Isotope effects on hydrogen absorption by Pd-4at.%Pt alloy

Isotope effects on hydrogen absorption were investigated for a Pd-4at.%Pt alloy by using a high vacuum microbalance. The absorption kinetics were well explained by a model assuming comparative contributions of the dissociative adsorption and associative desorption on the surface, and the diffusion into the bulk. The activation energies for adsorption were determined to be 29.1 and 32.8 kJmol^-1(H₂, D₂) for protium and deuterium, respectively. The activation energies for desorption were 48.1 kJmol^-1(H₂) and 49.0 kJmol^-1(D₂). Accordingly, the heat of absorption was evaluated to be -19.0 kJmol^-1(H₂) for protium and -16.2 kJmol^-1(D₂) for deuterium. The activation energies for diffusion were determined to be 28.7 kJmol^-1(H, D) for both protium and deuterium, but the frequency factor for deuterium was about 1.5 times greater than that for protium.