Competitive Molecular and Dissociative Hydrogen Chemisorption on Size Selected Doubly Rhodium Doped Aluminum Clusters

Abstract

The interaction of hydrogen with Al_nRh₂⁺ (n = 10–13) clusters is studied by mass spectrometry and infrared multiple photon dissociation (IRMPD) spectroscopy. Comparing the IRMPD spectra with predictions obtained using density functional theory calculations allows for the identification of the hydrogen binding geometry. For n = 10 and 11, a single H₂ molecule binds dissociatively, whereas for n = 12 and 13, it adsorbs molecularly. Upon adsorption of a second H₂ to Al₁₂Rh₂⁺, both hydrogen molecules dissociate. Theoretical calculations suggest that the molecular adsorption for n = 12 and 13 is not due to kinetic impediment of the hydrogenation reaction by an activation barrier, but due to a higher binding energy of the molecularly adsorbed hydrogen–cluster complex. Inspection of the highest occupied molecular orbitals shows that the hydrogen molecule initially forms a strongly bound Kubas complex with the Al_11-13Rh₂⁺ clusters, whereas it only binds weakly with Al₁₀Rh₂⁺

Hydrogen Chemisorption on Doubly Vanadium Doped Aluminum Clusters

“…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.…”

Few-atom cluster model systems for a hydrogen economy

“…In the figure, the activated molecular chemisorption is illustrated by taking place on the metal atom, although this is not strictly necessary for the process. studied transition metal doped aluminum clusters using IRMPD spectroscopy [113][114][115]. We observed that, with rhodium as the dopant, hydrogen binds molecularly onto the dopant for the smallest clusters, but dissociates for larger ones [114,115].…”

“…studied transition metal doped aluminum clusters using IRMPD spectroscopy [113][114][115]. We observed that, with rhodium as the dopant, hydrogen binds molecularly onto the dopant for the smallest clusters, but dissociates for larger ones [114,115]. As shown in Figure 6, the IR spectra of hydrogenated Al n Rh 2 + (n = 10 -13) clusters consist of three ranges of vibrational modes, roughly around 800 cm −1 , 1600 cm −1 , and 1900 cm −1 .…”