The adsorption and photodecomposition on semiconducting membranes of molecules with different functional groups were studied. Ceramic membranes, prepared by a sol-gel technique, were employed because this technique provides some control over surface area, porosity, and crystal form of these materials, properties which affect their photochemical behavior. Salicylic acid was chosen as a prototype molecule. 3-Chlorosalicylic acid, benzoic acid, phenol, and 4-chlorophenol were also investigated. The adsorption densities of salicylic and 3-chlorosalicylic acids decreased with both increasing solution pH and membrane firing temperature, in correlation with the number of positive adsorption sites on TiO2 surface. No adsorption for benzoic acid, phenol, and 4-chlorophenol was observed. Photodecomposition rates were found to depend on the adsorption characteristics of the organic compound; for salicylic acid the degradation rate diminished with increasing pH. Benzoic acid, whose degradation products adsorb on the oxide surface, showed a similar trend. No dependence on pH was detected for phenols. Methanol was found to affect the degradation rate of salicylic acid only under conditions of high pH. It is proposed that, for chemisorbing organics, the initial step of the photodecomposition process is governed by two different mechanisms that depend on the adsorption behavior of the organic compound at a particular pH. For adsorbed salicylate, an orbital configuration of the chelate ring is proposed to interpret direct electron transfer from the organic molecule to the semiconductor.
