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 AunMm (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 (ΔEads) and also calculated adsorption thermodynamics at standard conditions (298.15 K and 1 atm), i.e., internal energy of adsorption (ΔUads) and Gibbs free energy of adsorption (ΔGads). 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 = −ΔEads) 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.