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Abstract:
Oxygen core excitations in different molecular titania–silica model clusters are evaluated using density-functional theory (DFT). These results are compared with in situ X-ray absorption fine structure (NEXAFS) measurements near the O K-edge of titania model catalysts supported on mesoporous silica, SBA-15, with low Ti loading. The comparison allows for an analysis of structural details of the supported titania species. The silica support is found to contribute to the NEXAFS spectrum in an energy range well above that of the titania units, allowing for a clear separation among the corresponding spectral contributions. The different bridging and terminal oxygen species can also be easily distinguished in the theoretical NEXAFS spectra. The experimental NEXAFS spectrum for 3 wt % Ti loading exhibits a single broad peak in the O 1s to the Ti 3d–O 2p excitation range of 530–534 eV. This can be explained by the theoretical data for tetrahedrally coordinated but not for triply coordinated TiOx species, suggesting the presence of OH groups at the expense of titanyl oxygen in the titania surface structure. The experimental NEXAFS spectrum for low Ti loading differs substantially from that of TiO2 anatase bulk with octahedral TiO6 units where the observed double-peak structure is also reproduced by the calculations.
