The specific surface area of Cu2O particle affects the reduction degree, the lower the specific surface area, the higher reduction degree. Depending on the size distribution of the Cu2O particles, the potential window for simultaneous presence of Cu2O and Cu surfaces are different. While a strong reduction potential could lead to the complete reduction of Cu2O, the sluggish kinetics of oxygen diffusion from inside to surface makes the process complete in long time (depending on the size, from minutes to hours) in agreement with experimental observations . Comparing to the most recent experiments, the evidence by grazing incidence XPS can be seen as originated by defect species for O close to the surface and for the interstitial ones, and these configurations were also found in our simulations.