U.S. patent application number 10/118834 was filed with the patent office on 2003-05-15 for method and catalyst for producing single walled carbon nanotubes.
Invention is credited to Alvarez, Walter E., Balzano, Leandro, Herrera, Jose E., Resasco, Daniel E..
Application Number | 20030091496 10/118834 |
Document ID | / |
Family ID | 26816789 |
Filed Date | 2003-05-15 |
United States Patent
Application |
20030091496 |
Kind Code |
A1 |
Resasco, Daniel E. ; et
al. |
May 15, 2003 |
Method and catalyst for producing single walled carbon
nanotubes
Abstract
A catalyst composition and method of use of the catalyst
composition for producing single-walled carbon nanotubes (SWNTs).
The catalyst is cobalt (Co) and molybdenum (Mo) on a silica
support. The Mo occurs primarily as dispersed Mo oxide clusters on
the support while the Co is primarily in an octahedral
configuration in a CoMoO.sub.4-like phase disposed on the Mo oxide
clusters. In the method, the catalyst is used and the process
conditions manipulated in such a manner as to enable the diameters
of the SWNTs to be substantially controlled.
Inventors: |
Resasco, Daniel E.; (Norman,
OK) ; Alvarez, Walter E.; (Norman, OK) ;
Herrera, Jose E.; (Norman, OK) ; Balzano,
Leandro; (Norman, OK) |
Correspondence
Address: |
Dunlap, Codding & Rogers, P.C.
Attn: Christopher W. Corbett
Suite 420
9400 North Broadway
Oklahoma City
OK
73114
US
|
Family ID: |
26816789 |
Appl. No.: |
10/118834 |
Filed: |
April 8, 2002 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
60307208 |
Jul 23, 2001 |
|
|
|
Current U.S.
Class: |
423/447.3 ;
502/255; 502/313 |
Current CPC
Class: |
B01J 23/88 20130101;
B01J 23/882 20130101; B82Y 15/00 20130101; C01B 2202/02 20130101;
B82Y 40/00 20130101; D01F 9/1278 20130101; D01F 9/127 20130101;
C01B 2202/36 20130101; C01B 32/162 20170801; Y10S 977/748 20130101;
B82Y 30/00 20130101; Y10S 977/843 20130101; Y10S 977/742
20130101 |
Class at
Publication: |
423/447.3 ;
502/313; 502/255 |
International
Class: |
D01F 009/12; B01J
023/652 |
Claims
What is claimed is:
1. A catalyst composition, comprising: Co and Mo disposed on a
support material wherein the majority of the Mo occurs as dispersed
Mo oxide clusters and the majority of the Co occurs in a
CoMoO.sub.4-like phase with the Co therein primarily in an
octahedral configuration, and wherein the CoMoO.sub.4-like phase
occurs substantially disposed upon the dispersed Mo oxide
clusters.
2. The catalyst composition of claim 1 wherein the support material
is silica.
3. The catalyst composition of claim 1 wherein the molar ratio of
Co:Mo is less than 3:4.
4. A method of preferentially forming single walled carbon
nanotubes having a particular diameter, comprising: providing a
catalyst comprising: Co and Mo disposed on a support material
wherein the majority of the Mo occurs as dispersed Mo oxide
clusters and the majority of the Co occurs in a CoMoO.sub.4-like
phase with the Co therein primarily in an octahedral configuration,
and wherein the CoMoO.sub.4-like phase occurs substantially
disposed upon the dispersed Mo oxide clusters; and exposing the
catalyst in a reactor to a carbon-containing gas at a temperature
between about 700.degree. C. and about 800.degree. C. and
maintaining a CO.sub.2 concentration in the reactor below a
threshold CO.sub.2 concentration above which the conversion of
ionic Co to metallic Co is inhibited, wherein the majority of the
single walled carbon nanotubes thus formed have a diameter between
about 0.7 nm to about 0.9 nm.
5. The method of claim 4 wherein in the step of providing a
catalyst, the support material is silica.
6. The method of claim 4 wherein in the step of exposing the
catalyst to a carbon-containing gas, the reactor has a pressure
therein between about 1 atm and 7 atm.
7. The method of claim 4 wherein in the step of exposing the
catalyst to a carbon-containing gas, the threshold CO.sub.2
concentration in the reactor is 1%.
8. The method of claim 4 wherein in the step of exposing the
catalyst to a carbon-containing gas, the carbon-containing gas is
CO.
9. The method of claim 4 comprising the additional step of reducing
the catalyst by exposing the catalyst to a heated hydrogen gas.
10. A method of preferentially forming single walled carbon
nanotubes having a particular diameter, comprising: providing a
catalyst comprising: Co and Mo disposed on a support material
wherein the majority of the Mo occurs as dispersed Mo oxide
clusters and the majority of the Co occurs in a CoMoO.sub.4-like
phase with the Co therein primarily in an octahedral configuration,
and wherein the CoMoO.sub.4-like phase occurs substantially
disposed upon the dispersed Mo oxide clusters; and exposing the
catalyst in a reactor to a carbon-containing gas at a temperature
between about 800.degree. C. and about 900.degree. C. and
maintaining a CO.sub.2 concentration in the reactor below a
threshold CO.sub.2 concentration above which the conversion of
ionic Co to metallic Co is inhibited, wherein the majority of the
single walled carbon nanotubes thus formed have a diameter between
about 0.9 nm to about 1.2 nm.
11. The method of claim 10 wherein in the step of providing a
catalyst, the support material is silica.
12. The method of claim 10 wherein in the step of exposing the
catalyst to a carbon-containing gas, the reactor has a pressure
therein between about 1 atm and 7 atm.
13. The method of claim 10 wherein in the step of exposing the
catalyst to a carbon-containing gas, the threshold CO.sub.2
concentration in the reactor is 1%.
14. The method of claim 10 wherein in the step of exposing the
catalyst to a carbon-containing gas, the carbon containing gas is
CO.
15. The method of claim 10 comprising the additional step of
reducing the catalyst by exposing the catalyst to a heated hydrogen
gas.
16. A method of preferentially forming single walled carbon
nanotubes having a particular diameter, comprising: providing a
catalyst comprising: Co and Mo disposed on a support material
wherein the majority of the Mo occurs as dispersed Mo oxide
clusters and the majority of the Co occurs in a CoMoO.sub.4-like
phase with the Co therein primarily in an octahedral configuration,
and wherein the CoMoO.sub.4-like phase occurs substantially
disposed upon the dispersed Mo oxide clusters; and exposing the
catalyst in a reactor to a carbon-containing gas at a temperature
between about 900.degree. C. and about 1,000.degree. C. and
maintaining a CO.sub.2 concentration in the reactor below a
threshold CO.sub.2 concentration above which the conversion of
ionic Co to metallic Co is inhibited, wherein the majority of the
single walled carbon nanotubes thus formed have a diameter between
about 1.3 nm to about 1.7 nm.
17. The method of claim 16 wherein in the step of providing a
catalyst, the support material is silica.
18. The method of claim 16 wherein in the step of exposing the
catalyst to a carbon-containing gas, the reactor has a pressure
therein between about 1 atm and 7 atm.
19. The method of claim 16 wherein in the step of exposing the
catalyst to a carbon-containing gas, the threshold CO.sub.2
concentration in the reactor is 1%.
20. The method of claim 16 wherein in the step of exposing the
catalyst to a carbon-containing gas, the carbon-containing gas is
CO.
21. The method of claim 16 comprising the additional step of
reducing the catalyst by exposing the catalyst to a heated hydrogen
gas.
Description
RELATED REFERENCES
[0001] The present application claims the benefit of the filing
date of U.S. Provisional Application 60/307,208 filed on Jul. 23,
2001, the specification and drawings of which are expressly
incorporated by reference herein in their entirety.
BACKGROUND
[0002] Single-wall carbon nanotubes (SWNTs) exhibit exceptional
chemical and physical properties that have opened a vast number of
potential applications. Disproportionation of CO on several
bimetallic catalysts resulting in a high selectivity towards the
production of SWNTs at relatively low temperatures has been shown
in WO 00/73205 Al (corresponding to U.S. Ser. No. 09/389,553) and
PCT/USO1/17778 (corresponding to U.S. Ser. No. 09/587,257), each of
which is hereby expressly incorporated herein in its entirety and
each of which may contain subject matter related to the invention
claimed herein. Among the various formulations investigated therein
were catalysts comprising cobalt and molybdenum (Co and Mo)
supported on silica and having low Co:Mo ratios.
