Reaction Mechanisms in the Li_3AIH_6/LiBH_4 and AI/LiBH_4 Systems for Reversible Hydrogen Storage. Part 2: Solid-State NMR Studies

摘要

In Part 1, the promising hydrogen storage properties of the combined systems of Li_3AIH_6/LiBH_4 and AI/LiBH_4, exhibiting the favorable formation of AIB_2 during dehydrogenation, were presented based on TGA and XRD analyses. The present Part 2 describes the characterization of the intermediate and final products of the dehydrogenation and rehydrogenation of the above systems by multinuclear solid-state NMR characterization. This work has also verified that the presence of AI resulted in the re-formation of LiBH_4 occurring at a much lower temperature and H_2 pressure, under which conditions the dehydrogenation product from LiBH_4 alone does not show any degree of rehydrogenation. NMR studies mainly identified various reaction intermediates for LiBH_4 dehydrogenation/rehydrogenation reactions. Unlike the XRD studies, the AIB_2 formation, in particular, could not be unambiguously confirmed by NMR. ^(27)AI NMR showed that aluminum was mainly involved in various Li—AI alloy formations. The catalytic role of AI in the LiBH_4 hydrogen storage reactivity could be achieved by a reversible cycle of the AI + LiH ↔ LiAI + 1/2H_2 reaction.