Note: Descriptions are shown in the official language in which they were submitted.
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Description
HIGH YIELD VAPOR PHASE DEPOSITION METHOD FOR
LARGE SCALE SINGLE WALLED CARBON NANOTUBE PREPARATION
Government Interest
This work is in part supported by Office of Naval Research grant
#00014-98-1-0597 through the University of North Carolina in Chapel Hill,
North Carolina, United States of America. Thus, the United States government
has certain rights in the invention.
Technical Field
The present invention relates, in general, to a vapor phase deposition
method for the preparation of single walled carbon nanotubes, where the
method employs a metal catalyst on a support. More particularly, the present
invention relates to an improved method where the support comprises an
aerogel, such as an AI203 aerogel or an AI203/Si02 aerogel, as compared to
prior art methods that employed supports that are powders. The improved
method results in far higher yields of single walled carbon nanotubes than the
prior art methods.
Abbreviations
ASB aluminum tri-sec-butoxide
AFM atomic force microscope
(acac)2 bis(acetylacetonato)
cm centimeter
C centrigrade
CVD chemical vapor deposition
EtOH ethanol
g gram
kg kilogram
kV kilovolt
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m meter
ml milliliter
MW molecular weight
MWCNT multi-walled carbon nanotube
nm nanometer
psi pounds per square inch
SEM scanning electron microscope
SWCNT single walled carbon nanotube
sccm standard cubic centimeter per minute
STP standard temperature and pressue
Tpa tera pascal
TGA thermal gravimetric analyzer
TEM transmission electron microscope
Background of the Invention
Ever since the discovery by lijima in 1991 of the carbon nanotube, it has
been one of the most actively studied materials in today's research. See,
lijima, Vol. 354, Nature, pp. 56-58 (1991 ). This active study is not very
surprising given the outstanding chemical and physical properties that this
material possesses and its potential applications in many different fields.
For example, depending on the number of concentric walls of a
graphene sheet and the ways that a graphene sheet is rolled into a cylinder,
carbon nanotubes can be conductive behaving like metals or can be semi-
conductive. See, Dresselhaus et al., Science of Fullerenes and Carbon
Nanotubes, Academic Press, San Diego (1996).
Furthermore, experiments have shown that individual carbon nanotubes
can behave as quantum wires and can even be made into room temperature
transistors. See, Tans et al., Vol. 386, Nature, pp. 474-477 (1997) vis-a-vis
quantum wires and Tans et al., Vol. 393, Nature, pp. 49-52 (1998) vis-a-vis
transistors.
In addition, carbon nanotubes have been shown to possess superior
mechanical properties and chemical stability. Experimental measurements of
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carbon nanotubes for Young's moduli by AFM and for thermal vibrations
afforded respective values of 1.3 Tpa and 1.8 Tpa, which are higher than the
values for any other known material. See, Wong et al. Vol. 277, Science, pp.
1971-1975 (1997) vis-a-vis AFM and Treacy et al., Vol. 381, Nature, pp. 678
680 (1996) vis-a-vis thermal vibrations.
Consequently, the chemical stability, the superior mechanical properties,
the ballistic transport property of metallic-like behavior, and the rich
variation
in electronic properties due to different helicities make carbon nanotubes
ideal
candidates for high strength composite materials, and for interconnections and
functional devices in molecular electronics.
Although carbon nanotube materials possess many unique and
technically important properties, lack of a way to produce a sufficient amount
of materials has limited not only the study of the fundamental properties but
also the development of more practical applications. The discovery of a low
cost, high yield method for preparation of SWCNT material will certainly solve
one of the biggest problems facing this field in the past and open new
opportunities for a wide variety of applications.
Currently, carbon nanotubes are synthesized by three different
techniques: (1 ) arc discharge between two graphite electrodes, (2) CVD
through catalytic decomposition of a hydrocarbon or of CO, and (3) laser
evaporation of the carbon target. With respect to CVD, see, International
Publication No. WO 89/07163 to Synder et al.; U.S. Patent No. 4,663,230
(issued in 1987) to Tennent et al.; M. Terrones et al., Nature 388, 52-55
(1997); Z. F. Ren et al., Science 282, 1105-1107 (1998); J. Kong, A. Cassell,
and H. Dai, Chemical Physics Letters 292, 4-6 (1998); J. H. Hafner et al.,
Chemical Physics Letters 296, 195-202 (1998); E. Flahaut et al., Chemical
Physics Letters 300, 236-242 (1999); S. S. Fan et al., Science 283, 512-514
(1999); H. J. Dai et al., Chemical Physics Letters 260, 471-475 (1996); H. M.
