Equilibrium Isotope Effect for Hydrogen Absorption in Palladium

Abstract

Absorption isotherms at 323 K for the H−D−Pd system were measured by introducing H2 and D2 into Pd in sequence. The method using addition of isotopes to the system in sequence to investigate isotope exchange effects has not been previously reported. The equilibrium absorption pressure in the plateau region of the mixed-isotope system varies with the ratio of H/D in the solid phase. It lies between those of the single-isotope systems of H−Pd and D−Pd. Higher equilibrium pressures are associated with high D/H ratios in the solid phase. A model proposed previously (Luo, W.; Cowgill, D.; Causey, R.; Stewart, K. J. Phys. Chem., B 2008, 112, 8099) for mixed isotope hydride desorption, which correlates the equilibrium plateau pressure of the mixed H−D system with the fractions of D and H in the solid and the equilibrium plateau pressures of the single-isotope systems, is also successfully applied to absorption. When D2 is introduced into the H−Pd system in the plateau region, both the H−D exchange processes in the gas phase and net H (D) absorption take place. The former does not result in a total pressure change, but the latter creates a total pressure decrease. These reactions produce a D concentration increase in both the bulk Pd and the gaseous phase, as expected. Curiously, however, they also result in a counterintuitive small H concentration increase in bulk Pd and a decrease in gaseous H. Analogous results are obtained when the order of D2−H2 introduction is reversed. In the plateau region, isotope displacement does not take place. Once in the β-phase, isotope displacement does take place. The equilibrium isotope H−D partitions in the gas phase, H2, HD, and D2, are controlled by the equilibrium constant, KHD, and their equilibrium partitions among H and D between gas and bulk Pd are controlled by the separation factor, α.