An original growth mode of MWCNTs on alumina supported iron catalysts

Multi-walled carbon nanotubes (MWCNTs) have been produced from ethylene by fluidized bed-catalytic chemical vapor deposition (FB-CCVD) on alumina supported iron catalyst powders. Both catalysts and MWCNTs-catalyst composites have been characterized by XRD, SEM-EDX, TEM, Mössbauer spectroscopy, TGA and nitrogen adsorption measurements at different stages of the process. The fresh catalyst is an alumina/iron oxide powder composed of amorphous iron(III) oxide nanoparticles located inside the porosity of the alumina support and of a micrometric crystalline α-iron(III) oxide surface film. The beginning of the CVD process provokes a brutal reconstruction and simultaneous carburization of the surface film that allows MWCNT nucleation and growth. These MWCNTs grow aligned between the support and the surface catalytic film, leading to a uniform consumption and uprising of the film. When the catalytic film has been consumed, the catalytic particles located inside the alumina porosity are slowly reduced and activated leading to a secondary MWCNT growth regime, which produces a generalized grain fragmentation and entangled MWCNT growth. Based on experimental observations and characterizations, this original two-stage growth mode is discussed and a general growth mechanism is proposed.