U.S. patent application number 12/693915 was filed with the patent office on 2010-05-20 for carbon nanostructures manufactured from catalytic templating nanoparticles.
This patent application is currently assigned to Headwaters Technology Innovation, LLC. Invention is credited to Martin Fransson, Changkun Liu, Cheng Zhang, Bing Zhou.
Application Number | 20100125035 12/693915 |
Document ID | / |
Family ID | 37945044 |
Filed Date | 2010-05-20 |
United States Patent
Application |
20100125035 |
Kind Code |
A1 |
Zhang; Cheng ; et
al. |
May 20, 2010 |
CARBON NANOSTRUCTURES MANUFACTURED FROM CATALYTIC TEMPLATING
NANOPARTICLES
Abstract
Methods for manufacturing carbon nanostructures include: 1)
forming a plurality of catalytic templating particles using a
plurality of dispersing agent molecules; 2) forming an intermediate
carbon nanostructure by polymerizing a carbon precursor in the
presence of the plurality of templating nanoparticles; 3)
carbonizing the intermediate carbon nanostructure to form a
composite nanostructure; and 4) removing the templating
nanoparticles from the composite nanostructure to yield the carbon
nanostructures. The carbon nanostructures are well-suited for use
as a catalyst support. The carbon nanostructures exhibit high
surface area, high porosity, and high graphitization. Carbon
nanostructures according to the invention can be used as a
substitute for more expensive and likely more fragile carbon
nanotubes.
Inventors: |
Zhang; Cheng; (Pennington,
NJ) ; Fransson; Martin; (Princeton, NJ) ; Liu;
Changkun; (Lawrenceville, NJ) ; Zhou; Bing;
(Cranbury, NJ) |
Correspondence
Address: |
Workman Nydegger;1000 Eagle Gate Tower
60 East South Temple
Salt Lake City
UT
84111
US
|
Assignee: |
Headwaters Technology Innovation,
LLC
Lawrenceville
UT
|
Family ID: |
37945044 |
Appl. No.: |
12/693915 |
Filed: |
January 26, 2010 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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11539042 |
Oct 5, 2006 |
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12693915 |
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60724323 |
Oct 6, 2005 |
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60724315 |
Oct 6, 2005 |
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Current U.S.
Class: |
502/180 ;
423/448; 977/773 |
Current CPC
Class: |
B01J 21/185 20130101;
H01M 4/9083 20130101; B01J 23/40 20130101; C01B 32/05 20170801;
B82Y 30/00 20130101; H01M 4/96 20130101; B01J 21/18 20130101; B01J
37/084 20130101; B01J 23/74 20130101; Y02E 60/50 20130101 |
Class at
Publication: |
502/180 ;
423/448; 977/773 |
International
Class: |
C01B 31/04 20060101
C01B031/04; B01J 21/18 20060101 B01J021/18 |
Claims
1. A composition of matter comprising a plurality of carbon
nanostructures having a graphitic structure and a BET specific
surface area greater than about 120 m.sup.2/g, the carbon
nanostructures being manufactured according to a process
comprising: (i) forming a plurality of catalytic templating
nanoparticles by: (a) reacting a plurality of precursor catalyst
atoms with a plurality of organic dispersing agent molecules to
form complexed catalyst atoms; and (b) allowing or causing the
complexed catalyst atoms to form the templating nanoparticles; (ii)
forming intermediate carbon nanostructures by polymerizing a carbon
precursor in the presence of the templating nanoparticles; (iii)
carbonizing the intermediate carbon nanostructures to form a
plurality of composite nanostructures; and (iv) removing the
templating nanoparticles from the composite nanostructures to yield
the carbon nanostructures.
2. A composition of matter as defined in claim 1, wherein the
precursor catalyst atoms used to form the catalytic templating
nanoparticles comprise at least one of iron, nickel, or cobalt.
3. A composition of matter as defined in claim 1, wherein the
dispersing agent molecules used to form the catalytic templating
nanoparticles are capable of bonding with the precursor catalyst
atoms and comprise at least one functional group selected from the
group consisting of a hydroxyl, a carboxyl, a carbonyl, an amine,
an amide, a nitrile, a nitrogen with a free lone pair of electrons,
an amino acid, a thiol, a sulfonic acid, a sulfonyl halide, an acyl
halide, and combinations thereof.
4. A composition of matter as defined in claim 1, wherein the
dispersing agent molecules used to form the catalytic templating
nanoparticles comprise at least one member selected from the group
consisting of oxalic acid, malic acid, malonic acid, maleic acid,
succinic acid, glycolic acid, lactic acid, glucose, citric acid,
pectins, cellulose, ethanolamine, mercaptoethanol,
2-mercaptoacetate, glycine, sulfobenzyl alcohol, sulfobenzoic acid,
sulfobenzyl thiol, sulfobenzyl amine, polyacrylates,
polyvinylbenzoates, polyvinyl sulfate, polyvinyl sulfonates,
polybisphenol carbonates, polybenzimidizoles, polypyridine,
sulfonated polyethylene terephthalate, and combinations
thereof.
5. A composition of matter as defined in claim 1, wherein the
carbon precursor used to form the intermediate carbon
nanostructures comprises a hydrothermally polymerizable organic
substrate that comprises at least one of citric acid, acrylic acid,
benzoic acid, acrylic ester, butadiene, styrene, or cinnamic
acid.
6. A composition of matter as defined in claim 1, wherein the
carbon precursor used to form the intermediate carbon
nanostructures comprises at least one of
resorcinol-formaldehyde-gel, phenol resin, melamine-formaldehyde
gel, poly(furfuryl alcohol), or poly(acrylonitrile).
7. A composition of matter as defined in claim 1, wherein the
templating nanoparticles are formed prior to being mixed with the
carbon precursor.
