Reaction probability for exchange of hydrogen isotopes on palladium.

Abstract

Surface conductances measured for the diffusion of hydrogen in palladium have been used to calculate the probability, as a function of temperature, pressure, and hydrogen isotope, that a H, D, or T atom will enter bulk Pd as a result of a collision of a ${\mathrm{H}}_{2}$, ${\mathrm{D}}_{2}$, or ${\mathrm{T}}_{2}$ molecule with the Pd surface. This low probability is attributed to hydrogen penetrating a surface hydride phase under the driving force of the chemical potential of the gas consistent with the process having the exchange coefficient of the bulk material. These results were in good agreement with published experimental observations near 300 K and 100 kPa hydrogen pressure for D-H and H-D exchange. These exchange-probability calculations predict an isotope effect that inverts at elevated temperatures and, with decreasing temperature and increasing pressure, a marked decrease in the exchange probability.