Note: Descriptions are shown in the official language in which they were submitted.
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Catalysts f r the Large Scale Fa9'tDd$L9CtiO611 of High
Purity Carbon Nanotubes with ChemicaC Vapor Deposition
In the present invention, advanced catalysts fQr the production
of high purity and high quality carbon nanotubes vvith the
technique of chemical vapor deposition are described. -iuhe use
of these catalysts and the described methodology allows the
production of carbon nanotubes at high rates and wit'ri yields
per mass unit of catalytic material, which are rnuch higiner than
those achieved with other methods of carbori ri ,anc}t z.ab es
production and other catalysts. The high yieids, tl-1 E~ 11-1 igh
production rates and the very low cost of the catalysts that are
employed in the developed method lead to the produ-r:=tion of
materials that cost much less than commercially s:vai'~able
materials of similar or lower quality. The catalyst or the
catalytic substrate on which carbori is deposited and clrovsrs in
the form of nanotubes consists of the carrier or the :;U4:.>strate,
which is aluminum oxide (alumina) or one of the o=tl~ier rjietal
oxides that are usually employed as catalytic media, the active
phase which is iron oxide (preferably hematite but also i=any
other form) and a promoter, such as molybdenurri o;cide, "I"I-ie
ratio of these three componerits plays a very irnportai-ii: rOiE,~ in
the composition of the catalyst.
Background of the invention
The present inventiori refers to a method for the developmt,rit
of catalysts and catalytic substrates and their use r"or the
large-scale production of carbon nanotubes with c;hernical
vapor deposition. The developed nanotubes are eii:hc,r ciryc~lE-
wall or multi-wall, depending on the employed catalyst and the
employed hydrocarbon. In particular, the present inven=tion
refers to a procedure that leads to the developrnent of catalytic
substrates o=F specific composition. The catalytic substr~ate:a are
used for the deposition of carbon in nanotubes form on their
surface, in a way that provides the capability of hiqh yield
relative to the initial weight of the substrate. Carbon
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deposition and nanotubes synthesis take place with the methcd
of chemical vapor deposition. This method ensures the
production of carbon nanotubes of constant quali -l:y ,3- t a
relatively low temperature and atmospheric pressure arid gives
solution to the problem of carbon nanotubes higi,i pri:adUction
cost.
Many scientists characterize carbon nanotubes as the
"materials of the future". The nanotubes - cylindrical carbon
structures with diameters that range from 0.6 nanometers. {0.6
x 10-6 m) to 300 nanometers - are materials that ~:,,ornbine
exceptional mechanical and electrical properties. For this
reason, there is a lot of interest in the production r.if such
materials that can find many applications, as it can be seen in
various publications [1-3].
Carbon nanotubes are materials that exhibit unique propY.=Prties
such as high electrical and thermal conductivity. They also
have exceptionally high mechanical strength (100 tirnes, higher
than that of steel) and combine high surface area with low
weight. Thus, they can be employed in a variety of applications
including microelectronics (since they behave as conductive or
semiconductive materials depending on their structure),
batteries (Li storage), flat panel displays, hydrogen fLiel cells,
adsorption materials and membranes for separations. Carbon
nanotubes can also be used as components of composite
materia(s for reinforcement or modification of properties (e.g.
electrical conductivity of piastics), as microscope probes, in
materials of electromagnetic shields, and in high==;:strength
structures and applications.
A characteristic of the importance that is being given in the
potential applications of carbon nanotubes in the last years is
the data provided by several companies that are active in the
fields of investments and economic analysis [4-9]. Accordirig to
these data, 5-10 million dollars were invested on research for
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the development and production of carbon nanotubes in 2002,
while several tons of carbon nanotubes were globally produced,
the total cost of which amounts to millions of dollars.
