Synthesis of composite Pd-porous stainless steel (PSS) membranes with a Pd/Ag intermetallic diffusion barrier

Intermetallic diffusion of support elements (Fe, Cr and Ni) into the membrane layer during high temperature hydrogen separation applications is a key factor affecting the overall performance of Pd and Pd/Ag membranes supported on porous sintered metals. Pd/Ag membranes supported on porous stainless steel (PSS) supports were prepared by the bi-metal multi-layer (BMML) deposition technique, which involved the formation of a porous Pd–Ag composite layer by consecutive deposition of Pd and Ag layers with no intermediate surface activation. While the conventional electroless plating involved intermediate drying and activating steps between Pd and Ag depositions, BMML deposition provided continuous plating with alternating Pd and Ag baths. The synthesis of these membranes involved a finishing step of the application of a gas tight Pd layer upon the BMML giving the membrane high hydrogen selectivity and long-term durability due to the fact that the BMML formed an extremely effective intermetallic diffusion barrier. Several membranes prepared by this technique have been stable under hydrogen permeation conditions for over 500 h at temperatures exceeding 500 °C. He flux data and SEI micrographs, prior to application of the Pd top coat, showed that the BMML formed a graded support, without significantly changing the total resistance of the support. In addition, heat treatment studies coupled with the X-ray phase identification analysis showed the formation of a Pd/Ag alloy phase, which might further improve the hydrogen permeability.