Graduate School of Integrated Arts and Sciences, Hiroshima University, Higashi-Hiroshima, 739-8521, Japan.
J Chem Phys. 2011 Mar 28;134(12):124515. doi: 10.1063/1.3562122.
The hydrogen storage system LiH + NH(3) ↔ LiNH(2) + H(2) is one of the most promising hydrogen storage systems, where the reaction yield can be increased by replacing Li in LiH with other alkali metals (Na or K) in order of Li < Na < K. In this paper, we have studied the alkali metal M (M = Li, Na, K) dependence of the reactivity of MH with NH(3) by calculating the potential barrier of the H(2) desorption process from the reaction of an M(2)H(2) cluster with an NH(3) molecule based on the ab initio structure optimization method. We have shown that the height of the potential barrier becomes lower in order of Li, Na, and K, where the difference of the potential barrier in Li and Na is relatively smaller than that in Na and K, and this tendency is consistent with the recent experimental results. We have also shown that the H-H distance of the H(2) dimer at the transition state takes larger distance and the change of the potential energy around the transition state becomes softer in order of Li, Na, and K. There are almost no M dependence in the charge of the H atom in NH(3) before the reaction, while that of the H atom in M(2)H(2) takes larger negative value in order of Li, Na, and K. We have also performed molecular dynamics simulations on the M(2)H(2)-NH(3) system and succeeded to reproduce the H(2) desorption from the reaction of Na(2)H(2) with NH(3).
储氢系统 LiH + NH(3) ↔ LiNH(2) + H(2) 是最有前途的储氢系统之一，通过用其他碱金属（Na 或 K）替代 LiH 中的 Li 可以提高反应产率，Li、Na、K 的顺序依次递增。在本文中，我们通过基于从头算结构优化方法，计算了 M(2)H(2) 团簇与 NH(3)分子反应过程中 H(2) 脱附过程的势垒，研究了 MH 与 NH(3)反应的碱金属 M（M = Li、Na、K）依赖性。结果表明，势垒高度按 Li、Na、K 的顺序降低，Li 和 Na 之间的势垒差异相对较小，而 Na 和 K 之间的势垒差异较大，这一趋势与最近的实验结果一致。我们还表明，在过渡态时，H(2)二聚体的 H-H 距离取较大值，过渡态周围的势能变化变得更加柔和，顺序为 Li、Na、K。在反应之前，NH(3)中 H 原子的电荷几乎没有 M 依赖性，而 M(2)H(2)中 H 原子的电荷则按 Li、Na、K 的顺序取更大的负值。我们还对 M(2)H(2)-NH(3)体系进行了分子动力学模拟，并成功再现了 Na(2)H(2)与 NH(3)反应过程中的 H(2)脱附。