[0003] The Co--Mo system is previously known in the field of
catalysis due to its application in hydrotreating catalytic
processes. In that system, however, silica generally is not the
most suitable support, due to its weak interaction with the Co--Mo
components. Most studies on the Co--Mo catalysts have focused on
alumina-supported systems since alumina interacts with Co and Mo
with the appropriate strength to generate the HDS active species.
For that reason, alumina-supported Co--Mo catalysts are used in
industrial practice in the form of sulfides. Although the structure
of the sulfided Co--Mo catalysts is known almost at the atomic
level, that of the non-sulfided oxidic precursor has received less
attention. Using IR spectroscopy of adsorbed NO showed that the
interaction between Mo and the alumina in the oxidic state was not
greatly affected by the presence of Co. Further, in the oxidic
state, a major portion of the Co apparently is inside the alumina
lattice in a tetrahedral environment of oxygen ions and is not
exposed to the gas phase. However, silica-supported Co--Mo displays
a different behavior from that of the alumina-supported
catalysts.
[0004] It would therefore be desirable to have a detailed picture
of the structure of the Co--Mo catalyst so that the
silica-supported Co--Mo catalyst can be more effectively used in
the production of SWNTs.
[0005] Furthermore, it would be desirable to be able to control the
diameters of the SWNTs produced by the catalytic method. The
diameters of the SWNTs have important implications for their
thermal, mechanical and electrical properties. Control of the
diameters of the SWNTs will therefore result in better control of
the physical properties of SWNTs.
BRIEF DESCRIPTION OF THE DRAWINGS
[0006] FIG. 1: Lower panel: UV absorption spectra for two
Co:Mo/SiO.sub.2 calcined catalysts. Upper panel: (a): MoO.sub.3,
(b): (NH.sub.4).sub.6Mo.sub.7O.sub.24, (c): NaMoO.sub.4
references.
[0007] FIG. 2: Visible spectra for two calcined (oxidic) bimetallic
Co:Mo/SiO.sub.2 catalysts with different Co:Mo ratios(1:3 and 3:4)
and that for a monometallic Co(0.02% wt)/SiO.sub.2. The spectrum of
an .alpha.-CoMoO.sub.4 reference is included for comparison.
[0008] FIG. 3: Mo-edge (20,000 eV) XANES of two CoMo calcined
(oxidic) catalyst compared to .alpha.-CoMoO.sub.4 and
(NH.sub.4).sub.6Mo.sub.7O.su- b.24 references.
[0009] FIG. 4: Co K edge (7,719 eV) XANES of calcined
Co:Mo(1:3)/SiO.sub.2 catalyst compared to .alpha.-CoMoO.sub.4 used
as a reference.
[0010] FIG. 5: Co K edge XANES of a calcined Co:Mo(2:1)/SiO.sub.2
catalyst compared to those of .alpha.-CoMoO.sub.4, CoO and
Co.sub.3O.sub.4 references.
[0011] FIG. 6: Co K edge XANES of a calcined Co:Mo(2:1)/SiO.sub.2
catalyst (curve a) and a weighted linear combination of XANES from
.alpha.-CoMoO4 and Co.sub.3O4 (curve b). The contributions
resulting from the best fit and were 82% Co.sub.3O.sub.4 and 18%
.alpha.-CoMoO4.
[0012] FIG. 7: Fourier transforms of the k.sup.3 EXAFS data of the
Co K edge obtained on the calcined Co:Mo(2:1)/SiO.sub.2 catalyst
(solid line) and for Co.sub.3O.sub.4 reference (dotted line).
[0013] FIG. 8: Fourier transforms of the k.sup.3 EXAFS data of the
Co K edge, obtained on the calcined Co:Mo(1:3)/SiO.sub.2 catalyst
(dotted line) and on a .alpha.-CoMoO.sub.4 reference (solid
line).
[0014] FIG. 9: TPR profiles of several mono and bimetallic
cobalt/molybdenum catalysts. The TPR was conducted with 5%
H.sub.2/Ar at a heating rate of 8.degree. C./min.
[0015] FIG. 10: IR spectra of NO adsorbed on reduced monometallic
(Mo/SiO.sub.2 and Co/SiO.sub.2)and bimetallic (Co:Mo/SiO.sub.2)
catalysts. The catalysts were reduced under hydrogen at 500.degree.
C.
[0016] FIG. 11: IR bands of NO adsorbed on (a)
Co:Mo(2:1)/SiO.sub.2, (b) Co:Mo(3:4)/SiO.sub.2, and (c)
Co:Mo(1:3)/SiO.sub.2. The catalysts were reduced under hydrogen at
500.degree. C.
[0017] FIG. 12: Fraction of cobalt reduced to the metallic state as
a function of Co:Mo nominal ratio, as determined by XPS on samples
pretreated under H.sub.2 at 500.degree. C. and heated in He at
700.degree. C., without exposure to air.
[0018] FIG. 13: Fraction of molybdenum reduced to Mo(IV) and Mo(V)
as a function of Co:Mo nominal ratio, as determined by XPS on
samples pretreated under H.sub.2 at 500.degree. C. and heated in He
at 700.degree. C., without exposure to air.
[0019] FIG. 14: Surface atomic percentage of molybdenum (filled
circles) and cobalt (open circles) as function of the nominal Co:Mo
ratio, as determined by XPS on samples pretreated under H.sub.2 at
500.degree. C. and heated in He at 700.degree. C., without exposure
to air.
[0020] FIG. 15: Fourier transforms of the k.sup.3 EXAFS data of the
Co K edge obtained for several Co:Mo/SiO.sub.2 reduced catalysts
with different Co/Mo ratios. The data for a Co foil is included for
comparison.
[0021] FIG. 16: Fit in k-space for Co:Mo (2:1)/SiO.sub.2.
Experimental (triangles) and modeled EXAFS contribution around Co
(full line).
[0022] FIG. 17: Fit in k-space for Co:Mo (1:3)/SiO.sub.2.
Experimental (triangles) and modeled EXAFS contribution around Co
(full line).
[0023] FIG. 18: TEM images showing SWNTs produced by CO
disproportionation on a Co:Mo(1:3)/SiO.sub.2 catalyst.
[0024] FIG. 19: TEM images showing a mixture of SWNTs, MWNTs and
graphite produced by CO disproportionation on a
Co:Mo(2:1)/SiO.sub.2 catalyst.
[0025] FIG. 20: Fourier Transforms of the k .sup.3 EXAFS data
obtained for the K edge of Co for a fresh Co:Mo (1:2)/SiO.sub.2
catalyst reduced in hydrogen (500.degree. C.), and after the growth
of carbon nanotubes for reaction periods of 3 and 30 minutes. The
EXAFS data of a Co foil is included for comparison.
[0026] FIG. 21: Fourier Transforms of the k.sup.3EXAFS data
obtained for the K edge of Mo for a fresh Co:Mo(1:2)/SiO.sub.2
catalyst reduced in hydrogen (500.degree. C.), and after the growth
of carbon nanotubes for reaction periods of 3 and 30 minutes. The
EXAFS data of MO.sub.2C is included for comparison.
[0027] FIG. 22: Schematic description of the structure of the
reduced catalysts as derived from the characterization methods.
[0028] FIG. 23: Raman spectrum of diameters of SWNTs produced by
catalytic disproportionation of CO on Co:Mo/SiO.sub.2 at
750.degree. C., 850.degree. C. and 950.degree. C.
[0029] FIG. 24: Graphical description of the distribution of
diameters of SWNTs formed under conditions of 750.degree. C.,
850.degree. C. and 950.degree. C.
DETAILED DESCRIPTION OF THE INVENTION
[0030] The present invention is directed to a catalyst composition
and methods of using the catalyst composition for selectively
producing single walled carbon nanotubes (SWNTs). The present
invention is further directed to methods for selectively producing
SWNTs having diameters within a particular size range.
