Hydrogen permeation surface 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. 

Hydrogen can permeate selectively dense metal membranes, behaviour that permits the separation of hydrogen from gas mixtures. The mass transfer mechanism consists of several steps dissociation of hydrogen molecules into atoms, interaction of hydrogen atoms with the metal surface and their adsorption, diffusion of hydrogen into the metal lattice, and desorption of hydrogen atoms from the other metal surface and their recombination into molecules.The overall transport process through the metal wall is called permeation and is ruled by the expression ... 

Permeation of atomic hydrogen across metallic membranes from surface 1 to surface 2 is a multi-step process (Martin etal, 1996) where ad refers to surface-adsorption and ab to bulk-absorption ... 

Steps 5 to 7 are the same as steps 1 to 3 in the opposite direction The permeation mechanism can be simplified by considering only the series connection of three main steps (i) the surface resistance on the upstream side of the membrane due to the surface dissodation of molecular hydrogen and leading to the formation of sub-surface H ad-atoms (ii) the bulk impedance associated with the transport of hydrogen ad-atoms and (iii) the surface resistance on the downstream side of the membrane due to the surface recombination of hydrogen ad-atoms into molecular hydrogen and subsequent transport of gas away from the interface. 

A first limit rate expression is obtained when surface rate contributions are rate determining.The concentration profile is plotted in Fig. 18.22. Although the use of Henry s law (Equation [18.1]) as boundary conditions is usually limited to the case of molecular diffusion (polymer membranes), it can also be used to describe permeation across metallic membranes with surface rate-determining step (rds). In such cases, the dissociative physisorption step of H2 into H is assumed to be fast and at equilibrium. Steps (3) and (5) of the sorption mechanism are rds and the relationship between surface hydrogen ad-atoms and pressure is given by Equation [18.8] ... 

The progress of hydrogen to an efflux detector on the external pipe or vessel face involves three essential steps hydrogen entry into the vessel wall, permeation through it, and exit from the external surface to some means of detection. Factors governing this progress are summarised in Table 3.1. 

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