Unusual kinetics due to interface hydride formation in the hydriding of Pd/Mg sandwich layers

The hydriding of thick Mg films with a 50–100 A Pd film on top, as catalyst for H2 dissociation, was investigated by quartz‐crystal microbalance uptake measurements in a combined UHV and high‐pressure equipment. Unusual kinetics are observed: (i) the hydrogen uptake saturates far from MgH2 at room temperature and (ii) the saturation uptake decreases with increasing H2 pressure. The results are explained in terms of interface hydride formation at the Pd/Mg interface, which blocks hydrogen diffusion. A theoretical basis for this model is given by combining the dependence of hydrogen binding energy on local electron density, with the assumption that the lattice mismatch at the interface creates defect sites (e.g., vacancies) with low local electron density.