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, or n-octanoic acid films on aqueous 0.5 M NaCl with buoyant nanocrystalline n-TiO2-coated glass microbubbles was studied. The observed products and intermediates, as well as the observed inhibition of the air oxidation reaction by dissolved Fe3+ ions, show that not only holes but also electrons participate in the oxidation reaction and that molecular oxygen has two roles: it accepts the electron generated in a TiO2 crystallite and is reduced to a superoxide radical (O-2(.-) or HO2.); and it combines with the organic radical, generated upon the hole or (OH)-O-. radical reaction with the reactant, to produce an organoperoxy radical (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 the resulting reaction sequence. Because dissolved Fe3+ ions compete for the photogenerated electrons and oxidize superoxide to O-2, they reduce the CO2 yields in the photocatalytic air oxidation of the four reactants. Unlike the other reactions, the photocatalytic air oxidation of 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, radical-propagated, autoxidation reaction.