Multi-walled carbon nanotubes, three types of activated carbons, single wall carbon nanotube and reduced graphene oxides were used to synthesize nano-sized Co catalysts for H-2 preparation via NH3 decomposition. Catalyst samples were characterized by number of techniques such as N-2 physisorption, X-ray diffraction (XRD), X-ray photoelectron spectroscopes (XPS), Transmission electron microscopy (TEM), CO chemisorption, temperature-programmed reduction (H-2-TPR) and temperature-programmed desorption (N-2-TPD). The catalytic activities of the studied catalysts for H-2 production via NH3 decomposition were measured in a fixed-bed micro-reactor. Co catalyst supported on multi-wall carbon nanotubes has shown the highest catalytic activity. The Co particles size was significantly affected by the variation of the post-treatment temperature. The Co particles size in the range of 4.7-64.8 nm can be effectively controlled by varying post-treatment temperature between 230 and 700 degrees C. The maximum TOF of NH3 decomposition was registered on cobalt catalyst post-treated at 600 degrees C. Copyright (C) 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.