Metallic membranes permeation kinetics

The main observation from Table 2.1 is the enormous range of values of diffusion coefficients—from 10 1 to 10 30 cm2/s. Diffusion in gases is well understood and is treated in standard textbooks dealing with the kinetic theory of gases [24,25], Diffusion in metals and crystals is a topic of considerable interest to the semiconductor industry but not to membrane permeation. This book focuses principally on diffusion in liquids and polymers in which the diffusion coefficient can vary from about 10 5 to about 10-10 cm2/s. 

Basically, two different kinds of experiments can be carried out to analyze the kinetics of hydrogen absorbing metallic membranes (Fig. 18.25) (i) permeation experiment (the membrane is positioned between two reaction chambers) and (ii) sorption experiments (the membrane is not positioned between two reaction chambers but in a dead-end reactor). 

For automotive applications, there is a need to develop specific membranes with custom surface/bulk properties in order to meet kinetics requirements (in particular cold start and acceleration). As discussed in Section 18.3, the permeation mechanism consists of two main steps (i) a surface step characterized by a surface resistance Rg and (ii) a bulk (diffusion-controlled) resistance Rj). For permeation in transient conditions of flow, the ratio Rg/Ro (Rs is the surface resistance and R the bulk diffusion resistance) is critical because the two steps are connected in series. Schematically, Rg is rate-controlling in transient conditions and R is rate-controlling in stationary conditions of flow. The value of the surface resistance Rg is a function of surface state (chemical composition of surface and roughness factor defined as the dimensionless ratio of the surface of the true to the geometrical solid-gas interface). The value of the bulk resistance R is a function of bulk state (chemical composition and microstructure) and membrane thickness (5).Therefore, the development of metallic membranes with custom properties requires the adjustment of all these physical parameters. 

It is obvious that the pure-metal membranes can be easily prepared by conventional metallurgical processes in the configurations of tube and disk, which have a thickness greater than 25 pm and can be used as unsupported H2 permeation membranes. Because of the two obvious weaknesses of embrittlement and slow surface kinetic, the preparation of these metal-based H2 permeation membranes mainly focuses on preparation of alloys in order to improve the embrittlement and modification of the surface for improving the surface kinetic. The preparation of an extra modified layer on these pure non-Pd metal membranes is the subject of the current discussion. 

Figure 17 Time course of the kinetic experiment performed with surfactant substrate (19) in vesicular 20, Br upon addition of Cu(NO3)2 ([copper(II)] = 5 x 10 M, pH 5). The first part was run at 25 °C, i.e. above the of the membrane. At this temperature the kinetics are monophasic. The second part was run at 10 °C. If during the time at 25 °C copper(II) permeation occurred, monophasic kinetics would be expected since all the substrate (in the internal and external layers) is exposed to the metal ion. Since a biphasic process is observed, no permeation has occurred and the stay of the vesicles above f involves only flip-flop of (19)...

The membrane in permeation experiments is frequently coated with palladium on both sides to minimize kinetic limitations on the transfer of hydrogen between the adsorbed and absorbed states. The coating on the output side, as noted above, also prevents oxidation of the metal. Although the coatings are intended to facilitate surface reactions, the membrane itself must be thick enough to ensure that the rate of permeation is controlled by diffusion. The diffusivity should therefore be determined for membranes of varying thickness to verify that diffusion is rate[Pg.132]

The dependence of l/J on L can therefore be used to evaluate from the intercept provided that the value of 6 is known independently. If k hydrogen permeation is controlled only by surface entry kinetics when the membranes are too thin with respect to the hydrogen dififusivity for the metal of interest. 

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