1 Pollutant Oxidation Mechanism of Photocatalysts
As is well- known that, when light of the same or greater energy as the forbidden bandwidths contacts a semiconductor photocatalyst such as TiO2, charge separation of electrons and electron holes occurs. The electrons disperse on the surface of the photocatalyst and react with external substances, causing reductions and oxidations. As shown in Fig. 2, oxygen and water exist throughout the atmosphere and types of active oxygen such as OH radicals form, so normally only oxidation reactions occur. The oxidizing power of the OH radical is greater than that of chlorine and ozone, and a large portion of the multifarious environmental purification capabilities of photocatalysts come from this type of active oxygen.

Fig. 2 : Mechanism of photocatalysts
Fig. 2 : Mechanism of photocatalysts

The OH radical plays a very important role in atmospheric chemistry. Its atmospheric concentration is markedly low at 106 - 105 mol cm-3, but because it has something to do with the reaction process of all airborne substances, its speed of reaction with pollutants is well studied (Table 1). These values serve as an index for determining whether a photocatalyst can treat pollutants or not as well as for knowing relatively how fast it can do so.

Table 1 : Reaction speed constants of air pollutants and OH (25oC) (k : 10-13 cm3 molecule-1 s-1)
Pollutant k Pollutant k Pollutant k Pollutant k
CO 1.3 H2S 48 CH3COOH 8.0 C3H6 300
NO2 670 HCHO 92 CH4 0.06 CH3CCl3 0.1
NH3 1.6 CH3CHO 200 C2H6 2.5 CHCl=CCl2 21
SO2 20 CH3OH 7.9 C3H8 11 C6H6 10
CH3SH 330 C2H5OH 1.6 C2H4 90 Toluene 61

Photocatalyst technology
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2 Oxidation of Air Pollutants by Photocatalyst