A catalytic aspect, 'surface catalytic NO reactions assisted by gas phase CO molecules' on a Co2+ -ensemble/gamma-Al2O3 catalyst, was elucidated by successive density functional theory (DFT) calculations on a Co2+ -dimer/gamma-Al2O3 model surface. Unique cis-(NO)(2) dimeric species formed due to a specific alignment of the unoccupied orbitals on the Co2+ -dimer, whose vibrational frequencies were almost identical to those for conventional gem-dinitrosyl species. The cis-(NO)(2) species showed much higher reactivity with CO than the gem-dinitrosyl species, through the 2 pi*-2 pi* bonding between the two NO adsorbates. Detailed DFT calculations for the reaction pathways indicated an Eley-Rideal mechanism with an unusually large reactive cross-section. (c) 2007 Published by Elsevier B.V.