In Situ XAS and XRD Studies on the Structural Evolution of Ammonium Paratungstate

The bulk structural evolution during the decomposition of ammonium paratungstate (APT) under various oxidizing and reducing gases was elucidated by the complementary techniques, in situ XAS, in situ XRD, and TG/DSC, combined with mass spectrometry. In the temperature range from 300 to 650 K, the decomposition of APT proceeds nearly independently of the gas employed. At higher temperatures, an oxidizing gas results in the formation of crystalline triclinic WO3 as the majority phase at

773 K, while mildly reducing gases (propene, propene and oxygen, and helium) result in the formation of partially reduced and highly disordered tungsten bronzes. No further reduction is observed under propene or helium, and this indicates a strongly hindered oxygen mobility in the tungsten oxide lattice in the temperature range employed. The decomposition of APT under hydrogen results in the formation of WO2 and, eventually, tungsten metal. During the thermal treatment of APT, major changes occur at ca. 500 K where a complete structural rearrangement takes place that results in the destruction of the polyoxotungstate ion of APT and the formation of a tungsten oxide bronze. The rather low temperature for the formation of a three-dimensional lattice compared to the thermal treatment of common polyoxomolybdate precursors indicates the lower stability of the precursor and intermediates as ligands are removed. For tungsten to act as a potential structural or electronic promoter in molybdenum oxide based catalysts, tungsten needs to be incorporated in regular molybdenum oxide structures already at a very early stage of the catalyst preparation.

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