N2O和CO在Ga/ZSM-5上吸附和反应机理的密度泛函研究

摘要

A 3T-cluster model has been used to mimic the activation site of gallium ion-exchanged ZSM-5 zeolites (Ga/ZSM-5),then the geometries and energetics of all stationary points including adsorption complexes,reactants,intermediates,transition states,and products on the pathway of adsorption and reaction of N2O and CO on Ga/ZSM-5 were calculated by using UB3LYP and UM06-2x methods with 6-31+G?basis set for exploring the relevant mechanisms.Calculated results show that the competitive adsorption between η1-N and η1-O modes of N2O and η1-C mode of CO exists. Although the cyclic reaction between previously adsorbed N2O and incoming CO on Ga/ZSM-5 has large exothermicity of 348.1 kJ·mol-1,the reaction is difficult to take place at room temperature,since the turnover frequency (TOF) determining transition state (TDTS) (the TS for the decomposition of N2O) has too positive Gibbs free energy of 35.5 kJ·mol-1relative to the entrance channel of the reaction,and the cyclic reaction has a very low TOF of 23. 5 s-1. These calculated results agree well with the previously experimental observations.%采用3T簇模型来模拟Ga/ZSM-5分子筛的镓离子活性位,用密度泛函理论方法UB3LYP及UM06-2x在6-31+G?基组水平上,研究N2O与CO在镓离子交换分子筛Ga/ZSM-5上吸附、催化的反应路径中各驻点的结构和能量学以探讨有关机理.结果表明,当N2O和CO同时存在于Ga/ZSM-5上时,存在η1-N和η1-C与η1-O模式之间的竞争吸附;若N2O和CO先后分别吸附在Ga/ZSM-5和GaO/ZSM-5上则构成催化循环.因循环反应的决速过渡态(N2O的分解步过渡态)相对于反应的入口通道有太正的Gibbs自由能( +35.5 kJ·mol-1),虽然循环总放热为348.1 kJ·mol-1,但常温下整个催化循环的TOF仅为23.50 s-1,因此N2O的催化分解及还原在常温常压下难以发生.计算结果与实验事实一致.