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 concentration and pH. Salicylic acid chemisorbs at the particle interface, forming inner-sphere titanium(IV) salicylate surface complexes. The visible 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 surface 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 concentration, in the entire studied range. Chemisorption results are accounted for by a multisite surface complexation model in which two different surface titanium sites and three complexation modes are considered. The mismatch between salicylic acid surface excess values and photooxidation rates is interpreted in terms of the different reactivities of the titanium(IV) salicylate surface complexes and is attributed to the fastest hole capture by bidentate salicylate binding a single surface titanium ion. The advanced rationale illustrates the importance of the basic principles of coordination chemistry in the interpretation of apparent kinetic orders in photolyte concentration.