O2 Activation by Au5 Clusters Stabilized on Clean and Electron-Rich MgO Stepped Surfaces

We have studied with DFT supercell calculations the role of the charge and of the MgO Support in the activation of O-2 by a Au-5 cluster. We considered gas-phase Au-5 and Au-5(-) clusters and Au-5 deposited on a stepped MgO surface and on the same surface in the presence of a specific defect, a (H+)(e(-)) center. On this center, Au-5 forms a stable Au5- supported species. O-2 activation depends on the charge state of the gold cluster, but this is not a sufficient condition. In fact, it is only on Au-5(-)/MgO that O-2 becomes activated (superoxo species). The reason is that O-2(-) is bound at the interface between Au-5 and the oxide and benefits from the interaction with the ionic support, an effect which is missing on free Au-5(-). The activation of the O-O bond on supported anionic gold clusters does not result in a substantial reduction of the dissociation barrier which remains about 2 eV. The implications for low-temperature CO oxidation on very small Au clusters are discussed.