Iron oxide , defect structure

In contrast, the reddish-brown jerrihydrite (often wrongly termed amorphous iron oxide or hydrous ferric oxide (HFO) ) is widespread in surface environments. It was first described by Chukhrov et al. in 1973. Unlike the other iron oxides it exists exclusively as nano-crystals and unless stabilized in some way, transforms with time into the more stable iron oxides. Ferrihydrite is, thus, an important precursor of more stable and better crystalline Fe oxides. Structurally ferrihydrite consists of hep anions and is a mixture of defect-free, and defective structural units.The composition, especially with respect to OH and H2O, seems to be variable. A preliminary formula, often used, is FesOgH H2O. 

Ferric ferrocyanide, commonly known as Prussian blue (PB), was first synthesized > 300 years ago (39) and is still used in the manufacture of blueprints. Prussian blue is a prototypical mixed-valence compound with formula Fe 4[Fe (CN)6]3 I4H2O. In its canonical form, the pigment consists of ferrocyanide anions linked by Fe cations (Fig. 1) to form an extended pcu network. A defect structure that arises from the necessity of charge balancing in the cubic framework results in vacancies at 25% of the [Fe(CN)6]" sites (40). Analogous compounds are formed when one or both iron atoms are replaced by a variety of other metals. This substitution affords compounds of the formula M [My(CN)6]j where M and M can be Cr, Mn, Fe, Co, and many others, and where x andy depend on the identity and oxidation states of the metals. Because of their structural similarities we will refer to the entire class of compounds as blues PBs. 

The sodium tungsten bronzes already described are examples of incomplete lattice defect structures. Iron(II) oxide is rather similar it has a rock-salt structure but is always deficient in iron. Some Fe ions are always present to maintain electrical neutrality. Fe304 has the spinel structure which has the same arrangement of ions as FeO. Fe203. has also the same arrangement and oxidation of FeO to FcgOg consists of the replacement of Fe2+ ions by two thirds of their number of Fe ions. 

Transition metal catalysts, specifically those composed of iron nanoparticles, are widely employed in industrial chemical production and pollution abatement applications [67], Iron also plays a cracial role in many important biological processes. Iron oxides are economical alternatives to more costly catalysts and show activity for the oxidation of methane [68], conversion of carbon monoxide to carbon dioxide [58], and the transformation of various hydrocarbons [69,70]. In addition, iron oxides have good catalytic lifetimes and are resistant to high concentrations of moisture and CO which often poison other catalysts [71]. Li et al. have observed that nanosized iron oxides are highly active for CO oxidation at low tanperatures [58]. Iron is unique and more active than other catalyst and support materials because it is easily reduced and provides a large number of potential active sites because of its highly disordered and defect rich structure [72, 73]. Previous gas-phase smdies of cationic iron clusters have included determination of the thermochemistry and bond energies of iron cluster oxides and iron carbonyl complexes by Armentrout and co-workers [74, 75], and a classification of the dissociation patterns of small iron oxide cluster cations by Schwarz et al. [76]. 

A As iron is lost from the structure, the remaining iron in the structure is oxidized from Fe " to Fe ". The more oxidized ion is smaller owing to the contraction caused by the effect of the higher charge on the electron cloud. Also, as the defects are introduced randomly in the structure, the same overall structure is maintained. 

Structural sensitivity of the catalytic reactions is one of the most important problems in heterogeneous catalysis [1,2]. It has been rather thoroughly studied for metals, while for oxides, especially for dispersed ones, situation is far less clear due to inherent complexity of studies of their bulk and surface atomic structure. In last years, successful development of such methods as HREM and STM along with the infrared spectroscopy of test molecules has formed a sound bases for elucidating this problem in the case of oxides. In the work presented, the results of the systematic studies of the bulk/surface defect structure of the oxides of copper, iron, cobalt, chromium, manganese as related to structural sensitivity of the reactions of carbon monoxide and hydrocarbons oxidation are considered. 

Flexibility of the bulk/surface structure and reaction media effect. For such systems as manganese oxides, copper oxides, spinel iron oxides Fe304-y-Fe203 [4, 5, 24, 25 ], reaction media effect at enhanced temperatures (up to 400 °C ) and at prolonged (up to 10 h) exposures in reaction mixtures was found to remove all initial differences in the phase composition and defect structure. All extended defects were washed out due to interaction with a flux of point defects created by reaction media. As a result, a constant level of the catalytic activity was achieved for these oxide systems demonstrating apparent structural insensitivity of the reaction of CO oxidation. Hence, in this case, great flexibility of the oxide bulk structure allows to reach the same true steady state of the catalyst. 

The key to the extremely high activity for filament production found with FeO may well reside in the defect structure of this compound. In such a structure the oxygen atoms in the surface will be readily accessible to extraction by protons generated by the hydrocarbon decomposition reaction and as a result the oxide could rapidly attain at the surface an iron rich sponge-like arrangement i.e. the role of FeO is that of a precursor for a high surface area Fe catalyst formed ln-sltu. 

According to Niu and Millot (1999), at low/ 02, where the majority defects are iron interstitials, the prevailing oxygen defects are free oxygen vacancies. At high P02, where the majority defects are cation vacancies, the observed slope of 1/6 supports the presence of clusters of iron vacancies with oxygen vacancies formed as a result of Coulombic attractive forces. The main two points here are (a) oxygen diffusivities in select oxides (but not all) may follow a complex pattern as a function of Po2 md (b) information of this type is necessary in order to assess the relation between rates and defect structures. 

