Hydrogen Transport in Metallic Dense Membranes

Metallic membranes for hydrogen separation can be of many types, such as pure metals Pd, V, Ta, Nb, and Ti binary alloys of Pd, with Cu, Ag, and Y Pd alloyed with Ni, Au, Ce, and Fe and complex alloys of Pd alloyed with more than one metal [3], Body-centered cubic metals, for example, Nb and V, have higher permeability than face-centered cubic metals, for instance, Pd and Ni [26-29], Even though Nb, V, and Ta possess a permeability greater than that of Pd, these metals develop oxide layers and are complicated to be used as hydrogen separation membranes [29], Especially, the Pd and Pd-based membranes have in recent times obtained renovated consideration on account of the prospects of a generalized use of hydrogen as a fuel in the future [26], We emphasize on these types of membranes in this chapter. 

Hydrogen transport through Pd and Pd-based alloys comprises the next steps [30,31]. The H2 molecules during adsorption are dissociated on top of the metal surface, giving a proton to the interstitial sites and an electron to the metal conduction band (see Section 2.4.2). The second step is the diffusion of atomic H, since the proton will be surrounded by an electron cloud [32], through the bulk of the metal. Finally, an associative desorption process of H2 molecules occurs from the metal surface at the other end of the membrane. 

Permeability is an essential property of the materials that constitute the membrane and is independent of membrane thickness [23], Additionally, permeability can be described as the product of the diffusion coefficient and the solubility constant (see Equation 10.3) and is temperature dependent. Permeability can then be represented by the following Arrhenius-type expression (see Section 5.6.1) 

To conclude this section, it is necessary to state that Pd and Pd-based membranes are currently the membranes with the highest hydrogen permeability and selectivity. However, the cost, availability, their mechanical and thermal stabilities, poisoning, and carbon deposition problems have made the large-scale industrial application of these dense metal membranes difficult, even when prepared in a composite configuration [26,29,33-37], 

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