Oxygen surface mobility

Table 8.4. Relative oxygen surface mobility for some oxides at 400°C (base 10 for alumina)...

What became evident was that interactions between adsorbed particles can also exert an influence on their surface mobility and therefore the residence time at a particular site. The mean residence time of an isolated oxygen adatom at the Ru(0001) surface varies from 60 to 220 ms when a second oxygen adatom is located two lattice constants a0 apart from the first but only 13 ms when the... 

RuO2(110) exemplifies Langmuirian behaviour where the catalyst surface consists of equivalent sites statistically occupied by the reactants. This contrasts markedly with catalytic oxidation at metal surfaces, where oxygen transients, high surface mobility and island structures are dominant. The difference is in the main attributed to differences in surface diffusion barriers at metal and oxide surfaces. 

Oxygen chemisorption at caesiated Cu(110) indicates facile surface mobility and structural transformations16 at 295 K. For a caesium concentration of... 

Figueras et al. [Ill] emphasize the importance of the entropy of the oxygen bond, which can be considered as a measure of the surface mobility of oxygen. Unfortunately, their assumed positive correlation between entropy and selectivity is only based on two V2Os catalysts which differ with respect to the carrier (Si02 and A1203). 

Finally, a classification of catalysts by Matsuura [212] may be mentioned, in which the relation of adsorption entropy to heat of adsorption of butene-1 appears, surprisingly, to be linear. The conclusion can be drawn that moderate heats of adsorption (about 40—50 kcal mol 1) characterize suitable catalysts. Only here is the right combination of surface mobility and adsorption intensity found. Apparently, the oxygen is then tempered sufficiently to make a selective oxidation possible. Otherwise, the oxides are non-active (e.g. low heat of adsorption in FeP04 and low mobility) or active but non-selective because of high mobility coupled to a large heat of adsorption (e.g. Fe304). 

Film growth is under oxygen-rich conditions so the films are stoichiometric. The high partial pressure of the reactive gas limits the surface mobility, so film properties are not ideal. Furthermore, dopants are oxidized and thus, TCO films become non-conductive. 

Surface mobility of oxygen species on coked alumina... 

The increase of the heat of adsorption of oxygen when the adsorption temperature increases was explained in Section III, A by the enhancement of the outward mobility of surface ions and consequently by a surface modification. The present results show moreover that a permanent modification of the surface occurs at high temperatures (200-250°) when oxygen interacts with the solid and that removal of this oxygen does not restore the original surface structure. These modifications occur more rapidly at 250° than at 200°, probably because of the enhanced surface mobility at the higher temperature, and three catalytic runs and regeneration treatments produce at 250° the same modifications of surface structure of the catalyst [NiO(250°)] that four cycles of... 

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