In situ UV–Vis diffuse reflectance spectroscopy — on line activity measurements of supported chromium oxide catalysts: relating isobutane dehydrogenation activity with Cr-speciation via experimental design

Weckhuysen, Bert M.; Verberckmoes, An; Debaere, Jan; Ooms, Kristine; Langhans, Ivan; Schoonheydt, Robert A.

doi:10.1016/s1381-1169(99)00259-9

Cited by 97 publications

(78 citation statements)

References 22 publications

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“…Previous studies have estimated extents of reduction in metal oxides from measurements of near-edge spectral changes by using linear interpolations between spectra for stoichiometric oxides and for suboxides with stable structures formed by reduction in H 2 . [17][18][19][20][21][22][23][24][25][26][27][28] We report here a direct method for measuring the number of reduced centers during alkane oxidation catalysis on metal oxides. As described in a preliminary communication, 29 this method is based on measured changes in the pre-edge portion of UV-visible spectra.…”

Section: Introductionmentioning

confidence: 99%

Extent of Reduction of Vanadium Oxides during Catalytic Oxidation of Alkanes Measured by in-Situ UV−Visible Spectroscopy

et al. 2004

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In-situ UV-visible spectroscopy was used to measure the extent of reduction of active centers in VO x /γ-Al 2 O 3 during oxidative dehydrogenation (ODH) of propane. Prevalent extents of reduction (0.062 to 0.30 e -/V) are much smaller than required for the formation of stoichiometric V 3+ or V 4+ suboxides. Surface oxygen atoms are the most abundant reactive intermediates during propane ODH, as previously suggested by kinetic and isotopic studies. These measurements involved the rigorous calibration of UV-visible intensities in the pre-edge region using quantitative reoxidation of a small number of centers reduced in H 2 . Transients observed during changes in C 3 H 8 and O 2 concentrations indicate that only a fraction of the prevalent reduced centers (∼30-40%) are active in catalytic turnovers, while the rest are reoxidized in time scales much longer than turnover times. The number of catalytically relevant reduced centers depends only on C 3 H 8 /O 2 ratios, and not on individual reactant concentrations, indicating that oxygen vacancies are the predominant reduced centers and that hydroxyls and alkoxides are present at much lower concentrations. The fraction of V-atoms that exist as catalytically reduced centers and the rate of propane ODH (per exposed V-atom) increase with increasing vanadia surface density and domain size up to surface densities typical of polyvanadate monolayers (∼7.5 V/nm 2 ) and then reach nearly constant values at higher surface densities. This relation between the extent of reduction during catalysis and the propene formation rates confirms the redox nature of catalytic cycles and the exclusive kinetic relevance of the reduction part of the cycle, in which C-H bonds are activated using lattice oxygen atoms. This method for measuring the extent of reduction during catalysis using preedge features in the UV-visible spectrum provides greater sensitivity and time resolution than X-ray absorption and UV-visible spectroscopic methods based on near-edge spectral features. The approach and initial results seem generally applicable to oxidation reactions using lattice oxygens as reactive intermediates.

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Section: Introductionmentioning

confidence: 99%

Extent of Reduction of Vanadium Oxides during Catalytic Oxidation of Alkanes Measured by in-Situ UV−Visible Spectroscopy

et al. 2004

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“…Previously, the activity of supported chromia catalysts were investigated in isobutane dehydrogenation at 350 to 500 • C (15). The catalysts were monitored in situ by UVvis DR spectroscopy in a Praying Mantis DR attachment (Harrick) equipped with a photomultiplier detector.…”

Section: Figmentioning

confidence: 99%

“…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%

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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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“…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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In situ UV–Vis diffuse reflectance spectroscopy — on line activity measurements of supported chromium oxide catalysts: relating isobutane dehydrogenation activity with Cr-speciation via experimental design

Cited by 97 publications

References 22 publications

Extent of Reduction of Vanadium Oxides during Catalytic Oxidation of Alkanes Measured by in-Situ UV−Visible Spectroscopy

Extent of Reduction of Vanadium Oxides during Catalytic Oxidation of Alkanes Measured by in-Situ UV−Visible Spectroscopy

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

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

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