To convert harmful exhaust gases into less harmful gases before they are released into the atmosphere.

Catalytic converts use platinum, rhodium, and palladium catalysts.

A catalyst that has the same physical state as the reactants.

For example, sulfuric acid is a homogeneous catalyst in the production of esters because sulfuric acid and the reactants (an alcohol and a carboxylic acid) are all liquids.

A catalyst that has a different physical state from the reactants.

For example, iron is a heterogeneous catalyst in the Haber process because iron is a solid and the reactants (N₂ and H₂) are gases.

1. Diffusion of N₂ and H₂ towards the iron catalyst
2. Adsorption of N₂ and H₂ onto the iron surface.
3. Formation of new bonds between N and H atoms, forming NH₃.
4. Desorption of NH₃ from the iron surface.
5. Diffusion of NH₃ away from the iron catalyst.

NO + CO ➔ 1⁄2N₂ + CO₂

1. Iron in the Haber process - heterogeneous.
2. Nitrogen oxides in the oxidation of sulfur dioxide - homogeneous.
3. Platinum in catalytic converters - heterogeneous.
4. Fe²⁺ ions in the iodine-peroxodisulfate reaction - homogeneous.

Fe³⁺ ions catalyse this reaction by participating in two redox reactions:

1. Reduction of Fe³⁺ to Fe²⁺ by I^-:

2Fe³⁺ + 2I^- ➔ 2Fe²⁺ + I₂

1. Oxidation of Fe²⁺ back to Fe³⁺ by S₂O₈^2-:

2Fe²⁺ + S₂O₈^2- ➔ 2Fe³⁺ + 2SO₄^2-

The overall reaction is catalysed by both Fe²⁺ and Fe³⁺ so the redox reactions can take in either order.

Fe²⁺ or Fe³⁺ ions

NO₂ + 2CO ➔ 1⁄2N₂ + 2CO₂