Monte Carlo simulations are run until accepted additions and deletions exchange moves stabilize to the same count over the last 5 × 10 7 exchange moves. This criterion is not applied for higher pressures (1.0, 10 -2 , and 10 -5 atm), where the simulations are stopped before reaching this criterion due to low deletion acceptance rate. Throughout the study we will refer to the oxidized nanoparticles using the concept of oxygen ratio N O /N Pt instead of coverage. This choice is motivated by the difficulty of defining a proper surface area for the nanoparticle and the presence of subsurface oxygen atoms. Atomic oxygen tends to adsorb on low-index platinum surfaces on hollow sites (3-fold or 4-fold), this is highlighted in Figure where we report that for most pressures, more than 70% of the oxygen atoms are in a 3-fold coordination. We also report a low number of atomic oxygen binding on bridge sites (2-fold coordination) which does not seem to be impacted by the pressure, indicating that these are specific sites, where oxygen opportunistically binds.