Photoreactivity of porous metal-oxide frameworks
The photoreactivity of three different types of porous metal-oxide frameworks have been investigated. The porous metal-oxide frameworks assessed are germanate, titanosilicate and vanadosilicate materials. A number of materials were synthesised, ASU-7, AUG-1, AUG-2, NH4-Ge-PHA, Li-ex-Ge-PHA. Mesoporous germanates, K-Ti-Si-PHA, AM-6 and ETVS-10. All materials synthesised were characterised using a number of techniques; x-ray diffraction, electron microscopy, solid state NMR, FT-IR, Raman, UV-vis, EXAFS, XPS, TGA and DTA. The photoreactivity of selected materials were investigated using EPR spectroscopy. The photoreactivity of two forms of the germanate pharmacosiderite material (NH4Ge-PHA and Li-ex-Ge-PHA) was explored. These materials are shown to have limited potential as photocatalyst due to their limited photoreactivity and their low thermal stability. However, on comparison to the non-porous metal oxide (h-GeO2) an improvement in photoreactivity was observed. The titanosilicate material showed limited photoreduction in the presence of ethene and methanol. However, when irradiated in the presence of oxygen a relatively stable and intense mononuclear O- species is formed. This species is found as a result of positive holes trapped at lattice oxide ions. A trapped hole signal with this stability has not previously been reported. AM-6 is shown to be a fully substituted vanadium form of ETS-10. The vanadium present is vanadium (IV) ions in octahedral coordination linking to form V-O chains. EXAFS analysis shows that the vanadium is in fact in significantly distorted octahedral sites. It is also shown that the free electrons are delocalised along the length of the vanadium-oxygen chains within the structure. ETVS-10 is a partially substituted vanadium form of ETS-10. The vanadium-oxygen chains present are interrupted by titanium sites, resulting in a reduction of the delocalisation of electrons along the chains. A photoreactivity study of these materials was problematical due to the intensity of the vanadium (IV) signal this made analysis of changes occurring upon irradiation in oxygen and methanol complicated.