Structural Investigation of the Thermal Decomposition of Ammonium Heptamolybdate by in situ XAFS and XRD

Wienold, Julia; Jentoft, Rolf E.; Ressler, Thorsten

doi:10.1002/ejic.200390138

Cited by 79 publications

(56 citation statements)

References 33 publications

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“…The strongly reduced amplitude in the FT(v(k)*k 3 ) at higher distances may be caused by very small particles or a predominantly two-dimensional structure of the hexagonal MoO 3 modification in SBA-15. Treating AHM in air at 500°C results in the formation of a-MoO 3 , as previously reported [33], and hex-MoO 3 is formed as an intermediate phase at *350°C. However, hex-MoO 3 transforms upon further heating into the more stable a-MoO 3 modification.…”

Section: Xafssupporting

confidence: 81%

“…The MoO 3 species are formed within a confined space and the highly interesting question arises whether a metastable modification of the oxide is formed depending on particle sizes as observed for other oxides like TiO 2 . The thermal decomposition of bulk AHM in different gas atmospheres was recently investigated by in-situ X-ray absorption spectroscopy (XAFS) and in-situ X-ray diffraction [33]. Under static condition AHM is decomposed first to an amorphous phase followed by the formation of ammonium tetramolybdate (ATM) at about 230°C.…”

Section: Introductionmentioning

confidence: 99%

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The modification of MoO3 nanoparticles supported on mesoporous SBA-15: characterization using X-ray scattering, N2 physisorption, transmission electron microscopy, high-angle annular darkfield technique, Raman and XAFS spectroscopy

Huang

Bensch

Sigle³

et al. 2007

J Mater Sci

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MoO 3 was dispersed onto mesoporous SBA-15 by using ammonium heptamolybdate as MoO 3 source. The formation of MoO 3 was carried out by heating the loaded material to 500°C for 3 h in air. Below 13 wt% Mo loading, no reflections of MoO 3 occur in the X-ray powder patterns and even for high MoO 3 contents, the intensities of the reflections are much lower than expected for fully crystalline material. A detailed XAFS analysis reveals that at low Mo contents, the metastable hexagonal modification of MoO 3 is formed despite the high calcination temperature of 500°C. It is highly likely that the nanosize of the particles and the interaction between MoO 3 and SBA-15 stabilize the metastable modification of the material. Nitrogen physisorption experiments show the typical type-IV isotherms indicating that the mesoporosity of the materials is preserved despite the large amount of MoO 3 .Transmission electron micrographs demonstrate the presence of MoO 3 inside the SBA-15 support. The Raman spectra display a remarkable size-dependent intensity loss and several features give evidences for a bond formation between nano-sized MoO 3 particles and the silica support. Moreover, the spectroscopic details suggest the formation of (MoO 3 ) n oligomers.

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Section: Xafssupporting

confidence: 81%

Section: Introductionmentioning

confidence: 99%

The modification of MoO3 nanoparticles supported on mesoporous SBA-15: characterization using X-ray scattering, N2 physisorption, transmission electron microscopy, high-angle annular darkfield technique, Raman and XAFS spectroscopy

Huang

Bensch

Sigle³

et al. 2007

J Mater Sci

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“…In order to prepare and stabilize the appropriate molybdenum oxide phase, the behavior of the catalyst precursors during thermal treatment needs to be elucidated. Previously, we reported on the structural evolution of ammonium heptamolybdate (AHM) [3] and heteropolyoxomolybdates (HPOM) [4] during thermal activation. Ammonium paratungstate [(NH 4 ) 10 H 2 W 12 O 42 ·4H 2 O, APT] is a common precursor for the preparation of Mo/ tal.…”

Section: Introductionmentioning

confidence: 99%

“…[1] Binary molybdenum oxides have a limited catalytic applicability, mostly because of their tendency to readily transform into the less active molybdenum trioxide, MoO 3 , under the reaction conditions. Thus, it would be desirable to stabilize the active nanostructure of the molybdenum oxides (e.g.…”

