XAFS STUDY OF TI-SILICALITE - STRUCTURE OF FRAMEWORK TI(IV) IN THE PRESENCE AND ABSENCE OF REACTIVE MOLECULES (H2O, NH3) AND COMPARISON WITH ULTRAVIOLET-VISIBLE AND IR RESULTS

X-ray absorption at the Ti K edge (both XANES and EXAFS) of a very pur e Ti-silicalite containing a small fraction of Ti(IV) substituting for Si(IV) has been performed in order to Study the effect of the presenc e 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 det ail. 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-silica lite has been compared with a Ti-silicalite containing extraframework Ti and with other reference compounds such as anatase and amorphous Si O2/TiO2. Further information has been obtained from EXAFS determinatio n of first-shell bonding parameters.