Analysis of O₂ Adsorption on Binary−Alloy Clusters of Gold:  Energetics and Correlations

We report a B3LYP density-functional theory (DFT) analysis of O₂ adsorption on 27 Au_nM_m (m, n = 0−3 and m + n = 2 or 3; M = Cu, Ag, Pd, Pt, and Na) clusters. The LANL2DZ pseudopotential and corresponding double-ζ basis set was used for heavy atoms, while a 6-311+G(3df) basis set was used for Na and O. We employed basis-set superposition error (BSSE) corrections in the electronic adsorption energies at 0 K (ΔE_ads) and also calculated adsorption thermodynamics at standard conditions (298.15 K and 1 atm), i.e., internal energy of adsorption (ΔU_ads) and Gibbs free energy of adsorption (ΔG_ads). Natural Bond Orbital (NBO) analysis showed that all the clusters donated electron density to adsorbed O₂ and we successfully predicted intuitive linear correlations between the NBO charge on adsorbed O₂, O−O bond length, and O−O stretching frequency. Although there was no clear trend in the O₂ binding energy (BE = −ΔE_ads) on pure and alloy dimers, we found the following interesting trend for trimers:  BE (MAu₂) < BE (M₃) ≤ BE (M₂Au). The alloy trimers containing only one Au atom are most reactive toward O₂ while those with two Au atoms are least reactive. These trends are discussed in the context of the ensemble effect and coulomb interactions. We found an approximate linear correlation between the O₂ BE and charge transfer to O₂ for all 27 clusters. The clusters having strongly electropositive Na atoms (e.g., Na₃ and Na₂Au) donated almost one full electron to adsorbed O₂, and the BE is maximum on these clusters. Although O₂ dissociation is likely in such cases, we have restricted this study to trends in the adsorption of molecular O₂ only. We also found an approximate linear correlation between the charge transfer and BE versus energy difference between the bare-cluster HOMO and O₂ LUMOs, which we speculate to be a fundamental descriptor of the reactivity of small clusters toward O₂. Part of the scatter in these correlations is attributed to the differences in the O₂ binding orientations on different clusters (geometric effect). Relatively higher bare-cluster HOMO energy eases the charge transfer to adsorbed O₂ and enhances the reactivity toward O₂. The Frontier Orbital Picture (FOP) is not always useful in predicting the most favorable O₂ binding site on clusters. It successfully predicted the cluster−O₂ ground-state configurations for 10 clusters, but failed for the others. Finally, the energetics of fragmentation suggest that the bare and O₂-covered clusters reported here are stable.