O-2 activation by Au-5 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 O2 by a Au5 cluster. We considered gas-phase Au5 and Au5− clusters and Au5 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, Au5 forms a stable Au5− supported species. O2 activation depends on the charge state of the gold cluster, but this is not a sufficient condition. In fact, it is only on Au5−/MgO that O2 becomes activated (superoxo species). The reason is that O2− is bound at the interface between Au5 and the oxide and benefits from the interaction with the ionic support, an effect which is missing on free Au5−. 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