By a combination of theoretical modeling and XPS and SERS spectroscopic studies, it has been found that it is possible to stabilize metallic copper species under oxidizing reaction conditions by adjusting the atomicity of subnanometer copper clusters. Small Cu 5 clusters display low reactivity toward O 2 dissociation, being less susceptible to oxidation than larger Cu 8 or Cu 20 systems. However, in the presence of water this reactivity is strongly enhanced, leading to oxidized Cu 5 clusters. In that case, the interaction of Cu 5 with atomic O oxygen is weak, favoring recombination and O 2 desorption, suggesting an easier transfer of O atoms to other reactant molecules. In contrast, copper clusters of higher atomicity or nanoparticles, such as Cu 8 and Cu 20 , are easily oxidized in the presence of O 2 , leading to very stable and less reactive O atoms, resulting in low reactivity and selectivity in many oxidation reactions. Altogether, Cu 5 clusters are proposed as promising catalysts for catalytic applications where stabilization of metallic copper species is strongly required.

Sub-nanometric Cu clusters formed by endogenous reduction of Cu salts and Cu nanoparticles are active and selective catalysts for C-N, C-C, C-O, C-S, and C-P bond-forming reactions. Sub-nanometric Cu clusters have also been generated within a polymeric film and stored with full stability for months. In this way, they are ready to be used on demand and maintain high activity (TONs up to 10(4)) and selectivity for the above reactions. A potential mechanism for the formation of the sub-nanometric clusters and their electronic nature is presented.