On the H₂ interactions with transition metal adatoms supported on graphene: a systematic density functional study

The attachment of H₂ to the full set of transition metal (TM) adatoms supported on graphene is studied by using density functional theory. Methodology validation calculations on the interactions of H₂ with benzene and graphene show that any of the vdW corrections under study, the Grimme D2, D3, D3 with Becke-Jonson damping (D3BJ), and Tkatchenko-Scheffler methods, applied on the PBE functional, are similarly accurate in describing such subtle interactions, with an accuracy of almost 2 kJ mol^-1 compared to experiments. The PBE-D3 results show that H₂ physisorbs on especially stable d⁵ or d¹⁰ TMs. In other 5d metals, and the rightmost 3d and 4d ones, H₂ dissociates, and only for Y, Mn, Fe, and Zr the H₂ binds strongly enough for its storage in the so-called Kubas mode, where the H₂ bond is sensibly elongated. Other metals (Co, Ni, Ru, Rh and Pd) feature also an elongated Kubas mode, interesting as well for H₂ storage. Sc and Ti display a Kubas modes especially suited, given their lightness, for meeting the gravimetric requirements. The H₂ interactions with TM adatoms imply a TM → H₂ charge transfer, although the magnetic moment of the system tends to remain intact, except for the early 5d TMs, where the unpaired electron transfer seems to be associated with the H₂ bond breakage.