Photoadsorption of oxygen

Note should be taken of the general regularity observed by many authors on many systems. Photodesorption is always reversible, while photoadsorption is, as a rule, irreversible. This means that the molecules additionally adsorbed under illumination are retained on the surface for a sufficiently long time after the illumination is switched off. They can however be removed by heating. This was observed, for example, in the photoadsorption of oxygen on Ti02 (12, 14), on ZnO (15), and in many other cases. 

In some cases photoadsorption is masked by photodesorption. An example is provided by the photoadsorption of oxygen on Si02 first observed by Solonitzin (21, 22). In this case illumination apparently leads to photodisruption of the Si-OH bond and to desorption of the OH groups... 

A correlation of the amount of oxygen photosorbed with the content of interstitial zinc of the ZnO sample 50) and photoadsorption of oxygen at illumi-... 

Various studies concerned with the photoadsorption of oxygen at more or less dehyd-roxylated T1O2 powders allowed to identify a series of additional paramagnetic species, including 0 178,179) Q- 180-183) q- 175 181) q3- 180,181)... 

The conclusion that photoadsorption of oxygen is associated with the presence of OaidB) and subsurface interstitial zinc has prompted us to examine the photoeffect above 300° (118) where this second type of oxygen is considered to occur (Section III,B and Fig. 5). It was found that at 400°... 

Figure 5.32 Spectral dependencies of quantum yields of photoadsorption of oxygen (1), hydrogen (2) and methane (3) on powdered MgO. Reprinted with permission from Emeline et al. (2000a). Copyright (2000) American Chemical Society.

This suggests that on average about 6.6 molecules of oxygen react with the same surface-active centre during the time course of the photoreaction. Note that the TON for the photoadsorption of oxygen is greater than unity, as required for the process to be photocatalytic. 

Spectral dependence of the photoadsorption of oxygen on powdered samples of pure zinc oxide or sine oxide doped with lithium or aluminium has been studied by Zakharenko et al. [127] whose results provided... 

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