Highly active catalytic clusters are observed during the reaction mechanism study of the copper cluster Cux (x = 4-15) catalyzed N2O + CO → N2 + CO2 reaction. It was shown that N2O adsorbs on the copper cluster without an activation barrier and the nitrogen-oxygen bond is broken during the next step. The analysis of the chemical bonding showed that the oxide ion formation is an important driving force of the reaction. Among the different clusters Cu12 was the most active as the nitrogen-oxygen bond is cleaved without an energy barrier, while the nitrogen molecule is eliminated. It was shown that the resulting copper oxide cluster is reduced easily with carbon-monoxide. The elimination of the thus formed carbon-dioxide is thermodynamically a highly favored process, even at low temperature. Thus, Cu12 cluster is a potentially highly active catalyst at ambient condition for the N2O + CO → N2 + CO2 reaction.