Reactions of hydroxyl radicals on titania, silica, alumina, and gold surfaces

Reaction probabilities of OH. radicals under steady state conditions were d etermined over TiO2 (anatase and rutile), SiO2 (fused quartz), (alpha-Al2O3 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 x 10(-4) for TiO2 (anatase and rutile) , 2 x 10(-3) for SiO2, 5 x 10(-3) for alpha-Al2O3, and > 3 x 10(-2) for Au. The relative rate of H2O2(aq) decomposition was found to follow the sequen ce Au > alpha-Al2O3 > TiO2 similar or equal to SiO2, which, except for TiO2 , follows the sequence found for the OH. radical reaction probability. For all of the materials except TiO2, a radical mechanism is proposed which des cribes a common set of reactions for the removal of OH. and the decompositi on of H2O2. The first step in this mechanism is the coupling of OH. radical s to form H2O2. The overall rate of reaction for both OH. removal and H2O2 decomposition depends on the concentration of Oh on the surface. A theoreti cal 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 O H- anion. The adsorption energy may be as large as 155 kJ mol(-1). With TiO 2 as a catalyst and H2O2 as a reagent, an alternate mechanism that involves redox chemistry is believed to occur.