Heterogeneous Catalysis

Heterogeneous catalysis happens when the catalyst and reactants exist in different phases.

Common catalysts include metals, metal oxides, and acids that form bonds with chemisorbed species.

The process typically involves the chemisorption of one or more reactants, their transformation into a reactive form, followed by the desorption of products.

In many cases, the presence of a promoter enhances the catalytic activity by preventing the formation of large crystals on the catalyst surface, preserving the catalytic surface area. On the other hand, poisons bind strongly to active sites and inhibit the catalyst.

Additionally, the effectiveness of a catalyst depends on the enthalpy of adsorption of the reactants. If it is too low, the reactants do not bind effectively; if it is too high, they bind too tightly and cannot react easily.

Reactions on the catalyst's surface follow either the Langmuir-Hinshelwood or the Eley-Rideal mechanism.

The LH mechanism involves interactions between adsorbed molecules, while the ER mechanism involves a gas-phase molecule reacting with an already adsorbed molecule.