Pd and Pd–Cu membranes: inhibition of H 2 permeation by H 2S

Abstract

Low concentrations of H 2S decreased H 2 permeation through electroless-plated Pd and Pd–Cu alloy membranes by blocking H 2 dissociation sites. At high H 2S concentrations, a sulfur surface layer did not allow H 2 to even penetrate to the Pd–Cu surface. Sulfidation of Pd and Cu formed micron size pores and caused the membrane to fail. Failure depended on H 2S concentration, not time of exposure; the H 2S concentration that completely inhibited H 2 permeation was approximately 300 ppm for Pd–Cu alloys and 100 ppm for Pd surfaces. At lower H 2S concentrations, steady state permeation was obtained. Hydrogen sulfide exposure and H 2 post-treatment caused Cu segregation to the membrane feed side due to stress relaxation. For membranes with bulk Cu concentration above 60 wt%, the phase changed from body-centered cube (bcc) to face-centered cube (fcc), and this decreased H 2 permeability. Larger cracks and pores formed during H 2S exposure at 523 K than at 723 K due to a higher degree of Pd and Cu sulfidation, as predicted by thermodynamics. The presence of H 2O also increased sulfidation by aiding H 2S dissociation and blocking H 2 dissociation sites. Cast and rolled foils, which have smoother surfaces, were less prone to surface rearrangement by H 2S. Smoother surfaces have less surface area and adsorb less sulfur per unit area compared to rougher surfaces.