1. Adsorption: One of the reactants, say molecule A, adsorbs onto the catalyst surface. 2. Reaction: Molecule B, which is in the gas phase, directly collides with the adsorbed molecule A. 3. Desorption: The product molecules formed from the reaction between A and B desorb from the catalyst surface.
The stepwise process can be summarized as: \[ A_{ads} + B_{gas} \rightarrow AB + \text{surface} \]
