Ferrihydrite dehydroxylation

The surface area of synthetic hematite depends upon whether the oxide was produced by calcination or grown in solution. The temperature of (dry) heating influences the surface area. Hematites produced at 800-900 °C have areas < 5 m g due to sintering of the particles. Hematites obtained by dehydroxylation of the various polymorphs of FeOOH or ferrihydrite at temperatures lower than 500-600 °C are mesoporous and have much higher surface areas - up to 200 m g". Commercial hematites are usually produced by calcination and hence have a low surface area. 

Ferrihydrite Hematite, Maghemite Thermal dehydration/dehydroxylation Gas/vacuum... 

The transformation of ferrihydrite to hematite by dry heating involves a combination of dehydration/dehydroxylation and rearrangement processes leading to a gradual structural ordering within the ferrihydrite particles in the direction of the hematite structure. This transformation may or may not be facilitated by the postulated structural relationship between the two phases. EXAFS studies have shown, for example, that some face sharing between FeOg octahedra, characteristic of hematite, also exists in 6-line ferrihydrite (see chap. 2). 

All fine grained Fc oxides lose adsorbed water at characteristic temperatures of between 100 and 200 °C. Structural OH in gocthite and lepido-crocite is lost at 250-400°C by the dehydroxylation reaction 2OH O + H2O. Even fine grained oxides such as hematite contain some OH in the structure (Stanjek Schwertmann, 1992) and this is driven off over a wide temperature range. For Fe oxides endothermic peaks result from the release of adsorbed or structural water, whereas exothermic peaks come from phase transformations (e.g. maghemite to hematite) or from recrystallization of smaller crystals into larger ones. An example of this is observed during the transformation of ferrihydrite to hematite. 

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