Heterogeneous catalysis is when the catalyst and reactants are in different phases, usually a solid catalyst with gaseous or liquid reactants. The reaction occurs at the surface of the catalyst, and it involves several key steps:
Steps in Heterogeneous Catalysis
- Adsorption of reactants:
- Reactant molecules attach to the surface of the solid catalyst.
- There are two types:
- Physisorption: Weak van der Waals forces, reversible.
- Chemisorption: Strong chemical bonds, may be irreversible.
- Activation of reactants:
- The catalyst surface weakens bonds in the reactant molecules, making them more reactive.
- This reduces the activation energy of the reaction.
- Surface reaction:
- Adsorbed molecules react with each other on the catalyst surface to form intermediate species.
- The reaction occurs more easily because the molecules are held in the correct orientation.
- Formation of products:
- The intermediates transform into final product molecules while still on the catalyst surface.
- Desorption of products:
- Product molecules leave the catalyst surface.
- This frees the active sites for new reactant molecules to bind.
Example:
- Haber process: Ammonia synthesis uses an iron catalyst. Nitrogen and hydrogen gases adsorb on iron, react on the surface, and ammonia desorbs.
Key idea: The catalyst is never consumed; it only provides a surface where reactants can meet and react more efficiently.