[0031] The catalyst composition preferably comprises Co and Mo
disposed on a support material, preferably silica, wherein the
majority of the Mo occurs as dispersed Mo oxide clusters on the
support material and the majority of the Co occurs in a
CoMoO.sub.4-like phase with the Co therein primarily in an
octahedral configuration, and wherein the CoMoO.sub.4-like phase
occurs substantially as a layer upon the dispersed Mo oxide
clusters.
[0032] The method of forming single walled nanotubes comprises the
steps of providing a catalyst as described herein, exposing the
catalyst in a reactor to a carbon-containing gas such as CO at a
predetermined temperature, and maintaining a CO.sub.2 concentration
in the reactor below a threshold (maximum) CO.sub.2 concentration.
Above this threshold, the conversion of ionic Co to metallic Co is
inhibited and therefore interfering with the formation of
SWNTs.
[0033] In the method describe herein, the majority of the
single-walled carbon nanotubes thus formed have diameters within a
predetermined range. For example, when the reaction temperature is
between about 700.degree. C. and 800.degree. C. (for example at
750.degree. C. and wherein the pressure of the system is preferably
between about 1 atm to 7 atm), most of the SWNTs have diameters
between 0.7 nm and 0.9 nm. When the reaction temperature is between
about 800.degree. C. and 900.degree. C. (for example at 850.degree.
C. and wherein the pressure is preferably between about 1 atm to 7
atm), most of the SWNTs have diameters between about 0.9 nm and 1.2
nm. When the temperature is between about 900.degree. C. and
1,000.degree. C. (for example at 950.degree. C. and wherein the
pressure is preferably between about 1 atm to 7 atm), most of the
SWNTs have diameters between about 1.3 nm and 1.7 nm. The threshold
concentration of CO.sub.2 is preferably 1% or less. Preferably
before use, the catalyst is reduced, for example by exposure to
H.sub.2 gas at 500.degree. C.
[0034] More preferably, the threshold CO.sub.2 concentration, in
order of increasing preference, is 0.9% CO.sub.2, 0.8% CO.sub.2,
0.7% CO.sub.2, 0.6% CO.sub.2, 0.5% CO.sub.2, 0.4% CO.sub.2, 0.3%
CO.sub.2, 0.2% CO.sub.2 and 0.1% CO.sub.2.
[0035] Catalyst Preparation and Pretreatment
[0036] A series of monometallic and bimetallic (Co--Mo) catalysts
supported on silica was prepared by incipient wetness impregnation.
The bimetallic samples, prepared by co-impregnation of aqueous
ammonium heptamolybdate and Co nitrate solutions, had Co:Mo molar
ratios of 2:1, 3:4, 1:2, and 1:3. In this series, the amount of Mo
was kept constant for all catalysts at 4.6 wt %, while the amount
of Co was varied accordingly. Three monometallic catalysts were
prepared with loadings of 1.4 wt % Co, 0.02 wt % Co and 4.6 wt %
Mo, respectively. The SiO.sub.2 support obtained from ALDRICH had
an average pore size of 6 nm, BET area 480 m.sup.2/g, pore volume
0.75 cm.sup.3/g, and particle sizes in the range 70-230 mesh. After
impregnation, the solids were dried overnight at 120.degree. C. and
then calcined for 3 h at 500.degree. C. in flowing dry air.
[0037] The catalysts were investigated in three different forms,
the oxidic state, the reduced state, and the reacted (spent) state.
The catalyst in the oxidic state was treated by calcination in air
at 500.degree. C. The catalyst in the reduced state was first
calcined in air at 500.degree. C., then reduced by H.sub.2 flow for
1 h at 500.degree. C. and finally heated in He flow to 700.degree.
C. The catalyst in the reacted (spent) state was catalyst which had
been used to a point wherein SWNTs had been produced.
[0038] Catalyst Characterization
[0039] UV/Vis spectra of the solid samples were recorded using a
SHIMADZU double beam spectrometer UV-2101 with an integrating
sphere for diffuse reflectance. Barium sulfate was used as
reflectance standard. Several Mo and Co compounds, including
MoO.sub.3, Na.sub.2MoO.sub.4,
(NH.sub.4).sub.6Mo.sub.7O.sub.24.4H.sub.2O and .alpha.-CoMoO.sub.4
were used as references. Before each analysis, the samples were
dried in air at 120.degree. C.
[0040] The IR spectra of adsorbed NO were obtained on a BIO-RAD FTS
40 spectrometer, equipped with a diffuse reflectance cell (HARRICK
SCIENTIFIC CO. PRAYING MANTIS) with an in-situ reaction chamber.
Before the spectrum was acquired, the catalysts were reduced
ex-situ under H.sub.2 flow at 500.degree. C. for 1 h and then
heated up in He to 700.degree. C. using a ramp temperature of
10.degree. C./min. Then they were cooled down to room temperature
in He flow and transferred to the IR cell. To eliminate any
superficial oxidation caused by exposure to air during the
transfer, each sample was re-reduced in situ for 1 h at 500.degree.
C. in H.sub.2 flow, purged in He flow at that temperature, and then
cooled down to room temperature. Once cooled, the samples were
exposed to 3% NO in He for 30 min at room temperature and purged in
He for 30 min.
[0041] X-ray absorption data were obtained at the National
Synchrotron Light Source (NSLS) at Brookhaven National Laboratory,
using beam line X-18B equipped with a Si (111) crystal
monochromator. The X ray ring at the NSLS has an energy of 2.5 GeV
and ring current of 80-220 mA. The calcined, reduced and spent
samples were investigated by X-ray absorption. Both reduced and
spent samples were not exposed to air, but were directly
transferred from the reaction chamber to a He glove bag, where they
were wrapped in KAPTON tape and stored in He-purged sealed veils
until analysis. The EXAFS experiments were conducted in a stainless
steel sample cell at liquid nitrogen temperature. Six scans were
recorded for each sample. The average spectrum was obtained by
adding the six scans. The pre-edge background was subtracted by
using power series curves. Subsequently, the post-edge background
was removed using a cubic spline routine.
[0042] The spectra were normalized by dividing by the height of the
absorption edge. To obtain structural parameters, theoretical
references for Co--Co, Co--O, Mo--O, Mo--C, Mo--Mo and Co--Mo bonds
were obtained by using the FEFF and FEFFIT fitting programs from
the University of Washington (Rehr, J. J., Zabinsky, S. I., and
Albers, R. C., Phys. Rev. Lett. 69, 3397 (1992)). In this routine,
the Debye Waller factors for each bond type (s), the edge energy
difference (DEo), the coordination number N, and the difference in
bond distances (DR) with respect to the theoretical reference, were
used as fitting parameters. The quality of the fit was determined
using the r-factor, which gives a sum-of-squares measure of the
fractional misfit. Therefore, the smaller the r-factor, the better
the fit is. For good fits, the r-factor is always less than or
about 3%. The spectra of MoO.sub.3, Na.sub.2MoO.sub.4,
(NH.sub.4).sub.6Mo.sub.7O.sub.24, CoO, Co.sub.3O.sub.4 and
.alpha.-CoMoO.sub.4 were also obtained at liquid nitrogen
temperature and used as references.
[0043] X-ray photoelectron spectroscopy data were recorded on a
PHYSICAL ELECTRONICS PHI 5800 ESCA System with monochromatic A1K
.alpha. X-rays (1486.6 eV) operated at 350 W and 15 kV with a
background pressure of approximately 2.0.times.10-9 Torr. A 400
.mu.m spot size and 58.7 eV pass energy were typically used for the
analysis. Sample charging during the measurements was compensated
by an electron flood gun. The electron takeoff angle was 45.degree.
with respect to the sample surface. The pretreatment of the samples
was performed in a packed bed micro-reactor with an on/off valve at
each end of the reactor, which allowed for a quick isolation of the
samples after each treatment.
[0044] The reactor with the sample under He was transferred to a
glove bag; the sample (in powder form) was placed on a stainless
steel holder and kept in a vacuum transfer vessel (Model 04-110A
from PHYSICAL ELECTRONICS) to avoid any exposure to the atmosphere
before the analysis. For each sample, the binding energy regions
corresponding to Si (95-115 eV), Mo (220-245 eV) and Co (760-820
eV) were scanned. The binding energies were corrected by reference
to the C(1s) line at 284.8 eV. A non-linear Shirley-type background
was used for the area analysis of each peak. The fitting of the XPS
spectra was carried out with asymmetric peaks, using the MULTIPAK
software from PHYSICAL ELECTRONICS.
