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

Abstract

H diffusion constants, D H , have been obtained from steady-state fluxes through Pd membranes with the downstream side maintained at p H 2 ≈ 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 p H 2 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.

Hydrogen solubilities and permeabilities in un-oxidized and partially internally oxidized fcc Pd–Fe and alloys (393–523K)

“…The E * D values lie between those found by [5,8] and by Sakamoto et al [2]. The present higher temperature range will lead to smaller values of E * D [10]. In the present work E * D decreases slightly with X Fe and then increases.…”

“…They were then annealed in vacuo for 3 days at 1133 K, rolled into foil and then re-annealed for 2 days at 1133 K. These alloys form solid solutions over the range to be investigated, XFe = 0 to 0.15 [2,23]. The diffusion parameters were determined from the permeation through the alloy membranes with pup at the desired value and p down = 0 as described for Pd and Pd-Ag alloy membranes [10,24]. The equation employed for these boundary values is…”

“…It is known that D H , Fick's diffusion constant, depends on the H concentration [9] and it has been shown recently that the permeation rates where p H 2 upstream, p up , and downstream, p down , are finite and 0, respectively, can be corrected for the non-ideality to obtain D * H , Einstein's concentration-independent diffusion constant [10]. D * H values will be obtained here using this approach.…”

“…Since the H concentrations employed are not large, it will be assumed here that f cor = 1.0. Under the present boundary conditions, p upstream > 0 and p downstream = 0, the degree of non-ideality due to the thermodynamic factor varies with H penetration through the membrane which can be allowed for using [10] …”

Effect of the presence of light hydrocarbon mixtures on hydrogen permeance through Pd–Ag alloyed membranes

“…Indeed, the square-root law may only be applied to evaluate and predict the hydrogen permeation flux when the lattice diffusion of hydrogen is the controlling step while at the same time the PdeH system is "infinitely diluted" (very low H 2 concentration at the Pd lattice) [28]. In practice, H 2 solubility and diffusivity in the metal lattice are determined by the hydrogen concentration [28,29] and, thus, Richardson's coefficient differs from 0.5 even in the case of using thick Pd layers.…”