Density Functional Theory Molecular Cluster Study of Copper Interaction with Nitric Oxide Dimer in Cu−ZSM-5 Catalysts

Abstract

The various quantum chemical models of catalytic active site in Cu−ZSM-5 zeolites are analyzed. The density functional theory (DFT) is used to calculate the electronic structure of molecular cluster (HO)3Al−O−Cu−O−Cu modeling the catalytic active site in Cu−ZSM-5 zeolites and study the interaction and decomposition of NO. It is assumed that the rate-determining stage of the low-temperature selective catalytic reduction of NO is the formation of the π-radical (N2O2)- on electron donor sites of Cu−ZSM-5 catalysts. This is in good agreement with the high electron affinity of the molecular dimer ONNO (Ea = −1.5 eV) and is confirmed by the experimental data on the formation of surface anion π-radical (N2O2)- on electron donor sites of supported organo−zirconium surface complex. The DFT calculated electronic structure and excitation energy spectra for the model system (HO)3Al−O−Cu−O−Cu show that it is a satisfactory model for description of experimental UV−vis spectra of Cu−ZSM-5, containing (−O−Cu−O−Cu−) chain structures in the zeolite channels. The calculated reaction energy profile of ONNO adsorption and decomposition on the model catalytic active site shows the possibility of the low-temperature decomposition of dimer (NO)2 with low activation energy and the important role of copper oxide chains (−O−Cu−O−Cu−) in the channels of Cu−ZSM-5 zeolite during selective reduction of NO.