Low-temperature activation of molecular oxygen by gold clusters: a stoichiometric process correlated to electron affinity Original Research Article

The adsorption–desorption equilibria of di-oxygen (O2) on gold cluster anions (AuN−, N=2–22) have been measured in the high-pressure limit by pulsed flow-reactor methods at ambient temperature. Only certain clusters (N=2,4,6,8,10,12,14,18,20) exhibit measurable adsorption, and for most of these the reaction can be driven to completion (>90% conversion to AuNO2−). In no case could secondary adsorption be detected, i.e. a 1:1 reaction stoichiometry is strictly obeyed. The size (N) variation in the adsorption free energies is correlated with the known electron affinities of the gold clusters. Taken with the positive electron affinity of O2, these results suggest that O2 acts as a single-electron acceptor, yielding adsorbed superoxide (O2−), while the gold cluster strives to pair electrons (or close electron shells). This species could serve as an activated form for low-temperature oxygen-atom transfer reactions catalyzed by supported gold clusters, wherein the activation of molecular oxygen is regarded as a crucial step.