Preparation and characterization of composite palladium membranes on sinter-metal supports with a ceramic barrier against intermetallic diffusion

Abstract

Palladium composite membranes on sinter-metal supports were prepared by electroless plating. Zirconia (ZrO 2), yttria-stabilized zirconia (YSZ), and titania (TiO 2) were applied as porous barriers between the palladium membrane and the sinter-metal support to prevent intermetallic diffusion. The ceramic layers were coated by magnetron sputtering (MS-ZrO 2, thickness ∼2 μm), atmospheric plasma spraying (APS-YSZ, thickness ∼10–70 μm), and wet powder spraying (WPS-TiO 2, thickness ∼40–60 μm), respectively. They differ considerably in terms of thickness, pore size, surface roughness, and open porosity. The barriers and the final membranes were characterized by light-optical microscopy (LM), scanning electron microscopy (SEM), capillary flow porometry, in terms of the adhesion of the membrane layer, its gas-tightness, and by hydrogen permeation measurements. Moreover, the barrier function against intermetallic diffusion between the palladium and the metals from the support was demonstrated by annealing of membrane samples in pure hydrogen at 600 °C for up to 23 days. APS-YSZ was identified as the most promising barrier, followed by WPS-TiO 2 and MS-ZrO 2. Palladium membranes on APS-YSZ have to be significantly thicker (∼15–20 μm) than membranes on WPS-TiO 2 due to the extremely rough surface of the APS-YSZ layer. In contrast, the WPS-TiO 2 surface is smooth and uniform which allows the plating of very thin (∼8–10 μm) and selective palladium membranes. However, worse adhesion of the palladium membrane to the TiO 2 surface and indications of a change of the composition of the TiO 2 layer in intimate contact with the palladium membrane after exposure to hydrogen at high temperature give rise to doubt about their long-term stability. Pd/MS-ZrO 2 membranes showed intermetallic diffusion at locations where direct contact between the palladium and the sinter-metal support persisted. Among the three membrane types, Pd/WPS-TiO 2 demonstrated the best hydrogen permeance, i.e. 0.154 m N 3 m − 2 h − 1 P a − 0.5 , as well as the best H 2/N 2-permselectivity, ∼800, at 500 °C. However, the highest hydrogen permeability, as determined from the permeance and the nominal thickness of the palladium membrane, was reached by Pd/APS-YSZ ( 1.6 m N 3 μ m m − 2 h − 1 P a − 0.5 at 500 °C).