UV/Vis/near-IR spectroscopic characteristics of H4−xCsxPVMo11O40 (x = 0, 2) catalyst under different temperatures and gas atmospheres

Abstract

In order to understand the transformations of the Keggin-type H_4−xCs_xPV–Mo₁₁O₄₀ (x = 0, 2) compounds with rising temperature over a long time in a stream of different gas atmospheres, in situ UV/Vis/near-IR spectroscopic studies were carried out. Diffuse reflectance spectra were recorded using an improved spectrometer and a suitable microreactor. Visible and near-IR peak intensities, peak positions and the band gap energies were determined from apparent absorption spectra. Propene, isopropanol, water and the oxidation products were analyzed by GC. The experimentally observed blue shift of the visible absorption band in the region of crystal water loss and the increase in the near-IR absorption were explained on the basis of quantum-mechanical calculations of the shapes and positions of the charge transfer and d–d bands arising from Mo⁵⁺–Mo⁶⁺ and V⁴⁺–Mo⁶⁺ pairs in intact and ill-defined fragments of the Keggin structure. It was concluded that with removal of crystal water during the action of He, He/H₂O, propene, O₂/propene and increasing temperature, reduced species with protons located at the bridging oxygens promote a blue shift of the visible band, while a large number of ill-defined species form the near-IR part of the spectra in the temperature range 326–600 K.