**2. Titanium dioxide (TiO2) as a catalyst**

Generally, the semiconductor metal oxides are used as photocatalysts in photocatalysis. Fujishima et al., the pioneer of photocatalysis, employed titanium dioxide (TiO2) as a catalyst for the production of hydrogen gas by splitting water for the first time [22]. Water can not be decomposed by visible light because it is transparent to it. Water can only be decomposed if it is irradiated with light having a wavelength less than 190 nm. Fujishima and co-workers electrochemically decomposed water using TiO2 electrode. They reported that water can be electrochemically decomposed if a potential difference of 1.23 V is applied between anode and cathode. The potential difference of 1.23 V is equivalent to the energy of photons having wavelength about 1000 nm. Water can be electrochemically decomposed under any one of the following conditions.


Fujishima and co-workers used TiO2 electrode for electrothermal decomposition of water. They investigated the current–voltage curves under light condition and dark condition. They found that anodic current flowed under the irradiation of light having wavelength less than 415 nm. The energy of these radiation is equivalent to 3.0 eV. This energy is equal to the band gap of TiO2. On the basis of these observations, it was suggested that irradiation of light produced holes in the valence band of TiO2. Similarly, the production of oxygen at �0.5 V was also confirmed by various electrochemical measurement. They constructed an electrochemical cell. The TiO2 was used as electrode which was connected to a Pt electrode. The irradiation of surface of TiO2 electrode caused a current to flow from Pt electrode to TiO2 electrode. The flow of current from Pt electrode to TiO2 electrode suggested that production of oxygen takes place TiO2 electrode by oxidation reaction and production of hydrogen takes place at Pt electrode by reduction reaction. It was suggested irradiation caused decomposition of water in the absence of any external potential. The decomposition of water took place according to following reactions.

1.Production of hole and electron by excitation of TiO2

$$\text{TiO}\_2 \stackrel{2}{\rightarrow} \text{2h}^+ + \text{2e}^- \tag{1}$$

1.Production of oxygen by oxidation reaction at TiO2

$$\text{H}\_2\text{O} + 2\text{h}^+ \rightarrow \frac{1}{2}\text{O}\_2 + 2\text{H}^+ \tag{2}$$

1.Production of hydrogen by reduction at Pt

$$2\text{H}^+ + 2\text{e}^- \to H\_2 \tag{3}$$

The net decomposition reaction is

$$H\_2O + 2h\theta \to H\_2 + \frac{1}{2}O\_2\tag{4}$$

Since the work of Fujishima et al., TiO2 gained the attention of many researchers and presently it is the most used substance in the field of photocatalysis.

Titanium dioxide (TiO2) exists in three crystalline phases: Brookite, Anatase, and Rutile phase. The Brookite phase of titanium dioxide is unstable and therefore it is not used in photocatalytic applications. Anatase and Rutile phases of titanium dioxide are thermodynamically stable phases. The rutile phase is mostly used in photocatalytic applications due to its easy preparation and higher catalytic performance. Studies have shown that mixed phases of titanium dioxide are used as catalysts for higher photocatalytic performances. It is believed that mixed phases titanium dioxide exhibits higher catalytic performance due to the movement of photoinduced electrons from Rutile to Anatase phase of titanium dioxide. This movement of electrons prevents the recombination of positive holes and electrons and ultimately enhances the photocatalytic performance [23–25]. However, it has also been shown that electrons move from Anatase to Rutile phase in mixed-phase photocatalysts [26]. The band gap energy of the anatase and rutile phase of titanium dioxide is 3.2 eV and 3.0 eV, respectively. Irradiation of titanium dioxide with photons havening energy equal to or greater than 3.2 eV results in the excitation of an electron from the valence band to the conduction band. This excitation results in the formation of an electron–hole pair. These photo-induced charges move to the surface of titanium dioxide and promote a series of redox reactions. The positive holes lead to the formation of vacancies in titanium dioxide as well as excite the reduced species. The photoinduced electrons produce O2 • free radicals. These free radicals are highly reactive and unstable species, so they react further [27, 28]. This whole process can be summarized as follows.

*Photocatalytic Applications of Titanium Dioxide (TiO2) DOI: http://dx.doi.org/10.5772/intechopen.99598*

1.Production of positive hole and electron by excitation of TiO2

$$TiO\_2 + Irradiation \longrightarrow h^+(TiO\_2) + e^-(TiO\_2) \tag{5}$$

2.Recombination of positive hole and electron

$$h^+(\text{TiO}\_2) + e^-(\text{TiO}\_2) \to \text{Heat} \tag{6}$$

3.Production of OH radical by oxidation of water by reaction with positive hole

$$h^+(TiO\_2) + H\_2O \to OH^\bullet \tag{7}$$

4.Reduction of oxygen by reaction with electron

$$e^-(TiO\_2) + O\_2 \rightarrow O\_2^- \tag{8}$$

5.Production of OH radical by reaction of super oxide anion with water

$$O\_2^- + H\_2O \to OH^\bullet \tag{9}$$

6.Reaction of OH radicals with reactants

$$\text{Reactants} + \text{OH}^{\bullet \bullet} \to \text{Products} \tag{10}$$

The mechanism given above has been proposed based on electron spin resonance (ESR) and spin trapping studies. However, Ângelo [29] reported 82%

*Graphical representation of direct–indirect mechanism; a) direct transfer of positive holes, b) indirect transfer of positive holes.*

conversion of NO for a feed containing 75% NO and 25% RH with 20°C as dew point; the same work indicates that the water-adsorbed monolayer is reached for a relative humidity of 25%. If OH• were the main species in the conversion of NO, then the conversion of NO for dry feed would be much smaller. Hence, this study questions Eq. (3) in the above mechanism or otherwise the role of OH• radicals in photocatalysis. Montoya et al., [30] reported against the formation of OH• radicals by direct reaction of positive holes with water. They proposed a direct–indirect model (D-I) for titanium dioxide catalyzed reactions. The proposed mechanism is given and explained in **Figure 3**. They proposed two mechanisms for the transfer of interfacial charges.