[0045] H.sub.2-TPR experiments were conducted passing a continuous
flow of 5% H.sub.2/Ar over approximately 30 mg of the calcined
catalyst at a flow rate of 10 cm.sup.3/min while linearly
increasing the temperature at a heating rate of 8.degree. C./min.
The hydrogen uptake as a function of temperature was monitored
using a thermal conductivity detector, SRI Model 110 TCD.
[0046] Production and Characterization of Carbon Nanotubes
[0047] The production of SWNTs by CO disproportionation was
compared in a series of catalysts with Co:Mo ratios of 2:1, 1:2 and
1:3. For SWNTs production, 0.5 g of calcined (acidic) catalyst was
placed in a horizontal tubular reactor, heated with H.sub.2 up to
500.degree. C., and then heated by He flow up to 700.degree. C.
Subsequently, CO was introduced at a flow rate of 850 cm.sup.3/min
at 84 psi and kept under these conditions for a given period of
time, which ranged from 3 to 120 minutes. At the end of each run,
the system was cooled down by He flow. The total amount of carbon
deposits was determined by temperature programmed oxidation (TPO)
following the method of Kitiyanan et al. (Kitiyanan, B., Alvarez,
W. E., Harwell, J. H., and Resasco, D. E., Chem.Phys. Lett. 317,
497 (2000)). Transmission electron microscopy (TEM) was used for
characterizing the carbon deposits on the catalyst. The TEM images
were obtained in a JEOL JEM-2000FX TEM. For this analysis, a
suspension of the carbon-containing samples in isopropanol was
achieved by stirring the solid sample with ultrasound for 10 min.
Then, a few drops of the resulting suspension were deposited on a
grid and subsequently evacuated before the TEM analysis.
Results
[0048] Characterization of the Calcined (Oxidic) Catalysts
EXAMPLE 1
[0049] Diffuse Reflectance UV-Visible Spectroscopy (UV/V-DRS)
[0050] UV/V-DRS were used to study the state of both Mo and Co in
the oxidic form, after calcination in air at 500.degree. C. In
order to estimate the band energy gap of the Mo oxide compounds, it
has been recommended to use the square root of the Kubelka-Munk
function multiplied by the photon energy, and plot this new
function versus the photon energy. The position of the absorption
edge can then be determined by extrapolating the linear part of the
rising curve to zero.
[0051] The values thus obtained carry information about the average
domain size of the oxide nanoparticles. It has been shown that the
energy band gap decreases as the domain size increases. Therefore,
a comparison can be made between the energy of the samples under
investigation and those of references of known domain size. This
comparison is made in FIG. 1, which shows the absorption edges of
several MoO.sub.x species together with those of two different
Co:Mo/SiO.sub.2 catalysts. As expected, the band gap energies in
the reference series decrease as the domain size increases. Those
of the Co:Mo/SiO.sub.2 catalysts lie between those of
(NH.sub.4)6Mo.sub.7O.sub.24 and MoO.sub.3. From this comparison, it
can be inferred that the Mo species in the (oxidic) calcined
catalysts have relatively small domain sizes. The presence of a
small contribution of MoO.sub.3 species could not be ruled out
since a small tail can be observed below 3.0 eV, but most of the Mo
is in a high state of dispersion.
[0052] In addition to the charge-transfer bands due to Mo,
appearing in the UV region, the visible spectra of the bimetallic
catalysts present bands in the 500-750 nm region, which did not
appear for the Mo/SiO.sub.2 catalyst. These bands are associated
with Co species and have previously been ascribed to d-d
transitions (.sup.4T.sub.2g .sup.4A.sub.2g and
.sup.4T.sub.2g.sup.4T.sub.1g (P)) of high spin octahedral Co
complexes. FIG. 2 shows the DRS spectra in this region for three
calcined (oxidic) catalysts, Co:Mo(3:4)/SiO.sub.2,
Co:Mo(1:3)/SiO.sub.2 and Co/SiO.sub.2. The spectrum for the
Co:Mo(1:3) catalyst is very similar to that of .alpha.-CoMoO.sub.4,
which is typical of Co in an octahedral environment. The spectrum
for the Co:Mo(1:2) was almost identical to that of the Co:Mo(1:3)
catalyst, so only one of them is included in the graph.
[0053] By contrast, the shape of the spectrum of the Co:Mo
(3:4)/SiO.sub.2 catalyst was markedly different and exhibited the
appearance of a band at around 680 nm. This band was in turn the
dominant feature in the pure Co catalyst and should be associated
with Co.sub.3O.sub.4 species, which as shown below, are present in
the pure Co catalyst in the calcined state. Therefore, it can be
concluded that the catalysts with low Co:Mo ratio exhibit most of
the Co interacting with Mo. However, as the Co:Mo ratio increases,
free Co oxide begins to appear. In fact, the spectrum of the
Co:Mo(3:4)/SiO.sub.2 catalyst can be rationalized as a sum of
contributions from two types of species, one interacting with Mo
(main band at around 600 nm) and a second one in which the Co
oxidic species are segregated and not interacting with Mo (main
band at 680 nm). Similar conclusions have been previously drawn
from Raman spectroscopy and XRD data, which indicated that a
non-interacting Co phase is formed on Co--Mo/SiO.sub.2 catalysts at
high Co:Mo ratios (Jeziorowski, H., Knozinger, H., Grange, P., and
Gajardo, P., J. Phys. Chem. 84, 1825 (1980)).
EXAMPLE 2
[0054] X-ray Absorption Spectroscopy (EXAFS/XANES)
[0055] FIG. 3 shows the K-edges of Mo (E=20000 eV) for two
different calcined (oxidic) catalysts with Co:Mo molar ratios of
2:1 and 1:3. Absorption spectra for the two reference compounds,
.alpha.-CoMoO.sub.4 and ammonium heptamolybdate, have been included
for comparison. The absorption edges for both catalysts are
remarkably similar. They both exhibit a pre-edge feature, which is
also observed in the spectrum of ammonium heptamolybdate. This
pre-edge feature is typically observed in distorted octahedral
environments, such as that found in the heptamolybdate. It is due
to a 1s 4d bound-state transition that in the case of a perfect
octahedral geometry is formally forbidden. Accordingly, it is
barely present in compounds such as MoO.sub.3 and
.alpha.-CoMoO.sub.4. However, it becomes allowed when the d-states
of the metal mix with the p orbitals of the ligand, as in compounds
with distorted octahedral symmetries. Of course, a pre-edge feature
is always observed in Mo species with tetrahedral symmetry, such as
in Na molybdate. However, in such cases the feature is much more
pronounced than that observed in the present case. Therefore, it
can be concluded that in the bimetallic catalysts, Mo is mostly in
a structure similar to that of the heptamolybdate. Interestingly,
this is the same for both, the Co:Mo(2:1) and Co:Mo(1:3)
samples.
[0056] Next, the set of samples were investigated at the Co edge.
FIG. 4 compares the XANES spectra for the K-edge of Co (Eo=7709 eV)
in the calcined (oxidic) Co:Mo(1:3)/SiO.sub.2 catalyst and that in
the .alpha.-CoMoO.sub.4 reference. Except for some differences in
the size and shape of the first peak in the edge, both spectra look
remarkably similar. By contrast, the Co edge for the Co:Mo (2:1)
catalyst, containing excess Co, is very different from that of the
.alpha.-CoMoO.sub.4 reference.
[0057] As shown in FIG. 5, the XANES of the catalyst is in fact
very similar to that of Co.sub.3O.sub.4, although a small shoulder
appearing at around 7726 eV is more pronounced for the catalyst
than for the oxide. Interestingly, this shoulder coincides with the
white line of CoMoO.sub.4 and CoO species. A first approximation of
a XANES composed of two different phases can be obtained by simple
addition of the XANES of the individual components. A simple
fitting with a linear combination of contributions from
Co.sub.3O.sub.4 and CoMoO.sub.4 reproduces the XANES spectrum of
the Co:Mo (2:1) catalyst (see FIG. 6). This comparison indicates
that in the catalyst with Co excess, most of the Co is in the form
of Co.sub.3O.sub.4 and a small fraction as CoMoO.sub.4. When the
fitting was attempted using CoO as a third component, the best fit
did not include any contribution of this oxide. The EXAFS data was
in good agreement with the conclusions reached from XANES analysis.
