XAFS Study of Ti-Silicalite: Structure of Framework Ti(IV) in Presence and in Absence of Reactive molecules (H2O and NH3) and Comparison with U.V. Vis. and IR Results

X-ray absorption at the Ti K edge (both XANES and EXAFS) of a very pure Ti-silicalite containing a small fraction of Ti(IV) substituting for Si(IV) has been performed in order to study the effect of the presence or absence of ligands such as H₂O and NH₃ on the Ti(IV) coordination sphere. In particular, the effect of an outgassing treatment at 400 K and of the interaction with NH₃ has been studied and described in detail. It has been found that the Ti(IV) is fourfold coordinated in the samples outgassed at 400 K and expands its coordination sphere number with action of adsorbates. The process is partially reversible at room temperature. The pre-edge absorption of a well-manufactured Ti-silicalite has been compared with a Ti-silicalite containing extraframework Ti and with other reference compounds such as anatase and amorphous SiO₂/TiO₂. Further information has been obtained from EXAFS determination of first-shell bonding parameters.