KINETIC ISOTOPE EFFECTS ON HYDROGEN ABSORPTION AND DESORPTION BY Pd-ALLOYS

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The present authors have developed a new type of gas chromatography for separation of hydrogen isotopes. It can be operated near room temperature with any replacement gases by use of a column prepared from powder of Pt-8 at%Pd. It is important, however, to investigate in detail isotope effects on hydrogen absorption and desorption to improve the separation efficiency under much moderate operation conditions. The objective of the present research project is to understand mechanisms of hydrogen absorption and desorption, and to evaluate the kinetic as well as thermodynamic isotope effects.Materials considered in this program were Pt-based alloys : Pt-Pd and Pt-Ni alloys. The kinetics was measured using a vacuum microbalance and the thermodynamic properties of alloys were determined from PCT curves obtained by using a conventional constant volume apparatus.It was found that the rates of hydrogen absorption and desorption were determined by adsorption and desorption on the surface, and dif … More fusion in the bulk, where all of them contribute equally. Then the kinetics was described by a diffusion equations as<<numerical formula>>All of the reaction curves obtained in the present study could be reproduced well by this kinetic model. According to the kinetic analyses, the rate of diffusion were not so fast, about 100 times slower than that in pure Pd. The analyses showed that the rate of protium adsorption on the surface is about 1.5 times greater than that of deuterium, whereas protium desorbs from the surface about 1/2 slower than deuterium. As for the diffusion, deuterium can migrate in the bulk about 2 times faster than protium near room temperature for Pd-4at%Pt alloy. This alloy showed a large thermodynamic isotope effect : the isotope effect defined by a ratio of equilibrium pressures, P_<D_2>/P_<H_2>, was found to be about 5 around room temperature. The present research project reveled that the separation efficiency can be improved much by preparing the separation column from materials having large thermodynamic isotope effects, since the kinetic isotope effects are expected to be relative smaller. In this case, however, the present study also showed that powder particles for the column should be small enough to assure equilibrium conditions. These conclusions can be a guiding principle to improve the newly developed gas chromatograph for hydrogen isotope separation working at room temperature using any replacement gases. Less