Heterogeneous catalysis occurs when the catalyst and the reactant or reactants are in different physical states or phases. The adsorption theory of heterogeneous catalysis explains how catalysts work by utilizing their surface properties to enhance reaction rates.

According to the adsorption theory of heterogeneous catalysis, reactants in their gaseous or dissolved state get adsorbed onto the surface of a solid catalyst. This adsorption increases the concentration of reactants on the catalyst's surface, enhancing the probability of interaction between the reactants and increasing the reaction rate.

The adsorption process is exothermic, releasing some amount of energy as the reactants adhere to the catalyst’s surface. This released energy helps drive the reaction between the adsorbed reactant molecules, further increasing the reaction rate.

Heterogeneous catalysis occurs over 5 main steps:

• Diffusion of reactant molecules
• Adsorption of reactant molecules to the catalyst’s surface
• Formation of intermediate complex
• Breaking down of the intermediate complex and release of products
• Diffusion of product molecules