Metallic membranes hydrogen permeation mechanism

Figure 8.7 Mechanism of permeation of hydrogen through metal membranes...

Figure 8.2. Commonly accepted mechanism for the permeation of hydrogen through dense metal membranes.

Transport Mechanism of Hydrogen Through Dense Metal Membranes and Governing Permeation Equations... 

Evaluation of hydrogen absorption and permeation provides valuable information on the hydrogen evolution and hydrogen embrittlement mechanism. Devanathan-Stachurski [5,6] developed a technique to measure hydrogen permeation though metallic membranes. This technique is based on the assumption that the following conditions are established at each surface of the membrane under potentiostatic control ... 

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 ... 

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] ... 

Hydrogen can permeate selectively dense metal layers, and, accordingly, this phenomenon is exploited when metal membranes are applied for separating ultrapure hydrogen from gas mixtures [24-27]. The hydrogen mass transfer through dense metal membranes includes several transport mechanisms, which are discussed in the following sections. 

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. 

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