Cheng et al., Applied Physics Letters 72, 3282-3284 (1998); and A. M. Cassell,
J. A. Raymakers, J. Kong, and H. J. Dai, Journal of Physical Chemistry 8103,
6484-6492 (1999).
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Both the laser method and the arc method yield high quality SWCNTs.
However, both techniques suffer from the problem that it is hard to increase
the
production volume of the nanotube materials from laboratory scale to
industrial
scale.
On the other hand, based on published reports, the CVD method
appears to represent the best hope for large scale production of nanotube
materials. This method has been reported in the 1980's (by Tennent et al. in
U.S. Patent No. 4,663,230 (issued in 1987) and by M. S. Dresselhaus, G.
Dresselhaus, K. Sugihara, I. L. Spain, and H. A. Goldberg in Graphite Fibers
and Filaments. M. Cardona, et al., Eds., Springer Series in Materials Science
5 Springer-Verlag, New York (1988) vol. 5) for preparation of various carbon
materials such as carbon fibers and multi-walled carbon nanotubes with a yield
higher and on a scale larger than those reported for the laser method and the
arc method.
More recently in the 1990's, there have been reports of SWCNT
preparation by CVD (carbon monoxide or methane) and reports of SWCNT
preparation, mixed with a substantial amount of MWCNT preparation, by CVD
(benzene or ethylene). With respect to carbon monoxide CVD, see, H. J. Dai
et al., Chemical Physics Letters 260, 471-475 (1996) and P. Nikolaev et al.,
Chemical Physics Letters 313, 91 (1999). With respect to methane CVD, see,
A. M. Cassell, J. A. Raymakers, J. Kong, and H. J. Dai, "Large Scale CVD
Synthesis of Single-Walled Carbon Nanotubes", Journal ofPhysical Chemistry
8103, 6484-6492 (1999) and E. Flahaut et al., Chemical Physics Letters 300,
236-242 (1999). With respect to benzene CVD, see, H. M. Cheng et al.,
Applied Physics Letters 72, 3282-3284 (1998). With respect to ethylyne CVD,
see J. H. Hafner et al., Chemical Physics Letters 296, 195-202 (1998). Hence,
although the report for ethylene and the report for benzene each mentions
SWCNTs, they have the drawback that they are always mixed with a
substantial amount of MWCNTs.
Among these reported CVD methods, only the methane CVD method
has been reported to produce high purity and high quality SWCNT materials.
However, the reported yield of the methane CVD method is low, with the best
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results so far giving a total yield of 40% based on the weight of catalyst for
a
reaction time of 10 to 45 minutes, where the catalyst was supported on AI203 ,
powder or on AI203/Si02 powder and the catalystlsupport had a surface area
of about 100 m2/g. See, Cassell et al., supra.
Thus, a CVD method that affords high quality SWCNTs in a far higher
yield (such as at least about 100% for a reaction time of about 30 minutes)
would be desirable.
Summary and Objects of the Invention
Accordingly, the present invention provides vapor phase method that
employs a metal catalyst supported on an aerogel, for instance on AI203
aerogel and/or on AI203/SiOZ aerogel. The catalyst/support employed in the
present invention was made by solvent-gel synthesis with subsequent removal
of the liquid solvent by drying selected from the group consisting of
supercritical
drying, freeze drying and combinations thereof, with supercritical drying
being
preferred. The inventive method involves vapor phase. depositing on the
catalyst/support a carbon-containing compound. The compound should have
a molecular weight of 28 or less, and if the compound has a higher molecular
weight, then the compound should be mixed with H2. The vapor phase
depositing is with sufficient heat for a sufficient time, in order to produce
SWCNTs on the aerogel supported catalyst. Then, the SWCNTs may, if
desired, be removed from the aerogel supported catalyst. Typically, the
SWCNTs are produced in high yield, for instance, about 100% or greater,
based on the weight of the catalyst.
