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According to Fujishima et al. (2000), the ideal photocatalyst should possess the following properties.

• (a) Photo activity, low cost, and non-toxic nature: For large scale environmental clean up operations, the photocatalyst has to be available at low cost without toxic properties to flora and fauna when used at large concentrations.
• (b) Biological and chemical inertness: The catalyst used in the process of substrate treatment must not promote the formation of other complex and undesired substrates.
• (c) Stability towards photocorrosion: The catalyst must not undergo corrosion under prolonged light exposure; it has to be stable to tolerate the long light exposure which will be persistent throughout the process.
• (d) Suitability towards visible or near UV light: An excitation source which is essential for the initiation of reaction must be of low energy in the visible of near UV region so that the high radiation energy is not required.

Heterogeneous photocatalysis is type of interaction where the reactant and catalyst exist in different phases. This type of reaction generally involves the following five steps (Shan et al., 2010): (i) diffusion of reactants to the surface of catalyst, (ii) adsorption of reactants onto the surface, (iii) reaction on the surface, (iv) desorption of products from the surface, and (v) diffusion of products from the surface (Devilliers, 2006) (Fig. 1).

Fig. 1. 
Diagrammatic representation of Photocatalysis mechanism (Demeestere et al., 2007).

There are various indoor pollutants which can be responsible to cause various chronic health problems. Radons, formaldehyde, and VOC are common examples of indoor air pollutants. The number of aromatic VOC including benzene, toluene, and xylene (commonly called BTX) has been treated by the photocatalytic method (Pichat et al., 2000). The TiO₂ photocatalytic treatment of toluene is known to yield benzaldehyde, with high applications in synthetic industry (Sleiman et al., 2009). TiO₂ also has high capacity to deodorize the indoor air. The deodorization may be considered as the outcome of oxidation of the pollutant(Wei et al., 2010). A photo-reactor coated with TiO₂ fiber glass mesh in the presence of artificial radiation light at approximately 365 nm (UV) is found to be very efficient in the mineralization of BTX compounds (Pichat et al., 2000). This oxidation can be further enhanced under the presence of strong oxidants (like ozone) in the indoor environmental conditions. Doping of TiO₂ can further increase its catalytic efficiency (Park et al., 2004). VOCs in urban atmosphere are produced mainly by automobile exhausts. The majority of them are very stable recalcitrant compounds. TiO_2^-SiO₂ based photocatalysis was thus employed for the removal of VOC (Zou et al., 2006). The catalyst can act as adsorbing as well as catalyzing agent. The amounts of hydrocarbons and carbondioxide are observed as the end products by such photocatalytic treatment of VOC. The presence of NO (along with VOC) showed some interfering effects (Ao et al., 2003). In addition, it is also reported that humidity as well reactive nature of NO with VOCs had huge influence on the progress of reaction.

TCE is a very harmful widespread gaseous air pollutant, being carcinogenic in nature. When gas phase TCE is subjected to photodegradation in packed bed continuous flow reactor, it yielded many intermediate products like phosgene, dichloroacetyl chloride (DCAC), chloroform, hexachloroethane, alcohols, esters, aldehydes, carbon monoxide, and carbon dioxide (Wang et al., 2002). Phosgene emission can be reduced by using allopane clay suspended TiO₂. Here, phosgene is adsorbed on to surface of allopane clay catalyst, and it can be degraded gradually (Nishikiori et al., 2011). TCE degradation mechanisms usually begin with the Cl^- subtraction by the attack of hydroxyl radical of water present in air (Amama et al., 2001). TCE has also been removed by employing degussa titania coated reactor wall deposited on silica based support (Mohseni, 2005). A large amount of TCE can be removed by this process. The removal efficiency of TCE is very high due to the addition of silica support on degussa, as it provides higher interactive surface area for catalyst and reactant. The intermediates formed in the presence of silica-based support and in its absence can exhibit huge variations. Pure degussa titania interaction with TCE resulted in phosgene and dichloro acetyl chloride (DCAC) as end product (Mohseni, 2005). However, in the presence of silica based support, the DCAC was not formed. Instead, a very little amounts of carbon tetrachloride and chloroform were formed. As such, the efficiency of TCE photodegradation is very high, when TiO₂ is doped with high valency cations (Park et al., 2004).

