Key Aspects of Catalyst Design for the Selective Oxidation of Paraffins

Albonetti, Stefania; Cavani, Fabrizio; Trifirò, Ferruccio

doi:10.1080/01614949608006463

Cited by 428 publications

(288 citation statements)

References 55 publications

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“…A thermocouple located within the catalyst bed was used to measure local temperatures; a thermocouple inserted within the heater beneath the sample holder recorded temperatures as much as 100 K above sample temperatures, as previously reported. 17,19 Propane ODH experiments were conducted at 603 K using C 3 The H 2 reduction and TPO data were used to obtain calibration curves relating UV-visible intensities in the preedge and edge regions to the number of reduced centers for each sample. Quantification of the reduced centers by direct reoxidation following ODH is not sufficiently accurate, because purging the reactants and quenching the reaction alters the extent of reduction prevalent during steady-state catalysis.…”

Section: Methodsmentioning

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: Methodsmentioning

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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“…4. The edge energy values range from 2.5 to 3.6 eV, with the highest E g value indicating that the surface VO 4 species have a high degree of isolation for the supported 0.7 V/Al 2 O 3 catalysts. 15 The decreasing E g trend with increasing vanadia loading indicates a higher degree of polymerization (formation of bridging V-O-V bonds of polyvanadates), culminating in the formation of crystalline V 2 O 5 NPs at the highest loadings.…”

Section: Papermentioning

confidence: 99%

“…[2][3][4][5][6][7][8][9][10][11][12] The nature of supported VO x phases on oxide supports has received much attention in recent years. 5,13 Below monolayer coverage on oxide supports (<8 V nm −2 ), the supported VO x phase is generally present as isolated VO 4 and/or oligomer surface species. Poor dispersion of the VO x phase on the oxide support, however, may also result in V 2 O 5 NPs below monolayer coverage.…”

Section: Introductionmentioning

confidence: 99%

Anomalous reactivity of supported V2O5 nanoparticles for propane oxidative dehydrogenation: influence of the vanadium oxide precursor

et al. 2013

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The oxidative dehydrogenation (ODH) of propane to propylene by supported vanadia catalysts has received much attention in recent years, but different reactivity trends have been reported for this catalytic reaction system. In the present investigation, the origin of these differing trends are investigated with synthesis of supported V/SiO 2 , V/TiO 2 , and V/Al 2 O 3 catalysts prepared with three different vanadium

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“…[21][22][23] Also, none of the reported work has examined the reduction and oxidation cycles that occur during the oxidative dehydrogenation (ODH) of alkanes, a reaction that proceeds via a Mars-van Krevelen redox cycle using lattice oxygen atoms in metal oxides. [1][2][3][4][24][25][26][27][28][29][30] In this study, we report the use of UV-visible spectroscopy in a transient mode to study the dynamics of redox cycles for catalytically active centers involved in propane ODH on alumina-supported vanadia catalysts. The analysis of these data leads to the determination of the rate coefficients for the reduction and reoxidation of active centers.…”

Section: Introductionmentioning

confidence: 99%

In situ UV−Visible Spectroscopic Measurements of Kinetic Parameters and Active Sites for Catalytic Oxidation of Alkanes on Vanadium Oxides

et al. 2004

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In situ diffuse reflectance UV-visible spectroscopy was used to measure the dynamics of catalyst reduction and oxidation during propane oxidative dehydrogenation (ODH) on VO x /γ-Al 2 O 3 . Transients in UV-visible intensity in the near-edge region were analyzed using a mechanistic model of ODH reactions. Rate constants per site for the kinetically relevant reduction step (C-H bond activation) measured using this analysis are slightly larger than those obtained from steady-state ODH rates normalized by surface V. The ratio of these values provides a measure of the fraction of the V surface sites that are active for ODH (0.6-0.7, for V surface densities of 2.3-34 V nm -2 ). This suggests that some of the V atoms are either inaccessible or inactive. Reoxidation rate constants, which cannot be obtained from steady-state analysis, are 10 3 -10 5 times larger than those for the C-H bond activation reduction step.

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Key Aspects of Catalyst Design for the Selective Oxidation of Paraffins

Cited by 428 publications

References 55 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

Anomalous reactivity of supported V2O5 nanoparticles for propane oxidative dehydrogenation: influence of the vanadium oxide precursor

In situ UV−Visible Spectroscopic Measurements of Kinetic Parameters and Active Sites for Catalytic Oxidation of Alkanes on Vanadium Oxides

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