Here, we characterize the potential energy surface of CO oxidation on Pt 4 Ge n clusters (n = 0-4) supported on MgO(100). On Pt-based catalysts, CO oxidation usually proceeds via the bimolecular LH or ER mechanisms. In LH prior to the reaction both CO and O 2 are adsorbed on the surface, that is, co-adsorption of the reactants is required. Then the reactants diffuse to adjacent sites and react with each other to form CO 2 , which is released to the environment. Besides, the ER mechanism only involves O 2 adsorption followed by a CO attack in the gas-phase. As has been confirmed in the previous section, all clusters present small oxygen dissociation barriers. Hence, CO will be able to react not only with O 2 * but also with O*; in other words, both O 2 and O are available as oxygen sources. We must highlight that all the possible reaction mechanisms were characterized by considering two different structures for each Pt 4 Ge n /MgO, the isomers with the most stable IS and FS in the O 2 dissociation shown in Figures and. Pt 4 Ge n O 2 was used to study the routes involving CO + O 2 , while Pt 4 Ge n 2O was employed for the CO + 2 ✕ O paths. Since the starting point involves two oxygen atoms in either case, two steps will be needed to complete the catalytic cycle, i.e., the formation of two CO 2 molecules.