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Melsheimer, Jörg; Mahmoud, Sabri S.; Mestl, Gerhard; Schlögl, Robert

doi:10.1023/a:1019094621316

Cited by 47 publications

(35 citation statements)

References 21 publications

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“…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%

“…The detection of gaseous compounds downstream the reactor was performed using the highly structure-specific IMR-MS detector allowing the analysis of the products without the usual strong fragmentation [53]. The mass signals for water (18), carbon dioxide (44) and partially oxidized products (59) are displayed. As a general feature the onset temperature for the (-propyl) at about 600 K is caused exclusively by the oxidation of the sample consuming oxygen and not by the reaction of gas-phase propene.…”

Section: Salts Formed With Bulky and Instable Cationsmentioning

confidence: 99%

“…It is known from these studies that the DRS method detects even small changes in spectral features connected with water loss or chemical reduction. This method was applied to investigate the reduction-reoxidation of HPA and Cs 2 A by methanol and ethanol and to correlate structural changes with catalytic data [18]. The method exhibits limitations similar to those of other optical methods.…”

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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“…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: Salts Formed With Bulky and Instable Cationsmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

See 1 more Smart Citation

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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“…Diffuse reflectance spectroscopy in the ultraviolet and visible region is a suitable technique for studying electronic transitions of heterogeneous catalysts (13)(14)(15)(16)(17)(18). Additionally, the technique is potentially very sensitive for the detection of adsorbed reaction intermediates or side products, depending on the chromophores present in these species.…”

Section: Introductionmentioning

confidence: 99%

Isomerization of n-butane and of n-pentane in the presence of sulfated zirconia: formation of surface deposits investigated by in situ UV–vis diffuse reflectance spectroscopy

Ahmad

Melsheimer

Jentoft

et al. 2003

Journal of Catalysis

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Catalytic performance and formation of carbonaceous deposits were studied simultaneously during alkane isomerization over sulfated zirconia in a fixed bed flow reactor with an optical window for in situ UV-vis diffuse reflectance spectroscopy. The reactions of n-butane (5 kPa) at 358 and 378 K and of n-pentane (0.25 kPa) at 298 and 308 K passed within 5 h or less through an induction period, a conversion maximum, and a period of deactivation; a steady activity of 41 and 47 µmol g -1 h -1 (isobutane formation) and ≈2.5 µmol g -1 h -1 (isopentane, both temperatures) remained. UV-vis spectra indicate the formation of unsaturated surface deposits; the band positions at 310 nm (n-butane reaction) and 330 nm (npentane) are within the range of monoenic allylic cations. More highly conjugated allylic cations (bands at 370 and 430 nm) became evident during n-butane reaction at 523 K. The chronology of events suggests that the surface deposits are (i) a result only of the bimolecular and not the monomolecular reaction mechanism, and (ii) are formed in a competitive reaction to the alkane products.

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“…Techniques have been developed which allow the studying of catalysts truly in situ, i.e., at high temperatures and pressures in a stream of reactants. The most used techniques have been infrared spectroscopy (4), Raman spectroscopy (5,6), Mössbauer spectroscopy (7), nuclear magnetic resonance spectroscopy (8,9), X-ray absorption spectroscopy (10)(11)(12), and UV-visible (vis) spectroscopy (13)(14)(15). A challenge often remaining is the implementation of the techniques without changing the catalytic properties of the system under study.…”

Section: Introductionmentioning

confidence: 99%

Monitoring Chromia/Alumina Catalysts in Situ during Propane Dehydrogenation by Optical Fiber UV–Visible Diffuse Reflectance Spectroscopy

Puurunen

Beheydt

Weckhuysen

2001

Journal of Catalysis

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A continuous-flow reactor system with in situ monitoring of the catalyst was developed. The catalyst surface was analyzed by collecting UV-visible diffuse reflectance spectra through an optical fiber placed in the catalyst bed. The system was tested with a series of chromia/alumina catalysts (1 wt% Cr) under the conditions of propane dehydrogenation. The fast reduction of Cr 6+ to Cr 3+ under propane stream could be detected. Increase in the catalytic activity with the alumina pretreatment temperature was tentatively ascribed to the presence of θ-alumina, and decrease in the activity during calcination to incorporation of Cr 3+ inside the alumina support.

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Cited by 47 publications

References 21 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

Isomerization of n-butane and of n-pentane in the presence of sulfated zirconia: formation of surface deposits investigated by in situ UV–vis diffuse reflectance spectroscopy

Monitoring Chromia/Alumina Catalysts in Situ during Propane Dehydrogenation by Optical Fiber UV–Visible Diffuse Reflectance Spectroscopy

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