Dense metallic membranes preparation

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],... 

The manufacture of dense metal membranes or thin films can be effected by a number of processes casting/rolling, vapor deposition by physical and chemical means, electroplating (or electroforming) and electroless plating. By far, casting in combination with rolling is the predominant preparation and fabrication technique. It is noted that many of these processes have been demonstrated with palladium and its alloys because of their low oxidation propensity. Preparation of dense metal membranes is summarized in some detail as follows. 

This chapter focuses mainly on Pd-based MRs with respect to the gas permeation mechanism, membrane preparation, MR construction and operation, as well as applications in a variety of chemical reactions. In addition to a general description of Pd membranes and MRs, recent progress and critical issues in the dense metal membrane area will also be presented at the end of this chapter. 

Based on the different compositions, structures and configurations, the dense metallic membranes can be prepared using various methods. As described earlier, the dense metallic hydrogen separation membrane can be... 

Ni is a low-cost metal with lower permeability than Pd it is used for preparing both porous and dense metal membranes. Among its alloys, Ni-Nb-Zr has been investigated, and it has been demonstrated that the Nb in this alloy reduces the embrittlement while the Zr increases the permeability. 

Because membranes appHcable to diverse separation problems are often made by the same general techniques, classification by end use appHcation or preparation method is difficult. The first part of this section is, therefore, organized by membrane stmcture preparation methods are described for symmetrical membranes, asymmetric membranes, ceramic and metal membranes, and Hquid membranes. The production of hollow-fine fiber membranes and membrane modules is then covered. Symmetrical membranes have a uniform stmcture throughout such membranes can be either dense films or microporous. 

In this chapter membrane preparation techniques are organized by membrane structure isotropic membranes, anisotropic membranes, ceramic and metal membranes, and liquid membranes. Isotropic membranes have a uniform composition and structure throughout such membranes can be porous or dense. Anisotropic (or asymmetric) membranes, on the other hand, consist of a number of layers each with different structures and permeabilities. A typical anisotropic membrane has a relatively dense, thin surface layer supported on an open, much thicker micro-porous substrate. The surface layer performs the separation and is the principal barrier to flow through the membrane. The open support layer provides mechanical strength. Ceramic and metal membranes can be either isotropic or anisotropic. 

Thin film deposition for producing dense membranes has been presented in Sections 3.1.1 and 3.1.2. The processes can also be used to prepare porous membranes by adjusting the operating conditions. For example, transition metals and their alloys can be deposited on a porous ceramic, glass, or stainless steel support by the thin-film deposition process to produce porous metal membranes with small pore sizes [Teijin, 1984]. 

Now considering dense membranes, attention will be focused only on ceramic membranes, since a detailed description of the preparation and properties of the interesting and promising metal membranes have been described in detail in the preceding chapter of this book. Data concerning the permeability of Ag and Pd-alloy membranes, though, are listed in Table 2 for comparison. 

Tong J, Suda H, Haraya K, Matsumura Y (2005) A novel method for the preparation of thin dense Pd membrane on macroporous stainless steel tube filter. J Memb Sci 260 10-18 Ryi SK, Park JS, Kim SH, Cho SH, Park JS, Kim DW (2006) Development of a new porous metal support of metallic dense membrane for hydrogen separation. J Memb Sci 279 439-445... 

Concerning the preparation of thin membranes directly on porous supports, a lower thickness limit seemingly exists for which a dense metal layer can be obtained. This thickness limit increases with increasing surfaee roughness and pore size in the support s top layer." " Clearly, this relation puts strong demands on the support quality in terms of narrow pore size distribution, and the amount of surface defects. Therefore both pore size and roughness of the support surface are often reduced by the application of meso-porous intermediate layers prior to deposition of the permselective metal layer. This procedure facilitates the preparation of thin defect-free membranes beeause it is relatively easier to cover small pores by filling them with metal. It is therefore conceivable that for a certain low Pd-alloy thickness and support pore size, the H2 flux becomes limited by the support resistance. ... 

A similar experimental arrangement to that apphed by Joergensen et al. was used by Kikuchi et al. to compare the performance of palladium membranes prepared by electroless plating with platinum, palladium and mthenium membranes prepared by chemical vapour deposition [520]. The membranes were deposited onto commercially available alumina tubes. While the palladium membranes created by the plating technique appeared as an 8-pm thick dense layer on the alumina surface, the noble metals were deposited preferably inside the carrier pores by chemical vapour deposition. While the hydrogen/nitrogen selectivity was infinity for the plated membrane, it was about 280 for the platinum membrane created by chemical vapour... 

Currently, the membranes incorporated in MMRs are mainly zeolite and Pd-based dense metal ones. Incorporation of these membranes in microreactors can be achieved using one of the preparation methods described in previous chapters. 

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