Iron cation ordering

Mossbauer spectroscopy is a potentially very useful way to study the cation ordering in LDHs containing iron [293-295], although care must be taken in order to avoid misinterpreting the results as has often happened... 

Ganguly J. and Stimpfl M. (2000) Cation ordering in orthopyroxenes from two stony-iron meteorites implications for cooling rates and metal-silicate mixing. Geochim. Cosmo-chim. Acta 64, 1291-1297. 

In a recent work we were able to show that an electronic effect was detected between Bi2Mo30i2 and a mixed iron and cobalt molybdate with an enhancement of the electrical conductivity of the cobalt molybdate with the substitution of the cobaltous ions by the ferrous ions (7). However this effect alone cannot explain the synergy effect and we have investigated the influence of both the de ee of subtitution of the cobalt with the iron cations in the cobalt molybdate and the ratio of the two phases (for a given substituted cobalt molybdate) on the catalytic propert cs of the mixture.We have tried to characterize by XPS and EDX-STEM the catalysts before and after the catalytic reaction in order to detect a possible transformation of the solid. The results obtained are presented and discussed in this study. 

Magnetite exists in the spinel structure which can be represented by the formula (Fe " ") [Fe ,Fe " ]0, where the parentheses denote cations in tetrahedral lattice sites, and the brackets denote cations in octahedral lattice sites (J ). Figure 1 is a representation of the idealized spinel structure (note that the structure has been extended in the [001] direction for clarity). The oxygen anions form a cubic close-packed framework in which there are 2 tetrahedral vacancies and 1 octahedral vacancy per oxygen anion. From the above formula, it can be seen that one-eighth of the tetrahedral sites and one-half of the octahedral sites are occupied by iron cations. The ordered occupation of octahedral sites shown in Figure 1 facilitates electron hopping between ferrous and ferric cations at temperatures above 119 K( ). As a result, the oxidation state of these octahedral cations can be considered to be +2.5. 

For the linear type, a formalism found commonly in the recent literature regards the NO as an NO+ ion. For example, in the well-known nitroprusside ion [Fe(CN)5NO]2-, the metal is assigned an oxidation number of +2, which implies that it has a d6 electron configuration. While this is a plausible value, Mossbauer and other physical evidence5613 indicates that the effective oxidation number is closer to + 3. This is the number obtained if NO is considered to interact as an essentially neutral ligand. The idea that NO first transfers its n electron to iron in order to become NO+, that this cation then serves as an electron-donor, and that finally dn electron density is then transferred back to the very n orbitals from which an electron was initially removed seems to be an unnecessarily complicated way of describing the net electron distribution. Its only virtue seems to be that in this and certain other cases it leads to an intuitively satisfactory oxidation number for the metal. However, there are many cases where it leads to intuitively unsatisfactory oxidation numbers, viz., in Fe(NO)2(CO)2 or Co(NO)3 where oxidation numbers of — 2 and — 3 are obtained. 

Because solid solutions exist for many minerals, let us briefly digress and consider a very small sample of olivine that contains 100 silicate ions, 118 magnesium cations, and 82 iron cations. (Notice that the number of 2+ cations must be twice the number of 4-anions, in order to achieve electrical neutrality.) The empirical formula of this sample is... 

One of the most important aspects of nanoparticles in biomedical applications is their surface functionalization in order to improve their biocompatibility with biological entities, and Fourier infrared spectroscopy (FTIR) is very useful technique that provides information about iron oxides in their ground electronic state, and when this material is bonding with a polymeric coating provides information about mechanism of functionalized magnetic nanopartides. This technique is widely used in characterization nanopartides due to its simplicity and availability. In magnetite structure it provides information about the excitation of vibration or rotation of the trivalent and divalent iron cations and allows knowing the occupied sites when the divalent iron is replaced with other cations. 

Mossbauer spectroscopy in solid state chemistry under in situ conditions at high temperatures and at defined oxygen partial pressures have been made by Becker (2001) for order-disorder processes and their kinetics in nickel alu-minate spinel and magnetite. The study has also been carried out for heterogeneous solid olid and solid-gas reactions which relate to the formation of cobalt aluminate spinel and to redox processes in fayalite, Fe2Si04, respectively. The diffusion of the iron cations in Fc2Si04 has also been observed by means of Mossbauer spectroscopy. 

Ordered mesoporous carbon (CMK-3) has been used as a template to prepare magnetic nanocomposite materials. Wet impregnation of CMK-3 with iron nitrate solution, followed by evaporation of solvent, exposure to acetic add vapour and a sintering step, results in the formation of a hybrid magnetic material. The acetic acid will react with the iron cations dispersed... 

Reaction CT-2 Is rate determining for the MDR at potentials slightly less than E. The studies performed indicate that reaction CT-2 exhibits a reaction order of zero with respect to protons. For reaction CT-2 at 25°C, the relationship between the logarithm of flux and potential is linear with a slope of 170 10 mV.decade". The activation energy of reaction CT-2 was found to be 69 kJ.moM at a potential of -300 mV vs SCE. As discussed previously, this charge-transfer is believed to be a ion-transfer reaction involving the dissolution of the iron cations. The simplest representation of these reactions are given in equations (5) and (6)... 

In the presence of certain cations [sodium, potassium, lithium, calcium, aluminium, chromium, and iron(III)], co-precipitation of the sulphates of these metals occurs, and the results will accordingly be low. This error cannot be entirely avoided except by the removal of the interfering ions. Aluminium, chromium, and iron may be removed by precipitation, and the influence of the other ions, if present, is reduced by considerably diluting the solution and by digesting the precipitate (Section 11.5). It must be pointed out that the general method of re-precipitation, in order to obtain a purer precipitate, cannot be employed, because no simple solvent (other than concentrated sulphuric acid) is available in which the precipitate may be easily dissolved. 

In addition to effects on the concentration of anions, the redox potential can affect the oxidation state and solubility of the metal ion directly. The most important examples of this are the dissolution of iron and manganese under reducing conditions. The oxidized forms of these elements (Fe(III) and Mn(IV)) form very insoluble oxides and hydroxides, while the reduced forms (Fe(II) and Mn(II)) are orders of magnitude more soluble (in the absence of S( — II)). The oxidation or reduction of the metals, which can occur fairly rapidly at oxic-anoxic interfaces, has an important "domino" effect on the distribution of many other metals in the system due to the importance of iron and manganese oxides in adsorption reactions. In an interesting example of this, it has been suggested that arsenate accumulates in the upper, oxidized layers of some sediments by diffusion of As(III), Fe(II), and Mn(II) from the deeper, reduced zones. In the aerobic zone, the cations are oxidized by oxygen, and precipitate. The solids can then oxidize, as As(III) to As(V), which is subsequently immobilized by sorption onto other Fe or Mn oxyhydroxide particles (Takamatsu et al, 1985). 

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