Outstanding Features of Rhodium Clusters for Hydrogen Adsorption and Desorption Revealed by Thermal Desorption Spectrometry

The adsorption of H₂ molecules on cationic rhodium clusters, Rh_n⁺ , at 300 K and their desorption upon heating were experimentally investigated by using gas-phase thermal desorption spectrometry to assess the hydrogen storage capacity of Rh clusters. The results revealed that Rh_n⁺ could store a higher amount of H₂ with desorption occurring at significantly lower temperatures compared to other cluster such as V_n⁺ and Co_n⁺. In particular, Rh₆⁺ exhibited remarkable desorption characteristics. Density functional theory calculations revealed the geometrical structures of stable Rh₆H_m⁺ , showing that H₂ molecules predominantly adsorb dissociatively on bridge sites for m ≤ 8 as a metal hydride, while they start to adsorb molecularly for m ≥ 10 forming a σ complex. The adsorption energies of H₂ on Rh₆H_m⁺ estimated from the most stable isomer aligned well with the experimental findings.