Water assisted synthesis of double-walled carbon nanotubes with a narrow diameter distribution from methane over a Co–Mo/MgO catalyst

Double-walled carbon nanotubes (DWCNTs) with a narrow diameter distribution have been synthesized with the assistance of water vapor in a fluidized-bed reactor from methane over a Co–Mo/MgO catalyst. The results reveal that an appropriate amount of water enhances the carbon yield and also changes the diameter distribution of the DWCNTs. An excess amount of water depresses the formation of carbon nanotubes due to the stabilization of the Co–molybdate phase. Water facilitates the formation of adjacent Co and CoO phases, and therefore methane activation and carbon nanotube formation. The formation of adjacent Co and CoO phases results in smaller Co nanoparticles. DWCNTs with narrow diameter distribution are produced with this method at a low cost.