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Unraveling the Mechanisms of O2 Activation by Size-Selected Gold Clusters: Transition from Superoxo to Peroxo Chemisorption

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journal contribution
posted on 06.06.2012, 00:00 by Rhitankar Pal, Lei-Ming Wang, Yong Pei, Lai-Sheng Wang, Xiao Cheng Zeng
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 O2 via one-electron transfer from Aun to O2, whereas clusters with odd-numbered atoms are inert toward O2. Here we report spectroscopic evidence of two modes of O2 activation by the small even-sized Aun clusters: superoxo and peroxo chemisorption. Photoelectron spectroscopy of O2Au8 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 O2-chemisorbed isomers for O2Au8: the lower energy isomer involves a peroxo-type binding of O2 onto Au8, 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 O2 on gold clusters at Au8: O2Aun (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 Au20, which has a very low electron affinity. Hence, the two-dimensional (2D) Au8 is the smallest anionic gold nanoparticle that prefers peroxo binding with O2. At Au12, although both 2D and 3D isomers coexist in the cluster beam, the 3D isomer prefers the peroxo binding with O2.