8-FeOOH surface area

Fig. 9. Discharge and charging curves for a sintered iron electrode at a constant current of 0.2 A where the apparent geometrical surface area is 36 cm and porosity is 65%. A and B represent the discharging and charging regions, respectively. Overall electrode reactions, midpoint potentials, and, in parentheses, theoretical potentials at pH 15 ate Al, n-Fe + 2 OH Fe(OH)2 + 2, 0.88 V (1.03 V) B, Fe(OH)2 FeOOH + H+ +, 0.63 V (0.72 V) C,...

The precursor particles of Pt, Pt02 H20, were tried to be deposited on hematite (a-Fe203) supports (a) polycrystalline ellipsoid (A), (b) monocrystalline ellipsoid (B), (c) monocrystalline pseudocube, and (d) monocrystalline platelet. Also, the precursor particles of Pt were tried to be formed on other supports other than a-Fe203 (a) a-FeOOH, (b) P-FeOOH, (c) Zr02 (A) with rough surfaces, (d) Zr02 (B) with smooth surfaces, and (e) Ti02 (anatase). The mean sizes and yield of the precursor particles are summarized in Table 2 with the specific surface area of the supports. 

In the case of FeOOH and AI2O3, gold chlorocomplex adsorption was found to be surface area-dependent. By contrast, in the case of Fe304, gold chlorocomplexes were found to be reduced to the metallic state and thus be less dependent on surface area. 

As an example of an equilibrium calculation accounting for surface complexation, we consider the sorption of mercury, lead, and sulfate onto hydrous ferric oxide at pH 4 and 8. We use ferric hydroxide [Fe(OH)3] precipitate from the LLNL database to represent in the calculation hydrous ferric oxide (FeOOH /1H2O). Following Dzombak and Morel (1990), we assume a sorbing surface area of 600 m2 g-1 and site densities for the weakly and strongly binding sites, respectively, of 0.2 and 0.005 mol (mol FeOOH)-1. We choose a system containing 1 kg of solvent water (the default) in contact with 1 g of ferric hydroxide. 

The hydrous iron oxide has been characterized to have a specific surface area of 600 m2 g 1 and 0.2 moles of active sites per mol of FeOOH. Then the concentration of the active sites is... 

The amorphous iron oxide is observed to be considerably more photoactive than the crystalline oxide - presumably as a result of the greater number of surface-located ferric hydroxy chromophores (the BET surface area of the synthesized Y-FeOOH is only 34 m2/g... 

Synthetic 5-FeOOH has a surface area which ranges from 20-300 m g depending on the thickness of the crystals. In a series of seven synthetic feroxyhytes the surface area increased from 140 to 240 m g (EGME method) as the crystallinity decreased (Garlson and Schwertmann, 1980). 5-EeOOH displays interpartide porosity, i.e. slitshaped micro- or mesopores between the plate like crystals (Jimenez-Mateos et al., 1988 Ishikawa et al., 1992). Both TEM observations and t-plot analysis showed that 0.8 nm micropores formed upon dehydroxylation at 150 °G in vacuo. The surface area rose steeply as the temperature exceeded 100 °G and reached a value close to 150 m g at 200 °C at which temperature, the sample was completely converted to hematite. 

The surface area of synthetic hematite depends upon whether the oxide was produced by calcination or grown in solution. The temperature of (dry) heating influences the surface area. Hematites produced at 800-900 °C have areas < 5 m g due to sintering of the particles. Hematites obtained by dehydroxylation of the various polymorphs of FeOOH or ferrihydrite at temperatures lower than 500-600 °C are mesoporous and have much higher surface areas - up to 200 m g". Commercial hematites are usually produced by calcination and hence have a low surface area. 

A common feature of the dehydroxylation of all iron oxide hydroxides is the initial development of microporosity due to the expulsion of water. This is followed, at higher temperatures, by the coalescence of these micropores to mesopores (see Chap. 5). Pore formation is accompanied by a rise in sample surface area. At temperatures higher than ca. 600 °C, the product sinters and the surface area drops considerably. During dehydroxylation, hydroxo-bonds are replaced by oxo-bonds and face sharing between octahedra (absent in the FeOOH structures see Chap. 2) develops and leads to a denser structure. As only one half of the interstices are filled with cations, some movement of Fe atoms during the transformation is required to achieve the two thirds occupancy found in hematite. 

Model crystallites of a-FeOOH were created with systematic variations in the ratio of the two commonly developed (021) and (110) facets. The model particles for the two extreme cases in large (long thin particle) and small (short fat particle) (110)/(021) surface area ratios are illustrated in Fig. 10. These particles were protonated to... 

Figure 1. The oxidation rate of H2S by lepidocrocite is pseudo-first-order with respect to H2S. The experimental pseudo-first-order rate constant k<,/ is plotted as a function of the surface area concentration of y-FeOOH. The reaction rate depends on the surface area (A).

In summary, the reaction of H2S with y-FeOOH is a fast surface-controlled process. Equations 8 and 9 can be used to estimate an upper limit of sulfide oxidation rates in sediments with reactive iron (assuming reactive iron to be represented by lepidocrocite). The surface-area concentration A of reactive iron can be calculated according to... 

Properties Different shapes and structures (hematite or (3-FeOOH) were obtained, depending on the experimental conditions. TEM and SEM images available [1374]. BET specific surface area 10.4 mVg, length 1 pm, width 300 nm [1622], electron micrograph available [586]. 

Properties total iron 58.1%, 3-FeOOH structure confirmed by XRD [661], specific surface area 51 iiT/g (original and stored) [568], 51.6 mVg [1319], wheat-grain shape [661], TEM image available [661]. 

Boily, J.R, Persson. P. and Sjoberg, S., Benzenecarboxylate surface complexation at the goethite (a-FeOOH)/water interface. III. The influence of particle surface area and the significance of modeling parameters, J. Colloid Intetf. Sci., 2T1. 132, 2000. 

The chemical composition of different nodules used in the present study is widely varied (See Table 1). XRD patterns showed a few diffused peaks characteristic of S-MnOj, a-FeOOH and a-SiOj. As expected the samples with high silica contents showed more surface area than the samples with lower silica contents. The surface oxygen, however, does not vary widely. 

The oxides used in this study were a-Si02 (a-quartz), obtained commercially, and a-FeOOH (goethite), which was prepared in a manner similar to that of Forbes et al. (18). The silica was washed initially in Q.IN nitric acid. Both oxides were washed with double distilled water, dried at 100°C for 24 hr, powdered with a mortar and pestle, and passed through a 200 mesh (75 /xm) sieve. Powdered x-ray diffraction verified the existence of a-quartz and goethite. BET-Ng adsorption indicated specific surface areas of 1.7 m /g for silica and 85 m /g for goethite. Corresponding ZPC values, determined by electrophoresis and turbidity measurements, were 1.7 and 5.5. Dielectrics were taken to be 4.3 for silica and 14.2 for goethite (19). 

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