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要旨

The interaction of molecular hydrogen with doubly rhodium doped aluminum clusters, Al_nRh₂⁺ (n = 1 to 9), is investigated by a combination of time-of-flight mass spectrometry, infrared multiple photon dissociation spectroscopy, and density functional theory calculations. The reactivity of the Al_nRh₂⁺ clusters toward H₂ is found to be sensitive to cluster size, with sizes n = 1 to 4 and 7 being the most reactive. Al₃Rh₂⁺ and Al₄Rh₂⁺ are the only species that thermodynamically prefer molecular over dissociative H₂ adsorption. Calculated molecular adsorption energies of a single H₂ molecule correlate well with the experimental abundances of the hydrogenated species, and the potential energy profiles reveal that H₂ dissociation only has submerged barriers for n = 1, 2, and 7. In contrast, the molecularly hydrogenated complexes seem to be kinetically trapped for n = 5, 6, 8, and 9 due to significant energy barriers. This indicates that the initial molecular H₂ adsorption on the Rh atoms and thereafter dissociation are the determining steps for the hydrogenation reaction. An analysis of the cluster geometries reveals that the coordination environment and the steric factor of the Rh atoms are the main descriptors for the size-dependent reactivity of the Al_nRh₂⁺ clusters.