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.
ASJC Scopus subject areas
- Colloid and Surface Chemistry