LaNi5(111)表面结构及吸氢机理的第一性原理研究

First principle investigations on surface structure and mechanism of hydrogen adsorption of LaNi_5(111)

采用基于密度泛函理论的第一原理赝势平面波方法,对贮氢合金LaNi5及LaNi5(111)表面的电子结构进行计算,对H原子在LaNi5(111)表面吸附模型进行构型优化。结果表明:LaNi5(111)表面驰豫结构La原子向外凸出,Ni原子向里收缩,凹凸不平的表面层增加表面原子与H原子的接触面积,表面层的有效体积约增大2.3%,有利于H原子向块体内扩散;表面层有净余的0.5个电子,有利于表面层上的电子转移到H原子上;H2分子解离成两个H原子后在LaNi5(111)表面的平衡稳定结构与氢化物LaNi5H7晶体相同位置的结构极为相似;阐述H2分子在LaNi5(111)表面的解离吸附机理,其反应活化能约为0.27eV。

The electronic structures of LaNi5 hydrogen storage alloy and LaNi5（111） surface with hydrogen atoms were calculated by plane wave pseudo-potential method based on density functional theory. The results show that on the relaxed surface, La atoms protrude from surface and Ni atoms cave in, which enlarges the contacting area with H atoms. The effective volume of the surface layer is increased by 2.3%, which favors H atoms to diffuse into bulk from the surface. Calculated charge population presents negative charge on the surface, and the negative charge may transfer from the surface layer to H atoms. The stable structure by geometry optimizing after H2 molecule is dissociated into two H atoms on LaNis（111） surface presents similar structure with hydride LaNisH7 at the same position. The possible dissociation path and the mechanism of hydrogen-adsorbed are investigated with transition state method, and the activation energy of reaction is estimated as 0.27 eV.