An insight into separating H2 from natural gas/H2 mixtures using Mg-based systems

Abstract

The use of light, abundant, and relatively cheap Mg-based systems arouses great interest in hydrogen-economy-related applications such as hydrogen and heat storage. So far, MgH₂, capable of storing large amounts of H₂ (7.6 wt%), has been scarcely evaluated for its H₂ separation potential, which may be crucial for H₂ recovery from various H₂-containing gas mixtures. Herein, we reveal and discuss the ability of Mg-based systems to separate H₂ from CH₄-rich gas mixtures. Mg-Ni and Mg-Fe systems can separate ∼5.5 wt% of H₂ during the hydrogenation process and release pure H₂ (at least 99.9%) within the dehydrogenation process. Pure H₂ can, therefore, be obtained in a one-step separation system. In this study, we discuss the selection of the hydrogenation/dehydrogenation processes catalyst (Ni, Fe) as well as the optimal separation process temperature. The tested systems show satisfactory performance stability during cyclic H₂ separation from CH₄/H₂ and natural gas/H₂ gas mixtures. We also present the first investigation of the Mg-based systems (with Ni or Fe catalyst) after the cycled separation processes. The results of complementary techniques revealed H₂ separation-induced chemical and phase segregation in the studied materials. Moreover, we report the observation of networked MgH₂ microstructure formation. This research points out the potential of metal-hydrides in the H₂ separation sector as well as the challenges facing their application – especially those related to the presence of CO₂ impurity in the gas mixture. The unique and detailed description of processes taking place in a reactor during the separation process will significantly impact the design of future metal-hydrides-based scaled-up systems for H₂ separation.