Diffusion of H through Pd–Y alloy membranes

• Diffusion constants at infinite dilution decrease with atom fraction Y, XYXY.
• Fick's diffusion constants have been corrected by the thermodynamic factors leading to nearly constant, concentration-independent diffusion constants.
• The activation energies for diffusion increase with H content of the alloys.
• The pre-exponential factors increase with H content and XYXY.

Fick's diffusion constants, DHDH, for H in Pd–Y alloys decrease with the increase of atom fraction Y, XYXY, for the alloys investigated here (XY≤0.095)(XY≤0.095), however, the H permeabilities increase because the H2H2 solubility increase is greater than the decrease of the diffusion constant. DHDH values have been measured for some of the Pd  –Y alloys over a large range of H contents and, for several alloys, minima appear in plots of RTlnDH against ruprup arising from non-ideality, i.e.,   thermodynamic factor ≠1≠1, where rup=H-to-metalrup=H-to-metal mol ratio at the upstream surface of the membrane and rdown=0rdown=0. Concentration independent diffusion constants have been derived from the concentration-dependent ones and the thermodynamic factor.The effect of CO on the permeation of H through Pd–Y membranes has been determined and compared to the effect of CO on the permeability of a Pd0.77Ag0.23 alloy. The poisoning effect of CO is either greater or similar for the un-oxidized Pd0.77Ag0.23 alloy membranes and the internally oxidized Pd–Y alloy membranes, although any differences are small.