Role of the Oxygen Molecule and of the Photogenerated Electron in TiO2-Photocatalyzed Air Oxidation Reactions

Abstract

The photocatalytic air oxidation of n-octane, 3-octanol, 3-octanone, or n-octanoic acid films on aqeous 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 (O2⋅– or HO2⋅); and it combines with the organic radical, generated upon the hole or ⋅OH 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 dissolvd Fe3+ ions compete for the photogenerated electrons and oxidize superoxide to O2, 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 radical) initiated, radical propatated, autoxidation reaction.