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

摘要

The performances and long-term stability in the 350°-450 °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 H2 and permeability tests indicate a high hydrogen flux with an activation energy of about 14kJ/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 H2 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 H2:N2 mixtures as the feed is also shown. The presence of N2 induces a large decrease in the H2 permeation rate, greater than that expected by dilution effect. The membrane is stable in the 350°-400 °C temperature range with a high separation factor between H2 and N2, but at 450 °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 NH_x species which inhibit hydrogen diffusion in the intergrain boundary region, as suggested also from literature data.