U.S. patent application number 10/625069 was filed with the patent office on 2004-04-15 for process for producing crystalline graphite nanofibers.
Invention is credited to Baker, R. Terry K., Rodriguez, Nelly M..
Application Number | 20040071625 10/625069 |
Document ID | / |
Family ID | 27097820 |
Filed Date | 2004-04-15 |
United States Patent
Application |
20040071625 |
Kind Code |
A1 |
Baker, R. Terry K. ; et
al. |
April 15, 2004 |
Process for producing crystalline graphite nanofibers
Abstract
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:copper bimetallic, or an iron:nickel
bimetallic catalyst with a mixture of carbon monoxide and hydrogen
at temperatures from about 625.degree. C. to about 725.degree. C.
for an effective amount of time.
Inventors: |
Baker, R. Terry K.;
(Hopkinton, MA) ; Rodriguez, Nelly M.; (Hopkinton,
MA) |
Correspondence
Address: |
Henry E. Naylor
Kean, Miller, Hawthorne, D'Armond,
McCowan & Jarnan, L.L.P.
P.O. Box 3513
Baton Rouge
LA
70821-3513
US
|
Family ID: |
27097820 |
Appl. No.: |
10/625069 |
Filed: |
July 22, 2003 |
Related U.S. Patent Documents
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
|
|
10625069 |
Jul 22, 2003 |
|
|
|
09902113 |
Jul 10, 2001 |
|
|
|
09902113 |
Jul 10, 2001 |
|
|
|
09659441 |
Sep 8, 2000 |
|
|
|
6537515 |
|
|
|
|
Current U.S.
Class: |
423/447.3 |
Current CPC
Class: |
C01P 2004/16 20130101;
B82Y 40/00 20130101; C01B 32/05 20170801; Y10T 428/2918 20150115;
C01B 32/15 20170801; B82Y 30/00 20130101; D01F 9/1278 20130101 |
Class at
Publication: |
423/447.3 |
International
Class: |
D01F 009/12 |
Claims
What is claimed is:
1. A process for producing a substantially crystalline graphitic
nanofiber wherein at least a portion of which are comprised of
graphite sheets that are substantially parallel to the longitudinal
axis of the nanofiber, which process comprises reacting a mixture
of CO/H.sub.2 in the presence of a catalyst selected from the group
consisting of Fe, Fe:Cu bimetallic, and Fe:Ni bimetallic powder
catalysts for an effective amount of time at a temperature from
about 625.degree. C. to about 725.degree. C.
2. The process of claim 1 wherein said nanofibers are characterized
as having separate and non-continuous substantially graphite
sheets.
3. The process of claim 1 wherein said nanofibers are characterized
as having continuous substantially graphite sheets forming a
non-cylindrical multifaceted tubular structure.
4. The process of claim 1 wherein the catalyst is an Fe:Cu
bimetallic wherein the ratio of Fe to Cu is from about 1:99 to
about 99:1.
5. The process of claim 4 wherein the ratio of Fe to Cu is from
about 3:7 to about 7:3.
6. The process of claim 5 wherein the ration of Fe to Cu is about
7:3 and the temperature is about 650.degree. C.
7. The process of claim 1 wherein the catalyst is an Fe:Ni
bimetallic wherein the ratio of Fe to Ni is from about 1:99 to
about 99:1.
8. The process of claim 7 wherein the ratio of Fe to Ni is from
about 3:7 to about 7:3
9. The process of claim 1 wherein the ratio of CO to H.sub.2 is
from about 95:5 to about 5:95.
10. The process of claim 9 wherein the ratio of CO to H.sub.2 is
from about 80:20 to about 20:80.
11. The process of claim 5 wherein the ratio of CO to H.sub.2 is
from about 80:20 to about 20:80.
12. The process of claim 6 wherein the ratio of CO to H.sub.2 is
about 80:20.
13. The process of claim 1 wherein the crystallinity of the
nanofiber is greater than about 98%.
