Iron-oxide aerogel and xerogel catalyst formulations: Characterization by 57Fe Mössbauer and XAFS spectroscopies
| Autor: | Sumit Bali, Edward M. Eyring, Gerald P. Huffman, Frank E. Huggins |
|---|---|
| Rok vydání: | 2010 |
| Předmět: |
Iron
Inorganic chemistry Analytical chemistry Iron oxide Ferric Compounds Catalysis Analytical Chemistry law.invention Spectroscopy Mossbauer chemistry.chemical_compound law Mössbauer spectroscopy Calcination Instrumentation Spectroscopy Magnetite Temperature Aerogel Quadrupole splitting Hematite Atomic and Molecular Physics and Optics X-ray absorption fine structure X-Ray Absorption Spectroscopy chemistry visual_art visual_art.visual_art_medium Gels |
| Zdroj: | Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy. 76:74-83 |
| ISSN: | 1386-1425 |
| Popis: | Iron in various iron-oxide aerogel and xerogel catalyst formulations (> or =85% Fe(2)O(3); < or =10% K, Co, Cu, or Pd) developed for possible use in Fischer-Tropsch synthesis (FTS) or the water-gas-shift (WGS) reaction has been examined by (57)Fe Mossbauer spectroscopy. The seventeen samples consisted of both as-prepared and calcined aerogels and xerogels and their products after use as catalysts for FTS or the WGS reaction. Complementary XAFS spectra were obtained on the occurrence of the secondary elements in some of the same materials. A broad, slightly asymmetric, two-peak Mossbauer spectrum was obtained from the different as-prepared and calcined catalyst formulations in the majority of cases. Such spectra could only be satisfactorily fit with three quadrupole doublet components, but no systematic trends in the isomer shift and quadrupole splitting parameters and area ratios of the individual components could be discerned that reflected variations in the composition or preparation of the aerogel or xerogel materials. However, significant reductions were noted in the Mossbauer effective thickness (recoilless absorption effect per unit mass of iron) parameter, chi(eff)/g, determined at room temperature, for aerogels and xerogels compared to bulk iron oxides, reflecting the openness and lack of rigidity of the aerogel and xerogel structures. Mossbauer measurements for two aerogels over the range from 15 to 292K confirmed the greatly diminished nature of this parameter at room temperature. Major increases in the effective thickness parameter were observed when the open structure of the aerogel or xerogel collapsed during calcination resulting in the formation of iron oxides (hematite, spinel ferrite). Similar structural changes were indicated by increases in this parameter after use of iron-oxide aerogels as catalysts for FTS or the WGS reaction, during which the iron-oxide aerogel was converted to a mixture of nonstoichiometric magnetite and the Hagg carbide, chi-Fe(5)C(2), or nonstoichiometric magnetite, respectively. |
| Databáze: | OpenAIRE |
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