Wiistite-based catalyst

Magnetite based catalyst Wiistite based catalyst... 

The effect of carbon monoxide on the activities of wiistite and magnetite-based catalyst is shown in Fig. 1.21. When carbon monoxide is introduced into reaction system, the drop of activity for wiistite catalyst is less than that of magnetite-based catalyst (Fig. 1.21, AB section). Once carbon monoxide is removed from reaction system, wiistite catalyst recovers its activity more quickly than that of magnetite-based catalyst (Fig. 1.21, BC section). It can be concluded that the sensitivity of wiistite-based catalyst to CO poisoning is lower than magnetite-based catalyst. 

The basic technical characteristics of the wiistite-based catalyst (A301 and ZA-5) for ammonia synthesis are high activity at low-temperatme, and easy reduction. The following results could be obtained by comparison with the magnetite-based catalyst under the same conditions. 

Easy reduction. Its intrinsic reduction rate is 4.5 times that of the magnetite-based catalyst. Reduction temperatme of wiistite-based catalyst is lower by about 80°C 100°C than that of the magnetite-based catalyst, and the terminative... 

Table 1.14 presents comparison of the activity between wiistite-based catalyst (A301) and Ru-based catalyst. It is seen from Table 1.14, that ammonia concentrations in the outlet of reactor of A301 catalyst are the same as Ru-based catalyst under same pressures, temperatures and similar space velocities. 

Since 1992, the wiistite-based catalysts including ASOland ZA-5 have been widely applied in the world. Up to 2008, more than 20,000 tons of catalysts have been used by thousands of ammonia plants in China and all over the world. These catalysts can be used in plants with different capacity (from 25,000 tons per year to 450 thousands tons per year) and diameters of converters from d)600mm to 63,000 mm (Table 1.16). 

According to survey data from ammonia plants, compared with other catalysts, the industrial application of the wiistite-based catalyst reduces the reduction time, decreases the operation temperature and pressure, increases the net ammonia concentration and production capacity, reduce the energy consumption. Compared with the Ru/C catalyst, the wiistite-based catalyst has the advantages of cheap raw material, low cost of production and comparative high activity at low temperatures. Thus, it is promising for Further development and application. 

In summary, it seems that the structure-sensitivity and the surface reconstruction induced by nitrogen are due to the availability or to the creation of more active sites that induce C7 atoms. The authors found that the activity increase of wiistite-based catalyst, after reduction, also showed that there are more active sites including C7 atom. ... 

The result of simulation design calculation shows that it is feasible to replace AllO by ZA-5 with high activity in the synthesis reactors used originally for AllO catalyst.However, it should be noted that ZA-5 is a new wiistite-based catalyst operating at low-temperatures. Its superiority can be better if it is used at low-temperatures. Therefore, it is not necessary to use it at high-temperatures at... 

Chemical reaction. The main component of wiistite-based catalyst such as A301, ZA-5 etc. is Fei xO. The reduction reaction is as follows ... 

The author studied the reduction kinetics of A301, ZA-5 wiistite-based catalysts. A large number of experimental data confirm the applicability of SCM and its expressions. The following five experiments have been done in our lab. 

Discovery of Wiistite Based Fused Iron Catalysts... 

In 1986, Liu et at found that the iron catalyst with wiistite as the precursor has extremely high ammonia synthesis activity and rapid reduction rate, which led to the invention of wiistite (Fei xO) based catalyst for ammonia synthesis. The relationship between the activity and the iron oxides (Fe304, FeO and Fe203) and their mixtures in the precursor were studied systematically, and a hump type curve was found between the activity and the ratio (Fe +/Fe +). It was speculated that the monophase of iron oxide phase in the precursor is an essential condition for high activity of the catalyst and a uniform distribution of iron oxide phase and promoters is a key to make a better performance of catalyst. The hump type curve was interpreted by the ratio of phase compositions in the precursor, that is, the activity change of the fused iron catalyst depends essentially on the molecule ratio of different iron oxides but not on the atomic ratio of Fe + and Fe +, or Fe +/Fe +, in the precursor under certain promoters. Thus we found that Fei xO based catalyst with wiistite phase structure (Fei xO, 0.04 < x < 0.10) for ammonia synthesis has the highest activity among all the fused iron catalysts for ammonia synthesis. 

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. 

Table 1.10 shows a comparison between the wiistite based and the magnetite based catalyst. It is shown that the wiistite (Fei xO) based catalyst is a new generation of ammonia synthesis catalyst that is completely different from the magnetite (Fe304) based catalyst (including Fe-Co catalyst) in the chemical composition, crystal structure, physical-chemical property, and producing principle etc. 

TPR patterns of the Fei xO based catalyst and the magnetite based catalyst are shown in Fig. 1.19. It can be seen that the reduction peak of wiistite catalyst is shifted towards lower temperatures, thus confirming the advantage of this catalyst as to shorter reduction period in the industrial reactor. This result is in agreement with the better reducibility of wiistite with respect to magnetite. ... 

Table 1.14 Comparison of the activity of wiistite-based and Ru-based catalyst...

Without noble metals in the composition of the catalyst. The production cost of wiistite catalyst is much lower than that of cobalt-containing magnetite-based catalyst and Ru/AC catalysts. 

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. 

Among all catalysts with the iron oxides and their mixtures as precursor studied, Fei xO based catalyst with nonstoichiometric and wiistite structure has the fastest reduction rate and lowest reduction temperature. In a wiistite structure, large amounts of defects are iron ions, which enable the diffusion of Fe in oxide lattices, and will be preferable to electron transferences. This is the structural factor for the easy reduction of Fei xO based catalysts. 

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. 

In wiistite structure, a large number of defects are iron ions, which allow the diffusion of iron into the lattice, and it is beneficial for the transfer of electrons. This is the structural reason that why Fei xO-based catalyst is easy to be reduced. 

In order to investigate the catalytic activity of Ru catalysts, and compare with iron catalyst, we choose the representative iron catalyst A301 with wiistite as precursor as the reference sample. A301 has the highest activity among all of the iron-based catalysts for ammonia synthesis and now it has been widely used in ammonia synthesis industry. In order to get the reliable and comparable data of the evaluation of catalytic activity, the experiment was conducted under the same conditions and four samples were filled in four reactor contained in one shell. The results were shown in Table 6.41 and Figs. 6.56-6.58. 

---