Iron oxides and reduction performance of catalysts

The simple overall reduction reaction is a gas olid interface reaction. 

The product iron is much more dense as the educt magnetite. The metal forms after the initial reduction of a layer around the core of oxide and rapidly blocks the gas exchange from the outer gas-solid interface with the bulk. For this reason the mechanism cannot be based upon the removal of oxide ions from the lattice but requires the iron to move to a reaction interface at the gas-solid boundary. 

The mechanism is considered to involve diffusion of ions and electrons and to contain two types of sites. One type allows the oxidation of oxide ions and the other type generates diffusion of iron ions and their reduction. The overall processes are  

two conclusions are important from this mechanism derived from metallurgical samples of magnetite/wiistite synthetic mixtures. First, the elemental iron is essential to reduce magnetite with hydrogen gas at low temperature. This elemental iron is produced from thermal decomposition of the wiistite mixture in the precursor. Stability and bulk distribution of the wiistite determine the abundance of reaction interfaces in the polycrystalline solid. The grain structure and porosity of the final catalyst is mainly predetermined by the disposition of these reaction centers representing the nuclei of the iron metal crystallines. Second, the reaction involves movement of all iron ions and allows a complete bulk restructuring of the solid at low temperature. The topochemistry of the reduction process will determine 

The experimental results for catalysts with various iron oxides as precursors shown in Figs. 3.32-3.35 indicate that, when the catalyst precursor is composed of two of the different phases, temperature tm at the fastest reduction speed shows two of maximum values (Table 3.16) under the pure H2 atmosphere. They represent the reduction behaviors of FeO and Fes04, respectively according to the XRD measurements. Note that especially in Fig. 3.34 there are two tm values when the Fe +/Fe + ratios of samples are 0.89,1.10 and 2.16, respectively. This is because the precursors of the three samples are composed of two phases, e.g., FeO and Fe304 and both exist in independent phases. The reduction process is divided into several steps according to phases in order of FeO and then Fe304 i.e., FeO is reduced first 

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