Synthesis of multiwall carbon nanotubes over cobalt catalysts

CH4 pyrolysis over Co/Al2O3 (60 wt %) catalysts, prepared by different methods and doped with tervalent-metal oxides, was studied gravimetrically under isothermal conditions at 600–750°C and the atmospheric pressure. The process starts at the maximum rate and decays with time. The pyrolysis rate as a function of linear CH4 rate peaks at 40 cm/min. With the provision of the maximum process rate, the activation energy of CH4 pyrolysis over various catalysts falls within a range of 150–240 kJ/mol. For catalysts with identical compositions, the activity depends on their preparation. Transmission electron microscopy showed that the product was multiwall carbon nanotubes with diameters ranging from 6 to 30 nm. Mathematical equations were proposed to describe the process kinetics. A productivity of 12 g/h is achieved on a laboratory continuous reactor.