As shown, in FIG. 7, the Fourier Transform for the calcined
(oxidic) Co:Mo (2:1) catalyst is very similar to that of
Co.sub.3O.sub.4, indicating that this oxide is the predominant form
present when Co is in excess.
[0058] The results of the sample having a low Co:Mo ratio require
some further consideration. It is interesting to note that while
the XANES spectra for these catalysts look similar to that of
.alpha.-CoMoO.sub.4 from the Co side, they bear no resemblance with
this compound from the Mo side. One may rationalize this
contrasting behavior by proposing that while most of the Co in the
catalyst is forming a CoMoO.sub.4-like phase only a fraction of Mo
participates in this compound. The rest of the Mo would occur as
dispersed Mo oxide clusters. Apparently the Co is in a
CoMoO.sub.4-like phase because the XANES indicates that the local
environment of Co in the Co:Mo(1:3) catalyst is very similar to
that in CoMoO.sub.4, but the EXAFS data are significantly different
from that of the compound. This comparison is made in FIG. 8, which
shows the Fourier Transforms for the K-edge of Co in the calcined
(oxidic) Co:Mo(1:3)/SiO.sub.2 catalyst, together with that of
.alpha.-CoMoO.sub.4. The low intensity observed in the catalyst for
the peaks between 2.5 and 4 .ANG., clearly observable for the Co
molybdate, would indicate that bulk .alpha.-CoMoO.sub.4 is not
present, but rather a highly dispersed CoMoO.sub.4-like structure,
co-existing with Mo oxide clusters.
EXAMPLE 3
[0059] Temperature Programmed Reduction (TPR)
[0060] The reduction profiles of calcined monometallic Co/SiO.sub.2
and Mo/SiO.sub.2 catalysts together with that of the bimetallic
Co:Mo (1:3)/SiO2 catalyst are shown in FIG. 9 (upper panel). The
TPR profile of the Co monometallic catalyst shows two peaks at
360.degree. C. and 445.degree. C., which can be ascribed to the
reduction of Co oxide species. The reduction of the monometallic Mo
catalyst also exhibits two peaks, but they appear at much higher
temperatures than those of Co. Therefore, from the reduction
profiles it is possible to identify the presence of Co and Mo
species in the absence of interactions. Accordingly, the TPR of the
bimetallic Co:Mo (1:3)/SiO.sub.2 catalyst indicates that, in this
sample, the vast majority of Co oxide species are interacting with
Mo.
[0061] It is clear that while most of the Co in the monometallic
catalyst gets reduced below 500.degree. C., almost no reduction
takes place below that temperature in the bimetallic catalyst. In a
previous work it was reported that the reduction of interacting
Co--Mo supported species occurs at similar temperatures as those
assigned for the reduction of free Mo species. It has also been
proposed that the addition of Mo oxide to Co oxide inhibits the
reduction of the Co species because Mo.sup.6 + polarizes the Co--O
bonds, making them more ionic and consequently more difficult to
reduce. In agreement with the DRS and EXAFS/XANES data, TPR
indicates that a high degree of Co--Mo interaction is only observed
for the catalyst with a low Co:Mo ratio. As shown in FIG. 9 (lower
panel), as the Co:Mo ratio increases, a gradually increasing
fraction of segregated Co species is apparent from the peaks at
360.degree. C. and 445.degree. C., which are associated with the
reduction of non-interacting Co oxide.
EXAMPLE 4
[0062] Diffuse Reflectance Infrared Fourier Transform Spectroscopy
(DRIFTS)
[0063] The vibrational spectrum of adsorbed NO was used to
investigate the Co:Mo/SiO.sub.2 catalysts after the reduction
pretreatment in H.sub.2 at 500.degree. C. As mentioned above, both
Co and Mo are able to adsorb NO at room temperature, exhibiting
characteristic IR absorption bands that can be used to identify the
NO adsorption on each metal. It is generally agreed that Mo.sup.VI
does not adsorb NO, but Mo.sup.II, Mo.sup.III and Mo.sup.IV have
all been suggested as potential NO adsorption sites. Similarly,
both oxidic and reduced Co catalysts are able to adsorb NO. It has
been reported that adsorption on reduced Co results in bands at
slightly lower frequencies than on oxidized Co. However, it is not
always possible to determine the chemical state of Co based on NO
adsorption. Therefore, although DRIFTS of adsorbed NO may not be
the best technique to characterize the chemical state of Co and Mo,
it is certainly a powerful tool to quantify the degree of site
blocking of one of the two components by the other.
[0064] FIG. 10 shows the DRIFTS spectrum of NO adsorbed on two
reduced monometallic catalysts with 4.6 wt % Mo and 1.4 wt % Co,
respectively, and a reduced bimetallic catalyst with 4.6 wt % Mo
and 1.4 wt % Co (Co:Mo=1:2). On the monometallic Mo/SiO.sub.2
catalyst the bands corresponding to the symmetric and
anti-symmetric stretching modes of dinitrosyl species are clearly
observed. The symmetric mode exhibited a band at 1814 cm.sup.-1,
while the anti-symmetric mode generated a broad band centered at
around 1714 cm.sup.-1. The broadening of the anti-symmetric band
has been attributed to inhomogeneities on the surface that
influence the vibrational transition moment of the asymmetric mode
more than the moment of the symmetric mode. For the monometallic
Co/SiO.sub.2 catalyst, the bands of the dinitrosyl species appeared
at significantly higher wavenumbers (1880 and 1803 cm.sup.-1) than
those on Mo. In this case, the anti-symmetric mode is the dominant
band in the spectrum. Finally, the bimetallic Co:Mo(1:2)/SiO.sub.2
catalyst showed three absorption bands that roughly correspond to
those of the individual components. The dominant band appearing at
1806 cm.sup.-1 obviously has contributions from both the symmetric
band of NO adsorbed on Mo and the anti-symmetric band of NO
adsorbed on Co. A smaller band appeared at 1883 cm.sup.-1, which
can be associated with the symmetric mode on Co sites and a broader
band, which may include several individual bands, appeared in the
region 1730-1650 cm.sup.-1 and can only be associated to adsorption
on Mo.
[0065] The appearance of different components associated with this
asymmetric-mode band in the bimetallic catalyst contrasting to that
on Mo/SiO.sub.2 catalyst, could be explained in terms of Mo sites
with different degrees of coordinative unsaturation, or
alternatively, in terms of sites where Mo is influenced by Co to
various degrees. It is also interesting to note that the overall
intensity of the bands of the bimetallic Co--Mo catalysts is
consistently much lower than that of the monometallic Co and Mo
catalyst. This effect has been previously observed for
alumina-supported Co--Mo catalysts. In that case, a lower
dispersion of the active species on the bimetallic catalysts,
compared to that of the monometallic ones, was made responsible for
the observed loss in intensity.
[0066] FIG. 11 shows the FTIR spectra of NO adsorbed on different
reduced bimetallic catalysts, in which the Mo content was kept
constant while the Co content was increased. It is clear that as
the Co content increases the adsorption of NO over the Mo sites is
inhibited. A similar effect has been previously reported for the
sulfided Co:Mo/Al.sub.2O.sub.3 catalyst and interpreted as a
blockage of Mo sites by Co. Since the low-frequency anti-symmetric
NO stretching on Mo does not have contributions from species
adsorbed on Co, it can be used as an indication of the density of
Mo sites covered by Co. A clear decrease in the intensity of this
band is observed as the Co:Mo ratio increases, becoming practically
negligible when Co is in excess, e.g. for a Co:Mo ratio of 2:1. At
the same time, the bands for this catalyst appear at the same
wavelengths as those on the monometallic Co catalyst, i.e. 1880 and
1803 cm.sup.-1. A straightforward conclusion drawn from these
observations is that when Co is in excess, it almost completely
covers the Mo sites and forms a non-interacting species.
