Iron surface hydroxyl groups

Gas-phase chromatography shows that, in the absence of water, little CO2 is only formed at the beginning of the deposition process, presumably due to the reaction between the precursor and surface hydroxyl groups. However, the addition of water causes evolution of CO2 throughout the deposition process. XPS analyses demonstrate that the presence of water vapor not only limits carbon contamination but also permits avoidance of the formation of iron carbides however, with molybdenum and chromium it was not possible to completely suppress this side reaction at 493 K. The same XPS analyses reveal that mainly metallic Fe(0) is present in the deposit [13]. 

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... 

Iron and manganese oxides are characterized by high specific surface areas and high affinity of their surface hydroxyl groups for adsorption of a variety of trace elements. In addition to adsorption processes, oxidation reactions are catalyzed by these surfaces (18-20). The in situ precipitation and dissolution of these oxides are thus significant for the fate of various trace... 

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]. 

Figure 14 Determination by XPS of (a) potassium and (b) phosphate uptake per surface hydroxyl group on the oxidized iron surface as a function of pH. (Reprinted from Ref [154].)...

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