The present computational study investigates the mechanism of NO decomposition catalyzed by Cu-ZSM-5. It was considered that the active site could be of two kinds: (i) pairs of Cu+ ions located at opposite sides of the ten-membered rings of the zeolite, in the region at the intersection of the linear and sinusoidal channels; (ii) isolated Cu+ ions. DFT calculations were performed by using the B3LYP functional on cluster models suited to reproduce the geometrical constraints of the ZSM-5 structure. It was shown that both the Cu+-pair mechanism and that catalyzed by a single-Cu+ site proceed through the formation of N2O as a reaction intermediate, which further reacts with the oxygen atom of a ZCu-O-CuZ or ZCuO unit (Z = zeolite) to give O-2 + N-2. The latter step displayed a larger activation energy in the Cu+-pair mechanism, which is therefore unfavored although the pathway associated to Cu+-pairs occurs at lower energy. than that associated to a single-Cu+ site. Such a large activation energy is associated to the stability of the ZCu-O-CuZ fragment. A quantitative analysis, performed by means of the energetic span model of Kozuch and Shaik, reinforced the just outlined observations. (C) 2012 Elsevier B.V. All rights reserved.