Reaction pathways for hydrogen desorption from magnesium hydride/hydroxide composites: bulk and interface effects

This manuscript investigates the thermal desorption behaviour of MgH2/Mg(OH)(2) composites by means of thermal desorption spectroscopy. Besides the H2O and H-2 desorption events due to Mg(OH)(2) dehydration and MgH2 decomposition reactions, respectively, two additional H-2 desorption peaks arise at lower temperatures. These peaks are related to solid-state reactions between magnesium hydride and magnesium hydroxide through different channels. The low temperature H-2 peak (similar to 150 degrees C) is related to reaction between a H atom diffusing from MgH2 and a surface OH group, whereas the intermediate temperature H-2 peak (similar to 350 degrees C) is due to an interface reaction between the hydride and the hydroxide. The present work supports the theory that the onset of the H-2 desorption coming from MgH2 decomposition is controlled by an incubation process, consisting in the formation of catalytically active vacancies at the MgO/Mg(OH)(2) surface by dehydration. Possible ways to improve the H-2 desorption kinetics from MgH2 are discussed in the light of the results obtained.