ACCELERATION OF SECONDARY DARK REACTIONS OF INTERMEDIATES DERIVED FROM ADSORBED DYES ON IRRADIATED TIO2 POWDERS

Time-resolved diffuse reflectance flash photolysis been used to charac terize the kinetic behaviors of transients derived from several dyes ( eosin Y, thionine, oxazine 725, and nile blue A) adsorbed on titanium dioxide powder (Degussa P-25) suspended in aqueous solutions of variou s pH. Secondary reaction rates on these powders were compared with tho se observed on analogous colloidal TiO2 suspensions. Faster secondary reaction rates were observed on the powders, where the reaction surfac e is pseudo-two dimensional. Neither first nor second order decays wer e observed; instead, the observed kinetics depended on size distributi on of the powder. With N,N,N',N'-tetraphenylenediamine (TMPD), the dec ay rate of the singly oxidized cation radical TMPD+ was found to depen d on water concentration, thus implicating both direct electron transf er (ET) and OH-radical mediated oxidation pathways and complicating th e observed kinetics.