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

Abstract

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(2)Au(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(2)Au(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(2)Au(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).

Understanding of the oxygen activation on ceria- and ceria/alumina-supported gold catalysts: a study combining 18O/16O isotopic exchange and EPR spectroscopy

“…1). A decrease of the 18 O atomic fraction (α g ) was observed in the gas phase when the exchange was taking place. This behavior is not consistent with a homo-exchange reaction during which α g should remain constant.…”

“…Since it is well known that Al 2 O 3 support is not able to exchange oxygen at 723 K [33], these results show that the exchange is promoted by the interaction between Au nanoparticles and CeO 2 . The R e values, which were low for 5CeO 2 O concentration between the gas phase and the solid (depending on the mass of catalyst and the initial pressure of 18 O 2 ), which minimized the differences that could be detected after 20 min of exchange. Finally, we studied the effect of the ceria surface area with the results obtained on CeO 2 LSA.…”

“…OIE experiments were undertaken on 20 mg of catalyst, subjected to a 16 O 2 activation step at 873 K under atmospheric pressure for 1 h prior to cooling to the desired temperature, at which point the system was degassed and the isotopic mixture charged. The study of homoexchange (also called equilibration reaction) was performed using an equimolar mixture of 16 O 2 and 18 O 2 (99.9 % purity, supplied by Isotec), whereas for the study of the heteroexchange, the mixture was replaced by pure 18 O 2 . The masses 32, 34, and 36 m/z were monitored as a function of time to follow the exchange.…”

“…The m/z values of 28 and 44 were also recorded to check the absence of air or CO 2 . The atomic fraction of 18 O in the gas phase (α g ), the rate of exchange (R e ), and the number of O atoms exchanged (N e ) were calculated as described in previous references [29]. Typically:…”

“…One reason is the fact that the adsorbed oxygen species are not experimentally easy to identify in the conditions of the reaction. Some recent papers showed that the anion photoelectron spectroscopy [18] and infrared multiple photon dissociation [19] allowed determining which oxygenated species are involved in the activation process, but these spectroscopic studies have not been applied to ceria-supported gold catalysts. Computational method is a way to get around the experimental limitations, and very impressive demonstrations in terms of reactive intermediate species and mechanism scheme were reported [17,20].…”