Energy oxygen species

Fig. 4.7. Aluminium content of sputter deposited ZnO Al films. A target with a nominal A1 content of 2wt% has been used. The shadowed regions indicate the general behavior. The atomic concentration is calculated with and without considering the high binding energy oxygen species, which contributes to the O Is signal (see Sect. 4.2.2.2)...

It can be shown thermodynamically that ground state O or O2 do not react easily with ground state polymers [8]. It has also been shown experimentally by Golub [9] that the oxidation of a polymer in an oxygen plasma, where VUV and ions are present, is about 100 times faster than it is downstream of the plasma where there are no ions and very little VUV. The polymer must either be heated, or contain accessible free-radicals or a surface carbonyl for direct reaction with low-energy oxygen species. 

The backspillover O species on the Pt surface have an O Is binding energy 1.1 eV lower than on the same surface under open-circuit conditions. The Pt catalyst-electrode is surrounded by isoenergetic oxygen species both at the Pt/YSZ and at the Pt/vacuum interfaces.67... 

It is observed that higher potential values for the adatom redox process are correlated with a lower energy of the M—O bond, i.e., lower (less negative) enthalpy of formation of the adatom oxygenated species. In this regard, the discrepant behavior of Ge-Pt(lOO) may be related to the dilute nature of this adlayer, with a maximum coverage of only 0.25. 

Fig. 3. Binding energy distribution of several oxygen species occurring on emersed electrodes. Binding energies are taken for different substrates from various authors. In part after [15, 18].

Ruthenium like iridium is known for its ability of adopting various valence states which make these elements rather attractive in catalysis. Kim and Winograd [52] were the first who studied the chemical in XPS of different Ru compounds. The results of their extensive work still serve as reference for today s investigators. Kim and Winograd have identified binding energies of Ru-oxygen species (Table 1). 

Before analyzing the results of these, or similar, thermochemical cycles, the assumptions which have been made must be critically examined. Since the cycles are tested for different surface coverages, it is assumed first that the Q-0 curves represent correctly, in all cases, the distribution of reactive sites—the energy spectrum—on the surface of the adsorbent. This point has been discussed in the preceding section (Section VII.A). It is assumed moreover that, for instance, the first doses of carbon monoxide (8 = 0) interact with oxygen species adsorbed on the most reactive surface sites (0 = 0). This assumption, which is certainly not acceptable in all cases, ought to be verified directly. This may be achieved in separate experiments by adsorbing limited amounts of the different reactants in the same se-... 

Figure 3.15 O Is / Ag 3d5/2 XPS intensity ratio as a function of take-off angle for two oxygen species on polycrystalline silver. The data corresponding to an O 1 s binding energy of 528.4 eV are attributed to subsurface oxygen in Ag, the other with a binding energy of 530.5 eV to oxygen atoms adsorbed on the Ag surface (data from Baschenko et al. (39J).

Based on the experimental data and some speculations on detailed elementary steps taking place over the catalyst, one can propose the dynamic model. The model discriminates between adsorption of carbon monoxide on catalyst inert sites as well as on oxidized and reduced catalyst active sites. Apart from that, the diffusion of the subsurface species in the catalyst and the reoxidation of reduced catalyst sites by subsurface lattice oxygen species is considered in the model. The model allows us to calculate activation energies of all elementary steps considered, as well as the bulk... 

The extra oxygen decreased the activation energy of C-H bond scission of the methoxy, which is the rate-limiting step of the selective methanol oxidation. TPD spectra of CO indicate that extra oxygen species reduce the electron density of Mo atoms in MoNC rows. This modification causes the decrease of the activation energy for the methoxy dehydrogenation. The extra oxygen is... 

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