Results & discussion. Model II is also important for the discussion of coverage, since it allows an accurate evaluation of 𝜃 NH x (x = 0, 1, 2, 3) by considering the reverse reaction in the RDS and the ammonia adsorption reaction. For example, if step 3 is the RDS, then 𝜃 NH x = 0 in Model I. The effect of these adsorbed species on app Ea and the reaction order cannot be understood. On the other hand, Model II allows us to know the effect of 𝜃 NH x on app Ea and reaction order, and also allows us to describe a more accurate model when fitting the reaction rate to the model equation.