Screening of Subnanoscale Metal Hydride Formation for Late Transition Metals Using Dimer Cations─Group IX Element

The energetically stable structures of M₂H⁺ (M = Co, Rh, Ir; m = 2, 4, 6, ...) were investigated using density functional theory calculations, and possible reaction pathways for the sequential adsorption of H₂ molecules on M₂⁺ were proposed. Based on the most stable structures, adsorption energies of H₂ were calculated for each adsorption step, and the maximum numbers of adsorbed H atoms on Co₂⁺, Rh₂⁺, and Ir₂⁺ were estimated to be 14, 16, and 16, respectively. Compared to group XI elements (M = Cu, Ag, and Au), which are conceivably inert to H₂, more H atoms were bound to Co₂⁺, Rh₂⁺, and Ir₂⁺. The adsorption of H₂ on M₂⁺ (M = Co, Rh, Ir, or Cu) in the gas phase was investigated experimentally at 300 K using mass spectrometry. Although Rh₂⁺ and Ir₂⁺ stored numerous H₂ molecules as predicted by calculations, Co₂⁺ was found to adsorb no H atoms. It was probably due to the insufficient adsorption energy of Co₂⁺ and the kinetic effect in the H₂ adsorption process. Thus, computational calculations can overestimate the number of adsorbed H atoms.