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 NO2 in 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 (ZCu(2)O) was investigated. ZCu(2)O, 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, ZCu(2), with an activation energy of about 18 kcal mol(-1). The further addition of NO to the [Cu-O-Cu](2+) unit of ZCu(2)O occurs in the doublet state without activation energy and gives NO2. After desorption, which requires 39.9 kcal mol(-1), NO2 decomposes on a second ZCu(2)O site, giving NO again and O-2. Three reaction paths were defined for the latter reaction, with activation energies ranging from about 30 to 42-43 kcal mol(-1). Final O-2 desorption is endothermic. The effect of enthalpy and Gibbs free energy contributions at 298.15 and at 7731( 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 NO2 are 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. (C) 2018 Elsevier Inc. All rights reserved.