Catalyst magnetite ammonia synthesis

The early development of catalysts for ammonia synthesis was based on iron catalysts prepared by fusion of magnetite with small amounts of promoters. However, Ozaki et al. [52] showed several years ago that carbon-supported alkali metal-promoted ruthenium catalysts exhibited a 10-fold increase in catalytic activity over conventional iron catalysts under the same conditions. In this way, great effort has been devoted during recent years to the development of a commercially suitable ruthenium-based catalyst, for which carbon support seems to be most promising. The characteristics of the carbon surface, the type of carbon material, and the presence of promoters are the variables that have been studied most extensively. 

NH3 and Derivatives. As part of a systematic study of properties and functions of catalysts for ammonia synthesis, correlations have been sought between the promoter addition procedure and the physicochemical properties and activity of catalysts obtained by the fusion of natural magnetite. The results show that the manner of addition of the promoter affects, to a large extent, the investigated properties and catalyst activity. 

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. 

Preparation of an iron-based catalyst for ammonia synthesis by the rapid cooling of a magnetite/metal oxide promoter mixture. N. Pemicone, F. Ferrero, and A. Gennaro (Fertimont SPA). EP174716 (1986) US 4789657 (1988). 

Catalysts for ammonia synthesis are made by mixing natural or synthetic magnetite and promoters, and melting the mixture in an electric furnace. The cooled mass is broken up, sized, and in some cases pre-reduced before charging to the converter. The particle size is 2 to 10 mm, and the bulk density is in the range of 2.5 to 2.9 kg/liter. A typical chemical composition is as follows 68.4 percent total Fe, 3.16 percent AI2O3, 0.56 percent MgO, 0.50 percent Si02, 3.54 percent CaO, and 0.58 percent K2O. 

In contrast, numerous metal oxides—above all alumina (AI2O3) and magnesium oxide (MgO)—were identified as catalytic promoters. Moreover, combinations of several compounds may transform materials that by themselves have a neutral or even negative effect into promoters perhaps most notably, when combined with alumina, oxides of alkaline metals act as promoters. Appl marvels at how well Mittasch identified the optimal catalyst for ammonia synthesis for temperatures up to 530 °C and pressures up to 35 MPa. ° Most commercial catalysts on the market today are just slight variations on Mittasch s basic theme they use magnetite with 2.5-4% AI2O3, 0.5-f.2% K2O, 2.0-3.5% CaO, and 0-1.0% MgO (as well as 0.2-0.5% Si present as a natural impurity in the metal). ... 

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. 

The following discussion concentrates mainly on the ammonia synthesis reaction over iron catalysts and refers only briefly to reactions with non-iron catalysts. Iron catalysts which are generally used until today in commercial production units are composed in unreduced form of iron oxides (mainly magnetite) and a few percent of Al, Ca, and K other elements such as Mg and Si may also be present in small amounts. Activation is usually accomplished in situ by reduction with synthesis gas. Prereduced catalysts are also commercially available. 

Different reduction procedures apply if the catalyst is prereduced or when a combination of prereduced and unreduced catalyst is used. Whereas reduction of the bulk magnetite catalyst goes on over days, the reduction of the superficial oxidic layer of the prereduced catalyst is facile and may be accomplished within approximately one day if solely prereduced catalyst is charged. Often the first bed is charged with prereduced catalyst to enable fast reduction and onset of the ammonia synthesis reaction, which thereby liberates heat to support the endothermic reduction in the remaining part of the bed. 

The Haber-Bosch process has been known and used for over a century, and considerably little has changed over such a long period of time. In early work, Mittasch developed a highly active heterogeneous iron catalyst prepared from magnetite (Fe304) very similar catalyst formulations are still used in modern ammonia synthesis. It was also demonstrated that catalysts prepared from magnetite had superior catalytic activity in comparison to catalysts prepared from other iron oxides. 

Alwin Mittasch joined BASF in 1904 as a co-worker of Carl Bosch and started the search for suitable ammonia synthesis catalysts soon afterward. These efforts were considerably intensified after Haber s successful experiments but, at first, only with limited success. He mentioned (4) In particular iron failed, despite wide variations of the preparation conditions and admixtures. The breakthrough was obtained by accident A sample of Swedish magnetite left over from other experiments was investigated on November 6, 1909, by Mittasch s collaborator Dr. Wolf and exhibited remarkably high ammonia yields. The decisive patent application of January 9, 1910, says the following ... 

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. 

Since 1980s, ruthenium based catalysts discovered by British Petroleum of and Fei xO based catalyst developed by China had made new progresses on ammonia synthesis catalysts. Three technical routes were developed including the magnetite based (Fe3C4) route, Fei xC catalysts and Ru catalysts, and have achieved significant progresses, respectively. 

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. 

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. ... 

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