Density functional theory calculations for NO adsorption and NO-CO reaction on a Co2+-dimer/gamma-Al2O3(1 1 0) model catalyst were conducted to understand two new aspects of the increases in the amount and reduction rate of adsorbed NO by gas phase CO molecules which are undetectable at the Co2+-ensemble/gamma-Al2O3 catalyst surface. Three kinds of dinitrosyl adsorbates were found and assigned to normal gem-dinitrosyl species (gem-dinitosyl I and II) and an unique cis-(NO)(2) dimeric species. The gem-dinitrosyl II with a reconstructed structure involving CO-O-surf cleavage was considered as stable species experimentally observed, which is responsible for the spectator CO-promoted NO adsorption. We also found that the cis-(NO)(2) dimeric species specific for the Co2+-ensemble structure possesses a much higher reactivity than the gem-dinitrosyl species, enabling the facile reaction with CO that is very weakly trapped at the surface. In the cis-(NO)(2) dimeric species two adsorbed NO molecules on two adjacent Co2+ sites interact with each other due to the opposed orientation of the unoccupied d orbitals of the two Co2+, resulting in easy formation of a N-N bond through the 2 pi*-2 pi* hybridization. The NO adsorbates modify and activate the surface (Co2+ sites) to make pseudo-compounds (intermediates) with CO at the Co2+ sites in such a way that CO is not detected at the surface. The potential energy surface for the NO-CO reaction is presented and the transition states and intermediates are discussed. (C) 2010 Elsevier B.V. All rights reserved.