Iron oxide surfaces

Now the interaction of Zn(II) with the hydrous iron oxide surface can be readily computed because the surface charge is hardly affected by [=FeOZn+], because... 

The most direct evidence for surface precursor complex formation prior to electron transfer comes from a study of photoreduc-tive dissolution of iron oxide particles by citrate (37). Citrate adsorbs to iron oxide surface sites under dark conditions, but reduces surface sites at an appreciable rate only under illumination. Thus, citrate surface coverage can be measured in the dark, then correlated with rates of reductive dissolution under illumination. Results show that initial dissolution rates are directly related to the amount of surface bound citrate (37). Adsorption of calcium and phosphate has been found to inhibit reductive dissolution of manganese oxide by hydroquinone (33). The most likely explanation is that adsorbed calcium or phosphate molecules block inner-sphere complex formation between metal oxide surface sites and hydroquinone. 

The importance of bacteria in mediating Mn(II) oxidation in certain environments is evident. But, the mechanisms whereby bacteria oxidize Mn(II) are poorly understood. Some bacteria synthesize proteins or other materials that enhance the rate of Mn(II) oxidation (.52). Other strains of bacteria require oxidized manganese to oxidize Mn(II) (53), suggesting that they may catalyse the oxidation of Mn(II) on the manganese oxide surface. Other bacteria may catalyse the oxidation of Mn(II) on iron oxide surfaces, as iron is associated with manganese deposits on bacteria collected in the eastern subtropical North Pacific (54). 

Tab. 7.8 Techniques applied to examination of iron oxide surfaces...

Fig. 10.7 Schematic representation of the distribution of positive, negative and neutral surface hydroxyl groups on an iron oxide surface with a / i = 7.09 and /CI2 = H. 11.

Fig. 11.2 Modes of ligand coordination to the iron oxide surface and modes of coordination through COOH groups.

Core and valence level photoemission studies of iron oxide surfaces and the oxidation of iron. Surface Sd. 68 459—468 Bruno, J. Sturam, J.A. Wersin, P. Brand-berg, E. (1992) On the influence of carbonate on mineral dissolutions I. The thermodynamics and kinetics of hematite dissolution in bicarbonate solutions at T = 25°C. Geo-chim. Cosmochim. Acta 56 1139—1147 Brusic.V. (1979) Ferrous passivation. In Corrosion Chemistry, 153—184 Bruun Hansen, H.C. Raben-Lange, R. Rau-lund-Rasmussen, K. Borggaard, O.K. 

Pourbaix, M. (1974) Applications of electrochemistry in corrosion science and in practice. Corrosion Sci. 14 25-82 Prasad, J. Murray, E. Kelber, J.A. (1993) site selective chemistry of S, Cl and H2O at the iron oxide surface. Surface Sci. 289 10-18 Prasetyo, B.H. Gilkes, R.J. (1994) Properties of iron oxides from red soils derived from volcanic tuff in West Java. Aust. J. Soil Res. 32 781-794... 

Shaikhudtinov, Sh.K. Weiss, W. (2000) Adsor-balt dynamics on iron oxide surfaces studies by scanning tunnelling microscopy. J. Molecular Catalyses A. Chem. 158 129-133... 

Ranke, W. Weiss, W. (1999) Structure and reactivity of iron oxide surfaces. Faraday Discuss. 114 363-380... 

Zhao, J. Feng, Z. Huggins, F.E. Shah, N. Huffman, G.P. Wender, I. (1994b) Role of molybdenum at the iron oxide surface. J. Catalysis 148 194-197... 

Hence, these Qc values are a quantitative measure for the relative affinities of the various NACs to the reactive sites. Figs. 14.10e and/show plots of log Qc versus h(AtN02)/0.059 V of the 10 monosubstituted benzenes. A virtually identical picture was obtained for the log Qc values derived from an aquifer solid column and from a column containing FeOOH-coated sand and a culture of the iron-reducing bacterium, Geobacter metallireducens (GS15). Furthermore, a similar pattern (Fig. 14.10c) was found when correlating relative initial pseudo-first-order rate constants determined for NAC reduction by Fe(II) species adsorbed to iron oxide surfaces (Fig. 14.12) or pseudo-first-order reaction constants for reaction with an iron porphyrin (data not shown see Schwarzenbach et al., 1990). Fig. 14.12 shows that Fe(II) species adsorbed to iron oxide surfaces are very potent reductants, at least for NACs tv2 of a few minutes in the experimental system considered). 

In this study we performed initial rate experiments, reacting H2S with lepidocrocite (23). The consumption of H2S was measured continuously by using a pH2S electrode cell (25). To avoid interferences of pH buffer solutions with the iron oxide surface, the pH was stabilized by using a pH-stat that added appropriate amounts of HC1 to the solution. The added volume, which was also continuously monitored, provided information about the amount of protons consumed during the reaction. Dissolved iron was measured only in some runs. 

