water. For this photocatalytic application, among the common crystalline forms of
titania, anatase is generally recognized to be the most active phase.
2.3.2 Mechanisms of Photocatalysis and Principle
of Pollutant Degradation
Titanium dioxide photocatalysis and photodegradation mechanisms (Fig. 2.4a) are
well described in literature (Carp et al. 2004; Gaya and Abdullah 2008; Chong et al.
2010; Ajmal et al. 2014; Dong et al. 2015; Cai et al. 2017). Below we will just give a
short overview of the basic principles of both processes. The photocatalytic properties of TiO 2 are evidenced by Fujishima and Honda in 1972 because when the
photons energy of incident light is larger than the band gap of the TiO 2 particles, it
Fig. 2.4 Indirect (a) and direct (b) dye degradation processes. (Reprinted with permission from
Ajmal et al. 2014. Copyright 2014 Royal Society of Chemistry, RSC Advances)
2 Dyes Depollution of Water Using Porous TiO 2 -Based Photocatalysts
47