Catalytic and photocatalytic decomposition of ozone at room temperature over titanium(IV) oxide

Abstract

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 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.