EXAMPLE 5
[0067] X-ray Photoelectron Spectroscopy (XPS)
[0068] XPS can be used to determine the chemical nature of the
catalyst constituents and to roughly estimate their distribution on
the surface. For the first purpose, the binding energy of the
catalysts can be compared to those of reference compounds. In Table
1, the binding energies of the Co 2p.sub.3/2 and Mo 3d.sub.5/2
levels obtained for the catalysts in the reduced state are compared
to those of the reference compounds. The spectra corresponding to
the Co 2p.sub.3/2 levels for all the catalysts can be described in
terms of two contributions, one appearing at about 778 eV and other
at 781.5 eV. As shown in the Table 1, these binding energies are in
good correspondence with the binding energies that were obtained
for the metallic Co (778.2 eV) and the CoMoO.sub.4 (781.4 eV)
references, respectively. Similar values for these reference
materials have been previously reported in the literature.
1TABLE 1 Binding energies for reduced catalysts and reference
compounds. Binding Energies (eV) Mo 3d.sub.5/2 Co 2p.sub.3/2
Reference Samples Co 778.2 CoO 780.5 Co.sub.3O.sub.4 780.1
.alpha.-CoMoO.sub.4 232.5 781.4 MoO.sub.3 232.4 MoO.sub.2 229.1
(NH.sub.4).sub.6Mo.sub.7O.sub.24.4H.sub.2O 232.3 Reduced Catalysts
Mo/SiO.sub.2 229.2 231.0 233.4 Co:Mo(1:3)/SiO.sub.2 228.9 778.1
231.1 781.5 233.9 Co:Mo(1:2)/SiO.sub.2 228.3 777.9 230.8 780.7
233.3 Co:Mo(3:4)/SiO.sub.2 228.9 778.5 230.8 781.5 233.1
Co:Mo(2:1)/SiO.sub.2 228.7 778.4 230.9 781.4 233.2
[0069] The respective surface fractions of Co in the two chemical
states were obtained by fitting the spectra with asymmetric curves
centered at the corresponding binding energies. The fraction of Co
in the metallic state after reduction at 500.degree. C., as
determined from this analysis, is shown in FIG. 12 as a function of
the Co:Mo ratio. At low Co:Mo ratios, when the majority of the Co
oxide species are interacting with Mo, most of the Co remains in
the oxidic form, but as the Co:Mo ratio increases, a larger
fraction of Co gets reduced. In agreement with this trend and with
the TPR data shown above, previous reports have indicated that the
Co--Mo interacting phase is, in fact, more difficult to reduce than
Co oxide alone.
[0070] The assignment of the Mo 3d.sub.5/2 levels to different
chemical states of Mo is not as straightforward as that of Co. As
shown in Table 1, the spectra of the reduced catalysts can be
described in terms of three contributing peaks. The one at the
lowest binding energy appears in the same region as that of
Mo.sup.4+ in MoO.sub.2. The one at the highest binding energy
(233.1 to 233.9 eV) appears at slightly higher energy than that of
Mo.sup.6+in MoO.sub.3. This peak has been previously assigned to
Mo.sup.6+ species in an oxidic environment although some have
indicated that these species should have binding energies in the
region 232.5-232.7 eV. The remaining peak appearing at around 231
eV can be attributed to Mo in an intermediate state such as
Mo.sup.+5. From the fitting of the spectra of the different
catalysts with asymmetric curves centered at the indicated binding
energies, the fraction of reduced Mo (i.e., Mo.sup.4++Mo.sup.5+)
have been calculated and plotted, in FIG. 13 as a function of the
Co:Mo ratio. It is observed that, except for the monometallic
catalyst there is a slight decrease on the reducibility of Mo as
the Co:Mo ratio increases. That is, the Co--Mo interaction is also
evident from the Mo analysis, although the effect is not as
pronounced as for Co.
[0071] Table 2 indicates the surface atomic fractions of Co and Mo
on the catalyst after the reduction pretreatment. The intensity of
the Si peak remained almost constant for all samples, except for
the catalyst with a Co:Mo ratio of 2:1, in which it decreased. FIG.
14 shows the fractions of Co/Si and Mo/Si as a function of bulk
Co:Mo ratio. It is seen that, even though the Mo concentration in
the catalyst was kept constant, the Mo/Si ratio clearly decreased
as the amount of Co increased. In agreement with the DRIFTS data,
these results show that the addition of Co results in a gradual
coverage of Mo.
2TABLE 2 Surface atomic concentrations for reduced catalysts as
determined by XPS. Atomic concentration (%) Reduced Catalysts Mo Co
Si Mo/SiO.sub.2 1 0 30.41 Co:Mo(1:3)/SiO.sub.2 0.99 0.27 31.08
Co:Mo(1:2)/SiO.sub.2 0.94 0.29 31.04 Co:Mo(3:4)/SiO.sub.2 0.58 0.43
31.42 Co:Mo(2:1)/SiO.sub.2 0.33 0.35 29.77
EXAMPLE 6
[0072] X-ray Absorption Spectroscopy (EXAFS)
[0073] The EXAFS results for the reduced catalysts, shown in FIG.
15, reveal a definite trend, which corresponds with the XPS data.
The data indicate that, keeping the amount of Mo fixed, the
fraction of metallic Co increases with the Co:Mo ratio. The
appearance of clearly observable peaks in the range corresponding
to the 2nd and 3rd coordination spheres (0.25-0.45 nm), indicates
that larger Co.degree. clusters are present on the catalyst with
the higher Co content. To quantify this trend, the data for the
first, second and third coordination shells of Co on the
Co:Mo(2:1)/SiO.sub.2 were isolated by applying an inverse Fourier
transform over a restricted range of r (0.13-0.45 nm). The filtered
data were then fitted using FEFFIT program assuming that only
Co.degree. was present. As shown in FIG. 16, the quality of this
fit was excellent. The structural parameters resulting from the fit
are summarized in Table 3. From these results, we can conclude that
in reduced Co:Mo/SiO.sub.2, with high Co:Mo ratios, a large
fraction of Co is present in the form of metallic Co clusters.
3TABLE 3 Structural parameters resulting from the fitting of the
Co-edge EXAFS data obtained for the Co:Mo(2:1)/SiO.sub.2 catalyst.
In the table E.sub.0 represents the energy shift, sigma the
Debye-Waller factor and the r factor is a measurement of the fit
quality. Fitting (E.sub.0 = 8.2) Reference Coordination Phase Bond
number Distances Sigma r-factor Co.degree. Co--Co 7.8 2.492 0.0046
0.0014 (1.sup.st shell) Co--Co 2.4 3.523 0.0047 (2.sup.nd shell)
Co--Co 9.6 4.327 0.0059 (3.sup.rd shell)
[0074] To compare the structures of the Co phases in the catalysts
with low Co:Mo ratios with those of high Co:Mo ratios, filtered
EXAFS data were fitted for the Co:Mo (1:3) catalyst, as previously
described. In the first attempt, the EXAFS data were fitted using a
single phase, which was either metallic or oxidized Co. In both
cases, the results were unsatisfactory. A more complex model was
needed to obtain a good fit. In the second model used for fitting,
the simultaneous presence of metallic Co clusters and oxidized Co
species was considered. Satisfactory fits were only obtained with
the simultaneous presence of Co metallic clusters and a CoMoO.sub.4
phase. FIG. 17 illustrates the quality of the fit, while Table 4
summarizes the structural parameters determined from the analysis.
The resulting parameters indicate that in this catalyst, cobalt is
mainly forming a CoMoO.sub.4-like phase and a small fraction is
forming metallic Co.
4TABLE 4 Structural parameters resulting from the fitting of the
Co-edge EXAFS data obtained for the Co:Mo(1:3)/SiO.sub.2 catalyst.
Six different scattering paths were used for the Co--O (1.sup.st
and 2.sup.nd shells) and Co--Mo (2.sup.nd shell) pairs. Fitting
(E.sub.0 = 9.88) Coordi- nation Distances r- Phase Bond number
(.ANG.) Sigma factor .alpha.- Co--O (1.sup.st shell) 3 2.1 .+-.
