A catalytic converter is fitted to a vehicle exhaust to change the most harmful pollutants into less harmful gases.It contains a ceramic honeycomb coated with a thin layer of precious metals, usually platinum, palladium and rhodium. The honeycomb gives a very large surface area, so only a small mass of expensive metal is needed.
The metals are heterogeneous catalysts: they are solids, while the reactants are gases. Gas molecules are adsorbed onto the metal surface, bonds weaken and react, and the products are desorbed.Reactions in the converter:
Carbon monoxide and nitrogen monoxide react together:
2CO + 2NO → 2CO2 + N2
Unburnt hydrocarbons and carbon monoxide are oxidised:
2CO + O2 → 2CO2
C8H18 + 12½O2 → 8CO2 + 9H2O
Hydrocarbons can also react with NO:
C8H18 + 25NO → 8CO2 + 9H2O + 12½N2
(Check oxygen: 25 = 16 + 9; nitrogen: 25 = 2 × 12½.)Limitations:
They do not work well until they are hot, so short journeys give little benefit.
Lead poisons the catalyst by being adsorbed strongly and blocking the active sites, so only unleaded fuel can be used.
They do not remove carbon dioxide, a greenhouse gas; they actually convert CO and hydrocarbons into more CO2.
The precious metals are expensive and rare.
The metals are heterogeneous catalysts: they are solids, while the reactants are gases. Gas molecules are adsorbed onto the metal surface, bonds weaken and react, and the products are desorbed.Reactions in the converter:Carbon monoxide and nitrogen monoxide react together:
Unburnt hydrocarbons and carbon monoxide are oxidised:
Hydrocarbons can also react with NO:
(Check oxygen: 25 = 16 + 9; nitrogen: 25 = 2 × 12½.)Limitations:
They do not work well until they are hot, so short journeys give little benefit.
Lead poisons the catalyst by being adsorbed strongly and blocking the active sites, so only unleaded fuel can be used.
They do not remove carbon dioxide, a greenhouse gas; they actually convert CO and hydrocarbons into more CO2.
The precious metals are expensive and rare.