Catalytic synthesis of carbon nanotubes over Co, Fe and Ni containing conventional and sol-gel silica-aluminas

An attempt has been made to synthesise multiwalled carbon nanotubes using cobalt, iron and nickel supported on different types of silica-aluminas to investigate the rules governing their nanotube producing activity. Acetylene was used as the source of carbon. Decomposition of acetylene has been carried out at atmospheric pressure. The effect of reaction temperature in the 770-970 K range and the flow rate of the hydrocarbon has been investigated. The catalysts were analysed by XRD, UV-VIS, surface area and porosity measurements. Formation of carbon nanotubes was followed by electron microscopy. The amount of deposited carbon increased with increasing reaction temperature and the flow rate of acetylene, but decreased with increasing concentration of alumina in the catalyst support. Each catalyst showed high production of carbon nanotubes at 970 K; however, they were inactive at 770 K. The yield of tube formation was very low at 870 K. The high-resolution transmission electron microscopic (HREM) analysis showed that the outer diameter of the tubes generated varied from 8 to 40 nm, the tubes were multiwalled, and the number of the layers was between 8 and 30. Sol-gel derived samples were also found to be working catalysts, indicating the existence of an optimal metal particle size.