Moessbauer studies of iron-based ultrafine coal liquefaction catalysts
Moessbauer spectroscopy has been used to investigate the structure and size dispersion of a variety of ultrafine iron-based catalysts for direct coal-liquefaction. In the as-prepared state, iron was chemically incorporated into the coal as a FeOOH polymorph, while catalysts prepared separately were usually present as Fe[sub 2]O[sub 3]. The Moessbauer spectra of these catalysts showed pronounced superparamagnetic effects. The superparamagnetic relaxation spectra obtained at various temperatures were analyzed using a novel fitting model to determine the particle size distribution of these catalysts. The resulting size distributions are in the nanometer range and agree reasonably well with size information obtained by Scanning Transmission Electron Microscopy (STEM), SQUID Magnetometry, and X-Ray Diffraction (XRD). Investigation of a 30[angstrom] FeOOH catalyst by Moessbauer spectroscopy revealed that the recoilless fraction decreased dramatically with temperature, becoming effectively zero at 50K. This effect has been explained on the basis of a combined effect of the recoil of the particle and the particle motion due to thermal agitation. The mean square amplitude of particle-motion was derived as a function of temperature. It was shown that by application of pressure or by suspending the particles in a wax matrix, the recoilless fraction could be increased significantly. During direct coal liquefaction (DCL), the highly dispersed ferric iron rapidly reacts with H[sub 2]S to form pyrrhotite (Fe[sub (1[minus]x)]S), where x is the vacancy concentration. If sufficient sulfur is present in the reactor to convert all of the iron to pyrrhotite, the remainder is left in the form of a superparamagnetic oxide. Moessbauer studies were done on a number of catalysts after DCL or related hydrotreatment reaction to determine the structure and vacancy content of the pyrrhotites, which varied from 5 to 11% for the limited number of samples studied here.
- Research Organization:
- Kentucky Univ., Lexington, KY (United States)
- OSTI ID:
- 6980758
- Country of Publication:
- United States
- Language:
- English
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36 MATERIALS SCIENCE
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37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CHEMISTRY
400201* -- Chemical & Physicochemical Properties
CATALYTIC EFFECTS
CHALCOGENIDES
COAL LIQUEFACTION
IRON COMPOUNDS
IRON OXIDES
LIQUEFACTION
MAGNETIC PROPERTIES
MAGNETISM
MOESSBAUER EFFECT
OXIDES
OXYGEN COMPOUNDS
PARAMAGNETISM
PARTICLE SIZE
PHYSICAL PROPERTIES
RECOILLESS FRACTION
SIZE
SPECTRA
STRUCTURAL CHEMICAL ANALYSIS
TEMPERATURE DEPENDENCE
THERMOCHEMICAL PROCESSES
TRANSITION ELEMENT COMPOUNDS