8. A composition of matter as defined in claim 1, wherein at least
a portion of the templating nanoparticles are removed from the
composite nanostructures by etching with at least one of an acid or
a base to yield the carbon nanostructures.
9. A composition of matter as defined in claim 1, the carbon
nanostructures being composed of hollow multi-walled structures,
each multi-walled structure being formed from multiple graphitic
layers.
10. A composition of matter as defined in claim 1, wherein (a)
further comprises mixing and reacting a ground state metal
comprising the precursor catalyst atoms with the organic dispersing
agent molecules to form the complexed catalyst atoms and adding a
base to adjust the pH to above 8 and below about 13.
11. A composition of matter as defined in claim 1, further
comprising catalyst particles on the carbon nanostructures.
12. A composition of matter comprising a plurality of carbon
nanostructures having a graphitic structure and a BET specific
surface area greater than about 120 m.sup.2/g and being composed of
hollow multi-walled sphere-like carbon nanostructures, each
multi-walled sphere-like carbon nanostructure being formed from
multiple graphitic layers and having a single interior hole
defining an interior diameter of the sphere-like carbon
nanostructure, the carbon nanostructures being manufactured
according to a process comprising: (i) providing a plurality of
solid catalytic templating nanoparticles consisting essentially of
one or more types of metal catalyst atoms and optionally one or
more types of organic dispersing agent molecules; (ii) mixing the
solid catalytic templating nanoparticles with a carbon precursor
and polymerizing the carbon precursor in the presence of the solid
catalytic templating nanoparticles to form a plurality of
intermediate carbon nanostructures; (iii) carbonizing the
intermediate carbon nanostructures to form a plurality of composite
nanostructures; and (iv) removing the templating nanoparticles from
the composite nanostructures to yield the carbon
nanostructures.
13. A composition of matter as defined in claim 12, wherein the
metal catalyst atoms comprise at least one of iron, nickel, or
cobalt and the dispersing agent molecules comprise at least one
functional group selected from the group consisting of a hydroxyl,
a carboxyl, a carbonyl, an amine, an amide, a nitrile, a nitrogen
with a free lone pair of electrons, an amino acid, a thiol, a
sulfonic acid, a sulfonyl halide, an acyl halide, and combinations
thereof.
14. A composition of matter as defined in claim 12, wherein the
dispersing agent molecules comprise at least one member selected
from the group consisting of oxalic acid, malic acid, malonic acid,
maleic acid, succinic acid, glycolic acid, lactic acid, glucose,
citric acid, pectins, cellulose, ethanolamine, mercaptoethanol,
2-mercaptoacetate, glycine, sulfobenzyl alcohol, sulfobenzoic acid,
sulfobenzyl thiol, sulfobenzyl amine, polyacrylates,
polyvinylbenzoates, polyvinyl sulfate, polyvinyl sulfonates,
polybisphenol carbonates, polybenzimidizoles, polypyridine,
sulfonated polyethylene terephthalate, and combinations
thereof.
15. A composition of matter as defined in claim 12, wherein the
carbon precursor comprises a hydrothermally polymerizable organic
substrate that comprises at least one of citric acid, acrylic acid,
benzoic acid, acrylic ester, butadiene, styrene or cinnamic
acid.
16. A composition of matter as defined in claim 12, wherein the
carbon precursor comprises at least one of
resorcinol-formaldehyde-gel, phenol resin, melamine-formaldehyde
gel, poly(furfuryl alcohol), or poly(acrylonitrile).
17. A composition of matter as defined in claim 12, wherein the
solid catalytic templating nanoparticles are provided in an aqueous
medium having pH greater than 8 and less than about 13.
18. A composition of matter comprising: a plurality of carbon
nanostructures, the carbon nanostructures having a graphitic
structure and a BET specific surface area greater than about 120
m.sup.2/g, the carbon nanostructures being composed of hollow
multi-walled sphere-like carbon nanostructures, each multi-walled
sphere-like carbon nanostructure being formed from multiple
graphitic layers and having a single interior hole defining an
interior diameter of the sphere-like carbon nanostructure.
19. A composition of matter as defined in claim 18, at least some
of the sphere-like carbon nanostructures further including a
catalytic templating nanoparticle disposed within the single
interior hole.
20. A composition of matter as defined in claim 18, further
comprising catalyst particles on the carbon nanostructures.
Description
CROSS-REFERENCE TO RELATED APPLICATION
[0001] This application is a division of co-pending U.S. patent
application Ser. No. 11/539,042, filed Oct. 5, 2006, which claims
the benefit under 35 U.S.C. .sctn.119 of U.S. provisional
application Ser. No. 60/724,323, filed Oct. 6, 2005, and also of
U.S. provisional application Ser. No. 60/724,315, filed Oct. 6,
2005. The disclosures of the foregoing applications are
incorporated herein in their entirety.
BACKGROUND OF THE INVENTION
[0002] 1. The Field of the Invention
[0003] The present invention relates generally to carbon
nanomaterials. More particularly, the present invention relates to
carbon nanostructures that are manufactured using a carbon
precursor and a catalytic templating particle.
[0004] 2. The Related Technology
[0005] Carbon materials have been used in a variety of fields as
high-performance and functional materials. Pyrolysis of organic
compounds is well-known to be one of the most useful methods to
prepare carbon materials. For example, carbon materials can be
produced by pyrolyzing resorcinol-formaldehyde gel at temperatures
above 600.degree. C.
[0006] Most carbon materials obtained by pyrolysis of organic
compounds at temperatures between 600-1400.degree. C. tend to be
amorphous or have a disordered structure. Obtaining highly
crystalline or graphitic carbon materials can be very advantageous
because of the unique properties exhibited by graphite. For
example, graphitic materials can be conductive and form unique
nanomaterials such as carbon nanotubes. However, using existing
methods it is difficult to make these well-crystallized graphite
structures using pyrolysis, especially at temperatures less than
2000.degree. C.
