Surface-active centres

Formation of surface-active (colour) centres occurs via capture of free electrons and holes by the surface traps S. For completeness, creation of electron active centres S is indicated in reaction 5.59 (ksg). The centres interact with molecules M to yield the intermediate species M (reaction 5.61, ke ). Reaction 5.60 describes the recombination pathway for the decay of surface-active centres. 

Figure 5.18 Dependence of the amount of the laser pulse-induced surface-active centres Tij on laser pulse energy. Adapted with permission from Sahyun and Serpone (1997). Copyright (1997) American Chemical Society.

Charge-carrier and exciton trapping by pre-existing surface defects (S) yields surface-active centres (S and S" ) for adsorption and catalysis (step 4). 

Interaction of molecules in the gas (or liquid) phase, Mgas, with surface-active centres initiates surface chemical processes (step 5 Eley-Rideal mechanism). 

Reactions 5.101 and 5.105 represent the primary oxidative and reductive chemical events of the catalytic cycle, respectively, and reactions 5.99 and 5.103 show the generation of surface-active centres, whereas reactions 5.100 and 5.104 reflect the deactivation of such centres. Both generation and deactivation of surface-active centres constitute a significant pathway of snrface recombination of free carriers. 

At sufficiently high concentration of reagent M, when the chemical reaction dominates the physical pathway of the surface recombination of carriers, the reaction rate is equal to the rate of generation of surface-active centres e.g. surface OH radicals for oxidative processes and elecnon centres, and Ti for reductive reactions) and is proportional to the surface concentration of free charge carriers of the corresponding sign. From reaction 5.102 we obtain... 

The latter implies that under the above conditions the qnantnm yield depends on k, the concentration of surface-active centres [S], and the snrface concentration of charge carriers, ns (A is the fraction of light absorbed, i.e. absorptance, and p is the photon flow). It is therefore possible to query those factors that govern the snrface concentration of charge carriers, and the activity of the photocatalyst through the quantum yield (f>. For this, the functionality of the surface charge-carrier concentration and the parameters that affect it need to be examined. 

Determination of TON requires an estimate of the number of surface-active centres that participate in the reaction on the metal-oxide surface. This has represented a most challenging problem in heterogeneous photocatalysis because heretofore there has been no description of the nature, nor a quantitative determination of the number, of active centres in heterogeneous photocatalysis, and their concentration on the surface of a metal-oxide photocatalyst remains elusive. To complicate matters further, there is also some uncertainty about how much of the surface area of the metal-oxide photocatalyst is irradiated by the incident actinic light, and can therefore be active in photocatalysis. Accordingly, even if the concentration of active centres were known, it would normally be difficult to estimate the number of those centres actually involved in the photochemical events. 

In the third approach, the total number of surface-active centres was obtained by extrapolation of the kinetic curve that represents the accumulation of photoadsorbed oxygen species obtained from the TPD spectra (see Fig. 5.56). This gave 8.8 x 10 such centres at the limit of f °o, corresponding to the maximal number of surface-active centres, also in fair accord with the above estimates. 

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. 

The total number of surface colour centres of a given type, and hence surface-active centres (here of the electron type for adsorption of oxygen), is a dimensionless quantity since it also represents the product of the number of regular surface sites per cm of surface multiplied by the surface area in cm of the specimen (Serpone and Emeline, 2002). 

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