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In situ XAS study on the thermal treatment of molybdenum heteropolyacids



In Situ XAS Study on the Thermal Treatment of Molybdenum Heteropolyacids Poster Hasylab 2001 Preprint of the Department of Inorganic Chemistry, Fritz -Haber-Institute of the MPG (for personal use only) ( In Situ XAS Study on the Thermal Treatment of Molybdenum Heteropolyacids J. Wienold, O. Timpe, R.E. Jentoft, G. Mestl and T. Ressler Fritz-Haber-Institut der MPG, Department of Inorganic Chemistry, Faradayweg 4-6, 14195 Berlin,Germany Heteropolyacids (HPA) of the general composition (H/Cs)3+x[(P/Si)Vx(Mo/W)12-xO40]*yH2O; x=0,1,2; y=0-36 with a Keggin structure are used as catalysts for the oxidation of methacrolein and isobutyric acid to methacrylic acid on an industrial scale [1]. In this investigation we studied the influence of different reactant atmospheres on thermal decomposition products and on the decomposition mechanism. In this connection, the stability of the 'Keggin anion', from which the structures are built, is of special interest. In addition, the role of the Vanadium atoms in the Keggin structure was examined. Former investigations have shown that in situ XAS-studies in combination with the results of in situ XRD studies are a powerful tool to elucidate the evolution of structures in long range and short range order [2]. The HPA-samples were prepared according to the literature [3]. Samples with different amount of molybdenum substituted by Vanadium (x = 0,1,2), different number of Cesium (0,2,3) and with Silicon and Phosphorus as heteroatoms were examined. All samples were checked with XRD for impuritys. It was found that the diffraction patterns of the Cs-salts fits well to simulations, calculated from single crystal data of a HPA with Potassium, which was substiuted by Cesium for the calculations [ICSD 209]. The diffraction patterns of the free acids fits all well to simulations which are calculated from single crystal data of a free acid with Silicon

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