Adsorption photocatalysis

Table 20. Removal of pollutants using chitosan-ri02 catalyst through adsorption-photocatalysis process. 

Perovskites with specifically morphologies also exhibit other exeellent properties in gas sensoring, adsorption, photocatalysis, CO oxidation, and methanol reforming. 

Adsorption/desorption [24-25], as well as mass transfer phenomena [26] were also proved to be important for photocatalysis. 

Economic feasibility studies suggest that even at the present state of the art photocatalytic technology indeed can be competitive with the traditional carbon adsorption or incineration technologies in treatment of contaminated soil vapor extraction vents and small scale VOC-containing vents [28]. Rapid progress in basic and applied research in photocatalysis suggests... 

One of the most important fields of application of photocatalysis is the photodegeneration of organic compounds. These processes are used in particular for environmental decontamination, especially for wastewater treatment and air purification, because of the ability of semiconductors to totally degrade organics to C02, H 20, and inorganic anions under U V or visible light. This behavior is attributed to the photoinduced formation of radicals, such as OH, or to the adsorption and direct degradation of the pollutants. 

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. 

Photocatalysis uses semiconductor materials as catalysts. The photoexcitation of semiconductor particles generates electron-hole pairs due to the adsorption of 390 run or UV light of low wavelength (for Ti02). If the exciting energy employed comes from solar radiation, the process is called solar photocatalysis [21],... 

Other composite photocatalysts were prepared by mounting immobilized anatase particles on mesoporous silica and silica beads [189-191], The behavior of anatase-mounted activated carbons was also studied in detail [192-194], It was even suggested that carbon-coated anatase exhibits better performance in photocatalysis than anatase itself, demonstrating high adsorptivity, inhibition of interaction with organic binders, etc. [195,196],... 

Adsorption influences the reactivity of surfaces. It has been shown that the rates of processes such as precipitation (heterogeneous nucleation and surface precipitation), dissolution of minerals (of importance in the weathering of rocks, in the formation of soils and sediments, and in the corrosion of structures and metals), and in the catalysis and photocatalysis of redox processes, are critically dependent on the properties of the surfaces (surface species and their strucutral identity). 

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. 

A variety of models have been derived to describe the kinetics of semiconductor photocatalysis, but the most commonly used model is the Langmuir-Hinshel-wood (LH) model [77-79]. The LH model relates the rate of surface-catalyzed reactions to the surface covered by the substrate. The simplest representation of the LH model [Eq. (7)] assumes no competition with reaction by-products and is normally applied to the initial stages of photocatalysis under air- or oxygen-saturated conditions. Assuming that the surface coverage is related to initial concentration of the substrate and to the adsorption equilibrium constant, K, tire initial... 

Humidity has a significant influence on the photocatalytic oxidation of aromatic contaminants in the gas phase. This is of particular interest, because commercial photocatalytic systems will be required to operate under a broad range of relative-humidity levels. The specific influence of relative humidity on the photocatalytic reaction has generally proved rather difficult to quantify because water has a dual role It may compete with contaminants for surface adsorption sites (a negative influence) and it plays a role in the regeneration of surface hydroxyl groups during photocatalysis (a positive influence). 

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. 

Owing to adsorption, which concentrates dilute pollutants at the Ti02 surface where the active species are produced and/or can interact, photocatalysis is very appropriate in purifying/deodor-izing indoor air, as well as gaseous and aqueous effluents containing only traces of toxic and/or malodorous pollutants. 

In contrast to technologies that are exclusively based on adsorption or absorption phenomena and result in pollutant transfer with the need for supplementary treatments, photocatalysis completely mineralizes the organic pollutants or, at least, enables... 

Kinetic studies of photoreactions on semiconductor nanoparticles are important for both science and practice. Of scientific interest are the so-called quantum size effects, which are most pronounced on these particles shifting the edge of adsorption band, participation of hot electrons in the reactions and recombination, dependence of the quantum yield of luminescence and reactions on the excitation wavelength, etc. In one way or another all these phenomena affect the features of photocatalytic reactions. At present photocatalysis on semiconductors is widely used for practical purposes, mainly for the removal of organic contamination from water and air. The most efficient commercial semiconductor photocatalysts (mainly the TiC>2 photocatalysts) have primary particles of size 10-20 nm, i.e., they consist of nanoparticles. Results of studying the photoprocesses on semiconductor particles (even of different nature) are used to explain the regularities of photocatalytic processes. This indicates the practical significance of these processes. 

In an earlier review of the applications of photoluminescence (PL) techniques to the characterization of adsorption, catalysis, and photocatalysis (Anpo and Che, 1999), we addressed the basic principles of PL and the importance of PL measurements for understanding of (photo)catalytic processes. This chapter describes more recent developments and focuses on investigations of catalysts in the working state, with an emphasis on the role of local structure on photocatalytic reactions determined with PL and related techniques. 

The preceding review of PL which appeared in this series (Anpo and Che, 1999) gave the principles and applications of this technique to the investigation of solid surfaces in relation to adsorption, catalysis, and photocatalysis. 

Gogniat, G., Thyssen, M., Denis, M., Pulgarin, C. and Dukan, S. (2006) The bactericidal effect of Ti02 photocatalysis involves adsorption onto catalyst and the loss of membrane integrity. FEMS Microbiol. Lett. 258, 18-24. 

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