Raman spectroscopic investigation of the effect of H2O on the molybdenum surface species in MoO3/Al2O3 catalysts*1

Stencel, J.M.; Makovsky, Leo E.; Sarkus, T.A.; Vries, J. de; Thomas, Reji; Moulijn, J. A.

doi:10.1016/0021-9517(84)90259-8

Cited by 100 publications

(12 citation statements)

References 12 publications

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“…The positions of the first two bands are close to those observed for polymolybdates, although at higher frequencies. It is reported in the literature that the Raman bands of surface molybdates shift to higher frequencies with increasing degree of polymerization 17 or upon dehydration. , It is suggested, therefore, that the majority of surface species formed after 120 min of ball-milling is similar to that of polymeric molybdates, such as hepta- or octamolybdates.…”

Section: Resultsmentioning

confidence: 98%

Spreading of MoO₃ on γ-Al₂O₃ Induced by Mechanical Activation

Mestl¹,

Knözinger²

1998

Langmuir

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Section: Resultsmentioning

confidence: 98%

Spreading of MoO₃ on γ-Al₂O₃ Induced by Mechanical Activation

Mestl¹,

Knözinger²

1998

Langmuir

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“…For MoO 3 single crystals, IR-active modes are reported at 1002 (B 3u ) and 1010 cm -1 (B 2u ). , Hence, the observation of these bands may indicate weakening of the spectroscopic exclusion rule due to microcrystallinity leading to the appearance of new bands. An alternative assignment of the band at 1005 cm -1 to Al 2 (MoO 4 ) 2 − does not seem to be plausible, since this compound is only formed at higher temperatures and should also exhibit bands at 1028 and 830 cm -1 as well as a very intense band at 381 cm -1 .…”

Section: Resultsmentioning

confidence: 99%

Mechanically Activated MoO₃. 4. In Situ Characterization of Physical Mixtures with Al₂O₃

et al. 1996

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Mechanical activation of solids leads to an increased internal energy caused by the introduction of defects. This can result for example in a reduced surface melting temperature of activated particles, which in turn may affect their sintering behavior. Analogously the spreading behavior of MoO3 over Al2O3 may depend on the mechanical treatment during physically mixing the solids. Differences in the spreading over Al2O3 of unmilled MoO3 and MoO3 that was mechanically activated for 600 min were investigated by SEM/TEM, XPS, ESR, and in situ high-temperature Raman spectroscopy. SEM and EDX analyses of these physical mixtures make surface melting during the calcination very probable. In addition, analysis of XPS spectra also shows that spreading occurs under these conditions. However, spreading in the mixture with milled MoO3 is more effective. ESR spectroscopy shows that Mo5+ centers are reoxidized after calcining the mixtures with unmilled MoO3 in moist oxygen. For the mixture with milled MoO3 an additionally observed Mo5+ species in C 2v distorted sixfold coordination is stable against oxygen for many hours, independent of the presence or absence of water. This higher stability of this defect species against reoxidation is attributed to an improved stabilizing effect of the Al2O3 support due to a pronounced spreading of mechanically activated MoO3. High-temperature Raman spectroscopy of pure, unmilled MoO3 reveals that a transformation into polymeric species occurs at temperatures above 948 K. At 1053 K, melting is observed and the Raman bands of crystalline MoO3 are lost. Calcination of an unmilled physical mixture of 9 wt % MoO3 and Al2O3 at the considerably lower temperature of 823 K in dry O2 leads to the observation of Raman bands of an amorphous polymeric Mo surface melt. Quenching this sample to room temperature results in a Raman spectrum which is attributed to a glassy surface MoO3 phase. Calcination of the physical mixture milled for 3 h at 823 K also leads to a Raman spectrum of the surface melt. Quenching to 298 K does not lead to a considerable change of the spectrum, this being explained by a more effective spreading of the Mo phase in the mechanically activated mixture. This surface Mo phase is highly reactive toward H2O during rehydration at 298 K, which leads to the formation of a polymeric surface species. A long time spreading experiment at 723 K reveals that this process is considerably slower and less effective at this lower temperature.

