Catalysts magnetite catalyst precursor

The term ammonia catalyst commonly refers to the oxidic form consisting of magnetite and oxidic promoters. In fact this is only the catalyst precursor which is... 

The second difference between wiistite and magnetite precmsors is the roles of MgO. In Fei xO based catalysts, it is not only the cationic substitution of Fe + with Mg + ions, but also the fact that MgO can form a complete solid solution with FeO in the ranges of 0%-100%, and be well-dispersed into the catalyst precursor (Fei xO). This, to a certain extent, compensates the roles of AI2O3. 

The contents of the mixed structural promoters in iron oxides can determine the surface features, while the iron oxide precursors can influence the catalyst surface area, in order to perform the catalytic activity. Prom this point of view, the most optimum compositions of the structural promoters for wiistite catalysts are not identical to that of those magnetite catalysts. 

Pig. 7.31 Phase analysis of a technical catalyst precursor for ammonia synthesis K = KHC03, C = CaFe306, M = magnetite (Magnetite reflections are indexed in the upper diagram)... 

In summary, we see that the catalyst precursor consists of the dominant phases, magnetite and wustite, within small amounts of various ternary iron oxides and binary promoter additives. The crystallinity and texture of the samples differ widely. It is concluded that the initial solid state for the reduction process catalyst of activation is inhomogeneous for a given catalyst. It is further concluded that it is difficult to derive characteristic differences for a set of catalysts from XRD alone and also that very accurate quantitative chemical analyses are of limited value only, since they will not necessarily be representative for the catalyst charge as a whole. 

Turning to the macroscopic aspects of the reduction process, it has been noted that the porosity of the oxide, as defined by the percent of voids within a unit volume of magnetite, is a crucial parameter for the reducibility of an oxide. Reducibility may be defined as the time required to reduce a unit volume of iron oxide by 90% under standard conditions of temperature and gas flow. It is shown in Fig. 2.8a that the reducibility can vary over almost an order of magnitude with experimentally attainable variations in porosity. The shape of the curve suggests that two different reaction mechanisms for the extremes in porosity can exist. The catalyst precursors used in this study exhibit porosities below 5% when analyzed at room temperature. Thus, the following discussion concentrates on the mechanism appropriate to the low porosity extreme. 

Figure 2.9. The textural difference between magnetite and the catalyst precursor. The prereduction of wustite opens up voids and provides nucleation centers for the main reduction, which starts at ca 550 K. The promoter oxides prevent the crystallites of iron oxide from growing into large homogeneous and thus unreactive crystals.

Early catalysts were produced from calcined ferric oxide, potassium carbonate, a binder when required, and usually chromium oxide. Subsequently a wide range of other oxides replaced the chromium oxide typical compositions are shown in Table 7.5. The paste was extruded or granulated to produce a suitable shape and then calcined at a high temperature in the range 900°-950°C. Solid solutions of a-hematite and chromium oxide (the active catalyst precursors) were formed and these also contained potassium carbonate to inhibit coke formation. Catalyst surface area and pore volume were controlled by calcination conditions. It has been confirmed by X-ray diffraction studies that a-hematite is reduced to magnetite and that there is some combination of potash and the chromium oxide stabilizer. There is little change in the physical properties of the catalyst during reduction and subsequent operation. 

Dofour et al. [17] investigated the influence of synthesis method, precursor and effect of Cu addition on the WGS activity of Fe-Cr-Co catalysts. They prepared FeCr, FeCrCu, FeCrCo and FeCrCuCo formulations by oxidation precipitation method, using chloride (Cl) and sulphate (S) metal precursors. The catalytic activity results of FeCrCo and FeCrCuCo catalysts are presented in Figure 2.2. All the materials prepared from sulphate precursor showed higher carbon monoxide conversion than those synthesized with chloride. As expected Cu-promoted catalysts show better activity than Fe-Cr-Co catalysts. For the catalysts synthesized by chloride precursor, in the case of cobalt, incorporation of this metal into the magnetite lattice could improve the covalency of Fe and... 

In the past century, it was commonly believed that a catalyst has the best activity when its chemical composition and crystal structure of the precursor are most similar to those of magnetite. The relationship between the activity and the ratio... 

Although the precursor of wiistite based and magnetite based catalyst are different, their active states are the same i.e., a-Fe as shown in Fig. 1.12. 

The above-mentioned experimental results indicated that, in the ranges of Fe +/Fe + < 1 i.e., the first peak, the relation between catalytic activity and the Fe +/Fe + ratio is consistent with those results of traditional catalysts, in which the precursor is magnetite phases (Fig. 3.27). The facts of the decreasing activity with increasing of Fe +/Fe + ratio from 0.5 to 1, also coincides well with the results... 

Thereout, it is found that the catalyst has the highest activity among all the fused iron catalysts for ammonia synthesis when its chemical composition and crystal structure of the precmsor are those of wiistite (Fei xO)- It is called Fei xO or wiistite based ammonia sjmthesis catalysts, where the defect concentration x of iron ion is 0.04 experimental results break through the classical conclusion that lasted for more than 80 years, namely the catalyst has the best activity when its chemical composition and crystal structure of the precursor are most close to those of magnetite. It also provides a new approach for a novelcat-alytic system — wiistite Fei xO system for improving the performances of the fused iron catalysts. 

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