon carbon nanofiber of the present invention that is comprised of graphitic sheets that are substantially parallel to the longitudinal axis of the nanofiber.
• Figure Id is a representation of a faceted tubular carbon nanofiber of the present invention and is comprised of continuous sheets of graphic carbon but having multifaceted flat faces.
• the graphitic sheets are also substantially parallel to the longitudinal axis of the nanofiber.
• Figure le is a representation of a herringbone carbon nanofiber wherein the graphitic platelets or sheets are at an angle to the longitudinal axis of the nanofiber.
• the carbon nanofibers of the present invention possess novel structures in which graphite sheets, constituting the nanostructure, are aligned in a direction that is substantially parallel to the growth axis (longitudinal axis) of the nanofiber.
• the carbon nanofibers are sometimes referred to herein as “ribbon” nanofibers and multifaceted tubular nanofibers.
• the carbon nanostructures of the present invention are distinguished from the so-called “fibrils” or cylindrical carbon nanostructures.
• the terms “carbon nanofibers” and “carbon nanostructures” are sometimes used interchangeably herein.
• the graphite sheets that compose the nanostructures of the present invention are either discontinuous sheets or faceted flat-faced tubular structures.
• cylindrical carbon nanostructures are composed of continuous circular graphite sheets and can be represented by tube within a tube structure having a substantially hollow center.
• the carbon nanofibers of the present invention have a unique set of properties, that includes: (i) a nitrogen surface area from about 40 to 300 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 bulk metals in powder form wherein the metal is selected from the group consisting of iron, iron: copper bimetallics, and iron:nickel bimetallics. 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.
• Fe or the combination of Cu or Ni with Fe has such a dramatic effect on carbon nanofiber growth in the CO/H 2 system in the temperature range of about 625°C to about 725°C.
• Iron:copper catalysts are preferred for preparing the carbon nanostructures of the present invention.
• the average powder particle size of the metal catalyst will range from about 0.25 nanometers to about 5 micrometer, preferably from about 1 nanometers to about 3 micrometer and more preferably from about 2.5 nanometers 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 parallel to the longitudinal axis of the nanofiber, at temperatures from about 625°C to about 725°C in the presence of a mixture of CO/H 2 .
• the ratio of iron to either copper or nickel will typically be from about 1:99 to about 99: 1, preferably from about 5:95 to about 95:5, more preferably from about 3:7 to about 7:3; and most 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, copper, and nickel particularly the nitrates.
• 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 catalyst powders used in the present invention are preferably prepared by the co-precipitation of aqueous solutions containing appropriate amounts of iron, nickel and copper nitrates using ammonium bicarbonate.
• the precipitates were dried overnight at about 110°C before being calcined in air at 400°C to convert the carbonates into mixed metal oxides.
• the calcined powders are then reduced in hydrogen for 20 hours at 400°C. Following this treatment the reduced catalyst is 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).
• carbon nanostructures can be prepared by reacting a catalyst in a heating zone with the vapor of a suitable carbon-containing compound. While the art teaches a wide variety of carbon-containing compounds as being suitable, the inventors hereof have found that only a mixture of CO and H 2 will yield carbon nanofibers with unexpected high crystallinities in the unique structures of nanofibers of the present invention in the temperature range of about 625°C to about 725°C. That is, crystallinities greater than about 95%, preferably greater than 97% more preferably greater than 98%, and most preferably substantially 100%.
• an aqueous solution of an inorganic acid such as a mineral acid
• suitable mineral acids include sulfuric acid, nitric acid, and hydrochloric acid. Preferred is hydrochloric acid.
• intercalation involves incorporating an appropriate intercalation compound between platelets.
• Intercalation compounds suitable for graphite structures are comprehensively discussed in Applications of Graphite Intercalation Compounds, by M.hiagaki, Journal of Material Research, Vol 4, No.6, Nov/Dec 1989, which is incorporated herein by reference.
• the preferred intercalation compounds for use with the nanofibers of the present invention are alkali and alkaline-earth metals.
