MX2007009515A - Single-walled carbon nanotube catalyst - Google Patents

Abstract

An activated catalyst capable of selectively growing single-walled carbon nanotubes when reacted with carbonaceous gas is provided. The activated catalyst is formed by reducing a catalyst that comprises a complex oxide. The complex oxide may be of formula AxByOz, wherein x/y⿤2 and z/y⿤4, A being a Group VIII element and B being an element such that an oxide of element B is not reducible in the presence of hydrogen at a temperature less than or equal to about 900°C. Methods of making, uses for and carbon fibril-containing product made with these activated catalysts are also provided.

Description

SINGLE-WALL CARBON NANOTUBE CATALYST CROSS REFERENCE INFORMATION This application claims the benefit and priority of US Provisional Application No. 60 / 650,726, filed on February 7, 2005, which is incorporated herein by reference in its entirety. BACKGROUND OF THE INVENTION FIELD OF THE INVENTION The present invention relates to materials and methods for commercially preparing single-walled carbon nanotubes. More specifically, this invention relates to material comprising complex oxides which, when processed, are viable activated catalysts for products containing carbon fibrils. These products have both a Raman spectrum and characteristic transmission electron micrographs known to indicate the presence of single-walled carbon nanotubes. DESCRIPTION OF THE RELATED ART This invention pertains to the field of carbon nanotubes (also known as fibrils). Carbon nanotubes are vermicular carbon deposits having diameters less than 1.0 μ, preferably less than 0.5 μ, and preferably even greater than 0.2 μ. Carbon nanotubes can be either multiple-walled (that is, they have more than one graphene layer roughly parallel to the nanotube's axis) or single-walled (that is, they have only one layer of graphene parallel to the nanotube shaft). Other types of carbon nanotubes are also known, for example fishbone fibrils (for example, where the graphene layers are placed in a herringbone pattern, as compared to the axis of the tube), and so on. In the way they are produced, the carbon nanotubes can be in the form of discrete nanotubes, aggregates of nanotubes (ie, structure in microscopic, dense particles, comprising entangled carbon nanotubes) or a mixture of both. Carbon nanotubes are distinguished from commercially available continuous carbon fibers. For example, a diameter of continuous carbon fibers that is always greater than 1.0 μ and typically 5 to 7 μ is much larger than the diameter of carbon nanotubes that is usually less than 1.0 μ. Carbon nanotubes also have a much higher resistance and a higher conductivity than carbon fibers. Carbon nanotubes also differ physically and chemically from other forms of carbon, for example standard graphite and carbon black. The standard graphite, due to its structure, can be subjected to oxidation until almost complete saturation. In addition, carbon black is an amorphous carbon generally in the form of spheroidal particles having a graphene structure such as, for example, carbon layers around a disordered core. On the other hand, carbon nanotubes have one or more layers of ordered graphite carbon atoms positioned substantially concentrically around the cylindrical axis of the nanotube. These differences, among others, make graphite and carbon black an unsatisfactory predictor of the chemistry of carbon nanotubes. It has also been accepted that single walled and multiwall carbon nanotubes are also different from each other. For example, multi-walled carbon nanotubes have multiple layers of graphite along the nanotube axis while single-walled carbon nanotubes have only a single graphitic layer on the nanotube axis. The methods for the production of multi-walled carbon nanotubes also differ from the methods used to produce single-walled carbon nanotubes. Specifically, different combinations of catalysts, catalyst support, raw materials and reaction conditions are required to provide multiwall nanotubes vs. single wall carbon nanotubes. Certain combinations also provide a mixture of multi-walled carbon nanotubes and single-walled carbon nanotubes. As such, two characteristics are frequently examined to determine if this process will be commercially feasible for the production of a desired carbon nanotube on an industrial scale. The first feature is the catalytic selectivity (for example, will the catalyst primarily provide single-walled carbon nanotubes or multi-walled carbon nanotubes or other forms of carbon products?). A selectivity of at least 50% is preferred. The second characteristic is the catalyst yield (for example, the weight of carbon product generated per weight of catalyst used). The selectivity of single-wall nanotube catalyst can be measured through the evaluation of Raman spectrum signatures of fibril-containing products, which are informative for the differentiation of single-walled (perhaps two-walled) nanotubes from the multi-walled nanotubes. For example, "Diameter-Selective Raman Scattering from Vibrational odes in Carbon Nanotubes," Rao, A M et al; Science, vol. 257, p. 187 (1997); Dresselhaus, M.S., et al, "Single Nanotube Raman Spectroscopy," Accounts of Chemical Research 1. vol. 35, no. 12, pages 1070-1078 (2002), both are incorporated herein by reference. For example, a sample that has nanotubes of diameters small enough to be single-walled has a Raman spectrum that has: peaks in "radial breathing mode" (RBM) between 150 and 300 wave numbers , the area below the RBM peaks is at least 0.1% of the area below a characteristic G-band peak, and the intensity of the G-band peak is at least double the intensity of a characteristic D-band peak ( G / D of at least 2.0). The following references to multi-walled tube processes (MWNT) are incorporated by reference: Baker and Harris, Chemistry and Physics of Carbon, Walker and Thrower ed. , Vol. 14, 1978, p. 83; Rodríguez, N., J. ater. Research, Vol. 8, p. 3233 (1993); Oberlin, A. and Endo, M., J. of Crystal Growth, Vol. 32 (1976), pages 335-349; US Patent NO. 4,663,230 to Tennent et al; U.S. Patent No. 5, 171, 560 to Tennent et al; Lijima, Nature 354, 56, 1991; Weaver, Science 265. 