Performances and stability of a Pd-based supported thin film membrane prepared by EPD with a novel seeding procedure. Part 1-Behaviour in H2:N2 mixtures

The performances and long-term stability in the 350 degrees-450 degrees C temperature range of a Pd-Ag alloy thin film membrane are reported. The membrane was prepared by electroless plating deposition (EPD) on the inner walls of a tubular alumina asymmetric support using a novel seeding procedure based on Pd-only complexes. A defect-free film with thickness around 12 microns is obtained. Scanning electron microscopy (SEM) images evidence the presence of aggregates of small crystallites of few hundred nanometers. The membrane shows a stable behaviour for at least 260 h of time-on-stream in pure H-2 and permeability tests indicate a high hydrogen flux with an activation energy of about 14 kJ/mol. The permeability follows Sievert's law with an exponent of 0.5, indicating a bulk diffusion of hydrogen as the rate-controlling step. There is an initial increase in the transmembrane H-2 flux during the time-on-stream tests, due to hydrogen absorption in the Pd-Ag alloy which increases the permeation rate. The presence of two mechanisms of transport, within Pd-Ag alloy crystallites and in the intergrain boundary region, is discussed. The permeability behaviour using H-2:N-2 mixtures as the feed is also shown. The presence of N-2 induces a large decrease in the H-2 permeation rate, greater than that expected by dilution effect. The membrane is stable in the 350 degrees-400 degrees C temperature range with a high separation factor between H-2 and N-2, but at 450 degrees C shows an initial fast decrease of the selectivity with time-on-stream up to reach a nearly constant value. The effect is interpreted in terms of formation of NHx species which inhibit hydrogen diffusion in the intergrain boundary region, as suggested also from literature data. (C) 2008 Elsevier B.V. All rights reserved.