DFT calculations with the GGA-PBE exchange correlation functional were used to study H-2 adsorption on a Ni(1 1 1) surface, isolated Ni-13 cluster, and graphene-supported Ni-13. In comparison with Ni(1 1 1), hydrogen adsorption shows to be more stable on isolated Ni-13 and graphene-supported Ni-13. In the graphene-supported Ni-13, pseudo charge density difference calculations showed accumulation of charge density around the Ni-graphene interfacial region. Dissociative H-2 adsorption on Ni(1 1 1) and isolated Ni-13 appears to be a non-activated process, whereas an activation barrier is observed on the graphene-supported Ni-13. Additionally, the effect of pre-adsorbed hydrogen in H-2 adsorption in the mentioned systems was studied showing that it stabilizes the final state of adsorbed H and decreases the activation barrier. (C) 2018 Elsevier B.V. All rights reserved.