Hydrogen production by methane decomposition has been studied using a cobalt catalytic system prepared by urea precipitation. Upon calcination, the as-synthesized material converts to cobalt oxide catalyst precursor, and further thermal treatment with different reducing gas yields the actual metallic cobalt catalyst for the reaction. The reduction environment of the cobalt precursor has a substantial effect on the catalytic activity as well as on the type of carbon deposited over the catalyst. Moreover, after being reduced, the atmosphere used for catalyst pre-treatment has also a great influence in the H-2 production, the best catalytic activity being obtained when heating under a nitrogen atmosphere. These conditions lead towards a smaller size of the bulk Co nanoparticles and, therefore, to a higher surface area compared to the other two employed pre-treatment atmospheres. This results in the production of a high amount of H-2 (ca. 33 mol/mol of Co after 72 h of reaction at 600 degrees C) over the N-2 pre-treated catalyst. Accordingly, the size of the final Co nanoparticles is an essential factor determining their activity in methane decomposition, hence the reduction and/or pre-treatment conditions must be conveniently selected to avoid as much as possible the sintering and aggregative growth of the Co-based nanoparticles. Apart from hydrogen production, it is demonstrated the possibility of graphene formation by methane decomposition over Co catalysts when methane is used as reduction agent. (C) 2013 Elsevier B.V. All rights reserved.