A catalyst changes the rate of a reaction without appearing in the products and without changing the chemical equilibrium. A catalyst is a compound that lowers(or higher) the free activation enthalpy energy of the reaction.

The term photocatalysts consists of the combination of photochemisty and catalysis and impies that light and a catalyst are necessary to bring about or to accelerate a chemical transformation.
Photocatalyst can be induced photoreaction by the presence of a catalyst with the light with various photocatalysts, the titanum dioxide is wide used because it has stable about acid-base condition and harmless about human body.

 Titan is the 9th-most element on earth and titanium oxide is usually used to white pigment and white paint for cosmetics,For pigments, no response to lighrt-titanium oxide is used while for photocatalyst applications like air-purifiers, high photo reactive titanium oxide is used. And due to its low price, excellent wear-resistance, endurance ane no possibility of the secondary contamination for its abundance, it has safe and nonpoisonous feature. So titanium oxide is considered one of the most popular NANO products in future-oriented industry.

In particular, given the conditions and activeness of photocatalyst, it can be used semi-permanently, since it doesn't change itself even when a light hits. And its higher oxidation than CI2 and O3,excellent sterilization and function of decomposing all organic compounds into H2O and CO2 allow TiO2 to be one of the most popular photocatalyst materials.

Safety of TiO2

TiO2 used as photocatalyst is physically and chemically safe, the US certified it as food additions in 1968 and Japan did in 1983 so that it is used widely in making white chocolates and functional cosmetics including lipsticks. Particularly in Japan, company T produced photocatalyst-coated tiles after Food Analysis Center of Japan Food Research Laboratories and Mitsubishi Chemical Safety Institute Ltd. demonstrated its safety through skin simulating tests and acute oral toxicity test.

Safety of Activated Oxygen

Active oxygen always exists in the air and it is created in human body. And massive active oxygen can do harm to human body. Active oxygen created by photocatalyst is always on the surface of coated materials, not  in the air. And H2O2 is a good example to be used in daily lives. H2O2 sterilizes bacteria and other germs by creating active oxygen on human skins.