Surface Structures of 4-Chlorocatechol Adsorbed on Titanium Dioxide

TiO_2 has been extensively studied as a photocatalyst for the complete oxidation of a variety of organic pollutants commonly found in groundwater. In this paper, we investigate the surface structures formed between an organic substrate and TiO_2 in the context of understanding how these specific surface interactions affect photoreactivity. The surface complexes formed by 4-chlorocatechol (CT) sorbed on TiO_2 are investigated as a function of concentration and of pH. Singular-value decomposition of the IR spectra of CT adsorbed on TiO_2 indicates that a single bidentate chemisorbed species is present over the pH range of 2−10. The surface-bound species appears to have 40% covalent and 60% ionic bond character. The pH dependence of the adsorption isotherms is modeled using a generalized electric double-layer approach. The data are consistent with the formation of a bidentate binuclear surface group for solution CT concentrations below 50 μM followed by nonspecific multilayer partitioning at concentrations above 100 μM.