**1. Introduction**

340 Advanced Aspects of Spectroscopy

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Volatile organic compounds (VOCs) are emitted as gases from certain solids or liquids. VOCs include a variety of chemicals, some of which may have short- and long-term adverse health effects. Concentrations of many VOCs are consistently higher indoors (up to ten times higher) than outdoors.

The control of VOCs in the atmosphere is a major environmental problem. The traditional methods of VOCs removal such as absorption, adsorption, or incineration, which are referred to the new environmental condition have many technical and economical disadvantages. So in recent years, some new technologies called advanced oxidation processes (AOPs), such as biological process, photo-catalysis process or plasma technology, are paid more and more attention.

Advanced oxidation processes (AOPs) are efficient novel methods useful to accelerate the non-selective oxidation and thus the destruction of a wide range of organic substances resistant to conventional technologies. AOPs are based on physicochemical processes that produce in situ powerful transitory species, principally hydroxyl radicals, by using chemical and/or other forms of energy, and have a high efficiency for organic matter oxidation.

Among AOPs, photocatalysis has demonstrated to be very effective to treat pollutants both in gas and in liquid phase. The photo-excitation of semiconductor particles (TiO2) promotes an electron from the valence band to the conduction band thus leaving an electron deficiency or hole in the valence band; in this way, electron/hole pairs are generated. Both reductive and oxidative processes can occur at/or near the surface of the photo-excited semiconductor particle.

© 2012 Liang et al., licensee InTech. This is an open access chapter distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2012 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Photocatalytic degradation of VOCs on UV-illuminated titanium dioxide (TiO2) is proposed as an alternative advanced oxidation process for the purification of water and air. Heterogeneous photo-catalysis using TiO2 has several attractions: TiO2 is relatively inexpensive, it dispenses with the use of other coadjutant reagents, it shows efficient destruction of toxic contaminants, it operates at ambient temperature and pressure, and the reaction products are usually CO2 and H2O, or HCl, in the case of chlorinated organic compounds. Decomposition path of VOCs with UV/TiO2 or UV/TiO2/doped ions is shown in Fig. 1. However, the formations of by-products, such as CO, carbonic acid and coke-like substances, were often observed. These by-product formations are due to low degradation rate of intermediate compounds that are formed by the partial oxidation of VOCs. In order to improve the VOC degradation rate, some authors reported on the enhancement of VOC degradation through the addition of anions (dopant = S, N, P, etc), cations (dopant = Pt, Cu, Mg, etc), polymers or co-doped with several ions on TiO2, while the difference between doping agents has not been discussed yet.

**Figure 1.** Decomposition path of VOCs with UV/TiO2.

In this chapter, toluene, p-xylene, acetone and formaldehyde were chosen as the model VOCs because they were regarded as representative indoor VOCs for determining the effectiveness and capacity of gas-phase air filtration equipment for indoor air applications, the photo-catalytic degradation characters of them by TiO2/UV, TiO2/doped Ag/UV and TiO2/doped Ce/UV was tested and compared. The effects of hydrogen peroxide, initial concentration, gas temperature, relative humidity of air stream, oxygen concentration, gas flow rate, UV light wavelength and photo-catalyst amount on decomposition of the pollutants by TiO2/UV were analyzed simultaneously. Furthermore, the mechanism of titania-assisted photo-catalytic degradation was analyzed, and the end product of the reaction using GC-MS analysis was also performed.
