XPS-INVESTIGATION OF TITANIUM-MODIFIED MFI-TYPE ZEOLITES

The chemical bonding of titanium in MFI-type zeolites (Ti-silicalite) has been investigated using x-ray photoelectron spectroscopy (XPS). Different titanium silicalites as well as pure titania (anatase) have been analysed. We have found a distinctively higher binding energy of the Ti 2p photoelectrons in Ti-silicalite than in titania- or anatase-containing silicalite. Ti has the same oxidation state in all solids under study. We attribute the difference in binding energy to different oxygen coordinations of the titanium in the framework of Ti-silicalite (tetrahedral) and anatase (octahedral). This suggestion is supported by XPS-studies of TiO2-SiO2 glasses and TiO2-SiO2 mixed oxides where a similar shift was found. Additional Raman-spectroscopic investigations of the samples indicate the existence of tetrahedrally coordinated titanium by the appearance of a characteristic band at approximately 965 cm-1. Non-framework TiO2 species containing octahedrally coordinated titanium could also be distinguished by XPS. Further Ti incorporation is indicated by the expansion of the unit cell which is related to the Ti 2p intensity for the framework titanium.