For the case where N2 dissociation reaction (step 3) is rate-determining step (RDS), the following discussion is shown as an example. The case where steps 4-7 respectively are RDS is also discussed (see SI). When all reactions are reversible and one reaction step is the rate-determining reaction, the other reactions can reach equilibrium quickly from the quasi-equilibrium approximation. Therefore, 𝜃 𝑋 can be expressed as follows.