Comparative in situ X-ray diffraction (XRD) study of activation Fe, Co, and FeCo catalysts on different supports (Al2O3 and CaCO3) for multi-walled carbon nanotube (MWNT) growth combined with ex situ high-resolution transmission electron microscopy (HRTEM) characterization has been performed. The catalyst contained Fe as an active component (mono-component Fe catalyst) demonstrates the simultaneous formation of FeC alloys (austenite and tetragonal ferrite like structure) with their subsequent transformation into stable iron carbide Fe3C (cohenite). The active component of bimetal FeCo catalysts (Fe:Co = 2:1) is FeCo alloyed nanoparticles, which does not demonstrate carbide formation. The absence of carbides promotes carbon diffusion through metal particle providing much higher activity of bimetal FeCo catalyst in comparison with the mono-component Fe and Co catalysts. Tubes produced using the bimetal catalysts have a high degree of structural perfection and narrow diameter distribution. The most active metal catalyst particles formed under reaction conditions are solid or at least have crystalline core. Pechini-type method used for FeCo catalyst production provides much higher MWNT yields than it was described elsewhere.
Structure of active metal particles was proposed according to XRD data during MWNT growth on different type of catalysts with mono- and bimetal active component.