The reactants in heterogeneous photocatalysis in water are naturally occurring

species (such as oxygen, water molecules, and hydroxide ions (OH-)) as compared to

chemically supplied reactants (such as hydrogen peroxide and ozone) required for other

AOPs. When the surface of the photocatalyst absorbs a specific amount of energy, an

electron from the valence band jumps to the conduction band, thereby leaving a positive

"hole" in the valence band. These electrons (e-) and holes (h ) may either recombine,

releasing heat, or migrate to the surface of the catalyst. The reactive oxidative species are

then generated from reactions with the electrons or holes on the surface (see Figure 2-1).




 Acceptor-
 Reduction





 S}COxidation B
 hv Release of heat





 Do onorr
 Donor+


Figure 2-1. Schematic of oxidation and reduction occurring on the semiconductor
 surface.

 The primary oxidizing species in water is the hydroxyl radical (*OH) (Turchi and

Ollis, 1990). It is the most reactive of the oxidizing agents (see Table 2-1) and can be

generated via several pathways:

 H20 + h+ *OH + H+ (1)

 OH- + h *OH (2)