Mechanism photocatalytic

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). 

Ammonia and acetic acid in waste water give rise serious pollution problems which bring about eutrophication of rivers, lakes, etc [1, 2]. These have been treated by the conventional method of biological techniques, adsorption, and thermal incineration. A band of researchers have suggested that the ammonia molecules could be transferred to N2 using a photocatalytic redox mechanism as shown follows 4NH3 + 3O2 2N2 + 6H2O. However, it has been... 

Choi W, MR Hoffman (1997) Novel photocatalytic mechanisms for CHClj, CHBrj, and CCljCO degradation and fate of photogenerated trihalomethyl radicals on TiOj. Environ Sci Technol 31 89-95. 

Most of the so far designed photocatalytic systems can be divided into three categories simple molecular ones, organized molecular assemblies and semiconductor systems. In this section typical photocatalytic behaviour and reaction mechanisms will be discussed for photocatalysis with systems of all these types. 

The mechanism(s) by which these photocatalyzed oxidations are initiated remain uncertain. Early proposals have included involvement of either the photo-produced holes (h+) arising directly from semiconductor photo-excitation, or the (presumed) derivative hydroxyl radical (OH) which was argued to arise from the hole oxidation of adsorbed hydroxyls (h+ + OH-—> OH ). Recent subambient studies [4] with physisorbed chloromethane and oxygen suggest the dioxygen anion (02 ) as a key active species, and the photocatalytic high efficiency chain destruction of TCE is argued to be initiated by chlorine radicals (Cl) [5]. The chlorine-enhanced photocatalytic destruction of air contaminants has been proposed [1, 2, 6] to depend upon reactions initiated by chlorine radicals. 

FIG. 25 (a) Schematic representation for a photocatalytic mechanism based on shuttle photosensitizers at liquid-liquid interfaces. (Reprinted with permission from Ref. 182. Cop5right 1999 American Chemical Society.) (b) This mechanism is compared to the photo-oxidation of 1-octanol by the heterodimer ZnTPPS-ZnTMPyP in the presence of the redox mediator ZnTPP. (From Ref. 185.)... 

The formation of singlet oxygen through the Kautsky mechanism [78] was observed not only when sensibilization was induced by adsorbed dye molecules but also in case when sensibilization was provided by deposited oxides of transition metals [92, 93]. This is very important for understanding numerous heterogeneous and photocatalytic processes. 

Intensification can be achieved using this approach of combination of cavitation and advanced oxidation process such as use of hydrogen peroxide, ozone and photocatalytic oxidation, only for chemical synthesis applications where free radical attack is the governing mechanism. For reactions governed by pyrolysis type mechanism, use of process intensifying parameters which result in overall increase in the cavitational intensity such as solid particles, sparging of gases etc. is recommended. 

This facilitates the flow of degradable species through these tunnels onto the surface of the TiC>2 where electron could be donated to the holes of the anatase phase and the photocatalytic action in combination with the cavitational effect of the ultrasound can accelerate the fragmentations of pollutants. The details of this mechanism are however discussed at the end of chapter. Ultrasound also breaks TiC>2 particles to still smaller size and increases the active surface area manifold. 

Tang WZ, An H (1995) Photocatalytic degradation kinetics and mechanism of acid blue 40 by UV/Ti02 in aqueous solution. Chemosphere 31 4171-4183... 

Different mechanisms to explain the disinfection ability of photocatalysts have been proposed [136]. One of the first studies of Escherichia coli inactivation by photocatalytic Ti02 action suggested the lipid peroxidation reaction as the mechanism of bacterial death [137]. A recent study indicated that both degradation of formaldehyde and inactivation of E. coli depended on the amount of reactive oxygen species formed under irradiation [138]. The action with which viruses and bacteria are inactivated by Ti02 photocatalysts seems to involve various species, namely free hydroxyl radicals in the bulk solution for the former and free and surface-bound hydroxyl radicals and other oxygen reactive species for the latter [139]. Different factors were taken into account in a study of E. coli inactivation in addition to the presence of the photocatalyst treatment with H202, which enhanced the inactivation... 

Figure 4.12 Schematic representation of the proposed reaction mechanism for overall photocatalytic water splitting using 03 - redox mediator and a mixture of Pt-Ti02-anatase and Ti02-rutile photocatalysts. Adapted from [161] (2001) with permission from Elsevier.

Figure4.14 Schematic representation ofthe proposed mechanism ofthe photocatalytic reforming of glucose on Pt-Ti02 involving the formation of various radicals, aldehydes, and carboxylic acids. Adapted from [170] (2008) with permission from Elsevier.

Maness, P.C., Smolinski, S., Blake, D.M., Huang, Z., Wolfrum, E.J., and Jacoby, W.A. (1999) Bactericidal activity of photocatalytic Ti02 reaction toward an understanding of its killing mechanism. Applied and Environment Microbiology,... 

Our aim is to disclose the mechanism of the photocatalytic effect. It is necessary first to understand why and how illumination, in general, influences the course of a heterogeneous catalytic reaction by stimulating or, on the contrary, retarding it. One has to understand why the effect is positive in some cases (acceleration of the reaction) and negative in others (retardation of the reaction), and how the sign of the effect is determined. Furthermore, it is necessary to find out upon what factors, and in what manner, the magnitude of the effect depends. We shall try to answer all these questions. 

The photoadsorption effect as such does not constitute the subject matter of the present article. We shall consider it very briefly, only to the extent necessary to allow one to draw analogies between the mechanisms of the photoadsorptive and photocatalytic effects. The photoadsorptive effect has been studied sufficiently well. A brief summary of the experimental data will be given below. The mechanism of the phenomenon has been thoroughly discussed in a number of theoretical works from the standpoint of the electronic theory of chemisorption and catalysis C3,4,6-8). 

In the present article the photocatalytic reaction of water oxidation is examined from the standpoint of the electronic theory. We shall analyze here one of the possible mechanisms of the reaction [see reference (7[Pg.197]

This general recipe may be applied to any other photocatalytic reaction. This will require a knowledge of the electronic mechanism of the corresponding reaction in the dark. Such a mechanism is by no means always unambiguous and its choice should be based on a number of subsidiary considerations. The regularities of the photocatalytic effect may prove different depending on the electronic mechanism of the dark reaction involved. In such a case, a comparison of theory with experiment can yield additional information in favor of or against the supposed electronic mechanism. 

Domen, K., Kudo, A., and Onishi, T., Mechanism of photocatalytic decomposition of water into hydrogen and oxygen over nickel (II) oxide-strontium titanate (SrTiOs), /. Catal, 102,92,1986. 

The ambient temperature and the possible use of solar UV are the advantages of photocatalysis moreover, Ti02 is not toxic. The reaction mechanisms of Ti02 photocatalytic oxidation of azo dyes was similar to the biodegradation process of oxidation of azo dyes with OH radical. 

Mills G, Hoffmann MR (1993) Photocatalytic degradation of pentachlorophenol on Ti02 particles identification of intermediates and mechanism of reaction. Environ Sci Technol 27 1681-1689... 

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