Hydrogen into palladium, diffusivity

In a paper reporting the results of some simulations of diffusion of hydrogen into palladium [582], the authors describe their method of solution as the Treanor method. This is described in a few texts [314,351] and goes back to a paper by Treanor in 1966 [548]. 

Endothermic occlusion takes place by diffusion of hydrogen into a metal lattice which is very little changed by the process. In exothermic occlusion by palladium, however, the face-centred cubic lattice (a phase) of palladium, with lattice constant 3.88 A, will accomodate, below 100°C, no more than about 5 at. % hydrogen, and then undergoes a transition to an expanded phase ( 3 phase), with lattice constant 4.02 A and H/Pd = 0.5—0.6. The H—Pd system thus splits into a and 3 phases in the manner familiar for two partially miscible liquids. The consolute temperature (rarely observable for solid phases) is about 310°C at H/Pd = 0.22. The phase diagram is, however, not well established because formation of the... 

Sometimes there are slow processes at the surface of the diffusion medium which may sensibly alter the rate at which the diffusing substance leaves or enters the medium. For instance, under certain conditions the diffusion of hydrogen in palladium is fast compared with its rate of entry into the solid from the sorption layer. In this case it will be necessary to... 

Moreover, in the case of hydride intervention, still a further factor, namely the kinetics of hydrogen diffusion into the metal, influences also the overall kinetics by removing a reactant from a reaction zone. In order to compare the velocity of reaction of hydrogen, catalyzed by palladium, with the velocity of the same reaction proceeding on the palladium hydride catalyst, it might be necessary to conduct the kinetic investigations under conditions when no hydride formation is possible and also when a specially prepared hydride is present in the system from the very beginning. 

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