Ab initio study on the hydrogen desorption from MH-NH3 (M = Li, Na, K) hydrogen storage systems
JChemPhys_134_124515.pdf 1.42 MB
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_2H_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_2H_2 takes larger negative value in order of Li, Na, and K. We have also performed molecular dynamics simulations on the M_2H_2-NH_3 system and succeeded to reproduce the H_2 desorption from the reaction of Na_2H_2 with NH_3.
Journal of Chemical Physics
American Institute of Physics
Copyright (c) 2011 American Institute of Physics.