Probing defect sites on TiO2 with [Re-3(CO)(12)H-3]: Spectroscopic characterization of the surface species

Samples of the anatase phase of titania were treated under vacuum to create Ti3+ surface-defect sites and surface O- and O-2(-) species (indicated by electron paramagnetic resonance (EPR) spectra), accompanied by the disappearance of bridging surface OH groups and the formation of terminal Ti-3(+)-OH groups (indicated by IR spectra). EPR spectra showed that the probe molecule [Re-3(CO)(12)H-3] reacted preferentially with the Ti3+ sites, forming Ti4+ sites with OH groups as the [Re-3(CO)(12)H-3] was adsorbed. Extended X-ray absorption fine structure (EXAFS) spectra showed that these clusters were deprotonated upon adsorption, with the triangular metal frame remainina intact; EPR spectra demonstrated the simultaneous removal of surface O- and O-2(-) species. The data determined by the three complementary techniques form the basis of a schematic representation of the surface chemistry. According to this picture, during evacuation at 773 K, defect sites are formed on hydroxylated titania as a bridging OH group is removed, forming two neighboring Ti3+ sites, or, when a Ti4+-O bond is cleaved, forming a Ti3+ site and an O- species, with the Ti4+-OH group being converted into a Ti3+-OH group. When the probe molecule [Re-3(CO)(12)H-3] is adsorbed on a titania surface with Ti3+ defect sites, it reacts preferentially with these sites, becoming deprotonated, removing most of the oxygen radicals, and healing the defect sites.