Experiments were then performed to preadsorb the various forms of oxygen on an iron oxide surface. These were then followed by the butene adsorption-desorption experiments to determine the amounts of the two types of oxidation sites. Typical results are shown in Table V (6). The amounts of oxidation products are independent of the presence of preadsorbed oxygen. In fact, even the thermally desorbed isomers are not affected by the preadsorbed oxygen. This absence of effect of the preadsorbed oxygen was observed also in pulse experiments. It was found that the amounts of butadiene, C02, and butene isomers formed from a butene pulse passing over the catalyst at 100,200, or 300°C are independent of preadsorbed oxygen (6). 

Entrapped oxygen and water vapor in lubricants can act as anti-wear additives to form protective surface films. Metals are known to catalyze decomposition of certain lubricants, and decomposition temperatures may be reduced by 60°C or more. This is particularly true of bearing surfaces, on which the surface energy may be increased by stress-induced dislocations and by freshly exposed metal surfaces. An example of the way a tribochemical surface is affected by a polymerization process is vinyl chloride. If the load is increased from 0.1 to 0.5 kg in the presence of a vinyl chloride atmosphere, the Auger spectra of iron oxide surface shows a marked increase in the concentration of vinyl chloride on the surface (Buckley, 1981). 

Valentine RL, Wang HCA. Iron oxide surface catalyzed oxidation of quinoline by hydrogen peroxide. J Environ Eng 1998 124 31-38. 

Another technologically important reaction is the Fischer-Tropsch synthesis, with iron oxide being one of the components of some catalysts. A detailed understanding of the complex mechanism of this reaction can be obtained by studying the chemisorption of simple molecules on well-characterized surfaces by means of advanced surface-sensitive spectroscopic techniques. A few investigations of the interaction of small molecules (such as CO, CO2, H2O, O2, H2, and NO) (520-522) and organic molecules on iron oxide surfaces (523-527) have been carried out. 

Figure 8.5 Spectral radiancy of a blackbody, real bodies stainless steel (1400°C) and alumina (1200°C), and greybody approximations. Real body spectra were calculated based on emittance values from reference [5]. Greybody approximations (dot-dot-dashed lines) were based on emittances of 0.33 for alumina and 0.75 for stainless steel. The high emittance of stainless steel is a result of oxidation to form a rough iron oxide surface. The greybody approximation appears good for stainless steel and poor for alumina. This may not be the case for different temperatures where the most intense portion of the blackbody spectra shifts in wavelength the constancy of emittance differs in different regions of the spectrum.

Figure 3.25. Schematic showing the zero point of charge of an iron oxide surface and the charge generated as a function of added hydrogen (H+) or hydroxyls (OH ) (from Singh and Uehara, 1986, with permission).

Iron oxide surface below pzc Overall positive charge... 

Figure 7 Variable, pH-dependent charge on an iron oxide surface...

Figure 14 Chemisorption of a phosphate anion on to an iron oxide surface by ligand exchange...

Because iron oxide surfaces are excellent substrates for adsorption of metals from solution, sediments located under the hydrothermal particle plumes are highly enriched in iron and trace metals. The uptake of V, Cr and P onto these particles is a quantitatively important sink in the geochemical mass balance between dissolved river input and sedimentary output (Rudnicki and Elderfield, 1993). 

Dialysis against 5 mM TES buffer is done for at least 3 days with very frequent buffer changes (at least 15 times) above the transition temperature of the phospholipids used. During this step, the laurate molecules are slowly removed from the iron oxide surface and concomitantly replaced by phospholipid molecules (6, 7). At the end, a clear solution without any precipitate should be obtained. 

Iron oxide surface can coordinate with protons and hydroxide ions to form different surface groups. ... 

Johnson, R.L. et al., The adsorption of perfluorooctane sulfonate onto sand, clay, and iron oxide surfaces, J. Chem.Eng.Data, 52, 1165, 2007. 

In soils, autocatalytic oxidation of Mn + by Mn oxides seems to be a mechanism by which the formation of Mn oxides can be explained at pH values well below 8. There is also evidence that Mn and Fe tend to co-precipitate in oxides, possibly because iron oxide surfaces also catalyze the oxidation of Mn. In any event, the oxidizing agent for Mn " is molecular oxygen, either by a direct or indirect reaction path. The reason for this is clear from Figure 7.5. Except for NOs", which tends to be kinetically inert, O2 is the only sufficiently strong and common oxidant in soil to be able to oxidize Mn. ... 

---