Hematite synthesis

Throughout each experiment, the solution/suspension must be stirred constantly, preferably using an overhead stirrer. The experimental equipment must be installed beforehand and the solution for hematite synthesis already heated to close to 90 °C, The syntheses must be started immediately and while the oxides form, the pathways of formation are discussed. 

Wheeler D. A., Wang G., Ling Y, Li Y. and Zhang J. Z. (2012). Nanostmctured hematite synthesis, characterization, charge carrier dynamics, and photoelectrochemical properties. Energy Environmental Science, 5(5), 6682-6702. 

Shock-synthesis experiments were carried out over a range of peak shock pressures and a range of mean-bulk temperatures. The shock conditions are summarized in Fig. 8.1, in which a marker is indicated at each pressure-temperature pair at which an experiment has been conducted with the Sandia shock-recovery system. In each case the driving explosive is indicated, as the initial incident pressure depends upon explosive. It should be observed that pressures were varied from 7.5 to 27 GPa with the use of different fixtures and different driving explosives. Mean-bulk temperatures were varied from 50 to 700 °C with the use of powder compact densities of from 35% to 65% of solid density. In furnace-synthesis experiments, reaction is incipient at about 550 °C. The melt temperatures of zinc oxide and hematite are >1800 and 1.565 °C, respectively. Under high pressure conditions, it is expected that the melt temperatures will substantially Increase. Thus, the shock conditions are not expected to result in reactant melting phenomena, but overlap the furnace synthesis conditions. 

Lian, J.B., Duan, X.C., Ma, J.M., Peng, P., Kim, T.I. and Zheng, W.J. (2009) Hematite (alpha-Fe2C>3) with various morphologies ionic liquid-assisted synthesis, formation mechanism, and properties. ACS Nano, 3 (11), 3749—3761. 

The structure derived from a Rietveld fit of a neutron diffraction pattern of a 6-line ferrihydrite which showed more and sharper lines (Fig. 2.9, lower) than an XRD pattern, was in agreement with the structure proposed by Drits et al. (1993) except that it was not necessary to assume the presence of hematite in order to produce a satisfactory fit (Jansen et al. 2002). The unit cell of the defect free phase had a = 0.29514(9) nm and c = 0.9414(9) nm and the average domain size derived from line broadening was 2.7(0.8) nm. Since forced hydrolysis of an Fe solution at elevated temperatures will ultimately lead to hematite, it is likely that incipient hematite formation may occur under certain synthesis conditions. Neither these studies nor Mbssbauer spectroscopy, which showed only a singular isomer shift at 4.2 K characteristic of Fe, supported the presence of " Fe (Cardile, 1988 Pankhurst Pollard, 1992). However, the presence, at the surface, of some Fe with lower (<6) coordination, perhaps as tetrahedra (Eggleton and Fitzpatrick, 1988) which may have become unsaturated on heating, has been suggested on the basis of XAFS results (Zhao et al. 1994). 

Morphologies of synthetic hematite include plates and discs, rods, spindles, spheres, ellipsoids, double ellipsoids, rhombohedra, stars, cubes and peanuts. In the absence of additives, hexagonal plates, which are often rounded, and rhombohedra predominate. Each morphology can be obtained by more than one synthesis route. Two common ways of producing idiomorphic hematite crystals in aqueous systems... 

The specific surface area of a solid is the surface area of a unit mass of material, usually expressed as m g . There is an inverse relationship between surface area and particle size. Massive crystals of hematite from an ore deposit (e. g. specularite) may have a surface area 1 m g". As particle size/crystallinity is governed largely by the chemical environment experienced during crystal growth, the surface area of a synthetic iron oxide depends upon the method of synthesis and that of a natural one, upon the environment in which the oxide formed. 

The omnipresence of aluminium in weathering environments results in most of the Fe oxides in soils, except lepidocrocite, being Al-substituted. The possible range of substitution as deduced from synthesis experiments (see Chap. 3) viz. up to Al/ (Fe Al) of ca. 0.33 in goethite and up to Al/(Fe Al) of ca. 0.16 in hematite is also found in soil goethites and hematites. Where the two oxides coexist on a small scale... 

Synthesis of microcrystalline hematite and magnetite in organic solvents and effect of a small amount of water in the solvents. 

Single-crystalline maghemite (y-FezOs) nanotubes are obtained by a reduction and re-oxidation processes with hematite (a-FezOs) nanotubes as precursors. This approach provides a new strategy to synthesize single-crystalline nanotubes of nonlamellar-structured materials, which could be generally applicable to the synthesis of other inorganic tubular nanostructures. 

Iron(III) oxide crystallizes independently of the synthesis route in the a-modification (hematite) after calcination. Brilliant, intense colors are obtained with 50-250 nm layers of Fe203. Absorption and interference colors are formed simultaneously and vary with layer thickness of iron oxide. Especially, the red shades are extremely intensive because interference and absorption enhance each other (Fig. 79). It is possible to produce an intense green-red flop with different viewing angles at a layer thickness similar to a green interference [5.228]. 

The CO that is consumed in the process, besides forming hydrocarbons and C02 also caused the transformation of hematite to magnetite and subsequently the magnetite to iron carbide. Therefore, a better measure of Fischer-Tropsch synthesis (FTS) activity is the rate of CH4 formation plotted in Figure 28.1(b). It is seen that activation at 543 K makes the... 

When iron catalysts are exposed to FT synthesis reaction environments, the catalysts first transform from hematite into magnetite. The transformation into magnetite is rapid and occurs pseudomorphically where the shape of the hematite crystals is retained including their swiss-cheese morphology. The transformation from magnetite to carbide is slow and is affected... 

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