| Abstract
| - A detailed 57Fe Mössbauer study of the Mg0.8Fe0.2-yCoyAl2O4 (y = 0, 0.05, 0.1, 0.15, 0.2) solid solutions andof the CNT−Fe/Co−MgAl2O4 nanocomposite powders prepared by reduction in H2−CH4 has allowedcharacterization of the different iron phases involved in the catalytic process of carbon nanotube (CNT)formation and to correlate these results with the carbon and CNT contents. The oxide precursors consist ofdefective spinels of general formulas (Mg1-x-y2+Fex-3α2+Fe2α3+□αCoy2+Al23+)O42- . The metallic phase inthe CNT−Fe/Co−MgAl2O4 nanocomposite powders is mostly in the form of the ferromagnetic α-Fe/Coalloy with the desired composition. For high iron initial proportions, the additional formation of Fe3C andγ-Fe−C is observed while for high cobalt initial proportions, the additional formation of a γ-Fe/Co−C phaseis favored. The higher yield of CNTs is observed for postreaction α-Fe0.50Co0.50 catalytic particles, whichform no carbide and have a narrow size distribution. Alloying is beneficial for this system with respect to theformation of CNTs.
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