Tuning the photocatalytic activity of titanium dioxide by encapsulation inside zeolites exemplified by the cases of thianthrene photooxygenation and horseradish peroxidase photodeactivation

Abstract

A series of Y, Beta, mordenite and ZSM-5 zeolites containing different amounts of nanosized TiO₂ clusters have been prepared by TiO²⁺ exchange followed by condensation. The presence of TiO²⁺ and its subsequent oligomerization can be followed in Raman spectroscopy by the appearance of the band at ca. 530 cm⁻¹ and its replacement by others at 394, 460 and 637 nm. X-ray diffractograms of the solids show a considerable intensity decrease as the amount of TiO₂ present increases. High-resolution transmission electron microscopy (HRTEM) shows that no isolated TiO₂ particles are present in the case of zeolite Y, while the sample of mordenite at the highest Ti loading contains isolated TiO₂ particles distinguishable by HRTEM. Diffuse reflectance UV-Vis spectroscopy indicates that the bandgap and the apparent extinction coefficient depend remarkably on the Ti content as well as on the zeolite crystal structure. Quantum chemical calculations at the semiempirical level also predict large variations in the HOMO–LUMO energies as a function of the number of Ti atoms. It has been found that the photoactivity of TiO₂@Y and TiO₂@mordenite is higher than that of commercial anatase for the photooxygenation of thianthrene to thianthrene oxide (λ_ex > 200 nm), but smaller for the hoseradish peroxidase photodeactivation in aqueous buffer (λ_ex = 350 nm). These variations in the photocatalytic activity of TiO₂ illustrate the potential that inclusion of TiO₂ nanoclusters within zeolites has to modulate its photoactivity.