Metallic membranes challenges

However, dense metal membranes present a few challenges that have, to date, prevented their widespread industrial implementation. The best performing materials, Pd and Pd-alloys, are expensive. Pure Pd displays high H2 permeability and can operate at elevated temperatures, but suffers performance degradation because of poisoning and corrosion from exposure to conmum contaminants (H2S, NH3, CO, CO2, and Hg), hydride formation, and other factors, which will be described in following sections. Pd-alloys can display better chemical stability and mechanical strength than pine Pd, but often at the expense of permeability. 

The major issue found in testing is the corrosion of the foam material and resultant contamination of the membrane. The high manufacturing cost of the metal or carbon foam with the required pore shape, size, and distribution also is a challenge. Further study and testing of the corrosion mechanism, selection of appropriate coating, a capillary process involved in the tiny pores, and related water retention are necessary to identify whether the new material and concept can be finally applied in the plate. 

The present chapter will not deal with general topics of liquid crystals or crown ethers as this exceeds the scope of this volume. Interesting reviews and monographs on liquid crystals and their properties can be found in the literature [10-13]. The synthesis of crown ethers can be challenging. Most commonly, the synthetic routes are based on procedures established by Pedersen [14-17]. A review by Bradshaw [18] and a monograph edited by Patai [19] also cover the synthesis and properties of crown ethers. More recent reviews deal with the use of crown ethers as chemosensors [20, 21], potential antitumor agents [22], molecular wires [23], or carriers for the separation of metal ions in liquid membrane processes [24],... 

In view of the above applications and of the fact that extensive literature is actually available on the CVD of aluminum, copper, and tungsten, processing of nickel films has been chosen in this chapter as an introduction to the actual challenges in the CVD of metals. This field is large, especially if it is considered to include deposition on substrates such as preforms, membranes or, particles. Because of space limitations no attempt has been made to provide a comprehensive overview. Although the choice of materials to be discussed was of necessity subjective, it is expected that the present approach will be useful for the investigation of different cases of metal CVD. 

Some selective double alkoxides have been synthesized and can be utilized to prepare mixed metal oxide membranes. Preparation and stabilities of double or multiple metal alkoxides are sometimes challenging. Aluminum alkoxides are particularly interesting as they are reactive with other metal alkoxides [Bradley et al., 1978]. Some alkoxides... 

The combination of high temperature and chemical exposure poses a very challenging material problem that is quite common in high-temperature membrane reactor applications. The consequence of structural degradation as a result of such a combination not only affects the permeability and permselectivity but also leads to physical integrity or mechanical properties. These issues apply to both metal and ceramic membranes. 

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