14. The process of claim 5 wherein the crystallinity of the
nanofiber is greater than about 98%.
15. The process of claim 1 wherein the particle size of the
bimetallic powder is from about 0.25 nanometers to about 5
micrometers.
16. The process of claim 14 wherein the particle size of the
bimetallic powder is from about 2.5 nanometers to about 1
micrometer.
17. The product produced by the process of claim 1.
18. The product produced by the process of claim 6.
19. The product produced by the process of claim 12.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application is a division of U.S. Ser. No. 09/902,113
filed Jul. 10, 2001 which is a continuation-in-part of U.S. Ser.
No. 09/659,441 filed Sep. 8, 2000 now U.S. Pat. No. 6,537,515.
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention
[0003] 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:copper bimetallic, or an iron:nickel bimetallic catalyst with
a mixture of carbon monoxide and hydrogen at temperatures from
about 625.degree. C. to about 725.degree. C. for an effective
amount of time.
[0004] 2. Description of Related Art
[0005] Nanostructure materials, particularly carbon 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.degree. C. to about 1,200.degree. C.
[0006] 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 carbon-carbon structure comprised of carbon fibers having
carbon nanofibers grown therefrom. The examples in 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.
[0007] Also, U.S. Pat. No. 5,413,866 to Baker et al. teaches carbon
nanostructures characterized as having 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, 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.
[0008] 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 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.degree. C. All of
the above referenced US patents are incorporated herein by
reference.
[0009] 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
[0010] In accordance with the present invention, there is provided
substantially crystalline graphitic carbon nanofibers comprised of
graphite sheets that are substantially parallel to the longitudinal
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%.
[0011] In a preferred embodiment, the distance between the graphite
sheets is from about 0.335 and 0.40 nm.
[0012] Also in accordance with the present invention, there is
provided a process for producing substantially crystalline
graphitic carbon nanofibers which process comprises reacting a
mixture of CO/H.sub.2 in the presence of a bulk powder catalyst
comprised of iron, iron:copper bimetallic, or iron:nickel
bimetallic for an effective amount of time at a temperature from
about 625.degree. C. to about 725.degree. C.
[0013] In a preferred embodiment, the catalyst is an iron:copper
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.sub.2 is from
about 95:5 to about 5:95, preferably from about 80:20 to about
20:80.
BRIEF DESCRIPTION OF THE DRAWINGS
[0014] FIG. 1a 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.
[0015] FIG. 1b 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.
[0016] FIG. 1c 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.
[0017] FIG. 1d 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.
[0018] FIG. 1e 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.
DETAILED DESCRIPTION OF THE INVENTION
[0019] 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. On the other hand,
cylindrical carbon nanostructures, or "fibrils" are composed of
continuous circular graphite sheets and can be represented by tube
within a tube structure having a substantially hollow center. In
addition, 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.sup.2/g; (ii) an electrical resistivity
of 0.4 ohm.multidot.cm to 0.1 ohm.multidot.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.
[0020] 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.sub.2 reaction. Thus, it is unexpected that Fe or the
combination of Cu or Ni with Fe has such a dramatic effect on
carbon nanofiber growth in the CO/H.sub.2 system in the temperature
range of about 625.degree. C. to about 725.degree. C. Preferably
from about 650.degree. C. to about 725.degree. C., and more
preferably from about 670.degree. C. to about 725.degree. C.
Iron:copper catalysts are preferred for preparing the carbon
nanostructures of the present invention.
[0021] The average powder particle size of the metal catalyst will
range from about 0.25 nanometers to about 5 micrometer, preferably
from about 1 nanometer to about 3 micrometers and more preferably
from about 2.5 nanometers to about 1 micrometer. When the catalyst
is a bimetallic catalyst, 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.degree. C. to about 725.degree. C. in the presence of a
mixture of CO/H.sub.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.
[0022] 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.degree. C. before being calcined in air at 400.degree.
C. to convert the carbonates into mixed metal oxides. The calcined
powders are then reduced in hydrogen for 20 hours at 400.degree. 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.degree. C.).