• the limit to which the spacing of the graphite sheets will be increased for purposes of the present invention will be that point wherein the carbon nanofibers no longer can be characterized as graphitic. That is, the spacing can become so large that the carbon now has properties different than those of graphite. In most cases the electro-conductivity is enhanced. It is important for the practice of the present invention that the carbon nanofibers maintain the basal plane structure representative of graphite.
• the carbon nanostructures of the present invention contain a substantial number of edge sites, which are also referred to as edge regions.
• the edge regions of the nanostructures of the present invention can be made either basic (introduction of NFl groups) or acidic (addition of COOH " groups) by use of appropriate methods.
• oxygenated groups hydroxyl, peroxide, ether, keto or aldehyde
• These groups in turn can react with organic compounds to house unique structures for separations.
• Polar groups will promote the interaction of carbon edge atoms with other polar groups such as water.
• the interaction of graphitic materials with aqueous solutions can be greatly enhanced due to the presence of acid, basic or neutral functionality.
• polar groups in active carbon occurs in a random fashion, whereas the graphitic nanofibers of the present invention, such sites are located at the edges of the graphene layers.
• Addition of oxygenated groups can be achieved by selected oxidation treatments including treatment with peroxides, nitric acid, potassium permanganate, etc. Functionality can also be incorporated by electrochemical oxidation, at for example 2.3 volts for various periods of time. The nature of the groups will be dependent upon the oxidation time and the voltage.
• Polar sites can also be eliminated by reduction, out-gassing in vacuum at 1000°C or treatment in hydrazine at about 35°C. Following this procedure, the graphite nanofiber will become hydrophobic.
• composition of the gas phase was measured at regular intervals by taking samples of the inlet and outlet streams, which were then analyzed by gas chromatography using a 30m megabore (CS-Q) capillary column in a Varian 3400 GC unit. Carbon and hydrogen atom balances, in combination with the relative concentrations of the respective components, were applied to obtain the various product yields. In order to obtain reproducible carbon deposition data it was necessary to follow an identical protocol for each experiment.
• Table I shows the number of grams of carbon nanofibers per weight of catalyst produced after a period of 2 hours at each temperature. In each case the optimum yield of carbon nanofibers was generated at temperatures between 550°C and 600°C. The most active catalysts were those that contained a larger fraction of iron than copper.
• Example 5 In a further set of experiments the overall degree of crystallrnity of the carbon nanofibers produced from the interaction of selected Fe:Cu catalysts with a CO/H 2 (4: 1) mixture at 600°C for 2.0 hours was determined from temperature programmed oxidation of the nanofibers in C0 2 . The characteristics of the controlled gasification of carbonaceous solids in C0 2 provides a sensitive method of determining the structural perfection of such materials.
• Table V The data shown in Table V below indicates that the degree of crystallinity of carbon nanofibers generated from an Fe-Cu (7:3) catalyst is significantly higher than that of the same type of nanofibers grown under identical reaction conditions on a pure iron catalyst. Table V
• a carbon nanofiber having graphite sheets at an angle to the longitudinal axis of the nanofiber is referred to as a "herringbone structure”.
• Example 7 In another series of characterization studies, performed in a high resolution transmission electron microscope, samples of carbon nanofibers grown from the decomposition of CO/H 2 mixtures over a powdered iron catalyst at temperatures over the range 550 to 670°C were examined.
• Table VII The data presented in Table VII below indicates that there is a very narrow temperature window, 600 to 625 °C, where the structures of the nanofibers are produced exclusively in the form of platelet structures. Below this temperature the solid carbon product is found to consist of a mixture of herring-bone and platelet conformations, whereas at temperatures of 650°C there is a tendency for the structures to acquire a faceted tubular or ribbon arrangement, which becomes the only form at 670°C.

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• 2002
  • 2002-07-09 JP JP2003512478A patent/JP2004534914A/ja active Pending
  • 2002-07-09 WO PCT/US2002/021497 patent/WO2003006726A1/en active Application Filing
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