HYSTERESIS IN HYDROGEN PERMEATION THROUGH PALLADIUM MEMBRANES

A literature survey of hydrogen permeation through Pd-based membranes has shown some deviations from classical rules such as Sieverts' law. Controversial observations often occur with thin Pd-based membranes at elevated temperatures. An ultrathin palladium membrane (ca. 2 mu m) h as been prepared on the inner surface of a mesoporous ceramic tube by electroless plating. The membrane was permselective to hydrogen, allow ing us to examine the hydrogen permeation behaviour. The measured hydr ogen permeability was found to be proportional to the difference in pe rmeation pressures, by a power larger than 0.5, at elevated temperatur es. Evidence of hysteresis was discovered in hydrogen permeation throu gh the ultrathin, dense Pd membrane under varying pressure conditions. The permeation hysteresis at elevated temperatures seems not to be as sociated with the solubility hysteresis. An atomic-molecular permeatio n mechanism has been proposed to explain the permeation hysteresis at elevated temperatures. The atomic hydrogen dissolved in cavities or mi crocracks within the membrane may combine to form molecular-hydrogen s pecies. The dilatation of the palladium lattice resulting from the mot ion of molecular-hydrogen species is believed to be the cause of perme ation hysteresis. The higher power dependence of the permeability on p ressure is also understandable from the atomic-molecular permeation me chanism.