XPS characterization of a synthetic Ti-containing MFI zeolite framework: The titanosilicalites, TS-1

Following a previous investigation on micro- and mesoporous titanosilicates, we have here deepened the XPS and Xray-excited Auger electron spectroscopy (XAES) studies on the surface properties of microporous titanosilicalites-1 (TS-1) and reference silicalites-1 (S-1), both characterized by the MFI framework type. The aim was that of complementing the information already obtained on these compounds with other techniques such as X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), dielectric relaxation spectroscopy (DRS), electron spin resonance (ESR), and quantomechanical calculations. Notwithstanding the expected low photoelectron intensity due to the very low solubility of Ti (IV) ions in the MFI framework, an accurate XPS curve-fitting procedure and the simultaneous use of the XPS and Auger signals (Wagner plot) have made possible, both qualitatively and quantitatively, the distinction of two Ti (IV) sites: the framework tetrahedral species - Ti (IV) substituting Si (IV) - and the Ti(IV) tetrahedral species grafted to surface hydroxyl groups. Both species are prone to convert into octahedrally coordinated Ti (IV) species as a consequence of interaction with water and other ligands. Copyright © 2008 John Wiley & Sons, Ltd.