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

Dense Pd membranes are receiving significant attention for hydrogen separation in advanced coal-conversion processes because of their high hydrogen permeability and their nearinfinite selectivity. However, pure Pd is susceptible to hydrogen embrittlement as a result of the expansion of the Pd lattice during β-Pd-hydride formation. Furthermore, H2S, a coal gasification byproduct, severely inhibits hydrogen permeation through pure Pd. 10 PdCu alloys have exhibited resistance to hydrogen embrittlement and improved sulfur tolerance relative to Pd. 15 Pd70Cu30 and Pd47Cu53 (mol %) are two particularly attractive PdCu alloy compositions because of their sulfur tolerance and because of their relatively high hydrogen permeability. To separateH2 frommixed gas streams, a Pd-basedmembrane must dissociatively adsorb H2 on its upstream surface and associatively desorb H2 from its downstream surface. The energetics of H2 dissociation over pure Pd and pure Cu have been well characterized; H2 dissociates on Pd without significant activation barriers, 21 whereas H2 dissociation on Cu is hindered by a very large activation barrier (ΔEads q = 0.5 0.7 eV). 32 However, the effect of alloying Pd with Cu on the energetics of the H2 dissociation reaction is not well understood. In this work, the energetics of dissociative H2 adsorption and associative desorption on Pd, Cu, Pd70Cu30, and Pd47Cu53 foil surfaces have been investigated by microkinetic analysis of H2 D2 exchange (H2 + D2 f 2HD) rates measured at near ambient pressure and temperatures in the range 300 900 K.Our results confirm the presence of a large activation barrier to dissociative adsorption of H2 on the Cu surface, ΔEads q = 0.54 eV. In contrast, barriers to H2 dissociation over Pd, Pd70Cu30, and Pd47Cu53 are relatively small. We also show that the crystal structures of the Pd phases (β-PdH and α-PdH) and of Pd47Cu53 (body-centered cubic (B2) and face-centered cubic (FCC)) have a significant impact on the kinetics of H2 D2 exchange. Our results indicate that, although pure Cu has a very low H2 dissociation activity, Pd can be alloyed with as much as∼50 mol % Cu without significantly reducing H2 dissociation rates.