Study of Hydrogen Adsorption in Titanium, Nickel and Pd Cluster Supported on Graphene Monovacancies

A detailed description of the atomic structure and the energetics of H$_{\mathrm{2\thinspace }}$adsorption on Ti$_{\mathrm{4,}}$ Ni$_{\mathrm{4}}$, and Pd$_{\mathrm{4}}$ clusters on graphene monovacancies is presented. The large binding energy of that clusters on vacancies is a result of strong hybridization between the unsaturated carbon. We found that the binding energy of a single H$_{\mathrm{2}}$ is strongly dependent on the specific cluster. In particular, the H$_{\mathrm{2\thinspace }}$bond cleavage is favored by titanium clusters. On the other hand, the Ni and Pd clusters favours the formation of Kubas complexes. The analysis of the adsorption energies and H$_{\mathrm{2\thinspace }}$average bond lengths suggests that supported Ti$_{\mathrm{4\thinspace }}$cluster is a potential hydrogen storage candidate, being able to hold up to 6 molecules covalently with moderate average binding energy within the optimal range for an efficient cyclic adsorption/desorption process at room temperature and moderate pressures. These results give physical insigths about how the interaction between graphene monovacancies and metal clusters can be used to enhanced the hydrogen uptake, towards of design of new graphene-based material for hydrogen storage.