Reactions of Hydroxyl Radicals on Titania, Silica, Alumina, and Gold Surfaces
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Reaction probabilities of OH• radicals under steady state conditions were determined over TiO2 (anatase and rutile), SiO2 (fused quartz), α-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 × 10-4 for TiO2 (anatase and rutile), 2 × 10-4 for SiO2, 5 × 10-3 for α-Al2O3, and > 3 × 10-2 for Au. The relative rate of H2O2(aq) decomposition was found to follow the sequence Au > α-Al2O3 > TiO2 ≃ 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 describes a common set of reactions for the removal of OH• and the decomposition of H2O2. The first step in this mechanism is the coupling of OH• radicals 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 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 TiO2 as a catalyst and H2O2 as a reagent, an alternate mechanism that involves redox chemistry is believed to occur.
author list (cited authors)
Suh, M., Bagus, P. S., Pak, S., Rosynek, M. P., & Lunsford, J. H.