Photocatalysis is the induction and acceleration of reactions by the action of light. Photocatalysis involves absorption of light in a semiconductor with a suitable distribution of energy states. Upon irradiation of the semiconductor with light of energy exceeding the bandgap energy, an electron is promoted to the valence band and leaves a hole in the valence band. These photogenerated charge carriers can drive redox reactions either with molecules adsorbed on or close to the surface of the photocatalyst: for self-cleaning properties, dissolved or adsorbed contaminants are oxidized to yield ultimately CO2, mineral salts and mineral acids.
This phenomenon was first detected on single crystals of TiO2 in the rutile modification, but research has shown that nanostructured anatase TiO2 is the better catalyst.
TiO2 remains the most widely investigated photocatalyst so far.
Applications of photocatalysis
- Self cleaning surfaces
- Waste water treatment
- Superhydrophilic surfaces
Photocatalysis has been used in architecture and in water treatment mainly. In architecture self-cleaning windows and facades are among the main applications. Attempts have also been made with indoor photocatalysis. However, due to severely lower indoor UV levels, photocatalysis is significantly less effective. Efforts have been made to shift the bandgap energy of TiO2 into the visible light region by doping with e.g. W.
Another potential application of photocatalysis is waste water treatment by water oxidizing contaminants on a sunlight illuminated photocatalyst.
To quantify different photocatalysts, AAC has developed a test setup, where catalysts can be exposed to a model analyte.
AAC Photocatalysis Test Reactor
AAC has developed a flow through reactor to measure the photocatalytic activity of a photocatalyst. The reactor consists of a flow through cell where the photocatalyst is brought into close contact with dye solution. Through a quartz window the catalyst is illuminated with a Hg lamp. The dye solution is cycled through the system and the dye concentration is measured photometrically in a second flow cell.