Influence of hydrogen bonding upon the TiO2 photooxidation of isopropanol and acetone in aqueous solution

This study investigates the aqueous photocatalytic degradation of small polar organic compounds (SPOCs) that bear hydrogen-bonding capabilities but do not readily adsorb to the TiO2 catalyst. The effect of pH on the TiO2 surface hydroxyl speciation and surface acid/base equilibria was used to elucidate the possible role of hydrogen-bonding interactions in the degradation of acetone and isopropanol in aqueous TiO2 photocatalytic systems. The kinetic parameters describing the decomposition of these two model compounds were obtained by gas chromatographic analysis of their photoreaction systems and interpreted on the grounds of the Bronsted acid/base properties of the TiO2 Surface speciation and solute hydrogen-bonding numerical scales. The results showed that the fastest initial degradation rates of acetone and isopropanol occurred in a pH range where the optimal conditions for adsorption through hydrogen bonding to the TiO2 surface and optimum concentration of hydroxyl radicals (OH center dot) coincide. The fastest degradation constants were observed at pH 6.04 and 8.61 for acetone and isopropanol, respectively. The hypothesis of hydrogen bonding to surface hydroxyl groups presented in this study challenges the common assumption that these model compounds do not adsorb to surface sites, and that their oxidative pathways of degradation only occur via homogeneous-phase reaction with free OR radicals. (C) 2009 Elsevier Inc. All rights reserved.