Unraveling the mechanisms of O ₂ activation by size-selected gold clusters: Transition from superoxo to peroxo chemisorption

The activation of dioxygen is a key step in CO oxidation catalyzed by gold nanoparticles. It is known that small gold cluster anions with even-numbered atoms can molecularly chemisorb O ₂ via one-electron transfer from Au _n ^- to O ₂, whereas clusters with odd-numbered atoms are inert toward O ₂. Here we report spectroscopic evidence of two modes of O ₂ activation by the small even-sized Au _n ^- clusters: superoxo and peroxo chemisorption. Photoelectron spectroscopy of O ₂Au ₈ ^- revealed two distinct isomers, which can be converted from one to the other depending on the reaction time. Ab initio calculations show that there are two close-lying molecular O ₂-chemisorbed isomers for O ₂Au ₈ ^-: the lower energy isomer involves a peroxo-type binding of O ₂ onto Au ₈ ^-, while the superoxo chemisorption is a slightly higher energy isomer. The computed detachment transitions of the superoxo and peroxo species are in good agreement with the experimental observation. The current work shows that there is a superoxo to peroxo chemisorption transition of O ₂ on gold clusters at Au ₈ ^-: O ₂Au _n ^- (n = 2, 4, 6) involves superoxo binding and n = 10, 12, 14, 18 involves peroxo binding, whereas the superoxo binding re-emerges at n = 20 due to the high symmetry tetrahedral structure of Au ₂₀, which has a very low electron affinity. Hence, the two-dimensional (2D) Au ₈ ^- is the smallest anionic gold nanoparticle that prefers peroxo binding with O ₂. At Au ₁₂ ^-, although both 2D and 3D isomers coexist in the cluster beam, the 3D isomer prefers the peroxo binding with O ₂. © 2012 American Chemical Society.