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

本文作者开发了一种分离氢同位素的新型气相色谱法。它可以在室温下用任何替代气体操作，使用由%Pd的Pt-8粉末制备的柱。然而，在较为温和的操作条件下，详细研究同位素对氢吸收和解吸的影响对于提高分离效率是很重要的。本研究项目的目的是了解氢的吸收和解吸机理，并评估动力学和热力学同位素效应。本项目考虑的材料为pt基合金：Pt-Pd和Pt-Ni合金。动力学用真空微天平测量，热力学性能用常规恒体积仪测定合金的PCT曲线。结果表明，氢的吸收和解吸速率主要由表面吸附和解吸决定，而在整体中，两者的作用是相等的。然后用扩散方程描述动力学为<<数值公式>>本研究得到的所有反应曲线都可以用该动力学模型很好地再现。动力学分析表明，钯的扩散速度并不快，比纯钯的扩散速度慢约100倍。分析表明，质子在表面的吸附速率约为氘的1.5倍，而质子在表面的解吸速率约为氘的1/2。在室温下，Pd-4at%Pt合金中氘在体内的迁移速度比质子快2倍左右。该合金表现出较大的热力学同位素效应：在室温下，由平衡压力之比P_<D_2>/P_<H_2>定义的同位素效应约为5。本研究表明，在动力学同位素效应相对较小的情况下，利用热力学同位素效应较大的材料制备分离柱可以大大提高分离效率。然而，在这种情况下，本研究还表明，柱的粉末颗粒应该足够小，以确保平衡条件。这些结论可以作为改进新开发的气相色谱仪在室温下使用任何替代气体进行氢同位素分离的指导原则。少

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