Hydrogen permeation through surface modified Pd and PdAg membranes

The hydrogen permeation through surface modified Pd and Pd70Ag30 membranes has been studied at temperatures between 100 and 350 degreesC. Silver has b een evaporated on Pd and Pd70Ag30 foils with a thickness of 25 mum in order to study the role of the surface composition in comparison with the membra ne bulk composition. The Pd70Ag30-based membranes display the largest perme ation rates at temperatures below 200 degreesC, while Pd membranes with 20 Angstrom silver evaporated on the upstream side show the largest permeation rates above 200 degreesC. There are, consequently, different rate limiting processes above and below 200 degreesC: at temperatures below 200 degreesC , the bulk diffusion through the membrane is rate limiting, while at temper atures above 200 degreesC, the influence of the surface composition starts to become significant. It has further been concluded that a sharp silver co ncentration gradient from the surface to the bulk is important for the hydr ogen permeation rate at temperatures above 200 degreesC. Adding oxygen to the hydrogen supply will almost totally inhibit the hydrog en permeation rate when a pure Pd membrane surface is facing the upstream s ide, while for silver-containing surfaces the presence of oxygen has almost no effect. On a clean Pd surface, oxygen effectively consumes adsorbed hyd rogen in a water forming reaction. With Ag on the surface, no water formati on is detected. Co-supplied CO inhibits the permeation of hydrogen in a sim ilar manner on all studied membrane surfaces, independent of surface silver content. (C) 2001 Elsevier Science BN. All rights reserved.