Molecular adsorption of H-2 on small cationic nickel clusters

The adsorption of H2 on metal clusters is of interest in several fields, including metallurgy, catalysis and hydrogen storage. Hydrogen interacting with small Ni clusters is of special interest, as Ni is widely used as a hydrogenation catalyst. In general, reactions of H2 with extended Ni surfaces are well studied, both experimentally1 and theoretically.2 In practically all cases an H2 molecule dissociates upon adsorption on a Ni surface and the H atoms bind in high coordination sites. The dissociation of H2 on Ni is direct, i.e. it does not proceed via a molecular precursor state.1,3 H2 molecules can only bind molecularly on the step sites of a Ni(510) surface if this surface is fully saturated with H atoms.4 Small Ni clusters do not necessarily exhibit the same behavior as extended surfaces. Consequently, the reaction of gas-phase Ni clusters with H2 has been the subject of many studies.5–8 However, detailed information on H binding sites on Ni clusters is scarce. In agreement with what is observed for extended Ni surfaces, it has recently been confirmed by IR spectroscopy that H2 can be molecularly adsorbed on hydrogen saturated Ni clusters.9 It is not clear whether H2 binds molecularly to nickel clusters only once all sites for dissociative adsorption are blocked, or whether already the very first H2 molecule can bind molecularly.