1994; by Heer, Walt A., "Nanotubes and the Pursuit of Applications," MRS Bulletin, April, 2004, U.S. Patent No. 5,456,897 to Moy et al, U.S. Patent No. 6, 143, 689 to Moy et al, and U.S. Patent 5,569,635 to Moy et al. Processes for preparing single-walled carbon nanotubes (SWNTs) are also known. For example, "Single-shell carbon nanotubes of 1-nm diameter", lijima, S. and Ichihashi, T. Nature, vol. 363, p. 603 (1993); "Cobalt-catalysed growth of carbon nanotubes with single-atomic-layer walls," Bethune, D S, Kiang, C H, De Vries, M S, Gorman, G, Savoy, R and Beyers, R Nature, vol. 363, p. 605 (1993); U.S. Patent No. 5,424,054 to Bethune et al; Guo, T., Nikoleev, P., Thess, A., Colbert, D. T., and Smalley, R. E. , Chem. Phys. Letters 243: 1-12 (1995); Thess, A., Lee, R., Nikolaev, P., Dai, H., Petit, P., Robert, J., Xu, C, Lee, YH, Kim, SG, Rinzler, AG, Colbert, DT, Scuseria, GE, Tonarek, D., Fischer, JE, and Smalley, RE, Science, 273: 483-487 (1996); Dai., H., Rinzler, A.G., Nikolaev, P., Thess, A., Colbert, D.T., and Smalley, R.E., Chem. Phys. Letters 260: 471-475 (1996); U.S. Patent No. 6,761,870 (see also WO 00/26138) of Smalley et al; "Controlled production of single-wall carbon nanotubes by catalytic decomposition of CO on bimetallic Co-Mo catalysts," Chemical Physics Letters, 317 (2000) 497-503; U.S. Patent No. 6,333,016 to Resasco et al .; "Low-temperature synthesis of high purity single walled carbon nanotubes from alcohol," Maruyama et al Chemical Physics Letters, 360, pages 229-234 (July 10, 2002). These articles and patent documents are incorporated herein by reference. Processes currently known for forming single-walled tubes can not reach industrially acceptable levels of selectivity and yield under commercially viable reaction conditions. Recent literature contains disclosures describing the benefits of the use of catalytic precursors comprising solid solutions of transition metal (s) and non-reducible oxides (at practical temperatures). These solid solutions of mixed oxides must be calcined at relatively high temperatures to avoid the presence of oxide phases. Bacsa, R.R. et al, Chem. Phys. Letters 323: 566-571 (2000) and J. Am. Ceram. Soc, 85: 2666-69 (2002), both incorporated by reference, describe catalysts made by selective reduction (T> 800 ° C) in H2 / CH4 of "solid solutions between one or more transition metal oxides and a non-reducible oxide such as, for example, A1203, MgAl204 or gO ". The solid solutions were elaborated by synthesis of combustion, using the combustion of both precursors and a fuel (typically urea). Both transmission electron micrographs and Raman spectrum show the presence of a mixture of single wall / two wall tubes and a substantial amount of non-tubular amorphous products. Flahaut, et al, J. Materials Chemistry, 10: 249-252 (2000) describes the same catalyst synthesis as above except by providing a synthesis temperature "usually> 800 ° C". Coquay, et al, J. Phys Chem B, 106: 13199 (2002) and Coquay, et al J. Phys Chem B, 106: 13186 (2002) both identify that the use of C0304 in the oxide phase catalyses a production of nanofibers thick. This was a limitation of previous methods of flame synthesis to fabricate single-walled nanotube catalysts. Solid Mg! _xFexO solutions synthesized with flame catalyze the formation of single-walled nanotubes, while A2B04-type particles tend to provide only thick nanofibers. Electron micrographs of product made from solid Mgi_xFexO solutions synthesized with flames reveal that these catalysts provide only occasionally and not selectively SWNTs, consequently producing some SWNTS along with a broad spectrum of other carbonaceous products. [0014] Wang and Ruckenstein, Carbon, 40: 1911-1917 (2002) disclose and characterize a range of Co / Mg / O catalysts with different stoichiometries and calcination temperatures used in their preparation. They report the formation of filamentous carbon after decomposition of methane at 900 ° C for catalysts prepared in various ways. They found that phase A2B04 is formed only at the calcination temperature T <; 700 ° C and that only catalysts were calcined at T = 900 ° C. However, an XRD analysis revealed a solid solution of filamentous carbon but not single-walled nanotubes. The references cited above while using mixed metals as catalysts, disclose all and specifically conclude that solid solutions (in contrast to material in complex oxide phase) are favored to generate either tubular carbon products or filamentous carbon products. There is a need for a method for producing single wall carbon nanotubes with industrially acceptable levels of activity, selectivity and yield under commercially viable reaction conditions. None of the prior techniques discloses said methodology; The discovery of an acceptable process remains elusive despite ongoing global research to develop it. SUMMARY OF THE INVENTION An activated catalyst capable of promoting the growth of single-walled carbon nanotubes when reacted with carbonaceous gas and the method for preparing said activated catalyst is provided. The activated catalyst is formed by reacting a source of A with a source of B at a sufficiently low temperature to form a complex oxide having a formula AxByOz, wherein x / y = 2 and z / y = 4, A is an element of Group VIII, and B is a different element from A and is an element whose simple oxide, where B is in the same valence state as in the complex oxide, is not reducible in the presence of hydrogen gas to a temperature below about 900 ° C, and then by activation of the complex oxide by reduction of said complex oxide at a temperature of less than about 900 ° C. Element A may comprise cobalt, iron, nickel, or a mixture of they. Element B is selected from aluminum, lanthanum, magnesium, silicon, titanium, zinc, zirconium, yttrium, calcium, strontium and barium and may preferably be magnesium. The complex oxide may have a spinel crystallography, where the spinels comprise a group of oxides having very similar structures. The general formula of the spinel group is AB204. Element A represents a divalent metal ion, such as, for example, magnesium, ferrous iron, nickel, manganese and / or zinc. Element B represents trivalent metal ions such as, for example, aluminum, ferric ion, chromium and / or manganese. When A is cobalt and B is magnesium, the complex oxide may comprise Co2 spinel g04 and the calcination temperature (in air) may be less than about 800 ° C and greater than about 400 ° C. Catalyst reduction may occur Low hydrogen flow and activated catalyst can be passivated in an additional process step. A further embodiment discloses a method for manufacturing single-walled carbon nanotubes from the activated catalyst of the present invention. In an exemplary embodiment, a method for manufacturing single-walled carbon nanotubes is provided, said method comprising supplying a composition comprising a complex oxide having a formula AxByOz, wherein x / y = 2 and z / y = 4, A is an element of Group VIII; B is a different element from A and is an element whose simple oxide, where B is in the same valence state as in the complex oxide, is not reducible in the presence of hydrogen gas at a gas temperature of less than about 900 ° C; the reduction of said composition to form an activated catalyst; the contact of a carbonaceous gas with said activated catalyst under conditions suitable to promote the growth of single-walled carbon nanotubes, said suitable conditions include a pressure greater than about 1 atmosphere and less than about 10 atmospheres and a temperature greater than about 400 ° C and less than approximately 950 ° C, and to grow carbon nanotubes in said activated catalyst, said carbon nanotubes comprise single-walled carbon nanotubes. In another embodiment, a method for preparing single-walled carbon nanotubes is provided, said method comprising contacting a carbonaceous gas with an activated catalyst in a reaction zone under conditions suitable for the growth of carbon nanotubes of a single wall, said suitable conditions include a pressure greater than about 1 atmosphere and less than about 10 atmospheres and a temperature greater than about 400 ° C and less than about 950 ° C, said activated catalyst