Size-Dependent Hydrogen Adsorption and Desorption by Gas-Phase Iron Cluster Cations Revealed by Thermal Desorption Spectrometry

The adsorption of hydrogen on cationic iron clusters, Fe_<i>n</i>⁺, at 310 K and its desorption from Fe_<i>n</i>H_<i>m</i>⁺ clusters upon heating were investigated by using thermal desorption spectrometry (TDS) to elucidate the size-dependent hydrogen storage properties. Small clusters (<i>n</i> = 4 and 5) exhibited a higher hydrogen adsorption per Fe atom at 310 K and sequential H₂ release at high temperatures. The activation energies for desorption were estimated from the TDS curves, which indicated that Fe₄⁺ and Fe₅⁺ exhibited the strongest H₂ binding. The desorption energies of H₂ from Fe₄H_<i>m</i>⁺ obtained by density functional theory calculations were consistent with the activation energies. In contrast, no clear sequential release was observed for larger clusters (<i>n</i> = 7-9) because of the weaker binding of H₂ molecules to the H-deficient clusters. The reaction path of the adsorption of H₂ on Fe₇⁺ from molecular to dissociative adsorption exhibited an energy barrier of about 0.2 eV for the cleavage of the H-H bond. Hence, hydrogen may be trapped as a H₂ molecule in Fe₇H₂⁺.