Formation of multi-walled carbon nanotubes by Ni-catalyzed decomposition of methane at 600-750 deg C

Ni-catalyzed decomposition of methane at high temperatures was examined by using a thermogravimetric apparatus. The catalyst (10 wt.% Ni supported on spherical alumina) gave quite a high carbon nanotube (CNT) yield at the temperatures below 680 deg C. At > 700 deg C, however, carbon formation rate decreased with increasing the reaction temperature. Temperature-programmed reaction also showed the maximum CNT growth rate at ~ 690 deg C. This result ruled out the possibility that the apparent negative activation energy is caused by sintering of Ni particles. Detailed examination on the kinetic expression led us to a conclusion that the dissolution of carbon atoms formed by dissociation of methane into bulk of the nickel particles is the rate-determining step at high temperatures, while methane adsorption is the rate-determining step at lower temperatures. This idea also explains the fact that the carbon yield drastically decreased at high temperatures. The CNTs formed at these temperatures had thinner walls than those formed at lower temperatures. The latter fact also supports the idea that the solubility of carbon in the nickel particles decreases at high temperatures.