Electroless plating and permeation features of Pd and Pd/Ag hollow fiber composite membranes

A comprehensive study was carried out involving electroless plating (ELP) synthesis and permeation characterization of a series of Pd and Pd/Ag membranes supported on α-Al2O3 hollow fibers. A multi-fiber ELP apparatus with film thickness monitoring quantified Pd and Ag deposition rates over a range of conditions. The Pd deposition rate exhibited a maximum while the Ag growth rate demonstrated autocatalytic behavior and was enhanced by Pd. Electron micrographs reveal the grain size and morphology are sensitive functions of the plating temperature. Three membrane types synthesized by ambient temperature ELP were of high quality determined by permeation measurements (H2 flux, pure component and mixture separation factors) over a range of temperature, pressure, and time on stream: Pd (13 μm)/α-Al2O3, Pd-(12 wt.%)Ag (11 μm)/α-Al2O3, and Pd (5 μm)/α-Al2O3/α-Al2O3. Initial binary separation factors of 1000–10,000 were obtained with the low temperature ELP. The permselectivity for the Pd membrane decreased to 100 after 300 h of temperature and pressure cycling. The Pd/Ag alloy membrane exhibited high permselectivity for extended operation with the separation factor (α) decreasing from an initial value of 970 to 650 after 300 h with the H2 flux sustained at 0.5 mol/m2 s at 550 °C for a transmembrane pressure gradient (image) of ∼4 × 105 Pa. The thinner composite Pd/γ-Al2O3/α-Al2O3 exhibited high initial flux and permselectivity (0.5 mol/m2 s, α ∼ 1000 at 350 °C, image ∼ 4 × 105 Pa) with reduced non-selective transport compared to the Pd/α-Al2O3 fiber. A phenomenological model was used to quantify the contribution of non-selective Knudsen diffusion and convective transport.