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

PdCu alloy membranes are promising candidates for hydrogen separation from H 2 S-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, Pd 70 Cu 30, and Pd 47 Cu 53 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 (Δ E ads ⧧ = 0.54 ± 0.06 eV). The Cu content of the PdCu alloys and the crystal structures of the Pd-hydride phases (α- and β-PdH) and of Pd 47 Cu 53 (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, Δ E ads ⧧, on β-Pd-hydride (0.3 ± 0.1 eV), α-Pd-hydride (0.12 ± 0.04 eV), Pd 70 Cu 30 (0.09 ± 0.02 eV), B2 Pd 47 Cu 53 (0.15 ± 0.02 eV), and FCC Pd 47 Cu 53 (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 Pd 47 Cu 53 > Pd 70 Cu 30 > B2 Pd 47 Cu 53 > β-PdH > α-PdH) as the barriers to HD desorption, Δ E des ⧧, increase in the order FCC Pd 47 Cu 53 (0.46 ± 0.03 eV) < Pd 70 Cu 30 (0.52 ± 0.02 eV) < β-PdH (0.63 ± 0.03 eV) < B2 Pd 47 Cu 53 (0.67 ± 0.03 eV) < α-PdH (0.68 ± 0.06 eV). These results are significant because they demonstrate that, although the rate of H 2 adsorption on pure Cu is very low, Pd can be alloyed with as much as ∼50 mol % Cu without significantly reducing its activity for dissociative H 2 adsorption.