[0007] To acquire the graphite structure at lower temperature many
studies have been carried out on carbonization in the presence of a
metal catalyst. The catalyst is typically a salt of iron, nickel,
or cobalt that is mixed with carbon precursor. Using catalytic
graphitization, graphitic materials can be manufactured at
temperatures between 600.degree. C. and 1400.degree. C. Most
catalytic graphitization methods have focused on making graphite
nanotubes. However, the yield of crystalline materials is still
very low (e.g., for carbon nanotubes the yield is less than 2%).
These low yields make it difficult to use the nanomaterials in
making useful articles.
BRIEF SUMMARY OF THE INVENTION
[0008] The present invention relates to novel methods for
manufacturing carbon nanostructures using a carbon precursor and a
catalyst. The carbon nanostructures are formed around a plurality
of templating nanoparticles. In an exemplary embodiment, the
templating nanoparticles are manufactured from catalytic metal
atoms using an organic dispersing agent. The catalytic
nanoparticles advantageously function both as a nucleating site for
carbon nanostructure formation and as a catalyst during
carbonization and/or polymerization of the carbon precursor.
[0009] The novel methods of making carbon nanostructures according
to the present invention can include all or a portion of the
following steps: [0010] (i) forming a plurality of catalytic
templating nanoparticles by: [0011] (a) reacting a plurality of
precursor catalyst atoms with a plurality of organic dispersing
agent molecules to form complexed catalyst atoms; and [0012] (b)
allowing or causing the complexed catalyst atoms to form the
templating nanoparticles; [0013] (ii) forming one or more
intermediate carbon nanostructures by polymerizing a carbon
precursor in the presence of the templating nanoparticles; [0014]
(iii) carbonizing the intermediate carbon nanostructures to form a
plurality of composite nanostructures; and [0015] (iv) removing the
templating nanoparticles from the composite nanostructures to yield
the carbon nanostructures.
[0016] In the method of the present invention, the dispersed
templating nanoparticles are formed using a dispersing agent. The
dispersing agent is an organic molecule that includes one or more
functional groups that can bond with the catalyst atoms. In a
preferred embodiment, the one or more functional groups comprise a
hydroxyl, a carboxyl, a carbonyl, an amine, an amide, a nitrile, a
nitrogen with a free lone pair of electrons, an amino acid, a
thiol, a sulfonic acid, a sulfonyl halide, an acyl halide, or a
combination of any of these. The dispersing agent molecules bond
with the catalyst atoms to form a complex. The complexed catalyst
atoms then react or agglomerate to form solid catalytic templating
particles. The organic dispersing agent can control the formation,
size, and/or dispersion of the catalytic templating
nanoparticles.
[0017] In the method of the present invention, the catalytic
templating nanoparticles are used as a template for making the
nanostructures. When mixed with the carbon precursor, the
templating nanoparticles provide a nucleation site where
carbonization and/or polymerization can begin or be enhanced.
Because the templating nanoparticles are made from catalytic atoms,
the templating particles can advantageously serve as both a
nucleating site and as a catalyst for carbonization and/or
polymerization. This feature of the invention eliminates the need
to add templating particles and catalyst separately (e.g., silica
sol and metal salts). In this manner solid catalytic templating
particles avoid the situation where the separately added catalyst
atoms undesirably act as a nucleation site. The catalytic
templating nanoparticles of the present invention can
advantageously produce carbon nanostructures having more uniform
features (e.g., inner hole diameter) than carbon nanostructures
manufactured using existing methods.
[0018] In an exemplary embodiment, the method of the present
invention produces carbon nanostructures having a ring shape. The
ring shape can give the carbon nanostructures beneficial properties
such as high porosity and high surface area. Beneficial features
such as these make the carbon nanostructures useful as a support
material for a fuel cell catalyst. The high surface area allows for
high metal loadings while the high porosity improves the
performance of the fuel cell catalyst due to improved diffusion of
reactants. Their high electrical conductivity allows the
nanostructures to be used in the anode or the cathode of a fuel
cell. Carbon nanostructures can be substituted for carbon
nanotubes, which are typically more expensive and likely more
fragile.
[0019] These and other advantages and features of the present
invention will become more fully apparent from the following
description and appended claims as set forth below.
BRIEF DESCRIPTION OF THE DRAWINGS
[0020] To further clarify the above and other advantages and
features of the present invention, a more particular description of
the invention will be rendered by reference to specific embodiments
thereof which are illustrated in the appended drawings. It is
appreciated that these drawings depict only typical embodiments of
the invention and are therefore not to be considered limiting of
its scope. The invention will be described and explained with
additional specificity and detail through the use of the
accompanying drawings in which:
[0021] FIG. 1A is a high resolution TEM image of a plurality of
carbon nanostructures formed according to an exemplary embodiment
of the present invention;
[0022] FIG. 1B is a high resolution TEM image showing a close-up of
various carbon nanostructures of FIG. 1A;
[0023] FIG. 1C is a high resolution TEM image showing yet a closer
image of a carbon nanostructure of FIG. 1A;
[0024] FIG. 2A is a high resolution TEM image of a plurality of
carbon nanostructures formed according to an exemplary embodiment
of the present invention;
[0025] FIG. 2B is a high resolution TEM image showing a closer
image of various carbon nanostructures of FIG. 2A;
[0026] FIG. 3A is a high resolution TEM image of a plurality of
carbon nanostructures formed according to an exemplary embodiment
of the present invention;
[0027] FIG. 3B is a high resolution TEM image showing a closer
image of various carbon nanostructures of FIG. 3A;
[0028] FIG. 4A is a high resolution SEM image of carbon
nanostructures formed according to an exemplary embodiment of the
present invention showing them to be sphere-like in shape; and
[0029] FIG. 4B is a high resolution SEM image showing a closer
image of various carbon nanostructures of FIG. 4A.
DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS
I. Introduction and Definitions
[0030] The present invention is directed to methods of making
carbon nanostructures and the use of the carbon nanostructures as
catalyst supports (e.g., for fuel cell catalysts). Methods for
manufacturing carbon nanostructures generally include 1) forming a
plurality of solid catalytic templating particles by reacting
catalyst atoms with an organic dispersing agent, 2) forming
intermediate carbon nanostructures by polymerizing a carbon
precursor in the presence of the templating nanoparticles, 3)
carbonizing the intermediate carbon nanostructures to form
composite nanostructures, and 4) removing the templating
nanoparticles from the composite nanostructures to leave carbon
nanostructures. The carbon nanostructures manufactured using the
foregoing steps have one or more carbon layers forming a wall that
generally appears to define a carbon nanoring or truncated
tube-like structure when viewed as TEM images but which might be
characterized as hollow but irregular multi-walled sphere-like (or
spheroidal) nanostructures when the TEM images are analyzed in
combination with SEM images of the same material. In one
embodiment, the carbon nanostructures form clusters of grape-like
structures as seen in SEM images but which are known to be hollow
multi-walled nanostructures as shown by TEM images of the same
material.
[0031] For purposes of the present invention, a precursor catalyst
material is any material that can appreciably increase the rate of
carbonization of the carbon precursor when combined therewith.
Non-limiting examples of precursor catalyst materials include iron,
cobalt, and/or nickel.
[0032] Solid catalyst templating particles are particles where
substantially all of the templating particle are made from one or
more catalytic materials.
II. Components Used to Manufacture Carbon Nanostructures
[0033] The following exemplary components can be used to carry out
the above mentioned steps for manufacturing carbon nanostructures
according to the present invention.
[0034] A. Polymerizable Carbon Precursor
[0035] Any type of carbon material can be used as the carbon
precursor of the present invention so long as it can disperse the
templating particles, polymerize to form an intermediate
nanostructure, and become carbonized by heat-treatment. Suitable
compounds include single and multi-ring aromatic compounds such as
benzene and naphthalene derivatives that have polymerizable
functional groups. Also included are ring compounds that can form
single and multi-ring aromatic compounds upon heating. Functional
groups that can participate in polymerization include COOH,
C.dbd.O, OH, C.dbd.C, SO.sub.3, NH.sub.2, SOH, N.dbd.C.dbd.O, and
the like.
[0036] The polymerizable carbon precursor can be a single type of
molecule (e.g., a compound that can polymerize with itself), or the
polymerizable carbon precursor can be a combination of two or more
different compounds that co-polymerize together. For example, in an
exemplary embodiment, the carbon precursor can be a
resorcinol-formaldehyde gel. In this two compound embodiment, the
formaldehyde acts as a cross-linking agent between resorcinol
molecules by polymerizing with the hydroxyl groups of the
resorcinol molecules.
[0037] Other examples of suitable polymerizable precursor materials
include resorcinol, phenol resin, melamine-formaldehyde gel,
poly(furfuryl alcohol), poly(acrylonitrile), sucrose, petroleum
pitch, and the like. Other polymerizable benzenes, quinones, and
similar compounds can also be used as carbon precursors and are
known to those skilled in the art.
[0038] In an exemplary embodiment, the carbon precursor is a
hydrothermally polymerizable organic compound. Suitable organic
compounds of this type include citric acid, acrylic acid, benzoic
acid, acrylic ester, butadiene, styrene, cinnamic acid, and the
like.
[0039] B. Catalytic Templating Nanoparticles
[0040] As described below, the formation of the catalytic
templating particles generally includes reacting a plurality of
templating catalyst atoms with a plurality of dispersing agent
molecules in a solvent to form complexed catalyst atoms. The
complexed catalyst atoms then react to form nanoparticles.
[0041] 1. Carbon Precursor Catalyst Atoms
[0042] The precursor catalyst atom can be any material that can
cause or promote carbonization and/or polymerization of the carbon
precursor. In a preferred embodiment, the catalyst is a transition
metal catalyst including but not limited to iron, cobalt, or
nickel. These transition metal catalysts are particularly useful
for catalyzing many of the polymerization and/or carbonization
reactions involving the carbon precursors described above.
[0043] 2. Dispersing Agents
[0044] In addition to catalyst atoms, the catalyst complexes of the
present invention include one or more dispersing agents. The
dispersing agent is selected to promote the formation of
nanocatalyst particles that have a desired stability, size and/or
uniformity. Dispersing agents within the scope of the invention
include a variety of small organic molecules, polymers and
oligomers. The dispersing agent is able to interact and bond with
catalyst atoms dissolved or dispersed within an appropriate solvent
or carrier through various mechanisms, including ionic bonding,
covalent bonding, Van der Waals interaction/bonding, lone pair
electron bonding, or hydrogen bonding.
[0045] To provide the bonding between the dispersing agent and the
catalyst atoms, the dispersing agent includes one or more
appropriate functional groups. Preferred dispersing agents include
functional groups which have either a charge or one or more lone
pairs of electrons that can be used to complex a metal catalyst
atom, or which can form other types of bonding such as hydrogen
bonding. These functional groups allow the dispersing agent to have
a strong binding interaction with the catalyst atoms.
[0046] The dispersing agent may be a natural or synthetic compound.
In the case where the catalyst atoms are metal and the dispersing
agent is an organic compound, the catalyst complex so formed may be
an organometallic complex.
[0047] In an exemplary embodiment, the functional groups of the
dispersing agent comprise one or more members selected from the
group of a hydroxyl, a carboxyl, a carbonyl, an amine, an amide, a
nitrile, a nitrogen with a free lone pair of electrons, an amino
acid, a thiol, a sulfonic acid, a sulfonyl halide, or an acyl
halide. The dispersing agent can be monofunctional, bifunctional,
or polyfunctional.
