**5. Heterogeneous photocatalysis**

The heterogeneous photocatalysis is a process based on the absorption of UV-visible by a solid semiconductor. In the interface area between the solution and the electrically excited solid, a degradation reaction or transformation of pollutants may occur, without changing the chemical structure of the semiconductor (Custo et al., 2006).

According to Nogueira and Jardim (1998), a semiconductor is characterized by:


This generates vacancy electrons. The electron / vacancy pair migrates to the surface of the particle resulting in oxidation and reduction sites (Carp et al. 2004). Figure 1 illustrates schematically the behavior described above.

**Figure 1.** Electronic scheme of a photochemical process for heterogeneous photocatalysis (Ciola, 1981)

**Figure 2.** Crystal structures: anatase (a), rutile (b) and brookita (c)

TiO2 is a solid with a melting point of 1800 °C, and the ninth most abundant element corresponding to 0.63% of crust. The element described features 4 crystalline forms of anatase (tetragonal), brookita (orthorhombic), rutile (tetragonal) and TiO2 (B) monoclinic, as shown in Figure 2 (Carp et al., 2004).
