Heterogeneous photocatalysis defined

Since the numerator in eq. 5.84 expresses the rate of reaction, will depend on the reactant concentration. As correctly noted by Braun and co-workers (1991) and emphasised by Cabrera et al (1994), only for a zero-order reaction is uniquely defined at the given wavelength A because when the reaction rate depends on the reactant concentration, falls off over time. In homogeneous photochemistry, this problem is normally overcome by determining at small (less than -10%) conversions of reactants, a point not often respected in heterogeneous photocatalysis, where the focus is often on complete mineralisation (100% transformation) of the substrate, at least in studies of environmental interest that focus on the total elimination of organic pollutants in water. 

For example, the quantum yield (QY or d>) is a fundamental parameter in heterogeneous photocatalysis, whose definition should be carefully considered. A standard definition has been given in terms of a particular reaction, with a defined reactant or product, at a given wavelength A. [78] ... 

Semiconductors excited by appropriate energy photons. Two weU-defined AOP s systems which have special interest, because natural solar light can be used, are heterogeneous photocatalysis with Ti02 and homogeneous photocatalysis by the photo-Fenton process. These processes are covered in detail in the following sections. 

The efficacy of semiconductor devices in photocatalysis is typically described by the quantum yield (j). While determining this parameter in homogeneous photochemistry presents no great difficulties, in heterogeneous media the challenges have been insurmountable to many workers for several years owing to the lack of an appropriate protocol. This was remedied by Serpone and Salinaro (1999), and appropriate experimental details were enumerated by Salinaro et al. (1999). Another useful parameter, the relative photonic efficiency 4ei (defined in Section 5.4.1) also provided a method by which work from many laboratories in environmental photochemistry could be calibrated when the more fundamental parameter 4> could not be accessed because of experimental limitations (Salinaro et al, 1999). 

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