Surface Complexation at the TiO₂ (anatase)/Aqueous Solution Interface: Chemisorption of Catechol

Author： Rodríguez R. Blesa M.A. Regazzoni A.E.

ISSN： 0021-9797

Source： Journal of Colloid and Interface Science, Vol.177, Iss.1, 1996-01, pp. : 122-131

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Abstract

Catechol adsorbs at the TiO₂ (anatase)/aqueous solution interface forming inner-sphere surface complexes. The UV–visible differential reflectance spectrum of surface titanium–catecholate complexes presents a band centered at 420 nm 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 Γ_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 ≥ 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: ≡TiOH^1/3- and ≡OH^1/3+. Although both surface groups undergo protonation–deprotonation reactions, only ≡TiOH^1/3- are prone to chemisorption.