Adsorption and Activation of O₂ on Small Gold Oxide Clusters: the Reactivity Dominated by Site-Specific Factors

We experimentally explored adsorption and activation of O₂ on small anionic clusters AuO^- containing one to five gold atoms and between one and three oxygen atoms using an instrument including a magnetron sputtering cluster source, a micro flow reactor running at low temperature, and a time-of-flight mass spectrometer. Some species, including AuO^-, one isomer of Au₂O₂^-, Au₃O^-, one isomer of Au₃O₃^-, and Au₅O₂^-, can adsorb an O₂ molecule. We theoretically explored the structures of these active species and the inert ones appearing in the experiment by combining a structure search strategy based on the genetic algorithm and the density functional theory (DFT) calculations. Impressively, all active species observed in the experiment have a -O-Au site, in which the gold atom is a dangling or a vertex atom. Each -O-Au site can strongly adsorb one O₂ with its Au atom to form a straight-line structure -O-Au-O-, and the adsorbed O₂ is significantly activated by accepting one electron with one of its π_2p* orbitals. With no exception, all oxygen sites and the -O-Au-Au sites in AuO^- are inert. Analyses on the density of states (DOS) of representative species well interpret the physical origins of the activity of -O-Au and the inertness of -O-Au-Au. The observations that site-specific factors dominate the reactivity of gold oxide clusters with O₂ are in contrast to what happens in the reactions of Au^- with O₂, where clusters' reactivity is completely determined by their global spins and electron detachment energies. The new conclusions in this work offer a reference to understand the crucial O₂ activation processes in gold-based catalysts, since various gold oxide structures are commonly observed in these systems.