电场诱导(MgO)12储氢的从头计算研究

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 eV分别提高到场强为0.01 a.u.和0.02 a.u.时的0.12/0.11 eV 和0.20/0.26 eV. 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%.

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

(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 cation, 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%.