Two-controlling mechanisms model for hydrogen desorption in the Li4(NH2)3BH4 doped Mg(NH2)2–2LiH system

Abstract

The limiting step of the dehydrogenation process and the desorption kinetic model of the composite 0.7Mg(NH2)2–1.4LiH–0.2Li4(NH2)3BH4 under different hydrogen back pressures at low temperature (200 °C) were studied in this work. It was determined that a single mechanism model was not able to explain the behavior of the reaction at low and high reacted fractions simultaneously. A combination of two controlling mechanisms, which reproduce accurately the behavior of the system, was proposed. The rate equation deduced involves the contribution of a second grade Avrami model and a 3D diffusion model as a function of pressure and conversion. At low conversions, the limiting step of the reaction is the formation of the products. Once a thin layer of product is formed on the surface, the diffusion through it becomes the new limiting step. With pressure increase, the mechanism change occurs later during desorption due to the higher difficulty in creating nucleation points on a surface exposed to a higher concentration of hydrogen.