How Does Nanoporous Gold Dissociate Molecular Oxygen?

Nanoporous Au and other dilute AgAu alloys are highly active and selective oxidation catalysts. Their ability to dissociate O₂ is to a large extent unexplained, given that unsupported Au cannot generally dissociate O₂, while large ensembles of Ag atoms (>4) are generally necessary to lower the O₂ dissociation barrier significantly. Here, we identify a site on the surface of dilute AgAu alloys that is stable under reaction conditions and has a low O₂ dissociation barrier, in agreement with experimental measurements. Although Ag generally prefers to disperse throughout Au, the presence of adsorbed O near surface steps creates sites of high local Ag concentration, where the Ag atoms sit in the rows next to the step Au atoms. O₂ adsorbs on the Au step atoms, but the transition state involves significant Ag–O interaction, resulting in a barrier lower than expected from the adsorption energies of either the initial or final state