Thermodynamic and kinetic isotope effects on hydrogen absorption by Pd–Al2O3 pellets

Abstract

Palladium has been widely served as the separation material in the hydrogen isotope separation process on account of its unique hydrogenation characteristics. To reduce Pd pulverization effect and improve the gas flow rate, Pd–Al2O3 (Pd deposited on Al2O3) is commonly used as the separation material. It is crucial to grasp the hydrogen absorption and desorption thermodynamics and kinetic isotope effect of Pd–Al2O3 material, which favours raising the separation efficiency. In this work, the effect of PdO existing in the fresh Pd–Al2O3 on the thermodynamic character of Pd–Al2O3–H(D) system and the porous substrate effect on the hydrogen absorption kinetics of Pd–Al2O3 were investigated. It shows that PdO could totally reduce to Pd after primary hydrogen adsorption-desorption, leading to decreased hydrogen absorption capacity. However, it has no effect on the enthalpy and entropy values of Pd–Al2O3–H (D) system. The kinetic analysis of hydrogen adsorption of Pd–Al2O3 shows that the hydrogen absorption of Pd–Al2O3 could be fitted by Mample model, a special case of the JMAK model, and the interface-controlled model can be deduced. The porous substrate influences the rate of hydrogen adsorption of Pd–Al2O3. The SEM and XRD results show that after 60 hydrogen absorption-desorption cycles, the surface of Pd–Al2O3 pellet has a slight change with a pulverization rate of 3.3 wt%, although the high-strength alumina support can withstand the stress caused by Pd expansion during hydrogen absorption.