Hematite forms

An increasing intensity of the diffraction peaks of hematite is observed when comparing the dried and calcined catalyst as shown in Fig. 2(a), indicating that hematite forms at M er temperatures. No obvious diffraction peaks to lithium such as lithium iron oxide (LiFcsOg) could probably be ascribed to the small fraction of lithium or overlapped peaks betwem hematite and lithium iron oxide. The diffraction peak intensity of magnetite in tested catalysts increases significantly. 

We can write the overall reaction by which hematite forms, using the slopes-of-the-lines method discussed in Chapter 13. Initially, the reaction proceeds as,... 

Fig. 3.9 Effect of Al-substitution in synthetic hematites on (Left) the unit cell edge length a of hematites synthesized at various temperatures (Stanjek Schwertmann, 1992, with permission), and (Right) the magnetic hyperfine field Bhf of hematites formed at 70 °C and 1000°C dotted lines indicate 95% confidence limits (Murad Schwertmann 1986 with permission).

Al-hematites formed slowly from Al-ferrihydrite at 25 °C over 20 years, varied between rhombohedra at low substitution and multidomainic ellipsoids ca. 100 nm across with a grainy interior at higher substitution (Al/(Al-rFe) = 0.15) (Fig. 4.20e f) (Schwertmann et al. 2000). Allophane as a source of A1 had the same effect (Schwert-mann et al. 2000a). Mn substituted hematites grown from ferrihydrite were ellipsoidal in the presence of oxalate and platy in the presence of NaHCOa buffer (Cornell Gio-vanoli, 1987 Cornell et al., 1990). Cu substituted (0.09 mol mol" ) hematite grows as large (0.2 pm) rhombohedral crystals the crystal faces are most probably 102 or 104 (Fig. 4.20d) (Cornell Giovanoli, 1988). 

Hematite formed by dehydroxylation of oxide hydroxides at temperatures below 500-600 °C is porous. That formed by heating goethite in vacuo at 300 °C contains slit shaped meso pores which coalesce to circular macropores at temperatures >400°C (Naono and Fujiwara, 1980). At even higher temperatures, these pores are... 

Hematite forms by holding Fe " salt solutions at temperatures close to 100 °C ( forced hydrolysis ) (Matijevic Scheiner, 1978), from 2-line ferrihydrite in aqueous media at around pH 7, by high temperature solid-state transformation of var-... 

Fig. 14.15 The proportion of hematite formed from ferrihydrite in the pH range 2-12 and the temperature range 4-30 °C after 3392-4596 days of storage. The graph is interpolated from data at pH 2.5-12 in 1 pH unit steps and at 4, 10, 15 and 25°C. Increasing hematite in the mixture is indicated by a darker shade (Schwert-mann, unpubl.).

How factors such as the degree of ordering of ferrihydrite, and solution conditions, particularly pH and temperature affect the goethite/hematite ratio can provide information about the details of the process and also the conditions under which these two oxides might have formed in nature. The proportion of hematite formed after 15 hr at 100 °C increased from 43% to 95 % as the temperature at which the ferrihydrite was precipitated rose from 0 to 100 °C (Schwertmann Fischer, 1966). This suggests that with increasing temperature of precipitation, ferrihydrite dissolves less readily and the rate of dissolution (and hence goethite formation) falls. 

Hematite forms by a combination of aggregation-dehydration-rearrangement process for which the presence of water appears essential. Structural details about this process at 92 °C were obtained from EXAFS (Combes et al. 1989 1990) face-sharing between Fe octahedra developed before XRD showed any evidence for hematite. It is followed by internal redistribution of vacancies in the anion framework and by further dehydration. The dehydration process involves removal of a proton from an OH group and this in turn leads to elimination of a water molecule and formation of an 0X0 linkage. The local charge inbalance caused by proton loss is compensated for by migration and redistribution of Fe " within the cation sublattice. 

Direct proof for the participation of free water in the transformation to hematite was recently presented by Bao and Koch (1999) the oxygen of the hematite formed from 2-line ferrihydrite in the presence of water with a 5 0 of-8.0%o had the same isotope ratio as this water, showing that the oxygen came predominately from the water present during the transformation and not from the ferrihydrite precursor. 

In summary, there is considerable evidence to support the concept that in the presence of water, hematite forms from aggregated ferrihydrite by a short-range crystallization process within the ferrihydrite aggregate, with even adsorbed water being sufficient for the transformation to occur. The evidence is ... 

No hematite forms from sol particles, i.e. aggregation is essentia]... 

Figure 3.37. Potentiometric titrations of an iron oxide (hematite) formed under laboratory conditions (the point at which all three lines converge is representative of the PZC) (from Evangelou, 1995b, with permission).

Hematite forms competitively with akaganeite at reaction temperatures above 90 °C and goethite forms competitively if seed crystals of goethite are added (Atkinson et al., 1977). 

Figure 71. Sniall particles of twinned hematite formed in the electron microscope as a decomposition product of goethite 1215]...

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