Adsorption and Activation of O 2 on Small Gold Oxide Clusters: the Reactivity Dominated by Site-Specific Factors.

We experimentally explored adsorption and activation of O 2 on small anionic clusters Au x O y - 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 Au 2 O 2 - , Au 3 O - , one isomer of Au 3 O 3 - , and Au 5 O 2 - , can adsorb an O 2 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 O 2 with its Au atom to form a straight-line structure -O-Au-O-, and the adsorbed O 2 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 Au x O y - 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 O 2 are in contrast to what happens in the reactions of Au n - with O 2 , 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 O 2 activation processes in gold-based catalysts, since various gold oxide structures are commonly observed in these systems.