Iron oxides hydroxylation

Chen D, PA Frey (1998) Phenylalanine hydroxylase from Chromobacterium violaceum. Uncoupled oxidation of tetrahydropterin and the role of iron in hydroxylation. J Biol Chem 273 25594-25601. 

Voinov, M.A. et al. (2011) Surface-mediated production of hydroxyl radicals as a mechanism of iron oxide nanoparticle biotoxicity. Journal of the American Chemical Society, 133 (1), 35-41. 

Hydrated iron oxides can adsorb heavy metals. These adsorption properties arise from the presence of structural hydroxyl groups on their surface, which exhibit amphoterism (56) ... 

For iron oxides, IR spectroscopy is useful as a means of identification. Hematite crystals in films that were too thin (<70nm) to be characterized by XRD were shown by IR to be oriented with the c-axis perpendicular to the surface of the film (Yubero et al. 2000). This technique also provides information about crystal morphology, degree of crystallinity and the extent of metal (especially Al) substitution because these properties can induce shifts in some of the IR absorption bands. It is also widely used both to obtain information about the vibrational state of adsorbed molecules (particularly anions) and hence the nature of surface complexes (see Chap. 11) and to investigate the nature of surface hydroxyl groups and adsorbed water (see Chap. 10). Typical IR spectra of the various iron oxides are depicted in Figure 7.1. Impurities arising either from the method of preparation or from adsorption of atmospheric compounds can produce distinct bands in the spectra of these oxides -namely at 1700 cm (oxalate), 1400 cm (nitrate) and 1300 and 1500 cm (carbonate). 

The surface hydroxyl groups (whether they arise from adsorption of water or from structural OH) are the Junctional groups of iron oxides, i. e. they are the chemically re-... 

Fig. 10.1 S ingly coordinated, doubly coordinated, triply coordinated and geminal surface hydroxyl groups on iron oxides.

Adsorption and desorption reactions of protons on iron oxides have been measured by the pressure jump relaxation method using conductimetric titration and found to be fast (Tab. 10.3). The desorption rate constant appears to be related to the acidity of the surface hydroxyl groups (Astumian et al., 1981). Proton adsorption on iron oxides is exothermic potentiometric calorimetric titration measurements indicated that the enthalpy of proton adsorption is -25 to -38 kj mol (Tab. 10.3). For hematite, the enthalpy of proton adsorption is -36.6 kJ mol and the free energy of adsorption, -48.8 kJ mol (Lyklema, 1987). 

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.

Adsorption of cations on iron oxides (Table 11.3) may be specific or non specific. With non specific adsorption, there is at least one water molecule between the adsorbing species and the surface functional group. Specific adsorption involves interaction with deprotonated surface hydroxyl groups to form mono- and bi-nuclear, inner sphere complexes, i.e. 

Lewis, D.G. Cardde, C.M. (1989) Hydrolysis of Fe(III) solution to hydrous iron oxides. Aust. J. Soil Res. 27 103-115 Lewis, D.G. Farmer,V.C. (1986) Infrared absorption of surface hydroxyl groups and lattice vibrations in lepidocrodte (y-FeOOH) and boehmite (y-Al-OOH). Clay Min. 21 93-100... 

These results may be understood with a specific example. If we were interested in amorphous iron oxide with apA f of 7 and apA jf of 9 (Table 11.3) and a surface hydroxyl concentration, [=MOH], of 2 x 10-6 mol-m-2, we could estimate that this... 

During the course of studying the effect of crystallite sizes, attempts were made to produce very small unsupported iron oxide powders by lowering the calcination temperature of the iron hydroxyl gel that was precipitated from iron nitrate with ammonium hydroxide. However, catalysts calcined below 300°C still contain hydroxide, and they show high selectivity in butadiene production. For this reason, two catalysts, calcined at 250°C and 300°C, respectively, were studied in more detail. 

Reactions (2) and (3) indicate processes that regenerate Fe2+ in the catalytic cycle. As long as peroxide is available in the system, the iron species continually cycle between Fe2+ and Fe3+, unless additional reactions result in formation of insoluble iron oxides and hydroxides. The rate of formation of hydroxyl radical can be expressed as ... 

Among the experimental methods, that allow us to establish the concentration of surface hydroxyl groups are spectroscopy, methods based on chemical reactions, adsorption, and isotope exchange. The detailed survey of the applied experimental methods is presented by Boehm and Knozinger [36]. The densities of surface hydroxyl groups are presented in Table 2. Some of them, of the iron oxide structure, are beyond accepted hydroxyl group densities [39]. 

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