Lepidocrocite preparation

Reactivity of Fe(III)(hydr)oxide as measured by the reductive dissolution with ascorbate. "Fe(OH)3" is prepared from Fe(II) (10 4 M) and HCO3 (3 10 4 M) by oxygenation (po2 = 0.2 atm) in presence of a buffer imidazd pH = 6.7 (Fig. a) and in presence of TRIS and imidazol pH = 7.7 (Fig. b). After the formation of Fe(III)(hydr)oxide the solution is deaerated by N2, and ascorbate (4.8 10 2 M) is added. The reactivity of "Fe(OH)3 differs markedly depending on its preparation. In presence of imidazole (Fig. a) the hydrous oxide has properties similar to lepidocrocite (i.e., upon filtration of the suspension the solid phase is identified as lepidocrocite). In presence of TRIS, outer-sphere surface complexes with the native mononuclear Fe(OH)3 are probably formed which retard the polymerization to polynuclear "Fe(OH)3" (von Gunten and Schneider, 1991). 

Adsorption of Cyclohexane. The adsorption of cyclohexane was determined on synthetic lepidocrocite and its decomposition products prepared by heating it in vacuo for varying intervals of time at 190°, 300°, 400°, and 500°, and also on synthetic goethite and its decomposition products obtained in a similar manner by heating at 150°, 180°, 250°, 300°, and 500°. The adsorption was found to be physical in nature, and the isotherms are Type II of the Brunauer classification (3) in all cases except on iron oxide prepared from goethite at 300° and 500° here the isotherms are Type III. This finding is in contrast to the Type IV isotherms common to ferric oxide gels the difference may be due to the crystalline nature of the parent material. 

Typical results are shown in Figures 1 and 2. Thus Figure 1 represents the isotherms of cyclohexane on the two parent materials lepidocrocite and goethite at 35°, while Figure 2 represents the isotherms at the same temperature on iron oxide prepared from lepidocrocite by heating in vacuo at 300° for 0.5, 1, 3, 8, and 18 hours. The results agree with the equation of Brunauer, Emmett, and Teller (4) ... 

Figure 2. Adsorption isotherms of cyclohexane on ferric oxide prepared from lepidocrocite by decomposition at 300° C.

Figure 4. Effect of duration of heating on specific surface area of products prepared from lepidocrocite at various temperatures...

Figure 5. Effect of temperature of preparation on limiting surface areas of products prepared from lepidocrocite (I) and goethite (II)...

All chemicals were reagent grade, and the solutions were prepared with high-purity water from a Millipore system. The pH measurements were carried out with a combined glass electrode (Metrohm) standardized with pH-buffer solutions (Merck). The lepidocrocite suspensions were prepared according to the procedure developed by Brauer (21) by oxidation of a FeCL2 solution with NaNOa as the oxidant, in presence of hexamethylenetetramine at 60 °C for 3 h. To remove excess chloride, the lepidocrocite suspensions were washed sev-... 

To conclude, it is clear there is a strong dependence of the performance of synthetic hematite on the deposition technique. While methods such as spray pyrolysis and CVD consistently produce electrodes photoactive for water oxidation, solution-based methods such as sol-gel approaches have failed to produce especially photoactive hematite. This is certainly related to the quality of the prepared material in terms of crystallinity and impurity concentrations. Aqueous methods of preparing hematite typically pass through a phase containing iron hydroxide (e.g., akaganeite, lepidocrocite, or goethite) but primarily hematite is detected after at annealing at 500°C. However, it has been shown that at temperatures up to 800°C, a nonstoichiometric composition remains in hematite when prepared in this way... 

Fig. 3.12 Morin transition temperature vs. inverse average particle size for differently prepared hematite samples Black square prepared from decomposition of lepidocrocite for the other symbols, see Ref, [99])...

E. Van San, E. De Grave, R.E. Vandenberghe, H.O. Desseyn, L. Datas, V. Bandn, A. Rousset, Study of Al-substituted hematites, prepared from thermal treatment of lepidocrocite. Phys. Chem. Miner. 28, 488 97 (2001)... 

Although the direct reaction of titanium alkoxide with tetraalkylammonium hydroxide was a route to layered titania, another route has been investigated by Sasaki et al., Watanabe et al. This was an exfoliation of protonic lepidocrocite type layered ti-tanate, H Ti2 jc/4D /404-2H20, where represents a vacancy as shown in Figure 5-7. (Sasaki et al., 1997). The lepidocrocite type titanate was prepared by cation exchange of Cs Ti2 jc/4D /404 with HQ (Sasaki et al., 1995). Tetrabuthylammonium hydroxide... 

Saito N., Haneda H., Li D., Koumoto K. Characterization of zinc oxide micropatterns deposited on self-assembled monolayer template. J. Ceram. Soc. Jpn. 2002 110 386-390 Santos L.R.B., Santilli C.V., Pulcinelli S.H. Sol-gel transition in Sn02 colloidal suspensions viscoelastic properties. J. Non-Cryat. Solid 1999 247 153-157 Santos L.R.B., Craievich A.F., Santilli C.V., Pulcinelli S.H. Sol phase and sol-gel transition in Sn02 colloidal suspensions. J. Appl. Cryst. 2000 33 609-613 Sasaki T., Watanabe M., Michiue Y., Komatsu Y., Izumi F., Takeuchi S. Preparation of acid-base properties of a protonated titanate with the lepidocrocite-like layer structure. Chem. Mater. 1995 7 1001-1007... 

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