ADSORPTION AND PHOTOOXIDATION OF SALICYLIC-ACID ON TITANIUM-DIOXIDE -A SURFACE COMPLEXATION DESCRIPTION

The adsorption and photooxidation of salicylic acid on dispersed TiO2 (Degussa P-25) particles was studied as a function of substrate concen tration and pH. Salicylic acid chemisorbs at the particle interface, f orming inner-sphere titanium(IV) salicylate surface complexes. The vis ible differential diffuse reflectance spectra of the surface complexes present a band, with maximum absorption at 420 nm, which is assigned to the internal ligand to metal charge-transfer transition. The surfac e excess of salicylic acid increases with decreasing pH and levels off around pK(a1). At constant pH, the surface excess increases with the concentration of salicylic acid, the isotherm reflecting surface site heterogeneity. Photooxidation rates in air-saturated solutions, on the other hand, are independent of both pH and salicylic acid concentrati on, in the entire studied range. Chemisorption results are accounted f or by a multisite surface complexation model in which two different su rface titanium sites and three complexation modes are considered. The mismatch between salicylic acid surface excess values and photooxidati on rates is interpreted in terms of the different reactivities of the titanium(IV) salicylate surface complexes and is attributed to the fas test hole capture by bidentate salicylate binding a single surface tit anium ion. The advanced rationale illustrates the importance of the ba sic principles of coordination chemistry in the interpretation of appa rent kinetic orders in photolyte concentration.