Combined catalyst system for preferential growth of few-walled carbon nanotubes

The effect of a combined catalyst system on the synthesis of carbon nanotubes (CNTs) from methane (CH₄) was investigated using molybdenum trioxide (MoO₃) as a conditioning catalyst and molybdenum-doped iron supported on magnesia as the main catalysts. Without the conditioning catalyst, only single-walled CNTs with diameters smaller than 2 nm were formed on the main catalyst. With the conditioning catalyst, double-walled CNTs and few-walled CNTs with larger hollow diameters than 2 nm were produced with much higher yields. The combination of the two kinds of Mo-containing catalyst would more effectively transform CH₄ into reactive species related to the early stage of nanotube formation on the main catalyst, resulting in the increase of the yield, diameter and layer number of the CNTs.