Very high purity H2 can be produced from mixed gas streams, such as coal gasification syngas, by the use of dense Pd-based membranes which have the unique ability to dissociate molecular H2 and allow only atomic H to permeate through its bulk. To improve Pd's mechanical properties and its resistance to minor gas-phase components that can cause deterioration of membrane performance, such as H2S, Pd is often alloyed with other metals, including Cu.
In this work, we use the H2-D2 exchange reaction to probe the H2 dissociation activity of clean and sulfur-contaminated Pd, Cu and PdCu alloy surfaces. Over fixed beds of clean, diced Pd foils, the rate of H2-D2 exchange is limited by the desorption rate of product HD; over clean Cu, the reaction is limited by the relatively large barrier to dissociative adsorption. PdCu alloys display intermediate behaviors.
To provide a basis for understanding membrane performance in environments typical of the separation application, we also present kinetic analyses of H2-D2 exchange over Pd and PdCu surfaces in the presence of H2S. Our results contribute to the design basis for next generation purification devices.