Clean Water Programme

Photocatalytic Oxidation

Photo-catalytic oxidation technology

Photocatalytic oxidation process

The treatment of dilute water borne pollutants by semiconductor photocatalysis is one of the fast growing areas in term of academic research and commercial activity. The potential benefits of semiconductor photocatalysis can be reflected by the large number of literature generated in the past decade. Among many chalcogenides and metal oxide semiconductors, colloidal TiO₂ particles are frequently used as the photocatalyst. This is because TiO₂ is non-toxic, chemically stable, and possesses relatively high photocatalytic activity. Laboratory studies have demonstrated that organic compounds such as alcohols, carboxylic acid, phenolic derivatives, and chlorinated aromatics can be readily mineralized by TiO₂ into harmless carbon dioxide, water, and simple mineral acids, using molecular oxygen as the primary oxidant.

Mechanism of TiO2 photocatalysis

The mechanism of photocatalysis is illustrated below.

Mechanism of titanium dioxide photocatalytic degradation process

Under illumination, the TiO₂ photocatalyst absorbs photons with energy equal or higher than its band gap energy (l<385nm). This will delocalizes a valence electron and excites it to the conduction band of the semiconductor. These photoexcited charge carriers can initiate the degradation of the adsorbed chemical species by one or more forms of electron transfer reactions. However, Alternatively, they can recombine radiatively or non-radiatively and dissipating the input energy as heat.

TEM image of titanium dioxide particles

As in classical heterogeneous catalysis, the size of TiO₂ particles usually defines the surface area in a system available for adsorption and decomposition of the organic pollutants. In many occasion, nanosized TiO₂ particles are used (see the figure on top).