Surface oxide migration

All of these results are consistent with the notion that surface migration of titanium oxide species Is an Important factor that contributes to the suppression of carbon monoxide chemisorption. The H2 chemisorption experiments on 1-2 ML of Ft, where no migration Is observed, strongly Indicate that electronic (bonding) Interactions are also occurring. Thus, for the tltanla system, both electronic Interactions and surface site blocking due to titanium oxide species must be considered In Interpreting SMSI effects. 

The above rate equations confirm the suggested explanation of dynamics of silver particles on the surface of zinc oxide. They account for their relatively fast migration and recombination, as well as formation of larger particles (clusters) not interacting with electronic subsystem of the semiconductor. Note, however, that at longer time intervals, the appearance of a new phase (formation of silver crystals on the surface) results in phase interactions, which are accompanied by the appearance of potential jumps influencing the electronic subsystem of a zinc oxide film. Such an interaction also modifies the adsorption capability of the areas of zinc oxide surface in the vicinity of electrodes [43]. 

Reduced iron so formed would be expected to migrate rapidly out of the oxide lattice but, under the pH conditions of this study (pH 6.5) will be readily readsorbed to the oxide surface and, predictably, is not a major solution phase product. 

Thus, the possibility of adsorption is of primary importance. Adsorption may originate either from chelating properties of the organic substrate toward surface metal species or, because of the low hydrophobicity of the metal oxide surface, from the expulsion of the organic molecules from the solution for entropy reasons. Because there is depletion of substrate at the catalyst surface when degradation takes place, migration from the solution is assisted by a concentration difference in the two environments. 

Since the classification is essentially based on rates of catalytic reactions relative to rates of diffusion of redox carriers, there are oxidation reactions that are intermediate between the two limiting cases. We note that neither the molecular size nor the polarity of reactant molecules is the principal characteristic determining the type of catalysis. Although oxide ions migrate rapidly in the bulk, bulk type II catalysis is not observed for oxidation catalyzed by Bi-Mo oxides. In this case the rate-limiting step is a surface reaction. 

Mixed clusters much more active. Formation of bimetallic clusters depends on surface migration of Re oxide to hydrogen covered Pt particle and coalescence of migrating metal particles (especially on zeolites).41 ... 

Before this study was done, it was known that the presence of oxygen inhibited the reaction between water and uranium. However, it was incorrectly assumed (and mathematically inferred) from weight gain studies that the mechanism for the inhibition was the formation of a monolayer of adsorbed or chemisorbed oxygen atoms on the oxide surface that served to block the adsorption of water molecules [144]. The SIMS profiles in Fig. 4.44b made after the final exposure to 18OH2 clearly show that the lsO migrating species has traveled to the metal surface without inhibition, and additional reaction with the metal has not occurred to... 

Around neutral pH, by far the most common Fe(III) species are the oxides and hydroxides. Of these, a-Fc203 and /I-FeOOH have been most studied from a photochemical point of view [88-92], They are semiconductor oxides and irradiation in the visible promotes electrons from the valence band to the conduction one, leaving holes (h+) in the valence band [90,91]. Electrons and holes can either thermally recombine or migrate to the oxide surface, where they can be trapped by surface species and react with dissolved molecules [93]. In aerated solution, trapped electrons are likely to reduce oxygen to superoxide, while trapped holes can oxidise various molecules. Quite interestingly, when irradiated in the visible, g -Fc203 and /J-FcOOH are not able to transform phenol. However, in the presence of phenol and nitrite, quantitative yield in nitrophenols is observed as a consequence of the oxidation of nitrite to nitrogen dioxide [85]. 

Hachiya, K., M. Ashida, M. Sasaki, H. Kan, T. Inoue, and T. Yasunaga. 1979. Study of the kinetics of adsorption-desorption of lead(2+) ion on a gamma-aluminum oxide surface by means of relaxation techniques. J. Phys. Chem. 83 1866-1871. Heller-Kallai, L., and C. Mosser. 1995. Migration of Cu ions in Cu montmorillonite heated with and without alkali halides. Clays Clay Miner. 43 738-743. 

Strictly looking to oxygen surface migration on oxides, that is, assuming no bulk diffusion and no direct exchange, three types of exchange can occur according to either Boreskov [52], Winter [53] or Novakova [54] ... 

This is a purely empirical relationship based on Tammann s discovery in 1932 (ref. 78) that there was a minimum temperature at which a solid would undergo a solid-solid interaction. Since that time other workers reported that the rates of sintering of oxides increased markedly at about half the value of the absolute melting temperature. It was also found that under these circumstances defects in the surface of a solid became mobile enabling the surface migration of ions to take place. It should be realized that the Tammann temperature does not represent a discontinuity of behavior, but rather a temperature in the vicinity of which a rapid change in the rate of motion of ions or atoms occurs. 

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