High Pressure Hydrogen Permeance of Porous Stainless Steel Coated with a Thin Palladium Film via Electroless Plating

The high-pressure (100-2500 kPa) hydrogen permeance of two membranes, each composed of a thin palladium film (~22 μm) deposited on the oxidized surface of a porous stainless steel tubular substrate (0.2 μm grade support) has been determined over the 623 – 723 K temperature range. The hydrogen flux was proportional to the H2 partial pressure in the retentate raised to an exponent of ~0.55 for one membrane and ~0.64 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. H2/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 2700 kPa, the permeance of each membrane remained invariant at values of ~1.5x10 mol/(m s Pa) and ~2.9x10 mol/(m s Pa), then decreased by ~35% when the membrane was cooled back to 623 K, indicating some degradation of the membranes under the high-pressure testing conditions. SEM analysis revealed that extremes in the palladium film thickness ranged from about 10 to 50 μm 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. INTRODUCTION There are numerous possible industrial applications for palladium-based membrane reactors, including equilibrium-limited dehydrogenation reactions, reforming reactions and reactions that produce hydrogen as a product [1, 2]. For example, hydrogen permeable membranes are an integral part of coal gasification plants being promoted by the US DOE Vision 21 program that employ the water-gas shift reaction to produce high purity hydrogen. In such cases, membrane reactors permit high conversions of carbon monoxide and steam to hydrogen and carbon dioxide to be attained via the selective removal of hydrogen from the effluent gas mixture. Palladium-based membranes continue to be considered as membrane candidates because of their catalytic activity with respect to hydrogen dissociation, high hydrogen permeability and resistance to oxidation. Some degree of success has been obtained in dealing with the potential drawbacks in the application of palladium membranes. Hydrogen embrittlement of the membrane can be reduced through the use of highly permeable alloys such as 77wt% palladium 23wt% silver. The susceptibility of palladium surfaces to poisoning by trace concentrations of