Solution-diffusion mechanism palladium

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. 

As for the solution-diffusion mechanism, the hydrogen permeation through a palladium layer is a complex process consisting of adsorption and dissociation of hydrogen molecules, followed by diffusion of hydrogen atoms through the metal... 

The molecular transport of hydrogen in palladium membranes occurs through a solution/diffusion mechanism, which follows six steps ... 

Body centred cubic (BCC) metals, such as Ta, Nb, V and Zr, exhibit a hydrogen solubility higher than /ace centred cubic (FCC) metals, such as palladium, up to a factor of 10-100 (Dolan, 2010). Hydrogen permeation through these metals occurs by a solution-diffusion mechanism, similar to the one described above for palladium. These metals, however, exhibit a tendency towards embrittlement. Moreover, they are very reactive and can form carbides, nitrites and oxides when used in operating environments. In order to avoid this poisoning, BCC metals can be coated on both sides by a thin film (less than 1 pm) of a noble metal, which acts as a catalyst and can... 

Then Uemiya et al. [2] reported another Pd membrane reactor for WGS reaction using Fe-Cr oxide catalysts. The model of flow in the palladium membrane reactor is illustrated in Figure 6.3. They proposed that hydrogen is permeated through palladium membrane via a solution diffusion transport mechanism, and the rate of hydrogen permeation, /, per unit area of membrane, is written in terms of Fick s first law as follows ... 

The permeation of hydrogen through a palladium layer is by the solution and di sion mechanism. For the case of low H concentrations in the Pd-H phase, no a-to-p phase transformation, and the permeation controlled by the hydrogen diffusion through the bulk of palladium layer, the hydrogen flux can be expressed as... 

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