The most likely scenario, however, is the one where CO* reacts with predissociated O* (CO* + 2 ✕ O*), as shown in Figure . A detailed inspection reveals that in the TS of this channel CO* and O* are closer than in CO* + O 2 *. The reactants are bonded on top of neighbouring atoms and oriented parallel to each other, forming a cyclic Pt-C-O-Pt(Ge) structure. The length of the "to be formed" C-O bond lies between 2.75 and 2.90 Å. Such TS structure has also been observed previously in the CO oxidation by small Pd clusters. Contrary to the previous pathway, Pt 4 has to overcome now larger barriers of ~1.0 eV in both steps. Surprisingly, on Pt 4 Ge the first step proceeds almost barrierless, although the second oxidation is highly unfavorable. On the contrary, Pt 4 Ge 2 and Pt 4 Ge 3 show the best balance between the two steps with low barriers. Moreover, in this route the first IM formation is significantly less exothermic since the reaction starts from a very stable complex. Except for Pt 4 Ge 4 (0.85 eV), the first IM formation is still slightly exothermic, while the second step is predicted to be exothermic only on Pt 4 Ge 3 and Pt 4 Ge 4 (see Figure ). Another observation of critical importance is the fact that CO 2 removal becomes highly endothermic in Pt 4 , especially in the second step, as opposed to the alloys. This suggests a tendency to get poisoned by CO 2 .