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

Abstract

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 ≈ 0.10 MPa. ΔHH and ΔSH 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 H2 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-exponen...