Dense Palladium and Perovskite Membranes and Membrane Reactors
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other hand, glass membranes, such as Vycor and microporous glass, have Symmetrie pores. Materials commonly used as the porous support are alumina, granulär carbon, sintered metal, and Sili con carbide. The most widely used separating layers are alumina and zirconia. The ideal Separation factors for gas-phase Separation with porous membranes are generally low because the t r a n s p o r t through these membranes is mainly by Knudsen diffusion. On the other hand, microporous molecular-sieving mem branes, such as carbon molecular sieves,2 microporous glass,3,4 and zeolites,5,6 can have higher Separation factors, due to their molecular-sieving effects. This article will deal exclusively with dense membranes, with special attention to palladium membranes supported on porous stainless steel for hydrogen sepa rations and perovskite membranes for the conversion of methane to synthesis gas.
Palladium Membranes on Porous Stainless Steel Supports Palladium Deposition Because of their high hydrogen permeability and good chemical resistance, palladium and its alloys are the most commonly used materials for the preparation of dense metal membranes. 7 The use of an asymmetric composite mem brane with a porous support and thin Pd or Pd-alloy dense layer would provide both higher transmembrane flux and lower Pd loading in the membrane. Major techniques for the deposition of Pd and its alloys on a support include electroless plating, 8 spray pyrolysis, 9 metal-organic chemical vapor deposition (MOCVD),10 supercritical fluid transport-chemical
deposition (SFTCD),11 and sputter depo sition on polymeric membranes, 12 porous alumina, u ' 4 and anodic alumina. 1516 Additional discussion on other examples of the application of these techniques can be found in a review by Shu et al.' 7 Among the techniques just mentioned, electroless plating has the advantages of uniform deposition on complex shapes and large Substrate areas, hardness and good adhesion of the deposited film, and very simple equipment. The supports used for the deposition of Pd or Pd-alloy film include porous stainless steel (PSS), porous ceramics, and Vycor glass. PSS supports are preferred because of their resistance to cracking and ease of module construetion. More important, stain less steel and palladium have almost identical thermal-expansion coefficients, ensuring good mechanical properties of the composite membrane, especially during thermal cycling. Prior to electroless plating, foreign contaminants in the PSS support should be removed by mechanical treatment and Clearing.8 The procedure for electro less plating includes surface activation and Pd deposition. The purpose of sur face activation is to seed the PSS surface with palladium nuclei for the initiation of the autocatalytic process of reducing the metastable Pd salt complex on the target surface. Surface activation is gen erally aecomplished by successive immersion of the PSS in a SnCl2 Solution (for sensitizing), followed by an aeidie PdCl 2 Solution. The immersion of the PSS in the SnCl 2 Solution
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