Iron fused ammonia synthesis type catalyst

The fact that fused iron catalysts of the synthetic ammonia type were successively used in many investigations of hydrocarbon synthesis for both fluidized and fixed catalyst bed operations is of interest in different respects. Due to this fact it is possible to make use of the valuable experience obtained during development work of the ammonia synthesis (73). This applies to the reduction, the tendency to oxidize, and the effect of promoters and poisons, and to a certain extent also to questions regarding the reaction mechanism. 

Kummer et al. prepared the necessary carbided catalysts most of the studies were conducted with a promoted, fused, synthetic ammonia type catalyst containing 2.26 percent AI2O3, 0.62 percent Si02, and 0.21 percent Zr02 as promoters. CO chemisorption indicated that 35 percent of the catalyst surface was iron and 65 percent was promoter. Synthesis was affected at temperatures from 240 to 300 °C, 1 atm pressure and, in most cases, CO/H2 =1/1. Calculations for the fraction of the reaction via iron carbide as a... 

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. 

In order to reveal the intrinsic relation between the surface properties and textures with the hump-type activity curve, the specific surface area of both the ammonia synthesis fused iron catalyst with different iron oxides as precursors and their active components were measured by the means of low temperature physical adsorption of N2 and selective chemisorptions of CO, CO2 as shown in Table 3.17. 

Presently, the Fe-based ammonia synthesis catalysts used in industry are produced by traditional molten method, and are also known as fused iron catalysts. The advantages of molten method include simple processes, easy operation, low cost and excellent performance of product. Even though there are some differences in the chemical composition and catalytic performance of various types of fused iron catalysts, their manufacture process is mainly the same as follow. 12... 

The same type of fused iron catalyst may exhibit different structures and activities after reduction under different conditions (e.g., temperature, pressure, space velocity and gas composition etc.). Reduction condition is the external factor which affects the physical-chemical properties of catalysts. Thus, different reduction conditions are required for catalysts with different t3rpes, particle sizes or different types and content of promoters. The selection of the optimized reduction condition is very important to obtain a high performance for ammonia synthesis catalysts. It is the main reason to study the reductive performance and related kinetics of catalysts. 

At present, the two major types of ammonia synthesis catalyst are the fused iron catalyst and the ruthenium catalyst, but the fused iron catalyst is still the primary catalyst in use. A fused iron catalyst may be classified in several ways according to the operating temperature as medium-temperature and low-temperature according to the state before use as pre-reduction and oxidized state or according to shape as irregular shape and regular shape. The ruthenium catalyst is a low temperature catalyst, but is not widely used because of its high price. 

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