crystal. For example, a photocatalyst containing larger crystals is more efficient than one

with smaller crystals, even when both contain equal percentages of anatase. This could be

due to the larger migration distance of the holes and electrons to the surface of the

catalyst, thereby decreasing the possibility of recombination (Tanaka et al., 1991).

 Modifications in the catalyst surface have also been investigated for prevention of

electron-hole recombination. The addition of platinum and other transition metals have

been successfully arrayed on the titanium dioxide surface (Abrahams et al., 1985;

Okamoto et al., 1985; Martin et al., 1994; Suri et al., 1993). These metal additions have

an optimum at low weight percentages (less than 5%), above which the metal actually

hinders the photocatalytic ability.

 The isoelectric point or point of zero charge (PZC) represents the pH at which an

immersed solid oxide would have zero net charge, resulting in electrically equivalent

concentrations of positive and negative complexes on the surface. At a pH above the

PZC, interactions with cationic electron donors and acceptors will be favored; while

anionic electron donors and acceptors will be favored at pH below the PZC.

 The band gap is equal to the amount of energy required to activate the surface of

the catalyst. A band gap of 3.29eV corresponds to a wavelength of 378 nm. Therefore,

wavelengths below 378 nm (such as those emitted by UV light) would have the required

energy to activate the anatase phase of TiO2.

 2.3 Reactor Design

 A problem in the practical application of heterogeneous photocatalysis of

environmental pollutants is the design of a reactor that will maximize photocatalytic

efficiency while utilizing the least amount of energy. The simplest reactor configuration

uses an aqueous suspension or slurry of the photocatalyst. Most photocatalysis studies