MnO2上氧气第一个电子转移步骤的从头计算研究

采用裸露簇和嵌入簇模型, 对β-MnO₂ (001), (110), (111)三个晶面以及O₂在(110)晶面的单址吸附模式(Pauling和Griffths模式), 进行从头计算. 从β-MnO₂ (001), (110), (111)三个晶面的电子结构差异以及O₂在(110)晶面吸附的吸附能、几何结构、集居数以及净电荷数分析得到: (001), (110), (111)三个晶面中(110)晶面的催化活性最高, 其活性顺序为(110)＞(111)＞(001). 氧气在(110)晶面的吸附, Pauling和Griffths两种吸附模式均存在, 属于化学吸附中的离子吸附. 氧气与MnO₂固体间发生了单电子转移, 氧气得到电子被还原成O₂^-, 转移电子属于整个体系, 具有离域性.

Ab initio Study of the First Electron Transfer of O2 on MnO2 Surface

The oxygen adsorption on MnO₂ was studied in the light of quantum chemistry. The bare cluster of MnO₂ with the size of Mn₇O₁₄ was employed to evaluate the difference in the frontier molecular orbital energy, Mulliken population and net charge of MnO₂ three planes, (001), (110) and (111). The embedding cluster of Mn₃O₆＋Ca₇＋1096PCs was used to study the oxygen adsorption on MnO₂. Of three planes (001), (110) and (111) of MnO₂, the plane (110) has the highest HOMO energy. And thus, it is the easiest for electrons to effuse from the plane (110) of MnO₂ and transfer to O₂. The catalytic activity of MnO₂ to the first electron transfer of the oxygen reductive reaction (ORR) increases in the sequence of (001)＜(111)＜(110). The catalytic ability of MnO₂ to the first electron transfer of the ORR was confirmed by the calculations of adsorption energy, O—O bond length, Mulliken overlap population, and net charge on the concerned atoms after O₂ adsorption on MnO₂. The adsorption energy evolved after O₂ adsorption on MnO₂ is much larger than that evolved in reaction O₂＋e^－→O₂^-. It suggests that MnO₂ can strongly catalyze this reaction and thus more energy is evolved as it proceeds on MnO₂. The electron-transfer from MnO₂ to O₂ is a process of delocalization, that is, the one transferred electron is not only from the Mn atom at adsorption site but also from the Mn atoms of the bulk MnO₂ crystal cluster.