Carbon nanotubes are mainly produced by 1) sublimation of
graphite rodslelectrodes with arc discharge, 2) laser ablation,
3) catalytic decomposition of carbon-bearing gases (usuaiiy
hydrocarbons or carbon monoxide) with the use of inetal
catalysts supported on metal oxide substrates or suspended in
the gas phase (catalytic chemical vapor deposition), and 4)
decomposition of gaseous or liquid compounds with arc
discharge. Metal catalysts are not only employed in chernical
vapor deposition but in other techniques as well. The carbon
products that are obtained with arc discharge are mixtures of
single-wall and multi-wall nanotubes, fulierenes and relatively
high amounts of amorphous carbon. Similai- drawbacks are
encountered in the method of iaser ablation where iarge
amounts of amorphous carbon and multi-wall nanotube:::; with a
lot of structural defects are produced. Laser ablation i,,:. also a
costly technique with high power requirements. Despit(:,l the
fact that these methods can produce significant quantities of
nanostructured carbon, they consume a lot of energy and their
products have a low concentration of single-wall nanotubes and
a high concentration of multi-wall nanotubes. Nanotube
enrichment techniques have been developed, but ii-reir
complexity and their high cost affect significantly the final cost
of pure products, the prices of which prohibit their use in a
wide range of applications.
Methods based on chemical vapor deposition (CVD) can be
employed for the production of high quality carbon nanotubes
for various applications because of their capability for large
scale production and control of the synthesis procedure vvith
the use of the appropriate catalyst. Chemical vapor deposition
can lead to long (almost 2mm) carbon nanotubes of relatively
high purity, good alignment and uniformity throughout their
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length, and high carbon yield (in percentage of the overall
feed) in relatively mild conditions, compared to the methods of
arc discharge and laser ablation. For these reasons, c~VE) is
the most attractive method for the production of carbon
nanotubes in industrial scale.
The basic problem in the wide use of carbon nanotubes is their
high production cost, which is multiple of the cost of ~:aoid per
gram. The high production cost is mainly attributed tt) the use
of unsuitable and energy-consuming methods, as well as
unmanageable systems of reactors and catalysts, the
nanostructures production yield of which is lirnited. The high
cost of carbon nanotubes renders research in the field t:;f their
potential applications almost prohibitive.
The unique properties of these materials though, make thiam
very attractive for applications that involve composite
materials, and specifically, changes in their mechansr;al arzd
electrical properties with the incorporatiori of carbon
nanotubes in their structure. The use of carbon nanol:Uibe., in
hydrogen storage for fuel cell applications as well as in the
fabrication of nanoelectronic materials and parts has br-;en also
suggested.
The present invention erisures a very high yield in the process
of carbon nanotubes production, which is capable of pr=oviding
large quantities of these materials in a very short timE:, with a
relatively easy and inexpensive way. With the use of the new
catalysts that are described in the present inventiori and the
application of the presented method, the production cost of
carbon nanotubes is significantly reduced, at least by 20 times.
An additional advantage is that the production of nanotubes
takes place without the generation and emission of sigriificant
pollutants. The exceptionally high yield of the process results
from the large activity of the catalysts and the r::a talytic
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substrates, and their capability to absorb all carbon present in
the gaseous precursors.
In addition, the high yield of the method ensures a vey-y clean
5 product that excludes the need for any further purification
process, which in other cases is necessary. The purification
process increases the material cost, is time-consuming and can
cause degradation of the quality of nanotubes.
Description of the invention
Figures Caption
Figure 1. Schematic representation of the experimental
apparatus (vertical reactor) for chemical vapoi- deposition of
carbon nanotubes.
Figure 2. Scanning Electron Microscope image of carbon
nanotubes deposited frorn 31% C2H4 at 700 C on catalytic
substrate of A1203 I Fe /Mo.
