Effect of Au addition on hydrogen permeation and the resistance to H₂S on Pd-Ag alloy membranes

In order to make a detailed comparison between Pd-Ag and Pd-Ag-Au membranes according to their H₂ permeation properties and sulfide resistance Au was deposited by the electroless plating (ELP) technique onto one half of Pd-Ag membranes. Membranes' thicknesses are ranged between 2.45 and 3.13 µm. Permeation tests have been carried out from 400 to 600 °C under single gas conditions. The Pd_91.7Ag_4.8Au_3.5 membrane has shown a H₂ permeance of 4.71·10⁻³ mol s⁻¹ m⁻² Pa^0.5 at 600 °C, which is one of the highest values ever reported in the literature, where the Pd-Ag-Au membranes have exhibited higher hydrogen permeation rates compared to their respective Pd-Ag membranes above 550 °C. The H₂ permeation properties have been determined in terms of the degree of H₂S inhibition, up to 17 ppm, and subsequent H₂ flux recovery rate. Pd-Ag membranes alloyed with gold resisted 12.5 h of H₂S exposure showing recovery rates of 85% and 83% for Pd_91.5Ag_4.7Au_3.8 and Pd_90.5Ag_4.6Au_4.9 membranes, respectively, whereas the hydrogen flux of non-gold membranes decreased below detectable values. H₂/N₂ ideal perm-selectivity of the Pd-Ag membrane was reduced to 18 after H₂S tests (starting from > 1308) while Pd-Ag-Au membranes showed a better resistance to sulfur with H₂/N₂ selectivity values of 793 and 121 (starting from > 4115 and > 2557 respectively). No evidence of the formation of a crystalline sulfide phase on the Pd-Ag-Au alloy membrane surfaces was found in the XRD patterns after H₂S exposure and also XPS characterization did not show important changes in the composition before and after the H₂S exposure tests. However, SEM images showed a decrease in the thickness of the Pd-Ag membrane and signs of corrosion and roughening on its surface, while gold-alloyed membranes did not show any damage.