Article pubs.acs.org/JPCA
Theoretical Exploration of Hydrogen Loss from Al3H9 Christopher P. Nold and John D. Head* Department of Chemistry, University of Hawaii, Honolulu, Hawaii 96822, United States S Supporting Information *
ABSTRACT: The Al3H9 and Al3H7 potential energy surfaces were explored using quantum chemistry calculations to investigate the H2 loss mechanism from Al3H9, which provide new insights into hydrogen production from bulk alane, [AlH3]x, a possible energy storage material. We present results of B3LYP/6-311++G(d,p) calculations for the various Al3H9 and Al3H7 optimized local minima and transition state structures along with some reaction pathways for their interconversion. We find the energy for Al3H9 decomposition into Al2H6 and AlH3 is slightly lower than that for H2 loss and Al3H7 formation, but the calculations show that H2 loss from Al3H9 is a lower energy process than for losing hydrogen from either Al2H6 or AlH3. We found four transition state structures and reaction pathways for Al3H9 → Al3H7 + H2, where the lowest energy activation barrier is around 25−73 kJ/mol greater than the experimental value for H2 loss from bulk alane. Intrinsic reaction coordinate calculations show that the H2 loss pathway involves considerable rearrangement of the H atom positions around a single Al center. Three of the pathways start with the formation of an AlH3 moiety, which then enables a terminal H on the AlH3 to get within 1.1 to 1.2 Å of a nearby bridging H atom. The bridging and terminal H atoms eventually combine to form H2 and leave Al3H9. One implication of these H2 loss reaction pathways is that, since the H atoms in bulk alanes are all at bridging positions, if a similar H2 loss mechanism were to apply to bulk alane, then H2 loss would most likely occur on the bulk alane surface or at a defect site where there should be more terminal H atoms available for reaction with nearby bridging H atoms.
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involving approximately nine AlH3 molecules.23,24 Nonetheless, exploring H2 loss from Al3H9 should provide useful insight into how two H atoms can get close enough to interact with each other and combine to form H2 during the dehydrogenation mechanism for bulk alane. Although polymeric alane was synthesized in 19471 and gaseous AlH was characterized in 1954,7 it has only been in the last 20 years that experimental evidence has been obtained for molecular aluminum hydrides of the form AlxHy (x = 2−4; y ≤ 3x).7−15 Many of the species were identified by the laser ablation of aluminum to produce Al atoms in the presence of H2 that were then codeposited at low temperatures,
