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Title: ED-XAS Data Reveal In-situ Time-Resolved Adsorbate Coverage on Supported Molybdenum Oxide Catalysts during Propane Dehydrogenation

Abstract

Energy-Dispersive X-ray Absorption Spectroscopy (ED-XAS) data combined with UV/Vis, Raman, and mass spectrometry data on alumina- and silica-supported molybdenum oxide catalysts under propane dehydrogenation conditions have been previously reported. A novel {delta}{mu} adsorbate isolation technique was applied here to the time-resolved (0.1 min) Mo K-edge ED-XAS data by taking the difference of absorption, {mu}, at t>1 against the initial time, t=0. Further, full multiple scattering calculations using the FEFF 8.0 code are performed to interpret the {delta}{mu} signatures. The resulting difference spectra and interpretation provide real time propane coverage and O depletion at the MoOn surface. The propane coverage is seen to correlate with the propene and/or coke production, with the maximum coke formation occurring when the propane coverage is the largest. Combined, these data give unprecedented insight into the complicated dynamics for propane dehydrogenation.

Authors:
;  [1]; ;  [2]
  1. Department of Chemistry, George Washington University, Washington D.C. 20052 (United States)
  2. Inorganic Chemistry and Catalysis, Dept. of Chem., Utrecht University, Sorbonnelaan 16, 3584 CA Utrecht (Netherlands)
Publication Date:
OSTI Identifier:
21054697
Resource Type:
Journal Article
Resource Relation:
Journal Name: AIP Conference Proceedings; Journal Volume: 882; Journal Issue: 1; Conference: XAFS13: 13. international conference on X-ray absorption fine structure, Stanford, CA (United States), 9-14 Jul 2006; Other Information: DOI: 10.1063/1.2644610; (c) 2007 American Institute of Physics; Country of input: International Atomic Energy Agency (IAEA)
Country of Publication:
United States
Language:
English
Subject:
36 MATERIALS SCIENCE; ABSORPTION SPECTROSCOPY; ADSORPTION; ALUMINIUM OXIDES; CATALYSIS; CATALYSTS; COMPUTERIZED SIMULATION; DEHYDROGENATION; F CODES; MASS SPECTROSCOPY; MOLYBDENUM OXIDES; MULTIPLE SCATTERING; PROPANE; PROPYLENE; RAMAN SPECTROSCOPY; SILICON COMPOUNDS; SURFACES; TIME RESOLUTION; X-RAY SPECTRA; X-RAY SPECTROSCOPY

