Surface characterization of Pd–Ag composite membranes after annealing at various temperatures

Pd–Ag films (∼24% Ag, 20–26 μm thick) were deposited by sequential electroless plating onto porous tubular stainless steel substrates. Intermediate α- and γ-Al2O3 oxide layers were employed to modify the support pore size and to prevent intermetallic diffusion of the stainless steel components into the Pd–Ag layer. The aluminum oxides were applied to the substrate porous system by a vacuum assisted-coating method. Composite membranes annealed at temperatures between 500 and 600 °C were characterized for film structure (XRD), morphology (SEM), bulk and surface component distribution (EDS, XPS), and hydrogen permeance. Pd–Ag alloy formation progressed as annealing temperature was increased to 600 °C. Composition measurements within the Pd–Ag layer revealed preferential segregation of the Ag component to the top surface; this result is consistent with Agʹs lower surface free energy. No diffusion of stainless steel components into the Pd–Ag layer was observed, demonstrating the effectiveness of the oxide interdiffusion barrier. Hydrogen permeation tests of membranes annealed at 500 °C displayed high permeability and H2/N2 selectivity at operating temperatures between 400 and 450 °C. Permeabilities were higher but selectivities were lower for membranes annealed at 550 °C. This performance deterioration may be related to defects within the Pd–Ag layer caused by growth of dendritic Ag deposits.