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

H diffusion constants, D-H, have been obtained from steady-state fluxes through Pd membranes with the downstream side maintained at p(H2) approximate to 0. Good linearity of plots of H flux versus (I/d), where d is the thickness, attests to H permeation being bulk diffusion controlled in this temperature (423-523 K) and p(H2) range (<= 0.2 MPa). D-H values have been determined at constant p(up) 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 D-H values but the polished membranes give values about 12% lower. For diffusion in a concentration gradient J = -D-H*(c(H)/RT)(d mu(H)/dx) is the proper equation for the flux, where CH is the H concentration, rather than Fick's first law, J = -D-H(dc(H)/dx), where D-H and D-H* are the concentration-dependent and -independent diffusion constants. D-H* can be obtained from D-H using the thermodynamic factor, D-H(c(H)) = D-H*(partial derivative in p(H2)(1/2)/partial derivative ln c(H))(T). 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 D-H*. Procedures are given and utilized to determine D-H* from D-H(c(H)) when there is a large concentration gradient through the membrane. Activation energies for diffusion, E-D(c(H)), have been determined. E-D is found to increase with c(H), which is attributed to the thermodynamic factor. Published by Elsevier B.V.