5 - Recent progress on sulfur-resistant palladium membranes

Abstract

Palladium-based membranes have been proposed as potential solutions for H2 separation due to their exclusive H2 selectivity over other gas species. Over the past 2decades, Pd membranes of either pure Pd metal or various Pd alloys have been extensively investigated including fabrication method innovations, membrane stability improvements, H2 permeation optimization, and membrane reactor designs. High-quality Pd-based membranes have been successfully fabricated on various porous supports and the membrane products have been advanced from lab-scale research to the scale of pilot plant testing under practical conditions for H2 separation. In theory, a defect-free Pd membrane is the ideal candidate for achieving exclusive H2 selectivity due to the solution-diffusion mechanism. In practice, successful commissioning of H2-selective metal membranes needs to meet chemical and structural stability requirements when membranes are exposed to poisonous species like hydrogen sulfide, which is a typical impurity in syngas from coal gasification or from steam methane reforming. H2S can impose severe negative impacts on H2 permeation performance of Pd membranes. Pure Pd membranes can be rapidly poisoned after exposure to gas mixtures with H2S concentrations as low as parts-per-million (ppm) with permanent damage to membrane structures. A comprehensive understanding of poisoning mechanisms and identification of sulfur-resistant membrane materials are critical in promoting Pd membranes for large-scale industrial H2 separations. This chapter summarizes the development progress of sulfur-resistant Pd membranes with a focus on sulfur poisoning mechanism and its impact on H2 adsorption and permeation as well as the recent achievements on identifying and developing sulfur-resistant binary/ternary Pd alloy membranes. This chapter concludes with discussions on potential challenges of implementing Pd membranes for industrial-scale H2 separations.