Photocatalysis acceleration

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). 

Sonophotocatalysis is photocatalysis with ultrasonic irradiation or the simultaneous irradiation of ultrasound and light with photocatalyst. Tnis method includes irradiation with alternating ultrasound and light. Ultrasound effects on heterogeneous photocatalytic reaction systems have been demonstrated by Mason,1 Sawada et al.,2) Kado et al.,3) and Suzuki et al.4) In these papers, not only acceleration of photocatalytic reactions but increase in product selectivity by ultrasonic irradiation has also been reported. It was postulated that ultrasound effects, such as surface cleaning, particle size reduction and increased mass transfer, were the result of the mechanical effects of ultrasound.1,5) Lindley reviewed these and other effects.5)... 

Hattori, A., Y. Tokihisa, H. Tada and S. Ito (2000). Acceleration of oxidations and retardation of reductions in photocatalysis of a Ti02/Sn02 bilayer-type catalyst. Journal of the Electrochemical Society, 147(6), 2279-2283. 

A different type of synergy was foimd in a binary Cu(II) /Cr(VI) system imder N2 and UV light, acceleration of both Cr(VI) and Cu(II) reduction was observed (Goeringer et al., 2001). While no explanations were provided concerning this behavior, it can be proposed that the enhancement may be due to the formation of a layer or spots of metallic copper that promote photocatalysis. 

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. 

In a broad sense then, the label photocatalysis describes a photochemical process in which the photocatalyst accelerates the process, as any catalyst must do according to the definition of catalysis. 

Photochemical catalysis. The term photocatalysis may be mislead- ing. Moore and Pearson (1981) contend that catalysis by light is an improper phrase they argue that when the rate of a reaction is accelerated by a means I other than by a chemical species, it should not be classified as catalytic. However, j it is clear that some reactions, which are thermodynamically favorable (i.e., j AG°xn < 0), can be assisted by the interaction with light in the UV-visible range. These reactions are sometimes called photoassisted reactions. They can occur either homogeneously or heterogeneously. 

Homogeneous doping of semiconductor particles with a small amount of metal ions such as Fe " and V " prolongs the electron-electron-hole separation and hence increases the photocatalytic efficiency [46,47]. However, doping of Ti02 with metal ions such as Cr + and Sb + creates electron acceptor and donor centers that accelerate the charge recombination—an undesirable result for photocatalysis [48-50]. 

No general agreement exists on the definition of the term photocatalysis. Generally it implies that light and a catalyst are necessary to induce or accelerate a chemical transformation [1-9]. This definition includes photosensitization, a process by which a photochemical alteration occurs in one molecular entity as a result of initial absorption of radiation by another molecular entity called photosensitizer [10], but it excludes the photoacceleration of a stoichiometric thermal... 

Photocatalysis is the acceleration of a photoreaction in the presence of a catalyst such as Ti02. 

Photocatalysis refers to a process in which light and a catalyst complex bring about or accelerate a chemical reaction. The subject area has been extensively reviewed, and numerous examples of synthetically useful photocatalytic transformations have been cataloged and tabulated. Photocatalysis is also useful for the activation of small... 

The growth in the number of publications on photocatalysis has been accelerating steadily over the past quarter century, as seen in Fig. 1. The total number of papers that have appeared up to September 2000 is slightly more than 5000 and the number of patents is just more than... 

Finally the heterogeneous photocatalysis technique has been extended in scope to include the treatment of contaminated air and gas-phase pollutants. Photocatalyst suspensions, while they offer several advantages associated with huge surface area and consequently large reaction cross- sections, simplicity of reactor etc., are also limited in that a bias potential cannot be applied to further accelerate the photoprocess. Further, photocatalyst recovery after use is... 

For Fe-modifled Ti02 containing a carbon residue (FSC-series in Table 3.9), a marked acceleration of Ph decomposition was observed through the combination of two processes photo-Fenton [312-318] and photocatalysis in solution. 

Early photochemical observations with coordination compounds typically amounted to the finding that the known thermal reaction, be it redox decomposition or ligand substitution, was accelerated by light. Such behavior was often referred to as "photocatalysis", as we did in Ref. 1. Later, it seemed better to use more specific terms such as "photochemistry", "photo-induced reaction", "photosubstitution", etc. [19]. There was a progression in the sophistication of photochemical studies. With exceptions, early reports simply noted that the compound changed on exposure to... 

Among the first attempts to define photocatalysis more precisely Plotkinow s contributions have to be considered. In his classical textbook on photochemistry [2] he called all chemical reactions accelerated by the influence of light photo-catalytic ones. This definition may be regarded as starting point for all further discussions concerning the phenomenon of... 

As mentioned previously the term photocatalysis was used by Plotkinow to describe the acceleration of a chemical reaction due to the influence of light. Since this definition applies for any photochemical reaction there is no inventive power in it. Also a restriction of the acceleration by light only to endergonic reactions, as proposed by Balzani[3], does not improve the situation. 

Goutailler G., Guillard C., Daniele S., Hubert-PfalzgrafL.G. Low temperature and aqueous sol-gel deposite of photocatalytic active nanoparticulate Ti02. J. Mater. Chem. 2003 13 342-346 Hattori A., Tokihisa Y., Tada H., Ito S. Acceleration of oxidation and retardation of reductions in photocatalysis ofaTi02/Sn02 bilayer-type catalyst. J. Electrochem. Soc. 2000 147 2279-2283... 

A substance can be thought to be a catalyst when it accelerates a chemical reaction without being consumed as a reactant that is to say, it appears in the rate expression describing a thermal reaction without appearing in the stoichiometric equation [49], A catalyst is a compound that lowers the free activation enthalpy of the reaction. Then, photocatalysis can be defined as the acceleration of a photoreaction by the presence of a catalyst [38, pp. 362-375], This definition, as pointed out in [29, pp. 1-8], includes photosensitization, a process by which a photochemical alteration occurs in one molecular entity as a result of initial absorption of radiation by another molecular entity called the photosensitizer [13], but it excludes the photoacceleration of a stoichiometric thermal reaction irrespective of whether it occurs in homogeneous solution or at the surface of an illuminated electrode. Otherwise, any photoreaction would be catalytic [29, pp. 1-8]. Depending on the specific photoreaction, the catalyst may accelerate the photoreaction by interaction with the substrate in its ground or excited state and/or with a primary photoproduct. 

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