Nanotechnostructured Catalysts TiO Nanoparticles for Water Purification - Nanotechnologies for Water Environment Applications - page 69

mechanism with OH• as the major oxidizing species; on the contrary, at low pH level,
valence bend electron holes are considered as predominant oxidation species following
direct oxidation pathway (Tang and Huang, 1995).

Third, at acidic condition, TiO 2 particles are more likely to form aggregates due
to their intrinsic surface acidity properties. Thus, the available active surface area for
substrate and photon adsorption would be reduced (Fox and Dulay, 1993).

In general, the pH has varying effects on the photocatalytic process in water
purification systems. A wide range of optimal pH values (both acidic and alkaline) and
conflicting pH effects have been reported by different research groups. The different
observations reported by various researchers may be due to various experimental
conditions, such as differences in investigated pollutants, the type of photocatalyst and
supporting matrixes, the presence of electron accepters and interference ions, etc.
Therefore, the influence of pH on photocatalytic efficiency requires specific case-by-
case evaluation as to establish optimum operating pH before any application.

3.4.6 Temperature

Like most photoreaction, the true activation energy E t of photocatalytic oxidation
is zero, whereas the apparent activation energy E a is general very small, attributed to the
energy required for substrate adsorption, desorption, surface migration, and
rearrangement steps (Fox and Dulay, 1993). As the applications in water purification
using photocatalytic processes are generally operated under direct sun light, the
temperature is expected to be between 5 and 80 o C. At this temperature range, usually
weak dependence between temperature and degradation rate is observed (Dong and
Huang, 1995; Herrmann, 1999), as shown in Figure 3.15.

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-40 -20 0 20 40 60 80 100 120 140

Figure 3.15 Temperature influence on the degradation rate (Replotted from Herrmann,
1999).

Temperature, o C