comprises a reduced form of a complex oxide, said complex oxide has the formula AxByOz, where x / y = 2 and z / y = 4, A is an element of Group VIII; B is a different element of A and is an element whose simple oxide, in which B is in the same valence state as in the complex oxide, is not reducible in the presence of hydrogen gas at a temperature of less than about 900 °. C; and growing carbon nanotubes in said activated catalyst, said carbon nanotubes comprise single-walled carbon nanotubes. An additional embodiment offers an activated catalyst capable of promoting the selective growth of a product containing carbon fibrils when it reacts with carbonaceous gas. The activated catalyst is formed by the reduction of a catalyst comprising a complex oxide, wherein the product is characterized by a Raman spectrum showing RBM peaks between 150 and 300 wave numbers, which has the area below said peaks of RBM of at least 0.1% of the area under a characteristic G-band peak and having the intensity of the G-band peak at least double the intensity of a characteristic D-band peak. Additional modalities disclose methods to prepare, use and products formed through such use. Other improvements offered by the present invention compared to the prior art will be identified as a result of the following description that raises specific modalities. The description is not intended in any way to limit the scope of the present invention but only to provide examples. The scope of the present invention is indicated in the appended claims. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 illustrates the calcination of coatings of Co and g metal films on a silicon wafer. The final composition of the coating depends on the calcination temperature. Figure 2 illustrates Raman spectra of products containing carbon fibrils formed by catalytic decomposition of a carbonaceous gas in Co-Mg mixed oxide catalyst calcined in air at different temperatures. Above 400 ° C, the presence of single wall nanotubes is evidenced by the appearance of peak (s) of radial breathing mode (RBM) in the region of 150-300 cm "1 Figure 3 illustrates product Raman spectra containing carbon fibrils formed by catalytic decomposition of a carbonaceous gas manufactured in Co-Mg mixed oxide catalysts calcined in air at 800 ° C with and without hydrogen treatment and passivation. of a treatment with mild hydrogen improved the selectivity of single-walled nanotubes as shown by the higher intensity RBM peak Figure 4 illustrates a Raman spectrum of product made from a complex oxide oxide catalyst. Fe-Mg at 900 ° C in methane Figure 5 illustrates scanning electron micrographs of products containing carbon fibrils manufactured using Co-Mg (A) complex oxide catalyst and oxide catalyst. and Fe-Mg (B) complex at 900 ° C in methane. Both catalysts had been calcined in air at a temperature of 675 ° C for one hour. Figure 6 illustrates a transmission electron micrograph of product manufactured from an oxide catalyst of Co-Mg complex activated at 900 ° C in methane. Figure 7 illustrates reduction spectra of Co nitrate (Sample A) and Co acetate (Sample B) with 5% H2 / Ar as reducing carrier gas. Figure 8 illustrates the reduction spectrum of 9% Fe / Al203 with 5% H2 / Ar as reducing carrier gas. The first reduction (I) at 400 ° C indicated the transition from Fe203 to Fe304, followed by Fe304 to FeAl204 at 530 ° C (II) and finally to metallic Fe at 740 ° C (III). Figure 9 illustrates the Raman spectrum of a product containing high quality single-walled nanotubes formed by catalytic decomposition of a carbonaceous gas in 9% Fe / Al2C > 3 calcined at 800 ° C. The presence of high quality single-walled carbon nanotubes is evidenced through the presence of strong G and RBM bands with minimal D-band signal. DETAILED DESCRIPTION OF SPECIFIC MODALITIES For the purposes of this disclosure, a catalyst is a material or a composition that can be further processed to form carbon nanotubes (note that carbon fibrils that grew in steam, fibrils, graphite fibrils, linear fullerenes and buckytubes are considered here as terminology equivalent to nanotubes) catalytic decomposition of a carbonaceous gas. A carbonaceous gas is defined as a gas that consists of, contains, relates to or provides carbon. More specifically, prior to exposure (or simultaneously with exposure) of the material to the carbonaceous gas, the catalyst is "activated" in such a way that the formation of nanotubes is thermodynamically and kinetically favorable. Catalysts for the formation of carbon nanotubes are typically "activated" by a reduction process that alters or reduces the valence state of the material. As such, an "activated" catalyst is a reduced form of a catalyst or a catalyst that has been further processed to alter or reduce its valence state. In the literature, as well as in the production plant, it is commonly believed that microregions and even nanoregions (groups of atomic dimensions) of Group VIII elements (typically, iron, cobalt, nickel) offer excellent nucleation sites from which the nanotubes will grow easily. These regions may be metallic in nature from the outset or may be formed through the selective reduction of compounds containing Group VIII elements (the Group VIII element as a cation), in accordance with what is described above in the embodiments. The compounds disclosed herein are oxides. In addition, the compounds of interest are complex oxides which are defined herein as oxides of at least two elements (for these purposes, at least one of the elements is of Group VIII) which form a crystallographic cross-link within which the Group VIII atoms at specific periodic sites. Complex oxides are different from the simple oxides defined herein as compounds comprising a single element and oxygen. While simple oxide mixtures retain the crystal structures of each of the simple oxides when the complex oxides are mixed they frequently have different lattice structures and different crystal symmetry compared to simple oxides. Complex oxides are also different from solid solutions, the latter being defined here as structures in which the cations are randomly distributed without long-range periodic order. In other words, atoms in a solid solution composition can freely substitute at the various "sites" of the structure. There are several well known complex oxide crystallographic structures including, for example, spinel and type K2NiF4 (A2B04), rock salt (A2B03), and Perovskite (AB03). Although the remaining discussion focuses on the structural class comprising the cobalt magnesium system that forms spinel structures, the disclosure, modalities and appended claims contained herein are not limited to spine structure complex oxides. In the spinel structure, the oxygen atoms are placed in a dense, cubic centered face structure. There are two types of interstitial sites among the oxygen anions in this structure, named on the basis of the crystallographic symmetries that these sites possess. Prototypic magnesium aluminate spinel as defined herein (Al2 g04 - aluminum is A, magnesium is B in the spinel group composition A2B04), the magnesium ion has a valence state of +2 complex and the aluminum ion has a complex valence state of +3.

