A one-pot approach was established to prepare carbon nanotubes (CNTs) through the carbonization of polypropylene (PP) by the combined catalysts of activated carbon (AC) with Ni2O3. The combination of AC with Ni2O3 showed a synergistic catalysis on the catalytic conversion of PP to form CNTs. The effects of the content of AC and carbonization temperature on the yield of CNTs were studied. The morphology, phase structure and thermal stability of the obtained CNTs were analyzed by means of SEM, TEM, XRD, TGA and Raman. In this synergistic catalysis, the surface functional groups (especially carboxylic groups) of AC were proved to be the key factor. AC not only promoted the cracking of PP fragment radicals into light hydrocarbons and the dehydrogenation and aromatization of the resultant light hydrocarbons into aromatic compounds, but also promoted the formation of the intermediate aromatic compounds or polycyclic aromatic hydrocarbons (PAHs) from the reaction of light hydrocarbon products and aromatic compounds. Additionally, AC assisted in situ Ni catalyst (originated from the reduction of Ni2O3) catalyzing the dehydrogenation and aromatization of intermediate aromatic compounds or PAHs products to form CNTs. At last, a layer-by-layer assembled mechanism based on benzene rings for the growth of CNTs using PP as carbon source and combined AC/Ni2O3 as catalysts was proposed. This mechanism will help to understand the growth mechanism of CNTs using virgin or waste plastics as carbon sources. More importantly, this approach offers a new potential way for large-scale production of CNTs from waste plastics using cheap AC as a cocatalyst. (C) 2012 Elsevier B.V. All rights reserved.