When photons are implicated in a catalytic reaction, the system becomes a photocatalytic one. The catalyst may accelerate the photoreaction by interaction either with a substrate in its ground or an excited state, and/or with the primary photoproduct. Although not yet commercially viable, photocatalytic processes may provide an alternative to the traditional routes to water purification, destroying bacterial substances, and dissolved organics in waste streams. The process can be homogeneous, employing an oxidant and ultraviolet light, or heterogeneous, utilizing air, semiconductor particles (the photocatalyst), and ultraviolet radiation. These various pathways are discussed.
The mechanistic details of using semiconductor particles as light harvesters is reviewed, as are the mechanisms by which the photooxidation occurs. Examples of aqueous dissolved organic compounds photooxidized to carbon dioxide and water in the presence of titanium dioxide, air, and near‐ultraviolet radiation are given.