Size Dependent H2 Adsorption on AlnRh+ (n=1-12) Clusters

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 H-2 is found to be strongly sizedependent, with n = 1-3, and 7 being the most reactive. Furthermore, it is favorable for H-2 to adsorb molecularly on Al2Rh+ and Al3Rh+, while it prefers dissociative adsorption on other sizes. The initial molecular adsorption of H-2 is identified as the determining step for hydrogen interaction with the Al(n)Rh(+ )clusters, because the calculated molecular adsorption energies of H-2 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.