CATALYTIC AND PHOTOCATALYTIC DECOMPOSITION OF OZONE AT ROOM-TEMPERATURE OVER TITANIUM(IV) OXIDE

Dark and photoinduced decomposition of ozone (O3, ca. 80 ppm) in air over various titanium(IV) oxide (TiO2) powders has been studied with a flow-type reactor (1.5 dm3 min-1) at room temperature. The ozone decomposition in the dark was attributed to catalysis by TiO2. The catalytic activity, evaluated from % decrease in O3 concentration, increased with the increase in the specific surface area of TiO2 and also in the amount of surface hydroxyls. The fact that substitution of hydroxyls with fluoride anions reduced the activity of TiO2 (Merck, anatase) to less than half, suggested a mechanism including O3 decomposition on the surface hydroxyls. A TiO2 powder, contaminated with sulfate anion, exhibited a catalytic activity lower than expected from the specific surface area, but increased more than three-fold by washing with aqueous alkaline solution to eliminate sulfate anions and recover hydroxyls. The O3 decomposition under irradiation (> 200 nm) occurred not only by direct photolysis of O3 adsorbed on the TiO2 surface (mainly by 254 nm irradiation), but also by a photocatalytic reaction initiated by absorption of photons by TiO2 at <ca. 400 nm, as well as the dark catalytic decomposition. The O3 decomposition via the photocatalytic reaction was evaluated separately by cutting off the light of wavelength < 290 nm. The photocatalytic activity of TiO2 was independent of the specific surface area, but dependent on its crystal structure. The photocatalytic activity increased in the order anatase << anatase-rutile mixture < rutile. The mechanism of these O3 decomposition processes has been discussed.