Current theoretical models for the permeation behavior of hydrogen through palladium and palladium alloys predict that low temperature operation of thin membranes will result in permeation which was no longer controlled by the diffusion of hydrogen through the metal lattice. Specifically, the process of desorption from the downstream surface was predicted to become the dominant resistance to mass transfer. However, these models neglect the non-ideal absorption behavior of hydrogen in palladium which typically occurs at temperatures below 300C. In this work a model was developed which accounts for the non-ideal behavior of hydrogen in palladium–silver alloys (25% silver by weight) which were typically used for hydrogen purification. This model predicts that the diffusion-limited regime should be characterized by an increase in the activation energy to about 32kJ/mol below 200C. This prediction was supported by data available in the literature for thick (>125 μm) palladium–silver alloy membranes.

Non-Ideal Absorption Effects on Hydrogen Permeation Through Palladium–Silver Alloy Membranes, L.S.McLeod, F.L.Degertekin, A.G.Fedorov: Journal of Membrane Science, 2009, 339[1-2], 109-14