Limiting efficiency photocatalyst

Explain why photocatalysts based on Ti02 have limited efficiency for splitting water. 

From Equation (5), it is clear that the basic parameter that decides the light harvesting ability of the photocatalyst is its band gap. The ideal limiting efficiencies for conversion of solar radiation calculated by Equation (5) as a function of the band-gap wavelength for standard AM 1.5 solar irradiation in a single band-gap device are represented in Figure 9. 

One of the major limitations in semiconductor photocatalysis is the relatively low value of the overall quantum efficiency mainly due to the high rate of recombination of photoinduced electron-hole pairs at or near the surface. Some success in enhancing the efficiency of photocatalysts... 

We have already seen that photoactive clusters, e.g. CdS, can be introduced into vesicles and BLMs (Sect. 5.2 and 5.3). Similar support interactions are possible with both inorganic and organic polymeric supports. Photoactive colloidal semiconductor clusters can be introduced, for example, into cellulose [164], porous Vycor [165], zeolites [166], or ion exchange resins [167]. The polymer matrix can thus influence the efficiencies of photoinduced electron transfer by controlling access to the included photocatalyst or by limiting the size of the catalytic particle in parallel to the effects observed in polymerized vesicles. As in bilayer systems,... 

From the studies above, one can observe that the use of Fe has been somewhat limited and that there are still many areas to explore for better utilization of Fe in PC reactions. More specifically, there is a need for the development of new inexpensive techniques or procedures to increase the photocatalyst activity. These new procedures should make the process more efficient without the economic burden that photocatalyst doping brings about. Also, one can notice that most studies have focused on carboxylic acid species containing less than four carbons. It is of utmost importance to explore this area with more refractory molecules such as phenol and other hydroxylated aromatics to determine whether Fe can truly be applied to enhance the PC mineralization. 

Figure 9 The ideal limiting solar conversion efficiency for a single-band gap photocatalyst (Archer and Bolton, 1990).

Batch reactors are often used at laboratory-scale experiments, usually for studying reaction kinetics (Vorontsov and Dubovitskaya, 2004) or for demonsfrafing the efficiency of recently developed photocatalyst. Although their potential for industrial-scale treatment of air is very limited, they can still be quite useful for obtaining kinetic rates expressions. 

Proper definition of quantum yields (cp) requires the cai eful assessment of photons absorbed by the photocatalyst. Methods based on chemical actinometry ai e limited as they provide only the total rate of photons entering the reactor and do not account for various light scattering losses. The use of physical methods involving spec-trophotoradiometers and collimators (Salaices et al., 2001), as described in chapter IV, enables macroscopic energy balances, the proper assessment of irradiation energy absorbed by the photocatalyst and, consequently, an adequate definition of quantum efficiencies. 

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