The Kinetics of H2-D2 Exchange over Pd, Cu, and PdCu Surfaces

PdCu alloy membranes are promising candidates for hydrogen separation from H2S-containing gas mixtures. Dissociative H-2 adsorption and associative desorption are important steps in the process of hydrogen transport across PdCu membranes; however, the kinetics of H-2 adsorption and desorption on PdCu surfaces are not well understood. In this work, the kinetics and energetics of H-2 adsorption and desorption on Pd, Cu, and PdCu surfaces are investigated by microkinetic analysis of H-2-D-2 exchange (H-2 + D-2 -> 2HD) over fixed beds of Pd, Cu, Pd70Cu30, and Pd47Cu53 foils at near-ambient pressure and temperatures in the range 300-900 K. The rate of H-2-D-2 exchange over Cu, which is the least active H-2-D-2 exchange catalyst used in this study, is limited by the rate of H-2 adsorption due to the large activation barrier to dissociative H-2 adsorption on Cu (Delta E-ads(double dagger) = 0.54 +/- 0.06 eV). The Cu content of the PdCu alloys and the crystal structures of the Pd-hydride phases (alpha- and beta-PdH) and of Pd47Cu53 (body-centered cubic and face-centered cubic (FCC)) all have a significant impact on the kinetics of H-2-D-2 exchange. The activation barriers to dissociative H-2 adsorption, Delta E-ads(double dagger), on beta-Pd-hydride (0.3 +/- 0.1 eV), alpha-Pd-hydride (0.12 +/- 0.04 eV), Pd70Cu30 (0.09 +/- 0.02 eV), B2 Pd47Cu53 (0.15 +/- 0.02 eV), and FCC Pd47Cu53 (0.00 +/- 0.02 eV), are relatively small compared to the adsorption barrier on Cu. Over these surfaces, the rates of H-2-D-2 exchange are limited primarily by the rate of HD desorption. H-2-D-2 exchange activities in the desorption-limited cases decrease (FCC Pd47Cu53 > Pd70Cu30 > B2 Pd47Cu53 > beta-PdH > alpha-PdH) as the barriers to HD desorption, Delta E-des(double dagger), increase in the order FCC Pd47Cu53 (0.46 +/- 0.03 eV) < Pd70Cu30 (0.52 +/- 0.02 eV) < beta-PdH (0.63 +/- 0.03 eV)