Reactions of Hydroxyl Radicals on Titania, Silica, Alumina, and Gold Surfaces

Reaction probabilities of OH • radicals under steady state conditions were determined over TiO 2 (anatase and rutile), SiO 2 (fused quartz), α-Al 2 O 3, and Au surfaces. The OH • radicals were produced from water in a microwave cavity and were detected by laser-induced fluorescence spectroscopy. At 308 K the reaction probabilities were 2 × 10 -4 for TiO 2 (anatase and rutile), 2 × 10 -3 for SiO 2, 5 × 10 -3 for α-Al 2 O 3, and > 3 × 10 -2 for Au. The relative rate of H 2 O 2 (aq) decomposition was found to follow the sequence Au > α-Al 2 O 3 > TiO 2 ≃ SiO 2, which, except for TiO 2, follows the sequence found for the OH • radical reaction probability. For all of the materials except TiO 2, a radical mechanism is proposed which describes a common set of reactions for the removal of OH • and the decomposition of H 2 O 2 . The first step in this mechanism is the coupling of OH • radicals to form H 2 O 2 . The overall rate of reaction for both OH • removal and H 2 O 2 decomposition depends on the concentration of OH on the surface. A theoretical analysis using a cluster model for an Au surface has shown that the OH−Au chemisorption bond is dominantly ionic with the OH radical becoming an OH - anion. The adsorption energy may be as large as 155 kJ mol -1 . With TiO 2 as a catalyst and H 2 O 2 as a reagent, an alternate mechanism that involves redox chemistry is believed to occur.