Surface complexation at the TiO2 (anatase) aqueous solution interface: Chemisorption of catechol

Catechol adsorbs at the TiO2 (anatase)/aqueous solution interface forming inner-sphere surface complexes. The W-visible differential reflectance spectrum of surface titanium-catecholate complexes presents a band centered at 420 nn which corresponds to the ligand to metal charge transfer transition within the surface complexes. At pH values below pK(a1), the surface excess of catechol is almost insensitive toward pH and presents a Langmuirian dependence with the concentration of uncomplexed catechol. The ratio T-max:N-s (N-s being the measured density of available OH surface groups) indicates a prevailing 1 to 2 ligand exchange adsorption stoichiometry. In the range pH greater than or equal to pK(a1), the catechol surface excess decreases markedly with increasing pH. Formation of 1 to 1 surface complexes produces an excess of negative surface charge that is revealed by the shift of the iep to lower pH values. The reported data, which are supplemented with information on the charging behavior of TiO2 suspended in indifferent electrolyte solutions, are interpreted in terms of the multi-site surface complexation model. In this model, two types of surface OH groups are considered: = TiOH1/3- and = OH1/3+. Although both surface groups undergo protonation-deprotonation reactions, only = TiOH1/3- are prone to chemisorption. (C) 1996 Academic Press, Inc.