Density Functional Study of NO Decomposition With Cu-Exchanged Zeolites

Abstract

Cu ions exchanged into zeolites like ZSM-5 show high catalytic activity for the decomposition of environmentally harmful NO to harmless N$\sb2$ and O$\sb2.$ The present work is an attempt to study this phenomenon using a first-principles quantum mechanics-based density functional method. A small cluster model is proposed and used to examine the properties of zeolite-bound Cu ions, and their interactions with various species relevant to the decomposition process. Vibrational frequencies of the adsorbed species are compared with measured frequencies in an effort to assess the models and to help interpret infrared spectroscopy results. In addition, an orbital symmetry analyses coupled with transition state and intrinsic reaction coordinate searching techniques are used to assess the plausibility of proposed reaction mechanisms, and to elicit novel insights into other likely reaction pathways. According to conventional belief, the Cu-bound N-down gem-dinitrosyl species decomposes to N$\sb2$ and O$\sb2$ through a series of steps. We, however, find evidence for a more likely pathway initiated by the formation of a short-lived and difficult to detect O-down intermediate, and have mapped out a multi-step reaction pathway. The redox mechanism proposed here has reasonable energetics and activation barriers for each individual mechanistic step, and involves two successive O-atom transfers to an isolated zeolite-bound Cu$\sp+$ center, yielding sequentially N$\sb2$O and Cu-bound O, followed by N$\sb2$ and Cu-bound O$\sb2.$ We believe that the generic redox mechanism proposed here could play a role in other non-zeolitic nitrogen oxide transformation reactions as well.