How to Synthesize Ammonia

In the ammonia synthesis process, pure hydrogen-nitrogen mixture is compressed to high pressure and, under the influence of a catalyst, synthesized into ammonia. This synthesis is crucial for producing liquid ammonia and forms the core of the entire ammonia production process. The synthesis reaction occurs at high pressure with ammonia synthesis catalysts, with ammonia content typically at 10%~20%. Due to the low ammonia content post-reaction, a loop process with unreacted hydrogen-nitrogen gas is employed.

The thermodynamics indicate that low temperature and high pressure favor the ammonia synthesis reaction. However, without a catalyst, the activation energy is high, and the reaction scarcely occurs. Iron catalysts change the reaction kinetics, reducing the activation energy, enabling a significant reaction rate. One possible mechanism involves nitrogen molecules chemically adsorbing on the iron catalyst's surface, weakening the chemical bonds between nitrogen atoms. Subsequently, chemically adsorbed hydrogen atoms react with surface nitrogen molecules, gradually forming -NH, -NH2, and NH3 on the catalyst surface, and ammonia molecules are desorbed to produce gaseous ammonia.

In the absence of a catalyst, the activation energy for ammonia synthesis is approximately 335 kJ/mol. With iron catalysts, the reaction occurs in two stages: the first stage has an activation energy of 126 kJ/mol to 167 kJ/mol, and the second stage's activation energy is 13 kJ/mol. Changes in the reaction pathway (formation of unstable intermediate compounds) reduce the activation energy, accelerating the reaction rate.

Catalytic ability is termed catalytic activity. Some believe that once a catalyst is made, its properties remain unchanged, allowing indefinite use. In reality, many catalysts mature, starting with low activity and gradually reaching a normal level; this is the catalyst's maturation period. After a stable period, the catalyst activity decreases until it is no longer usable, marking its aging. The time during which the activity remains stable is the catalyst's lifespan, which varies based on preparation methods and operating conditions.

During the stable activity period, catalysts often experience a significant drop in activity or even destruction due to contact with trace impurities, a phenomenon known as catalyst poisoning. Poisoning is categorized as temporary or permanent. For example, in iron catalysts for ammonia synthesis, O2, CO, CO2, and water vapor can temporarily poison the catalyst. However, using a pure hydrogen-nitrogen mixture can restore the catalyst's activity, making this poisoning temporary. Conversely, compounds containing P, S, and As can permanently poison iron catalysts. Once poisoned, the catalyst typically loses all activity, making it challenging to recover even with pure hydrogen-nitrogen gas. Catalyst poisoning severely disrupts normal production. To prevent poisoning, industrial practices involve purifying reactant materials, necessitating additional equipment and increasing costs. Consequently, developing new catalysts with strong resistance to poisoning is a vital research area.