What is the reaction mechanism of ammonia synthesis catalysts?

A catalyst is a substance that accelerates the rate of a reaction without changing the substance itself. It induces a change in a chemical reaction, either by making the reaction faster or by performing the reaction at a lower temperature. In the presence of a catalyst, molecules require less energy to complete a chemical reaction.

Catalysts play an extremely important role in the modern chemical industry, and nowadays catalysts are used in the production of almost half of all chemical products. For example, iron catalysts are used in the production of ammonia, vanadium catalysts are used in the production of sulphuric acid, and different catalysts are used in the polymerisation of ethylene and in the production of the three main synthetic materials, such as butadiene rubber.

It is now believed that one possible mechanism for the ammonia synthesis is firstly the chemisorption of nitrogen molecules on the surface of the iron catalyst, which weakens the chemical bonds between the nitrogen atoms. Then the chemisorbed hydrogen atoms continuously interact with the nitrogen molecules on the surface, gradually generating -NH, -NH2 and NH3 on the surface of the catalyst, and finally the ammonia molecules are desorbed on the surface to generate gaseous ammonia. The above reaction pathway can be simply expressed as.

xFe + N2 → FexN

FexN + [H] suction → FexNH

FexNH + [H]suction → FexNH2

FexNH2 + [H]suction FexNH3xFe + NH3

In the absence of a catalyst, the activation energy of the ammonia synthesis reaction is high, about 335 kJ/mol.Upon addition of an iron catalyst, the reaction proceeds in two stages, the formation of a nitride and a nitrogen hydride. The activation energy of the reaction is 126 kJ/mol to 167 kJ/mol for the first stage and 13 kJ/mol for the second stage.The reaction rate is accelerated due to the change in the reaction pathway (generation of unstable intermediate compounds), which reduces the activation energy of the reaction.