Magnetite thermal stability

Ammonia synthesis catalysts have traditionally been based on iron and have been made by the reduction of magnetite (Fe304). The difference between different commercially available products lies in optimized levels of metal oxide promoters that are included within the magnetite structure. These metal oxides promote activity and improve the thermal stability of the catalyst. Typical promoters are alumina (AI2O3X potassium oxide (K2O), and calcium oxide (CaO). The interactions between the many components in the catalyst can radically affect 1) the initial reducibility, 2) the level of catalyst activity that is achieved, 3) the long-term catalyst performance and 4) the long-term catalyst stability204. 

Thermal Stability It is known that corrosion inhibitor effectiveness varies with temperature. This can be due to chemical degradation, decreased adsorption at higher temperature, or changes in surface properties (e.g., iron carbonate converts to magnetite at higher temperature). 

Figure 1.20 compares the thermal-stability of three catalysts. It is found that the activity of Fei xO-based catalyst almost remains the same following operation at 500°C for 20h, indicating that the Fei xO-based catalyst exhibits similar thermal stability as the magnetite based catalysts. 

Here, 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... 

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