二金属簇PdM^+(M=Cu,Ag)活化甲烷C—H键的密度泛函研究

A DFT study on methane activation by bimetallic clusters PdM^+(M=Cu,Ag)

采用密度泛函B3LYP方法计算了PdM+(M=Cu,Ag)+CH4→H2+[Pd(μ-CH2)M]+的反应机理.结果发现Pd原子很容易插入到CH4的C—H键中,产生中间体[MPd(CH3)H]+,此中间体在反应中是势能面上的稳定构型;它进一步反应,第二个C—H键被活化,产生分子-离子复合物[H2-Pd(CH2)M]+,其活化能较高(约175 kJ.mol-1),是整个反应的速率控制步骤.通过对反应物电子结构的分析,解释了对C—H键活化能垒的高低.最后,计算结果与PtM+(M=Cu,Ag)+CH4体系进行了比较.

The reaction mechanisms of PdM^＋（M=Cu,Ag）＋CH4→H2＋[Pd（μ-CH2）M]^＋ have been studied from density functional theory（DFT） B3LYP calculations. The reaction path in which the intermediates transfer from one to another via transition states has been rationalized by their structure and vibrational frequency analysis. In the first step, the reactants give molecule-ion complex [PdMCH4]^＋,[PdMCH4]^＋ could undergo oxidative addition, cleaving a C-H bond and yielding the insertion product [MPd（CH3） H]^＋ , which is the stable structure on potential energy surfaces. The third step is a reductive elimination, leading to a molecule-ion complex [H2-Pd（μ-CH2 ）M]^- , and this step is the rate-determination step in the whole reaction path. Predicted high activation energies are understood through the electronic configurations of their reactive precursors. Finally, the results of these calculations are compared with those of PtM^＋ clusters.