Probing Defect Sites on TiO2 with H3Re3(CO)12:
Spectroscopic Characterization of the Surface Species
Kongkiat Suriye,1,2 Rodrigo J. Lobo-Lapidus,1 Gregory J. Yeagle,3 Piyasan Praserthdam,2 R. David Britt,3 and Bruce C. Gates1,*
1Department of Chemical Engineering and Materials Science, University of California, One Shields Ave, Davis, CA 95616,
2Reaction Engineering, Department of Chemical Engineering, Faculty of Engineering, Chulalongkorn University, Bangkok 10330, Thailand,
3Department of Chemistry, University of California, One Shields Ave., Davis, CA 95616.
Chemistry—A European Journal, 14, 1402 (2008).
Samples of the anatase phase of titania were treated under vacuum to create Ti3+ surface defect sites and surface O- and O2- species (indicated by electron paramagnetic resonance (EPR) spectroscopy), accompanied by the disappearance of bridging surface OH groups and the formation of terminal Ti3+–OH groups (indicated by infrared spectroscopy). EPR spectra showed that the probe molecule H3Re3(CO)12 reacted preferentially with the Ti3+ sites, forming Ti4+ sites with OH groups as the H3Re3(CO)12 was adsorbed. Extended X-ray absorption fine structure (EXAFS) spectra showed that these clusters were deprotonated upon adsorption, with the triangular metal frame remaining intact; EPR spectra demonstrated the simultaneous removal of the surface O- and O2- 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 H3Re3(CO)12 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.