Hydrogen Storage Mechanism by Gas-Phase Vanadium Cluster Cations Revealed by Thermal Desorption Spectrometry

The adsorption of hydrogen on gas-phase vanadium cluster cations, V⁺ (n = 3-14), at 300 K and desorption of hydrogen from hydride clusters, VH⁺, upon heating were observed experimentally by combined thermal desorption spectrometry and mass spectrometry analyses. The ratio m/n was approximately 1.3 for all n values at 300 K, which was reduced to approximately zero at 1000 K. For n = 4, stable cluster geometries of V₄H⁺ (m = 0, 2, 4, and 6) were investigated by DFT calculations, revealing that V₄ adopted a trigonal pyramidal structure and the H atoms adsorbed mainly on the μ₂ bridge sites. The adsorption reaction pathway of one H₂ molecule on V₄⁺ was also investigated. The experimentally estimated desorption energies of the H₂ molecules were consistent with their calculated binding energies. Among the observed hydride clusters, V₆H₈⁺ was found to be significantly thermally durable, probably because of its close-packed octahedral V₆ core structure, with H atoms occupying all hollow sites.