Effect of Ni loading and reaction temperature on the formation of carbon nanotubes from methane catalytic decomposition over Ni/SiO₂

Since their discovery carbon nanotubes (CNT) have attracted much attention due to their singular physical, mechanical and chemical properties. Catalytic chemical vapor deposition (CCVD) of hydrocarbons over metal catalysts is the most promising method for the synthesis of CNT, because of the advantages of low cost and large-scale production and the relatively low temperature used in the process, compared to the other methods (laser ablation and discharge between graphite electrodes). In this study, CNT were synthesized by CCVD using Ni supported on SiO₂ as a catalyst. The carbon deposited in the reaction was analyzed by Raman spectroscopy, thermogravimetric analysis (TGA), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The effects of reaction temperature and Ni loading on the carbon nanotube formation were evaluated. The catalyst with 5% Ni favored high yield of CNT at lower temperature, with abundant “multi-walled carbon nanotubes” (MWNTs) at 625 °C, while single-walled carbon nanotubes (SWNTs) and MWNTs were obtained at 650 °C. With an increase in the reaction temperature a marked decrease in the yield of CNT was observed, probably due to the sintering of the catalyst. The catalyst with 1% Ni gave SWNTs with a high degree of order at all reaction temperatures, but in low quantity.