As for the reducing/oxidating processes, our computational result match the experimental values even regarding the timescale of the events. XAS combined to EELS analysis demonstrate the metallic nature of the 7 nm NP after 30 min at -0.8 VRHE. After 1 h the EXAFS of these particles retrieves a coordination number for Cu of 8.1 ± 1.6. As EXAFS corresponds to an average of bulk and surface atoms, the observation means that under steady-state conditions Cu catalyst contains a large fraction of undercoordinated sites. This corresponds to our calculation on the equivalent system for which we find the average coordination number is 9.08 and complete oxygen depletion in 7.3 h. In addition, the small Cu structures (7nm) fully reoxidize after 5 h of air exposure 6 while the larger structures form Cu@Cu2O shell structures (2 nm oxide shell). This also agree with our computed reoxidation shell of 2.1 nm. EXAFS analysis show that the removal of oxygens ends up with materials that contain a very large amount of disorder. Electrocatalytic experiments show that the disordered grains were responsible for the undercoordinated sites active in CO2RR. In air exposure experiments these defects can react with O2 leading to the spontaneous incorporation of oxygen in the lattice. In our simulations, once O2 is split, the penetration of O in these disordered layers is rather fast, with the oxidation layer reaching saturation within 0.4 ns.