Further studies of the formation of filamentous carbon from the interaction of supported iron particles with acetylene

A combination of controlled atmosphere electron microscopy and Mössbauer spectroscopy has been used to investigate the characteristics of supported α-iron and γ-iron particles during the formation of carbon filaments via decomposition of acetylene. γ-iron was found to exhibit a higher intrinsic activity than α-iron for this reaction when the metal was supported on graphite. In both systems, however, catalytic action decreased significantly at temperatures in excess of 700°C. Major changes were observed in the catalytic behavior of the metal particles when they were supported on silica. The rate of formation of carbon filaments from the α-iron/silica system showed a uniform increase up to 900°C. Mössbauer spectroscopy analysis of similarly treated samples revealed that under these conditions α-iron was the only metallic phase present, even though experiments were conducted through a temperature region where the transformation of α-iron to γ-iron can occur, suggesting that silica stabilizes the α-form of iron. In contrast, the catalytic activity displayed by γ-iron particles supported on silica was considerably reduced over that found for the corresponding graphite supported system. The results of this study are discussed in terms of some of the factors controlling the growth characteristics of filamentous carbon.