Hydrogen Storage Capacity of Cobalt Cluster Ions.

The adsorption of H 2 on gas-phase Co n ± ( n = 5-12) clusters at 300 K and the desorption of H 2 from Co n H m ± upon heating were studied experimentally by combining thermal desorption spectrometry and mass spectrometry to elucidate the hydrogen storage property of the Co clusters. Hydrogen atoms adsorbed well on Co n + ( n = 5, 10-12) and Co n - ( n = 5-12) at 300 K to form Co n H m ± . The atomic ratios, m / n , for Co n H m - (0.9-1.4) were higher than those for Co n H m + (0.2-1.1). According to density functional theory (DFT) calculations, the first few H 2 molecules had a tendency to dissociatively adsorb onto the Co clusters. Further, the bonding nature of the H atoms was ionic, similar to the H atoms in the metallic hydrides. In contrast, H 2 , adsorbed molecularly, was explained in terms of σ complex formation. H 2 molecules were desorbed from the clusters upon heating. The temperature dependences showed that Co n H m - released H 2 at a higher temperature (700-800 K) than Co n H m + (600-700 K), suggesting that Co n - should have a higher affinity to hydrogen than Co n + . The desorption temperatures were lower than those of V n H m + , which was consistent with the fact that the adsorption energies of H 2 were lower for the Co clusters than those for the V clusters. The low adsorption energies were ascribed to their large highest occupied molecular orbital-lowest unoccupied molecular orbital (HOMO-LUMO) gaps in the Co clusters.