Peroxo-like species

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. 

Even nondissociative (molecular) adsorption may be accompanied by an activation barrier if, for example, the reaction proceeds from a physisorbed state into a chemisorbed state (trapping-mediated adsorption). In the system 02/Pt(l 11), for example, the O2 molecule may be chemisorbed either in a superoxo-like or a peroxo-like state [18]. It was found that with low kinetic energies of the incident molecules, both types of surface species are formed, while at higher kinetic energies the more strongly held peroxo-like species is favored, thus reflecting correlations between incident translational energy and preferred trajectories for adsorption [19]. 

Fig. 4.10. Tentative model describing the initial growth of ZnO on different substrates that is consistent with the different evolution of the O Is signal (Fig. 4.9) and the different reactivitiy at the interface (see Sects. 4.3.2.1, 4.4.1, and 4.5.2). A peroxo-like surface species is observed during growth on ZnO and CdS substrates but not on ImSs and Cu(In,Ga)Se2 substrates. The differences are attributed to the abilities of the surfaces to dissociate the adsorbed O2 molecules...

The catalytic activity increased with time over 2-3 hours (at 323 K) and 70-90 min (at 373 K) followed by a period of relative stability. Naito et al. [23] have suggested that oxygen molecules on small gold particles behave as a peroxo-like adsorbed species, which enhances the dissociation of hydrogen molecules. Moreover, the products are similar to those obtained... 

Snbsequent detailed kinetic stndies revealed that the reaction mechanism for the hydroxy-lation of arenes is mnch more complicated than that indicated above Furthermore, the active intermediate is likely an anion radical species formed upon interaction of two molecules of the vanadium peroxo complex. The sequence of the various steps is indicated in equations 17-24. The steps indicated in equations 17-21 refer to a radical chain which accounts for decomposition of the peroxo complex to form dioxygen, whereas the subsequent steps are those required for the functionalization of the substrate. 

The nature of the titanium-containing active site has been investigated with different techniques, including theoretical calculations. The formation of a hydroperoxidic species or of a bidentate side-on titanium peroxo structure was suggested by many authors . Alternatively, some DFT calculations indicated an undissociated molecule of H2O2 weakly interacting with Ti centers or an active Ti-O-O-Si peroxo moiety as a reactive site . Recently, Lin and Frei reported the first direct detection, obtained using in situ FT-infrared spectroscopy, of a Ti-OOH moiety as active species in the oxidation of small olefins like ethylene or propylene . 

---