Within the spinel structure, all magnesium ions are found in tetrahedral interstitial sites while all aluminum atoms are in distinct octahedral interstitial sites. Simple oxides, magnesia, MgO and alumina, A1203 have very different crystalline structures (cubic and hexagonal, respectively) from the aluminate spinel. The simple oxides are defined here as having a simple valence state of oxides of magnesium of +2 and a valence state of simple aluminum oxide of +3. Accordingly, the valence state of simple magnesium oxide is the same as the valence state of magnesium complex oxide. A person skilled in the art will understand that the temperature and actual calcination conditions (atmosphere, etc.) to form a complex oxide such as spinel, rock salt, or perovskite will depend on the chemical interactions between elements A and B. A lower formation temperature will tend to result from stronger interactions between the elements. The cobalt-magnesium oxide system has been of interest to nanotube manufacturers for some time. This is due to the fact that the nanotubes can be relatively easily separated from residual Co-Mg support material after performing a catalytic decomposition. Examples 1 and 2 describe preparations of cobalt-magnesium catalysts and Example 3 reports x-ray diffraction data obtained from the samples of Example 1 calcined at various temperatures. The calcination temperature effectively determines which phases will predominate in the prepared catalyst sample. For example, a solid solution begins to form when the calcination is carried out in air at a temperature above about 800 ° C, while an inverse spinel of formula Co2Mg04 is formed at temperatures between 400 ° C and 800 ° C, more dominantly in the samples with a Co / Mg ratio of approximately 2. The interaction between Co and Si can be influenced significantly by several Co precursors, and preparation procedures, and in some cases, leads to the formation of Co silicate complex which is much more stable in reducing environment compared to other oxides of Co. For example, Journal of Catalysis, vol. 162, 220-229, 1996, van Steen discovered that during the impregnation step the surface cobalt silicate precursor was formed through a reaction between surface silanol groups and aqueous cobalt complexes. A solution of Co acetate with a mild pH can favor the interaction and cause the formation of a greater amount of silicate. In addition, it was found on the part of Girardon and colleagues, Journal of Catalysis, vol. 230, 339, 2005, that after impregnation and drying, cobalt exists in complexes coordinated octahedrally in catalysts prepared from cobalt nitrate or cobalt acetate. The decomposition of octahedral complexes results in the appearance of C03O4 crystallites and cobalt silicate species. The distribution of cobalt between crystalline C03O4 and the cobalt silicate phase in the oxidized samples depends on the exothermicity of salt decomposition in air and the temperature of the oxidant pretreatment. The C03O4 crystallite is the dominant phase in samples prepared through the endothermic decomposition of cobalt nitrate supported. The high exothermicity of decomposition of cobalt acetate leads primarily to cobalt silicate of amorphous complex, little reducible (of type C02SÍO4). It is also believed that stable Co-oxides such as Co-Si complex oxide will stabilize Co under a mild or even severe reducing environment, thereby avoiding the sintering of metallic Co and retaining its fine particles suitable for the growth of carbon nanotubes. a single wall. For the case of C03O4 supported on Si02, due to the lack of strong interaction between Co and Si oxide, C03O4 tends to be reduced under very mild condition, therefore when hydrogen is in contact with gases containing carbon, reduced Co particles can be subjected to rapid sintering to form larger particles unsuitable for the growth of single-walled nanotubes. In addition to Co-Mg or Co-Si or similar system such as for example Fe-Mg, Fe-Si, Fe-Al, a system can also undergo a complex oxide formation after a series of calcination or reduction, a process frequently found in the catalytic growth of carbon nanotubes. Tang, et. al, in Journal of Catalysis, vol. 106, 440, 1987, have reported the observation of change in the chemical status as well as the crystallography of Fe species in a Fe-Al oxide system during calcination and reduction. After depositing on an alumina support, the Fe species will be in the form of Fe203 on the surface of A1203. A programmed reduction of temperature indicated that Fe species can be subjected to changes in multiple stages in chemical state before being totally reduced in metallic iron at a temperature higher than 850 ° C. The initial reduction will reduce Fe203 in Fe304 followed by reduction to Fe (III) -Fe (II) oxides and transition in FeAl204. Thus, due to the strong interaction between Fe and Al, especially the formation of a complex oxide, FeAl20, j, the resulting metallic Fe particles can be very fine and stable even at very high temperatures, for example, >850 ° C, a condition usually favorable for the growth of nanotubes when a reagent containing carbon is introduced.

