Composite Palladium and Palladium-alloy Membranes for High Temperature Hydrogen Separations

Introduction As we are approaching the era of the hydrogen economy, hydrogen is not only an important industrial chemical but also becoming one of the future’s key energy resources. It is expected that the future worldwide demand for hydrogen will increase greatly. Dense composite Pd and Pd/alloy membranes supported on a porous substrate, in particular, porous stainless steel (PSS), are especially suited for high temperature hydrogen separation and membrane reactor applications. Most early work on hydrogen permeation in palladium involved the use of palladium foils as summarized in detail by Lewis (1). However, there are a number of advantages of using composite palladium membranes supported on porous substrates, in particular, porous stainless steel, over palladium foils and tubes. These advantages include thinner membrane layer, better mechanical strength and higher hydrogen flux. Furthermore, there are additional main advantages for using porous stainless steel (PSS) supports that include the resistance to cracking and the simplicity of module construction. Composite Pd/PSS membranes, welded from both ends with non-porous stainless steel tubes, can be very easily assembled and integrated into a process. Additionally, the thermal expansion coefficient of stainless steel is very close to that of palladium, insuring good mechanical properties of the composite membrane during temperature cycling. This paper reviews the synthesis by electroless plating and characterization of composite Pd and Pd-alloy with a special emphasis on porous stainless steel (PSS) support, and long-term thermal stability. In addition, the unique features of the controlled insitu oxidation technique to create an intermetallic diffusion barrier are discussed.