Density Functional Theory Study of Transformations of Nitrogen Oxides Catalyzed by Cu-Exchanged Zeolites

A previously reported density-functional-theory-based model of NO decomposition in Cu-exchanged zeolites (Schneider, W. F.; et al. J. Phys. Chem. B 1997, 101, 4353) is extended to consider more generally the Cu-zeolite catalyzed chemistry of nitrogen oxides. The catalyst active site is considered to be an isolated, zeolite (Z)-bound Cu ion, which can exist in either a reduced (Z--Cu(I)) or an oxidized (Z--Cu(II)-O-) state. Three different cluster models are used to study the affinity of ZCu and ZCuO for gaseous molecules (e.g., NO, NO2, or N2O), the structures and vibrational spectra of the stable complexes thus formed, and the possible reactivity between active sites and gaseous species. The reduced and oxidized states are found to react with nitrogen oxides via two types of O atom transfer reactions, one in which ZCu adds an O atom to form ZCuO, and the other in which ZCuO adds an O atom to form ZCu + O2 via a dioxygen (ZCuO2) intermediate. Potential energy surfaces for several key reactions are explored, and the results combined into a mechanistic model which can be used to rationalize much of the known catalytic chemistry of nitrogen oxides on Cu zeolites.