Hydrogen permeation and stability in ultra-thin Pd-Ru supported membranes

In this paper, we report the performance of supported Pd-Ru membranes for possible applications to hydrogen purification and/or production. For this purpose, we fabricated three ultra-thin a-alumina-supported membranes by combined plating techniques: a Pd- Ag membrane (3 mu m-thick ca.) and two Pd-Ru (1.8 mu m-thick ca.). The former is set as a benchmark for comparison. The membranes were characterised using different methodologies: permeation tests, thermal treatment and SEM analysis. Preliminary leakage tests performed with nitrogen has revealed that the two Pd-Ru membranes, namely PdRu#1 and PdRu#2, show a non-ideal (non-infinite) selectivity, which is relatively low for the former (around 830 at 400 degrees C) and sufficiently high for the latter (2645 at 400 degrees C). This indicates a relevant presence of defects in the PdRu#2 membrane, differently from what observed for the Pd-Ag and PdRu#1 ones. The permeation tests show that the hydrogen permeating flux is stable up to around 550 degrees C, with an apparently unusual behaviour at higher temperatures (600 degrees C), where we observe a slightly decrease of hydrogen flux with an increase of the nitrogen one. Moreover, a peculiar bubble-shaped structure is observed in the metal layer of all membranes after usage by means of SEM image analysis. This is explained by considering the effect of the Pd-alloy grain surface energy, which tends to minimise the exposed surface area of the grain interface by creating sphere-like bubble in the lattice, similar to what occurs for soap bubbles in water. The above-mentioned decrease in hydrogen flux at 600 degrees C is explained to be caused by the bubble formation, which pushes the alloy deeper in the support pores. (C) 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.