Thus, it is an object of the invention that in a preferred embodiment
SWCNTs are obtained in high yields heretofore unobtainable. This yield is far
higher than that of the prior art CVD method, which resulted, at best, in a
yield
of about 40% based on the weight of the catalyst.
Hence, it is an advantage that this inventive discovery affords a way to
prepare SWCNT materials on a large scale, i.e., industrial scale, with low
cost.
Some of the objects and advantages of the invention having been
stated, other objects will become evident as the description proceeds, when
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taken in connection with the laboratory examples and drawings described
below.
Brief Description of the Drawing
Figure 1 is a graph showing typical TGA yield curves for (a) as-prepared
and (b) purified SWCNT materials in air, made in accordance with the inventive
method.
Figure 2 is a graph showing weight gain versus reaction time at
900°C
with a methane flow at 1158 sccm for a SWCNT material prepared by the
inventive method.
Figures 3a and 3b are, respectively, photographs taken through a
microscope showing (a) a SEM image and (b) a TEM image of a SWCNT
sample prepared by the inventive method on an AIZ03 aerogel supported Fe/Mo
catalyst. The sample was prepared at about 900°C under a CH4 flow. The
flow rate was 1158 sccm. The reaction time was 30 minutes.
Detailed Description of the Invention
The present invention provides single walled carbon nanotubes using
a novel vapor phase method in which a particular catalyst/support is employed
in deposition of a carbon-containing compound. In a preferred embodiment,
the present invention provides a dramatic increase in the yield of single
walled
carbon nanotubes as compared to the prior art method that uses powder for a
support.
By the term single walled carbon nanotubes is meant what is
conventionally known in the art. Moreover, with the inventive method, it is
not
intended to exclude that a minor amount, for instance < 1 %, of multi-walled
carbon nanotubes may be concurrently produced.
A suitable carbon-containing compound may be one that is vapor at STP
or may be one that is capable of being converted into vapor at reaction
conditions. Preferably, the compound is one that has a molecular weight of 28
or less. Examples are CO, CH4, and combinations thereof. If the compound
has a molecular weight greater than 28, for instance benzene (MW = 78) or
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ethylene (MW = 30), then the compound should be admixed with H2, for
instance, 50% by volume H2.
To effect the preferred embodiment of a high yield of about 100% or
more, a sufficient flow rate of the carbon-containing compound should be
employed, and may range from about 900 sccm to about 1300 sccm.
A sufficient time may range from about 0.25 hours to about 7 hours. A
sufficient temperature may range from about 750°C to about
1000°C. The
yield may be about 200%, about 300%, or even higher.
A suitable catalyst is any metal catalyst known in the art for making
nanotubes. A preferred metal catalyst may be Fe/Mo, Fe/Pt, and
combinations thereof. A suitable support is any aerogel as that term is
conventionally adopted in the art to mean a gel with air as dispersing agent
prepared by drying. The aerogel support could be a powdered support
converted to an aerogel by known methods. As discussed in more detail
below, the drying may be supercritical drying or may be freeze drying, but it
is
not intended to include drying that results in a xerogel. A preferred aerogel
support may be AI203 aerogel support, AI203/Si02 aerogel support, and
combinations thereof.
As shown in Figure 1, the yield of the SWCNT material was measured
by heating up the prepared SWCNT material under flowing air in a TGA. The
total yield of SWCNT material, which yield is shown on the vertical axis as a
weight gain, was calculated by the weight loss between 300°C and
700°C,
which temperature is shown on the horizontal axis, where the SWCNT material
burned in air, divided by the weight left at 700°C, which was
presumably the
weight of the catalyst and support materials.
Purification of the material prepared in the inventive method was also
studied. Because of the highly amorphous nature of the aerogel support
prepared in the inventive method, removing the catalyst and support from the
SWCNT material turned out to be quite easy. The support can be removed by
stirring in dilute HF acid, refluxing in another dilute acid (such as HN03),
or
refluxing in dilute base, such as NaOH solution. Figure 1 shows the TGA result
of the material refluxed in 2.6 M HN03 for about 4 hours, followed by
filtration.
