Interaction between Li2Mg(NH)2 and CO: Effect on the hydrogen storage behavior of the Li4(NH2)3BH4 doped Mg(NH2)2-2LiH composite

Abstract

Metal hydrides have been studied as a promising solution for the hydrogen recovery from gas mixtures of industrial processes. However, scarce information is available about the behavior of amides as hydrogen purification material. In this work, the tolerance against CO, hydrogen sorption kinetics and the thermodynamics of the Li4(NH2)3BH4 doped Mg(NH2)2-2LiH composite after repetitive dehydrogenation and rehydrogenation cycles with 0.1 mol% of CO-H2 mixture were investigated. A progressive degradation of the hydrogen storage capacity of the material and an improvement in the dehydrogenation rate (50%) was observed for the composite after 20 cycles of the CO containing gas mixture. The formation of Li2CN2 and MgO, which are the main responsible for the deterioration of the hydrogen storage properties, was confirmed by Fourier transform infrared spectroscopy (FTIR) and X-ray powder diffraction (XRPD). For the first time, the reaction between pure CO and the dehydrogenated product Li2Mg(NH)2 was demonstrated. This reaction is fast and produces mainly Li2CN2 and MgO as solid products. When a high CO pressure was diluted with H2 (mol ratio 1:8), the reactivity of Li2Mg(NH)2 with CO was notably reduced and mainly MgO formation was detected. No clear reaction of CO with Mg(NH2)2 was detected by FTIR, XRPD and volumetric measurements. Evaluation of the reactivity of CO with LiNH2-LiH, Mg(NH2)2-2LiH and LiBH4 systems provides the following decreasing ranking of reactivity: [NH]2− > [NH2]− > [BH4]−.