The present invention involves catalysts and catalytic
substrates for the production of carbon nanotubes vvith fi.he
technique of chemical vapor deposition, by errti) loying
hydrocarbons, alcohols as well as other molecules that contain
carbon. The catalyst or the catalytic substrate on which carbon
is deposited and grows in the form of nanotubes consists of
the carrier or the substrate, which is aluminum oxide (alumina)
or one of the other metal oxides that are usuaiiy employed as
catalytic media, the active phase of which is irori or iron oxide
(preferably hematite but also any other form) and a pr-orrioter,
such as molybdenum or niolybdenum oxide. The ratio of these
three components plays a very important role in the
composition of the catalyst. The concentration of iron or its
oxide in the carrier (e.g. AI203) is between 5 and 90%,
preferably between 25 and 75%. For example, the employed
catalyst can be a natural material that contains alumina and
iron at the desired ratio, as the red mud iri which i:he
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AIzO3/FeZO3 ratio is 26.4/73.6. The ratio of molybderiurn over
iron (or of their i-espective oxides) is between 0 and 1/'( ,
preferably between 1/10 and 1/3.
The preparation of the catalytic substrate takes place through
dissolution of the right amounts of hydrous nitric salts of iron
(Fe(NO3)3 9H20) and aluminum (A!(N03)3 OFI2O) in asn'iall
volume of methanol or water. The resulting mixture is mixed
with an aqueous solution that contains ammonium rnr.aly'bdate
tetrahydrate ((NH4)6Mo7O24 4H2O). The solution dries in room
temperature for a week until complete evaporation oF ao:-)1:hanol
is achieved, and the remaining mud is baked at: 300-700 :1 for
30 minutes under heliurn flow. The baked material is cooled
under inert gas flow, and subsequently it is groi.ind in a
compact mortar until it turns into a fine red powdr:~~r. This
powder is the catalyst, which is theri placed in the reactor tt:ar
the production of carbon nanotubes.
With the above described method, the iron, alurninurn and
molybdenum of the catalytic substrate are converted to the
respective oxides during the heating of the rria teria i.
Subsequently, the iron oxide (possibly hematite (Fe203)) that
has been formed should be reduced to iron or iron c,arbide in
order to initiate the deposition process. The reductioVI of iron
oxide can take place either with the use of the hydroc:virbon
that is employed for the production of carbon nanotubes or with
the use of hydrogen prior to the beginnirig of the production
process. There are two alternative ways that can be ei-nployed
for this purpose: '1 ) a two-step process which includes (a)
heating of the catalyst (at 500-900 C) in ineri: atmosphere and
(b) exposure of the catalyst in a mixture of a hydrocarboei and
hydrogen that results in the reduction of hematite, anci 2) a
three-step procedure which includes (a) heating of the catalyst
in inert atmosphere, as above, (b) reduction of the cai:alyst in
inert gas/hydrogen flow at a temperature that ranges from 200
to 700 0 C and (c) exposure of the catalyst in a mixture that
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consists of a hydrocarbon species and an inert gas (e.g.,
helium or nitrogen) or a mixture of a hydrocarbon sl::recivs,
hydrogen and an inert gas. In the above mentioned procedures,
the carbon-bearing gas (hydrocarbon) is preferably ethyiene or
methane, and the mixture at which the catalyst is exposed
contains hydrogen at a percentage that ranges between 10 and
200 % of the hydrocarbon concentration.
For the production of carbon nanotubes, the catalyst or the
catalytic substrate that was prepared according to thr: above
described method is placed in a suitable reactor, a;~~ for
example the one that is shown in Figure 1. The specific reactor
consists of a vertical quartz tube (1) with 15 mm tnl:ernal
diameter, which is heated by a two-temperature zone furnace
with 22 cm length (2). Two K-thermocouples (3) are ernployed
for temperature measurements and are placed at the center of
each heating zone. The temperature is coritrollec.l by a
temperature controller (4). The rate of the deposition of carbon
nanotubes on the catalytic substrate (5) is measured
gravimetrically by i-ecording the change in the weight of the
catalytic substrate. In the apparatus that is presented in Figure
1, the reactor tube is being coupled to an electrccnic
microbalance of 1 microgram (pg) sensitivity (CAHN D-101)(5)
for continuous monitoring of the weight of the deposit, ur-itil
100 grams (7). The catalytic substrate is placed iri a::ah;Wow
container made of quartz or platinum, or other inert and
resistant material, which is hung from the sample arni of the
microbalance with a thin wire (8) aligned to the reactor axi.,ti.
