As depicted in Figure , CO(g) attack to O 2 * always occurs from the C end. As CO approaches the molecules, the O-O bond starts activating until the TS is reached. It should be remarked that the TS is effectively achieved only under certain angles of attack (∠O-C-O* between 120º and 125º). In the TS, CO(g) is found at a distance of ~1.70 Å from O 2 . The CO internal bond is almost unchanged while O-O is activated to ~1.68 Å. After the energy barrier is successfully overcome, CO 2 is directly ejected from the surface without the formation of an intermediate state (IM). The corresponding energy barriers for this route are rather small; Pt 4 has a barrier of 0.25 eV, whereas in the nanoalloys it lies in the range of 0.30-0.60 eV. In contrast, in the most likely route, the CO(g) + 2 ✕ O* route (Figure ), the barriers on Pt 4 increase up to 0.80 eV. Pt 4 Ge (second step) and Pt 4 Ge 4 (first step) also show high barriers of 1.0 eV, whereas in the clusters with medium Ge content they remain below 0.60 eV. All steps are found to be exothermic. These results suggest that Pt 4 Ge 2 and Pt 4 Ge 3 are the most active compositions. Nonetheless, as long as there is predissociated O*, i.e., CO poisoning is avoided, the reaction will move forward in all cases and thus, the ER channel will proceed similarly regardless of the PtGe composition.