Superoxo-like

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. 

From the EEL spectra, the activation energy for conversion from the peroxo-like state to the atomically dissociated state (Ep in Fig. 22) was estimated to be <0.29 eY. It was also reasoned by Nolan et al. that the activation energy for the superoxo-like potential well (Es) should have a similar magnitude to Ep. This assertion was based on the EEL spectra, which indicate that Ev must be similar in magnitude to Es, since both peak features are seen and that heating of the sample does not result in a rapid reduction of the superoxo-like (870 cm-1) peak and rapid growth of the peroxo-like (690 cm-1) peak. 

As a complement to these Pt(l 1 1) studies, Nolan et al. investigated 02 dissociation on Pd(l 1 1) [81]. The ability of both surfaces to bind oxygen in similar molecular forms (peroxo-like and superoxo-like) at high coverages, could provide additional support for the sequential precursor mechanism proposed for Pt(l 1 1). EELS and dissociation probability data obtained from this study produced trends similar to that observed for the Pt(l 1 1) experiments [75, 76], supporting the same sequential precursor mechanism described above. 

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]. 

Computational studies of oxygen adsorption on Pt(lll) were reported by Eichler and Hafher [38] with the GGA-PW91 functional and a slab model. The study reported two distinct but energetically almost degenerate chemisorbed molecular precursor state t5q>es for O2 on Pt(ll 1) at distances of 1.8 -1.9 A. The first type was a superoxo-like (O2) paramagnetic precursor formed at the bridge site (t-b-t) with the molecule parallel to the surface (see t-b-t site in Figure 5.3) ... 

The structure of the active component, manganese pyrophosphate, has been reported in the literature (24). It is layer like with planes of octahedrally coordinated Hn ions being separated by planes of pyrophosphate anions (P20y ). Examination of models of this compound gave calculated Hn-Hn thru space distances of 3.26 and 3.45 angstroms, a metal-metal distance close to that found for binuclear dibridged peroxo- and superoxo- complexes of cobalt ( ). 

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