Nanotechnostructured Catalysts TiO Nanoparticles for Water Purification - Nanotechnologies for Water Environment Applications - page 65

investigated the degradation rate of 4-chlorophenol (4-CP) with various initial TiO 2
loadings; they found that the optimal degradation rate was obtained with the initial TiO 2
concentration of 1000 ppm (Dong and Huang, 1995), as shown in Figure 3.12. This can
be rationalized by the competition between the active TiO 2 surface sites and the light
penetration into the suspension in the slurry reaction system. The availability of active
surface sites increases with the catalyst loading, while light penetration capability
decreases for the same reactor design. Further more, the aggregation of the nano-sized
TiO 2 particles is another factor affecting the photoactivity in terms of TiO 2 loading. The
aggregation rate of the suspended particles is closely related to the concentration, which,
in turn, compromises the available catalyst surface for adsorption and reaction (Dong
and Huang, 1995; Konstantinou and Albanis, 2004; Lin et al., 2006).
Regarding the photocatalytic reactor with immobilized TiO 2 , the overall
degradation rate is limited by the diffusivity of reagents and photo-penetration inside of
the film. Although a thicker immobilized photoactive film has plausible influence on
total active sites, the diffusivity of reagents and the distribution of light irradiation will
be negativity affected (Camera-Roda and Santarelli, 2007). Furthermore, thicker films
lead to a longer charge carrier pathway, which enhances the possibility of electron and
hole recombination and lowers the quantum yield. Therefore, like particle size effect,
the film thickness is a critical factor determining the photocatalyst activity of the
immobilized TiO 2 film.
3.4.3 Light Irradiation
Ollis et al. (1991) and Hoffmann et al. (1995) reviewed the effect of light
intensity on the photocatalytic degradation rate and quantum efficiency. Their
observations are summarized as follows: (a) at low light intensities, the reaction rate
shows direct dependence on the intensity of light irradiation; (b) at intermediate light
intensities, the rate would depend on the square root of the light intensity; and (c) at high
light intensities, the rate is independent of light intensity. Moreover, the measured
quantum efficiency of the photocatalytic reaction decreases with an increase in light
intensity. For instance, in one CHCl 3 degradation study the measured quantum
efficiency decreases from 0.56 to 0.02 when the light intensity increases from 2.8 to 250
E/L-min (Hoffmann et al., 1995). This is explained partially by the electron-hole
recombination process. At low light intensity reactions, the electron-hole recombination
effect is negligible. However, as lighted intensity increases, the electron-hole pair
recombination can compete with charge carriers transfer due to high intensity of
electron-hole pairs, thereby causing lower effect on the reaction rate and quantum yield
(Konstantinou et al., 2002).
Another important factor of the incident irradiation is the light wavelength. In
general, shorter wavelengths are preferred and generally offer better degradation rates;