Lattice magnetite, substitution

In the proposed two-step process, it is important to attain high efficiencies for conversion of coal to COx (CO -I- CO2) in the first-step reaction and then for conversion of CO2 to CO in the second-step reaction by an external heat input. From the thermodynamic conditions and the low cost, the redox pair of Fe304/a-Fe was one of the promising redox systems for the two-step process, but it still required the operating temperature above 1200°C[2]. It is well known that many kinds of metal ions can be incorporated into the spinel lattice structure of magnetite by replacing ferrous or ferric ions. There is the possibility that metal-substitution for Fe or Fe " in magnetite causes a phase transition to the metallic phase, which proceeds readily even at low temperatures and improves the conversion efficiencies of coal and CO2 to CO in the two-step process. 

The solid state structure of magnetite, a spinel(2.), contains iron cations in two different oxidation states (Fe " and Fe ) and in two lattice sites of different coordination (octahedral and tetrahedral) therefore, the catalytic surface of this material may be expected to provide a variety of possible sites capable of acting as adsorption or reaction centers. Also, it has been demonstrated that substitution of other cations for iron can significantly alter the catalytic activity for WGS (4,5). 

In another study, the effect of silica incorporation into the Fe O lattice was studied (5,49,50) A 20% Fe O on silica catalyst was prepared using conventional techniques. Ir was found that while direct oxidation of the catalyst at 800 K produced the expected a -catalyst was previously reduced in CO/CO2 to produce magnetite, then subsequent oxidation resulted in the formation of y-Fe203 Figure 10 shows MOssbauer spectra of this catalyst after various thermal treatments. In these spectra, the central doublets were demonstrated to be a result of small iron oxide particles which were superparamagnetic at the conditions where the spectrum was recorded. The suppression of the y - oOg to a -Fe203 transition is characteristic of the substitution of roreign cations into the mag-... 

When a series of silica supported-magnetite catalysts of varying iron oxide particle size were investigated, it was determined that Si substitutes into the magnetite lattice according to the following reaction(49) ... 

The effect of Si substitution on the turnover frequency for WGS is shown in Figure 11. The turnover frequencies plotted in this figure were based on the magnetite surface area as determined by the NO chemisorption technique. The turnover frequencies shown for unsupported Fe O indicate that the factor of 10 decline in activity for the silica-supported catalysts is not a particle size effect, but instead is a consequence of the substitution of Si into the lattice. However, when the adsorption of CO/COo at 663 K was used to titrate the surface sites instead of NO, the resulting turnover frequencies were essentially constant as shown in Figure 12. Accordingly, the CO/CO2 mixture apparently titrates the sites active for WGS. Clearly, the number of active sites is decreased markedly as the particle size decreases in the silica-substituted magnetite catalysts. 

Magnetite possesses an inverse spinel structure with oxygen ions forming a face-centred cubic closely packed structure. The formula for describing Fe occupancy is (Fe " ) [Fe ", Fe ]04 where the parentheses ( ) stand for cations at tetrahedral sites while brackets [ ] denote cations at octahedral lattice sites. Stoichiometric magnetite has all available substitutional sites occupied by Fe and Fe ions. Non-stoichiometric magnetites also exist, with various numbers of available sites being either vacant or occupied by impurity ions. 

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