Diffusion of H through Pd membranes: Effects of non-ideality

H diffusion constants, DH, have been obtained from steady-state fluxes through Pd membranes with the downstream side maintained at image ≈ 0. Good linearity of plots of H flux versus (1/d), where d is the thickness, attests to H permeation being bulk diffusion controlled in this temperature (423–523 K) and image range (≤0.2 MPa). DH values have been determined at constant pup and also at constant H content. H fluxes through Pd membranes with three different surface treatments have been investigated (polished (unoxidized), oxidized and palladized) in order to determine the effects of these pre-treatments. The palladized and oxidized membranes give similar DH values but the polished membranes give values about 12% lower. For diffusion in a concentration gradient J = −image(cH/RT)(dμH/dx) is the proper equation for the flux, where cH is the H concentration, rather than Fickʹs first law, J = −DH(dcH/dx), where DH and image are the concentration-dependent and -independent diffusion constants. image can be obtained from DH using the thermodynamic factor, image. In the commonly employed situation, where there is a large difference in concentrations between the upstream and downstream sides of a membrane, the thermodynamic factor varies with distance through the membrane and this should be allowed for in obtaining image. Procedures are given and utilized to determine image from DH(cH) when there is a large concentration gradient through the membrane. Activation energies for diffusion, ED(cH), have been determined. ED is found to increase with cH, which is attributed to the thermodynamic factor.