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“…In comparison to ACN, a slight amount of H 2 O can be negligible. Thus, the physical adsorption of H 2 O on the Mo/γ-Al 2 O 3 catalyst also is neglected . DBT adsorbed on the catalyst is primarily physical adsorption; , therefore, DBT adsorption on the Mo/γ-Al 2 O 3 catalyst is much weaker than the chemisorption of H 2 O 2 .…”

Section: Results and Discussionmentioning

confidence: 99%

“…To overcome the disadvantages of HDS, varied methods, such as adsorptive desulfurization, extractive desulfurization (EDS), oxidative desulfurization (ODS), ,, and biodesulfurization, are being investigated the world over to produce ultraclean fuels. Among these techniques, ODS is regarded as a potential method to obtain ultralow-sulfur fuel oils. − In the ODS process, DBT and its derivatives are oxidized to sulfones in mild conditions − and then the products are removed by adsorption, decomposition, or solvent extraction. , Heterogeneous catalysts attract a great deal of attention, owing to their recycling use in the ODS system, ,, especially Mo-based catalysts with high oxidation activity and high selectivity. − Aqueous H 2 O 2 as a green and cheap oxidant is used for ODS. ,, However, organosulfur compounds in oil cannot fully contact H 2 O 2 in water. To make sure H 2 O 2 , oil, and catalyst mixed enough, violent stirring or ultraphonics is necessary .…”

Section: Introductionmentioning

confidence: 99%

Kinetic Modeling of the Extraction–Oxidation Coupling Process for the Removal of Dibenzothiophene

Guo

Zhong

et al. 2016

Energy Fuels

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Extraction–oxidation desulfurization (EODS) technology would be a potential industrial application for achieving ultralow-sulfur fuel oils. A combined extraction–oxidation system for dibenzothiophene removal from model fuel with H2O2 as an oxidant, acetonitrile as an extractant, and Mo/γ-Al2O3 as a catalyst was performed in a batch reactor. To study this complex heterogeneous system, the chemical reaction and mass transfer on each phase were individually assessed. Kinetics of the EODS system for dibenzothiophene in model fuel was developed, comprising the chemical kinetic mechanism and mass transfer effects. The kinetic parameters of the kinetic model, such as the kinetic constants and apparent activation energy, were determined. The EODS system in the experimental condition is determined by both the extraction process and catalytic oxidation reaction, because the terms involving the intrinsic resistance of the chemical reaction are no more important than mass transfer resistance.

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Raman spectroscopic investigation of the effect of H2O on the molybdenum surface species in MoO3/Al2O3 catalysts*1

Cited by 100 publications

References 12 publications

Spreading of MoO₃ on γ-Al₂O₃ Induced by Mechanical Activation

Spreading of MoO₃ on γ-Al₂O₃ Induced by Mechanical Activation

Mechanically Activated MoO₃. 4. In Situ Characterization of Physical Mixtures with Al₂O₃

Kinetic Modeling of the Extraction–Oxidation Coupling Process for the Removal of Dibenzothiophene

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Raman spectroscopic investigation of the effect of H2O on the molybdenum surface species in MoO3/Al2O3 catalysts*1

Cited by 100 publications

References 12 publications

Spreading of MoO3 on γ-Al2O3 Induced by Mechanical Activation

Spreading of MoO3 on γ-Al2O3 Induced by Mechanical Activation

Mechanically Activated MoO3. 4. In Situ Characterization of Physical Mixtures with Al2O3

Kinetic Modeling of the Extraction–Oxidation Coupling Process for the Removal of Dibenzothiophene

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Product

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Spreading of MoO₃ on γ-Al₂O₃ Induced by Mechanical Activation

Spreading of MoO₃ on γ-Al₂O₃ Induced by Mechanical Activation

Mechanically Activated MoO₃. 4. In Situ Characterization of Physical Mixtures with Al₂O₃