Nomenclature of Photochemical AOPs

Photochemical processes of water treatment can be classified into three principal categories photooxidation, photoreduction and direct photodegradation (Bolton and Cater, 1994). Common processes of reduction reactions induced by UV/ 

VIS radiation include the addition of one or more electrons to an electronically excited species or the photochemical hydrogenation of a substance. Photo-initiated reductions are reactions in which the substrate itself is not electronically excited (Verhoeven, 1996). 

Oxidation processes that are driven by UV/VIS radiation are called photooxidation reactions. According to Verhoeven (1996), three classes of photooxidation reactions may be distinguished (Fig. 5-10)  

As already menhoned above, AOPs mainly rely on photo-inihated oxidahons (Fig. 5-10, reactions 4). However, depending on the particular situahon of water or air contamination, photo-induced oxidation and photooxygenahon reachons may compete effechvely with photo-initiated oxidations generating a complex mechanishc scheme (cf. Zepp, 1988). 

In catalyzed photolysis either the catalyst molecule (Fig. 5-11, situation B) or the substrate molecule (Fig. 5-11, situation C), or both, are in an electronically excited state during the catalytic step. The electronically excited catalyst molecule is produced via photon absorption by a nominal catalyst (Fig. 5-11, situation B). The reaction of substrate to product is catalytic with, respect to the concentration of the electronically excited catalyst species. It is non-catalytic in photons and therefore, continuous irradiation is required to maintain the catalytic cycle. The quantum yield of product formation Product is equal to or less than unity. Titanium dioxide photocatalysis is the most widely applied example of this type, with Ti02 representing the nominal catalyst that must be electronically excited by photon absorption with formation of the electron hole pair Ti02 (hvb + cb), being the active catalytic species (cf Fig. 3-17 and Fig. 5-9, reaction 1). The oxidation of substrates by the combination of UV/VIS radiation and an appropriate photocatalyst is often called photocatalytic oxidation (PCO). 

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