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 H-2 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 mu m. Permeation tests have been carried out from 400 to 600 degrees C under single gas conditions. The Pd91.7Ag4.8Au3.5 membrane has shown a H-2 permeance of 4.71.10(-3) mol s(-1) m(-2) Pa-0.5 at 600 degrees 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 degrees C. The H-2 permeation properties have been determined in terms of the degree of H2S inhibition, up to 17 ppm, and subsequent H-2 flux recovery rate. Pd-Ag membranes alloyed with gold resisted 12.5 h of H2S exposure showing recovery rates of 85% and 83% for Pd91.5Ag4.7Au3.8 and Pd90.5Ag4.6Au4.9 membranes, respectively, whereas the hydrogen flux of non-gold membranes decreased below detectable values. H-2/N-2 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 H-2/N-2 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.