EXAMPLES Example 1 Four (4) grams of magnesia (Martin Marietta MagChem 50) were pulped with deionized water at a temperature of 80 ° C for 3 hours and then allowed to cool. 29 grams of cobalt nitrate Co (NO3) 2 · 6H2O (Alpha Chemical) dissolved in deionized water were slowly added to the Mg (OH) 2 / MgO paste while the mixture was subjected to constant stirring. 6N ammonia was used to adjust the pH of the paste to maintain it at approximately 8-9. The resulting paste had a pink color and was filtered and washed 2 times with 1N ammonium acetate by new paste formation and new filtration. The filter cake was dried at 100 ° C for 24 hours and then calcined at various temperatures from 200 ° C to 900 ° C for 4 hours. The nominal composition of the calcined catalyst is 57% by weight in Co and 11.8% by weight of Mg (molar ratio Co / Mg ~ 2). Additional samples were prepared using the same procedure to achieve a Co / Mg molar ratio of 1, 0.5, 0.1 and 0.01, respectively. EXAMPLE 2 Four (4) grams of magnesia were placed in a flask maintained at 80 ° C with constant stirring using a magnetic bar. 29 Grams Co (NO3) 2 · 6H20 (Alpha Chemical) were dissolved in 200 mL of methanol and slowly added to the flask. After addition of all the solution, the paste was maintained at a temperature of 80 ° C with constant stirring in order to remove all the solvent. The resulting powder was further dried at 110 ° C for 24 hours and then calcined at various temperatures from 200 ° C to 900 ° C for 24 hours. The nominal composition of the calcined catalyst is 57% Co and 11.8% Mg (molar ratio of Co / Mg ~ 2). Additional samples were prepared using the same procedure to achieve a Co / Mg molar ratio of 1, 0.5, 0.1 and 0.01, respectively. EXAMPLE 3 The phase analysis of samples elaborated in Examples 1 and 2 is carried out using X-ray diffraction techniques in a Rigaku 300 X-ray diffractometer equipped with Cu blank for generation of X-rays and Ni-monochromator to remove X-rays dispersion. The samples prepared from Example 1 and calcined under various conditions were pressed into sample holders and XRD spectra were collected. Table 1 summarizes the data obtained. The data correlate well with data previously obtained by Wan and Ruckenstein, Carbón, 40: 1911-1917 (2002), a reference cited above. Table 1 XRD phase analysis of Co / Mg catalyst prepared at different calcination temperatures Calcination temperature Crystalline phase 200 ° C Co203, gO 400 ° C Co2Mg04, MgO 600 ° C C03O4 (trace), Co2Mg04, MgO 800 ° C Co2Mg04, MgO, (Co, Mg) 0 solid solution (?) 900 ° C (Co, Mg) 0 solid solution, MgO It is instructive to study Table 1 to understand the relevant phase equilibria in the Co / Mg system /OR. Simple oxides, Co203 (cobalt with a simple oxide valence status of +3) and MgO are stable up to a calcination temperature slightly exceeding 200 ° C. While in this discussion all the calcination is carried out in air, all the modalities disclosed they are not limited to the use of an air atmosphere, as will be evident to a person with ordinary knowledge in the field. Crystallographically speaking, this is analogous to the example of magnesium aluminate spinel described above (MgO and A1203 have stability there.) Magnesium oxide remains stable during much higher temperature excursions. Above about 400 ° C (the exact temperature seems to depend on the processing method, for example, a mechanical state requires a higher temperature than the impregnation of a nitrate in MgO, the cobalt oxide (formula Co304) becomes the simple oxide stable Some of the cobalt cations acquire a simple oxide valence state of +2; leading to a reverse spinel crystallography, where some of the cobalt ions will fill octahedral sites and some will fill tetrahedron sites. With increasing calcination temperature, more and more MgO will decompose with magnesium anions that diffuse into the cobalt oxide structure occupying octahedral sites. Since the +2 ions occupy the octahedral sites, this structure is known as the reverse spinel. It approaches an equilibrium at increasing calcination temperatures, the formation of complex Co2Mg04 is carried out until completion. Once the inverse spinel structure was filled, Co2Mg04 would have half of the cobalt ions occupying octahedral sites and half of them in tetrahedral sites. At still higher calcination temperatures > 800 ° C the complex oxide becomes unstable and the formation of a solid solution is favored. Note that some magnesia remains in calcined samples at all temperatures in this study. Figure 1 illustrates how a precursor containing a Group VIII 2 (in Figure 1 (a)) can be placed on a silicon wafer (or other suitable material) 1. The Group VIII element for this modality and illustration is cobalt . A second precursor 3 is placed on the precursor 2 as a source of element B. The assembly 4 is then exposed to temperatures within a range of about 200 ° C to about 900 ° C to react to prepare a Co-Mg 5 catalyst. Example 4 offers additional details on the generic form for preparing a catalyst in this manner. Example 4 A cobalt and Mg wire (Purity> 99.9999%) was placed in a metal evaporator and both metals were sequentially evaporated in a tungsten filament, the temperature being controlled by current. A quartz setter was used to measure and monitor the thickness of the resulting film. In a common experiment, 5 nm of Co and 10 nm of Mg were deposited on a Si substrate, where the Co / Mg molar ratio is about 1/1 (Figure 1). The coated Si wafers were then placed in an oven and calcined in air at 200 ° C, 400 ° C, 600 ° C and 800 ° C, separately. Examples 5 and 6 are illustrative of a catalytic decomposition process carried out on samples prepared in Examples 1 or 2 (Example 5) or in Example 4 (Example 6). Inert gas is maintained until a reaction temperature of 900 ° C is reached. The carbonaceous gas introduced in these Examples was methane. However, many other known reactive gases can also function and the use of such gases is well known in the art. Example 5 A 0.05 gram sample made from Example 1 or 2 that had been calcined in air at a temperature of 400 ° C for one hour was placed in a frit in a vertical quartz reactor of 2.54 cm (1 inch) an argon flow of 200 mL / min. The temperature was then raised to 900 ° C by a Lindberg tube furnace, and the inlet gas was changed to methane at 500 mL / min, the reaction was allowed to continue for 30 minutes before it was turned off. After the reaction, the powder sample was collected and subjected to analysis using a Raman laser spectrometer and transmission electron microscope. Example 6 A sample of 1.27 cm x 1.27 cm (0.5"x 0.5") cut from Example 4 was placed in a frit in a quartz reactor of 2.54 cm (1 inch) under argon flow at 200 mL / min. The temperature was then rapidly raised to 900 ° C, and the gas inlet was changed to methane at 500 mL / min. The reaction was allowed to continue for 30 minutes before turning it off. After the reaction, the wafer sample was subjected to analysis using a Raman laser spectrometer. Figure 2 illustrates Raman spectra of products containing carbon fibrils formed in Examples 5 and 6 and discussed below as Example 7. Example 7 A Raman spectrometer equipped with continuous He-Ne laser with a wavelength of 632.8 nm was used to collect Raman excitement. A type