[0023] It is known that 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.sub.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.degree. C. to about 725.degree. C.
That is, crystallinities greater than about 95%, preferably greater
than 97% more preferably greater than 98%, and most preferably
substantially 100%.
[0024] After the nanofibers are grown, it may be desirable to treat
them with an aqueous solution of an inorganic acid, such as a
mineral acid, to remove any excess catalyst particles. Non-limiting
examples of suitable mineral acids include sulfuric acid, nitric
acid, and hydrochloric acid. Preferred is hydrochloric acid.
[0025] It is within the scope of this invention to increase the
spacing between the graphite sheets by any suitable means, such as
by intercalation. 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. Inagaki, Journal of Material Research, Vol 4, No.6,
November/December 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.
[0026] A major advantage of the graphite nanofibers of the present
invention over other graphitic materials is their flexibility with
regard to modification of surface chemistry. For example, 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 NH.sub.4.sup.+
groups) or acidic (addition of COOH.sup.- groups) by use of
appropriate methods. Furthermore, the presence of oxygenated groups
(hydroxyl, peroxide, ether, keto or aldehyde) that are neither
acidic nor basic in nature can impart polarity to the graphite
structure. 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. As a consequence, the interaction of graphitic materials
with aqueous solutions can be greatly enhanced due to the presence
of acid, basic or neutral functionality.
[0027] The distribution of polar groups in active carbon
(non-graphitic) 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.degree. C. or treatment in hydrazine at about
35.degree. C. Following this procedure, the graphite nanofiber will
become hydrophobic. Theodoridou and coworkers, (Met. 14, 125
(1986)), demonstrated that very efficient surface oxidation of
carbon fibers can be achieved by d.c. oxidation or repetitive
anodic oxidation and cathodic reduction of the material in acidic,
alkaline or neutral aqueous media. It was believed that this method
had the advantage over other procedures in that thick layers of
surface oxides could be produced without damaging the fiber
structure. These workers also capitalized on the conductive
properties of graphitized carbon fibers to introduce various noble
metals onto such materials via the use of electrochemical
procedures. The possibility of controlling the functionality of the
graphite surface could have a direct impact on both the chemistry
of the supported metal particles and their morphological
characteristics.
[0028] The present invention will be illustrated in more detail
with reference to the following examples, which should not be
construed to be limiting in scope of the present invention.
[0029] Gas flow reactor experiments were carried out in a
horizontal quartz tube (40 mm i.d. and 90 cm long) contained in a
Lindberg tube furnace, at temperatures over the range of about
450.degree. C. to 700.degree. C. Gas flow rates to the reactor were
regulated by MKS mass flow controllers. In a typical experiment, 50
mg of given catalyst powder was dispersed in a substantially
uniform manner along the base of a ceramic boat, which was
subsequently placed at the center of the reactor tube. After
reduction of the sample at 600.degree. C. for 2 hours, the system
was flushed with helium and brought to the desired temperature
level before being reacted with in the CO/H.sub.2 mixture for a
period of 2 hours. The total amount of solid carbon formed in any
given experiment was determined at the completion of the reaction
by weight difference. The 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 30
m 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.
[0030] The structural details of the carbon materials resulting
from the interaction of the CO/H.sub.2 mixtures with the various
powdered bimetallic catalysts were examined in a JEOL 2000 EX II
transmission electron microscope that was fitted with a high
resolution pole piece capable of providing a lattice resolution of
0.18 nm. Temperature programmed oxidation studies (TPO) of the
various carbon materials were carried out in a Cahn 2000
microbalance in the presence of a CO.sub.2/Ar (1:1) mixture at a
heating rate of 5.degree./min. The degree of crystallization of a
given type of carbon nanostructure was determined from a comparison
of the oxidation profile of two standard materials, amorphous
carbon and single crystal graphite when treated under the same
conditions.
