A DFT study on the mechanism of NO and N2O decomposition catalysed by Cu(I) pairs in Cu-ZSM-5: Revisited reactivity at the M6 ring

Abstract

By DFT calculations, a new coordination was found for a pair of Cu+ ions at the M6 ring of ZSM-5, with Al located at the T11 and T11’ positions. This coordination is 27 kcal mol−1 more stable than other structures previously reported in the literature. In the present structure, one of the Cu+ ions is tetracoordinated to the framework oxygens, whereas the other one is nearly pentacoordinated. After NO adsorption, Cu+ ions are split apart and do not show the typical reactivity of a Cu-pair. The bridged [Cu‒O‒Cu]2+ structure is 30 kcal mol−1 less stable than the structures where only one of the copper ions is oxidised. Because of the high degree of coordination of both copper ions, the adsorption energy of NO does not exceed 10–15 kcal mol−1. Two mechanisms for NO decomposition were defined for the present site, but when contributions to Gibbs free energy were accounted for, all intermediates and transition states turned out to be strongly destabilised at 773.15 K, with a consequent drop of the reaction rate. Since M6 rings and similar structures are widely represented within ZSM-5, the findings of the present work rule out many possibilities in the search of the actual active site for NO decomposition.