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Growth Mechanism of Carbon Nanotubes over Gold-Supported Catalysts

Carbon nanotubes have been widely synthesized by chemical vapor deposition (CVD) in the presence of transition-metal catalysts, such as Fe and Co, because they have strong catalytic activities for both the decomposition of hydrocarbon feedstock and the formation of graphitic carbons. Recently, gold (Au) was reported to catalyze the growth of multi-wall and single-wall carbon nanotubes (MWNTs and SWNTs, respectively). Because of the low solubility of carbon in Au and the unique catalytic activity observed for Au nanoparticles, the growth mechanism and yield of nanotubes over Au catalysts are intriguing. In this paper, we investigated the catalytic activity of Au supported on various metal oxides, alumina (Al2O3), silica (SiO2), titania (TiO2), and magnesia (MgO), with different hydrocarbon feedstocks, methane (CH4), ethylene (C2H4), and acetylene (C2H2). We found that Au-supported catalysts have limited activity for the decomposition of the carbon source from the gas analyses and that only the catalyst supported on Al2O3 and SiO2 gave a small number of MWNTs when reacted with C2H4 or C2H2. The formation mechanism of MWNTs over the Au-supported catalysts is also discussed.