On the nucleation step in the Mg–MgH2 phase transformation

Scanning electron microscopy was used to support kinetic analysis in order to ascertain the physical mechanisms involved in the Mg–MgH2 phase transformation. Kinetic asymmetry in hydrogen absorption and desorption was explained by the different requirements for fast nucleation in the two reactions. In particular, MgH2 nucleation in Mg is assisted by crystal defects, while free surface is the only nucleation site in the reverse reaction. This explains, for the first time, the different role of the metal catalyst in the two reactions.