The mechanism of NO and N2O decomposition catalyzed by short-distance Cu(I) pairs in Cu-ZSM-5: A DFT study on the possible role of NO and NO2in the [Cu–O–Cu]2+active site reduction

The reactivity between NO and the oxidized form of a short-distance dinuclear Cu-ZSM-5 catalyst (ZCu2O) was investigated. ZCu2O, which contains the [Cu–O–Cu]2+bridge coordinated at the opposite T11 positions of the M6 ring of ZSM-5, is obtained by the spin-forbidden decomposition of N2O on the reduced form of the catalyst, ZCu2, with an activation energy of about 18 kcal mol−1. The further addition of NO to the [Cu–O–Cu]2+unit of ZCu2O occurs in the doublet state without activation energy and gives NO2. After desorption, which requires 39.9 kcal mol−1, NO2decomposes on a second ZCu2O site, giving NO again and O2. Three reaction paths were defined for the latter reaction, with activation energies ranging from about 30 to 42–43 kcal mol−1. Final O2desorption is endothermic. The effect of enthalpy and Gibbs free energy contributions at 298.15 and at 773 K was also shown and discussed. According to the present calculations, the [Cu–O–Cu]2+bridge can easily be broken by reaction with NO but the desorption and further decomposition of NO2are characterized by energetics which make the above mechanism slower than the spin-allowed decomposition of N2O on similar sites, already reported in the literature. The above conclusions were based on a kinetic analysis according to the Energetic Span Model.