What is the role of ammonia catalysts in the activation of the ammonia synthesis reaction

In the absence of a catalyst, the ammonia synthesis reaction has a high activation energy of about 335 kJ/mol. upon addition of an iron catalyst, the reaction proceeds in two stages, the formation of nitrides and nitrogen hydrides. The activation energy of the reaction is 126kJ/mol,167kJ/mol for the first stage and 13kJ/mol for the second stage.Due to the change in the reaction pathway (generation of unstable intermediate compounds), the activation energy of the reaction is lowered and hence the rate is accelerated. The catalytic ability of a catalyst is generally referred to as catalytic activity. It has been thought that once a batch of catalysts has been made, they can be used forever because their chemical properties and quality remain unchanged before and after the reaction. In fact, many catalysts reach a normal level of activity from small to large during the process of use, which is the maturation period of the catalyst. Then, the catalyst activity remains stable for a period of time and then declines until it ages and can no longer be used. The period of time during which the activity remains stable is the lifetime of the catalyst, which varies according to the method of preparation and the conditions of use of the catalyst. During the period of stabilized activity, the catalyst is often exposed to a small amount of impurities and the activity decreases significantly or is even destroyed, this phenomenon is called poisoning of the catalyst. This phenomenon is called catalyst poisoning. It is generally believed that the poisoning is caused by the active center on the catalyst surface being occupied by impurities. Poisoning is divided into two kinds: temporary poisoning and permanent poisoning. For example, for the iron catalyst in ammonia synthesis reaction, O2, CO, CO2 and water vapor can make the catalyst poisoned. However, when using pure hydrogen and nitrogen gas mixture through the poisoned catalyst, the activity of the catalyst can be restored, so this poisoning is temporary poisoning. On the contrary, compounds containing P, S, can make the iron catalyst permanent poisoning. After the catalyst is poisoned, it often loses its activity completely, and even if it is treated with pure hydrogen and nitrogen gas mixture, the activity is difficult to recover. Catalyst poisoning will seriously affect the normal production. In order to prevent the poisoning of catalyst in the industry, the reactant material should be purified to remove the poison, which will increase the equipment and raise the cost. Therefore, the development of new catalysts with strong anti-poisoning ability is an important issue.

Ammonia is an important chemical raw material, ammonia industry in the national economy occupies an important position, so the ammonia process and catalyst improvement to reduce energy consumption, improve economic efficiency has a huge impact abroad.