Isotopic Exchange in Porous and Dense Magnesium Borohydride

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 H→D exchange. In'situ Raman measurements on high-surface-area porous Mg(BH₄)₂ in 0.3'MPa D₂ have shown that the isotopic exchange at appreciable rates occurs already at 373'K. This is the lowest exchange temperature observed in stable borohydrides. Gas-solid isotopic exchange follows the BH₄^-+D^.→BH₃D^-+H^. mechanism at least at the initial reaction steps. Ex'situ 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 H→D exchange reaction at the surface and it is a rate-limiting steps for hydrogen desorption and absorption in Mg(BH₄)_2. The H-to-D exchange in high-surface-area complex ionic hydride Mg(BH₄)₂ exhibits exceptionally high rates at low temperature and pressure. The gas-solid H→D exchange at the surface is thus significantly faster than the solid-state H(D) diffusion in the bulk. The latter can be regarded as the rate-limiting step for hydrogen desorption and absorption in Mg(BH₄)₂.