An investigation into the photocatalytic properties of microporous titanosilicate materials

Photocatalysis is currently an extensively studied research area with a variety of potential commercial/industrial applications ranging from the production of H2 from water as a clean energy source to the treatment of waste water by solar collector reactors. For all these potential applications semiconductors, particularly TiO2 and ZnO, have attracted the most attention due to their high reactivity. However these materials are all wide-band gap semiconductors and thus require high energy electromagnetic radiation (UV light) in order to promote photocatalysis. They are also dense nonporous materials with low surface areas. Microporous titanosilicate materials have recently been seen as viable alternatives to these semiconductors since they have large surface areas and their band gaps can be modified for low energy electromagnetic radiation.;In this thesis we examine three different microporous titanosilicate materials, TSP, Sitinakite, and ETS-10. We will show that all of these materials have surface areas that are much larger than TiO2 or ZnO. In addition, these materials will also be shown to have unique photocatalytic properties that are determined by the structure, particularly the connectivity of the titanium octahedra. One disadvantageous photocatalytic property is a higher band gap than TiO2 and ZnO that results from quantum confinement. These unique photocatalytic properties will also be shown to lead to partial oxidation products for the reaction of model volatile organic compounds (VOCs), methanol and ethylene, with oxygen. Since only partial oxidation is achieved and the band gap energy is higher than that of TiO2 and ZnO, methods for modifying the photocatalytic properties of these materials will be discussed.;The use of transition metals as dopants into the structure of ETS-10 is presented as a method for lowering the band gap energy and achieving visible light photocatalysis. In this thesis we investigate the use of two different transition metals, vanadium and chromium. We show that both metals isomorphously substitute into the ETS-10 structure by replacing titanium and that vanadium is unique since it can completely replace titanium forming an analogous ETS-10 structure known as AM-6. In the case of vanadium we show that the substitution occurs with two different oxidation states, V4+ and V 5+, which each have different coordination. In chromium the substitution occurs with only one oxidation state, Cr3+, all with the same coordination. We show that these transition metals lower the band gap energy and add new visible light transitions. This lowering of the band gap is shown to occur by the addition of d orbitals just below the conduction band while some of the visible light transitions are shown to be caused by d-d transitions that arise from V4+ and Cr3+. These new transitions and the lowering of the band gap are shown to give the vanadium-incorporated ETS-10 materials visible light reactivity with the mixed vanadium-titanium samples being shown as the most active. The chromium-incorporated ETS-10 materials are shown to be inactive under visible light.;In order to solve the problem of partial oxidation post synthesis methods are discussed which change the photocatalytic properties of these materials to allow complete oxidation. These post synthesis methods usually involve adding defects to the crystal structure by acid treatment so as to expose more of the titanium octahedra. Ammonium ion exchange on ETS-10 and AM-6 is presented as the best post synthesis procedure since it exposes the titanium/vanadium octahedra but does not completely destroy the structure. For both ETS-10 and AM-6 no change in overall crystal structure is observed after ion exchange; however, changes in the local coordination of the titanium/vanadium octahedra are observed. There is also a red shift in the band gap energy of ETS-10 and no change in the band gap of AM-6. No further changes in the coordination of the titanium octahedra or overall structure are observed after breakdown of the ammonium ions to ammonia and the proton form of ETS-10 (H-ETS-10) upon heating. In AM-6 however, the vanadium octahedra change from an octahedral coordination to a tetrahedral coordination upon breakdown of the ammonium ions, which results in changes to the intensity of several of the X-ray diffraction peaks for the dehydrated sample. These changes in local coordination of the titanium/vanadium octahedra upon breakdown of the ammonium ions are shown to achieve complete oxidation of methanol and ethylene in oxygen.;Finally this thesis discusses the viability and general aspects of these photocatalytic microporous titanosilicate materials and the methods used to modify them. Future recommendations are also given on ways to improve the synthesis and modification methods of these microporous materials.