A first-principles study of the catalytic mechanism of the dehydriding reaction of LiNH2 through adding Ti catalysts

Abstract

Experiments on a ball milled mixture with a 1:1 molar ratio of LiNH2 and LiH with a small amount (1 mol %) of Tinano, TiCl3 and TiOnano2 have revealed a superior catalytic effect on Li–N–H hydrogen storage materials. In the x-ray diffraction profiles, no trace of Tinano, TiCl3 and TiOnano2 was found in these doped composites, by which we deduced that Ti atoms enter LiNH2 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 LiNH2 system. The results show that Ti substitution can reduce the dehydrogenation reaction activation energy of LiNH2 and improve the dehydrogenating properties of LiNH2. Based on the analysis of the density of states and overlap populations for LiNH2 before and after Ti substitution, it was found that the stability of the system of LiNH2 is reduced, which originates from the increase of the valence electrons at the Fermi level (EF) and the decrease of the highest occupied molecular orbital (HOMO)–lowest unoccupied molecular orbital (LUMO) gap (ΔEH–L) near EF. The catalytic effect of Ti on the dehydrogenating kinetics of LiNH2 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.