电场诱导(MgO)_(12)储氢的从头计算研究

The electric field effect on the hydrogen storage of (MgO)_(12) by ab intio calculations

管状(MgO)12是(MgO)n的幻数团簇,非常稳定.为研究电场对其储氢性能的影响,本文在B3LYP/6-31G**水平上研究了电场中H2在(MgO)12管状结构上的吸附性质.结果表明(MgO)12能承受强电场而保持管状结构并被极化,其偶极矩增大为场强0.01 a.u.和0.02 a.u.时的9.21和19.39 deb(1 deb=3.33564×10-30C·m).H2能稳定吸附在单个Mg/O原子上.无电场时H2在Mg上为侧位吸附,而在O上为端位吸附;电场中,H2在Mg和O上均为端位吸附,且其分子取向沿外电场方向.由于(MgO)12及H2均被电场极化,因此H2在(MgO)12部分位置上的吸附强度显著提高.H2在3配位的Mg/O上的吸附能由无电场时0.08/0.06 e V分别提高到场强为0.01 a.u.和0.02 a.u.时的0.12/0.11 e V和0.20/0.26 e V.电子结构分析表明H2吸附在Mg原子上时,向团簇转移电荷,电场极化效应是其吸附能较无电场时增大的主要原因.吸附在O原子上时,一方面由于O阴离子极化效应更强;另一方面,H2从(MgO)12得到电荷,其价轨道与团簇价轨道重叠形成化学键,因此电场效应更显著.电场中(MgO)12最多能吸附16个H2,相应的质量密度为6.25 wt%.

（MgO）12 in a tube structure is one of the magic number clusters of （MgO）n and exhibits particular stability. To study the electric field effect on the hydrogen storage properties of （MgO）12, the H2 adsorption behavior on the surface of the tube （MgO）12 in an external electric field is explored at the level of B3LY/6-31G＊＊. In the external electric field, the （MgO）12 keeps the frame of tube structure but with little distortion, implying that the （MgO）12 cluster can sustain the strong electric field for hydrogen storage. The NBO analysis also indicates that （MgO）12 is polarized by the external electric field; and its dipole momenta increase to 9.21 and 19.39 Debye at the field intensities of 0.01 and 0.02 a.u., respectively. Without the external electric field, H2 can be adsorbed on Mg atoms in the end on modes, while on O atoms in the top on modes. When the external electric field is applied, whether H2 is adsorbed on Mg or O atoms, the stable adsorption structures are all top on modes and the molecular orientation of H2 is turned to the direction of the external electric field. Because （MgO）12 and H2 are effectively polarized by the external electric field, the adsorption strength of H2 on some adsorption sites are enhanced remarkably. The adsorption energies of H2 on the three-coordinated Mg/O are promoted from 0.08/0.06 eV in free field to 0.12/0.11 eV and 0.20/0.26 eV at field intensities of 0.01 a.u. and 0.02 a.u., respectively. Electronic structure analysis reveals that when H2 is adsorbed on Mg atoms, this process denotes charges moving to the cluster, and the improvement of adsorption interaction of H2 on Mg atoms is mainly due to the polarization effect. While the adsorption on O atoms, on the one hand implies the polarization effect of O anion is stronger than that of Mg cations, on the other hand, H2 receives charges from （MgO）12 and its valence orbitals also take part in the bonding with the valence orbitals of the cluster. Thus the structures of H2 adsorbed on O atoms are more stable. In an external electric field, （MgO）12 can adsorb sixteen H2 molecules at most, and the corresponding mass density of hydrogen storage reaches 6.25wt%.