Browsing by Author "Pike, KJ"

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    Microcrystalline hexagonal tungsten bronze. 1. Basis of ion exchange selectivity for cesium and strontium
    (American Chemical Society, 2009-07-06) Griffith, CS; Luca, V; Hanna, JV; Pike, KJ; Smith, ME; Thorogood, GJ
    The structural basis of selectivity for cesium and strontium of microcrystalline hexagonal tungsten bronze (HTB) phase NaxWO3+x/2·zH2O has been studied using X-ray and neutron diffraction techniques, 1D and 2D 23Na magic angle spinning (MAS) nuclear magnetic resonance (NMR) spectroscopy, and radiochemical ion exchange investigations. For the HTB system, this study has shown that scattering techniques alone provide an incomplete description of the disorder and rapid exchange of water (with tunnel cations) occurring in this system. However, 1D and 2D 23Na MAS NMR has identified three sodium species within the HTB tunnels—species A, which is located at the center of the hexagonal window and is devoid of coordinated water, and species B and C, which are the di- and monohydrated variants, respectively, of species A. Although species B accords with the traditional crystallographic model of the HTB phase, this work is the first to propose and identify the anhydrous species A and monohydrate species C. The population (total) of species B and C decreases in comparison to that of species A with increasing exchange of either cesium or strontium; that is, species B and C appear more exchangeable than species A. Moreover, a significant proportion of tunnel water is redistributed by these cations. Multiple ion exchange investigations with radiotracers 137Cs and 85Sr have shown that for strontium there is a definite advantage in ensuring that any easily exchanged sodium is removed from the HTB tunnels prior to exchange. The decrease in selectivity (wrt cesium) is most probably due to the slightly smaller effective size of Sr2+; namely, it is less of a good fit for the hexagonal window, ion exchange site. The selectivity of the HTB framework for cesium has been shown unequivocally to be defined by the structure of the hexagonal window, ion exchange site. Compromising the geometry of this window even in the slightest way by either (1) varying the cell volume through changes to hydration or sodium content or (2) introducing disorder in the a−b plane through isomorphous substitution of molybdenum is sufficient to reduce the selectivity. Indeed, it is our hypothesis that this applies for all cations which are strongly bound by the HTB framework. © 2009, American Chemical Society
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    Structural evolution and stability of sol-gel biocatalysts
    (Elsevier, 2006-11-15) Rodgers, LE; Knott, RB; Holden, PJ; Pike, KJ; Hanna, JV; Foster, LJR; Bartlett, JR
    Immobilisation strategies for catalytic enzymes are important as they allow recovery and reuse of the biocatalysts. In this work, sol-gel matrices have been used to immobilise Candida antarctica lipase B (CALB), a commonly used industrial enzyme. The sol-gel bioencapsulate is produced through fluoride-catalysed hydrolysis of mixtures of tetramethylorthosilicate (TMOS) and methyltrimethoxysilane (MTMS) in the presence of CALB, yielding materials with controlled pore sizes and surface chemistries. Sol-gel matrices prolong the catalytic life and enhance the activity of CALB, although the molecular basis for this effect has yet to be elucidated due to the limitations of analytical techniques applied to date. Small angle neutron scattering (SANS) allows such multi-component systems to be characterised through contrast matching. In the sol-gel bioencapsulate system at the contrast match point for silica, residual scattering intensity is due to the CALB and density fluctuations in the matrix. A SANS contrast variation series found the match point for the silica matrix, both with and without enzyme present, to be around 35%. The model presented here proposes a mechanism for the interaction between CALB and the surrounding sol-gel matrix, and the observed improvement in enzyme activity and matrix strength. Essentially, the inclusion of CALB modulates silicate speciation during evolution of the inorganic network, leading to associated variations in SANS contrast. The SANS protocol developed here may be applied more generally to other encapsulated enzyme systems. © 2006, Elsevier Ltd.
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    Structure and dehydration of the pyrochlore system NaW2−yMoyO6+δ·nH2−zO between 10 and 675 K
    (Elsevier, 2008-07) Thorogood, GJ; Kennedy, BJ; Luca, V; Blackford, MG; van de Geeste, SK; Finnie, KS; Hanna, JV; Pike, KJ
    The temperature dependence of the structure of the pyrochlore NaW2−yMoyO6+δ·nH2−zO has been investigated using a variety of diffraction and spectroscopic methods. The positions of OH−/H2O molecules in the structure have been determined. Increases in temperature induce small lattice parameter changes, which are thought to result from movement of the H2O molecules in the pyrochlore lattice. Crown Copyright © 2008 Published by Elsevier Ltd.