First Principles Study on the Adsorption of H2 Molecules on Mg3N2 Surface

Abstract

The first principles density theory calculations have been performed to investigate different Mg3N2 surface and the corresponding properties of H2 adsorption. The calculation of surface energy present that Mg3N2(011) is the most stable surface. The result show that the H2 parallel to the surface is a favorable adsorption and the most stable structure is H2 adsorbed onto the Model Ⅱ surface, which have the lowest energy. There are three main modes of chemical adsorption: The first adsorption mode is that H2 is dissociated into two H, and each H connect with N atom respectively to form double NH. This is the best adsorption model, which mainly results from the interaction between the H 1s orbit and N 1s, 2p orbits. By the analysis of the charge distribution variation H atom and N atom lose electrons, Mg obtain electrons. The second mode, H2 dissociated partly and the two H are adsorbed onto the same N forming one NH2, forms covalent bond. From the analysis of the bond population, we conclude that the covalent bonds strengthen the structure of NH. In other words, the hydrogen desorption of NH2 is easier than NH. H2 is fully dissociated in the third mode. One H atom is adsorbed onto N forming a NH group, which is connected by covalent bond, while the other H atom is adsorbed onto Mg forming MgH, which is forming ionic bond. The reaction energy barrier show that there is no competition among the three adsorption modes. The model of forming two NH is the easiest pathway, which have the lowest reaction energy barrier of 0.848 eV. The second is that the adsorption of H2 molecules on the surface forming NH2 have the reaction energy barrier of 1.596 eV. The most unlikely adsorption model is that H2 is dissociated and forming the structure of NH+MgH, which have the reaction energy barrier of 5.495 eV. In addition, H2 also can be physically adsorbed onto Mg3N2(011) surface.