EXAMPLE 1
[0031] In the first set of experiments selected Fe:Cu catalysts
were heated in the presence of a CO/H.sub.2 (4:1) mixture at
temperatures ranging from 450.degree. C. to 700.degree. C. Table I
below 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.degree. C. and 600.degree. C. The most
active catalysts were those that contained a larger fraction of
iron than copper.
1TABLE I Effect of Temperature on the amount of Carbon Nanofibers
(grams/grams of Catalyst) from the Decomposition of CO/H.sub.2 over
selected Fe:Cu Powders Temperature (.degree. C.) Fe:Cu (1:9) Fe:Cu
(3:7) Fe:Cu (7:3) 450 1.10 1.15 1.31 500 2.55 4.15 10.83 525 4.48
550 6.14 9.81 12.02 600 7.86 10.15 11.55 625 5.07 650 3.72 4.21
4.40 700 1.24 1.15 1.31
EXAMPLE 2
[0032] A second series of experiments was carried out at
550.degree. C. under conditions where selected Fe:Cu catalysts were
heated in CO/H.sub.2 mixtures in which the percent of H.sub.2 was
progressively increased. The data presented in Table II below shows
that the number of grams of carbon nanofibers per weight of
catalyst produced after 2.5 hours reached a maximum for each system
when the reactant gas contained between 20 to 50% of hydrogen.
2TABLE II Effect of Percent H.sub.2 in the CO/H.sub.2 reactant
mixture on the amount of Carbon Nanofibers (grams/grams of
Catalyst) formed over Fe:Cu Catalysts at 550.degree. C. Catalyst
20% H.sub.2 50% H.sub.2 80% H.sub.2 Pure Fe 17.53 16.86 14.16 Fe-Cu
(7:3) 16.63 17.23 12.96 Fe-Cu (5:5) 16.41 15.74 12.14 Fe-Cu (3:7)
13.78 13.71 12.51 Fe-Cu (1:9) 8.7 10.41 10.79
EXAMPLE 3
[0033] Another set of experiments was performed at 600.degree. C.
under conditions where selected Fe:Cu catalysts were heated in
CO/H.sub.2 mixtures in which the percent of H.sub.2 was
progressively increased. The data presented in Table III below
shows that in this case the number of grams of carbon nanofibers
per weight of catalyst produced after 2.5 hours reached a maximum
for each system when the reactant gas contained 20% of
hydrogen.
3TABLE III Effect of Percent H.sub.2 in the CO/H.sub.2 reactant
mixture on the amount of Carbon Nanofibers (grams/grams of
Catalyst) formed over Fe:Cu Catalysts at 600.degree. C. Catalyst
20% H.sub.2 33% H.sub.2 50% H.sub.2 67% H.sub.2 80% H.sub.2 Fe-Cu
(1:9) 7.86 7.37 7.11 5.26 3.96 Fe-Cu (3:7) 10.15 8.91 7.44 6.35
4.05 Fe-Cu (7:3) 11.85 9.33 8.99 4.77 3.23
EXAMPLE 4
[0034] In a set of experiments carried out at 600.degree. C. for 2
hours it was found that the number of grams of carbon nanofibers
per weight of catalyst produced after 2.5 hours with a CO/H.sub.2
mixture was dependent upon the percentage of copper in the Fe:Cu
bimetallic catalyst. It can be seen from Table IV below that as the
fraction of copper exceeds 40% there is a gradual decrease in
carbon nanofiber yield. It can also be seen that a catalyst
containing pure copper does not produce carbon nanofibers.
4TABLE IV The effect of catalyst composition on carbon nanofiber
formation from the Fe-Cu catalyzed decomposition of CO/H.sub.2
(4:1) after 1.0 hours at 600.degree. C. % Copper in catalyst Grams
of carbon nanofibers/grams catalyst 0 8.8 30 11.65 50 11.60 70
10.25 80 9.10 90 7.35 95 4.70 100 0
EXAMPLE 5
[0035] In a further set of experiments the overall degree of
crystallinity of the carbon nanofibers produced from the
interaction of selected Fe:Cu catalysts with a CO/H.sub.2 (4:1)
mixture at 600.degree. C. for 2.0 hours was determined from
temperature programmed oxidation of the nanofibers in CO.sub.2. The
characteristics of the controlled gasification of carbonaceous
solids in CO.sub.2 provides a sensitive method of determining the
structural perfection of such materials. 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.
