Oxygen species diffusion

The concentration of Oad atoms on the PbO surface increases and a concentration gradient is formed as a result. Under the influence of fliis gradient, oxygen species diffuse into the empty layers of the PbOn crystal lattice (Fig. 2.19). 

In systems in which anionic diffusion prevails (Fig. 1.81b), metal is consumed by direct reaction to form the diffusing oxygen species... 

In that pioneering study28 it was not possible to obtain a time resolved distribution of the backspillover oxygen species despite the fast, 40 ms, time resolution of the video-frames. If the spillover distance is 100 pm, this implies surface spillover oxygen diffusivities as high as 10 3 cm2/s. If, however, microcracks exist in the film, which is very likely, then the spillover distance is much shorter and thus much lower diffusivities would suffice to escape time-dependent detection. 

A PP sample after ozonization in the presence of UV-irradiation becomes brittle after 8 hrs of exposure, whereas the same effect in ozone is noticeable after 50-60 hours.Degradation of polymer chain occurs as a result of decomposition of peroxy radicals. The oxidation rapidly reaches saturation, suggesting the surface nature of ozone and atomic oxygen against of PP as a consequence of limited diffusion of both oxygen species into the polymer. Ozone reacts with PP mainly on the surface since the reaction rate and the concentration of intermediate peroxy radicals are proportional to the surface area and not the weight of the polymer. It has been found that polyethylene is attacked only to a depth of 5-7 microns (45). 

The reactions of radicals with oxygen are diffusion-controlled it-S-S. Moreover, as has been previously shown, the isobutyryl radical a could readily be captured by a nitroxide. It is therefore not easy to see why reaction between oxygen and the species a does not also occur. 

D diffusion coefficient of oxygen species in the solid catalyst lattice (m2/sec)... 

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 photoreactivity of the involved catalyst depends on many experimental factors such as the intensity of the absorbed light, electron-hole pair formation and recombination rates, charge transfer rate to chemical species, diffusion rate, adsorption and desorption rates of reagents and products, pH of the solution, photocatalyst and reactant concentrations, and partial pressure of oxygen [19,302,307], Most of these factors are strongly affected by the nature and structure of the catalyst, which is dependent on the preparation method. The presence of the impurities may also affect the photoreactivity. The presence of chloride was found to reduce the rate of oxidation by scavenging of oxidizing radicals [151,308] ... 

The role of adsorbed oxygen species in the mechanism of alkane transformation, on the contrary, is more questionable. The effect induced by the substitution of O2 with N2O and IR indications are in agreement with this interpretation, but, on the other hand, activated electrophilic oxygen species form on reduced sites, preferably in tetrahedral coordination (79). The partial reduction of tetrahedral V =0 with formation of tetrahedral v after propane oxidative dehydrogenation can be observed using UV-Visible diffuse reflectance, ESR and V-NMR spectroscopies. It is thus not possible to assign unequivocally the active species in propane selective activation to a tetrahedral V =0 species or to or V -0-0 species formed in the... 

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