0.18 0.0029 0.0323 CoMoO.sub.4 Co--Co (1.sup.st shell) 1 3.03
0.0035 & 3.15 Co--Mo (1.sup.st shell) Co--O (2.sup.nd shell) 3
3.7 .+-. 0.05 0.0019 Co--Mo (2.sup.nd shell) 3.2 3.9 .+-. 0.13
0.0027 Co.degree. Co--Co (1.sup.st shell) 1.5 2.50 0.0035 Co--Co
(2.sup.nd shell) 2.2 3.65 0.0023 Co--Co (3.sup.rd shell) 1.6 4.35
0.0033
[0075] Production of Carbon Nanotubes by Catalytic
Disproportionation of CO
EXAMPLE 7
[0076] Influence of Co:Mo Ratio on SWNT Selectivity
[0077] The ability of the different catalysts to produce SWNTs by
CO disproportionation was tested by passing pure CO over the
catalyst at 700.degree. C. Before the reaction, the catalysts were
calcined in air at 500.degree. C., then reduced in H.sub.2 at
500.degree. C., and then heated in He flow up to the reaction
temperature. At the end of a 2-h reaction period, the spent
catalyst containing the carbon deposits was cooled down in He flow.
The characterization of the carbon deposits was done by using two
main techniques that we have previously used and tested. They are
temperature programmed oxidation (TPO) and transmission electron
microscopy (TEM). We have previously shown that from the TPO
results we can obtain a quantitative measure of the carbon yield
and selectivity towards SWNTs. These results are summarized in
Table 5 and illustrate the strong influence of the Co:Mo ratio on
SWNTs selectivity. The TEM observations totally support the TPO
results. As shown in FIGS. 18 and 19, a contrasting difference is
observed on the carbon structures produced with the two different
samples. While the sample with a Co:Mo ratio of 1:3 exhibited a
high density of SWNTs, the sample with a Co:Mo ratio of 2:1 mainly
produced MWNTs and graphitic carbon deposits. In the first case,
one can observe the
[0078] parallel lattice fringes characteristic of SWNTs. In the
second case, a spacing of 0.34 nm can be observed between the
fringes, which is characteristic of the space between individual
walls in multi-walled nanotubes.
5TABLE 5 Carbon yield and selectivity for two catalysts with
different Co:Mo ratio. The yield is defined as mass of total
deposited carbon per mass of catalyst. The selectivity to SWNT is
the mass of SWNT per total mass of carbon deposits. % % MWNT
Amorphous and Total carbon Catalyst carbon % SWNT graphite yield.
Co:Mo(1:3)/SiO.sub.2 18.7 70.7 10.6 18.25% Co:Mo(2:1)/SiO.sub.2
14.8 15.2 70.0 16.35%
[0079] It is noteworthy that when the Co:Mo(1:3)/SiO.sub.2 catalyst
that exhibited a high yield and selectivity towards SWNTs was
employed without the reduction step or with an exceedingly high
reduction temperature, poor SWNTs yields were attained. It is
suggestive that the TPR indicates that the Co reduction in the
Co:Mo (1:3) does not start below 500.degree. C. Some degree of
reduction is apparently necessary, since a totally oxidized
catalyst is not efficient. Alternatively, a high degree of
reduction of the catalyst, causing the appearance of metallic Co,
is also detrimental for the selectivity towards SWNTs.
[0080] Characterization of the Spent (Reacted) Catalysts
EXAMPLE 8
[0081] Formation of Metallic Co and Mo Carbide
[0082] The EXAFS results for the spent catalysts are shown in FIG.
20. The Fourier Transform of the EXAFS data for the K edge of Co
(E.sub.o=7,709 eV) for the Co:Mo (1:2) catalyst show that after the
pretreatment and before the reaction with CO, a significant
fraction of Co remains oxidized. However, as the reaction proceeds,
metallic Co begins to form and after 30 min., the Fourier Transform
is indicative of pure Co metal. Undoubtedly during the formation of
carbon nanotubes over the selective catalyst, Co is gradually
reduced to the metallic state. The detailed structural analysis of
the Co EXAFS data indicated that as the reaction proceeds the Co
reduction is accompanied by an increase in metal particle size.
Evidence for this process is presented in Table 6 where an increase
on the Co--Co coordination number as a function of reaction time is
observed.
6TABLE 6 Structural parameters for the Co:Mo(1:2)/SiO.sub.2
catalyst after different reaction periods, as obtained from the
EXAFS data analysis. N.sub.M--M, N.sub.M--C and N.sub.M--O
represent the coordination number between the metals (Mo or Co)
with another metal atom, a C atom and an O atom respectively.
Sample Edge Coordination Distances Sample Treatment (M) E.sub.0
Sigma N.sub.M--M N.sub.M--O N.sub.M--C (.ANG.) r-factor
CoMo(1:2)/SiO.sub.2 Spent Co 4.2 0.0068 5.8 -- -- 2.47 0.043 3 min
0.0025 -- 2.2 -- 2.05 CoMo(1:2)/SiO.sub.2 Spent Co 9.4 0.0065 9.2
-- -- 2.50 0.020 30 min 0.0003 -- 1.1 -- 2.07 Mo.sub.2C Reference
Mo 6.6 0.0063 11.6 -- -- 2.97 0.022 0.0106 -- 5.8 2.08
CoMo(1:2)/SiO.sub.2 Spent Mo 7.1 0.0075 6.7 -- -- 2.98 0.038 30 min
0.0075 -- 3.3 -- 2.09 0.0039 -- -- 0.9 1.68
[0083] In the same way, as illustrated in FIG. 21, after a 30-min
reaction period, the Fourier Transforms of the EXAFS data of Mo
(E.sub.o=20,000 eV) reveal a drastic change, developing peaks that
correspond exactly to those of the Mo.sub.2C reference. The
transformation of oxidized Mo species into Mo carbide during the
reaction is indeed very clear, with a small fraction of unconverted
Mo oxide remaining in the catalyst. This conversion can be clearly
seen from the EXAFS analysis data of the spent catalyst summarized
in Table 6. The best fit of the experimental data was obtained when
the coordination sphere of the Mo was described as composed as
Mo--C, Mo--Mo, and Mo--O. This indicates that the conversion to Mo
carbide is not complete and a fraction of Mo remains in the spent
catalyst as an oxidized species.
[0084] Characteristics of Oxidic Catalysts
[0085] The UV-Vis and the X-ray absorption data indicate that, in
all the calcined samples, Mo is in an octahedral environment in the
6+ oxidation state. Furthermore, from the values of the adsorption
edge energy it can be concluded that the molybdenum oxide species
are dispersed as small clusters with average domain size somewhat
larger than that of the heptamolybdate ion, but not large enough to
form bulk MoO.sub.3. Regardless of the Co:Mo ratio in the
catalysts, the majority of Mo occurs as these oxidic clusters. It
is apparent that the degree of dispersion of Mo is more a result of
the loading and degree of interaction with the support, rather than
a consequence of the extent of interaction with Co. A contrasting
picture is obtained from the characterization of Co, particularly
in the catalysts with low Co:Mo ratio. In this case, both, the
UV-Vis DRS and the XANES data obtained on the catalyst with a Co:Mo
ratio of 1:3 demonstrated that most of the Co is in an environment
similar to that in .alpha.-CoMoO.sub.4 i.e., closely interacting
with Mo. However, the slight but clear differences between the
spectra of the catalyst and the .alpha.-CoMoO.sub.4 reference
suggest that the similarity is limited to the local environment and
nature of ligands. In fact, the EXAFS data demonstrate that bulk
.alpha.-CoMoO.sub.4 is not present in the bimetallic catalysts.
Bulk .alpha.-CoMoO.sub.4 has only been observed by XRD in
silica-supported catalysts at high metal contents.
[0086] All these results form the basis of a model of the calcined
(oxidic) catalysts with low Co:Mo ratios (e.g., preferably less
than 3:4) wherein Co is in the form of a CoMoO.sub.4-like layer on
top of dispersed Mo oxide clusters. Therefore, most of the Co is in
an interacting phase, while only a fraction of Mo is in such a
phase. Therefore, the characterization of Mo does not reflect a
strong interaction with Co, while the characterization of Co shows
in fact a high degree of interaction with Mo.
[0087] In contrast, when the Co is in excess, more than one type of
Co species is present in the catalyst. The UV-Vis-DRS data show
that in addition to the interacting phase of Co, which dominates at
low Co:Mo ratios, a non-interacting phase begins to form as the Co
content increases. The X-ray absorption data (FIG. 7) indicate that
this non-interacting species is Co.sub.3O.sub.4, which has been
previously detected by Raman spectroscopy and X-ray diffraction in
Co:Mo/SiO.sub.2catalysts.
