Photocatalysis defined

Catalysis (qv) refers to a process by which a substance (the catalyst) accelerates an otherwise thermodynamically favored but kiaeticahy slow reaction and the catalyst is fully regenerated at the end of each catalytic cycle (1). When photons are also impHcated in the process, photocatalysis is defined without the implication of some special or specific mechanism as the acceleration of the prate of a photoreaction by the presence of a catalyst. The catalyst may accelerate the photoreaction by interaction with a substrate either in its ground state or in its excited state and/or with the primary photoproduct, depending on the mechanism of the photoreaction (2). Therefore, the nondescriptive term photocatalysis is a general label to indicate that light and some substance, the catalyst or the initiator, are necessary entities to influence a reaction (3,4). The process must be shown to be truly catalytic by some acceptable and attainable parameter. Reaction 1, in which the titanium dioxide serves as a catalyst, may be taken as both a photocatalytic oxidation and a photocatalytic dehydrogenation (5). 

From these results and others from UV-vis absorption spectroscopy and differential pulse polarography (data not shown), Garbin et al. (2007) concluded that HS can act as photocatalyst to pesticide photolysis in aqueous solution only for specific ranges of concentration (as seen in Figure 16.12), which in turn depended on the HS and pesticide chemical characteristics. Under ultraviolet and visible radiation, this photocatalysis is based on photogeneration of -OH radicals, and the susceptibility of pesticide molecules to -OH attacks defines the efficiency of the photocatalysis. 

In the introduction, the model systems will be defined to familiarize the reader with the approach in order to appreciate the connection to real-world catalysis. Following the introduction, we will demonstrate via four case studies various fundamental aspects in thermal and photocatalysis whereby studies on model systems might become important to unravel the foundations of reaction mechanisms. 

A significant problem in studies on photocatalysis is the definition of positive hole. Positive hole is defined as a defect of an electron (i.e., a positive hole must be included in a substance, while an electron is a real substance). Therefore, not only h produced by photoinduced band-to-band transition in solid materials but also a hydroxyl radical, which is a one-electron deficient hydroxyl anion, can be a positive hole. If this definition is accepted, there should be no difference in the photocatalsrtic oxidation mechanisms between direct hole transfer and surface-adsorbed hydroxyl radical reaction, since it is well known that the surface of a metal oxide is covered with chemically or physically adsorbed water and a positive hole passing through this water layer into a solution may be a hydroxyl radical or its protonated or deprotonated species (Fig. 4). Actually, hydroxyl... 

The term synergetic effect in photocatalysis could be defined as follows when more than two kinds of photocatalysts are used as a mixture, the overall photocatalytic activity exceeds the sum... 

Photocatalysis can be defined as acceleration of a photoreaction by the presence of a catalysf [3, 5]. The catalytic dimension may originate either because of the quantity of consumed photons or because of the quantity of one added substance [6]. The most restrictive situation corresponds to a transformation, photoinduced with a catalytic quantity of photons, provided that a catalytic quantity of an exogenous substance is added to the reaction mixture. The two other possibilities, where the catalytic dimension comes either from the number of consumed photons or from the presence of an added substance in catalytic quantities, are also well identified. 

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). 

Lawless et al. (1991) concluded that adsorbed OH radicals (surface-trapped holes) are the major oxidising species in photocatalysis, while so-called free hydroxyl radicals (if there are any) play only a minor role. This makes sense since the OH radical reacts with Ti02 at a diffusion-controlled rate, the desorption of OH from the Ti02 surface to the solution phase seems highly unlikely. The surface-trapped hole, as defined by reaction 5.15, accounts for most of the observations which had previously... 

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. 

Quantum yield and quantum efficiency (significant value in photocatalysis (Chapter VI). In principle, these parameters should be defined with the... 

Photocatalysis can be defined as follows A change in the rate of chemical reactions or their generation imder the action of light in the presence of substances called photocatalysts - that absorb light quanta and are involved in the chemical transformations of the reactants [4]. Typical photocatalysts or photosensitisers are semiconductor materials. There are many chemical compounds which can act as photocatalysts, but only a very few of these materials are photochemically and chemically stable semiconductor photocatalysts, one compound dominates titania (titanium dioxide) Ti02. 

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] ... 

The degree of intensification is quantified by comparing the enhanced rate of disappearance of 4-CP in the integrated process with the sum of the rates that are obtained in PV alone and in photocatalysis alone. This sum represents an optimistic (conservative in the comparison) evaluation of the rate that would be achieved by operating sequentially with the two processes. An intensification factor, can be defined as... 

Various experiments, carried out with different photocatalytic reactors and commercial pervaporation membranes, by varying the relative weight of pervaporation with respect to photocatalysis, have demonstrated that the intensification factor depends solely on the parameter 5 previously defined, as is apparent in Fig. 3.7. 

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