of Raman at ~ 1580 cm "1 10 is present in all types of graphite sample such as highly oriented pyrolytic graphite (HOPG), pyrolytic graphite, and carbon. This peak is commonly referred to as the "band" "G". The peak at 1355 cm-1 11 occurs when the material contains defects in the graphene planes or from the edges of the graphite crystal. This band is commonly referred to as the "band" "D" and the position of this band depends strongly on the laser excitation wavelength. "Radial breathing modes (RBM)" were observed (typically below 300 cm-1) with single-walled nanotubes, where all carbon atoms are subjected to an equal radial displacement. A small change in the frequency of laser excitation produces a resonant Raman effect. Therefore, in most cases, it is possible to distinguish multi-walled carbon nanotubes (NT) of carbon nanotubes (SWNT) from Raman spectroscopy from the presence or absence of RBM and division in the G-band. Raman spectra of products manufactured in silicon wafer unambiguously indicated the frequencies characteristics of S NTs when the catalyst was calcined at 400 ° C or more. As illustrated in Table 2, this is consistent with the presence of Co2Mg04 complex oxide and (Table 2). Weak D band demonstrates that samples in the state in which they were synthesized contain a very small amount of amorphous carbonaceous materials. In other words, it is likely that high purity SWNTs were synthesized. In summary, the product containing carbon fibrils having nanotubes of diameters small enough to be single-wall (or possibly double-walled) has a Raman spectrum that has: peaks of "radial breathing mode" (RMB, its acronym in English) in 150 and 300 wave numbers, The area below the RBM peaks is at least 0.1% of the area below the characteristic G-band peak, the intensity of the G-band peak is at least twice the intensity of a characteristic D-band peak (G / D of at least 2.0). The Raman spectrum, described in Example 7 and shown in Figure 2, reveals 4 to 5 components in 138, 192, 216, 256, and 283 cm "1, respectively The expression: CORBM = (223.75 / d) cm-1, where ???? is frequency of radial breathing mode (RBM) in cm "1 and" d "is the diameter of SWNT in nm, can be used to calculate the diameters of SWNT. According to this formula, the peaks at 138, 192, 216, 256, and 238 cm "1 correspond to the SWNTs with diameters of 1.62, 1.17, 1.04, 0.87, and 0.79 nm, respectively Nanotubes with diameters of 1.17 nm (peak at 192 cm "1 6) seem to dominate based on the relative peak height. Table 2 Correlation between complex oxide phase in a catalyst with SWNT growth after activation and decomposition Calcination condition Crystalline phase containing Co2Mg04 Crystalline phase containing solid solutions (Co, MG) Single wall nanotube growth 200 ° C No No No 400 ° C Yes, trace No Yes, low selectivity 600 ° C Yes, some No Yes, average selectivity 800 ° C Yes, majority Not clear Yes, high selectivity Figure 3 illustrates a Raman spectrum of a product containing carbon fibrils formed in Example 6 plus additional processing steps of reduction and passivation of the catalyst before catalytic decomposition; the discussion is included below as Example 8. Example 8 A sample from Example 4, calcined at 800 ° C in air for one hour, was then placed in a 2.54 cm (1 inch) quartz reactor under a hydrogen flow of 100 mL / min, and heated slowly to 250 ° C for 30 minutes. The reduced sample was then passivated using 2% 02 / Ar. The treated sample was then placed in a reactor following the procedure described in Example 6 to grow nanotubes from a single wall. Peak height 6 ', compared to peak height 6, shows a significantly improved single-wall characteristic for those grown from this treated sample. Meanwhile, the band D region was found to be smaller and sharper. This is another indication of a substantial improvement in the growth selectivity of single-walled carbon nanotubes. Examples 9-13 describe embodiments for which element A of group VIII is iron instead of cobalt. Example 9 The same procedures were applied as in Example 1 and 2 and Co (N03) 2"6H20 was replaced with 40.4 grams of Fe2 (N03) 3 · 9H20.Example 10 The same procedure as described in Example 4 was applied. and the Co wire was replaced by iron wire (purity >; 99.9999%). EXAMPLE 11 The same procedure as that described in Example 5 was applied to manufacture single-walled nanotubes with catalyst from Example 9. Example 12 The same procedure as described in Example 6 was applied to prepare single-walled nanotubes with catalyst of Example 10. Example 13 In Figure 4, the Raman spectra of products made from the Fe-Mg catalyst system (Example 12) presented results similar to those of the Co-Mg system (Example 6) except that the Raman peak height corresponding to single-walled nanotubes having a diameter of 1.17 nm (peak 6") is lower than previously observed for a catalyst activated with Co-Mg in Figure 2. Figure 5 illustrates micrographs electronic scan products containing carbon fibril produced by catalytic decomposition of methane in catalysts activated from the Co-Mg system (Figure 5A) and the Fe-Mg system (Figure 5B) and co mint as in Example 14 below. Example 14 Figure 5A and B show low amplification SEM images of the product containing carbon fibrils as synthesized produced by catalytic reaction of CH4 in an oxide catalyst of Co-Mg and Fe-Mg complex at a temperature of 900 ° C. They indicate a large quantity of entangled carbon filaments with lengths of several tens of microns. A high resolution transmission electron micrograph (HRTEM) of SWNTs formed using a catalyst comprising a Co-Mg complex oxide phase is shown in Figure 6 and is described in the embodiment of Example 15. Example 15 Figure 6 shows a typical HRTEM image of the product containing carbon fibril as synthesized from Example 5. Examination of such HRTEM images indicates that the carbon filaments produced are mainly SWNT materials consisting of SWNTs and small amounts of discrete SWNTs isolated Examples 16 and 17 described the results obtained from samples having lower Co-Mg ratios than in the case of the stoichiometric spin, specifically 0.1 and 1, respectively. Example 16 0.05 gram of sample prepared from Example 2 was placed with a Co / Mg ratio of 0.1 in a vertical quartz reactor of 2.54 cm (1 inch). The sample was first calcined at 400 ° C for one hour, and then the reactor temperature was rapidly raised to 850 ° C under argon flow of 200 mL / min. Once the temperature had reached 850 ° C, the inlet gas was changed to CO (99.95%) at 300 mL / min and the reaction continued for 15 minutes before being turned off. After the reaction, the product was heavy. The carbon yield was measured and it was established that it was 0.5. The growth selectivity of single-walled carbon nanotubes was estimated to be greater than 70% (in accordance with that determined from the Raman spectrum and HRTE analysis). Example 17 0.05 grams of sample prepared from Example 2 was placed with Co / Mg ratio of 1 in a vertical quartz reactor of 2.54 cm (1 inch). The sample was first calcined in air at 200 ° C for 1 hour and then the reactor was purged with argon at 200 mL / min and the temperature was lowered to 250 ° C. 56% H2 / Ar was then introduced into the reactor at 100 mL / min. After 2 hours of hydrogen reduction, the