Growth of carbon nanotubes by catalytic decomposition of CH4 or CO on a Ni-MgO catalyst
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By using a Ni-MgO catalyst, carbon nanotubes with small and even diameter could be prepared from catalytic decomposition of CH4 or CO. These carbon nanotubes prepared by this method are more or less twisted, with the outer diameter at 15-20 nm, and the tube length up to 10 mu m. The results of XRD measurements and pulse reaction testing indicated that the NiO and MgO components in this catalyst precursor formed, due to their highly mutual solubility, a NixMg1-xO solid solution. The high dispersion of Ni-species in this solid solution and the effect of valence-stabilization by the MgO crystal field would be in favor of inhibiting deep reduction of Ni2+ to Ni-0 and aggregation of the Ni-0 to form large metal particles at the surface of catalyst, making the carbon nanotubes grown on this catalyst relatively small and even in size of diameter. The experimental results also indicated that, in the growing process of carbon nanotubes, the rate-determining step was dependent upon the conditions of preparation (i.e. feedgas used, reaction temperature, flow-rate of the feedgas, etc.). The growth mechanism of the carbon nanotubes on the Ni-MgO catalyst is discussed together with the experimental results. (C) 1997 Elsevier Science Ltd.