Hydrogen permeation bulk diffusion step

Volumetric steady-state gas permeation tests at elevated temperatures are typically used to characterize the performance of paUadium membranes. Electrochemical methods are also effective, even at high temperatures [90, 166]. Permeation through palladium depends on a solution-diffusion mechanism, including the steps of chemisorption and dissociation into atoms, absorption into the metal, diffusion through the metal lattice, transfer from the bulk metal to the opposite side, and recombination into molecules for desorption [167, 168]. Difiusion of molecular hydrogen through boundary layers adjacent to the surface is also necessary. 

However, to relate CPC to the membrane permeance according to eqn (14.1), it is necessary to choose an appropriate form of the bulk driving force. Concerning this issue, it should be considered that, when the rate-determining step of the overall permeation process is the diffusion in the Pd-based layer and, also, the atomic hydrogen concentration inside membrane... 

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