Hydrogen transport through a selection of thin Pd-alloy membranes: Membrane stability, H2S inhibition, and flux recovery in hydrogen and simulated WGS mixtures

Resistance to H2S is important for the use of Pd membranes in fossil-fuel power stations with integrated carbon capture. In the current article we present the H2S inhibition and subsequent H-2 flux recovery of thin (similar to 2 mu m) pure Pd, Pd77Ag23, Pd85Au15 and Pd70Cu30 membranes. Upon H2S exposure in H-2/N-2 feed mixtures, all the investigated thin Pd-alloy membranes exhibited a sharp decrease of the H-2 flux depending of the H2S concentration. At 2 ppm, the H-2 flux reduces to a value equal to around 20-40% compared to the H-2 flux obtained in the absence of H2S; at 20 and 100 ppm H2S in the feed the H-2 flux only equals 10-20% of its original value. The pure Pd membrane shows an instant failure during the exposure to 100 ppm H2S. After removal of 2 ppm H2S from the feed stream the H-2 flux through the Pd, Pd85Au15 and Pd70Cu30 membranes recovers within 2 h. After the exposure to 20 ppm H2S only the H-2 flux through the pure Pd, Pd85Au15 and Pd70Cu30 membranes recovers to the same extent. As opposite to these three alloys, the H-2 flux through the Pd77Ag23 alloy is recovering much more slowly. When exposed to a simulated sour WGS mixture, the Pd85Au15 membrane experiences a loss of H-2 flux down to 10-15% of the value obtained applying 62% H-2 in N-2 as feed. The extent of H2S poisoning is thus of the same magnitude in WGS mixtures and H-2/N-2 mixtures. No sign of unselective transport is observed for this membrane after 100 h of operation in a sour WGS mixture. Post-process characterisation show signs of surface roughening combined with grain growth, whilst multiple small pits are found primarily in the grain boundaries of the membrane feed surface after the long-term operation. This study demonstrates that thin Pd-Au and Pd-Cu membranes survive sour atmospheres, but at a cost of a significant loss of H-2 flux. (c) 2012 Elsevier B.V. All rights reserved.