In this study data are presented which support the view that a special configuration of dimeric Cu(I) species in Cu-ZSM-5 catalysts may be the active site for NO decomposition. Such dimeric Cu(I) species, produced by self-reduction of Cu(II) during the thermal activation of Cu-ZSM-5 catalysts in He or under vacuum, are the sites where N-2 is generated from two adsorbed NO molecules, and from which O-2 desorbs from the catalyst at the steady state. It was found that these dimeric Cu(I) species adsorb N-2 irreversibly at low temperature (273-325 K). DFT calculations for cluster models obtained from the crystallographic structure of orthorhombic ZSM-5 suggested that Cu(I)-N-2-Cu(I) species may be IR-silent or have a very low extinction coefficient, as suggested by previous diffuse reflectance FT results. The isosteric heat of adsorption of N2 on Cu-ZSM-5, obtained with the Clausius-Clapeyron equation from adsorption isotherms in the temperature range of 273-325 K, is 10.3 kcal mol(-1) at a coverage of 0.4 mol N-2/mol Cu. This value is much larger than the heat of condensation of N-2 at 77 K (1.33 kcal mol(-1)) and significantly higher than the heat of adsorption of N-2 on Li-ZSM-5 (8.68 kcal mol(-1) at zero coverage) obtained with the Clausius-Clapeyron equation from adsorption isotherms in the temperature range of 196-273 K. The turnover frequency (TOF) for the NO decomposition, calculated assuming that the active sites are those titrated by the irreversible adsorbed N2 at 273 K, is independent of the copper loading and of the ZSM-5 Si/Al atomic ratio. The present model is also supported by the evidence that Cu-Y, Cu-S-1, and Cu-[Al]-MCM-41 catalysts, containing Cu(I) species that do not adsorb irreversibly N-2 at 273-325 K, were found to be inactive or scarcely active (Cu-Y) for the NO decomposition reaction under the same experimental conditions as employed for Cu-ZSM-5. (c) 2005 Elsevier Inc. All rights reserved.