Permeable materials palladium

Knapton, A.G., Palladium alloys for hydrogen diffusion membranes—A review of high permeability materials, Plat. Met. Rev., 21,44-50,1977. 

US Patent 6,183,542 was issued in 2001 for a palladium membrane process. This process provides an apparatus that can handle high flow rates of gas, per unit area of membrane, while using a minimal amount of hydrogen-permeable material. This is accomplished by using stainless steel mesh elements to reinforce the thin-walled, palladium or palladium alloy membranes. This process also provides the ability to withstand large pressure gradients in opposite directions and thus will make it easier to clean membranes that have been clogged with contaminants. 

US Patent 6,183,542 was issued in 2001 for a palladium membrane process. This process provides an apparatus that can handle high flow rates of gas while using a minimal amount of hydrogen-permeable material. 

At 700°C, the permeability of palladium is approximately 3.0 xlO- [mol/(m s Pa )] and the permeability of iron is 5.0x10- [mol/(m s Pa"0] [Buxbaum and Marker, 1993]. Figure 4 illustrates the hydrogen permeability of palladium, iron and the proton conducting ceramic material as a function of inverse temperature. The driving force for hydrogen flux in each membrane material is... 

The DT reactor needs several kg tritium as starting material. A likely technique involves the irradiation of a Li-Al alloy in a high flux thermal fission reactor which produces both tritium and He (17.43) These can be separated on the basis of their different vapor pressures, different permeability through palladium, or through their different chemical reactivities. 

It was not equally obvious that dense ceramic hydrogen-permeable membranes would be of similar interest. There are clearly needs for hydrogen purification membranes, but polymers and microporous materials as well as metals such as palladium and its alloys appeared to fill these needs. In addition, possible candidates for dense ceramic hydrogen-permeable materials were not as appealing as the oxygen-permeable ones in terms of performance and stability. 

Table I. Hydrogen permeability for Palladium containing materials.

Ionic liquids have already been demonstrated to be effective membrane materials for gas separation when supported within a porous polymer support. However, supported ionic liquid membranes offer another versatile approach by which to perform two-phase catalysis. This technology combines some of the advantages of the ionic liquid as a catalyst solvent with the ruggedness of the ionic liquid-polymer gels. Transition metal complexes based on palladium or rhodium have been incorporated into gas-permeable polymer gels composed of [BMIM][PFg] and poly(vinyli-dene fluoride)-hexafluoropropylene copolymer and have been used to investigate the hydrogenation of propene [21]. 

Membrane processes are based on the selective transmission characteristics of the membrane material for different molecules, whereby the most effective membranes are usually also the most expensive. For example, the purest hydrogen can be captured by palladium membranes with suitable additives, but their low permeability make it necessary to use large membrane surfaces and high pressures, which result in high costs. 

Metal-based PRBs involve the introduction of metals, usually zero-valent iron, but sometimes metal wool, palladium, or other metals to chemically react with the target contaminant(s), causing chemical adsorption with and/or destruction of the contaminants. These materials are typically permeable to water and thus avoid the groundwater management and flow problems associated with impermeable barriers. 

Kurungot et al. [48] developed a novel membrane material and a catalytic membrane reactor for the partial oxidation of methane. The driver of the development was the fact that rates of reforming reactions are much higher compared with the low permeability of conventional palladium membranes [49], Silica was previously recognized as a low-cost alternative to palladium [50], Additionally, the conventional... 

Ceramic and semiconductor thin films have been prepared by a number of methods including chemical vapor deposition (CVD), spray-coating, and sol-gel techniques. In the present work, the sol-gel method was chosen to prepare uniform, thin films of titanium oxides on palladium Titanium oxide was chosen because of its versatility as a support material and also because the sol-gel synthesis of titania films has been clearly described by Takahashi and co-workers (22). The procedure utilized herein follows the work of Takahashi, but is modified to take advantage of the hydrogen permeability of the palladium substrate. Our objective was to develop a reliable procedure for the fabrication of thin titania films on palladium, and then to evaluate the performance of the resulting metalloceramic membranes for hydrogen transport and ethylene hydrogenation for comparison to the pure palladium membrane results. 

As discussed earlier, many composite porous membranes have one or more intermediate layers to avoid substantial penetration of fme particles from the selective layer into the pores of the bulk support matrix for maintaining adequate membrane permeability and sometimes to enhance the adhesion between the membrane and the bulk support The same considerations should also apply when forming dense membranes on porous supports. This is particularly true for expensive dense membrane materials like palladium and its alloys. In these cases, organic polymeric materials are sometimes used and some of them like polyarilyde can withstand a temperature of up to 350X in air and possess a high hydrogen selectivity [Gryaznov, 1992]. 

Pd alloys are preferred over pure Pd as the membrane materials because of several considerations. First of all, pure palladium can become embrittled after repeated cycles of hydrogen sorption and desorption. Second, the hydrogen permeabilities of certain Pd-alloys are higher than those of pure palladium. Third, the catalytic activities of the alloy membranes, in many cases, exceed that of palladium alone. Finally, palladium is very expensive. Alloying with other metals makes it more economically attractive for... 

Thermal stability. Thermal stability of several common ceramic and metallic membrane materials has been briefly reviewed in Chapter 4. The materials include alumina, glass, silica, zirconia, titania and palladium. As the reactor temperature increases, phase transition of the membrane material may occur. Even if the temperature has not reached but is approaching the phase transition temperature, the membrane may still undergo some structural change which could result in corresponding permeability and permselectivity changes. These issues for the more common ceramic membranes will be further discussed here. 

The alloying of palladium with some other metals permits one to overcome the disadvantages of pure palladium and to prepare the materials with a hydrogen permeability above that of palladium. The insertion of a second and a third component into the palladium membrane may increase its mechanical strength, the hydrogen solubility, and catalytic activity of the membrane toward hydrogen dissociation. This was discussed in many original papers and reviews [26-36]. 

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