adion complexes, since definition of these factors would The structures and compositions of Pb(II) adsorption complexes facilitate thermodynamic, mechanistic, and kinetic descripand surface binding sites on a-Al 2 O 3 (0001) and (11V 02) surfaces tions of surface reactions. In addition, it would be useful to were investigated in the presence of water using grazing-incidence understand the relationships between the reactivities of these X-ray absorption fine structure (GI-XAFS) spectroscopy. Pb(II) surface species and their structures and compositions, since ions were found to adsorb in an inner-sphere mode on a-Al 2 O 3 this knowledge could be used in a predictive capacity. His-(11V 02) but as outer-sphere complexes on a-Al 2 O 3 (0001). The distorically, it has been very difficult to characterize these proptance between the outer-sphere complexes and the surface places erties because standard structure characterization techniques useful constraints on double-layer properties of water. A bondwere not sufficiently surface-sensitive to directly probe the valence model is described that relates the reactivities of surface functional groups and adsorption complexes to their molecular molecular structures and compositions of species at oxidestructures and compositions, and places constraints on the stoichiwater interfaces, particularly under reactive conditions (e.g., ometries of adsorption reactions, including proton release. The low concentrations of adsorbates and at ambient or higher EXAFS and modeling results suggest that Pb(II) and Co(II) ions pressures and temperatures). Until recently, theories to debond to [Al Al Al nO 01/2 ú -and [Al-OH /1/2 2 ] surface functional groups. scribe aqueous-oxide surface reactions were based on indi-In contrast, [ Al Al ú OH] groups complex Co(II) but not Pb(II). The rect measurements of surface processes using techniques results indicate the importance of using structurally defined sursuch as metal ion uptake measurement, potentiometric titraface sites to describe reactions at oxide-water interfaces. ᭧ 1997 tion, and electrophoretic mobility measurement. Recently, Academic Press spectroscopic techniques have been developed that can pro-Key Words: Pb(II); Co(II); Al 2 O 3 ; surfaces; adsorption; bond vide direct observations of surface species. Nevertheless, valence; hydrogen bond; hydroxyl group; XAFS; grazing-incidence little is known about the structures and compositions of sur-XAFS; oxide-water interface; surface functional group.
face functional groups and adion sorption complexes on oxide surfaces, and the factors that control their reactivities. Definition of these aspects would be an important contribu-