Hydrogen Release from Ammonia Alane-Based Materials: Formation of Cyclotrialazane and Alazine

Abstract

In previous papers (Nguyen et al. J. Phys. Chem. C 2008, 112, 5662–5667 and J. Phys. Chem. C 2009, 113, 18914–18926), formation of H2 molecules from ammonia alane monomer (AAl) and dimers (AAl)2 was shown to be facilitated by the addition of one or more alane or ammonia molecules that can play the role of efficient bifunctional catalyst. Ammonia alane emerges as a good starting compound for building up materials for chemical hydrogen storage (CHS). Further exploration of the products based on the H2 release from ammonia alane were carried out using coupled-cluster theory computations together with the aug-cc-pVTZ basis set (based on MP2/aug-cc-pVDZ optimized geometries). Our ab initio MO calculations for the first time led to the identification of cyclotrialazane [(H2AlNH2)3], alazine [(HAlNH)3], and its oligomer [H2Al(HNAlH)2NH2] that are produced along the multistep dehydrogenation processes from the reactions of ammonia alane and AlH3NH2AlH2NH3. The formation of alazine (homologue of borazine) as the final product in our H2 elimination reactions is an important feature because of its long-debated existence. The present reaction path analyses show that the formation of this compound is an important phenomenon for explaining the entire dehydrogenation process.