Reduction of Mg from a MgO/MgAl2O4 support by atomic hydrogen permeation through thin-film Pd membranes

In this work, we discuss an observed reduction of magnesium from a MgO/MgAl2O4 ceramic substrate by permeating hydrogen in thin-film Pd membranes. Membranes were fabricated via electroless plating of Pd on a porous MgO/MgAl2O4 substrate and tested under pure hydrogen permeation conditions between 500 °C and 700 °C. It was observed that a rapid and severe hydrogen flux decline occurred at 700 °C, whereat up to 96% of the hydrogen flux was lost within 40 h of operation. Using energy dispersive x-ray spectroscopy and x-ray photoelectron spectroscopy, it was concluded that the flux loss occurred due to alloying and a subsequent surface segregation of Mg in the Pd film. In contrast, a control test in which the membrane was soaked in hydrogen at 700 °C without permeation resulted in only very slight or potentially no MgO reduction or alloying with the Pd. The significantly enhanced reduction of MgO/MgAl2O4 during membrane operation is attributed to the chemical potential provided by atomic hydrogen permeating through the membrane.