Dirk Niemeyer

The synthesis and structure of a single-phase, nanocrystalline MoVW mixed-oxide catalyst of the Mo5O14 type

The different preparation steps are characterized to the single phase, crystalline, ternary oxide (MoVW) 5O14, which is important for catalytic, mild selective oxidation reactions. For the synthesis of this oxide, solutions of ammonium heptamolybdate, ammonium metatungstate, and v anadyl oxalate were spray-dried followed by different thermal treatments. The structures of the materials formed at each preparation step, starting from the precursor to the final product, were studied using scanning and transmission electron microscopy, X -ray powder diffraction, thermal analysis, and Raman spectroscopy. Raman spectroscopy was also applied to shed some light into the aqueous chemistry of the mixed precursor solutions. Raman data indicates that a molecular structure is already formed in solution which seems to be closely related to that of the final crystalline Mo5O14-type oxide. X-ray diffraction revealed that the thermal treatment steps strongly affect the degree of crystallinity of the ternary Mo5O14 oxide. Transmission electron microscopy with energy dispersive microanalysis confirmed the presence of V and W in the molybdenum oxide particles and gave evidence for the (010) plane as the most developed face of the crystals of this phase. Details of the structural transformation of this system at the different preparation and calcination steps are discussed in relation to their performance in the selective partial oxid ation of acrolein to acrylic acid.

Nanoclusters as Precursors to (MoVW)5O14: In situ and chemical characterisation of the systems of a single phase oxidation catalyst

Abstract The ternary oxide (MoVW)5O14 is considered to be highly significant for catalytic, mild selective oxidation reactions. By using UV/Vis, ESR and 95Mo NMR spectroscopies the coordination chem-istry of the molybdate species in the precursor solutions was studied. This work has established that mixing the precursor solutions (ammonium hepta molybdate, ammonium meta tungstate and vanadyl oxalate) forms a polymeric network in which the vanadyl species act as a linker between the molybdate species. Addition of tungsten atoms significantly enhances the polymerisation proc-ess