Enhanced Stability of Cu Clusters of Low Atomicity against Oxidation. Effect on the Catalytic Redox Process

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 Cu5 clusters display low reactivity toward O2 dissociation, being less susceptible to oxidation than larger Cu8 or Cu20 systems. However, in the presence of water this reactivity is strongly enhanced, leading to oxidized Cu5 clusters. In that case, the interaction of Cu5 with atomic O oxygen is weak, favoring recombination and O2 desorption, suggesting an easier transfer of O atoms to other reactant molecules. In contrast, copper clusters of higher atomicity or nanoparticles, such as Cu8 and Cu20, are easily oxidized in the presence of O2, leading to very stable and less reactive O atoms, resulting in low reactivity and selectivity in many oxidation reactions. Altogether, Cu5 clusters are proposed as promising catalysts for catalytic applications where stabilization of metallic copper species is strongly required.