Model of hydrogen permeation behavior in palladium membranes

A model has been constructed for H-2 permeation through Pd which accounts f or external mass transfer, surface adsorption and desorption, transitions t o and from the bulk metal, and diffusion within the metal. Reasonable value s for all rate parameters have been estimated based on surface science and membrane literature. In the absence of external mass transfer resistance, n early diffusion-limited permeation is expected for clean Pd for temperature s above approximately 573 K and membrane thicknesses down to I mu m Low-tem perature permeation is limited by desorption while adsorption is only expec ted to impact permeation at very low upstream H-2 partial pressures, or und er conditions of substantially reduced sticking due to surface contaminatio n. The efficiency of external gas-phase mass transfer is a critical element in permeation flux and apparent Sievert's law behavior for Pd membranes ap proaching 10 mu m and thinner, especially at low H-2 partial pressures or w hen a porous support is present. Comparison of the calculations with litera ture results for Pd membranes less than 10 mu m thick reveals that permeati on rates well below those expected are often observed, indicating the impor tance of factors related to fabrication methods, such as film microstructur e, grain size, and surface or grain boundary contamination. (C) 1999 Elsevi er Science B.V. All rights reserved.