Meeting Program – March 2013
James B. Miller
Department of Chemical Engineering
Carnegie Mellon University
Abstract – Separation of hydrogen from mixed gas streams is a key unit operation in the generation of carbon-neutral fuels and electricity from fossil- and bio-derived feedstocks. Dense Pd membranes have received significant attention for the separation application in advanced gasification processes. Pd’s near-perfect selectivity reflects its unique interactions with H2: molecular H2 dissociates on the catalytic Pd surface to create H-atoms, which dissolve into and diffuse through the Pd bulk, to eventually recombine on the downstream side of the membrane. In practice, Pd suffers from several limitations, including high cost, structural instability, and deactivation by minor components of the mixed gas, most notably H2S. Alloying with minor components, such as Cu, can be an effective strategy for improving membrane performance.
In collaboration with scientists at the National Energy Technology Laboratory, we have combined membrane performance testing, computational modeling, and H2 dissociation activity characterization to provide fundamental understanding of the interactions of H2 and H2S with Pd and PdCu alloys. We have shown that H2S influences membrane performance by two distinct mechanisms: surface deactivation, which inhibits the dissociative adsorption of H2, and reaction with the metal to form a low-permeability sulfide scale. The mechanism that dominates depends on both alloy composition and operating conditions. Significantly, the surface of the sulfide scale is itself active for H2 dissociation. Atomistic modeling of the dissociation process provides context for this observation, showing that while the energetic barrier for H2 dissociation is higher on Pd4S than on Pd, there exist reaction trajectories with relatively low barriers that can sustain the separation sequence at acceptable rates. Microkinectic analysis of H2-D2 exchange conducted over Pd and a series of PdCu alloys, both in the presence and absence if H2S, confirms this finding and provides insight into the role of the Cu minor component in imparting S-tolerance to the alloy.
Finally, we have developed a high throughput capability to explore alloy properties over broad, continuous composition space, based on Composition Spread Alloy Film (CSAF) libraries of model separation alloys. CSAFs are thin (~100 nm) films with compositions that vary continuously across the surface of a compact (~1cm2) substrate. Using a unique multichannel microreactor for spatially resolved measurement of reaction kinetics across CSAF surfaces, we have characterized the kinetics of H2-D2 exchange across continuous Pd1-xCux and Pd1-x-yCuxAuy composition space.