Size Dependent H₂ Adsorption on Al_<i>n</i>Rh⁺ (<i>n</i> = 1–12) Clusters

Abstract

The interaction of hydrogen with singly rhodium doped aluminum clusters AlnRh+ (n = 1–12) is investigated experimentally by a combination of time-of-flight mass spectrometry and infrared multiple photon dissociation (IRMPD) spectroscopy. Density functional theory (DFT) is employed to optimize the geometric and electronic structures of bare and hydrogenated AlnRh+ clusters and the obtained infrared spectra of hydrogenated clusters are compared with the corresponding IRMPD spectra. The reactivity of the AlnRh+ clusters toward H2 is found to be strongly size-dependent, with n = 1–3, and 7 being the most reactive. Furthermore, it is favorable for H2 to adsorb molecularly on Al2Rh+ and Al3Rh+, while it prefers dissociative adsorption on other sizes. The initial molecular adsorption of H2 is identified as the determining step for hydrogen interaction with the AlnRh+ clusters, because the calculated molecular adsorption energies of H2 correlate well with the experimental abundances of the hydrogenated clusters. Natural charge populations and properties of the AlnRh+ clusters are analyzed to interpret the observed size-dependent reactivity.