Enhancement of the Long-Term Permeance, Selectivity Stability, and Recoverability of Pd–Au Membranes in Coal Derived Syngas Atmospheres

Two new large scale (200 cm2) composite Pd and Pd–Au membranes were prepared and tested in an actual coal derived, but desulfurized, syngas in order to further quantify permeance loss due to species other than sulfur and to determine the nature of the contaminants. As in our previous work, membranes were tested at the National Carbon Capture Center (NCCC) in Wilsonville, Alabama. Before the syngas test, the Pd and Pd–Au membranes had thicknesses of 7 and 6.6 μm, H2 permeances at 450 °C of 17.7 and 29.2 N m3 m–2 h–1 bar–0.5, and H2/He selectivities higher than 2700 and 160 000, respectively. The two membranes produced H2 at an exceptionally high purity level of 99.8–99.9%. The selectivity of the Pd–Au membrane was stable for over 473 h in an actual syngas atmosphere at 450 °C and 12.6 bar demonstrating the high robustness and suitability of these membranes in industrial environments. However, as seen in our previous study, the two membranes showed a decrease in H2 permeance upon syngas introduction (ranging from 40 to 50%), indicating the presence of a fast surface poisoning process. The XPS analysis of a Pd coupon attached to the Pd membranes, and therefore exposed to the same syngas as the Pd membranes, revealed the presence of Mg, Na, Hg, O, C, and S on its surface. Furthermore, depth profile analysis revealed the presence of C at a concentration level of 4 atom % at a depth of 1.1 μm. Tests in pure H2 atmosphere at 450 °C after syngas exposure resulted in a moderate permeance recoverability for the pure Pd membrane (77%), while an outstanding 100% recoverability was achieved with the new Pd–Au alloy. Compared to our previous study, the Pd–Au membrane had a thicker Au layer on its surface (approximately 0.5 μm instead of 0.2 μm) that was intended to better mitigate surface poisoning by syngas contaminants. The average Au content of the layer with the Au gradient (2.2 μm in thickness) was determined by XRD and equaled 23 atom %.