For the production of carbon nanotubes in a larger scale, and
provided that the precise monitoring of the rate of carbon
nanotubes deposition on the catalytic substrate is riot
necessary, a vertical or horizontal quartz tube of larger
diameter is employed as the reactor, without the use of a
sensitive microbalance. The catalytic substrate is inseri:ed in a
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suitable quartz container, which is placed in the midclie o-f the
quartz tube.
The gas mixture that contains the carbon source is supplied to
the reactor through an appropriate system of preS sure
controllers (9, '10), valves (11), a pump (12) and mass flow
controllers (13). This system determines the gas composition
and flow. The stream that contains the carbon source (e.g,,
ethylene, methane or another hydrocarbon, or alr..nhol or
carbon monoxide) (14) is mixed with an inert gas (15), and if
chosen with hydrogen (16), and the total stream is driven into
the reactor where it flows above the quartz contairi(:~r that
encloses the catalyst. T'he gas comes in contact with the
catalyst and carbon nanotubes are produced. The clasead.i,,, by==
products of the production reaction are safely driven to the
exhaust line (17). It should be pointed out that the apparatus
described above, as well as the reactor are only giveti as an
example. Any suitable arrangement and any hydrocarbon would
produce carbon nanotubes at the same rate and with t1-1i-; same
quality, provided that the employed catalyst was -rhe one
described in the present invention.
Any hydrocarbon or alcohol or other organic or irtorganic
material that contains carbon can be used as carbon source,
Better results are obtained when employing ethylene. For
example, when the above ethylene mixture, with a
concentration of 31% in ethylene, is supplied to the reactor
that contains the above described catalyst, the yield of the
production of carbon nanotubes surpasses 2000% -relative to
the initial weight of the mixture of the oxides that comprise; the
catalytic substrate - in less than 20 minutes. The nanotubes
that are produced this way are multi-wall carbon nanotubes,
and their purity exceeds 95%. Their diameter ranges frorn 'i 5 to
nanometers, and their length is of several micrometers as it
35 is shown in the pictures that were taken with a scanning
electron microscope (Figure 2).
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The use of methane as hydrocarbon and a sir-nilar procedure
lead to the production of mixtures of single-wall and muli:i-wall
carbon nanotubes with a 700% yield relative to the initial
weight of the mixture of the oxides that comprise the c:atalytic
substrate. The methane concentration in the mixture of tf7e
deposition reaction is 36%. The rate of the deposition reaction
for this case is lower than that of the case where ethylene is
employed. However, the purity (88%) and the quality of the
nanotubes are very good. Scanning electron microscopy
revealed the presence of carbon tubes with diameters in the
range of 10-40 nariometers. The use of Raman spectroscopy
proved the existence of single-wall nanotubes.
Example 1: Production of Mul$i-waiB Carbon Nanot:ubas =t'rorru
Ethylene
Preparation of the Catalytic Substrate
The catalytic substrate for carbon deposition was prepared as
following: in high purity methanol solution of approximately 8-
10 ml volurne, 3.71 g of iron nitrate (Fe(N03)3 9H20) and
1.948 g of aluminum nitrate (AI(N03) 9H20) were dissoived.
In addition, 0.18 g of ammonium molybdate tetrahydrate
((NH4)6M07024 4H2O) were initially dissolved in 3-5 mf of
water and subsequently added to the methanol solution. The
resulting solution was left in environmental conditiorIs urrtil
methanol evaporated, and the generated mud was placed in a
shallow quartz container and heated at 700 C for 30 rriinutes
under helium flow. Subsequently, the solid material was cooled
slowly to room temperature and it was then ground in a
compact mortar. This procedure led to the creation of a red
powder, which contained Fe203 and A1203 in a ratio of 74/26
whereas the Fe/Mo ratio was equal to 5/1.
