Ag^+和ZSM-5沸石相互作用及其对NO吸附的理论研究

Theoretical Study on Interaction between Ag^+ and ZSM-5 Zeolite and Its Adsorption of NO

首次采用密度泛函理论方法和模型簇法研究了Ag+和ZSM-5沸石的相互作用以及含Ag沸石分子筛对NO的吸附作用。所有模型簇采用B3LYP混合方法对氢原子在3-21G基组水平上、对硅、铝、氧、氮原子在6-31G(d)基组水平上进行了优化和频率分析。考虑到过渡金属Ag的内层电子相对论效应比较明显和计算速度因素的影响,采用有效核势法(ECP)处理了Ag原子。Ag+与沸石分子筛模型簇的两种配位方式的结构与能量及其对NO不同吸附方式的能量差别进行了着重比较研究。研究结果表明,Ag+与沸石分子筛骨架以二配位的键合方式结合在结构上是最优的、能量上是有利的,是研究NOx分子在Ag改性沸石分子筛上优先选择的模型簇。NO在Ag沸石分子筛模型簇的四种不同吸附方式所得的吸附热表明,NO分子以端基O与Ag配位是最有可能的吸附方式。

The density functional theory and the cluster model method have been employed to investigate the interactions between Ag+ and ZSM - 5 zeolite and its adsorption of NO molecule. Full optimization and frequency analysis of all cluster models have been carried out by using B3 LYP hybrid method at 3 -21G basis level for hydrogen atoms and 6 -31G(d) basis level for silicon, aluminum, oxygen, and nitrogen atoms. The effective core potential ( ECP) method was invoked to calculate silver atom in order to take consideration of remarkable inner electron relativistic effect of silver atom and calculation speed. The structures and energy changes of two different coordination forms between Ag + and ZSM - 5 zeolite as well as different NO adsorption modes on Ag -ZSM - 5 zeolite have been comparatively studied. The calculated results showed that the optimized structure was Ag+ binding to zeolite framework with twofold coordination, which was beneficial to stability in terms of energy. The twofold coordinated model cluster may be firstly considered when the interaction between NOx and silver modified ZSM - 5 zeolite was studied. The adsorption heat of NO on Ag - ZSM - 5 model cluster with four different modes showed that the most possible adsorption mode was the terminal oxygen of NO coordination with Ag.