Section: Introductionmentioning

confidence: 99%

In Situ XAS and XRD Studies on the Structural Evolution of Ammonium Paratungstate During Thermal Decomposition

et al. 2005

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Keywords:In situ studies / X-ray absorption spectroscopy / X-ray diffraction / Tungsten / Oxidation / Decomposition / Structure-activity relationships / Heterogeneous catalysisThe bulk structural evolution during the decomposition of ammonium paratungstate (APT) under various oxidizing and reducing gases was elucidated by the complementary techniques, in situ XAS, in situ XRD, and TG/DSC, combined with mass spectrometry. In the temperature range from 300 to 650 K, the decomposition of APT proceeds nearly independently of the gas employed. At higher temperatures, an oxidizing gas results in the formation of crystalline triclinic WO 3 as the majority phase at 773 K, while mildly reducing gases (propene, propene and oxygen, and helium) result in the formation of partially reduced and highly disordered tungsten bronzes. No further reduction is observed under propene or helium, and this indicates a strongly hindered oxygen mobility in the tungsten oxide lattice in the temperature range employed. The decomposition of APT under hydrogen results in the formation of WO 2 and, eventually, tungsten me-

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“…During decomposition of ammonium heptamolybdate, hex-MoO 3 is formed as an intermediate which rapidly transforms into α-MoO 3 at temperatures above 623 K [25]. The latter is accompanied by the evolution of ammonia or water from the channels of hex-MoO 3 .…”

Section: Local Structure Of As-prepared Mo Oxide Species On Vgcnfmentioning

confidence: 99%

Structure and properties of a Mo oxide catalyst supported on hollow carbon nanofibers in selective propene oxidation

Ressler

Walter

Scholz

et al. 2010

Journal of Catalysis

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In situ X-ray absorption spectroscopy (XAS) under reaction conditions of selective propene oxidation was employed to elucidate the local structure of as-prepared and activated molybdenum oxide supported on hollow vapor-grown carbon nanofibers (VGCNF). The local structure of asprepared MoxOy-VGCNF was very similar to that of hexagonal MoO3. During heat treatment in propene-and oxygen-containing atmosphere, asprepared MoxOy-VGCNF transforms into activated MoxOy-VGCNF above 623 K. The local structure around the Mo centers in activated MoxOy-VGCNF is similar to that of α-MoO3. Temperature-and time-dependent XAS measurements showed a rapid transformation from hex-MoO3 to α-MoO3 supported on VGCNF under reaction conditions. Subsequently, the resulting activated MoxOy-VGCNF catalyst exhibited a slowly increasing average oxidation state. The latter coincided with the formation of acrylic acid, which is hardly detectable during catalysis on regular, binary α-MoO3. Moreover, activated MoxOy-VGCNF is much more active in the selective oxidation of propene compared to α-MoO3. The correlation between catalytic selectivity and average oxidation state as a result of suitable reduction-oxidation kinetics corroborates the importance of structural complexity rather than chemical complexity.

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Structural Investigation of the Thermal Decomposition of Ammonium Heptamolybdate by in situ XAFS and XRD

Cited by 79 publications

References 33 publications

The modification of MoO3 nanoparticles supported on mesoporous SBA-15: characterization using X-ray scattering, N2 physisorption, transmission electron microscopy, high-angle annular darkfield technique, Raman and XAFS spectroscopy

The modification of MoO3 nanoparticles supported on mesoporous SBA-15: characterization using X-ray scattering, N2 physisorption, transmission electron microscopy, high-angle annular darkfield technique, Raman and XAFS spectroscopy

In Situ XAS and XRD Studies on the Structural Evolution of Ammonium Paratungstate During Thermal Decomposition

Structure and properties of a Mo oxide catalyst supported on hollow carbon nanofibers in selective propene oxidation

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