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

Palladium composite membranes on sinter-metal supports were prepared by electroless plating. Zirconia (ZrO2), yttria-stabilized zirconia (YSZ), and titania (TiO2) 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-ZrO2, thickness ∼2 μm), atmospheric plasma spraying (APS-YSZ, thickness ∼10-70 μm), and wet powder spraying (WPS-TiO2, 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-TiO2 and MS-ZrO2. Palladium membranes on APS-YSZ have to be significantly thicker (∼15-20 μm) than membranes on WPS-TiO2 due to the extremely rough surface of the APS-YSZ layer. In contrast, the WPS-TiO2 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 TiO2 surface and indications of a change of the composition of the TiO2 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-ZrO2 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-TiO2 demonstrated the best hydrogen permeance, i.e. 0.154mN3m−2h−1Pa−0.5, as well as the best H2/N2-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 (1.6mN3μmm−2h−1Pa−0.5 at 500 °C).