First-principle density functional theory (DFT) calculations on the electronic state and structure of a [CO2+](2)/gamma-Al2O3 model catalyst have been performed in relation to catalysis for unique NO-CO reactions on a Coll ensemble/gamma-Al2O3 catalyst. The DFT calculations reveal that a bulk structure of gamma-Al2O3 is energetically most favorable when aluminum vacancies are evenly dispersed at octahedral sites, and that the (110) plane is exposed as a topmost layer by its neutrality. Two Co2+ ions on the (110) surface are supported adjacently to each other in a tetrahedral symmetry. The calculations also demonstrate that the vacant d orbitals of the two Coll ions are directed toward each other, which brings about an adsorbate-adsorbate interaction between two molecules which adsorb on each of the Coll ions. This may be an origin of the unique aspect of Co2+ ensemble/gamma-Al2O3 catalysis.