As indicated in Table 10.18, a zinc-rich primer is often recommended. It can be an organic zinc-epoxy or an inorganic zinc-ethyl-silicate primer. Zinc-rich primers are also used as so-called shop primers, or prefabrication primers, for temporary protection of semi-manufactured steel goods. After fabrication, e.g. of welded steel structures, the shop primer surface must be cleaned (degreased), and possible shop primer defects and weld joints have to be blast cleaned and coated with a primer before the whole structure is painted. Iron oxide is also used as a pigment in some shop primers. These must not be overpainted with a zinc-rich paint. 

Bruemmer et al. (55) studied Ni, Zn, and Cd sorption on goethite, a porous iron oxide known to have defects within the structure in which metals can be incorporated to satisfy charge imbalances. At pH 6, as reaction time increased from 2 hours to 42 days (at 293K), sorbed Ni increased from 12 to 70% of Ni removed from solution, and total increases in Zn and Cd sorption over this period increased 33 and 21%, respectively. The kinetics of Cd, Zn, and Ni were described well with a solution to Pick s second law (a linear relation with the square root of time). Bruemmer et al. (55) proposed that the uptake of the metal follows three-steps (i) adsorption of metals on external surfaces (ii) solid-state diffusion of metals from external to internal sites and (iii) metal binding and fixation at positions inside the goethite particle. They suggest that the second step is the rate-limiting step. However, they did not conduct microscopic level experiments to confirm the proposed mechanism. In view of more recent studies, it is likely that the formation of metal-nucleation products could have caused the slow metal sorption reactions observed by Bruemmer et al. (55). 

Among all catalysts with the iron oxides and their mixtures as precursor studied, Fei xO based catalyst with nonstoichiometric and wiistite structure has the fastest reduction rate and lowest reduction temperature. In a wiistite structure, large amounts of defects are iron ions, which enable the diffusion of Fe in oxide lattices, and will be preferable to electron transferences. This is the structural factor for the easy reduction of Fei xO based catalysts. 

As previously described, FeO is an oxygen excessive (Fe defect) non-stoichiometric iron oxide (Fei xO), which shows the rock-salt type face center cubic structure (fee). The unit cell of Fei xO are constituted from four Fei xO molecules, where there are eight tetrahedral interspaces A site) and four octahedral interspaces B sites) with 0 closely packing onto NaCl-type cubic lattices. 

The reduction performance of catalyst is closely related with the composition of its precursor in hydrogen flow. As mentioned earlier, this is due to the different reduction mechanisms for catalysts with different precursors. All precursors of iron oxide such as Fe304, Fei xO and their mixture are possible for fused iron catalysts, while the sequence of the reduction rate as well as the reduction temperature is Fei xO > Fe304 > mixture. Apparently, the catalysts with non-stoichiometric Fei xO with wiistite structure as precursors have the fastest reduction rate and the lowest reduction temperature. As mentioned before, the defects of iron ions in lattice of Fei xO has serious impact on its reduction properties. It can be seen from Fig. 5.13 that the reduction process is faster and more complete when the amount of the defects is larger in wiistite. 

When Rietveld analysis is used to amend the structure of chromium-doped iron oxide samples, the structural parameters of pure 7-Fe203 is used as initial parameters. 7-F6203 has a spinel structure of Fe304 with iron ions defect, and Fc304 has the space group Fd-3m, lattice constants a = 0.8333nm, Z = 8. The parameters of atomic coordinates are listed in Table 7.16. 

The broadening of the spectrum upon treatment at 790 K is similar to observations made when calcium carbonate single crystals are subject to argon ion bombardment. The defects induced in the calcium oxide component of the catalyst cannot be caused by an initial reduction of the oxide, because calcium oxide is stable to hydrogen at 790 K. The defects may be indicative of the formation of a ternary calcium iron oxide since there is no observed increase in the dispersion of calcium, which would be associated with a disintegration of the CaO crystals. It should be pointed out that the other structural promoter element, aluminum, exhibits the same spectral changes in its A12 emission. The dispersion of the alumina increases, however, with reduction of the catalyst, particularly when the wet reduction method is applied (see Table 2.1). 

It has been known for a long time that two ciystalline solids, one which of starts at the surface of the other, often show mutual orientation relationships this is the process of epitaxy. Studies such as thin layers of oxides formed by oxidation of a metal, for example, have shown such a process. Figure 11.6 shows the example of the schematic orientations of cubic iron oxide (FeO) formed on the 001 side of the cubic iron. The side of the iron oxide cube is 2.86 A. The parameter of FeO is 4.29 A. In the isolated crystal, FeO iron atoms are spaced at 4.29 / VI = 3.89 A intervals, resulting in a slight deformation of the iron oxide crystal. If the layer of FeO is thick, the defect fades away from the interface the oxide has found its own structure. 

For both NeuCa and OxiCa calcined at 850 and 1100°C, iron is detected by XRD only as Ca2Fe20s phase. Iron in the (2+, 2.5+) oxidation state, as well as free Fe20s iron oxides, are not detected. This evidence may be due to the possibility of brownmillerite structure to stabilize the oxygen defects [Hirabayashi et al, 2006] and to allow high oxygen mobility. These data are confirmed by Mdssbauer analysis, not shown here. 

On the other hand, pit initiation which is the necessary precursor to propagation, is less well understood but is probably far more dependent on metallurgical structure. A detailed discussion of pit initiation is beyond the scope of this section. The two most widely accepted models are, however, as follows. Heine, etal. suggest that pit initiation on aluminium alloys occurs when chloride ions penetrate the passive oxide film by diffusion via lattice defects. McBee and Kruger indicate that this mechanism may also be applicable to pit initiation on iron. On the other hand, Evans has suggested that a pit initiates at a point on the surface where the rate of metal dissolution is momentarily high, with the result that more aggressive anions... 

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