[0048] Examples of suitable monofunctional dispersing agents
include alcohols such as ethanol and propanol and carboxylic acids
such as formic acid and acetic acid. Useful bifunctional dispersing
agents include diacids such as oxalic acid, malic acid, malonic
acid, maleic acid, succinic acid, and the like; dialcohols such as
ethylene glycol, propylene glycol, 1,3-propanediol, and the like;
hydroxy acids such as glycolic acid, lactic acid, and the like.
Useful polyfunctional dispersing agents include sugars such as
glucose, polyfunctional carboxylic acids such as citric acid,
pectins, cellulose, and the like. Other useful dispersing agents
include ethanolamine, mercaptoethanol, 2-mercaptoacetate, amino
acids, such as glycine, and sulfonic acids, such as sulfobenzyl
alcohol, sulfobenzoic acid, sulfobenzyl thiol, and sulfobenzyl
amine. The dispersing agent may even include an inorganic component
(e.g., silicon-based).
[0049] Suitable polymers and oligomers within the scope of the
invention include, but are not limited to, polyacrylates,
polyvinylbenzoates, polyvinyl sulfate, polyvinyl sulfonates
including sulfonated styrene, polybisphenol carbonates,
polybenzimidizoles, polypyridine, sulfonated polyethylene
terephthalate. Other suitable polymers include polyvinyl alcohol,
polyethylene glycol, polypropylene glycol, and the like.
[0050] In addition to the characteristics of the dispersing agent,
it can also be advantageous to control the molar ratio of
dispersing agent to the catalyst atoms in a catalyst suspension. A
more useful measurement is the molar ratio between dispersing agent
functional groups and catalyst atoms. For example, in the case of a
divalent metal ion two molar equivalents of a monovalent functional
group would be necessary to provide the theoretical stoichiometric
ratio. In a preferred embodiment, the molar ratio of dispersing
agent functional groups to catalyst atoms is preferably in a range
of about 0.01:1 to about 100:1, more preferably in a range of about
0.05:1 to about 50:1, and most preferably in a range of about 0.1:1
to 20:1.
[0051] The dispersing agents of the present invention allow for the
formation of very small and uniform nanoparticles. In general, the
nanocatalyst particles formed in the presence of the dispersing
agent are less than 1 micron in size. Preferably the nanoparticles
are less than 100 nm, more preferably less than 50 nm and most
preferably less than 20 nm.
[0052] During pyrolysis of the carbon precursor, the dispersing
agent can inhibit agglomeration and deactivation of the catalyst
particles. This ability to inhibit deactivation can increase the
temperature at which the nanocatalysts can perform and/or increase
the useful life of the nanocatalyst in the extreme conditions of
pyrolysis. Even if including the dispersing agent only preserves
catalytic activity for a few additional milliseconds, or even
microseconds, the increased duration of the nanocatalyst can be
very beneficial at high temperatures, given the dynamics of
carbonization.
[0053] 3. Solvents and Other Additives
[0054] The liquid medium in which the catalytic templating
nanoparticles are prepared may contain various solvents, including
water and organic solvents. Solvents participate in particle
formation by providing a liquid medium for the interaction of
catalyst atoms and dispersing agent. In some cases, the solvent may
act as a secondary dispersing agent in combination with a primary
dispersing agent that is not acting as a solvent. In one
embodiment, the solvent also allows the nanoparticles to form a
suspension. Suitable solvents include water, methanol, ethanol,
n-propanol, isopropyl alcohol, acetonitrile, acetone,
tetrahydrofuran, ethylene glycol, dimethylformamide,
dimethylsulfoxide, methylene chloride, and the like, including
mixtures thereof.
[0055] The catalyst composition can also include additives to
assist in the formation of the nanocatalyst particles. For example,
mineral acids and basic compounds can be added, preferably in small
quantities (e.g., less than 5 wt %). Examples of mineral acids that
can be used include hydrochloric acid, nitric acid, sulfuric acid,
phosphoric acid, and the like. Examples of basic compounds include
sodium hydroxide, potassium hydroxide, calcium hydroxide, ammonium
hydroxide, and similar compounds.
[0056] It is also possible to add solid materials to assist in
nanoparticle formation. For example, ion exchange resins may be
added to the solution during catalyst formation. Ion exchange
resins can be substituted for the acids or bases mentioned above.
Solid materials can be easy separated from the final iron catalyst
solution or suspension using simple techniques such as
centrifugation and filtration.
III. Manufacturing Carbon Nanostructures
[0057] The carbon nanostructures of the present invention can be
manufactured using all or a portion of the following steps: (i)
forming a plurality of dispersed catalytic templating nanoparticles
by reacting a plurality of precursor catalyst atoms with a
plurality of dispersing agent molecules, (ii) mixing the plurality
of catalytic templating nanoparticles (e.g., iron particles) with a
carbon precursor (e.g., citric acid) and allowing or causing the
carbon precursor to polymerize to form a plurality of intermediate
nanostructures, (iii) carbonizing the intermediate nanostructures
to form a plurality of composite nanostructures, and (iv) removing
the templating nanoparticles from the plurality of composite
nanostructures to yield the carbon nanostructures.
[0058] A. Providing Catalytic Templating Nanoparticles
[0059] The process for manufacturing the nanoparticles can be
broadly summarized as follows. First, one or more types of
precursor catalyst atoms and one or more types of dispersing agents
are selected. Second, the precursor catalyst atoms (e.g., in the
form of a ground state metal or metal salt) and dispersing agent
(e.g., in the form of a carboxylic acid or its salt) are reacted or
combined together to form catalyst complexes. The catalyst
complexes are generally formed by first dissolving the catalyst
atoms and dispersing agent in an appropriate solvent and then
allowing the catalyst atoms to bond with the dispersing agent
molecules. The various components may be combined or mixed in any
sequence or combination. In addition, a subset of the components
can be premixed prior to addition of other components, or all
components may be simultaneously combined.
