Magnetite crystals

HTS catalyst consists mainly of magnetite crystals stabilized using chromium oxide. Phosphoms, arsenic, and sulfur are poisons to the catalyst. Low reformer steam to carbon ratios give rise to conditions favoring the formation of iron carbides which catalyze the synthesis of hydrocarbons by the Fisher-Tropsch reaction. Modified iron and iron-free HTS catalysts have been developed to avoid these problems (49,50) and allow operation at steam to carbon ratios as low as 2.7. Kinetic and equiUbrium data for the water gas shift reaction are available in reference 51. 

All boiler system waterside surfaces need the protection given by the smooth, hard, tenaciously adherent magnetite layer. The magnetite film sometimes may sparkle because of the precipitation of fine magnetite crystals onto the metal-oxide surface. Magnetite film formation is best achieved under stable, low-oxygen content operating conditions at a pH level of 10.5 to 11.5 (possibly up to 12.0). 

Many other creatures also possess a magnetic sense. Certain insects, turtles, and fishes, as well as some birds and mammals, respond to the earth s field. Several of these organisms contain localized magnetite crystals as well, but in only one vertebrate is there firm evidence linking the crystals to a magnetic sense. Like many... 

Fig. 4.25 Octahedral magnetite crystals produced hydrothermally at 250°C from 0.01 M Fe2(S04)3 solution in the presence of 0.4 M triethanolamine, 2.4 M NaOH and 0.85 M N2H4 (Sapieszko, Matijevic, 1980 with permission courtesy E. Matijevic).

Fig. 4.26 Left Rounded magnetite crystals obtained by slow oxidation (over 150 days) of a 0.05 M FeCl2 solution at pH 11.7 and room temperature (Schwertmann Murad, 1990 with permission) Right Magnetite octahedra produced by oxidation of a 0.5 M FeS04 solution with KNO3 in 1.43 M KOH at 90°C (Schwertmann. Cornell, 2000 with permission).

Fig. 7.14 Normalized decrease of the magnetization for magnetites of different origin. Whole cells refer to magnetotactic bacteria, extracted to biogenic magnetite extracted form these. The number indicates the size of synthetic magnetite crystals in nm (modified from Moskovitz etal., 1993, with permission).

A feature of this transformation is the influence of magnetite crystal size on the nature of the reaction products (Feitknecht, 1964 Gallagher et ak, 1968 Gillot et ak, 1978). At 200-250 °C, crystals smaller than 300 run transformed via the mixed phase to maghemite which in turn transformed to hematite at temperatures above 500 °C. 

In acid media (pH 2) magnetite crystals ca. 10 nm across transform topotactically to maghemite via an adsorption reaction which traps mobile electrons from the bulk material and reduces interfacial Fe the Fe ions that form are selectively leached into solution (Jolivet Tronc, 1988). Electron delocalization also induces ferrihydrite in contact with small magnetite particles to transform into a spinel layer (Belleville etal., 1992). 

Deep sea sediments may contain magnetite which may be not only of detrital origin, but may also contain a contribution from former magnetotactic bacteria. Petersen et al. (1986) have identified single-domain, magnetite crystals in Eocene to Quaternary sediments from the South Atlantic, which are very similar to biomagnetite in recent marine bacteria (see Chap. 17). 

The magnetite crystals are well developed (euhedral), and this ensures that they act as single magnetic domains (SD) and produce remanent magnetization in sediments. The average number of magnetite crystals/cell in 220 cells of the microaero-... 

Biogenic magnetite may persist once the organism that produced it has died and may, therefore, contribute to the natural magnetic remanence of sediments (Stolz et al., 1986). The discovery in a calcareous Martian ( ) meteorite found in Antarctica, of magnetite crystals with properties very similar to these biogenic magnetites, sup-... 

Pinheiro, E.A. de Abreu Filho, P.P. Galem-beck, F. Correa da Silva, E. Vargas, H. (1987) Magnetite crystal formation from iron(III) hydroxide acetate. A CRS study. Langmuir 3 445-448... 

Chemical vapour transport has been used to grow magnetite crystals using the reaction of magnetite with hydrogen chloride gas ... 

FIGURE 3.13 Growth of magnetite crystals using chemical vapour transport... 

Beads of cross-linked polystyrene containing magnetite crystals (Fe304, a ferromagnetic material) have been prepared and successfully used for solid-phase synthesis [125-127]. The magnetic beads could be readily separated from the reaction mixture with a magnet. 

Yushkin (1971) proposes for proper interpretation of hardness anisotropy the construction of hardness indicatrices or comparison of hardness formulae, in which the numbers give the hardness in a particular crystallographic direction. For the example of Ural magnetite crystal, such a hardness formula is written as follows ... 

Fig. 6.1.12. Dependence of hardness of natural face in magnetite crystal before and after polishing on load applied to indenter.

The points representing the data plotted in the coordinates log P0l vs. mass of O in the surface layer lie on a straight line. Thus the logarithmic adsorption isotherm is obeyed. A particular feature of this system is that the state of free surface, 6 — 0, corresponds to a complete removal of oxygen from the monoatomic surface layer of the Fe304 lattice and the state of occupied surface, 0 = 1, corresponds to the oxide with the stoichiometric composition in the surface layer. Thus Z in (344) is an oxygen vacancy on the surface of a magnetite crystal. 

Figure 28.7 Low-magnification TEM image of sample after activation at 543 K. The carbide nodules growing out of the magnetite crystals are clearly visible. The diffraction pattern in the inset shows spots corresponding to magnetite and the location of the carbide ring.

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