Photoreactivity studies of microporous titanosilicate materials

• Main Author: Krisnandi, Y. K.
• Published: University of Aberdeen 2004
• Subjects: 541.3
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.590942

Photoreactivity studies of microporous titanosilicate materials have been carried out on two types of Engelhard titanosilicate materials, ETS-10 and ETS-4. The as-synthesised samples of both materials contain exchangeable Na⁺ and K⁺ cations in the extra-framework positions that maintain the charge balance. In these studies, the ETS-10 and ETS-4 sample were subjected to ammonium and sodium back exchange treatments to modify the degree of defects within the samples. Characterisation studies on ETS-10 samples were performed using x-ray diffraction, FTIR and Raman spectroscopy, XAS spectroscopy, solid state NMR, XPS and electron microscopy. It was shown that the level of defects does not affect the long range order of the structures. However, the defects created in the ion exchange samples caused irreversible modifications to the structure producing more distorted and exposed TiO₆ sites, particularly in the external surface of the crystals. The modifications in the structure of ETS-4 were not as extensive as those occurring in ETS-10. <i>In-situ</i> EPR experiments showed that in the presence of organic molecules, photoreduction of Ti⁴⁺ in the framework to Ti³⁺ occurred only in ion-exchanged ETS-10 samples and the more extensively when H⁺ ions were presented in the pores. Subsequent admission of oxygen removed the Ti³⁺ signals and formed superoxide ion (O₂). The EPR experiments with ETS-4 materials, on the other hand, showed that in the presence of organic molecules, photoreduction of Ti⁴⁺ ® Ti³⁺ took place in all forms. Finally, it was concluded that to enhance the photoreactivity of ETS-10 and ETS-4 it was necessary to increase the concentration of defects in order to create more exposed Ti sites within the structure. For ETS-10 in particular, maintaining accessible pores is important if a selective reaction is preferred.