Nanoscale carbon formation from various hydrocarbons over nanocrystalline Co/MgO catalyst

A series of nanocrystalline MgO samples containing 1.2-15.9% carbon were prepared by MgO carbonization at 500 degrees C in 1,3-butadiene flow diluted with argon in a 1:75 ratio. For the sample with 15.9% C almost all the MgO surface was covered with carbon. An increase of the MgO carbonization rate from 500 to 750 degrees C resulted in the growth of the carbon deposition rate. Still, the carbon deposits covered the MgO surface in a fairly uniform way. This is proved by the very high surface area (2000-2500 m(2)/g) of porous carbon obtained by dissolving MgO from the carbonized samples with hydrochloric acid. Nanocrystalline MgO was used as a support for synthesis of Co/MgO catalysts. The concentration of supported cobalt was varied from 2.5 to 30%. The nature and formation kinetics of carbon deposits from butadiene over Co/MgO catalysts were studied. High-resolution transmission electron microscopic (HRTEM) studies showed that carbonization of the Co/MgO catalyst in butadiene diluted with argon in the temperature range of 600-750 degrees C resulted in the formation of carbon nanotubes and a carbon film decorating the MgO surface. Unlike butadiene, methane was not found to carbonize the MgO surface at temperatures below 800 degrees C. Carbonization of the Co/MgO catalysts with methane in the temperature range of 600-800 degrees C resulted in the formation of carbon nanotubes. As reduction of the catalysts with hydrogen was found to yield highly dispersed cobalt nanoparticles with a narrow particle size distribution, the resulting carbon nanotubes are characterized by high uniformity. The number of layers in the nanotubes varied from one layer to six to eight layers depending on the reduction conditions and carbonization temperature. In the case of methane, a carbonization temperature increase from 600 to 800 degrees C led to a decrease of the carbon nanotube diameter from 4-5 to 1-2 nm and the number of layers from one to three layers to one layer. The observed decrease of the nanotube diameter with the temperature increase was explained assuming the key role of the graphite phase nuclei in the formation of carbon nanotubes.