Photocatalysis/photocatalyst catalyst

The challenges to be faced in air-purification systems using photocatalysis involve the treatment of relatively large gas flows in devices with low pressure drops, good catalyst irradiation, and efficient reactant species as well as good photocatalyst contacting [51-53]. 

However, the pathways for these reactions, particularly in the gas phase, have been only -.rtially characterized. In a wide variety of these reactions, coordinatively unsaturated, highly reactive metal carbonyls are produced [1-18]. The products of many of these photochemical reactions act as efficient catalysts. For example, Fe(C0)5 can be used to generate an efficient photocatalyst for alkene isomerization, hydrogenation, and hydrosilation reactions [19-23]. Turnover numbers as high as 3000 have been observed for Fe(C0)5 induced photocatalysis [22]. However, in many catalytically active systems, the active intermediate has not been definitively determined. Indeed, it is only recently that significant progress has been made in this area [20-23]. 

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... 

In classical kinetic theory the activity of a catalyst is explained by the reduction in the energy barrier of the intermediate, formed on the surface of the catalyst. The rate constant of the formation of that complex is written as k = k0 cxp(-AG/RT). Photocatalysts can also be used in order to selectively promote one of many possible parallel reactions. One example of photocatalysis is the photochemical synthesis in which a semiconductor surface mediates the photoinduced electron transfer. The surface of the semiconductor is restored to the initial state, provided it resists decomposition. Nanoparticles have been successfully used as photocatalysts, and the selectivity of these reactions can be further influenced by the applied electrical potential. Absorption chemistry and the current flow play an important role as well. The kinetics of photocatalysis are dominated by the Langmuir-Hinshelwood adsorption curve [4], where the surface coverage PHY = KC/( 1 + PC) (K is the adsorption coefficient and C the initial reactant concentration). Diffusion and mass transfer to and from the photocatalyst are important and are influenced by the substrate surface preparation. 

Practical application of the results of photocatalysis will depend much on the capability of the catalysts to work under visible fight conditions. Therefore, the design of new photocatalysts either by modification of the existent TiOz or by discovery of new materials will be a key step in developing... 

Applications of titania nanotube arrays have been focused up to now on (i) photoelectrochemical and water photolysis properties, (ii) dye-sensitized solar cells, (iii) photocatalysis, (iv) hydrogen sensing, self-cleaning sensors, and biosensors, (v) materials for photo- and/or electro-chromic effects, and (vi) materials for fabrication of Li-batteries and advanced membranes and/or electrodes for fuel cells. A large part of recent developments in these areas have been discussed in recent reviews.We focus here on the use of these materials as catalysts, even though results are still limited, apart from the use as photocatalysts for which more results are available. 

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. ... 

Various efforts to apply photocatalysis to photoenergy conversion are described in Part III. Synthetic chemistry utilizing photocatalysis by semiconductors has been attracting attention as discussed in Chapters 11 and 12. The merits of the photocatalysts for synthetic chemistry are (a) multiple processes are possible, (b) catalysts can be separated easily and re-used, and (c) the reactions can proceed under ambient conditions, etc. (Chapter 11). In Chapter 12 photolysis and sonolysis are combined to obtain specific effects in addition to photochemical reactions. 

The catalytic reaction involving light absorption by a substrate and catalyst species is called photocatalysis [29], At the end of the reaction cycle, the photocatalyst is regenerated to its original state. Sometimes, however, the agent that initiates certain chemical transformations under the action of light is itself consumed in the reaction or process then it is called a photoinitiator [127-129]. 

As research into gaseous photocatalysis progressed a potential major disadvantage was the possibility of catalyst deactivation. Einaga et al. [208] concluded that the key factors which influenced catalyst deactivation were the formation of carbon deposits on the photocatalyst and their decomposition to CO. The photo oxidation rate of benzene decreased with decreasing humidity due to the increasing amount of carbon deposits on the catalyst, however, photo irradiation in humidified air decomposed the deposits and regenerated the catalyst [208]. 

The formation of halohydrins can be promoted by peroxidase catalysts.465 Recently 466 it has been shown that photocatalysis reactions of hydrogen peroxide decomposition in the presence of titanium tetrachloride can produce halohydrins. The workers believe that titanium(IV) peroxide complexes are formed in situ, which act as the photocatalysts for hydrogen peroxide degradation and for the synthesis of the chlorohydrins from the olefins. The kinetics of chlorohydrin formation were studied, along with oxygen formation. The quantum yield was found to be dependent upon the olefin concentration. The mechanism is believed to involve short-lived di- or poly-meric titanium(IV) complexes. 

In catalyzed photolysis either the catalyst molecule (Fig. 5-11, situation B) or the substrate molecule (Fig. 5-11, situation C), or both, are in an electronically excited state during the catalytic step. The electronically excited catalyst molecule is produced via photon absorption by a nominal catalyst (Fig. 5-11, situation B). The reaction of substrate to product is catalytic with, respect to the concentration of the electronically excited catalyst species. It is non-catalytic in photons and therefore, continuous irradiation is required to maintain the catalytic cycle. The quantum yield of product formation Product is equal to or less than unity. Titanium dioxide photocatalysis is the most widely applied example of this type, with Ti02 representing the nominal catalyst that must be electronically excited by photon absorption with formation of the electron hole pair Ti02 (hvb + cb), being the active catalytic species (cf Fig. 3-17 and Fig. 5-9, reaction 1). The oxidation of substrates by the combination of UV/VIS radiation and an appropriate photocatalyst is often called photocatalytic oxidation (PCO). 

The difficulty of quantum yield determination in heterogeneous photocatalysis is related to reflection and scattering of UV/VlS radiation by suspended catalyst grains. Consequently, Serpone and Salinaro (1999) proposed a detailed protocol for the determination of photonic efficiencies (quantum yields) or relative photonic efficiencies, suggesting the photocatalyzed degradation of phenol as a standard reaction (using Ti02 as photocatalyst). 

Photocatalysis A catalytic reaction triggered or enhanced by illuminating the system with visible or ultraviolet irradiation. This reaction involves normally the electronic excitation of the catalyst via the absorption of photons and an interfacial charge transfer to an adsorbed species. Typically, the photocatalyst is not consumed in the reaction. 

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. 

On the basis of the above discussion, in its most simple description photocatalysis implies a catalysed process preceded by absorption of a photon by a material acting as the catalyst. Where the photocatalyst is a semiconductor nanoparticulate system, e.g. TiOi, which is the material treated in most detail in this chapter, absorption of photons of energy greater than 3.2 eV (for anatase 3.0 eV for the rutile polymorph) leads to formation of conduction-band electrons and valence-band holes, which subsequently diffnse to the particle surface in competition with bnlk recombination. 

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