Hydrogen Adsorption and Dissociation on AlnRh₂⁺ (n = 1 to 9) Clusters: Steric and Coordination Effects

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The interaction of molecular hydrogen with doubly rhodium doped aluminum clusters, Al n Rh 2 + ( 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 n Rh 2 + clusters toward H 2 is found to be sensitive to cluster size, with sizes n = 1 to 4 and 7 being the most reactive. Al 3 Rh 2 + and Al 4 Rh 2 + are the only species that thermodynamically prefer molecular over dissociative H 2 adsorption. Calculated molecular adsorption energies of a single H 2 molecule correlate well with the experimental abundances of the hydrogenated species, and the potential energy profiles reveal that H 2 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 H 2 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 n Rh 2 + clusters.