Nanotechnostructured Catalysts TiO Nanoparticles for Water Purification - Nanotechnologies for Water Environment Applications - page 50

potential of h tr + . Moreover, the presence of oxygen in water as an electron scavenger
can out-compete h tr + for commonly reducible substrates kinetically (Fox and Dulay,
1993; Demeestere et al., 2007). Hence, in this chapter, we only focus on the
photocatalytic oxidation.

3.2.3 Reaction Kinetics

Even though various complex kinetics models have been developed (Dong and
Huang, 1995; Herrmann et al., 1998), most researchers have reported that the
photocatalytic oxidation of various pollutants can be fitted by the Langmuir-
Hinshelwood (L-H) model as described by Eq. 3.15 (Peral et al., 1997; Yu and Savage,
2000; Houas et al., 2001; Konstantinou et al., 2002; Konstantinou and Albanis, 2004;
Son et al., 2004; Ferguson et al., 2005; Choi et al., 2006; Li et al., 2006; Valente et al.,
2006; Belessi et al., 2007; Demeestere et al., 2007; Xu et al., 2007; Yu and Lee, 2007)
without consideration of detailed fundamental mechanistic steps as discussed above.

dC k ' KC

r == + (Eq. 3.15)

dt 1 KC

where, r is the oxidation rate of the reactant, C is the concentration of the reactant, t is
the reaction time, k' is the apparent reaction rate constant, and K is the adsorption
coefficient of the reactant in question.

Konstantinou and Albanis (2004) concluded that the L-H model could be applied
to describe the photocatalytic degradation of organic compounds under the following
four possible scenarios: (a) the reaction takes place between two adsorbed substances; (b)
a surface adsorbed substance and a radical in solution reaction; (c) a reaction between a
radical linked to TiO 2 surface and a substrate in solution; and (d) both species are in the
liquid phase.

3.3 Intrinsic Photocatalytic Activity

As discussed above, the intrinsic activity of TiO 2 or any other photocatalyst is
resulted from multi-competitive steps, including catalyst bandgap, electrons and holes
recombination, and interfacial charge transfer and charge carrier traveling time. Many
strategies have been developed to maximize the photoactivity in terms of quantum
efficiency. These include: (a) bandgap engineering, which extends the usable sunlight
spectrum and increases the electron holes yield per unit sunlight irradiation; (b) holes
recombination rate reduction, which is normally achieved by the presence of impurity
elements functioning to increase the holes trapping and delay the electron/holes
recombination; (c) increase of the charge carrier transfer rate in the bulk solution or on