[0060] In one aspect of the invention, the catalyst complexes may
be considered to be the complexed catalyst atoms and dispersing
agent, exclusive of the surrounding solvent. Indeed, it is within
the scope of the invention to create catalyst complexes in a
solution and then remove the solvent to yield dried catalyst
complexes. The dried catalyst complexes can be reconstituted by
adding an appropriate solvent.
[0061] In an exemplary embodiment, the components are mixed for a
period of about 1 hour to about 14 days. This mixing is typically
conducted at temperatures ranging from 0.degree. C. to 200.degree.
C. Preferably the temperature does not exceed 100.degree. C.
[0062] The precursor catalyst atoms are typically provided in the
form of an iron salt such as iron chloride, iron nitrate, iron
sulfate, or the like. These compounds are often soluble in an
aqueous solvent. Formation of the catalyst nanoparticles using
metal salts can lead to the formation of additional by-products
from the release of the anion. If desired, formation of an anion
can be avoided by using a metal powder (e.g., iron). Typically the
only significant by-product of the catalyst preparation using iron
metal is hydrogen gas, which is evolved during the mixing
procedure. If the catalyst particles are made using a material that
evolves hydrogen gas or another gas, the mixture is typically
vented and/or exposed to air periodically (or continuously) during
the preparation procedure.
[0063] In an exemplary embodiment, the nanocatalyst particles are
in an active form once the mixing step is complete or upon further
reduction using hydrogen, for example. In a preferred embodiment,
the nanocatalyst particles are formed as a suspension of stable
active metal nanocatalyst particles. The stability of the
nanocatalyst particles prevents the particles from agglomerating
together and maintains them in suspension. Even where some or all
of the nanocatalyst particles settle out of solution over time, the
nanocatalyst particles can be easily re-suspended by mixing.
[0064] A base can be added (e.g., concentrated aqueous ammonia) to
adjust the pH of the solution to between about 8 and about 13, and
more preferably between about 10 and about 11. The higher pH can be
useful for precipitating the precursor catalyst atoms in a finely
divided manner.
[0065] The catalytic templating nanoparticles are capable of
catalyzing polymerization and/or carbonization of the carbon
precursor. The foregoing procedure for preparing the catalytic
templating particles can assist in arranging the catalyst atoms
into particles that are catalytically active. In contrast, the
inventors have found that some commercially available reagents
(e.g., at least one commercially available iron citrate) do not
have satisfactory catalytic activity.
[0066] B. Polymerizing the Carbon Precursor
[0067] The catalytic templating nanoparticles are mixed with a
carbon precursor (e.g., citric acid) under conditions suitable for
the carbon precursor to polymerize around the templating
nanoparticles. Because the templating nanoparticles are
catalytically active, the templating nanoparticles can
preferentially accelerate and/or initiate polymerization of the
carbon precursor near the surface of the templating particles.
[0068] The concentration of catalytic templating nanoparticles in
the carbon precursor is typically selected to maximize the number
of carbon nanostructures formed while still producing uniformly
shaped nanostructures. The amount of catalytic templating particles
can vary depending on the type of carbon precursor being used. In
an exemplary embodiment the molar ratio of carbon precursor to
catalyst atoms is about 0.1:1 to about 100:1, more preferably about
1:1 to about 30:1.
[0069] The precursor composition is allowed to cure for sufficient
time such that a plurality of intermediate carbon nanostructures
are formed around the templating nanoparticles. The time needed to
form intermediate nanostructures depends on the temperature, the
type and concentration of the catalyst material, the pH of the
solution, and the type of carbon precursor being used. During
polymerization, the intermediate carbon nanostructures can be
individual organic structures or an association of nanostructures
that break apart during carbonization and/or removal of amorphous
carbon.
[0070] Ammonia added to adjust the pH can also effect
polymerization by increasing the rate of polymerization and by
increasing the amount of cross linking that occurs between
precursor molecules.
[0071] For hydrothermally polymerizable carbon precursors,
polymerization typically occurs at elevated temperatures. In a
preferred embodiment, the carbon precursor is heated to a
temperature of about 0.degree. C. to about 200.degree. C., and more
preferably between about 25.degree. C. to about 120.degree. C.
[0072] An example of a suitable condition for polymerization of
resorcinol-formaldehyde gel (e.g., with iron particles and a
solution pH of 1-14) is a solution temperature between 0.degree. C.
and 90.degree. C. and a cure time of less than 1 hour to about 72
hours. Those skilled in the art can readily determine the
conditions necessary to cure other carbon precursors under the same
or different parameters.
[0073] In an exemplary embodiment the polymerization is not allowed
to continue to completion. Terminating the curing process before
the entire solution is polymerized can help to form a plurality of
intermediate nanostructures that will result in individual
nanostructures, rather than a single mass of carbonized material.
However, the present invention includes embodiments where the
carbon precursor forms a plurality of intermediate carbon
nanostructures that are linked or partially linked to one another.
In this embodiment, individual nanostructures are formed during
carbonization and/or during the removal of amorphous carbon.
[0074] Forming intermediate carbon nanostructures from the
dispersion of templating nanoparticles causes formation of a
plurality of intermediate carbon nanostructures having unique
shapes and sizes. Ultimately, the properties of the nanostructure
depend at least in part on the shape and size of the intermediate
carbon nanostructure. Because of the unique shapes and sizes of the
intermediate carbon nanostructures, the final nanostructures can
have beneficial properties such as high surface area and high
porosity, among others.
