O2 Adsorption on AunRh+ (n = 3-8) Clusters: A Distinctive δ-like Bonding Interaction at Rh Sites.

We investigated reactions of AunRh+ (n = 3-20) with O2 using mass spectrometry and density functional theory calculations. The results reveal a structural transition from surface Rh sites with high spin density to encapsulated ones with quenched spin. For n = 3-8, O2 adsorption on surface Rh involves a distinctive face-to-face interaction between Rh 4d and O2 π* orbitals, resembling a δ-like bond. The abrupt reactivity quenching at n = 9 arises from an inert, high-coordination, half-encapsulated Rh, while for n > 9 it stems from an inert, fully encapsulated Rh, both exhibiting negligible spin density. These findings demonstrate that surface Rh atoms on gold with low coordination numbers can activate O2 into a nondissociative side-on configuration, offering mechanistic insights for the rational design of Rh-Au catalysts for low-temperature oxidation.