Palladium-alloy materials

Palladium-alloy materials have been recently introduced as a promising ORR cathode electrocatalyst for replacing Pt [91-98], Pd alloys are considerably less expensive than Pt. Besides, experimental studies indicate that they have high methanol tolerance for direct methanol fuel cells (DMFCs) in which the methanol crossover to the cathode significantly decreases the cell s efficiency [99, 100]. 

In dentistry, palladium alloys are widely used as alternatives to base metal alloys in the manufacture of crowns and bridges as weU as the replacement of lost or damaged teeth (see Dental materials). Such alloys contain over 80% palladium, and hence offer significant cost benefits over alloys containing a high proportion of gold. 

Bessing, C., Bergman, M. and Thoren, A. Potentiodynamic Polarization Analysis of Low-gold and Silver-Palladium Alloys in Three Different media. Dental Materials, 3, 153-159 (1987)... 

Metzger, P. R., Vrijhoef, M. M. A. and Greener, E. H. Corrosion Resistance of Three High-Palladium Alloys , Dental Materials, 1, 177-179 (1985)... 

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. 

Some of the materials that have been examined as catalysts include Pure Platinum, Platinum-Iridium Alloys, Various Compositions of Platinum-Rhodium Alloys, Platinum-Palladium Alloys, Platinum-Ruthenium Alloys, Platinum-Rhenium Alloys, Platinum-Tungsten Alloys, FejOj-M CVI Oj (Braun Oxide), CoO-Bi20j, CoO with AI2O3, Thorium, Cerium, Zinc and Cadmium. 

Nanostructured aluminum [74—78], iron [74] and aluminum-manganese alloys [74] have been prepared from a Lewis acid A1Q3/[BMIM]Q mixture (65 mol% AICI3, 35 mol% [BMIMJC1) whereas palladium alloys have been deposited from a Lewis basic system (45 mol% Aid , 55 mol% [BMIMJC1). The electrochemical cell and all parts which are in contact with the electrolyte have to be built from inert materials. As cathode material glassy carbon can be used. A constant ion concentration in the electrolyte can be realized by the use of a sacrificial anode consisting of the... 

Simonet, J., Poizot, P. and Laffont, L. (2006) A copper-palladium alloy usable as cathode material mode of formation and first examples of catalytic cleavages of carbon-halide bonds. J. Electroanal. Chem. 591, 19-26. 

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... 

Sample holders used in the Stone instruments are shown in (d)- /). Small cups are used in (d) to contain samples from 10 to 200 mg in mass the cups are constructed from aluminum, stainless steel, nickel, or platinum and or palladium alloys. For smaller samples, 0.1 to 20mg, the highly sensitive ring thermocouple holder, as shown in ( ), is used. The sample dishes can be made from aluminum, stainless steel, or platinum by the investigator using a simple press and die. True dynamic gas atmosphere control is featured in the sample holder in (/). The gas flow is through the sample and reference materials it cannot be used with samples that fuse, however. 

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. 

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. 

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