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As shown in Figure 2, for a typical growth time of about 60 minutes at
about 900°C, the average yield using this catalyst/support was about
200%.
The maximum yield (weight gain) was found to be about 600% for about 6.5
hours of growth. The inventive method showed a yield of significantly better
than the values previously reported values by A. M. Cassell, J. A. Raymakers,
J. Kong, H. J. Dai, Journal of Physical Chemistry 8 103, 6484-6492 (1999)
Kong, Cassell, and Dai, Chemical Physics Letters 292, 4-6 (1998).
As shown in Figures 3a and 3b, the quality of the prepared SWCNT was
characterized by SEM imaging and TEM imaging.
More particularly, as depicted in Figure 3a, the SEM image of the as-
prepared SWCNT material showed a tangled web-like network of very clean
fibers. The diameters of the fibers appeared to be in the range of about 10 to
about 20 nanometers. It is noted that the SEM image was of as-grown
materials; no purification was performed before the imaging.
Furthermore, as depicted in Figure 3b, the TEM image of the SWCNT
material showed that the fibers observed in the SEM image were actually
bundles of single walled carbon nanotubes. The diameters of the nanotubes
measured from the high resolution TEM images were between about 0.9 and
about 2.7 nm.
Both the SEM and the TEM images showed the SWCNT materials
possessed characteristics similar to those of high quality single walled
carbon
nanotube materials prepared in the laser method (see, A. Thess et al., Science
273, 483-487 (1996) and T. Guo, P. Nikolaev, A. Thess, D. T. Colbert, and R.
E. Smalley, Chemical Physics Letters 243, 49-54 (1995)) and the arc method
(see, M. Wang, X.L. Zhao, M. Ohkohchi, and Y. Ando, Fullerene Science &
Technology 4, 1027-1039 (1996) and C. Journet et al., Nature 388, 756-758
(1997)).
The fact that the SEM image showed only nanotubes, but no amorphous
carbon overcoat, indicated that the catalyst/support surface was substantially
fully covered with nanotube materials. However in the TEM image, for samples
with a weight gain, i.e., yield, higher than about 300%, an amorphous carbon
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overcoat was observed, which probably can be eliminated during production
of SWCNTs and/or removed after production of SWCNTs.
Moreover, the inventive method reflects that a drying process of the wet
gel, as discussed below in the laboratory examples, is a necessary step in
preparing the high performance catalysts on aerogel supports, as employed in
the inventive method. The drying may be achieved by supercritical drying, such
as by C02 supercritical drying, ~ or by ethanol supercritical drying, or
alternatively, may be achieved by freeze drying, such as by freeze drying
using
water, and combinations thereof.
However, it is not intended to include drying that results in a xerogel.
Fricke, Aerogels, Springer-Verlag, Berlin, Heidelberg, New York, Tokyo (1986)
and N. Husing, U. Schubert, Angew. Chem. Int. Ed. 37, 22-45 (1998) discuss
that merely evaporating the liquid solvent at ambient conditions (i.e., about
STP) would cause the gel to shrink due to the collapse of the porous
structures
by the strong forces from surface tension at the liquid/gas interfaces within
the
pores in the gel, and this shrinkage would significantly reduce the total
surface
area and pore volume of the dried material, which is normally called xerogel.
On the other hand, in the supercritical drying process, which is
performed at a temperature well above STP and should also be at a pressure
well above STP, the liquid solvent in the wet gel is put into the
supercritical
state, for instance, under a carbon dioxide blanket. Therefore, there are
substantially no liquid/gas interfaces in the pores during drying. The
original
porous structure in the wet gel is thus substantially maintained in the
resultant
dried catalysts/aerogels.
Also, as more and more nanotubes were grown on the surface of the
aerogel supported catalyst, the diffusion of the carbon-containing compound,
i.e., methane or carbon monoxide in the examples below, to the
catalyst/support became more difficult. Furthermore, since as noted in the
above discussion of Figures 3a and 3b, an amorphous carbon deposition was
observed on the nanotubes at a longer growth time, this carbon overcoat
probably further reduced the diffusion rate of the carbon-containing compound
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to the catalyst/support. This overcoat would explain why the growth rate
slowed down versus time as shown in Figure 2.
