A first-principles study of the catalytic mechanism of the dehydriding reaction of LiNH 2 through adding Ti catalysts

Abstract

Experiments on a ball milled mixture with a 1:1 molar ratio of LiNH 2 and LiH with a small amount (1 mol %) of Ti nano , TiCl 3 and TiO nano 2 have revealed a superior catalytic effect on Li–N–H hydrogen storage materials. In the x-ray diffraction profiles, no trace of Ti nano , TiCl 3 and TiO nano 2 was found in these doped composites, by which we deduced that Ti atoms enter LiNH 2 by partial element substitution. A first-principles plane-wave pseudopotential method based on density functional theory has been used to investigate the catalytic effects of Ti catalysts on the dehydrogenating properties of LiNH 2 system. The results show that Ti substitution can reduce the dehydrogenation reaction activation energy of LiNH 2 and improve the dehydrogenating properties of LiNH 2 . Based on the analysis of the density of states and overlap populations for LiNH 2 before and after Ti substitution, it was found that the stability of the system of LiNH 2 is reduced, which originates from the increase of the valence electrons at the Fermi level ( E F ) and the decrease of the highest occupied molecular orbital (HOMO)–lowest unoccupied molecular orbital (LUMO) gap (Δ E H–L ) near E F . The catalytic effect of Ti on the dehydrogenating kinetics of LiNH 2 may be attributed to the reduction of average populations between N–H per unit bond length (nm −1 ), which leads to the reduction of the chemical bond strength of N–H.