From the Monte Carlo run we selected a total of 31 oxidized configurations spanning a range from 383 to 896 atoms. Due to the difficulties associated with calculating coverage for configurations including subsurface and core oxygen atoms, we refer to our systems using the concept of oxygen ratio (N O /N Pt ). When needed, the coverage was estimated by using a technique involving the alpha shape to find both oxygen and platinum atoms at the outer shell of the nanoparticles (Supporting Information). For all our oxidized nanoparticles the Gibbs free energy per oxygen atom was calculated using Equation .