The reactions on the catalyst surface are assumed to be based on the Langmuir-Hinshelwood mechanism, and the ammonia synthesis reaction (N 2 + 3H 2 ⇌ 2NH 3 ) is divided into the following seven elementary reaction steps (step 1-7) . The * represents a surface site and the X * represents a reaction intermediate on the surface. If the surface coverage of X is expressed as 𝜃 𝑋 , the coverage of vacant sites 𝜃 𝑉 is defined as follows.