Electron transfers in a TiO2-containing MOR zeolite: synthesis of the nanoassemblies and application using a probe chromophore molecule

Abstract

New assemblies constituted by a microporous matrix of mordenite (MOR) zeolite on which TiO₂ nanoclusters are deposited were synthesized using ionic oxalate complexes and TiCl₃ titanium precursors. The samples were used to investigate the transfer of electrons produced by spontaneous or photo-induced ionization of a guest molecule (t-stilbene, t-St) occluded in the porous volume towards the conduction band of a conductive material placed nearby, in the pores or at least close to their entrance. The reaction mechanisms were compared in these Ti-rich solids and in a Ti-free mordenite sample. The characterization by XRD, N₂ physisorption, TEM, XPS and DRIFT spectroscopy of the supramolecular TiO₂/MOR systems before t-St adsorption showed the preservation of the crystalline structure after Ti addition and thermal activation treatments. They also revealed that titanium is mainly located at the external surface of the zeolite grains, in the form of highly dispersed and/or aggregated anatase. After incorporation of the guest molecule in the new assemblies, diffuse reflectance UV-visible and EPR spectroscopies indicate that the electron transfer processes are similar with and without TiO₂ but strongly stabilized t-St˙⁺ radicals are detected in the TiO₂-MOR samples whereas such species were never detected earlier in TiO₂-free mordenite using these techniques. The stabilization process is found to be more efficient in the sample prepared with TiCl₃ as the precursor than with titanium oxalates. It is proposed that the proximity of TiO₂ with the formed t-St˙⁺ radicals provokes the stabilization of the radical through capture of the ejected electron by the semi-conductor and that confinement effects can also play a role.