ii. Non-metal based catalysts (Ceramic and semiconductor catalysts)

Among non-metallic catalysts SiC is most widely used. The Fig.5 shows the carbon nanostructure formation over silicon carbide by carbothermal reduction of silica. At first stage SiO₂ is reduced to SiC via a carbothermal reaction. In next step SiC nano particle coalesce and finally carbon nanotube growth occurs on surface of SiC particles through carbon precipitation and/or SiC decomposition. The carbon generation reaction is given below.

Fig. 5. Schematic representation of CNT formation over silica

MgO, Al₂O₃, zirconia, magnesium borates etc. are also reported to be used to grow carbon nanotubes.