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Preparation and performance of Palladium membranes with an intermediate layer of alumina

pubblicazioni - Poster

Preparation and performance of Palladium membranes with an intermediate layer of alumina

Preparazione e caratterizzazione a permeazione di membrane composite al palladio per utilizzo ad elevata temperatura, ottenute mediante deposizione di un layer di allumina su un supporto poroso di acciaio inossidabile.

Palladium and palladium alloys are considered the most important materials for high quality hydrogen extraction from a mixture of gases. Nevertheless the fabrication of a dense, thin and stable palladium membrane is still challenging. Porous stainless steel or in general metal supports are good candidates for the development of inorganic membranes for high temperature applications because of their good mechanical properties and easy and reliable assembly into modules. Drawbacks for commercially available porous stainless steel supports include high nominal pore size, a wide pore size distribution and high surface roughness. The development of thin membranes for high temperature gas separation could be achieved through the deposition of a suitable intermediate ceramic layer on the support: this could lead to the desired morphology by decreasing pore dimensions and levelling surface roughness. Moreover the intermediate ceramic layer will act as an barrier between the metal support and the Pd membrane to avoid metallic interdiffusion, thus increasing membrane long-term stability at high temperatures. Several studied are ongoing to carry out stable ceramic supports on the surface of porous metal supports. Our study deals with the development of palladium composite membranes for high-temperature separations through deposition of alumina barrier layers on porous stainless steel supports. The deposition of the alumina barrier layer was obtained through dip-coating of tubular supports with colloidal dispersions and boehmite sols. Dip-coating conditions, the role of chemical composition on viscosity and on the resulting morphology of the deposited layer have been investigated. An activation layer was obtained by a boehmite sol seeded with PdCl2. Palladium was grown by electroless plating technique. Pd layers, with a thickness ranging for 10 to 15μm, were deposited on the alumina-modified supports. Surface morphology has been characterized by electron microscopy and gas permeance measurements were carried out after each preparation step. The stability of the ceramic layers has been evaluated through thermal cycling. Results indicate that a thorough control of viscosity by adding water-soluble polymers is fundamental to get a satisfying covering of the support surface avoiding the formation of cracks and defects. Morphological evidences demonstrate that the successive depositions of sol-gel systems with decreasing viscosity lead to a good leveling of the support surface roughness. The Pd/alumina/stainless steel membranes were tested at high temperature to assess hydrogen separation properties up to 500 C. Results show H2 fluxes of up to 10-6 mol∙s-1∙Pa-1∙m-2, compared to 10-9 mol∙s-1∙Pa-1∙m-2 for He and N2. Membrane stability has proven to be high and room-temperature He permeance has remained in the 10-9 range after several high temperature cycles. This work has been financed by the Research Fund for the Italian Electrical System under the Contract Agreement between RSE and the Ministry of Economic Development stipulated on July 29, 2009.

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