Structural Evolution of Ammonium Paratungstates during Thermal Decomposition

Mixed metal oxide systems (e.g. Mox(V,W)yO3-z) are employed for the partial oxidation of light alkenes. Ammonium paratungstate (APT) and ammonium heptamolybdate (AHM) are used as precursors for the production of WO3 and MoO3, respectively. The catalytic activity of these materials may depend on the treatment of the precursors. Therefore, studies of the decomposition process in order to identify and quantify tungsten oxide phases and their formation under various atmospheres, reveal correlation between catalytic activity and structural evolution of APT is very important. Here, we present results obtained from bulk structural studies on the thermal decomposition of APT in various reducing and oxidizing atmospheres. In this work the decomposition of APT is studied in situ by X-ray diffraction (XRD) and X-ray absorption spectroscopy (XAS). Using these two complementary methods allows us to follow the evolution of the short-range and long-range structure of the phases formed during the decomposition of APT, to elucidate the evolution of the primary and secondary structure under different conditions. 

Experimental 

Ammonium paratungstate (APT), (NH4)10H2W12O42*4H2O (OSRAM) was used as purchased. Transmission X-ray absorption spectra were measured in situ with the sample pellet in a flow reactor (4 ml total volume) under a controlled reactant atmosphere. In situ XAS experiments were performed at the W LIII edge (10.204 keV) (Hamburger Synchrotron Radiation Laboratory, HASYLAB, beamline X1), using a Si (311) double crystal monochromator. Temperature programmed decomposition was carried out at temperatures between 300 and 773 K in atmospheres of pure helium, 5 % hydrogen in helium, 20 % oxygen in helium, 10 % propene in helium, and 10 % propene/10% oxygen in helium. For the in situ XAS measurements APT was mixed with boron nitride and pressed into 5 mm in diameter pellets. Analysis of the gas phase was carried out with a quadrupol mass spectrometer, QMS 200 (Pfeifer), with a time resolution of ~ 2 s. Further details about the experimental XAS set-up used can be found in .