The interaction of metal oxide surfaces with complexing agents dissolved in water

Upon exposure to liquid water or to aqueous solutions, the surfaces of meta l oxide particles or films undergo a series of chemical reactions that are dictated to a large extent by the chemistry of the metal ions involved. The se reactions involve surface :hydroxylation and hydration (dissociative and non-dissociative water chemisorption), chemisorption of solutes and charge transfer reactions. The present review focuses on the chemisorption of ani ons, which is a surface complexation reaction. In simple cases, chemical eq uilibria may be written, and quantified by heterogeneous stability constant s that resemble the analogous homogeneous ones. This approach has been prac ticed for more than 20 years, and in selected cases values are available fo r a discussion of stability trends, even though the stability constant valu es are sensitive to double-layer modeling and to the history of the metal o xide used. Most of the stability constants have been derived in conventiona l ways from measurements of the corresponding adsorption isotherms, a proce dure that does not provide structural information. Modeling of the shape an d pH dependence of adsorption isotherms has been however used to propose va rious modes of adsorption, in order to derive, for instance, the speciation of surface complexes as a function of ligand concentration and pH. Present ly, structural techniques are available to probe directly into the structur e of the surface ensembles; the use of W-vis, IR, magnetic and surface spec troscopies, together with EXAFS and SEXAFS has provided credence to the sur face complexation approach, as discussed in the present review for selected cases. In particular, attenuated total refection FTIR has proved to be a p owerful tool to derive the surface speciation in selected cases. The reacti vity patterns of the surface complexes is being currently explored. The cat alysis of eater hydrolysis, the rates and mechanisms of oxide dissolution, heterogeneous charge transfer reactions and the photocatalytic reactions of oxidation of organic compounds can all, in certain cases, be described as reactions of specific surface complexes; some relevant examples are discuss ed. (C) 2000 Elsevier Science S.A. All rights reserved.