Photoreactivity of porous metal-oxide frameworks

• Main Author: Yeates, Rachel Marie
• Published: University of Aberdeen 2005
• Subjects: 541.3533
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.415549

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 (<i>h</i>-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.