A DFT study on the mechanism of NO decomposition catalyzed by short-distance Cu(I) pairs in Cu-ZSM-5

The complete NO decomposition catalyzed by short-distance Cu+ pairs in Cu-ZSM-5 was studied by means of DFT calculations. After adsorption of two NO molecules, an hyponitrite species is formed. Further decomposition of hyponitrite occurs with activation energies ranging from about 4 to 24 kcal mol−1, depending on the initial geometry of the substrate-catalyst complex. An oxidized form of the catalyst, [Cu[sbnd]O[sbnd]Cu]2+ and a copper-coordinating N2O molecule are obtained. Further N2O decomposition may occur with oxygen transfer from N2O to [Cu[sbnd]O[sbnd]Cu]2+ and formation of N2 and O2, both adsorbed on the catalyst. Three different kinds of transition states were identified for the latter step, which appears to be rate-determining due to activation energies ranging from 39–40, to 44–45, and to 50–52 kcal mol−1, respectively. After this, N2 desorption occurs easily, whereas O2 desorption is endothermic (from 28.8 to 36.5 kcal mol−1), the highest value being associated to reductive O2 desorption from a peroxide-like complex. It turned out that the best way for N2O elimination is the direct, spin-forbidden decomposition on a reduced Cu+⋯Cu+ pair, with formation of [Cu[sbnd]O[sbnd]Cu]2+ and N2, as already suggested in the literature. The problem of how the reduced catalyst may be regenerated is left open. © 2017 Elsevier B.V.