(574c) Controlling Sulfur Corrosion of Pd-Cu Hydrogen Separation Membranes with Ultra-Thin Metal Films

Palladium-copper (Pd-Cu) alloy hydrogen separation membranes are promising alternatives to pure Pd membranes due to their higher permeability to hydrogen and their improved resistance to bulk corrosion from H₂S—a common contaminant in hydrogen-containing gas streams—relative to pure Pd. However, the catalytic surface of Pd-Cu alloys is poisoned by ppm concentrations of H₂S, which severely inhibits hydrogen transport across the membrane. In this talk, I will discuss our efforts to engineer sulfur-tolerant catalytic coatings for Pd-Cu alloy membranes. Thin metal (Pd, Mo, Ni, Co, Cr) films with thicknesses ranging from ~monolayer up to 1000 nm thickness were deposited onto Pd-Cu substrates. The composite metal/Pd-Cu membranes were then exposed to 1000 ppm H₂S/10% He/balance H₂ in a membrane permeation apparatus to evaluate the sulfur tolerance of hydrogen transport across the composite membranes. The interactions of H₂S with the composite membranes were characterized by compositional analysis of the membranes before and after permeation testing. The thickest Pd film (1000 nm) was converted to a Pd₄S-like compound, which significantly increased rates of hydrogen transport across the membrane relative to the unmodified Pd-Cu membrane during H₂S exposure. However, the reaction of the Pd film with H₂S was more complex than expected, and the H₂ flux across the composite membrane was lower than predicted. The metal element and the film thickness had a strong influence on both the interaction of the composite membranes with H₂S and the hydrogen permeation behavior of the membrane. Monolayer-thick films of all of the metals compromised the corrosion resistance of the Pd-Cu substrate and catalyzed the bulk reaction of Pd-Cu with H₂S to form two different metal-sulfide corrosion products—Cu₂S and Pd₁₃Cu₃S₇. These results demonstrate that (1) Pd₄S is a promising sulfur-tolerant coating material, and (2) the Pd-Cu substrate is not thermodynamically corrosion resistant; sulfidation of the Pd-Cu substrate is kinetically limited by a surface reaction that is catalyzed by the thin metal films.