Fabrication of a thin palladium membrane supported in a porous ceramic substrate by chemical vapor deposition

A thin, gas-tight palladium (Pd) membrane was prepared by the counter-diffusion chemical vapor deposition (CVD) process employing palladium chloride (PdCl2) vapor and H-2 as Pd precursors. A disk-shaped, two-layer porous ceramic membrane consisting of a fine-pore gamma-Al2O3 top layer and a coarse-pore alpha-Al2O3 substrate was used as Pd membrane support. A 0.5-1 mu m thick metallic membrane was deposited in the gamma-Al2O3 top layer very close to its surface, as verified by XRD and SEM with a backscattered electron detector. The most important parameters that affected the CVD process were reaction temperature, reactants concentrations and top layer quality. Deposition of Pd in the gamma-Al2O3 top layer resulted in a 100- to 1000-fold reduction in He permeance of the porous substrate. The H-2 permeation flux of these membranes was in the range 0.5-1.0 x 10(-6) mol m(-2) s(-1) Pa-1 at 350-450 degrees C. The H-2 permeation data suggest that surface reaction steps are rate-limiting for H-2 transport through such thin membranes in the temperature range studied.