Dehydrogenation mechanisms and thermodynamics of MNH<inf>2</inf>BH<inf>3</inf> (M = Li, Na) metal amidoboranes as predicted from first principles

Abstract

Electronic structure calculations have been used to determine and compare the thermodynamics of H2release from ammonia borane (NH3BH3), lithium amidoborane (LiNH2BH3), and sodium amidoborane (NaNH2BH3). Using two types of exchange correlation functional we show that in the gas-phase the metal amidoboranes have much higher energies of complexation than ammonia borane, meaning that for the former compounds the B-N bond does not break upon dehydrogenation. Thermodynamically however, both the binding energy for H2release and the activation energy for dehydrogenation are much lower for NH3BH3than for the metal amidoboranes, in contrast to experimental results. We reconcile this by also investigating the effects of dimer complexation (2×NH3BH3, 2×LiNH2BH3) on the dehydrogenation properties. As previously described in the literature the minimum energy pathway for H2release from the 2×NH3BH3complex involves the formation of a diammoniate of diborane complex ([BH4]-[NH3BH2NH3]+). A new mechanism is found for dehydrogenation from the 2×LiNH2BH3dimer that involves the formation of an analogous dibroane complex ([BH4]-[LiNH2BH2LiNH2]+), intriguingly it is lower in energy than the original dimer (by 0.13 eV at ambient temperatures). Additionally, this pathway allows almost thermoneutral release of H2from the lithium amidoboranes at room temperature, and has an activation barrier that is lower in energy than for ammonia borane, in contrast to other theoretical research. The transition state for single and dimer lithium amidoborane demonstrates that the light metal atom plays a significant role in acting as a carrier for hydrogen transport during the dehydrogenation process via the formation of a Li-H complex. We posit that it is this mechanism which is responsible, in condensed molecular systems, for the improved dehydrogenation thermodynamics of metal amidoboranes. © 2011 the Owner Societies.