A computational investigation of the hydrogen loss mechanism from alane : a hydrogen storage material

Abstract

Aluminum hydride or alane, AlH3, has potential use as a hydrogen storage material. The alane undergoes a single step dehydrogenation reaction AlH3→Al+3/2H2. While looking like a simple reaction, the dehydrogenation mechanism is not clearly understood. Quantum chemistry calculations were used to explore H2 loss from various potential energy surfaces (PES): AlH3, Al2H6, Al3H9, Al4H12 and Al4H10 to provide insight into the hydrogen production mechanism. We present the results of B3LYP/6-311+ +G(d,p) calculations for the various optimized local minima and transition state (TS) structures along with H2 loss and interconversion reaction pathways. Two pathways for the reaction Al2H6→Al2H4+H2 with activation barriers lower in energy than for the removal of H2 from AlH3 were found. The energy for the Al3H9 decomposition into Al2H6 and AlH3 was found to be slightly lower than for H2 loss and Al3H7 formation, but calculations show that H2 loss from Al3H9 was a lower energy process than for losing hydrogen from either Al2H6 or AlH3. There were four TS structures found for the reaction pathway Al3H9→Al3H7+H2, where the lowest activation energy was around 73 kJ/mol greater than the experimental value for H2 loss from bulk alane. There were fourteen transition state structures found for Al4H12 and eight TS structures for Al4H10 PES located. The lowest energy activation barriers found for these reactions were 34 and 50 kJ/mol greater than experimental results for the Al4H10 and Al4H12 PES respectively. Intrinsic reaction coordinate calculations show that the H2 loss pathway involves considerable rearrangement of the H atom positions around a single Al center. Several of the pathways included the formation of an AlH3 moiety which enables a terminal H on the AlH3 to interact with a nearby bridging H atom. The bridging and terminal H atoms eventually combine to form H2. Along with the H2 loss reactions that involve the AlH3 moiety being formed, we found a few H2 loss pathways that do not form an AlH3 moiety during the H2 loss reaction.