Photocatalysis mechanisms

The diameter of the electrospun Ti02 nanofibers has also been found to play an important role on their photocatalytic activity. Pan and co-workers prepared pure anatase Ti02 nanoflbers with different diameters (92, 120, 205, 245 nm) via an electrospinning process and followed the calcination treatment only by adjusting the content of tetrabutyl titanate in the precursor solution [17]. The photocatalytic property studies showed that the activity increased with the increasing of the fiber diameter up to about 200 nm and then it decreased (Fig. 15.2). The photocatalysis mechanism might be due to dynanfics of the recombination of electron/hole on the surface sites. 

Schneider J, Matsuoka M, Takeuchi M et al (2014) Understanding Ti02 photocatalysis mechanisms and materials. Chem Rev 114 9919-9986... 

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). 

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 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. 

In 1991 and 1993, Ziessel123,129 reported mechanistic studies for the photocatalysis of the Ir(m) complexes, and proposed the mechanism shown in Scheme 41 to explain the results. A condensed version of the work was arranged by Vlcek.132 Many intermediates were isolated or detected spectroscopically using NMR, infrared spectroscopy, or UV-Vis, and XRD was employed to determine the detailed... 

Photobleach mechanism, 19 203 Photobleach reversal grains, 19 201 Photocatalysis, 19 73-106. See also Photocatalysts Photoreactors aqueous pollutants eliminated and mineralized by, 19 89t catalyst modifications in, 19 94-95 catalysts in, 19 75-76 challenges in, 19 101-102 fate of photo-holes in titania, 19 82-85 in fine chemistry applications, 19 102 influence of oxygen pressure in, 19 82 ion doping in, 19 94-95 mass of catalyst in, 19 77-78 noble metal deposit in, 19 94 parameters governing kinetics in, 19 77-82... 

The deposition of nanostructured Ti02-based material directly on glass is expected to achieve enhanced photocatalysis and mechanical strength. Sputtering and a combination of sputtering and sol-gel techniques seem to be the most adequate for such a purpose [91,97,98], In the latter case, porous alumina Trims on glass formed by anodizing sputter-deposited A1 layers were used as templates in the successive sol-gel process. 

The above mechanism suggests that the presence of adsorbed oxygen 02(ads) is essential for photocatalysis. It allows an increase of hole lifetime by reaction with an electron and the formation of oxidizing OH radicals. 

Fig. 3.77. Proposed degradation pathways for MeOr and MeRed during photocatalysis. The schemes illustrate that two mechanisms of degradation are independently active. Reprinted with permission from R. Comparelli et al. [143],...

Serpone N, Pelizzetti E (1989) Photocatalysis Fundamentals and Applications, Wiley, New York Schiavello M, Dordrecht H (1985) Photoelectrochemistiy, Photocatalysis, and Photoreactors Fundamentals and Developments Kluwer Academic, Boston, MA Linsebigler AL, Lu G, Yates JT (1995) Photocatalysis on Ti02 surfaces principles, mechanisms, and selected results. Chem Rev 95 735-758... 

In early works, the reactivity associated widi TiO2 photocatalysis was assigned to free OH radicals, but more recently die oxidative species has been identified as bound HO radicals and photogenerated holes. Reaction of the substrate with these species would require either die direct adsorption of organic compounds to the semiconductor interface or Fickian diffusion of the substrate to the semiconductor surface under depletion conditions. However, these two pathways imply different mechanisms, and in some cases different reaction products or intermediates. 

The majority of studies on the TiOz photocatalysis of organophosphorus compounds rely primarily on the disappearance of the initial substrate coupled with the monitoring of the mineralization products. Because of the complex mixtures formed in the TiOz photocatalysis of these compounds, there is limited information on the reaction pathways and mechanisms involved in the degradation processes. A recent report by Konstantinou et al. identified a number of intermediate products in the degradation of dichlofenthion and bromophos methyl by TiOz... 

When titanium oxides are irradiated with UV light that is greater than the band-gap energy of the catalyst (about X < 380 nm), electrons (e ) and holes (h+) are produced in the conduction and valence bands, respectively. These electrons and holes have a high reductive potential and oxidative potential, respectively, which, together, cause catalytic reactions on the surfaces namely photocatalytic reactions are induced. Because of its similarity with the mechanism observed with photosynthesis in green plants, photocatalysis may also be referred to as artificial photosynthesis [1-4]. As will be introduced in a later section, there are no limits to the possibilities and applications of titanium oxide photocatalysts as environmentally harmonious catalysts and/or sustainable green chemical systems. ... 

The absorption of light by semiconductors cremes electron-hole pairs e h+) which can be separated because their components diffuse in different directions. The energies of these moieties can be stored by several mechanisms or used in photocatalysis or photosynthesis for nitrogen fixation, formation of amino acids, methanol, etc. The efficiencies of such conversions depend almost entirely upon the semiconductor material, and as yet these efficiencies are too low for significant application. Currently the most promise is demonstrated by the use of titania on a platinum substrate or single crystals of strontium titanate. See also Photoelectric Effect. 

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)... 

Recently, liquid water was decomposed to hydrogen and oxygen stoichiometrically and continuously by irradiations of ultrasound and light with particulate photocatalyst.n) This reaction system is thought to be a joint one for sonolysis and photocatalysis. Furthermore, this system also is a hybrid of mechanical effects and chemical effects. In this chapter, the effect of ultrasound on photocatalytic reaction is considered. The joint system of sonochemical and photocatalytic reactions, in particular s explained. 

In order to understand the reaction mechanism of photocatalysis, a simple kinetic model can be presented as follows. 

If a two-step mechanism, as shown in Eqs. (12.7) and (12.8), occurs, each product will be produced independently by alternating irradiation of ultrasound and light. An alternating irradiation was performed for overall water splitting under argon atmosphere. Ultrasonic wave must be irradiatea first followed by photoirradiation. This oraei of irradiation was decided from tbe scheme of sonophotocatalysis of water, as shown in Eqs. (12.7) and (12.8). As shown in Fig. 12.12, H2 and 02 were evolved by sonolysis and photocatalysis, respectively Thus each gaseous product was collected independently. 

As reported by Augugliaro et al. [64] the photocatalysis can be combined with chemical or physical operations. In the first case, when the coupling is with ozonation [65, 66], ultrasonic irradiation, photo-Fenton reaction or electrochemical treatment, which influence the photocatalytic mechanism, an increase of the efficiency of the process is obtained. 

Coupling photocatalysis with a physical technologies, such as biological treatment [67, 68], membrane reactor [39] or physical adsorption, the combination does not affect the mechanisms but increases the efficiency of the whole process. 

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