Oxygen, adsorption on oxides

Small changes in impurity content did not affect this rate but the presence of water vapor and changes in partial pressure of oxygen were critical (61,62). Steam and various impurities and binders also affect the oxidation of siUcon carbide (63). Differences have been observed in oxygen adsorption on the different SiC crystal faces (64). 

Nevertheless, there is a whole series of experimental results enabling one to propose and substantiate a sufficiently general model consistently describing the effect of oxygen adsorption on electric conductivity of partially reduced oxides observed in experiments. Let us con sider these data. 

In surface science, work function measurements are considered to be rather sensitive towards changes of the sample surface. Work function measurements are used to follow adsorption processes and to determine the dipole established at the surface. During oxygen adsorption and oxide formation the sign of the work function change allows one to distinguish between oxygen atom adsorbed on the surface or sub-surface [30]. 

The IR data observed for oxygen adsorbed on oxides depend on a number of factors, such as the nature of the oxide (20a). the pretreatment conditions (20b), and the temperature of oxygen adsorption (149a), and this results in a wide range of frequencies. 

The heat and entropy associated with the above reaction implied by the temperature dependence of the critical oxygen activity were —46 kcal mol 02 1 (-192 kJ mol 02 ) and -45 kcal K-1 mol O 1 (-188 kJ K-1 mol 02- ) in close agreement with the results of Berry.81 Oxygen adsorption and oxide formation on platinum have also been studied using exchange currents in a solid electrolyte cell.82... 

Fig. 19. Illustration of carbon monoxide adsorption followed by oxygen adsorption on metal oxides [W. G. Garner, J. Chem. Soc. 1239 (1947)].

Electrocatalyzed oxygen reduction proceeds always in the adsorbed state. Figure 17 depicts the three different modes of oxygen adsorption on a metal or metal oxide surfaces, which are supposed to be of relevance for cathodic oxygen reduction (113). So-called Griffith adsorption (I), which has been observed by Gland and co-workers (114) on Pt(l 11) surfaces due to interac-... 

Another school of thought believes that the efficacy of boric oxide in afterglow suppression is related to its high ionization energy/or electron affinity active sites for oxygen adsorption on the char surface may be deactivated by boric oxide via electron transfer. It was reported that boric oxide increases the oxidation temperature of crystalline carbon from 700°C to 800°C.109... 

An interesting application of adsorption microcalorimetry was used by these researchers to examine changes in adsorption behavior of graphite-supported iron/rhodium bimetallic catalysts as a function of oxidation and reduction treatments. The differential heat of oxygen adsorption on the bimetallic catalysts after various treatments was compared to the values obtained for the monometallic materials to determine the relative contributions to the total adsorption. Reduction at 673 K produced an alloy whose... 

The oxidation rate of the micro- and nanopowders was found to be faster than that of bulk single-crystal silicon, hich can be rationalized as follows. At the initial stages of the oxide growth oxygen adsorption on micro- and nanoparticles leads to SiO formation. The oxidation is then due to an enhanced oxidant diffusion over the particle surface. During the oxidation of the powders, the amount of the oxidant penetrating into the bulk of a particle is so small that its contribution to the oxidation process in the bulk of the particle is insignificant. In this case, the oxidation of the micro- and nanopowders is... 

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