Thermally and chemically induced structural transformations of Keggin-type heteropoly acid catalysts

Mestl, Gerhard; Ilkenhans, Thomas; Spielbauer, D.; Dieterle, Martin; Timpe, Olaf; Kröhnert, Jutta; Jentoft, Friederike C.; Knözinger, Helmut; Schlögl, Robert

doi:10.1016/s0926-860x(00)00793-6

Cited by 120 publications

(105 citation statements)

References 93 publications

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“…The investigation of Raman spectra [13] shows that the formation of ill-defined clusters takes place at higher temperatures. Therefore the transformations of the spectra in the range 326 < T < 663 K are attributed to these clusters.…”

Section: Determination Of Spectroscopic Characteristicsmentioning

confidence: 98%

“…Unfortunately, however, the title compound in particular with x = 0, is thermally unstable, which makes it especially difficult to determine the catalytically active center. Many efforts have been made to investigate the thermally unstable state, for example, by XRD [2,4], TG-DTA coupled with IMR-MS and DSC [2,5], IR [4,[6][7][8], EPR [9,10] and Raman [11][12][13]. To improve the stability and catalytic activity of such compounds, cations were introduced in the secondary structure [14,15].…”

Section: Introductionmentioning

confidence: 99%

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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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Section: Determination Of Spectroscopic Characteristicsmentioning

confidence: 98%

Section: Introductionmentioning

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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“…An adsorbate-type structure of the active site allowing dynamical response will be useful whereas a closely packed 3-D crystalline state will hinder such a process with high activation barriers. Such ''adsorbatelike'' active phases [77,78] can be identified with probe molecule reactions [79][80][81][82][83][84][85][86] and have been rationalized for covering some bulk catalysts [13,42,[87][88][89][90][91] as consequence of surface energy minimization.…”

Section: Size and Functions Of Active Sitesmentioning

confidence: 99%

Active Sites for Propane Oxidation: Some Generic Considerations

Schlögl

2011

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Based on experimental observations about structure and dynamics of the reactive surface of the M1 phase some considerations are made about the nature and size of active sites. In analyzing the stoichiometry of the reactions following activation of propane it occurs that for dehydrogenation small sites are desirable being just sufficient to re-activate oxygen without kinetic hindrance. For deeper oxidation to acrylic acid (AA) the sites should be larger to accommodate all redox equivalents and oxygen species required for one transformation. A qualitative model for size and composition of the active site is made. It is likely that the active site consists on a VxOy species of strictly 2-D nature. This follows the structural suggestion for the active site on the a-b-plane of the M1 structure. The role of Te in moderating the active site is discussed. The suggestions are discussed in comparing requirements for oxidative dehydrogenation of propane (ODP) with those for AA synthesis.

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“…Mixed phases of partially salified HPA have been proposed forming a core-shell system of the Cs 3 A-salt and the free acid under catalytic conditions [25]. Furthermore, it has been suggested that the molybdenum cations [28] as well as the vanadium cations [29,30] migrate from the Keggin anion to cationic positions in the structure [31,32] and that the heteropoly acids become partially reduced under reaction conditions [33,34]. This migration of cations from the Keggin anion has been speculated to be responsible for the catalytic activity of the heteropoly oxomolybdates with Keggin structure [25,29,30,33,35,36,37].…”

Section: Introductionmentioning

confidence: 99%

“…Furthermore, it has been suggested that the molybdenum cations [28] as well as the vanadium cations [29,30] migrate from the Keggin anion to cationic positions in the structure [31,32] and that the heteropoly acids become partially reduced under reaction conditions [33,34]. This migration of cations from the Keggin anion has been speculated to be responsible for the catalytic activity of the heteropoly oxomolybdates with Keggin structure [25,29,30,33,35,36,37]. The result of the migration is a cubic molybdenum autosalified heteropolyacid (Pn-3m, a = 11.853 Å) derived from triclinic H 3 [PMo 12 O 40 ]*13H 2 O under reduction conditions with propene and hydrogen at temperatures above 600 K. This way the in situ studies on the electronic structure of the active state of HPA are corroborated by direct in situ geometric structural studies using Xray diffraction and EXAFS.…”

Section: Introductionmentioning

confidence: 99%

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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Thermally and chemically induced structural transformations of Keggin-type heteropoly acid catalysts

Cited by 120 publications

References 93 publications

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

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

Active Sites for Propane Oxidation: Some Generic Considerations

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

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