Influence of dehydration effects on the optical spectra of H4PVMo11O40 in the visible and near infrared range: intra- and intercenter optical transitions in the V–Mo cluster

Klokishner, Sophia; Melsheimer, Jörg; Ahmad, Rafat; Jentoft, Friederike C.; Mestl, Gerhard; Schlögl, Robert

doi:10.1016/s1386-1425(01)00469-3

Cited by 16 publications

(22 citation statements)

References 32 publications

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“…With increasing temperature broad absorptions develop first in the Vis spectral range and at higher temperatures also in the NIR region. Similar behaviour was observed for H 4 [11,18,19]. Usually, the observed spectra are decomposed into Gauss profiles due to their strong broadening.…”

Section: Electronic Structure By In Situ Uv-vis-near-ir Spectroscopy supporting

confidence: 75%

“…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) …”

Section: In Situ Xasmentioning

confidence: 99%

“…These bands designate binuclear metal centers bridged by at least one oxygen atom in which the formal oxidation states are d 0 and d n for the metals. The shape and the intensity of the IVCT bands are determined by vibrational coupling parameters and by parameters characterizing the electron transfer between different metal sites of the Keggin molecule [11,19]. As the intensity of an IVCT transition is proportional to the square of the electron transfer parameter, the most intense charge transfer bands arise from the transitions in which the only one d 1 electron is excited from its original site to a neighboring one.…”

Section: Data Analysis Using Semi-empirical Theorymentioning

confidence: 99%

“…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.…”

Section: Introductionmentioning

confidence: 99%

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

Jentoft

Klokishner

Kröhnert

et al. 2003

Applied Catalysis A: General

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The present study focuses on the evidence about the existence of Keggin ions under various reactive conditions. The stability of the hydrated parent heteropoly acid (HPA) phases is probed in water, by thermal methods in the gas phase, by in situ X-ray diffraction and in situ EXAFS. An extensive analysis of the in situ optical spectra as UV-Vis-NIR in diffuse reflectance yields detailed information about the activated species that are clearly different from Keggin ions but are also clearly no fragments of binary oxides in crystalline or amorphous form. Infrared spectroscopy with CO as probe molecule is used to investigate active sites for their acidity. Besides -OH groups evidence for electron-rich Lewis acid sites was found in activated HPA. All information fit into a picture of a metastable defective polyoxometallate anion that is oligomerised to prevent crystallisation of binary oxides as the true nature of the "active HPA" catalyst. The as-synthesized HPA crystal is thus a pre catalyst and the precursor oxide mixture is the final deactivated state of the catalyst.

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Section: Electronic Structure By In Situ Uv-vis-near-ir Spectroscopy supporting

confidence: 75%

“…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) …”

Section: In Situ Xasmentioning

confidence: 99%

Section: Data Analysis Using Semi-empirical Theorymentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

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

Jentoft

Klokishner

Kröhnert

et al. 2003

Applied Catalysis A: General

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“…The peak maximum of this band is located in the range of 740-780 nm and a shoulder can be distinguished in the range of 900-1000 nm. A semiempirical model that considers a dimeric cluster V 4+ -Mo 6+ was employed for the simulation of the spectra [28]. Appreciable spectroscopic changes between 600 and 1300 nm were attributed to the increase in the parameter characterising electron transfer between the V 4+ and Mo 6+ ions (electron transfer parameter) due to water loss.…”

Section: Peak Intensity Versus Time On He Stream At Room Temperaturementioning

confidence: 99%

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

Melsheimer

Kröhnert

Ahmad

et al. 2002

Phys. Chem. Chem. Phys.

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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.

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Concepts in Selective Oxidation of Small Alkane Molecules

Schlögl

2009

Modern Heterogeneous Oxidation Catalysis

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Influence of dehydration effects on the optical spectra of H4PVMo11O40 in the visible and near infrared range: intra- and intercenter optical transitions in the V–Mo cluster

Abstract: Appreciable changes in the band intensity are attributed to the increase of the electron transfer parameter due to water loss. Quite good agreement with the experiment was found for a reasonable set of transfer and vibronic parameters.

Cited by 16 publications

References 32 publications

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

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

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

Concepts in Selective Oxidation of Small Alkane Molecules

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