CVD Synthesis and Gas Permeation Properties of Thin Palladium/Alumina Membranes

Thin (0.5-5-μm) palladium (Pd) membranes were prepared inside pores or on the surface of mesoporous (pore size 4-6 nm), sol-gel-derived γ-Al₂O₃ supports by the chemical vapor deposition (CVD) method using palladium acetylacetonate and palladium chloride (together with hydrogen) as Pd precursors. When Pd is deposited in the form of metal plugs inside pores of γ-Al₂O₃, He permeance of the composite membranes increases exponentially with temperature, indicating activated diffusion through the microporous metal deposit defects, while only modest permselectivity for H₂ is observed. When Pd is deposited in the form of a metal film on the surface of γ-Al₂O₃, He permeance is governed by Knudsen-viscous flow through meso/macroporous metal film defects. The H₂ permeance of different palladium membranes prepared by CVD appears to increase with palladium crystal grain size. The highest H₂ permeance and H₂:He permselectivity for membranes prepared in this study are about 1.0-2.0 × 10^-7 mol · m^-2 · s^-1 · Pa^-1 and 200 - 300 at 300°C, respectively.