An investigation of carbon nanotubes obtained from the decomposition of methane over reduced Mg_1−xM_xAl₂O₄ spinel catalysts

Carbon nanotubes produced by the treatment of Mg_1−xM_xAl₂O₄ (M = Fe, Co, or Ni; x = 0.1, 0.2, 0.3, or 0.4) spinels with an H₂–CH₄ mixture at 1070 °C have been investigated systematically. The grains of the oxide-metal composite particles are uniformly covered by a weblike network of carbon nanotube bundles, several tens of micrometers long, made up of single-wall nanotubes with a diameter close to 4 nm. Only the smallest metal particles (<5 nm) are involved in the formation of the nanotubes. A macroscopic characterization method involving surface area measurements and chemical analysis has been developed in order to compare the different nanotube specimens. An increase in the transition metal content of the catalyst yields more carbon nanotubes (up to a metal content of 10.0 wt% or x = 0.3), but causes a decrease in carbon quality. The best compromise is to use 6.7 wt% of metal (x = 0.2) in the catalyst. Co gives superior results with respect to both the quantity and quality of the nanotubes. In the case of Fe, the quality is notably hampered by the formation of Fe₃C particles.