inlet gas was changed back to argon and the temperature was quickly raised to 850 ° C. Once the temperature had reached 850 ° C, CO (99.95%) was introduced to the reactor at 300 mL / min and the reaction was allowed to proceed for 15 minutes before being turned off. After the reaction, the product was heavy. The carbon yield was measured and determined to be 1. It was estimated that the growth selectivity of single wall carbon nanotubes was greater than 90% (in accordance with that determined from the Raman spectrum and HRTEM analysis). Example 18 illustrates the sensitivity of the processing steps involved. Example 18 A 1.27 cm x 1.27 cm (0.5"x 0.5") sample cut from Example 4 is placed in a frit in a 2.54 cm (1 inch) quartz reactor under an argon flow of 200 mL / min. . The temperature is then rapidly raised to 700 ° C, and the inlet gas is changed to ethylene / H2 / Ar (0.5 / 2 / 97.5) at 500 mL / min. Let the reaction continue for 15 minutes before switching off. After the reaction, both the Raman spectrum and the SEM analysis showed that the product consisted of a mixture of single-walled and multi-walled carbon nanotubes. Example 19 Co-nitrate and Co-acetate were applied as catalyst precursors to form Co-oxides supported on silica. An ethanol solution of Co acetate and Co nitrate with 3% equivalent of metal based on Si02 was impregnated in fumed silica and followed by calcination in air at 400 ° C. Two different products resulted from this process, black powder a from nitrate (sample A) and pink powder from acetate (sample B). XRD diffraction indicates that the black powder contained C03O4 while the pink powder contained C0SÍO3, a trioctahedral layered silicate or stevensite. EXAMPLE 20 A programmed temperature reduction was carried out (TPR9 on a Quanta Chrome Autosorb 1C with 5% H2 / Ar as a reducing carrier gas.) The spectra are presented in Figure 7. Clearly, different Co precursors have provided a reduction profile. When nitrate is applied, the resulting Co species has the form of C03O4 and can be reduced under mild conditions while the acetate precursor produces a more stable Co species on the silica surface, specifically, Co silicate, Complete reduction of Co species requires a much higher temperature than in the case of C03O4. "Example 21 A Co / Sio2 catalyst was placed in a 1-inch tube reactor and the temperature was rapidly elevated to 850 ° C under a Argon atmosphere, Immediately after the temperature reached 850 ° C, the carrier gas was changed to methane and the reaction continued for 30 minutes. of Raman to characterize the product from the reaction, and two dramatically different carbon products were obtained from the two catalysts. The product made from methane when it was catalyzed by C0304 / Si02 appeared to be amorphous by nature; no Raman signature of single wall nanotubes was found. On the other hand, a clear characteristic of a single wall was presented in the product manufactured from the silicate catalyst of Co. Example 22 Salts of Al (N03) 3 · 9H20 and Fe (N03) 3 · 9H20 with 3% equivalent, 6% and 9% Fe in metallic base versus A1203 were dissolved in 25mL of deionized water. Then, this nitrate mixture was added concurrently with a 20% solution (NH4) 2C03 to a three-necked round bottom flask containing 200 mL of deionized water under strong agitation. The pH of the resulting paste was maintained at ~ 6 by controlling the rate of carbonation addition. After adding all the solutions in nitrate, the paste was stirred for an additional 15 minutes, followed by filtration and drying at 80 ° C. After calcination in argon at 500 ° C, the samples were set aside for reaction tests. Example 23 The reduction of Fe-Al oxide was studied by using a programmed temperature reduction with 5% H2 / Ar as reducing carrier gas. The 9% Fe / Al203 spectrum was shown in Figure 8. As can be seen in the spectrum, three main reduction steps were revealed. The first reduction (I) at 400 ° C indicated the transition from Fe203 to Fe304, followed by Fe304 to FeAl204 at 530 ° C (II) and finally to metallic Fe at 740 ° C (III). Example 24 The 9% calcined Fe / Al203 was placed in a 2.54 cm (1 inch) tube reactor. After complete purging with Ar, the reactor was heated to 500 ° C under a flow of 5% H / Ar. When the temperature reached 500 ° C, the carrier gas was again Ar and the reactor was heated rapidly to 800 ° C. At 800 ° C, the carrier gas was changed to CO and the reaction was carried out for 30 minutes. The black product was analyzed by Raman spectroscopy. As you can see in Figure 9, the product contains high quality single wall nanotubes with strong G and RMB bands and minimal D band signal. The selective reduction of the complex oxide materials disclosed herein requires that during activation only the Group VIII element is reducible under decomposition conditions. Therefore, an element B must be limited to elements capable of forming simple oxides of element B where the valence state for B in the simple oxide is equivalent to the valence state for B in the complex oxide, and which are not reducible in the presence of oxygen gas at a temperature less than or equal to about 900 ° C. Such B elements include aluminum, lanthanum, magnesium, titanium, zinc, zirconium, yttrium, calcium, strontium and barium. Furthermore, even when the selective reduction of the complex oxide is defined here in terms of reducibility of the B element in hydrogen at a specific temperature, it will be understood that the actual catalytic decomposition may occur, in other embodiments, in an atmosphere conformed by a wide range of carbonaceous gases other than methane and other hydrocarbons. The catalytic areas formed through the selective reduction can be "totally" reduced to the Group VIII element itself or they can be quite rich areas in this element to expulsion from other materials. The morphology, size and spacing of such regions within an activated catalyst are probably critically important for the resulting selectivity and yield of products containing fibrils. Without limiting ourselves to a particular theory, it is reasonable to state that the morphology, size and spacing of the element-rich regions from the selective reduction will vary, for a given recipe for additional processing, depending on the Group VIII element previously located in the states crystallographic specific at specific sites in the structure or is randomly distributed through the catalyst in a "solid solution". In addition, since the prior art comments seemingly detrimental effects of thickening (increase in size) of reduced areas for example from simple oxide, for example cobalt oxide (formula CO3O4), the presence of complex oxide in the catalyst to promote SWNT training is required. Without wishing to be bound by a particular theory, it is believed that without an appropriate chemical and, perhaps physical interaction between, the complex oxide catalyst (rich in A) and its support (rich in B), the microregions or nanoregions of activated catalyst in Group VIII (rich in A) will have a tendency to agglomerate to form larger regions (sintering) when heated to a temperature greater than or equal to half its melting temperature (° K). A strong interaction with the support will tend to stabilize these small catalytic regions even at such temperatures.

