Influence of the composition of a H2-CH4 gas mixture on the catalytic synthesis of carbon nanotubes-Fe/Fe3C-Al2O3 nanocomposite powders

C-NTa-Fe/Fe3C-Al2O3 nanocomposite powders have been prepared by selective reduction of an alpha-Al1.9Fe0.1O3 solid solution in H-2-CH4 gas mixtures (0, 1.5, 3, 4.5, 6, 9, 12, 14, 16, 18, 24, 30 and 45 mol% CH,). The powders have been studied using macroscopic and microscopic techniques. The C-NTs are arranged in very long bundles homogeneously dispersed in the composite powder. Most C-NTs have less than four walls and are free of pyrolytic or amorphous carbon deposits. The inner diameter is in the range 1-6 nm, which could indicate that the catalyst particles active for C-NTs formation are in this size range. The reduction of the Fe3+ ions to metallic Fe is highly favoured by the presence of CH, in the reduction atmosphere. There are more Fe3C than alpha-Fe particles located at the surface of the matrix grains for CH4 contents higher than 4.5 mol%, however, the exact nature of the catalytically active particles remains an open question. Compositions in the range 9-18 mol% CH4 give the best results.