NO₂ is harmful and reactive compound and is emitted from various industrial sources (Latza et al., 2009). For removal of NO₂, its photocatalytic reduction to NO by cuppric ion loaded thin TiO₂ films was studied (Ohko et al., 2009). As cupric ion can induce photocatalytic reduction of NO₂ to NO in air, it can also act as oxidizing agent to yield HNO₃ from NO. As such, cuppric ion can be used to provide delicate redox conditions. However, oxidation efficiency of NO to HNO₃ decreased, as the accumulation of HNO₃ proceeds. In an alternative method, the photocatalytic oxidation of NO can take place under some specified hydrothermal conditions. The P25 degussa titania, commercially produced by a German company (Anatase and Rutile in ratio of 3 : 1), showed best oxidation of NO, if the catalyst is calcined at temperature of 200 degree for the duration of 24 hrs(Wu et al., 2008). The application of photocatalytic concrete material in urban places has also been very effective in the treatment of NO₂ at highly polluted urban areas (Ballari et al., 2010).

Hydrogen sulphide is a colorless, poisonous, flammable gas with the characteristic foul odor of rotten egg (Kourtidis et al., 2008; Lambert et al., 2006). It often results from the bacterial breakdown of organic matter in the absence of oxygen, such as in swamps and sewers (anaerobic digestion) (Chaiprapat et al., 2011). It also occurs in volcanic gases, natural gas, and some well waters. H₂S can also be subject to photodegradation by chromium doped molecular seives in the presence of TiO₂ (Portela et al., 2008). The chromium doped mesoporous crystalline molecular (Cr-MCM) sieves, prepared by hydrothermal method, are impregnated with TiO₂. During the photodegradation, there was no oxidation of H₂S to sulfurdioxide. However, the accumulation of sulphates deactivated the catalytic property of chromium. As another approach of H₂S treatment, its photocatalytic decomposition was carried out in the presence of CdS catalyst. It allows the decomposition and photodegradation simultaneously with the production of H₂ (Bai et al., 2010).

Benzene is a flammable, colorless and carcinogenic compound (Bird et al., 2005). It is naturally produced, when wood is completely burned (during forest fires) or volcanic eruptions (Capaccioni et al., 2004). The commenest exposure to benzene is through the inhalation of cigarette smoke or car exhaust (Edwards and Jhantunen, 2001) (Weisel, 2010). This extremely toxic compound was discovered in the gas emitted by burning coal (The Lancet, 1904).

The photocatalytic oxidation of benzene can be carried out utilization of doped TiO₂ catalysts (Zhong et al., 2007; Fu et al., 1995). The doped catalyst exhibited high decomposition of benzene than bare TiO₂. It is highly reactive in the presence of sunlight, indicative of high potential in solar photocatalysis. The addition of Pd in to TiO₂ increases photocatalytic activity by more than 2.3 times than pure TiO₂ (Zhong et al., 2009).

Choloroaniline is a typical semi-volatile aromatic amine (Kataoka, 1996). It is precursor for various antibacterial products. It is strong irritant with the potential for tissue damage. The photocatalytic degradation of chloramines containing gases can be achieved very rapidly by TiO₂ catalyst (An et al., 2011). Cholorobenzene and phenols are formed as intermediates of this degradation. Here the degradation as well as detoxification of the gaseous aromatic amines is simultaneously observed by this method.

Air usually contains high amount of micro-organisms. There are various airborne diseases which are transmitted through air (like tuberculosis, whooping cough, etc) (Martinez et al., 2008). The application of photocatalysis for the disinfection of microbes has been exercised. The Ag doped TiO₂ acts as a very good disinfecting agent (Akhavan, 2009). The catalyst coated filters when passed over recirculation air, can induce disinfection of various microorganisms (e.g., Bacillus cereus, Staphylococcus aureus, Escherichia coli, and Aspergillus niger) which are used as indices for disinfecting property (Vohra et al., 2006). Photocatalysis is also very efficient in the inactivation of microbial growth. The principle of disinfection of photocatalyst can be expressed as the disintegration of cells by the activity of OH radical which oxidizes the intracellular components of micro-organisms (Cheng et al., 2007). Thus, this method can be very beneficial under rapidly infectious conditions.