First-principle Study Of H₂Adsorption On The Surface Of Metal Nitride

Because of the high hydrogen storage quality and certain reversibility, the M-N-H (M means the metal elements of Ⅰ-Ⅳ) of metal complexes hydrogen storage materials draws a lot of attention in recent years. Currently, it is very limited about the reaction mechanism of hydrogen storage of the M-N-H complex. In this paper, DFT(Density functional theory) is adopted to study the electronic structure, adsorption energy and the reaction mechanism of Mg3N2、Li3N before and after H2is adsorbed.The adsorption of H2on two kinds of Mg3N2(110) crystal surface is studied by first principles. Adsorption sites, adsorption energy, and electronic structure of the Mg3N2(110)/H2systems are calculated separately. It is found that H2is mainly adsorbed as chemical adsorption, on these sites the H2molecules are dissociated and the H atoms tend to the two N top respectively forming two NH. Or the H atoms tend to the same N forming one NH2. There are also some physical adsorption sites. On one of the bridge sites of Mg3N2(110) surface it is more favorable than the other sites. On this site, H atoms tend to the two N top respectively forming two NH, this process belongs to strong chemical adsorption. The interaction between H2molecule and Mg3N2(110) surface is mainly due to the overlap-hybridization among H Is, N2s, and N2p states, covalent bonds are formed between N and H atoms.The adsorption of H2on pure Mg3N2(110) crystal surface and Li-substituting-Mg3N2(110) crystal surface are comparatively studied by first principles. Adsorption sites, adsorption energy, and electronic structure of the two systems are calculated separately. There are three kinds of chemical adsorption, one type is that two H atoms are absorbed by the same N atom forming NH2. The second type is that each H atom is absorbed by two N atoms forming NH. The third type is that a H atom is absorbed a N atom while the other H atom is absorbed by a Mg atom. The adsorption energy of the substituted one is larger which means a more stable system than that of pure Mg3N2. On the2-N-b site, the interaction of NH is weaken which means it is helpful releasing H; On the7-N-t site, the interaction of NH is strengthened which means it is not helpful releasing H.The adsorption of H2on pure Li3N (001) crystal surface and Mg-substituting-Li3N (001) crystal surface are comparatively studied by first principles. Adsorption sites, adsorption energy, and electronic structure of the two systems are calculated separately. There are two kinds of chemical adsorption, one type is that two H atoms are absorbed by the same N atom forming NH2while the other type is that each H atom is absorbed by two N atoms forming NH. On the stable adsorption sites, the adsorption energy of the Mg-substituting-Li3N (001) surface is smaller, and the overlap bands of H and N move to deep level associated with band gap disappearing, which means less stability compared to the pure Li3N (001) surface.