5TABLE V Percent reactivity of carbon nanofibers in CO.sub.2 as a
function of reaction temperatures Carbon Material 805.degree. C.
900.degree. C. 950.degree. C. 1000.degree. C. 1050.degree. C.
Nanofibers from Fe 29.1% 52.0% 72.8% 86.2% 100.0% Nanofibers from
5.2% 12.8% 30.6% 57.0% 100.0% Fe-Cu (7:3)
EXAMPLE 6
[0036] In a series of characterization studies performed in a high
resolution transmission electron microscope, small sections of
carbon nanofibers grown from the decomposition of CO/H.sub.2
mixtures at 600.degree. C. over various metal and bimetallic
catalyst systems were examined and representative micrographs taken
of each sample. A compilation of the observations made from
inspection of several micrographs from each sample is given in
Table VI below. Also included for comparison purposes are
corresponding data for nanofibers grown from the interaction of the
same series of catalysts with C.sub.2H.sub.4/H.sub.2 at 600.degree.
C.
6TABLE VI Comparison of structural features of carbon nanofibers
from the decomposition of CO/H.sub.2 (4:1) and
C.sub.2H.sub.4/H.sub.2 (4:1) over various metal and bimetallic
catalysts at 600.degree. C. Nanofiber Structure Catalyst
C.sub.2H.sub.4/H.sub.2 CO/H.sub.2 Fe No nanofiber growth Platelet
Ni Straight amorphous No nanofiber growth nanofibers Co Straight
amorphous No nanofiber growth nanofibers Fe--Ni Straight coiled
& branched Faceted Tubular/Ribbon "herring-bone" Ni--Cu
Straight coiled & branched No nanofiber growth "herring-bone"
Co--Cu Amorphous straight, No nanofiber growth Coiled &
branched Fe--Cu Straight coiled & branched Platelet
"herring-bone"
[0037] 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
[0038] 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.sub.2
mixtures over a powdered iron catalyst at temperatures over the
range 550 to 670.degree. C. were examined. The data presented in
Table VII below indicates that there is a very narrow temperature
window, 600 to 625.degree. 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.degree. C. there is a
tendency for the structures to acquire a faceted tubular or ribbon
arrangement, which becomes the only form at 670.degree. C.
7TABLE VII Characteristics of carbon nanofibers produced from the
iron catalyzed decomposition of a CO/H.sub.2 (4:1) mixture as a
function of reaction temperature Catalyst Temperature (.degree. C.)
Nanofiber Structure Fe 550 Herring-bone & Platelet Fe 580
Herring-bone & Platelet Fe 600 Platelet Fe 625 Platelet Fe 650
Platelet & Faceted Tubular/Ribbon Fe 670 Faceted
Tubular/Ribbon
EXAMPLE 8
[0039] 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.sub.2
mixtures over a powdered iron-copper (7:3) catalyst at temperatures
over the range 550 to 670.degree. C. were examined. The
observations from these experiments are presented in Table VIII
below.
8TABLE VIII Characteristics of carbon nanofibers produced from the
iron-copper (7:3) catalyzed decomposition of a CO/H.sub.2 (4:1)
mixture as a function of reaction temperature Catalyst Temperature
(.degree. C.) Nanofiber Structure Fe-Cu (7:3) 550 Herring-bone
& Platelet Fe-Cu (7:3) 575 Platelet Fe-Cu (7:3) 600 Platelet
Fe-Cu (7:3) 625 Platelet Fe-Cu (7:3) 650 Platelet & Faceted
Tubular/Ribbon Fe-Cu (7:3) 670 Faceted Tubular
* * * * *