In summary, discovered was a new method employing a form of
catalyst/support that can be used in a vapor phase deposition method to
prepare single walled carbon nanotubes, preferably in yields greaterthan those
obtainable by prior art processes. The yield was typically improved by at
least
a factor of 2.5 and often 5, compared with a similar catalyst supported on
AI203
powder.
Laboratory Examples
Materials.
All materials used in the laboratory examples were research grade
materials purchased from different suppliers.
Aluminum tri-sec-butoxide (abbreviated below as ASB), Fe2(S04)3.4H20,
and bis(actylacetonato)dioxomolybdenum (abbreviated below as Mo02(acac)2)
were purchased from Sigma/Aldrich Chemicals.
Reagent grade nitric acid, ammonium hydroxide, and ethanol were
purchased from VWR Scientific Products.
High purity methane, carbon dioxide, and hydrogen were supplied by
National Welders Inc.
EXAMPLE I
Catalyst/Support Preparation.
Catalysts/supports were prepared using the solvent-gel technique , as
reported in D. J. Suh and J.T. Park, Chemistry of Materials 9, 1903-1905
(1997) followed by supercritical drying. Optionally, some were dried by freeze
drying.
In a typical experiment, 23 g of ASB were dissolved in 200 ml of ethanol
as the liquid solvent in a round bottom flask under reflux conditions. Then,
0.1
ml of concentrated HN03, diluted with 1 ml of water and 50 ml of ethanol, was
added into the mixture.
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The resultant was refluxed for 2 hours, until a clear solution was formed,
followed by adding 1.38 g of Fe2(S04)3.4H20 and 0.38 g of MoOZ(acac)2 into
the mixture. The amount of Fe and Mo were chosen so that the molar ratio of
Mo:Fe:AI = 0.16:1:16. After refluxing for 2 more hours, the mixture was cooled
to room temperature and then 5 ml of concentrated NH40H, diluted with 5 ml
of water, was added into the mixture under vigorous stirring, in order to
enhance that the dissolved metal salts would form nm sized hydroxide particles
and would attach to the aerogel. Within a few minutes, a gel formed.
The resultant was left to age for about 10 hours before the supercritical
drying step was performed under the following conditions.
First, the catalyst/support wet gel was sealed in a high-pressure
container, which was then cooled to about 0°C and pressurized to fill
the
container with liquid COz, at about 830 psi (about 59.4 kg/cm2). A solvent
exchange step followed, in order to exchange the ethanol liquid solvent in the
gel with liquid C02, by flushing the container with liquid COZ a few times.
Then, the container was warmed up to between about 50°C and about
200°C, which is above the critical temperature (31 °C) of C02,
and the pressure
was kept between about 1500 psi and 2500 psi (between about 106.4 kg/cmZ
and 176.8 kg/cm2), which is above the critical pressure (1050 psi, 74.8
kg/cm2)
of C02. The system was held at these conditions for a short time before the
pressure was slowly reduced while the temperature was kept the same.
Finally, the temperature was reduced to room temperature. Then, each
catalyst (in metal hydroxide form) on aerogel support was calcined at
500° C
for 30 minutes, to effect conversion to the metal oxide form. Then before
being
used for SWCNT growth, conversion to the metal form was effected by
reduction under H2 for 30 minutes at 900° C. The pressure at that stage
was
about 830 psi (about 59.4 kg/cm2). Each catalyst/support prepared this way
was a catalyst supported on a highly porous, very fine, free-flowing aerogel
with
a surface area of from about 500 m2/g to about 600 m2/g.
Alternatively, instead of C02, some samples were supercritically dried
with ethanol or dried with freeze drying as follows.
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Ethanol supercritical drying: A 100 ml high pressure and high
temperature container was used. At least 35 ml of the wet gel was added in
the container. Before heating, N2 was used to flush the system to drive the
air
out. Then the whole system was sealed and heating was started. After the
temperature reached 260°C, the system was maintained at that
temperature
for about 30 minutes before the EtOH was released slowly. The releasing
process took about 15 minutes. The, the system was cooled down gradually
and the aerogel supported catalyst taken out. The yield of nanotubes for this
was similar to the one dried with C02 .
