Trends in the Reactivity of Molecular O-2 with Copper Clusters: Influence of Size and Shape

The adsorption and dissociation of molecular O-2 on copper clusters of varying size and morphology (Cur 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 lc* 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-100 adsorption 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.