What are the main components of ammonia synthesis catalysts and what is the role of promoters?

Before reduction, it is mainly composed of Fe3O4, Al2O3, K2O, and CaO. It appears as a grayish-black solid with a metallic luster, in the form of amorphous particles; it is magnetic, with a particle size of 1.5–3.0 mm. Among them, Al2O3 is the carrier, while K2O and CaO are catalyst promoters

Before reduction, the ammonia synthesis catalyst exists in the form of iron oxides, whose main components are ferric oxide (Fe2O3) and ferrous oxide (FeO). In addition, various accelerants are also added to the catalyst. (1) Composition of iron oxide: The composition of iron oxide before reduction has a significant impact on the activity of the ammonia-catalyzed reduced product. According to the test results, the catalyst exhibits the best activity after reduction when the ratio of ferrous iron to ferric iron is equal to or close to 0.5. (2) Composition and function of accelerators: Accelerators, also known as catalyst promoters, have no activity on their own, but when added to a catalyst they can improve its physical structure, thereby enhancing the catalyst’s activity. Common promoters used in ammonia synthesis catalysts include aluminum oxide (Al2O3), potassium oxide (K2O), and calcium oxide (CaO), among others. When aluminum oxide is added to ammonia synthesis catalysts, it can form a solid solution with iron oxide. When the iron catalyst is reduced, iron oxide is reduced to active iron, while aluminum oxide remains unreduced and acts as a scaffold, thereby preventing the growth of fine iron crystals, increasing the catalyst’s surface area, and enhancing its activity. The addition of potassium oxide to the catalyst facilitates the active adsorption of nitrogen, thereby increasing the catalyst’s activity; it also reduces the adsorption of ammonia by aluminum oxide in the catalyst.

What are the main components of ammonia synthesis catalysts? Answer: The main components of the catalyst before reduction are ferrous oxide (FeO) and ferric oxide (Fe2O3), with ferrous oxide accounting for 24%–38%, and the Fe2+/Fe3+ ratio generally ranging from 0.47 to 0.57. The main component of the catalyst after reduction is sponge-like elemental iron. What are the components of the co-catalyst? Answer: What are the functions of the various components in alumina (Al2O3), magnesium oxide (MgO), potassium oxide (K2O), calcium oxide (CaO), and silicon dioxide (SiO2) catalysts? Answer: ① When the Fe2+/Fe3+ ratio is close to or equal to 0.5, the reduced form exhibits the best activity; at this point, the molar ratio of FeO/Fe2O3 is 1, which corresponds to the composition of Fe3O4. However, with the addition of promoters, the FeO/Fe2O3 ratio fluctuates between 24–38%, and this has little effect on the catalyst. On the other hand, the thermal stability and mechanical strength of the catalyst decrease as the content of low-valent iron decreases. ②Alumina (Al2O3): It can form a solid solution with Fe2O3; Fe2O3 is reduced while Al2O3 remains unreduced. It acts as a framework, preventing the growth of fine iron crystals, thereby increasing the specific surface area of the catalyst and enhancing its activity. However, the addition of Al2O3 slows down the reduction rate of the catalyst and makes it difficult for the ammonia formed on the catalyst surface to be released. ③Magnesium oxide (MgO): It acts in a similar manner to Al2O3 and is a structural promoter. ④Potassium oxide (K2O): an electron-type promoter that facilitates the active adsorption of nitrogen, thereby enhancing the catalyst’s activity and reducing the adsorption of ammonia by Al2O3 during catalysis. ⑤Calcium oxide (CaO): Reduces the melting point and viscosity of the melt, and improves the thermal stability and resistance to toxicity of the catalyst. It helps with the uniform distribution of Al2O3. ⑥Silica (SiO2): It serves to neutralize basic components such as K2O and CaO, while also enhancing the catalyst’s resistance to water damage and sintering.