H₂-D₂ Exchange Kinetics in the Presence of H₂S over Pd₄S, Pd₇₀Cu₃₀, and Pd₄₇Cu₅₃ Surfaces

Abstract

H 2 S retards H 2 transport across Pd membranes used for H 2 separation by forming a Pd 4 S scale that has both low H 2 dissociation activity and low H-atom permeability. As hydrogen purification membranes, alloys such as Pd 70 Cu 30 and Pd 47 Cu 53 are attractive alternatives to pure Pd due to their resistance to formation of bulk sulfides. Nonetheless, exposure to H 2 S can decrease the permeability of Pd 70 Cu 30 and Pd 47 Cu 53 membranes, presumably by poisoning the catalytic activity of their surfaces for H 2 dissociation. In this work, the effect of H 2 S on H 2 dissociation on Pd 4 S, Pd 70 Cu 30 , and Pd 47 Cu 53 surfaces is investigated by microkinetic analysis of H 2 -D 2 exchange kinetics over Pd 4 S, Pd 70 Cu 30 , and Pd 47 Cu 53 surfaces in the presence of varying concentrations of H 2 S at nearambient H 2 +D 2 pressure. We show that the rate of H 2 -D 2 exchange over all three surfaces is significantly suppressed when H 2 S is added to H 2 /D 2 /Ar reactant mixtures and that the suppression of H 2 -D 2 exchange increases with increasing H 2 S concentration. Microkinetic analysis of the H 2 -D 2 exchange kinetics reveals that H 2 S decreases the rate of H 2 -D 2 exchange by two distinct mechanisms: (1) increasing the intrinsic barrier to H 2 dissociation and (2) blocking H 2 dissociation sites.

Estimating Bulk-Composition-Dependent H2 Adsorption Energies on CuxPd1–x Alloy (111) Surfaces

“…We neglect the B2 phase in this work. We expect that the fcc(111) orientation is predominant at the surface of the polycrystalline CSAF used in the experimental portion of this study . Hence, we focus our modeling on the basis sites in an fcc(111) surface.…”

“…To illustrate this method, we look specifically at the CuPd system that has been well studied experimentally due to its application as an extremely selective separator of H 2 gas from syngas streams. − We prepared a composition spread alloy film (CSAF) mapping out the CuPd bulk composition space and determined the adsorption energy of H 2 as a function of bulk composition through analysis of H 2 –D 2 exchange kinetics. We then compare our computationally estimated bulk-composition-dependent adsorption energies with these experimental results.…”

Effect of Au addition on hydrogen permeation and the resistance to H2S on Pd-Ag alloy membranes

“…Post-characterization based on XRD, XPS and SEM analysis was used to investigate the effect of the H 2 S exposure on the membrane after all the tests were completed. For the M2 (Pd 96.1 Ag 3.9 ) membrane exposed to 1 ppm of H 2 S, a decrease of 38% in the H 2 permeation rate was observed, caused by the interaction of H 2 S with active sites on the membrane surface blocking the H 2 dissociation sites [18], [35], [36], [37]. After the initial decrease, no further significant decrease in the flux was measured during the exposure.…”

H2–D2 Exchange Activity and Electronic Structure of AgxPd1–x Alloy Catalysts Spanning Composition Space

“…The transport of H 2 through Pd-based membranes involves three principal steps: dissociative adsorption of H 2 on the upstream side of the membrane surface into H atoms, diffusion of H atoms through the bulk, and finally, associative desorption of H 2 from the downstream surface. , This transport process is known to be limited by the bulk diffusion of H atoms, as manifested by its half-order dependence on H 2 pressure, P H2 . Decreasing the membrane thickness increases the order of the H 2 pressure dependence and increases the permeability of Ag–Pd alloys. ,, For ultrathin Ag–Pd alloy films with thicknesses <500 nm, the H 2 transport was reported to be first-order in P H2 over the temperature range 373 K – 523 K .…”