Mobile Species in NaAlH₄

Abstract

Sodium\nalanate (NaAlH₄) is the archetypical complex\n(ionic-covalent) hydride compound for hydrogen storage applications.\nHowever, the details of the reactions of this compound upon hydrogen\ncycling remain unclear. Recently, a new mobile aluminum-containing\nspecies termed S105 was discovered in the hydrogen chemistry of sodium\nalanate using in situ NMR with S105 a likely facilitator of Al-atom\ntransport in NaAlH₄. Here, we find that hydroxide impurities\nplay a crucial role in the formation of the mobile species. Specifically,\nin bare NaAlH₄, S105 is present after melting (under sufficient\nhydrogen pressure to block the dehydriding reaction) only in samples\neither exposed to H₂O vapor or mixed with metal hydroxides.\nWe find that the ²⁷Al line position of S105 is close to\nthat of NaAlH₄ (after correcting for second-order quadrupole\neffects), indicating that S105 involves very mobile AlH₄^– tetrahedra (rotationally and translationally).\nWe propose that hydroxide impurities promote fast diffusion of nearby\nAlH₄^– units, similar to enhanced motions\nseen in NaH; the hydroxides also react with NaAlH₄ to form\nNaH and subsequently produce Na₃AlH₆, which\nis always found to accompany S105. Our measurements reveal that the\nonly chemical components of S105-containing alanate apart from hydroxides\nare NaAlH₄ and Na₃AlH₆. Presence\nof the S105 species in NaAlH₄ samples also leads to faster\ndehydriding in hot, undoped NaAlH₄ solid, pointing to an\nenhancement of the hydrogen reaction kinetics by S105.