Palladium alloy membranes materials

Itoh, M., Saito, M., Tajima, N., Machida, K. (2007). Ammonia synthesis using atomic hydrogen supplied from silver-palladium alloy membrane. Materials Science Forum, 561-565, 1597-1600. 

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. 

Shown in Table 8.6 arc some literature data on the use of dense membrane reactors for liquid- or multi-phase catalytic reactions. Compared to gas/vapor phase application studies, these investigations are relatively few in number. Most of them involve hydrogenation reactions of various chemicals such as acetylenic or ethylenic alcohols, acetone, butynediol, cyclohexane, dehydrolinalool, phenylacetylene and quinone. As expected, the majority of the materials adopted as membrane reactors are palladium alloy membranes. High selectivities or yields are observed in many cases. A higher conversion than that in a conventional reactor is found in a few cases. 

Corrosive reaction streams. In some application environments, the reactive or corrosive nature of one or more of the reaction components in a membrane reactor can pose a great technical challenge to the selection as well as the design of the membrane element Feed streams often contain some Impurities that may significantly affect the performance of the membrane. Therefore, attention should also be paid to the response of the selected membrane material to certain impurities in the reactant or product streams. Care should be taken to pretreat the feed streams to remove the key contaminants as far as the membrane is concerned in these cases. For example, palladium alloy membranes can not withstand sulfur- or carbon-containing compounds at a temperature higher than, say, 500 C [Kamcyama et al., 1981]. Even at lOO C, the rate of hydrogen absorption (and, therefore, permeation) in a pure palladium disk is... 

Membrane failure modes have been discussed above, and the connection to module design has also been discussed. Poor design for cyclic durability will be measured by the customer - the operating costs will be adversely affected by the requirement to replace membranes prematurely. Although the cost of membrane replacement should be offset by a recycle credit, the cost of materials and labor, and potential lost productivity, is stQl likely to be significant. The credit for recy-cUng palladium alloy membranes may be as great as 95% of the market value of the palladium (for foil membranes, perhaps only 85% of the market value if the palladium alloy is deposited onto a porous substrate). Environmentally, recycle also offers benefits versus recovery and purification of palladium from ore. 

Advanced organic and inorganic membranes and materials include polymers of intrinsic microporosity (PIMs), microporous PVDF, perovskite and palladium alloy membranes [45]. PIM membranes have displayed both high permeability with high selectivity for various gas mixtures. Major commercial and promising applications of membrane GS are delineated below [43—45] ... 

A wide variety of materials (polymers, zeolites, ceramics and metals) have been reported for various gas separation applieations. Polymeric membranes are the only ones whieh are extensively used. Inorganic and metallic membranes have limited applieations in the gas separations. Palladium alloy membranes have been applied for the purifieation of hydrogen and mixed metal oxide membranes have been developed for high temperature air separations by ion transport meehanism. The state-of-the-art of gas separation membrane materials is presented in a reeent review article. ... 

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

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Electroplating. Basically in electroplating, a substrate is coated with a metal or its alloy in a plating bath where the substrate is the cathode and the temperature is maintained constant Membranes from a few microns to a few millimeters thick can be deposited by carefully controlling the plating time, temperature, current density and the bath composition. Dense membranes made of palladium and its various alloys such as Pd-Cu have been prepared. Porous palladium-based membranes have also been made by deposition on porous support materials such as glass, ceramics, etc. 

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... 

Palladium is an expensive metal and this imposes limits on the thickness of material that can be used for hydrogen purification in competition with other industrial methods. Emonts et al. estimated that films less than about 5 p,m in thickness need to be used in a fuel-cell methanol reformer [7], while Criscuoli et al. [8] concluded that 20 p,m is an upper limit for membranes to be economically competitive. These economic estimates overlook the possibility of recycling the palladium or palladium alloy. This becomes a very real possibility in the use of free-standing membranes rather than composite structures with other metals or ceramics. Recycling prospects probably increase the thickness constraint to something between 5 jxm and 8 p.m, a value that is also consistent with factors such as limitations on the volume of space occupied by a multiple membrane assembly. 

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