Magnetite-based catalyst

In principle, metals or metal alloys are suitable as ammonia catalysts, above all those from the transition-metal group [233] (Table 14). Metals or metal compounds for which the chemisorption energy of nitrogen is neither too high nor too low show the greatest effectiveness (Figs. 14, 15), [234], [235], but only the magnetite-based catalyst proved suitable for industrial use. 

Water Gas Shift over Magnetite-Based Catalysts... 

Unfortunately, at present, while the bulk structure of magnetite is well understood, little is known about the surface structure of magnetite-based catalysts. In general, the surface may be nonstoichiometric (FeoO ) with iron cations in both octahedral and tetrahedral sites ( ). 

In view of the above discussion, the following qualitative model may be suggested to describe the regenerative mechanism over magnetite-based catalysts ... 

This review has focused on recent research directed toward characterization of the active sites for water-gas shift over magnetite-based catalysts. The reaction can be described by a regenerative mechanism wherein gas phase or weakly adsorbed CO reduces anion sites and steam oxidizes the resultant surface oxygen vacancies. Kinetic relaxation techniques indicate this to be a primary pathway. The sites which participate in this reaction comprise only about 10% of the BET monolayer, and these sites can be titrated using CO/CO2 adsorption at 663 K. In contrast, the total cation site density is effectively titrated with NO at 273 K. In fact, the ratio of the extent of CO/CO2 adsorption to the extent of NO adsorption provides a measure of the fraction of the magnetite surface which is active for water-gas shift. 

In 1986, Zhejiang University of Technology made an important breakthrough on iron catalyst, invented a novel Fei j 0 based catalyst system.In 1992, the first Fei a 0 based catalyst (A301) at low temperatures and pressures was successfully developed, which was superior to the best magnetite-based catalysts in the world. In 1998, they further developed ZA-5 catalyst, and the running temperature was further decreased, which established the technical foundation for low pressure ammonia synthesis process. 

The results mentioned above are in agreement with the Mossbauer spectroscopy of these samples shown in Fig. 1.11(a). The Mossbauer spectroscopy of magnetite based catalyst consists of two typical hexa-finger peaks of Fe304 as in Fig. 1.11(b), and that of Fei xO catalyst consists of one t3rpical diss3unmetrical double peak of Fei xO as Fig. 1.11(a) presents. 

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 chemical composition, crystal phases and some structure parameters of Fei xO catalyst and magnetite based catalyst are listed in Tables 1.7-1.9. 

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. 

Table 1.10 Comparison of wiistite based and magnetite based catalyst...

The effects of temperatme, pressme and space velocity on the activity of Fei xO based catalyst and magnetite based catalyst are shown in Figs. 1.13-1.15. 

From Fig. 1.13, it can be seen that the activity of Fei xO based catalyst is much higher than those of the magnetite based catalysts. For example, ammonia concentration reached about 19.15% over the Fei xO based catalyst imder 15 MPa,... 

In the temperature range of 400°C-460°C, typical of a modern low-pressure ammonia synthesis unit, the reaction rate of Fei xO based catalyst is, on the average, 70% higher than that of the magnetite-based catalyst. ... 

In addition, it is seen from Fig.1.13 that the activity of Fei xO based catalyst at 400°C equals to the highest activity of magnetite based catalyst at 455°C, i.e.. 

The activity of Fei xO based catalyst at different pressures is shown in Fig. 1.14. It can be seen that the activity of Fei xO based catalyst is the highest at both high and low pressures. This kind of catalyst could be used in a wide range of operating pressures. The reaction pressure may be decreased by more than 3.5 MPa as compared to the magnetite based catalyst with the same ammonia yield. 

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. ... 

The initial reduction temperatures of the Fei xO-based and magnetite based catalysts are almost same, and 344.9°C and 348°C, respectively. 

The terminal reduction temperature of Fei xO-based catalyst is 490°C, which is 147°C lower than 637°C of the magnetite based catalyst, indicating that Fei xO-based catalyst has a much lower reduction temperature. The reduction degrees of Fei xO based catalyst and the magnetite based catalyst are 98.89%... 

The reduction rate of the Fei xO based catalyst is 4.5 times higher than that of the magnetite based catalyst. 

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. 

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. 

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