In the Eley-Rideal mechanism, a molecule from the gas phase (A) approaches the catalyst surface where another molecule (B) is already adsorbed. The gas-phase molecule then reacts directly with the adsorbed molecule to form the product (P), which subsequently desorbs from the surface. This model contrasts with the Langmuir-Hinshelwood model, where both reactants need to be adsorbed on the surface before reacting.
