Hydrogen Storage Capacity of Cobalt Cluster Ions

The adsorption of H₂ on gas-phase Co^± (n = 5-12) clusters at 300 K and the desorption of H₂ from CoH^± 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 = 5, 10-12) and Co^- (n = 5-12) at 300 K to form CoH^±. The atomic ratios, m/n, for CoH^- (0.9-1.4) were higher than those for CoH⁺ (0.2-1.1). According to density functional theory (DFT) calculations, the first few H₂ 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, H₂, adsorbed molecularly, was explained in terms of σ complex formation. H₂ molecules were desorbed from the clusters upon heating. The temperature dependences showed that CoH^- released H₂ at a higher temperature (700-800 K) than CoH⁺ (600-700 K), suggesting that Co^- should have a higher affinity to hydrogen than Co⁺. The desorption temperatures were lower than those of VH⁺, which was consistent with the fact that the adsorption energies of H₂ 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.