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°C. Silver has been evaporated on Pd and Pd70Ag30 foils with a thickness of 25 μm in order to study the role of the surface composition in comparison with the membrane bulk composition. The Pd70Ag30-based membranes display the largest permeation rates at temperatures below 200°C, while Pd membranes with 20 Å silver evaporated on the upstream side show the largest permeation rates above 200°C. There are, consequently, different rate limiting processes above and below 200°C: at temperatures below 200°C, the bulk diffusion through the membrane is rate limiting, while at temperatures above 200°C, the influence of the surface composition starts to become significant. It has further been concluded that a sharp silver concentration gradient from the surface to the bulk is important for the hydrogen permeation rate at temperatures above 200°C.