Sulfur Poisoning of Pd and PdCu Alloy Hydrogen Separation Membranes
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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.