Thermal Decomposition of Ammonium Paratungstate Tetrahydrate Traced by In Situ UV/Vis Diffuse Reflectance Spectroscopy

In situ UV/Vis diffuse reflectance spectra of ammonium paratungstate tetrahydrate at increasing temperatures were recorded in oxidative as well as in reductive atmospheres. From the edge energy of the ligand‐to‐metal charge transfer transition of tungsten ions, the number of bridging W–O–W bonds, N, was determined. The increase of N with temperature indicates a stepwise enhancement of the condensation of WO6 octahedra to finally form WO3 in an oxidative atmosphere. The graphic illustrates the stepwise increasing number of nearest tungsten ions due to thermally induced condensation of WO6 octahedra, which was calculated from the edge energies of the ligand‐to‐metal charge transfer transitions of tungsten ions. These optical transitions, in turn, were determined from the in situ UV/Vis diffuse reflectance spectra while heating in oxidative and reductive atmospheres.