Jentoft, R.

Ressler, T.

Wienold, J.

English

MPG.PuRe

  EuropacatV Limerick   Preprint of the Department of Inorganic Chemistry, Fritz -Haber-Institute of the MPG (for personal use only) (www.fhi-berlin.mpg.de/ac) Influence of different parameters on the decomposition of Ammonium heptamolybdate  J. Wienold, R.E. Jentoft, T. Ressler Fritz-Haber-Institut der Max-Planck-Gesellschaft, Department of Inorganic Chemistry, Faradayweg 4-6, 14195 Berlin, Germany  Ammonium heptamolybdate tetrahydrate (AHM), NH4Mo7O24*4H2O, is a suitable model system for the more complex systems which have additions of Wolfram and Vanadium. The latter are the precursors for the preparation of oxides of the type (Mo/W/V)O3-x which are used as catalysts for the partial oxidation of light alkenes on an industrial scale. The activity of the oxides might be influenced by their constitution, i.e. the composition, the structure, the particle size and crystallinity. These properties in turn, might be influenced by the decomposition and the activation process respectively [1]. Important parameters therein are the reactant atmosphere and heating and flow rates. In this investigation the decomposition process is studied by TG/DTA, in situ XRD and in situ XAS measurements. The gas phase composition is monitored with mass spectroscopy simultaneously in all three methods. The use of the two complementary methods XRD and XAS enables to follow the evolution on the long range order scale and on the short range order scale [2]. This facilitate to describe the phases formed more complete. The influence of different reactant atmospheres (inert and 5 % hydrogen, 20 % oxygen, 5 % propene in helium respectively) are examined. Intense studies with TG/DTA under variation of the heating and flow rate gave additional information on the kinetics of the decomposition steps. It could be shown that the constitution of the products, which are in the case of AHM molybdenum oxides with different stoichiometry, are mainly dependent on the reactant atmosphere and on the partial pressure or more precise the dwell time of decomposition products in the gas phase in a given temperature range. The latter is influenced by the flow rate as well as the heating rate. The crystallisation process of one of the intermediates is controlled by the partial pressure of the decomposition products in the gas phase. Depending on this, the succeeding decomposition process proceeds via variable amounts of different intermediates. The reactant atmosphere influence in turn the phase formation after the spontaneously decomposition of the last intermediate and thus the constitution of the products.   [1] J.-L. Li, T. Inui, Appl. Catal. A, 1996, 137, 105 [2] T. Ressler, R.E. Jentoft, J. Wienold, M.M. Günter, O. Timpe, J. Phys. Chem. B, 2000, 104, 6360    

Influence of different parameters on the decomposition of ammonium heptamolybdate

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