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 H4-xCsxPVMo11 (x=0,2) compounds with rising temperature over long time stream of different gas athmospheres, 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, iso-propanol, water and the oxidation products were analyzed by GC. The experimental observed blue shift of the visible absorption band in the range of crystal water loss and the increase of 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 Mo 5+ -Mo 6+ and V 4+ -Mo 6+ 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.

The structure of molybdenum-heteropoly acids under conditions of gas-phase selective oxidation catalysis: a multi-method in situ study

“…For CsH 3 [PVMo 11 O 40 ]*nH 2 O treated in He from RT to 663 K the apparent absorption increases strongly with temperature as can be seen in Figure 14. Simultaneously, the near-IR absorption bands of the crystal water disappear completely.…”

“…All spectroscopic measurements were carried out with Spectralon ® (Labsphere) as reference material. HPA samples (7)(8)(9)(10)(11)(12)(13)(14)(15)(16)(17)(18)(19) …”

“…These fragments can be detected by mass spectroscopy and used as indicators for the decomposition of the Keggin structure. A series of salts with the general composition (NR 4 ) 2 H 2 [PVMo 11 O 40 ]*nH 2 O is subjected to temperature programmed reaction analysis (R: -H, -methyl, -ethyl, -npropyl, -n-butyl). Characterization by TG (in inert atmosphere or in 50 % oxygen, heating rate 5 K/min) reveals a distinct dependence of the thermal stability upon the nature of the cation, i. e. upon the residue R. The DTG traces presented in Figure 6 reveal the sample with the bulkiest cation to be the least stable among the alkylammonium salts.…”

“…This can be studied in situ using optical spectroscopy. An essential advance in the studies of optical spectra of catalysts of different stages of their transformation was achieved by applying the in situ diffuse reflectance UV/Vis/near-IR spectroscopy (DRS) [10,11], which proved to be a suitable technique for probing both d-d and charge transfer transitions at reaction temperature and under realistic gas compositions [12]. A number of groups have reported exsitu optical spectra [13,14,15,16,17].…”

“…Due to strong coupling of outer valence electronic states with the vibrational states of the solid the bands are additionally broadened. A further loss in resolution caused by data acquisition above room temperature has to be accepted as consequence of the dynamic nature of the activated systems [11,19]. The multiple redox states of activated HPA give rise to band systems strongly overlapping and thus requiring data analysis based on theoretical predictions in order to derive meaningful electronic structural information.…”