Non-random distribution of adsorbates on catalytic surfaces: The role of adsorbate mobilities on reaction rates

An important step in any Langmuir-Hinshelwood process is the surface migration of reactants to a critical distance at which a reaction can occur with reasonable probability* It is usually assumed that once the reactants reach catalytic surface, they diffuse rapidly and the main barrier for the product formation on the surface is in the reaction step. In highly exothermic surface reactions, the barrier for the reaction step is often not very large. In such cases, the surface reaction may be diffusion-controlled. This gives rise to a non-random distribution of the adsorbates on the catalyst surface. A model to describe the chemical reaction under such conditions is proposed. The consequences of limited adsorbate mobility are studied through two examples. It is shown that limited adsorbate mobilities can significantly alter the actual reaction rate realized.