The Role of Surface Basal Planes of Layered Mixed Metal Oxides in Selective Transformation of Lower Alkanes: Propane Ammoxidation over Surface <i>ab</i> Planes of Mo−V−Te−Nb−O M1 Phase

Shiju, Raveendran N.; Liang, Xin Hua; Weimer, Alan W.; Liang, Chengdu; Dai, Sheng; Guliants, Vadim V.

doi:10.1021/ja800575v

Cited by 73 publications

(50 citation statements)

References 17 publications

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“…Catalytic experiments trying to verify the assignment of the minority {001} faces as location of active sites gave conflicting results. [20][21][22] Approximately 80% of the catalyst surface area accounts for the lateral surface of the cylindrical M1 particles. Electron microscopy has shown that the latter is characterized by a stepped morphology presumably resulting in similar terminating metal-oxo arrangements like on the basal plane.…”

Section: Introductionmentioning

confidence: 99%

Surface chemistry of phase-pure M1 MoVTeNb oxide during operation in selective oxidation of propane to acrylic acid

Hävecker

Wrabetz

Kröhnert

et al. 2012

Journal of Catalysis

162

183

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The surface of a highly crystalline MoVTeNb oxide catalyst for selective oxidation of propane to acrylic acid composed of the M1 phase has been studied by infrared spectroscopy, microcalorimetry, and in-situ photoelectron spectroscopy. The acid-base properties of the catalyst have been probed by NH3 adsorption showing mainly Brønsted acidity that is weak with respect to concentration and strength of sites. Adsorption of propane on the activated catalyst reveals the presence of a high number of energetically homogeneous propane adsorption sites, which is evidenced by constant differential heat of propane adsorption qdiff,initial=57 kJ mol -1 until the monolayer coverage is reached that corresponds to a surface density of approximately 3 propane molecules per nm 2 at 313 K. The decrease of the heat to qdiff,initial=40 kJ mol -1 after catalysis implies that the surface is restructured under reaction conditions. The changes have been analyzed with high-pressure in-situ XPS while the catalyst was working applying reaction temperatures between 323 and 693 K, different feed compositions containing 0 mol.-% and 40 mol.-% steam and prolonged reaction times. The catalytic performance during the XPS experiments measured by mass spectrometry is in good agreement with studies in fixed bed reactors at atmospheric pressure demonstrating that the XPS results taken under operation show the relevant active surface state. The experiments confirm that the surface composition of the M1 phase differs significantly from the bulk implying that the catalytically active sites are no part of the M1 crystal structure and occur on all terminating planes. Acrylic acid formation correlates with surface depletion in Mo 6+ and enrichment in V 5+ sites. In presence of steam in the feed, the active ensemble for acrylic acid formation appear to consist of V 5+ oxospecies in close vicinity to Te 4+ sites with an element ratio Te/V=1.4. The active sites are formed under propane oxidation conditions and are embedded in a thin layer enriched in V, Te, and Nb on the surface of the structural stable self-supporting M1 phase.

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Section: Introductionmentioning

confidence: 99%

Surface chemistry of phase-pure M1 MoVTeNb oxide during operation in selective oxidation of propane to acrylic acid

Hävecker

Wrabetz

Kröhnert

et al. 2012

Journal of Catalysis

162

183

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“…It is not clear as yet [42][43][44] that the active sites of the M1 phase are exclusively located on the (001) basal faces (''a-b plane'') of the needle morphology [42] but there is strong and general belief that the (001) face contains the motif of the active site that may exist in the termination structure of the whole surface of the material. For this reason the exact crystallographic analysis [40,45] of the basal plane structure not only in terms of its connectivity but also in terms of its cation ordering statistics [45][46][47] is of particular relevance.…”

Section: Introductionmentioning

confidence: 99%

Active Sites for Propane Oxidation: Some Generic Considerations

Schlögl

2011

Top Catal

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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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“…Another factor impacting catalysis is the location of these sites. In one study, atomic layer deposition with alumina was employed to passivate the ab planes [60]. These passivated catalysts show very low conversion (<15%) with almost >95% selectivity to CO x , compared to catalysts where the ab planes are exposed where propane conversion of 40%-65% and acrylonitrile selectivity of 50%-70% were achieved [60].…”

Section: Propane Ammoxidation To Acrylonitrilementioning

confidence: 99%

Characterization of MoVTeNbOx Catalysts during Oxidation Reactions Using In Situ/Operando Techniques: A Review

et al. 2017

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Light alkanes are abundant in shale gas resources. The bulk mixed metal oxide MoVTe(Sb)NbO x catalysts play a very important role in dehydrogenation and selective oxidation reactions of these short hydrocarbons to produce high-value chemicals. This catalyst system mainly consists of M1 and less-active M2 crystalline phases. Due to their ability to directly monitor the catalysts under the relevant industrial conditions, in situ/operando techniques can provide information about the nature of active sites, surface intermediates, and kinetics/mechanisms, and may help with the synthesis of new and better catalysts. Sophisticated catalyst design and understanding is necessary to achieve the desired performance (activity, selectivity, lifetime, etc.) at reasonable reaction conditions (temperature, pressure, etc.). This article critically reviews the progress made in research of these MoVTe(Sb)NbO x catalysts in oxidation reactions mainly through in situ/operando techniques and suggests the future direction needed to realize the industrialization of these catalysts.

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The Role of Surface Basal Planes of Layered Mixed Metal Oxides in Selective Transformation of Lower Alkanes: Propane Ammoxidation over Surface ab Planes of Mo−V−Te−Nb−O M1 Phase

Cited by 73 publications

References 17 publications

Surface chemistry of phase-pure M1 MoVTeNb oxide during operation in selective oxidation of propane to acrylic acid

Surface chemistry of phase-pure M1 MoVTeNb oxide during operation in selective oxidation of propane to acrylic acid

Active Sites for Propane Oxidation: Some Generic Considerations

Characterization of MoVTeNbOx Catalysts during Oxidation Reactions Using In Situ/Operando Techniques: A Review

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