High quality double-walled carbon nanotubes (DWNTs) were synthesized by decomposition of methane over cobalt (Co) nanoparticles supported on porous MgO nanoparticles. The growth was significantly influenced by catalyst concentration and MgO type. Catalysts with 2.5-5 wt% Co loaded MgO (pore size: similar to4 nm) efficiently grow DWNTs with diameters of 2-4 nm, high graphitization, clean surfaces, and clean inside channels. The preliminary separation of DWNTs from MgO, Co and other carbonaceous nanoparticles has been carried out. Energy dispersive X-ray spectroscopy and X-ray diffraction were employed to analyze the chemical composition of the material's before and after purification. (C) 2002 Elsevier Science B.V. All rights reserved.