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

Abstract

The adsorption–desorption equilibria of di-oxygen (O 2) on gold cluster anions ( Au N −, 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 Au N O 2 −). 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 O 2, these results suggest that O 2 acts as a single-electron acceptor, yielding adsorbed superoxide (O 2 −), 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.