ROLE OF THE OXYGEN MOLECULE AND OF THE PHOTOGENERATED ELECTRON IN TIO2-PHOTOCATALYZED AIR OXIDATION REACTIONS

The photocatalytic air oxidation of n-octane, 3-octanol, 3-octanone, o r n-octanoic acid films on aqueous 0.5 M NaCl with buoyant nanocrystal line n-TiO2-coated glass microbubbles was studied. The observed produc ts and intermediates, as well as the observed inhibition of the air ox idation reaction by dissolved Fe3+ ions, show that not only holes but also electrons participate in the oxidation reaction and that molecula r oxygen has two roles: it accepts the electron generated in a TiO2 cr ystallite and is reduced to a superoxide radical (O-2(.-) or HO2.); an d it combines with the organic radical, generated upon the hole or (OH )-O-. radical reaction with the reactant, to produce an organoperoxy r adical (ROO(.)). The superoxide radical, though by itself a relatively ineffective oxidizing agent, combines with the organoperoxy radicals to form an unstable tetraoxide that decomposes. CO2 evolves early in t he resulting reaction sequence. Because dissolved Fe3+ ions compete fo r the photogenerated electrons and oxidize superoxide to O-2, they red uce the CO2 yields in the photocatalytic air oxidation of the four rea ctants. Unlike the other reactions, the photocatalytic air oxidation o f n-octanal is not inhibited by Fe3+; that is, it does not involve the superoxide radical. It is a hole- (or (OH)-O-. radical) initiated, ra dical-propagated, autoxidation reaction.