This study attempts to develop a membrane for use in the high temperature separation of hydrogen from a gaseous mixture. For high temperature applications, inorganic membranes are necessary, and palladium and palladium silver alloy metal membranes exhibit the highest selectivity for hydrogen. The problems with these metallic membranes are that temperature or pressure cycles can cause them to crack, and their surfaces can be poisoned by sulfur containing molecules in the gaseous mixture. To avoid cracking, metallic membranes are usually fabricated to large thicknesses which imparts a structural rigidity. However large membrane thicknesses lower the flux to the point where the use of thick membranes becomes economically unattractive. In order to develop a membrane with high flux and selectivity, the study suggests forming membranes in which metallic Pd and Pd/Ag films have been placed inside the pores of a macroporous ceramic substrate. Two types of porous ceramic membranes will be used. The first is a one layer . alumina disk with a pore diameter in the range of .2 - .7 m. The second is a two layer disk consisting of a thin (5 m), small pore (5 nm) layer lanthana modified y Al2O3 layer atop a thick (1-2 mm), large pore (.2 - .7 m) . alumina support. The metal will be placed in the ceramic support by using a CVD counterdiffusion process in which PdCl2 or a mixture of PdCl2 and AgBr will diffuse through the pores from one face of the ceramic support and H2 will diffuse from the opposite face. When the gases meet somewhere inside the membrane, the reduction reactions: PdCl2(g) + H2(g) --- Pd(s) + 2HCl(g) AgBr(g) + 1/2 H2(g) --- Ag(s) + HBr(g) will produce the metal (or metal alloy) which will span the interstitial space of the pore and form a gas tight layer. The layers, supported by the ceramic membrane, should be more durable and less susceptible to chemical attack.