[0075] C. Carbonizing the Intermediate Nanostructures
[0076] Once the intermediate nanostructures are obtained, they are
carbonized by heating to produce carbonized composite
nanostructures. In an exemplary embodiment, the intermediate
nanostructures are heated to a temperature between about
500.degree. C. and about 2500.degree. C. During the heating
process, atoms such as oxygen and nitrogen are volatilized or
otherwise removed from the intermediate nanostructure and the
carbon atoms are rearranged or coalesced to form a carbon-based
structure.
[0077] In a preferred embodiment, the carbonizing step produces a
graphite based nanostructure. The graphite based nanostructure has
carbon atoms arranged in sheets of sp.sup.2 hybridized carbon
atoms. The graphitic layers can provide unique and beneficial
properties, such as electrical conduction and structural strength
and/or rigidity.
[0078] D. Removing the Templating Nanoparticles and/or Amorphous
Carbon to Yield Carbon Nanostructures
[0079] In a final step, the templating nanoparticles and/or
extraneous amorphous (i.e., non-graphitic) carbon are removed from
the composite nanostructures. Typically, the templating
nanoparticles are removed using acids or bases such as nitric acid,
hydrogen fluoride, or sodium hydroxide. The method of removing the
templating nanoparticles or amorphous carbon depends on the type of
templating nanoparticle or catalyst atoms in the composite.
Catalyst atoms or particles (e.g., iron particles or atoms) can
typically be removed by refluxing the composite nanostructures in
5.0 M nitric acid solution for about 3-6 hours.
[0080] Any removal process can be used to remove the templating
nanoparticles and/or amorphous carbon so long as the removal
process does not completely destroy the carbon nanospheroidal
and/or nanoring structure. In some cases it can be beneficial to at
least partially remove some of the carbonaceous material from the
intermediate nanostructure during the removal process. It is not
presently known at what point in the method that the annular shape
is formed, whether it is during the polymerization step,
carbonation step, or nanoparticle removal step.
IV. Carbon Nanostructures
[0081] The methods of the present invention produce a multi-walled
carbon nanostructure having useful properties such as unique shape,
size, and electrical properties. In a preferred embodiment, the
carbon nanostructures can be a regular or irregularly shaped
annular structure having a hole therethrough (i.e., a nanoring or
hollow multi-walled, sphere-like or spheroidal structure). The
carbon nanostructures of the present invention are particularly
advantageous for some applications where high porosity, high
surface area, and/or a high degree of graphitization are desired.
Carbon nanostructures manufactured as set forth herein can be
substituted for carbon nanotubes, which are typically far more
expensive.
[0082] The size of the nanostructure is determined in large part by
the size of the templating nanoparticles used to manufacture the
carbon nanostructures. Because the carbon nanostructures form
around the templating nanoparticles, the hole or inner diameter of
the carbon nanostructures typically corresponds to the outer
diameter of the templating nanoparticles. The inner diameter of the
carbon nanostructures can be between 0.5 nm to about 90 nm. For
certain applications such as fuel cells, the inner diameter is
preferably between about 1 nm and about 50 nm.
[0083] FIGS. 1A-1C, 2A-2B, and 3A-3B show TEM images of exemplary
carbon nanostructures made according to the methods of the present
invention, the details of which are described in Example 1 below.
FIGS. 4A-4B show SEM images of exemplary nanostructures made
according to the present invention, the details of which are
described in Example 1 below.
[0084] The generally annular shape of the carbon nanostructures is
shown in the TEM images of FIGS. 1A-1C, 2A-2B, and 3A-3B. The
generally sphere-like shape of the carbon nanostructures is shown
in the SEM images of FIGS. 4A-4B. In many of the carbon
nanostructures shown in the TEM images, the outer ring diameter is
between about 10 nm and about 60 nm and the pore size is about 10
nm to about 40 nm. However, the present invention includes
nanostructures having larger and smaller diameters. Typically, the
carbon nanostructures have an outer diameter that is less than
about 100 nm to maintain structural integrity.
[0085] The thickness of the nanostructure wall is measured from the
inside diameter of the wall to the outside diameter of the wall.
The thickness of the nanostructure can be varied during manufacture
by limiting the extent of polymerization and/or carbonization of
the carbon precursor as described above. Typically, the thickness
of the carbon nanostructure wall is between about 1 nm and 20 nm.
However, thicker and thinner walls can be made if desired. The
advantage of making a thicker wall is greater structural integrity.
The advantage of making a thinner wall is greater surface area and
porosity.
[0086] The wall of the carbon nanostructure can also be formed from
multiple graphitic layers. The TEM images in FIGS. 1A, 1B, and 1C
clearly shows multiple layers. In an exemplary embodiment, the
carbon nanostructures have walls of between about 2 and about 100
graphite layers, more preferably between about 5 and 50 graphite
layers and more preferably between about 5 and 20 graphite layers.
The number of graphitic layers can be varied by varying the
thickness of the carbon nanostructure wall as discussed above. The
graphitic characteristic of the carbon nanostructures is believed
to give the carbon nanostructures beneficial properties that are
similar to the benefits of multi-walled carbon nanotubes (e.g.,
excellent conductivity). They can be substituted for carbon
nanotubes and used in virtually any application where carbon
nanotubes can be used but often with predictably superior
results.
