Hysteresis in hydrogen permeation through palladium membranes

Abstract

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 µm) has been prepared on the inner surface of a mesoporous ceramic tube by electroless plating. The membrane was permselective to hydrogen, allowing us to examine the hydrogen permeation behaviour. The measured hydrogen permeability was found to be proportional to the difference in permeation pressures, by a power larger than 0.5, at elevated temperatures. Evidence of hysteresis was discovered in hydrogen permeation through the ultrathin, dense Pd membrane under varying pressure conditions. The permeation hysteresis at elevated temperatures seems not to be associated with the solubility hysteresis. An atomic–molecular permeation mechanism has been proposed to explain the permeation hysteresis at elevated temperatures. The atomic hydrogen dissolved in cavities or microcracks within the membrane may combine to form molecular-hydrogen species. The dilatation of the palladium lattice resulting from the motion of molecular-hydrogen species is believed to be the cause of permeation hysteresis. The higher power dependence of the permeability on pressure is also understandable from the atomic–molecular permeation mechanism.