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J. Phys. Chem. C 2009, 113, 20076–20080
Equilibrium Isotope Effect for Hydrogen Absorption in Palladium Weifang Luo,* Donald F. Cowgill, and Rion A. Causey Deptartment of Hydrogen and Metallurgical Sciences, Sandia National Laboratories 7011 East AVenue, LiVermore, California 94551 ReceiVed: June 15, 2009; ReVised Manuscript ReceiVed: August 17, 2009
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 singleisotope 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, R. 1. Introduction Hydrogen isotope effects attract research attention because of its importance for both fundamental and technical reasons.1-13 The most prominent equilibrium hydrogen isotope effect is observed for the hydrogen-Pd system. The hydrogen isotope effect observed from the absorption/desorption isotherms is of interest since the isotherms of mixed isotopes deviate from those of the single isotopes, which provide useful information for understanding the mechanism of isotope exchange. One of the potential applications of the hydrogen isotope exchange is for heavy hydrogen isotope enrichment, and this requires an understanding of the mechanism and patterns of the exchange over a wide range of hydrogen concentrations in Pd, including all R, R + β, and β phases. Sieverts et al. reported absorption isotherms for H-D-Pd at the ratios of PH2/PD2 ) 1:1 and 1:3 and the isotherms in the subsequent desorption at a temperature of 373 K.14 The H/D ratios in the gas and solid phases, however, were not reported. The changing slope in these isotherms in the R-β phase regions is obvious, but no discussion was given as to its origin. Desorption isotherms at 323 K for mixed Pd hydrides, Pd(HxD1-x)y (0 < x 0.6) were reported previously.1 A simple model was proposed1 that described the dependence of the desorption plateau pressure on a linear combination of the isotopic fractions in the solid and the plateau pressures of the single-isotope-Pd systems; that is, H-Pd and D-Pd. This paper focuses on H-D exchange in the gas and solid phases during absorption. * Corresponding author. Phone: (925) 294-3729. Fax: (925) 294-3410. E-mail:
[email protected].
Isotope exchange in the H-D-Pd system takes place on the Pd surface, resulting in isotope composition variations in both the gas and solid phases before the system reaches equilibrium. The following equations describe the exchange processes:
H2 + D2 S 2HD
(1)
H2 + Ds S HD + Hs
(2)
HD + Ds S Hs + D2
(3)
Here the subscripts “g” and “s” denote the quantities in gas and solid phases, respectively, and Hg ) PH2 + (1/2)PHD, Dg ) PD2 + (1/2)PHD. In this article, KHD′ and R′ are introduced to denote the nonequilibrated quantities of PHD2/PH2PD2 and Dg/ Ds/Hg/Hs, respectively. When a system is at equilibrium, KHD′ ) KHD and R′ ) R. Previous experimental results of H-D exchange on a clean Pt surface and in β-phase Pd14 indicate that the equilibrium of eq 1 is rapidly achieved; that is, PHD2/PH2PD2 ) KHD. The exchanges in eqs 2-3 in β-Pd produce isotope composition variations in both gas and bulk Pd but do not result in a total pressure rise or fall.14 The observed equilibrium values14 are close to those reported in the literature. This confirms that R determines the equilibrium partitions of H and D between the gas and solid phases; that is, Hg, Hs, Dg, and Ds, whereas KHD determines the equilibrium partition of H and D among the three species, PH2, PHD, and PD2, in the gas phase.
10.1021/jp905614x CCC: $40.75 2009 American Chemical Society Published on Web 10/21/2009
Hydrogen Absorption in Palladium
J. Phys. Chem. C, Vol. 113, No. 46, 2009 20077
Figure 1. (a) H2-D2 absorption profiles at 323 K. The blue arrow indicates the time when the first D2 dose was introduced, and the brown arrow indicates the time the plateau region ends. Pressures of H2 (blue), HD (pink), and D2 (green) and KHD′ and R′ values (light-blue and red, the right-hand y-axes) are included. (b) An insert for the area marked by the square in panel a.
2. Experimental Section
99.9995% from Matheson Inc.) were used for the isotherm measurements. A Sieverts’ apparatus was used to monitor the hydrogen absorption. The volume of the sample chamber for exchange was ∼54 mL at room temperature. Gas pressures in the sample vessel were monitored by MKS pressure gauges (MKS Instruments Inc.). The pressure in the system was in the range of 0 to 1.5 × 105 Pa during isotherm determinations. The isotopic composition in the gas phase of the sample vessel was monitored by a residual gas analyzer, RGA-200 (Stanford Research Systems Inc.). A minimal amount of gas from the sample vessel was sent to the RGA by a flow-restricted valve as described elsewhere1 to ensure that the gas pressure in the RGA remained below 6 × 10-4 Pa, as required by the RGA and to ensure the gas composition or pressure in the sample vessel was unaffected by the RGA sampling. Noise in the RGA measurement leads to errors in the pressure data, which affects the exchange calculations. The estimated magnitude of the error for the partial pressures from the RGA is
