Iron oxide systems, ammonia synthesis

Reforming is completed in a secondary reformer, where air is added both to elevate the temperature by partial combustion of the gas stream and to produce the 3 1 H2 N2 ratio downstream of the shift converter as is required for ammonia synthesis. The water gas shift converter then produces more H2 from carbon monoxide and water. A low temperature shift process using a zinc—chromium—copper oxide catalyst has replaced the earlier iron oxide-catalyzed high temperature system. The majority of the CO2 is then removed. 

Figure 8. SEM surface images of partly crystallized sections of an activated Fe Zr alloy used for ammonia synthesis [23, 24J The main image reveals the formation of a stepped iron metal structure with a porous zirconium oxide spacer structure An almost ideal transport system for gases into the interior of the catalyst is created with a large metal-oxide interface which provides high thermal and chemical stability of this structure The edge contrast in the 200 keV backscatlered raw data image arises from the large difference in emissivity between metal and oxide It is evident that only fusion and segregation-crystallization can create such an interface structure.

Successful ammonia conversion required discovery of a catalyst, which would promote a sufficiently rapid reaction at 100-300 atm and 400-500°C to utilize the moderately favorable equilibrium obtained under these conditions. Without this, higher temperatures would be required to obtain sufficiently rapid rates, and the less favorable equilibrium at higher temperatures would necessitate higher pressures as well, in order to obtain an economic conversion to ammonia. The original synthesis experiments were conducted with an osmium catalyst. Haber later discovered that reduced magnetic iron oxide (Fe304) was much more effective, and that its activity could be further enhanced by the presence of the promoters alumina (AI2O3 3%) and potassium oxide (K2O 1%), probably from the introduction of iron lattice defects. Iron with various proprietary variations still forms the basis of all ammonia catalyst systems today. 

In 1920s, the studies on the catalysts for ammonia sjmthesis were performed sporadically in BASF, instead, the company mainly focused on the organic synthesis under high pressm-es and the new fields in heterogeneous catalysis. Dm-ing the development of ammonia synthesis catalysts, researchers provided valuable information about the dm-ability, thermal stability, sensitivity to poisons, and in particular to the concept of promoter. Mittasch smnmarized the roles of various additives as shown in Fig. 1.9. The hypothesis of successful catalyst is multi-component system proposed by Mittasch was confirmed to be very successful. Iron-chromium catalysts for water gas shift reaction, zinc hromium catalystfor methanol synthesis, bismuth iron catalysts for ammonia oxidation and iron/zinc/alkali catalysts for coal hydrogenation were successively developed in BASF laboratories. 

The ease of reduction and subsequent performance of the catalyst will be strongly influenced by the extent and nature of the interaction between the metal and the oxide support. In ammonia synthesis studies the most commonly used oxides have been alumina and magnesia. " Some work has also been carried out with basic materials such as calcia, etc., although they have little commercial potential due to their low surface areas. In the case of magnesia and alumina, high and reasonably stable dispersions can be produced but complete reduction is sometimes very difficult to achieve. In the case of coprecipitated iron/alumina the spinels formed can only be reduced at temperatures above 1073 The reducibility of the supported systems also tends to be retarded to a greater extent by low vapor pressures of water than less highly dispersed catalysts. ... 

The synthesis of uncoated and surface-modified silica particles via the hydrolysis of tetraethoxysilane (TEOS) in a homogeneous alcoholic solution of water and ammonia is well documented in the literature [1-7]. TEOS and other metal aUtoxides (including those of titanium and iron) can be hydrolyzed in reverse microemulsion systems for the production of gels or metal oxide particles [8-16]. Systems incorporating surfactant aggregates have also been exploited in the synthesis of a number of other ultrafine particulate materials [13,15,17-20]. 

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