CO and C3H6 poisoning of hydrogen permeation across Pd77Ag23 alloy membranes: A comparative study with pure palladium

In this work, we compare the poisoning (inhibiting) effects of CO and C3H6 on H2 permeation across Pd77Ag23 and Pd foil membranes under identical conditions to investigate the influence of alloying Pd with Ag on its sensitivity to poisoning by these contaminants. H2 flux measurements show that H2 permeation across Pd77Ag23 is significantly more sensitive to CO poisoning, but less sensitive to C3H6 poisoning, than Pd. CO inhibits H2 permeation across both Pd and Pd77Ag23 primarily by adsorbing on the membrane surfaces—exclusively on Pd atoms—and by blocking H2 dissociative adsorption at these sites. Density functional theory calculations show that CO binds more strongly to the Pd3Ag(1 1 1) surface, both bulk-terminated and Pd-terminated, than to the Pd(1 1 1) surface. Thus, H2 permeation across Pd77Ag23 is more sensitive to CO poisoning than Pd because there is a higher coverage of CO and fewer available sites for H2 adsorption than on Pd. C3H6 poisoning of H2 permeation across Pd77Ag23 and Pd is much more complex than CO poisoning and involves decomposition of C3H6 and deposition of at least four different carbonaceous species. In contrast to the CO site blocking mechanism, C3H6 poisoning of Pd and Pd77Ag23 is primarily an electronic effect where the carbonaceous deposits modify the electronic structure of the Pd77Ag23 and Pd surfaces making them less active for H2 dissociation. Thus, even though Pd77Ag23 accumulates ∼ 10 × more carbon during C3H6 exposure than Pd, due to the higher carbon solubility of Pd77Ag23, the Pd77Ag23 surface is more resistant to deactivation from carbon deposits than Pd. This work highlights the importance of benchmarking, which is surprisingly rare in the Pd-alloy membrane literature, but is critical for understanding alloying effects.