Sulfur Poisoning of Pd and PdCu Alloy Hydrogen Separation Membranes

Separation of high purity H₂ from coal-derived syngas is a key unit process in next generation gasification processes. Dense Pd and Pd₄₇Cu₅₃ (mol%) alloy membranes are promising candidates for separating H2 from mixed gases due to their rare combination of high permeability and high selectivity to H₂ separation. However, ppm concentrations of H₂S, a coal gasification byproduct, can severely inhibit hydrogen transport across Pd and Pd₄₇Cu₅₃ membranes in the temperature range that the membrane would need to be operated in a gasification process (~500 to ~700 K). This thesis is an experimental investigation of hydrogen transport (H₂ dissociation and H atom permeation) across Pd and Pd₄₇Cu₅₃ alloy membranes. The objective of this thesis was to understand how H2S inhibits hydrogen transport across Pd and Pd₄₇Cu₅₃ alloy membranes.

H₂S slows hydrogen transport across Pd membranes mainly by producing a Pd4S film on the surface of the Pd membrane that is about an order-of-magnitude less permeable to hydrogen than Pd. In contrast to Pd, the Pd₄₇Cu₅₃ alloy is resistant to bulk sulfidation; H₂S slows hydrogen transport across Pd₄₇Cu₅₃ by decreasing the rate of H₂ dissociation on the Pd₄₇Cu₅₃ surface. H₂S also decreases the rate of H₂ dissociation on the Pd₄S surface, but not as significantly as that on the Pd₄₇Cu₅₃ surface

At high temperatures (~900 K), the rate of hydrogen permeation through the Pd₄₇Cu₅₃ alloy is not significantly affected by 1000 ppm H₂S. It is likely that the sulfur tolerance exhibited by the Pd₄₇Cu₅₃ alloy at high temperatures is due to its resistance to bulk sulfidation. However, our results indicate that the Pd₄₇Cu₅₃ alloy is not thermodynamically resistant to sulfidation. Thin metal (Fe, Ni, Co, Cr, Mo, Pt) films on the surface of the Pd₄₇Cu₅₃ alloy catalyzed its corrosion to Pd₁₃Cu₃S₇ and Cu2S during exposure to H₂S, which suggests that sulfidation of Pd₄₇Cu₅₃ is kinetically limited by a surface reaction. This result is important because understanding why Pd₄₇Cu₅₃ alloy membranes are resistant to sulfidation may lead to the development of membrane materials that can tolerate the harsh conditions of coal gasification processes.