Isotopic Exchange in Porous and Dense Magnesium Borohydride

摘要

Magnesium borohydride (Mg(BH4)(2)) is one of the most promising complex hydrides presently studied for energy-related applications. Many of its properties depend on the stability of the BH4- anion. The BH4- stability was investigated with respect to HD exchange. Insitu Raman measurements on high-surface-area porous Mg(BH4)(2) in 0.3MPa D-2 have shown that the isotopic exchange at appreciable rates occurs already at 373K. This is the lowest exchange temperature observed in stable borohydrides. Gas-solid isotopic exchange follows the BH4-+(DBH3D-)-B-.+H-. mechanism at least at the initial reaction steps. Exsitu deuteration of porous Mg(BH4)(2) and its dense-phase polymorph indicates that the intrinsic porosity of the hydride is the key behind the high isotopic exchange rates. It implies that the solid-state H(D) diffusion is considerably slower than the gas-solid HD exchange reaction at the surface and it is a rate-limiting steps for hydrogen desorption and absorption in Mg(BH4)(2.