Stability and Reversible Oxidation of Sub-Nanometric Cu₅ Metal Clusters: Integrated Experimental Study and Theoretical Modeling**

Sub-nanometer metal clusters have special physical and chemical properties, significantly different from those of nanoparticles. However, there is a major concern about their thermal stability and susceptibility to oxidation. In situ X-ray Absorption spectroscopy and Near Ambient Pressure X-ray Photoelectron spectroscopy results reveal that supported Cu₅ clusters are resistant to irreversible oxidation at least up to 773 K, even in the presence of 0.15 mbar of oxygen. These experimental findings can be formally described by a theoretical model which combines dispersion-corrected DFT and first principles thermochemistry revealing that most of the adsorbed O₂ molecules are transformed into superoxo and peroxo species by an interplay of collective charge transfer within the network of Cu atoms and large amplitude “breathing” motions. A chemical phase diagram for Cu oxidation states of the Cu₅-oxygen system is presented, clearly different from the already known bulk and nano-structured chemistry of Cu.