Iron oxide, precipitation magnetite

Synonyms Black magnetic oxide Black oxide, precipitated Black rouge Cl 77499 Ethiops iron Ferric ferrous oxide Ferrosoferric oxide Iron oxide Iron (II, III) oxide Iron (III) oxide Iron (II) oxide, black Iron (II, III) oxide, black Iron oxide magnetic Iron oxides (FesOJ Magnetite Pigment black 11 Triiron tetraoxide Classification Syn. iron oxide Empirical FejO,... 

The magnetite film may, under some circumstances, be layered. For example, in power boilers, under conditions of high steaming rate and low steam-water velocity, a secondary film of precipitated particulate iron oxide may form over the original magnetite film. 

For a precipitated iron catalyst, several authors propose that the WGS reaction occurs on an iron oxide (magnetite) surface,1213 and there are also some reports that the FT reaction occurs on a carbide surface.14 There seems to be a general consensus that the FT and WGS reactions occur on different active sites,13 and some strong evidence indicates that iron carbide is active for the FT reaction and that an iron oxide is active for the WGS reaction,15 and this is the process we propose in this report. The most widely accepted mechanism for the FT reaction is surface polymerization on a carbide surface by CH2 insertion.16 The most widely accepted mechanism for the WGS reaction is the direct oxidation of CO with surface 0 (from water dissociation).17 Analysis done on a precipitated iron catalyst using bulk characterization techniques always shows iron oxides and iron carbides, and the question of whether there can be a sensible correlation made between the bulk composition and activity or selectivity is still a contentious issue.18... 

If precipitation is carried out at ca. 90 °C while air is passed into the mixture at ca. pH > 7, black iron oxide pigments with a magnetite structure and a good tinting strength are obtained when the reaction is stopped at a Fe0 Fe203 ratio of ca. 1 1. 

Raman chemical imaging can be employed to access the homogeneity and the structural stability in terms of oxidation rates, onset of hematite, and organic contamination of as-precipitate and oxidized iron oxide samples. Oxidation-related Raman features have been established by comparative study of bulk oxides and nanoparticles attained in two different oxidation states, suggesting that the solid nanophase synthesized had a mixed magnetite-maghemite composition [52]. 

The molar ratio of Fc203/Fe0 in the BIFs is less than 1.0 in all but one of the 28 analyses of Isua iron formation reported by Dymek and Klein (1988). In the one exception the ratio is 1.17. Unless the values of this ratio were reduced significantly during metamorphism, the analyses indicate that magnetite was the dominant iron oxide, and that hematite was absent or very minor in these iron formations. This, in turn, shows that some of the hydrothermal Fe was oxidized to Fe prior to deposition, but that not enough was oxidized to lead to the precipitation of Fc203 and/ or Fe oxyhydroxide precursors, or that these phases were subsequently replaced by magnetite. 

The process(es) by which the precursor(s) of magnetite in these iron formations precipitated is not well understood. A possible explanation involves the oxidation of Fe by reaction with seawater to produce Fe and H2 followed by the precipitation of green rust —a solid solution of Fe(OH)2 and Fe(OH)3—and finally by the dehydration of green rust to magnetite (but see below). Figure 4 shows that the boundary between the stabihty field of Fe(OH)2 and amorphous Fe(OH)3 is at a rather low value of pe. The field of green rust probably straddles this boundary. Saturation of solutions with sideiite along this... 

Because of the expense and scarcity of cobalt, emphasis has shifted to the use of iron. Iron catalysts may be prepared for commercial applications by precipitation from solution, from magnetite (FesOO ore, or from magnetite obtained by fusion of iron oxides, or by oxidation of metallic iron with steam. An active catalyst prepared at the U.S. Bureau of... 

When steel or iron is exposed to an atmospheric environment, a thin layer of magnetite, Fe304, is formed, covered by a layer of FeOOH. Atmospheric oxygen then penetrates though the almost water-free, porous outer layer of FeOOH and oxidizes the magnetite to hydrated ferric oxide, Fe203, or FeOOH. The presence of Fe " in the electrolyte initiates the precipitation of various corrosion products. The electrochemical mechanism of atmospheric corrosion of iron suggested by Evans is briefly summarized in this chapter [8]. 

---