Potassium, rubidium and cesium hydrides as dehydrogenation catalysts for the lithium amide/magnesium hydride system

Abstract

In this study, the effectiveness of several alkali metal hydrides (KH, RbH and CsH) for improving the hydrogen desorption properties of a 2LiNH2/MgH2 mixture was studied. Results showed that the relative effectiveness of these additives in decreasing the hydrogen desorption temperature, lowering the activation energy and increasing desorption rates from the mixtures is in the order: RbH > KH > CsH > Un-catalyzed. Modeling studies showed that diffusion through a Li2Mg(NH)2 product layer is the rate-controlling process. It is believed that the alkali elements: K, Rb and Cs partially replace the Li in the product layer. This may have an inductive effect in which the N–H bond is weakened thus leading to lower desorption enthalpies. The lattice expansion caused by substitution of the larger alkali elements for Li may also allow for faster diffusion and increased desorption rates.