Photocatalysis
Photocatalysis is acceleration of chemical reactions taking place with participation of photocatalysts under ultraviolet, visible or infrared light. Lights quanta are absorbed by photocatalyst. Excited photocatalyst transfer electron or energy to reaction participants. This results in initiation of chemical reactions that normally proceed at a very low rate. Below different classes of photocatalytic reactions are considered in more detail.
Photochemistry
Photochemistry is a branch of chemistry that studies chemical reactions taking place as a result of ultraviolet, visible or infrared light absorption by the reaction participants. Photochemical reactions can be written as shown in the following equation.
A + B + hν ⇒ C + D,
where A and B are reagents, C and D are products, hν is a light quantum that is absorbed by one of the reagents.
Photocatalysis
Photocatalysis differs from photochemistry in that light quantum is absorbed not by a reagent but by photocatalyst. The simplest mechanism of photocatalytic reaction is depicted by the following scheme.
Pc + hν ⇒ Pc*,
Pc* + A ⇒ Pc + P,
where Pc is photocatalyst in its ground state, Pc* is photocatalyst in excited state, hν is light quantum, A is reagent, and P is product.
Photocatalyst can be an atom, molecule, nanoparticle or bulk solid material.
Similar to catalysis, photocatalysis can be homogeneous or heterogeneous. Homogeneous photocatalysis proceeds with photocatalyst residing in the same phase as the other reaction participants. For example, liquid phase homogeneous photocatalysis proceeds with reagents and photocatalyst dissolved in a solution, or photocatalyst dissolved in liquid reagent. Heterogeneous photocatalysis occurs with participation of photocatalyst and reaction participants in different phases. Typically, photocatalyst is solid and reaction participants are gaseous or liquid.