Freeze drying: The ethanol in the wet gel was replaced by water through
solvent exchange. Then, the sample was frozen with liquid nitrogen and put
in a freeze dryer (Freezone Plus 6, Labconco, Kansas City, Missouri, United
States of America). It took a few days to dry the sample totally and the yield
of this was lower than the one dried with COZ.
SWCNT Growth.
SWCNTs were prepared in a simple vapor phase deposition setup made
of a tube furnace and gas flow control units. In a typical growth experiment,
about 50 mg of a catalyst/support sample were put into an alumina boat inside
a quartz tube. Each sample was individually heated to reaction temperature,
under an Ar flow at a flow rate of about 100 sccm, and then, the Ar was
switched to H2 (about 100 sccm flow rate) for 30 minutes, before switching to
a methane flow (about 1000 sccm) for 30 minutes. An individual sample was
heated for each temperature of about 800°C, about 850°C, about
900°C, and
about 950°C.
The reaction was carried out for the desired time before the methane
flow was turned off and the Ar flow turned on and the temperature reduced to
room temperature. Each resultant was then weighed and characterized.
Characterization.
SWCNT samples were fully characterized using TEM imaging and SEM
imaging.
TEM imaging was performed on a Philip CM-12 microscope operating
at 100 kV. The samples for TEM imaging were prepared by sonicating about
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1 mg of material in 10 ml of methanol for 10 minutes and drying a few drops of
the suspension on a holy-carbon grid.
SEM imaging was performed on a Hitachi S-4700 microscope with a
beam energy of 4 kV by placing the as-grown materials on conductive carbon
tape.
The yield of the SWCNT material with respect to the catalyst was
measured on a thermal gravimetric analyzer (model SDT 2960, purchased from
TA Instruments) under flowing air with a heating rate of
5°C/minute. The
observed yield, measured by TGA, was 100.2% as depicted in Figure 1.
EXAMPLE II
The procedure of Example I was substantially repeated, except this time
with a methane flow for about 60 minutes (instead of about 30 minutes) and a
temperature of about 900°C (instead of various temperatures of about
800°C
about 850°C, about 900°C, and about 950°C) and a flow
rate of about 1158
sccm (instead of about 1000 sccm), during SWCNT growth. The yield
measured by TGA was about 200%.
EXAMPLE III (COMPARISON)
Also, a catalyst/support made from the same AI203 wet gel, but dried
differently to make xerogel, was compared. The aerogel supported catalyst
showed a yield of about 200% of high purity SWCNT under a methane flow at
about 900°C for about 60 minutes, as reported by Example I. On the
other
hand, the xerogel supported catalyst showed a weight gain of <5% under the
same conditions.
EXAMPLE IV
The procedure was repeated as per the Fe/Mo catalyst supported on
AI203 aerogel, but this time with the Fe/Mo catalyst instead supported on Si02
aerogel prepared by a similar method.
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The weight gain of the catalyst on Si02 aerogel under the same
conditions, about 900°C under a methane flow for about 60 minutes, was
almost
10%. Thus, it appears that although a Si02 aerogel support works (i.e., about
10%), it is preferred with the inventive method to employ an AI203 aerogel
support or an AI203/SiOz aerogel support to obtain improvements that are far
superior (i.e., weight gain of about 100% or greater).
EXAMPLE V
The procedure of Example I was substantially repeated, except this time
with CO instead of CH4. Also, the temperature of the CO flow was about
850°C, with a CO flow rate of about 1200 sccm for about 200 minutes.
The
result was a yield of about 150%.
EXAMPLE VI
The procedure of Example I was substantially repeated, except this time
with AI203/SiOZ as the aerogel support, instead of AI203 as the aerogel
support.
Substantially the same results were obtained, except there was more
amorphous carbon.
EXAMPLE VII (COMPARISON)
It is believed that more amorphous carbon resulted in Example VI since
in a comparison, AI203/SiOZ aerogel (without any metal catalyst) was tried
with
methane for 30 minutes at 900°C and this converted the methane to
amorphous carbon.
It will be understood that various details of the invention may be
changed without departing from the scope of the invention. Furthermore, the
foregoing description is for the purpose of illustration only, and not for the
purpose of limitation - the invention being defined by the claims.
