The Pt-Ge clusters were supported on a MgO(100) 4 × 4 × 3 slab composed of 6 layers. The bottom two layers of the support were fixed at the positions of the bulk, while the top four layers were allowed to relax. Periodic images in z direction were separated with ca. 15 Å of vacuum. The same ensemble of Pt 4 Ge n /MgO(100) (n = 0-4) isomers as reported in ref. 35 were used in this work. CO and O 2 molecules were adsorbed in all possible orientations on these catalysts with the global optimization toolkit PGOPT. PGOPT uses the bond length distribution algorithm (BLDA) to generate a pool of initial adsorption geometries. Transition states were located with the climbing image nudged elastic band (CI-NEB) method. It must be mentioned that in a few cases a preliminary CI-NEB run with low convergence criteria was performed followed by a tighter convergence calculation using the improved dimer method. Due to the high computational cost, we only included the 2 uppermost MgO layers and the criteria for the forces was set to 0.05 eV Å -1 . The nature of the transition states was confirmed with a vibrational analysis, making sure that they only displayed a single imaginary frequency along the reaction coordinate.