By forming a complex oxide system, not only is there a strong interaction between A and B, but also, as discussed above, each metal site is separated in an orderly manner which can further improve the sintering resistance . Accordingly, a selective (or controlled) reduction will result in the formation of small metal particles from component A, separated and stabilized by metal oxide much less reducible from B. Terms and expressions that have been used are used as terms of description and not limitation and there is no intention to use such terms or expressions to exclude equivalents of the features shown and described as portions thereof, recognizing that various modifications are possible within the scope of the embodiments of the invention presented in the claims Attached

Claims (19)

1. CLAIMS 1. A method for manufacturing single wall carbon nanotubes, said method comprising: supplying a composition comprising a complex oxide having a formula AxByOz, wherein x / y = 2 and z / y = 4, A is a Group VIII element; B is a different element from A and is an element whose simple oxide, wherein B is in the same valence state as in the complex oxide, is not reducible in the presence of hydrogen gas at a temperature below about 900 ° C; reducing said composition to form an activated catalyst; contacting a carbonaceous gas with said activated catalyst under conditions suitable for growing single-walled carbon nanotubes, said suitable conditions include a pressure greater than about one atmosphere and less than about 10 atmospheres and a temperature greater than about 400 ° C and less than about 950 ° C; and growing carbon nanotubes in said catalyst, said carbon nanotubes comprise single-walled carbon nanotubes.
2. 2. The method according to claim 1, wherein said A is cobalt, iron or nickel.
3. 3. The method according to claim 1, wherein said B is aluminum, lanthanum, magnesium, silicon, titanium, zinc, zirconium, yttrium, calcium, strontium or barium.
4. 4. The method according to claim 1, wherein said carbon nanotubes comprise at least 50% single-walled carbon nanotubes.
5. 5. The method according to claim 1, wherein said B is magnesium.
6. 6. The method according to claim 5, wherein said A is cobalt.
7. 7. The method according to claim 6, wherein the complex oxide is Co2Mg04.
8. The method according to claim 1, wherein said reduction step and said contacting step occur contemporaneously.
9. 9. A method for manufacturing single-walled carbon nanotubes, said method comprising: contacting a carbonaceous gas with an activated catalyst in a reaction zone under conditions suitable for growing single-walled carbon nanotubes, said suitable conditions they include a pressure greater than about 1 atmosphere and less than about 400 ° C and less than about 950 ° C, said activated catalyst comprises a reduced form of a complex oxide, said complex oxide having a formula AxByOz, wherein x / y = 2 yz / y < 4, A is an element of Group VIII; B is a different element from A and is an element whose simple oxide, in which B is in the same valence state as in the complex oxide, is not reducible in the presence of hydrogen gas at a temperature below about 900 ° C; and growing carbon nanotubes in said activated catalyst, said carbon nanotubes comprise single-walled carbon nanotubes.
10. 10. The method according to claim 9, wherein said A is cobalt, iron or nickel.
11. The method according to claim 9, wherein said B is aluminum, lanthanum, magnesium, silicon, titanium, zinc, zirconium, yttrium, calcium, strontium or barium.
12. 12. The method according to claim 9, wherein said carbon nanotubes comprise at least 50% of single-walled carbon nanotubes.
13. The method according to claim 9, wherein said B is magnesium.
14. The method according to claim 13, wherein said A is cobalt.
15. 15. The method according to claim 6, wherein the complex oxide is Co2Mg04.
16. 16. A method for preparing a catalyst for use in a process for the manufacture of single-walled carbon nanotubes, said method comprising the steps of: reacting a source of A with a source of B at a temperature sufficiently low to form a complex oxide that has a formula AxByOz, where x / y <; 2 and z / and < 4, A is an element of Group VIII; B is a different element from A and is an element whose simple oxide, in which B is in the same valence state as in the complex oxide, is not reducible in the presence of hydrogen gas at a temperature below about 900 ° C; and activating the complex oxide by reducing said complex oxide at a temperature below about 950 ° C.
17. 17. A method for preparing a catalyst for use in a process for the manufacture of single-walled carbon nanotubes, comprising the step of reducing an oxide of complex of formula AxByOz, at a temperature lower than 950 ° C, where x / y = 2 and z / y = 4, A is a Group VIII element, and B is a different element of A and is an element whose simple oxide, where B is in the same state of valence as in the complex oxide, is not reducible in the presence of hydrogen gas at a temperature below about 900 ° C.
18. 18. A catalyst manufactured in accordance with the method of claim 16.
19. 19. A catalyst manufactured in accordance with the method of claim 17.