Catalysts for vapor-phase deposition of multi-walled carbon nanotubes are considered. A method for synthesis of iron-containing catalyst by coprecipitation of aluminum, magnesium, and iron(II) hydroxides is suggested.Carbon nanotubes (CNT), which were only available in milligram amounts several years ago, are gradually becoming a product of large-tonnage synthesis. Simultaneously with the increase in the production scale, new application fields of CNT are being revealed. For example, Hyperion Catalysis International Co. (USA) manufactures CNT 10315 nm in diameter and CNT-containing polymeric formulations. Introduction of 235% CNT into polymeric materials makes them electrically conducting and thereby hinders accumulation of static electricity [1]. Mechanical properties of CNT-filled polymeric materials have been studied [23 4]. CNT and CNT-based composites are under intensive scientific and technological development in China [5310].According to the results of [9], the most valuable are fine CNT less than 20 nm in diameter. In the best variants reported in numerous publications devoted to synthesis of multi-walled CNT by the chemical vapor deposition technique, the CNT diameter is 10 320 nm. The authors of the present study are not aware of any method for obtaining multi-walled CNT with an average diameter of less than 10 nm.It is commonly believed, even though this issue remains insufficiently studied, that the diameter of a growing nanotube is determined by the size of the cluster of a catalytically active metal, e.g., iron, cobalt, or nickel. Presumably, the most efficient way to obtain and stabilize nanosize clusters of catalytically active metals is their immobilization on some highly dispersed or porous support, e.g., aluminum oxide, silicon, magnesium, or carbon. For this purpose, a dispersed support is impregnated with solutions of metal compounds (nitrates, acetylacetonates, salts of organic acids, etc.) On being introduced into a hot reactor, these compounds decompose to give, commonly, metal oxides, from which catalytically active metal clusters are formed in a reducing atmosphere (as a rule, a mixture of hydrogen and hydrocarbons). Such a procedure for fabrication of catalysts for synthesis of multi-walled CNT was described, e.g., in the patent [11]. Aluminum oxide with a particle size of about 10 nm was used as a highly dispersed support, and nitrates and acetylacetonates of iron, molybdenum, manganese, and chromium, and mixtures of these, as compounds of catalytically active metals. Ethylene mixed with hydrogen served as a source of carbon. The mass of the multi-walled CNT obtained was tens of times that of the catalytically active metals, and the diameter of the resulting CNT was in the range from 3.5 to 70 nm. The method described has the following disadvantage. As a result of impregnation and drying, particles of the material undergo aggregation and the size of metal oxide clusters formed in pyrolysis may markedly exceed the particle size of the highly dispersed support. Thus, the nanotubes obtain...

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A mechanochemical method for fabrication of catalysts for vapor-phase deposition of porous carbon nanofibers from acetylene is suggested.Carbon nanotubes (CNT) and nanofibers (CNF) are the most promising materials for energy-storage devices: chemical power cells, supercapacitors, fuel cells, and hydrogen adsorbers [138]. The energy storage capacity is determined by the structure of the carbon materials. For example, the highest capacity for carbon has been observed for nanofibers in which the carbon layers are perpendicular to the fiber axis or have a [fir-tree] orientation. A reproducible capacity for hydrogen of about 15 wt % has been achieved [8].Porous CNF with a developed surface have been synthesized. For example, chemical activation of CNT and CNF with molten potassium hydroxide makes it possible to raise manyfold the specific surface area through burning-out of a part of graphene layers and their unfolding [9, 10]. In [11], carbon nanofibers with porous walls in the form of honeycomb were obtained by chemical vapor deposition.The main factors determining the structure of CNT and CNF in vapor-phase deposition are the composition and structure of the catalyst used. The conventional methods for fabrication of catalysts for the process in question (aerosol techniques are disregarded) are as follows: impregnation of porous or highly dispersed mineral substrates with solutions of compounds of catalytically active metals (most frequently, iron, cobalt, and nickel) [12], coprecipitation of a mixture of metal hydroxides [13], and thermal decomposition of a mixture of metal salts and organic acids [14]. The forming homogeneous or microheterogeneous mixtures of metal oxides give rise, under the conditions of vapor-phase deposition (temperature, reductive atmosphere), to clusters of catalytically active metals, on which CNT or CNF of certain structure grow. In the process, the structure is determined not only by the nature and size of clusters of the catalytically active metals, but also by the nature of a substrate. For example, it was shown in [15] that a mixed nickel-aluminum oxide deposited onto oxidized carbon nanotubes catalyzes growth of carbon nanotubes, and nanofibers grow on this mixed oxide without a carbon substrate. It was demonstrated in [16] that a thin layer of iron, deposited in a vacuum on a substrate made of a mixed indium-tin oxide, catalyzes growth of carbon nanocoils. At the same time, comparatively straight CNT or CNF are formed on other iron-containing catalysts.A disadvantage of the impregnation or deposition from solution is that particles of the substrate or catalyst undergo aggregation upon drying. For example, highly dispersed pyrogenic oxides of aluminum and silicon, used as supports, lose fluffiness and turn into dense lumps of a xerogel upon impregnation with aqueous or methanol solutions and drying. In growth of CNT or CNF on catalysts of this kind, there appears a tendency toward agglomeration of the carbon product into more or less dense concretions, which is unfavorable for its use...

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