Decomposition of methane over iron catalysts at the range of moderate temperatures: The influence of structure of the catalytic systems and the reaction conditions on the yield of carbon and morphology of carbon filaments

Decomposition of high-purity methane in the presence of alpha -Fe-based cat alysts to produce filamentous carbon was investigated. The reaction was stu died in the temperature range of 650 to 800 degreesC. Filamentous carbon wa s demonstrated to form at temperatures not lower than 680 degreesC in the p resence of both bare alpha -Fe and catalysts based thereon with admixtures of various hard-to-reduce oxides (SiO2, Al2O3, ZrO2, and TiO2). The maximal carbon yield, 45 g per g of iron, was obtained with the Fe/SiO2 catalyst c omprising silica in amounts of 15 wt%. XRD and high-resolution electron mic roscopy were used for studying the carbon deposits. The data obtained allow ed the conclusion on the essential influence of the chemical nature of the hard-to-reduce oxide admixture in the iron catalyst on the microstructure a nd morphology of the carbon filaments. Depending on the admixture, specific shapes of filaments and nanotubes predominated. Carbon nanotubes with thin walls built up by coaxial cylindrical graphene layers were formed in large amount over the Fe/Al2O3 catalyst. Centers of growth of octopus-like nanot ubes were observed. (C) 2001 Academic Press.