High pressure hydrogen permeance of porous stainless steel coated with a thin palladium film via electroless plating

The high-pressure (100-2800 kPa) hydrogen permeance of two membranes, each composed of a thin palladium film (similar to22 mum) deposited on the oxidized surface of a porous stainless steel tubular substrate (0.2 mum grade support) has been determined over the 623-723 K temperature range. The hydrogen flux was proportional to the H-2 partial pressure in the retentate raised to an exponent of similar to0.55 for one membrane and similar to0.64 for the other, indicating that the transport of hydrogen through the composite membrane was primarily limited by bulk diffusion. Overall, the hydrogen permeance of these membranes was within a wide range of values previously reported with thin film palladium membranes of comparable thickness. The first membrane exhibited no detectable helium flux at hydrogen partial pressures less than 350 kPa for a retentate stream composed of 90% hydrogen and 10% helium. H-2/He selectivity decreased to values as low as 12, however, at total transmembrane pressure differentials as great as 2800 kPa. As the membranes were heated from 623 to 723 K under pressures of up to 2800 kPa, the permeance of each membrane remained invariant at values of similar to1.5 x 10(-4) and similar to2.9 x 10(-4) mol/(m(2) s Pa-0.5), then decreased by similar to35% when the membrane was cooled back to 623 K, indicating some degradation of the membranes under the high-pressure testing conditions. Scanning electron microscopy (SEM) analysis revealed that extremes in the palladium film thickness ranged from about 10-50 mum with palladium "fingers" extending into the pore structure anchoring the palladium layer to the support. Although surface characterization could not pinpoint the source of the degradation, intermetallic, diffusion could not be ruled out in spite of the presence of the oxide layer. (C) 2004 Elsevier B.V. All rights reserved.