Citation Formats

Ramaker, David, Gatewood, Daniel, Beale, Andrew M., and Weckhuysen, Bert M. ED-XAS Data Reveal In-situ Time-Resolved Adsorbate Coverage on Supported Molybdenum Oxide Catalysts during Propane Dehydrogenation. United States: N. p., 2007. Web. doi:10.1063/1.2644610.
Ramaker, David, Gatewood, Daniel, Beale, Andrew M., & Weckhuysen, Bert M. ED-XAS Data Reveal In-situ Time-Resolved Adsorbate Coverage on Supported Molybdenum Oxide Catalysts during Propane Dehydrogenation. United States. doi:10.1063/1.2644610.
Ramaker, David, Gatewood, Daniel, Beale, Andrew M., and Weckhuysen, Bert M. Fri . "ED-XAS Data Reveal In-situ Time-Resolved Adsorbate Coverage on Supported Molybdenum Oxide Catalysts during Propane Dehydrogenation". United States. doi:10.1063/1.2644610.
@article{osti_21054697,
title = {ED-XAS Data Reveal In-situ Time-Resolved Adsorbate Coverage on Supported Molybdenum Oxide Catalysts during Propane Dehydrogenation},
author = {Ramaker, David and Gatewood, Daniel and Beale, Andrew M. and Weckhuysen, Bert M.},
abstractNote = {Energy-Dispersive X-ray Absorption Spectroscopy (ED-XAS) data combined with UV/Vis, Raman, and mass spectrometry data on alumina- and silica-supported molybdenum oxide catalysts under propane dehydrogenation conditions have been previously reported. A novel {delta}{mu} adsorbate isolation technique was applied here to the time-resolved (0.1 min) Mo K-edge ED-XAS data by taking the difference of absorption, {mu}, at t>1 against the initial time, t=0. Further, full multiple scattering calculations using the FEFF 8.0 code are performed to interpret the {delta}{mu} signatures. The resulting difference spectra and interpretation provide real time propane coverage and O depletion at the MoOn surface. The propane coverage is seen to correlate with the propene and/or coke production, with the maximum coke formation occurring when the propane coverage is the largest. Combined, these data give unprecedented insight into the complicated dynamics for propane dehydrogenation.},
doi = {10.1063/1.2644610},
journal = {AIP Conference Proceedings},
number = 1,
volume = 882,
place = {United States},
year = {Fri Feb 02 00:00:00 EST 2007},
month = {Fri Feb 02 00:00:00 EST 2007}
}
  • Oxidative dehydrogenation (ODH) of propane was studied on zirconia-supported molybdenum oxide catalysts. The structure of the ZrO{sub 2} support and of the dispersed MoO{sub x} species was characterized by X-ray diffraction and by Raman and UV-visible spectroscopies. The structure of dispersed molybdena depends on the Mo surface density and on the temperature at which catalyst precursors are treated in air. Polymolybdate domains were detected by Raman at Mo surface densities below 5 Mo/nm{sup 2}. At higher surface densities, MoO{sub 3} and ZrMo{sub 2}O{sub 8} are present; their relative concentrations depend on the pretreatment temperature. Below 773 K, MoO{sub 3} ismore » the predominant structure at high surface densities, but ZrMo{sub 2}O{sub 8} forms above 773 K. UV-visible edge energies decrease with increasing surface density for samples containing polymolybdate species, suggesting that MoO{sub x} domains become larger as the Mo surface density increases. ODH turnover rates decrease with increasing Mo surface density on samples containing polymolybdate species and MoO{sub 3}. This trend is accompanied by an increase in the initial propene selectivity and in the vibrational frequency of Mo=O bonds. Higher Mo{double{underscore}bond}O vibrational frequencies reflect stronger Mo=O bonds, which show lower ODH reactivity; therefore, the lower ODH reaction rates (per Mo atom) at higher Mo surface densities arise from the lower reactivity of Mo=O bonds, while higher initial propene selectivities arise either from the decrease of exposed Mo-O-Zr bonds or the lower reactivity of Mo=O bonds as the size of MoO{sub x} domains increases with increasing Mo surface density. At similar Mo surface densities, samples containing predominantly ZrMo{sub 2}O{sub 8}/ZrO{sub 2} show higher turnover rates and lower initial propene selectivities than those containing MoO{sub 3} species because the vibrational frequency of the Mo=O bond for ZrMo{sub 2}O{sub 8}/ZrO{sub 2} also decreased with increasing Mo surface density, ultimately due to the increase of the particle size which leads to lower propane accessibility.« less
  • No abstract prepared.
  • Kinetics and reaction mechanisms of homologation, hydrogenolysis, and dehydrogenation of ethane were studied on reduced molybdenum oxides supported on SiO/sub 2/ and SiO/sub 2/-Al/sub 2/O/sub 3/. It was found that homologation of ethane to propane occurs via (1) dehydrogenation of ethane to ethylene, (2) homologation of ethylene to propylene, and (3) hydrogenation of propylene to propane. The rate-determining step was found to be hydrogenation of adsorbed propylene or that of adsorbed C/sub 2/H/sub 5/ species with an activation energy of 10.7 kcal/mole. In addition to homologation products, significant amounts of hydrogenolysis and dehydrogenation reaction products were also found. Dehydrogenation ofmore » ethane to ethylene had an activation energy of 25 kcal/mole, and it was quite similar to that for desorption of ethylene. Consequently, the dehydrogenation rate was assumed to be controlled by desorption of ethylene formed.« less
  • Potassium promotion of iron oxide catalysts supported on magnesium oxide results in considerably more active and selective 1-butene dehydrogenation catalysts. Upon promotion the activation energy was found to decrease from 194 to 156 kJ/mol. KFeO[sub 2] appeared to be the active phase under dehydrogenation conditions. No reduction of KFeO[sub 2] was observed. KFeO[sub 2] shows high 1-butene dehydrogenation activity, yet is not sufficiently effective to suppress coking entirely. For that purpose the presence of highly dispersed potassium carbonate at the catalyst surface is a prerequisite. Under identical dehydrogenation conditions, a commercial unsupported catalyst, S-105, which contains the more easily reduciblemore » KF[sub 11]O[sub 13], is reduced to Fe[sub 3]O[sub 4]. Compared with this unsupported S-105 catalyst, the supported catalysts show significantly higher 1,3-butadiene selectivities at comparable conversion levels, which is to be attributed to the different natures of their respective active phases.« less
  • Catalysts of Mo supported on MgO-{gamma} Al{sub 2}O{sub 3} were studied in the oxidative dehydrogenation of propane to propene. The catalysts were active and very stable, but the dehydrogenation selectivity was reduced by the formation of carbon oxides. Characterizations by XRD, XPS, Raman spectroscopy, electron paramagnetic resonance, and BET surface measurements were performed. The catalyst preparation method led to large and stable magnesium molybdate particles on the surface. EPR and XPS measurements gave clues about the fact that the active centers for the reaction include Mo{sup 5+} ions. A scheme for the surface architecture is proposed.