Sorption of Co(II) on Metal Oxide Surfaces: I. Identification of Specific Binding Sites of Co(II) on (110) and (001) Surfaces of TiO2 (Rutile) by Grazing-Incidence XAFS Spectroscopy

We have examined the molecular-scale details of aqueous Co(II) surface complexes and the types of surface sites to which these complexes bind on the (110) and (001) surfaces of single-crystal TiO2 (rutile) using polarization-dependent grazing-incidence X-ray absorption fine structure (GI-XAFS) spectroscopy under ambient conditions in a humid atmosphere. On both surfaces, Co(II) adsorbs at sites corresponding to Ti-equivalent positions in an extension of the rutile structure. This result suggests that even if different crystallographic surfaces of metal oxides have strongly differing adsorption properties for gaseous species in ultra-high vacuum, they can have similar properties for adsorption of metal ions in aqueous solution, probably due to the tendency of liquid water to heal defects and satisfy the bonding requirements of coordinatively unsaturated surface atoms. Using a bond valence approach in combination with the XAFS results, we have proposed specific surface reactions for sorption of Co(II) on the (110) and (001) rutile surfaces as a function of pH and Co surface concentration. No evidence was found for well-ordered Co(II)-hydroxide-like precipitates that would show Co–Co pair correlations, or for Co–Ti pair correlations similar to those in anatase, as have been observed in other studies. These results demonstrate the utility of GI-XAFS spectroscopy on adsorbed metal ions at submonolayer surface coverages for determining the types of reactive sorption sites on metal oxide surfaces.