Interaction of NO molecules with a copper-containing zeolite, a theoretical ab initio study

Ab initio cluster quantum chemical calculations at the HF level were performed for one, two and three NO molecular interactions with a Cu-containing zeolite. The three different cluster models which represent substructures of Cu-containing zeolites take into account both the different oxidation and coordination states of the Cu active site. It was shown that Al substituted at Si causes essential perturbations to the zeolite framework structure even with an extra-framework Cu cation. The increase in the coordination state of the Cu atom leads to further stabilization of the whole cluster due to its interaction with a next-neighbor basic O atom. The adsorption energy for one NO molecule on the mono- and di-coordinated Cu+ active site of the Cu+/zeolite are 7.6 and 6.4 kcaI/mol, respectively and is accompanied by a slight charge transfer from the NO molecule to the surface. However, its adsorption on a preadsorbed water-containing Cu+/zeolite or adsorption of two NO molecules on the Cu+ active site was found to be unfavorable by energetics due to some lateral interactions between them. There is no way of initiating the direct decomposition reaction from two adsorbed NO molecules to produce N-2 and O-2 since it is symmetrically forbidden and is accompanied by high energy consumption. We have suggested that an adsorption complex of three NO molecules on the Cu+ active site of the Cu+/zeolite can be considered as a possible candidate for NO decomposition into N-2 and O-2. (C) 1998 Elsevier Science B.V. All rights reserved.