The Electronic Factor in Alkane Oxidation Catalysis

Eichelbaum, Maik; Hävecker, Michael; Heine, Christian; Wernbacher, Anna Maria; Rosowski, Frank; Trunschke, Annette; Schlögl, Robert

doi:10.1002/anie.201410632

Cited by 46 publications

(63 citation statements)

References 33 publications

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“…The high density of adsorption sites on M1 apparently renders possible concerted reactions, which involve oxygen insertion. Such an ensemble effect together with the balanced oxygen activation result in improved selectivity to acrylic acid over MoVTeNb M1 oxide.…”

Section: Concluding Discussionmentioning

confidence: 99%

“…Vanadium oxide supported on an insulating support behaves electronically more like a single‐site catalyst, whereas the selectivity over a semiconducting catalyst, such as M1, can be controlled by the redox level of the (dynamic) surface layer that has an impact on the surface potential barrier, which again determines charge transfer between adsorbed molecules and the catalyst as will be explained in detail below . Comparable dynamic properties have not been proven so far on monolayer catalysts under reaction conditions.…”

Section: Concluding Discussionmentioning

confidence: 99%

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Functional Analysis of Catalysts for Lower Alkane Oxidation

et al. 2017

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The catalytic performance of 1) crystalline MoVTeNb oxide that exhibits the electronic properties of a n-type semiconductor, 2) submonolayer vanadium oxide supported on meso-structured silica (SBA-15) as an insulating support, and 3) surface-functionalized carbon nanotubes that contain neither a redox active metal nor bulk oxygen, but only surface oxygen species have been compared in the oxidative dehydrogenation of ethane and propane under equal reaction conditions. The catalytic results indicate similarities in the reaction network over all three catalysts within the range of the studied reaction conditions implying that differences in selectivity are a consequence of differences in the rate constants. Higher activity and selectivity to acrylic acid over MoVTeNb oxide as compared to the other two catalysts are attributed to the higher density of potential alkane adsorption sites on M1 and the specific electronic structure of the semiconducting bulk catalyst. Microcalorimetry has been used to determine and quantify different adsorption sites revealing a low V_surface/C₃H_{8 ads} ratio of 4 on M1 and a much higher ratio of 150 on silica-supported vanadium oxide. On the latter catalyst less than one per cent of the vanadium atoms adsorb propane. Barriers of propane activation increase in the order P/oCNT (139 kJ mol⁻¹)≤M1 (143 kJ mol⁻¹)<6V/SBA-15 (162 kJ mol⁻¹), which is in agreement with trends predicted by theory

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Section: Concluding Discussionmentioning

confidence: 99%

Section: Concluding Discussionmentioning

confidence: 99%

Functional Analysis of Catalysts for Lower Alkane Oxidation

et al. 2017

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“…Minimization of rates of undesired pathways requires co-ordinated design of bulk electronic properties and surface dynamics of oxidation catalysts. 30 C-H bond activation may involve multiple mechanisms including carbenium or carbonium intermediates and homolytic splitting of C-H bonds at metal oxide surface functional groups under formation of radical species. 31,32 Model calculations, generally based on small cluster models, favour the homolytic pathway over transition metal oxide catalysts.…”

Section: Ethanol Oxidationmentioning

confidence: 99%

Hydrothermal synthesis of bi-functional nanostructured manganese tungstate catalysts for selective oxidation

Lunkenbein

Kröhnert

et al. 2016

Faraday Discuss.

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The mechanism of C-H activation in selective oxidation reactions of short-chain alkane molecules over transition metal oxides is critically affected by the balance of acid-base and redox sites at the surface of the catalyst. Using the example of manganese tungstate we discuss how the relative abundance of these sites can be controlled via synthetic techniques. Phase-pure catalysts composed of the thermodynamic stable monoclinic MnWO4 phase have been prepared using hydrothermal synthesis. Variation of the initial pH value resulted in rod-shaped nano-crystalline MnWO4 catalysts composed of particles with varying aspect ratio. The synthesis products have been analysed using transmission electron microscopy, X-ray diffraction, infrared, and photoelectron spectroscopy. In situ Raman spectroscopy was used to investigate the dissolution-re-crystallization processes occurring under hydrothermal conditions. Ethanol oxidation was applied to probe the surface functionalities in terms of acid-base and redox properties. Changes in the aspect ratio of the primary catalyst particles are reflected in the product distribution induced by altering the fraction of acid-base and redox sites exposed at the surface of the catalysts in agreement with the proposed mechanism of particle growth by re-crystallization during ageing under hydrothermal conditions.

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“…The generalisation of using a bulk structural motif as lead for searching the phase space of complex oxides for performance catalysts is, unfortunately, not universally valid as was documented recently [48] in the case of butane oxidation. It is re-assuring that the physical method of determining in situ the charge carrier dynamics provided the insight [49,50] why the structures synthesized were unable to perform well as catalyst. The optimization of the M1 catalyst was frequently the subject [51,52] of combinatorial searches.…”

Section: The Empirical Conceptual Basismentioning

confidence: 99%

Selective Oxidation: From a Still Immature Technology to the Roots of Catalysis Science

Schlögl

2016

Top Catal

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The design of heterogeneous selective oxidation catalysts based upon complex metal oxides is governed at present by a set of empirical rules known as “pillars of oxidation catalysis”. They serve as practical guidelines for catalyst development and guide the reasoning about the catalyst role in the process. These rules are, however, not based upon atomistic concepts and thus preclude their immediate application in for example computer-aided search strategies. The present work extends the ideas of the pillar rules and develops the concept of considering a selective oxidation catalyst as enabler for the execution of a reaction network. The enabling function is controlled by mutual interactions between catalyst and reactants. The electronic structure of the catalyst is defined as a bulk semiconductor with a surface state arising form a terminating over layer being different from the structure of the bulk. These components that can be identified by in situ analytical methods form a chemical system with feedback loops, which is responsible for generating selectivity during execution of the reaction network. This concept is based upon physical observables and could allow for a design strategy based upon a kinetic description that combines the processes between reactants with the processes between catalyst and reactants. Such kinetics is not available at present. Few of the constants required are known but many of them are accessible to experimental determination with in situ techniques

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The Electronic Factor in Alkane Oxidation Catalysis

Cited by 46 publications

References 33 publications

Functional Analysis of Catalysts for Lower Alkane Oxidation

Functional Analysis of Catalysts for Lower Alkane Oxidation

Hydrothermal synthesis of bi-functional nanostructured manganese tungstate catalysts for selective oxidation

Selective Oxidation: From a Still Immature Technology to the Roots of Catalysis Science

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