Sintese de nanotubos de carbono por CVD utilizando catalisadores à base de ferro e molibdênio suportados em matrizes cerâmicas

AUTOR(ES)
FONTE

IBICT - Instituto Brasileiro de Informação em Ciência e Tecnologia

DATA DE PUBLICAÇÃO

26/02/2010

RESUMO

Molybdenum is known for its synergistic effect in the synthesis of carbon nanotubes (CNs) by chemical vapor deposition (CVD method). When added to typical catalysts like iron, nickel, and cobalt, even in small quantities, it is increases the yield of these nanostructures. The presence of Mo also has an influence on the type and number of CN walls formed. Although this effect is widely documented in the literature, there is not yet a consensus about the mechanism of action of molybdenum in catalytic systems. The objective of the present work is to study the influence of molybdenum on the catalytic activity of iron nanoparticle-based catalysts supported on magnesium oxide (Fe/MgO system) in the synthesis of carbon nanotubes by the CVD method. The Mo concentration was systematically varied from null to molar ratio values four times greater than the quantity of Fe, and the obtained material (catalysts and carbon nanotubes) were broadly characterized by different techniques. In order to also study the influence of the preparation method on the final composition of the catalytic system phases, the catalytic systems (Fe/MgO e FeMox/MgO) were synthesized by two different methods: co-precipitation and impregnation. The greatest CN yields were observed for the catalysts prepared by coprecipitation. The difference was attributed to better dispersion of the Fe and Mo phases in the catalyst ceramic matrix. In the precipitation stage, it was observed the formation of layered double hydroxides whose concentration increased with the Mo content up to the ratio of Mo/Fe equal to 0.2. This phase is related to a better distribution of Fe and Mo in this concentration range. Another important characteristic observed is that the ceramic matrix is not inert. It can react both with Fe and Mo and form the iron solid solution in the magnesium oxide and the phases magnesium-ferrite (MgFe2O4) and magnesium molybdate (MgMoO4). The MgFe2O4 phase is observed in all catalytic systems, while the MgMoO4 phase is observed in systems with Mo/Fe ratios greater than 0.2. In spite of the differences between the two methods of preparation, the influence of molybdenum is practically the same in the two series of catalysts studied. In both cases, the reaction yield was directly proportional to the molybdenum concentration. When the Mo concentration, however, was much higher than the Fe concentration, the CN synthesis yield decreased. The highest yields, therefore, were found when the Mo/Fe ratio was equal to 1. We propose that excess molybdenum leads to the formation of Mo metallic agglomerates that do not catalyze the CN synthesis by chemical vapor deposition. We also observed that the presence of molybdenum brought about the formation of multi-walled carbon structures (multi-walled nanotubes MWNT and bamboo-like structures), while iron promoted the preferential formation of nanotubes with one SWNT or few walls. Besides carbon and MgO nanostructures, iron carbide (Fe3C) and molybdenum carbide (Mo2C) (catalysts containing Mo) were also formed in all of the samples grown with CVD, and the quantity of Mo2C increased with the increase in the Mo content in the catalyst. Based on the results obtained and the literature, two distinct regimes of action of Mo in the studied catalysts on the CVD carbon nanotubes synthesis from ethylene are proposed, when carried out in the conditions used in this work: 1) catalytic systems containing only Fe or small concentrations of Mo (Mo/Fe = 0, 0.02, 0.05, and 0.10). In this system, part of the Mo-containing species associate with the Fe phases present in the catalysts. During the decomposition of ethylene, Fe associates in particles with metallic Mo or Mo carbide. This association brings about the formation of MWNTs and carbon nanostructures with a higher degree of defects. The iron particles from the magnesium-ferrite phase or from the Fe solid solution in Mg bring about the formation of SWNTs and CNs with only a few walls. 2) Catalytic systems containing larger concentrations of Mo (Mo/Fe = 0.20, 0.50 and 1.00, and 4.00). In this system, two catalytic sites participate in the formation of CNs. In the first site, Fe nanoparticles from the magnesium-ferrite phase and the iron solid solution in Mg catalyze the synthesis of SWNTs or CNs with few walls. In the second site, Mo nanoparticles (from the magnesium molybdate phase), whether associated or not with Fe nanoparticles, catalyze the formation of MWNTs and other structures containing graphite multilayers.

ASSUNTO(S)

quimica do estado condensado nanotubos materiais carbono síntese ferro molibdênio nanotubes materials carbon synthesis iron molybdenium

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