Thermodynamics of Hydrogen Solution and Diffusion in Oxidized and Unoxidized Pd-W Alloys

Equilibrium hydrogen isotherms have been measured from 423 to 523 K for two fcc Pd-W alloys, Pd0.98W0.02 and Pd0.96W0.04, over H content ranges available up to pH2 approximate to 0.10 MPa. Delta H-H and Delta S-H values have been determined from the H2 isotherms as a function of the H content of the alloys. After measurement of the isotherms, the Pd0.98W0.02 and Pd0.96W0.04 alloys were internally oxidized. The H-2 solubilities in the internally oxidized alloys were measured and evidence is given for the formation of H-bronzes, HxWO3, from the nanosized WO3 precipitates within the Pd matrix. Pd-W alloy membranes prepared for H diffusion measurements were partially internally oxidized in order to improve their resistance toward gaseous contaminants. An equation has been derived to obtain the H diffusion constant for the inner, unoxidized alloy layer within the three layer membrane resulting from internal oxidation. This equation has been applied to data for the two internally oxidized Pd-W alloys. Activation energies and pre-exponential factors have been derived for H diffusion constants through the inner, unoxidized Pd-W alloy layers.