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A new CO oxidation mechanism occurring at (MgO 2Mg) 2+ oxide clusters in different zeolites is modeled at both the periodic DFT (PDFT) level for Mg-phillipsite and with the cluster approach for zeolite Y and mordenite. Using PDFT, we obtained a value of reaction activation energy (15.3 kcal/mol) that is lower than the one obtained using the isolated cluster approach (35 kcal/mol). The main reason of the lower barrier is the strong CO chemisorption, which was not obtained with the cluster approach. The lower oxidation activity of the Mg zeolite forms is due to the smaller number of active singlet MgO XMg species compared to those in the other alkaline earth zeolites. © 2011 Published by Elsevier B.V.