The basis of photocatalysis is the photo-excitation of a semiconductor that is solid as a result of the absorption of electromagnetic radiation in the near UV spectrum. Under near UV irradiation, a suitable semiconductor material may be excited by photons possessing energies of sufficient magnitude to produce conduction band electrons and valence band holes. These charge carriers are able to induce reduction or oxidation respectively.
Many Authors found that there is no need to bubble the air through the reaction mixture, as the performance does not depend on aeration. The absorption of oxygen by the surface of solution is sufficient for photo-catalytic oxidation (PCO). This means that the absorption of oxygen by the liquid phase is not the stage limiting the process rate.
Titanium dioxide, both in the forms of anatase and rutile, is one of the most widely used metal oxides in industry. Its high refractive index in the visible range permits preparation of thin films, and thus its use as a pigment material. On the other hand, its use as a catalyst support or as a catalyst and photo-catalyst itself is well known.
The pH value has a dominant effect on the photocatalytic reaction because many properties, such as the semiconductor’s surface state, the flat-band potential, the dissociation of organic contaminant, are all strongly pH dependent. The solution matrix can influence the photocatalytic reaction rate of a particular compound in several ways. PCO has been found to be the best in terms of the process rate under the conditions of pH 3.0 when landfill leachate is treated by either H2O2/UV or TiO2/H2O2/UV.
The optimum pH for the most effective PCO depends strongly on the character of the compound to be oxidized. Thus, aromatic amino compounds behave differently than phenolics.