Energetics of Stable and Metastable Low Temperature Iron Oxides and Oxyhydroxides
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Mat. Res. Soc. Symp. Proc. Vol. 481 01998 Materials Research Society
EXPERIMENTAL PROCEDURE The a-FeOOH, y-FeOOH, ct-Fe 2 0 3 , yFe 20 3, Fe 30 4 are well-characterized commercial materials. 13-FeOOH phase was a natural sample provided by Huiming Bao. The structure and the composition of the samples studied are summarized in Table 1. Table 1. Characteristics of the studied minerals Mineral name
Composition
Specific surface area m2/g
Magnetite
Fe 30 4,0.08H 2 0
1.0
Hematite
Maghemite-1 Maghemite-2 Goethite- I Goethite-2 Akageneite Lepidocrocite
a-Fe 20
3
y-Fe 20 3,0.04H 2 0 y-Fe 20 3,0.04H 20 a-FeOOH,0.09H 20 oc-FeOOH,0.08H 20 13-FeOOH,0. 13H 2 00.032C1 'y-FeOOH,0. 10H 20
1.0
24.4 27.6 41.2 39.2 40.2 35.2
The enthalpies of formation and the enthalpies of reaction (oxidation and/or dehydroxylation) were determined using a high temperature Calvet type calorimeter [9] modified over the past twenty years by incremental improvements [10]. The calibration factor of the calorimeter was obtained by dropping pellets of known mass (and heat content) of alumina (Aldrich, 99.99%) stabilized in the corundum phase by heating 24 h at 1117 K. Drop solution calorimetry at 979 K, with sodium molybdate as solvent, was used to get the sum of heat content, heat of reaction and enthalpy of solution. Samples were pressed into pellets of 10-20 mg and dropped, from room temperature, into the hot zone of the calorimeter. We used a relatively high flow of oxygen (1.3 cm 3/sec) through the calorimeter to make sure that the oxidative transformation to the stable a-Fe 20 3 phase at 979 K was complete. The end of reaction was judged by the return of the baseline to the initial value. Transposed temperature drop calorimetry, in which the samples were dropped from room temperature into an empty Pt crucible in the calorimeter, was employed to determine the enthalpy of oxidation and dehydroxylation. The measured heat effect was a combination of the heat content and the enthalpy of reaction (oxidation, transition, or dehydroxylation). Some of samples studied contain physisorbed water, as shown by thermogravimetric analyses. As Mc.Hale and al. [11] showed for aluminum oxides, adsorbed water can affect the energetics. The correction for adsorbed water is discussed below. At the surface of the iron oxides or oxyhydroxides, at least three types of H20 adsorption can occur: chemisorbed H2 0 (hydroxyl ions bonded to Fe3+ in various configuration), chemisorbed molecular H2 0 (hydrogen bound), physisorbed molecular H2 0. The TGA analyses (see above) suggest that the samples contain predominantly two kinds of H20: weakly bound (physisorbed) molecular H20 presumably at the surface, which is removed at low temperature (373 K) and structural OH in the oxyhydroxides. There is no evidence for a significant amount of chemisorbed water or hydroxyl at the surface. Accordingly, we approximate the heat of adsorption of the "extra" H 20 (the physisorbed surface water) by the heat of condensation of
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gaseous water. Then, the heat of desorption, when
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