中性团簇(CeO_2)_m(m=1～3)活化甲烷C—H的密度泛函理论计算

Density Functional Theory Studies on the C—H Bond Activation of Methane by(CeO_2)_m( m =1—3)

采用密度泛函理论（DFT）计算了CH4在电中性（Ce O2）m（m=1~3）团簇上的活化情况,并对其机理进行了探讨.计算结果表明,甲烷C—H键在团簇上的活化为亲核加成模式,电子由团簇流向甲烷C—H反键轨道,使甲烷C—H键削弱而得以活化,反应的过渡态为四中心结构.团簇的桥氧位活化甲烷C—H键的活性大于端氧位,而三重桥氧位的活性高于二重桥氧位.团簇中作用位点Ce和O原子的电荷布居与其活化甲烷C—H的能力密切相关.溶剂的存在不仅降低了甲烷C—H活化自由能垒,而且使与甲烷作用的团簇各位点的活性差异缩小.

Although the rare earth oxide Ce O2-based nano-catalysts have exhibited good performances for the activation of C—H of methane at low temperatures,the nature of the active sites and the C—H activation mechanisms are not clear. In this work,we employed the density functional theory（ DFT） method to investigate the activation of C—H of CH4 and its mechanism at the electroneutral clusters（ Ce O2）m（ m = 1—3）. The results show that the activation of C—H of methane on the clusters obeys the nucleophilic addition modes with the tetra-center structured transition state,in which the electrons are transferred from the clusters to the antibonding orbital of CH4,then weakening and activating the C—H of methane. The bridge oxygen sites of the clusters display the higher activity toward the C—H of methane than the terminal oxygen sites,and the threefold bridge sites show the greater activity for C—H activation of methane than the two-fold bridge sites. The charge population of the involved Ce and O atoms in the clusters is closely correlated to their ability toward the C—H activation of methane. In addition,not only decreases the solvation of the clusters the energy barrier for C—H activation of methane,but also makes the activity difference between the active sites of the clusters for C—H activation of methane be smaller.