26.2 Homogeneous and Heterogeneous Catalysts
🎯 Learning outcomes
Candidates should be able to:
- Explain that catalysts can be homogeneous or heterogeneous
- Describe the mode of action of a heterogeneous catalyst, including adsorption of reactants, bond weakening and desorption of products, for example:
- (a) iron in the Haber process
- (b) palladium, platinum and rhodium in the catalytic removal of oxides of nitrogen from car exhaust gases
- Describe the mode of action of a homogeneous catalyst as being used in one step and reformed in a later step, for example:
- (a) atmospheric oxides of nitrogen in the oxidation of atmospheric sulfur dioxide
- (b) Fe²⁺ or Fe³⁺ in the I⁻/S₂O₈²⁻ reaction
🧪 Homogeneous catalysts
Example: hydrogen ions catalyse the hydrolysis of esters (the reverse of esterification, shown below). Reactants, products and catalyst are all in the aqueous phase.
Atmospheric NOₓ and sulfur dioxide
The iodide–peroxydisulfate(VI) reaction
Iodide reacts slowly with aqueous peroxydisulfate(VI) ions to give iodine and sulfate ions:
Transition-metal cations such as Fe³⁺(aq) catalyse it by providing two faster steps:
⚗️ Heterogeneous catalysts
Example: the decomposition of aqueous hydrogen peroxide is catalysed by solid manganese(IV) oxide.
🏭 The Haber process
Iron is the heterogeneous catalyst. It is promoted with small amounts of potassium oxide and aluminium oxide. The iron is a solid and the reactants are gases, so they are in different phases.
Finely divided iron has a large surface area, giving more active sites where reactant molecules can be adsorbed.
- N₂ and H₂ are adsorbed onto the iron surface.
- Their bonds are weakened, so the atoms react more easily.
- NH₃ forms on the surface and desorbs.
🔬 How a heterogeneous catalyst works
- Adsorption: reactant molecules attach to the catalyst surface.
- Bond weakening: bonds in the adsorbed molecules become weaker.
- Reaction: the reactants react more easily on the surface.
- Desorption: products leave the surface, freeing the active sites.
🚗 Catalytic converter
Platinum, palladium and rhodium are used as catalysts in catalytic converters. Exhaust gases pass over the catalyst surface, where nitrogen oxides are adsorbed. Their bonds are weakened and reactions occur on the surface.
The nitrogen oxides react to form less harmful products, including N₂. These desorb from the surface, leaving the active sites free again.
📌 Exam tips & quick summary
| Feature | Homogeneous | Heterogeneous |
|---|---|---|
| Phase of catalyst | Same phase as reactants | Different phase from reactants |
| How it works | Participates in reaction steps and is regenerated | Reactants adsorb onto catalyst surface |
| Key idea | Used → regenerated | Adsorb → react → desorb |
| Example | Fe²⁺/Fe³⁺ in solution | Fe in Haber process |