The adsorption and dissociation of molecular O 2 on copper clusters of varying size and morphology (Cu n with n=3-8, 13, and 38 atoms) has been systematically investigated using several DFT approaches. Different modes of adsorption of molecular O 2 are found for each atomicity of the copper clusters, with their relative stability depending on the cluster morphology: bridge conformations are stabilized on planar clusters, while hollow h-100 and h-111 modes are preferentially formed on 3D clusters. O 2 adsorption energies correlate with the HOMO energy of the isolated copper clusters, but not with their atomicity. On the other hand, the degree of activation of the O-O bond, and therefore the activation energy barrier necessary to break it, depends on the charge transferred to the π* MO of O 2 , which in turn is determined by the mode of adsorption of O 2 on the metal. Cluster morphology appears then as the key factor determining the catalytic activity of copper, since the most activating h-111 and h-100adsorption modes leading to lower barriers are preferentially stabilized on 3D clusters, no matter their size.All computational levels considered, including periodic and molecular calculations, lead to similar trends and conclusions.properties of small copper clusters, in some cases combined with photoelectron spectroscopy measurements, can be found in the literature. [25][26][27][28][29] The reactivity of copper clusters, and its evolution with the number of atoms in the cluster, has been less investigated. The interaction of CO and O 2 with Cu n clusters with n ≤ 10 atoms and the dissociative chemisorption of H 2 on Cu n clusters of up to 15 atoms have been explored in detail, and a higher reactivity of copper clusters with an odd number of atoms has been reported with few exceptions. [30][31][32][33][34][35][36] As regards mechanistic studies, only the dissociation of H 2 O on Cu 7 37 and the mechanism of CO oxidation on Cu 6 and Cu 7 clusters 38 have been reported in the literature.We present now a systematic theoretical study of O 2 adsorption and dissociation on Cu n clusters with n=3-8, 13, and 38 atoms, this last model being considered here the size limit between clusters and nanoparticles. Different modes of adsorption of molecular O 2 have been considered for each atomicity of the copper clusters, and the transition states for dissociation of the O-O bond have been calculated with the objective of finding a trend with particle size in the reactivity of copper clusters.There is, however, a methodological issue in the systematic study of metal clusters of increasing size, which is mainly related to the scaling of the computational cost with the number of electrons in the system. Density functional theory (DFT) 39, 40 based methods are usually chosen because they provide reasonable accuracy at a relatively moderate computational cost. However, the cost of a DFT calculation based on atom-centered Gaussian orbitals scales with N 3 , being N the number of functions in the basis set. [41][42][43] This means that e...

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.