Peroxo-like adsorbed state

It should be noted that no direct transformation from the superoxo-like state to the peroxo-like state was observed in this experiment. From the EEL spectra results it was not possible for these investigators to separate the contribution from the superoxo- and peroxo-like peaks to the rise in the atomically adsorbed oxygen peak. Therefore, a direct pathway from the superoxo-like 02 state to the atomically adsorbed state cannot be dismissed as a possibility. Thus, the assertion by Nolan et al. that the molecularly bound 02 progresses sequentially from the superoxo-like state to the peroxo-like state is a hypothesis. However, this sequential progression does appear to be a very attractive explanation, as a superoxo-like species arises from the contribution of one electron to the antibonding p orbital of the 02 molecule and the peroxo-like species arises from the contribution of two electrons to this antibonding 7T orbital. 

At higher temperatures the peroxo species dissociates completely and forms atomic oxygen which can be either present as surface-adsorbed (a) species or as surface-intercalated (y) species [18]. The overall sticking coefficient into these selectively reacting species is low as the likely processes for depleting the precursor states are desorption or bulk-dissolution [12]. 

Other DFT calculations have shown that co-adsorption of H2O and O2 on Aug clusters, free or supported on MgO(lOO), leads to the formation of an 02- -H20 complex involving partial proton sharing or proton transfer, and leading to a hydroperoxy-like complex (HO2) [178]. This favors the activation of the 0—0 bond, i.e. the bond extension to values characteristic of a peroxo- or superoxo-Uke state. Consequently, the reaction with CO can occur with a small activation barrier of -0.5 eV, either through an Eley-Rideal mechanism if O2 is adsorbed on the top face of Aug clusters or through a Langmuir-Hinshelwood mechanism if O2 is adsorbed on the periphery of the cluster. 

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