[0088] Characteristics of Reduced Catalysts
[0089] The DRIFTS data of adsorbed NO give further evidence of the
interaction between Co and Mo in the reduced catalysts with low
Co:Mo ratios. When the position of the symmetric-stretching band of
dinitrosyl adsorbed on Co is compared for the monometallic Co and
bimetallic Co--Mo catalysts, a shift of about 8 cm.sup.-1 to higher
wavenumbers is observed on the bimetallic catalyst (FIG. 10).
Furthermore, the binding energy obtained by XPS for the fraction of
Co that remains unreduced after the reduction treatment in the
bimetallic catalyst (around 781.4 eV) corresponds to that of Co
interacting with Mo oxide, rather than of a non-interacting Co
oxide species (780.1-780.5 eV).
[0090] However, as the amount of Co in the reduced catalyst
increases, the shift in the IR main band to higher wavenumbers
becomes gradually smaller, until the position of the band coincides
with that of the monometallic Co/SiO.sub.2 catalyst, i.e. 1803
cm.sup.-1. At the same time, the intensity of the band ascribed to
the Mo sites gradually decreases until it practically disappears
for the catalyst with a Co:Mo ratio of 2:1. This trend is
consistent with the XPS data, which indicate that the Mo sites are
progressively covered by Co as the Co:Mo ratio increases. It has
been proposed that cobalt molybdate is present in the
silica-supported catalyst, and that the non-interacting Co or Mo
species (depending on which metal is in excess) agglomerate over
CoMoO.sub.4 forming a geode-like structure. The results provided
herein clearly show that when Mo is in excess, this is not the
case. The DRIFTS data show that the Co sites are strongly
influenced by the presence of Mo but they are not covered by it. On
the other hand, a geode-like structure could be postulated for the
catalysts when Co is in excess. That is, Co oxide could partially
cover the Co molybdate species.
[0091] From the combined analysis of the XPS, TPR, and EXAFS data,
one can infer that the reducibility of the Co species is strongly
influenced by the presence of Mo in the catalyst. The amount of
reducible Co clearly increases with the Co:Mo ratio. This trend can
be explained in terms of the formation of an interacting Co
molybdate-like species in the oxidized catalyst, which should be
more difficult to reduce than non-interacting Co oxide. At the same
time, as shown in FIG. 13, the reducibility of the Mo species is
also affected by the presence of Co, indicating that although the
techniques that probe bulk Mo do not show an interaction with Co
(XANES, UV/V DRS), an interaction indeed exists at the catalyst
surface and it is more pronounced at low Co:Mo ratios.
[0092] Combining all the information obtained from FTIR, XPS, UV/V
DRS, H.sub.2-TPR and XAS, a coherent representation of the catalyst
structure, as it stands right before the beginning of the SWNTs
reaction, can be envisioned and is schematically represented in
FIG. 22. Three main phases have been identified in the reduced
catalyst just before reaction: dispersed molybdenum oxide clusters,
cobalt interacting with molybdenum oxide in a cobalt molybdate-like
structure, partially covering the Mo oxide clusters, and segregated
metallic cobalt particles. The fraction of each of these species
depends on the Co:Mo ratio.
[0093] The CoMo (1:3) and CoMo (1:2) species of FIG. 22 represent
preferred embodiments of the invention wherein Co and Mo are
disposed upon a support material wherein the majority of the Mo
occurs as dispersed Mo oxide clusters and the majority of the Co
occurs in a CoMoO.sub.4-like phase with the Co therein primarily in
an octahedral configuration, and wherein the CoMoO.sub.4-like phase
occurs substantially (i.e. preferably at least 50%) disposed upon
the dispersed Mo oxide clusters.
[0094] Relationship Between Catalyst Morphology and Selectivity
Towards SWNTs
[0095] When Co and Mo co-exist in the catalysts, especially when Mo
is in excess, SWNTs are formed with high selectivity. The results
obtained by EXAFS on the spent Co:Mo(1:2)/SiO.sub.2 catalyst,
clearly show that Co suffers a severe transformation under reaction
conditions. Before reaction, but after pretreatment in H.sub.2 at
500.degree. C. and then in He at 700.degree. C., a considerable
fraction of Co is still in the oxide state closely interacting with
Mo on the surface, as shown by FTIR, XPS and XAS. After 3 min under
reaction conditions, a significant growth in metallic Co was
observed, although some oxidized Co was still present. After 30
min, the particles were even larger and essentially all the Co
became metallic. Simultaneously Mo that was initially in the
oxidized state is converted to the carbidic form, as clearly
demonstrated by EXAFS.
[0096] The role of Co in the Co:Mo catalyst is the activation of CO
gas. However, when Co is in the form of large metal aggregates, it
has the tendency to generate mostly multiwalled carbon nanotubes
(MWNTs), carbon filaments and graphite. When Mo is present in the
catalyst but Co is not in excess, a well-dispersed Co.sup.+2
species in the form of a Co molybdate-like phase is stabilized. The
formation of this interacting Co molybdate-like species plays a
determining role in the catalyst selectivity towards the formation
of SWNTs.
[0097] The effect of having Co stabilized on this cobalt-molybdate
like environment results in minimized reduction and formation of
large metallic aggregates. The importance of preventing the
reduction of Co is evident when the selective catalyst is compared
with a non-selective one (e.g., wherein the Co:Mo ratio is 2:1). In
the non-selective catalyst, most of the Co in the oxidic state
after calcination is in the form of Co.sub.3O.sub.4 and, as in the
monometallic Co/SiO.sub.2 catalyst, this oxide is converted into
large metallic Co clusters upon reduction. By contrast, on the
SWNT-selective catalyst (lower Co:Mo ratios), a concerted mechanism
should take place during the reaction. As the CO disproportionation
starts during catalysis, Mo oxide is converted into Mo carbide.
This transformation breaks up the Co molybdate-like structure,
allowing for the reduction of Co by CO. However, in the selective
catalyst, the Co ions are now highly dispersed and in the presence
of high concentration of CO in the gas phase. This environment is
favorable for the production of SWNTs instead of the sintering that
would normally occur during a high temperature reduction process.
Extremely small metal Co clusters are released into the gaseous Co
environment and catalyze the formation of SWNTs.
[0098] On the very small Co clusters, substantial production of
MWNTs, carbon filaments, and graphite does not occur. Such
production of MWNTs, carbon filaments, or graphite normally occurs
on larger metallic Co clusters, following the well-known mechanism
for carbon filament growth. That is, the metal clusters begin to
decompose CO therein producing Co carbide particles, which then
tend to precipitate graphite at their ends in the form of
cylindrical filaments.
[0099] Relationship Between Reaction Temperature and SWNTs
Diameters
[0100] It is a novel feature of the present invention that the
average diameters of SWNTs formed using the catalyst described
herein can be controlled selectively by selection of a particular
reaction temperature, and by controlling the level of exposure of
the catalyst to CO.sub.2 during the reaction. As noted previously,
the level of CO.sub.2 in the reaction vessel used to conduct SWNTs
formation should be held below a maximum threshold, i.e.,
preferably less than a threshold of 1% CO.sub.2 in the reaction
vessel.
[0101] When the reaction temperature is between about 700.degree.
C. and about 800.degree. C. (and preferably between 725.degree. C.
and 775.degree. C.), the majority of the SWNTs have diameters
between about 0.7 nm (.+-.0.1 nm) and about 0.9 nm (.+-.0.1 nm).
When the reaction temperature is between about 800.degree. C. and
900.degree. C. (and preferably between 825.degree. C. and
875.degree. C.), the majority of the SWNTs have diameters between
about 0.9 nm (.+-.0.1 nm) and about 1.2 nm (.+-.0.1 nm). When the
reaction temperature is between about 900.degree. C. and
1000.degree. C. (and preferably between 925.degree. C. and
975.degree. C.), the majority of the SWNTs have diameters between
about 1.3 nm (.+-.0.1 nm) and about 1.7 nm (.+-.0.1 nm). FIGS. 23
and 24 show the ranges of diameters of SWNTs formed at 750.degree.
C., 850.degree. C., and 950.degree. C.
[0102] Changes may be made in the composition and the operation of
the various catalysts described herein or in the methods described
herein without departing from the spirit and scope of the invention
as defined in the following claims.
* * * * *