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Title: Elucidation of Anchoring and Restructuring Steps during Synthesis of Silica-Supported Vanadium Oxide Catalysts

Authors:
; ; ; ; ; ;
Publication Date:
Research Org.:
Argonne National Lab. (ANL), Argonne, IL (United States). Advanced Photon Source (APS)
Sponsoring Org.:
UNIVERSITY
OSTI Identifier:
1329399
Resource Type:
Journal Article
Resource Relation:
Journal Name: Chemistry of Materials; Journal Volume: 28; Journal Issue: 15
Country of Publication:
United States
Language:
ENGLISH

Citation Formats

Love, Alyssa M., Carrero, Carlos A., Chieregato, Alessandro, Grant, Joseph T., Conrad, Sabrina, Verel, René, and Hermans, Ive. Elucidation of Anchoring and Restructuring Steps during Synthesis of Silica-Supported Vanadium Oxide Catalysts. United States: N. p., 2016. Web. doi:10.1021/acs.chemmater.6b02118.
Love, Alyssa M., Carrero, Carlos A., Chieregato, Alessandro, Grant, Joseph T., Conrad, Sabrina, Verel, René, & Hermans, Ive. Elucidation of Anchoring and Restructuring Steps during Synthesis of Silica-Supported Vanadium Oxide Catalysts. United States. doi:10.1021/acs.chemmater.6b02118.
Love, Alyssa M., Carrero, Carlos A., Chieregato, Alessandro, Grant, Joseph T., Conrad, Sabrina, Verel, René, and Hermans, Ive. 2016. "Elucidation of Anchoring and Restructuring Steps during Synthesis of Silica-Supported Vanadium Oxide Catalysts". United States. doi:10.1021/acs.chemmater.6b02118.
@article{osti_1329399,
title = {Elucidation of Anchoring and Restructuring Steps during Synthesis of Silica-Supported Vanadium Oxide Catalysts},
author = {Love, Alyssa M. and Carrero, Carlos A. and Chieregato, Alessandro and Grant, Joseph T. and Conrad, Sabrina and Verel, René and Hermans, Ive},
abstractNote = {},
doi = {10.1021/acs.chemmater.6b02118},
journal = {Chemistry of Materials},
number = 15,
volume = 28,
place = {United States},
year = 2016,
month = 8
}
  • Geometrical structures of vanadium oxide and niobium oxide clusters supported on silica and alumina are investigated by the ab initio molecular orbital method. Optimized geometries are searched by using the energy gradient technique. For the bare cluster and the clusters supported on silica, the monoxo form is more stable than the dioxo form independent of the transition-metal atoms, whereas the possibility of greater stability for the dioxo form is suggested on alumina. Stabilization of the dioxo form is ascribed to the coordinative bonding from the oxygen atom to the three-coordinated aluminum atom within the cluster and the resulting four-membered ringmore » formation. The calculated bond lengths for the V=O, V-O, Nb=O, and Nb-O bonds are compared with those estimated from EXAFS/XANES measurements, and the agreement is better for the silica-supported clusters than the alumina-supported clusters.« less
  • The effect of Cs modification in the selective oxidation of butane at 520[degrees]C was studied over SiO[sub 2]-supported catalysts containing about 6 or 0.6 wt% vanadium atoms using a butane/oxygen/helium mixture of 4/8/88. Dehydrogenation products (unsaturated C[sub 4]'s) and carbon oxides were the primary products. For the supported sample of high vanadium loading where a significant amount of vanadium existed as V[sub 2]O[sub 5] crystallites under reaction conditions, Cs modification significantly increased the selectivity for dehydrogenation and decreased the extent of reduction of the vanadium oxide crystallites in the reaction mixture, but had only small effects on the activation energymore » or rate of reaction. Unsupported V[sub 2]O[sub 5] catalysts modified by Cs at a Cs/V ratio of 0.023 also showed enhanced dehydrogenation selectivity, a lower degree of reduction at steady state, and a significantly different temperature-programmed reduction profile. Cs was found to be segregated on the surface of the crystallites. For the supported samples of low vanadium loading, Cs modification decreased the intensity of the Raman peak that was indicative of highly dispersed vanadia species and increased the activation energy of the reaction. These data were consistent with the picture that Cs modification enhanced the formation of vanadia on the support. 25 refs., 6 figs., 3 tabs.« less
  • Oxygen chemisorption experiments on prereduced silica- supported vanadia catalysts have been performed in order to determine the optimum conditions for measuring the relative dispersion of vanadium oxide on the silica substrate. By following the extent of vanadia reduction microgravimetrically, it is shown that an appropriate prereduction of the catalyst at 823 K under hydrogen is fundamental in obtaining a reproducible well-defined stoichiometrically reduced vanadium oxide system (V{sup 3+}). AT 643 K the reduction process is very much slower and requires very long times in order to approach a plateau. Measurement of the binding energies of the V{sub 2}p{sub 3/2} coremore » level spectra indicates that high-temperature oxygen chemisorption (643 K) results in a reoxidation of the prereduced vanadium oxide species within crystallites leading to an overestimation of the number of exposed vanadium sites. Chemisorption at low temperatures (298 or 195 K) does not affect the stoichiometrically reduced vanadium oxide species within crystallites, thus allowing the number of exposed surface vanadium sites to be measured. 25 refs., 5 figs., 1 tab.« less
  • The molecular structure of silica-supported vanadium oxide (VOx) catalysts over wide range of surface VOx density (0.0002 8 V/nm2) has been investigated in detail under dehydrated condition by in situ multi-wavelength Raman spectroscopy (laser excitations at 244, 325, 442, 532, and 633 nm) and in situ UV-Vis diffuse reflectance spectroscopy. Resonance Raman scattering is clearly observed using 244 and 325-nm excitations while normal Raman scattering occurs using excitation at the three visible wavelengths. The observation of strong fundamentals, overtones and combinational bands due to selective resonance enhancement effect helps clarify assignments of some of the VOx Raman bands (920, 1032,more » and 1060 cm-1) whose assignments have been controversial. The resonance Raman spectra of dehydrated VOx/SiO2 show V=O band at smaller Raman shift than that in visible Raman spectra, an indication of the presence of two different surface VOx species on dehydrated SiO2 even at sub-monolayer VOx loading. Quantitative estimation shows that the two different monomeric VOx species coexist on silica surface from very low VOx loadings and transform to crystalline V2O5 at VOx loadings above monolayer. It is postulated that one of the two monomeric VOx species has pyramidal structure and the other is in partially hydroxylated pyramidal mode. The two VOx species show similar reduction-oxidation behavior and may both participate in redox reactions catalyzed by VOx/SiO2 catalysts. This study demonstrates the advantages of multi-wavelength Raman spectroscopy over conventional single-wavelength Raman spectroscopy in structural characterization of supported metal oxide catalysts.« less
  • No abstract prepared.