Size Dependent H₂ Adsorption on Al_nRh⁺ (n = 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 towards 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.

Hydrogen Chemisorption on Doubly Vanadium Doped Aluminum Clusters

“…[7]. Here we will discuss only the doubly doped clusters and the effects emerging from the presence of the two, as will be shown below, directly Figure 1b, which shows the fraction F(H 2p ) of Al n V 2 + that adsorb p (with p = 1 or 2) hydrogen molecules, and which is defined as has been attributed to cage formation [9,20,21]. For the singly doped clusters, the onset of cage formation, i.e.…”

“…Understanding the size-dependence of the reactivity of transition metal (doped) clusters in the gas phase is generally not an easy task due to the possibility of low-lying geometric or electronic states (possibly even of a different multiplicity), whose potential energy surfaces (PES) during the reaction with hydrogen might cross that of the ground state. Nevertheless, complementing the experimental data with ab initio calculations of the thermodynamics of the reaction often goes a long way in explaining the observations [8,9]. Furthermore, one can identify features in the electronic structure, which reveal why some clusters react strongly with a given reagent, and why others do not [7].…”

“…DFT calculations are performed by employing the Gaussian 09 software package [15] with the generalized gradient approximated (GGA) exchange correlation functional as devised by Perdew, Burke, and Ernzerhof (PBE) [16]. This functional was previously adopted in our work for H 2 adsorption on singly vanadium doped and singly and doubly rhodium doped aluminum clusters [7][8][9]. Global optimizations were carried out for Al…”

“…n = 2 and n ≥ 10, were reactive towards hydrogen. Previous work on rhodium doped clusters showed that the doubly doped species are more reactive than the singly doped ones [8,9]. Also for Ti-doped sodium alanate, computational studies suggest that the active site consists of at least two neighboring Ti atoms [11].…”

“…Recent combined experimental and computational studies of singly vanadium and singly and doubly rhodium doped aluminum clusters [7][8][9] showed that hydrogen can chemisorb dissociatively, as well as molecularly via the Kubas interaction [10]. Such molecular side-on bound H 2 -complexes on transition metal atoms have been first described by Kubas and termed 'non-classical' in contrast to the 'classical' complexes containing atomic H-ligands.…”