Successive cobalt increments from 3.1% up to 40.0% in a commercial hydrotreating catalyst containing originally 3.1%Co and 10.5%Mo on gamma-Al2O3 was investigated at a reaction temperature of 700 degrees C and atmospheric pressure for the catalytic decomposition of natural gas to CO/CO2 free hydrogen and carbon nanomaterials. The fresh and used catalysts were characterized by XRD, BET, TEM and TG-DTA analysis. The catalytic performance data showed that the increase of cobalt concentration improved the catalytic dissociation activity and longevity toward hydrogen production. All surface properties declined with successive addition of cobalt in the catalysts. XRD results showed that the crystallinity was remarkably enhanced and Co3O4 phases predominate upon addition of the cobalt precursor. The catalytic activity was found to be primarily dependent on the extent and degree of isolation of Co3O4 phases on the catalyst surface which reflects that the current reaction is a metal catalyzed one. The influence of metal content on the total carbon yield and the degree of graphitization of the resulting CNTs was investigated by TGA, XRD, Raman spectroscopy and TEM analysis. (C) 2013 Elsevier Ltd. All rights reserved.