The ER mechanism takes place when CO in the gas-phase attacks O 2 * or predissociated O* on the surface. Given the small size of the nanoclusters explored in this work, the variety of co-adsorption configurations is limited, and hence the ER mechanism may actually be a competitive mechanism within this size regime. Figure shows the reaction pathways for both CO(g) + O 2 * and CO(g) + 2 ✕ O* for all Pt 4 Ge n considered in this section. For the sake of clarity, we only show the results for the first CO 2 formation in both routes; the second CO 2 formation is depicted in Figure .