Production of Carbon Nanotubes
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The material that resulted from the above procedure was
employed as the catalytic substrate in the process of carbon
nanotubes production. A catalyst quantity equal to 2.8 rn'j was
placed in a shallow platinum container, which was hung from
5 the sample arm of the microbalance with a thin wire aligned to
the axis of the reactor, which was placed inside a furnace. The
reactor was a quartz tube of 15 mm internal diameter and 22
cm length. The catalytic substrate was heated under 200 sccm
helium (He) flow until the temperature reached 700 l:".. After
10 approximately 30 min and with the temperature having reached
700 C, an ethylene-helium mixture, in which the C2H4/He ratio
was 63/137, was allowed to enter the reactor at 200 scorn total
flow by opening the respective valve. Initially, and fc7r .:ibout
one minute, a slight weight loss (0.15 mg) was observed that is
correlated to the procedure of catalyst activation via the
process of the reduction of the initial oxide. Subsequently, a
continuous increase of the weight of the material iri the
container was observed and after 7 min it reached 48.6 n-ig.
The increase of the weight of the material in the contairier as a
result of the carbon deposition was more than 18 tirne s the
weight of the catalytic substrate that was initially place(:l in it.
The material was characterized without being previously
subjected to any treatment for the removal of the catalytic
substrate, soot, or other carbon forms that were possibly
generated during the deposition process. The presc,ance of
multi-wall carbon nanotubes was confirmed with scanning
electron microscopy (SEM). The average diameter of the
nanotubes was estimated to be 10-20 nm and their lengf:h a few
pm. The characterization of the material - as obtained afte:r the
deposition process - with Raman spectroscopy revealed
characteristics of graphitic forms of carbon. The specific
surface area of the material was measured to be 230 m2/g.
Example 2: Production of Single-wao8 Carfooru Nanratubes
from Methane
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In a second procedure, for the production of single-wall carbon
nanotubes, a quantity of the catalytic substrate (tE-iat was
prepared with the procedure described in Example 1) equa{ to
2.4 mg was placed in a shallow platinum container and
positioned in the same apparatus that was describeca in
Example 1. The catalytic substrate was heated under 200 sc.crn
helium (He) flow until 700 C. After approximately 30 min and
with the temperature having reached 700 C, a rneth ei ri r.,-
helium-hydrogen mixture, with CH4/H2/He ratio ecl u ,.Ql to
73/67/60, was allowed to enter the reactor at 200 scc.;rn total
flow by opening the respective valve. Initially, and for about
five minutes, a slight weight loss (around 0.3 mg) was
observed that is correlated to the procedure of catalyst
activation. Subsequently, a continuous increase of the ve,ffwight
of the material in the container was observed and after 22 miri
it reached 4.8 mg.
The material was characterized without being previously
subjected to any treatment for the removal of the catalytic
substrate, soot, or other carbon forms that were possibly
generated during the deposition process. The diameter of the
observed tubes was 15 rim, which is a characteristic size of
single-wall carbon nanotube bundles. The size r.) f the
nanotubes is considered to be less thari 2 nm whereas their
length was estimated to be a few pm. The characterization of
the material - as obtained after the deposition process - with
Raman spectroscopy revealed the absence of an-i orphous
carbon and structural defects, as well as the presence ol' Pyllõiiti-
wall carbon rianotubes.
References
1. R.H. Baughman, A.A. Zakhidov and W.A. cle Heer, "Carbon
Nanotubes - The Route toward Applications", Science, 297
(2002).
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2. J. Kong, A.M. Cassell and H. Dai, Chem. Phys. Lett., 292,
567-574 (1998).
3. M. Su, B. Zheng and J. Liu, Chem. Phys. Lett., 322, -321-326
(2000).
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&
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31
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