The influence of HS-AlF3 on the decomposition reaction of MgH2

The influence of nanoscopic aluminium fluoride prepared via a sol-gel-synthesis route developed by our group on the thermal decomposition of MgH2 used as hydrogen storage material has been comprehensively investigated. Combined XRD and MAS NMR experiments have been performed to follow structural changes in ball milled MgH2/HS-AlF3 mixtures at thermal decomposition. Although the behaviour of Lewis acidic metal fluorides is discussed being catalytic, our results with HS-AlF3 clearly give a different picture. We found strong evidence for a decrease of the decomposition temperature in combination with a significant acceleration of the decomposition rate; however, the action of at least AlF3 is not catalytic in nature. As shown, a fluoride against hydride exchange is taking place, thus transforming AlF3 irreversibly into AlH3, which is followed by decomposition into metallic Al (MgAl alloy).