[0087] The carbon nanostructures also have a desired length. The
length of the carbon nanostructure is the length of the hole as
measure along the axis of the hole. If the carbon nanostructure is
lying flat or horizontal, the length of the carbon nanostructure is
the height of the carbon nanostructure. In a preferred embodiment,
the length of the carbon nanostructure is limited by forming the
carbon nanostructures from substantially spherical templating
nanoparticles. Carbon nanostructures formed from spherical
templating nanoparticles typically only have a length that is
approximately the same as the outer diameter of the carbon
nanostructure. Such a result can be obtained because of the
substantially even polymerization and/or carbonization about the
templating nanoparticle. With regard to what appear to be carbon
nanorings in the TEM images, the length typically does not exceed
the outer diameter of the carbon nanoring because the length and
the outer diameter typically grow at substantially the same rate
during polymerization. Carbon nanostructures that have a length
that is less than or about equal to the outer diameter can be
advantageous because of their large surface area and/or because
they can better facilitate diffusion of reactants and reaction
products as compared to, e.g., carbon nanotubes.
[0088] Another feature of the carbon nanostructures of the present
invention is the formation of a non-tubular wall. As shown in the
TEM images of FIGS. 1A, 1B, and 1C, and also the SEM images of
FIGS. 4A and 4B, the graphitic layers form a substantially solid
wall. This is in contrast to attempts by others to make a carbon
nanostructure where the ends of a carbon nanotube are connected to
make a ring. Carbon nanostructures having tubular walls create
undesirable strain that can affect structural integrity and other
properties of the nanostructure. For example, reports in the
literature suggest that kinks in the ring shaped nanotubes prevent
formation of carbon nanostructures smaller than 70 nm in diameter.
In any event, the terms "carbon nanoring" and "carbon
nanostructures" shall exclude ring-like structures formed by
joining opposite ends of a carbon nanotube.
[0089] In addition to good electron transfer, the carbon
nanostructures of the present invention have high porosity and
large surface areas. Adsorption and desorption isotherms indicate
that the carbon nanostructures form a mesoporous material. The BET
specific surface area of the carbon nanostructures can be between
about 80 and about 400 m.sup.2/g and is preferably greater than
about 120 m.sup.2/g, and typically about 200 m.sup.2/g, which is
significantly higher than the typical 100 m.sup.2/g observed for
carbon nanotubes.
[0090] The high surface area and high porosity of the carbon
nanostructures manufactured according to the present invention
makes the carbon nanostructures useful as a support material for
nanoparticle catalysts. Improved diffusion of reactants and/or
electrons through the support material improves the efficiency with
which substrates and electrons can be transferred to the catalytic
surface of the nanoparticles. Consequently, the supported catalysts
of the present invention perform better than nanoparticles
supported on traditional supports such as carbon black.
[0091] As discussed in U.S. application Ser. No. 11/351,620, filed
Feb. 9, 2006, the disclosure of which is incorporate herein,
another use for carbon nanostructures manufactured according to the
invention is as a solid particulate filler material added to a
polymeric material (e.g., as a replacement for carbon black or
carbon nanotubes). Preliminary testing of polymeric materials that
were filled with carbon nanostructures according to the invention
indicates that such filled polymeric materials have substantially
reduced surface resistance compared to polymers filled with a
comparable quantity of carbon black or carbon nanotubes.
V. Examples
[0092] The following examples provide formulas for making carbon
nanostructures according to the present invention.
Example 1
[0093] Example 1 describes a method for making carbon
nanostructures using solid catalytic nanoparticles. A 0.1 M iron
solution was prepared using 2.24 g iron powder, 7.70 g citric acid,
and 400 ml water. The iron-containing mixture was mixed in a closed
bottle on a shaker table for 7 days, with brief interruption (e.g.,
1-2 minutes) to open the container to vent hydrogen and allow air
into the vapor space in the bottle. 100 ml of the iron solution was
slowly added to a mixture of 6.10 g of resorcinol and 9.0 g of
formaldehyde. 30 ml of ammonium hydroxide was added drop-wise with
vigorous stirring. The pH of the resulting suspension was 10.26.
The slurry was then cured at 80-90.degree. C. (oil bath) for 3.5
hours to form an intermediate carbon nanostructure. The
intermediate carbon nanostructure was collected by filtering and
then dried in an oven overnight and then carbonized at 1150.degree.
C. under N.sub.2 flow for 3 hour. The resulting composite
nanostructure was refluxed in 5M HNO.sub.3 for 6-8 hours and then
treated with 300 ml of mixture
(H.sub.2O/H.sub.2SO.sub.4/KMnO.sub.4, molar ratio=1:0.01:0.003) at
90.degree. C. for 3 hours. Finally, the carbon nanostructures were
washed with water, and dried in an oven for 3 hours. The procedure
yielded 1.1 g of carbon nanostructure product (i.e., carbon
nanorings and/or hollow multi-walled sphere-like structures).
[0094] The carbon nanostructures manufactured in Example 1 were
then analyzed, first by TEM and later by SEM. TEM images of the
nanorings from Example 1 are shown in FIGS. 1A-1C, 2A-2B and 3A-3B.
As seen in the TEM images, the method of the present invention can
produce carbon nanostructures that appear to be predominantly
ring-shaped (i.e., "nanorings") and nanostructures of uniform size.
The SEM images of FIGS. 4A-4B of the same carbon nanostructures
indicate that the nanostructures are actually sphere-like (or
spheroidal) rather than ring-shaped. Because the sphere-like
multi-walled carbon nanostructures have a hole in the middle, as
shown by the TEM images, they are not "nano onions", which are
solid.
Example 2
[0095] In example 2, carbon nanostructures were manufactured in a
process similar to Example 1, except that the intermediate carbon
nanostructures were carbonized at 850.degree. C. for 4 hours. The
procedure yielded 1.04 g of carbon nanostructure product (i.e.,
sphere-like multi-walled carbon nanostructures and/or carbon
nanorings).
[0096] The present invention may be embodied in other specific
forms without departing from its spirit or essential
characteristics. The described embodiments are to be considered in
all respects only as illustrative and not restrictive. The scope of
the invention is, therefore, indicated by the appended claims
rather than by the foregoing description. All changes which come
within the meaning and range of equivalency of the claims are to be
embraced within their scope.
* * * * *